A. Ermirch. R. Landgraf and H.-J. Ruhle (Eds.) Progress in Brain Research, Vol. 91 0 1992 Elsevier Science Publishers B.V. All rights reserved
69 CHAPTER 11
Vasopressin involvement in central control of blood pressure Q.J. Pittman and B. Bagdan Neuroscience Research Group and Department of Medical Physiology, University of Calgary, Calgary, Alberta T2N 4NI, Canada
Introduction There is now considerable evidence that arginine vasopressin (AVP) is involved in the central control of blood pressure. Firstly, it is found in regions of the brain known to be involved in cardiovascular regulation (Sofroniew, 1985). Secondly, an injection of the peptide into the lateral ventricles, intrat hecal space, or directly into circumscribed brain tissue sites causes elevation of blood pressure and heart rate in both anesthetized and conscious rats (reviewed in Pittman et al., 1987). These responses appear to be sympathetically mediated, as they are accompanied by increases in plasma catecholamines (King et al., 1985; Martinet al., 1988). They can be abolished by intravenous injection of a-adrenergic antagonist or ganglionic blocking agents (Riphagen and Pittman, 1989a) and they are accompanied by increases in renal nerve activity (Riphagen and Pittman, 1989b). A role for endogenous AVP in control of blood pressure has been more difficult to demonstrate. This chapter will focus on some of the recent evidence demonstrating a role for endogenous AVP as a neurotransmitter involved in neuronal pathways involved in blood pressure control. We will first of all show evidence that putative vasopressinergic neurons that project to central autonomic nuclei receive baroreceptor information. We will then examine the evidence that activation of central nuclei containing vasopressin neurons
elevates blood pressure. We will furthermore discuss the evidence that activation of such nuclei causes AVP release in appropriate post-synaptic areas and that this released AVP is responsible, in part, for the elevations in blood pressure. Finally, we will review the evidence that other physiological and pathophysiological stimuli which alter blood pressure, may do so through neuronal pathways involving AVP. Cardiovascular influences neuronal activity
on paraventricular
The paraventricular and supraoptic nuclei of the hypothalamus contain vasopressin and oxytocin synthesizing neurons which project to the posterior pituitary where their peptide products are secreted into the blood stream. These nuclei have been particularly amenable for electrophysiological investigation and such studies have provided definitive evidence that vasopressinergic, neurohypophyseal neurons respond to elevations in blood pressure with a reduction in neuronal activity (reviewed in Poulain and Wakerley, 1982). Electrophysiological studies have also been useful for determining whether centrally projecting PVN neurons respond in a similar manner to cardiovascular perturbations. A substantial number of descending neurons in the PVN project to the nucleus tractus solitarius/dorsal vagal complex (NTS/DVC). Kannan and Yamashita (1983) examined the responses
70
of 5 1 such cells to an increase in pressure of the “isolated” carotid sinus and found that such stimulus excited two neurons and inhibited seven. Lawrence and Pittman (1985) also examined the responses of this population of neurons to a number of stimuli and observed one unit which was activated by hemorrhage. In view of the fact that the synaptic interactions within the NTS of these descending neurons are not known, it is difficult to interpret these responses with respect to their potential participation in cardiovascular control. Also, it must be cautioned that extracellular recordings do not permit one to identify the peptidergic nature of the cell under study; indeed, it is calculated that only approximately 10% of the descending PVN neurons are immunoreactive for vasopressin and/or oxytocin (Sofroniew and Schrell, 1981). More recently, we have examined the responses to baroreceptor stimulation of another population of descending PVN neurons, that which projects t o the rostra1 ventrolateral medulla. This is an area which is believed to play a pivotal role in cardiovascular regulation as it is a major site of convergence of inputs arising from peripheral receptors and higher brain centers for transmittal to the pre-ganglionic sympathetics (Dampney, 1990). To date, of forty neurons tested in this pathway, to increases in blood pressure caused by intravenous injection of the alpha agonist, methoxamine, or decreases in blood pressure caused by intravenous injection of sodium nitoprusside, five neurons have responded with reproducible alterations in electrical activity in response to these stimuli. As was previously observed in studies on the PVN-NTS/DVC projection, a substantial number of neurons was unresponsive to cardiovascular stimuli. In contrast, in these same animals, it was possible to reproducibly obtain inhibitory responses in neurohypophyseal neurons following elevations in blood pressure. Examples of such neurons are shown in Fig. 1.
Effects of paraventricular stimulation If neurons in such pathways are involved in cardiovascular control mechanisms, and given that in-
Y
m P
E
‘I
A
o
2 200r
4
w 0 - 01
-m 0
- 2 m i4n
5 min
-
4
5 min
Fig. 1. Ratemeter records from single PVN neurons identified using antidromic invasion criteria on neurohypophyseal ( A )or projecting to the ventral lateral medulla ( E , C ) . Lower records in each panel display blood pressure responses of the rat in response to i.v. injection of the a-agonist methoxamine, at the times indicated by arrows. The PVN neuron in A displayed the typical and abrupt cessation of activity in response to the increase in blood pressure. The PVN neuron in B , which projects to the VLM displays the typical lack of response to blood pressure alteration seen in this population of neurons. Cdisplays response typical of a small number of VLM projecting PVN neurons which reduce activity during abrupt increases in blood pressure.
traventricularly injected AVP has pressor actions, activation of the paraventricular nucleus with electrical or chemical stimulation should elevate blood pressure. This was first shown in cats by Ciriello and Calaresu (1980) when they observed that electrical stimulation of the paraventricular nucleus elicited increases in arterial pressure and heart rate and also inhibited the reflex vagal bradycardia elicited by the stimulation of the carotid sinus nerve. These results were subsequently replicated in rats using both electrical stimulation (Lawrence et al., 1984; Pittman and Franklin, 1985) and chemical stimulation
71
(Gelsema et al., 1989) with an excitatory amino acid to rule out the possibility that these effects were due to activation of fibers of passage. It must be noted that, despite the preponderance of evidence that stimulation of the paraventricular nucleus elevates blood pressure, isolated reports have occurred indicating that such stimulation may cause depressor responses (Yamashita et al., 1987; Katafuchi et al., 1988).While individual laboratories have attributed these differences to type and level of anesthesia (e.g., Kannan et al., 1989), it is also possible that location of the stimulating electrode in the magnocellular part of the PVN rather than the parvicellular, autonomic areas may account for these differences (cf., Porter and Brody, 1986a,b). Release of AVP As the PVN synthesizes a number of different transmitters, among which are a number which have been shown to be active in producing centrally mediated cardiovascular effects (cf., Howe, 1985), it must be shown that such stimulation causes AVP release in terminal areas. The first such demonstration came from our perfusion studies of the intact spinal cord in anesthetized rats, when we were able to demonstrate that electrical stimulation of the paraventricular nucleus caused increased quantities of immunoreactive AVP (and oxytocin) to appear in spinal cord perfusates (Pittman et al., 1984). Subsequently, Neumann et al. (1988) observed increased quantities of immunoreactive AVP in push-pull perfusates of the septa1 area following PVN stimulation; similar findings were obtained by Landgraf et al. (1990) during push-pull perfusion of the NTS/DVC. It is important to point out that, in interpreting such data, electrical stimulation of the PVN is expected to release AVP into the circulation concurrent with its possible release into brain areas. However, it was possible to demonstrate that elevation in the level of blood-borne peptide following intravenous injection did not cause the appearance of an elevated level of peptide recovery in the push-pull perfusate, a finding in accord with the generally accepted view that peptides in the circulation do not
cross the blood-brain barrier (this, of course, does not rule out a possible penetration of the brain at one of the circumventricular organs lacking a bloodbrain barrier). Actions of AVP antagonist While the above studies provide unequivocal evidence in favor of a role for AVP as a neurotransmitter in appropriate areas of the brain involved in cardiovascular control, they do not prove that the released peptide is responsible for the increases in blood pressure that occur subsequent to PVN stimulation. To achieve this end, it is necessary to demonstrate that one can interfere with the action of the released peptide and thereby alter the magnitude of the pressor response. With respect to AVP, we are fortunate in that a relatively specific antagonist of the V, receptor has been synthesized (Sawyer and Manning, 1984). Using the antagonist d(CH2)5Tyr(Me)AVP, Pittman and Franklin (1985) were able to reduce the magnitude of the pressor response and tachycardia to PVN stimulation by introducing the antagonist directly into the NTWDVM area. It was of interest that it was impossible to completely block the induced pressor responses; we interpreted these findings to mean that a portion of the pressor response was due to release of AVP into the NTS/ DVC with the remaining pressor activity due either to release of another peptide in this area or to release of AVP in other parts of the brain (in addition to the NTS/DVC) involved in cardiovascular control. Porter and Brody (1986a) attempted similar experiments by introducing the antagonist into the intrathecal space and were able to demonstrate a blockade of the cardiovascular effects of PVN stimulation. However, since it also reduced the effects produced by stimulation of other brain sites which do not contain vasopressin cell bodies, they suggested that the antagonist has non-specific neurodepressant actions in the spinal cord. However, they apparently did not consider the possibility that stimulation of areas outside the PVN could produce cardiovascular effects in part through activation of afferent pathways to the PVN or to release of vaso-
12
pressin from descending fibers in terminal areas. More recently, we have examined this problem in more detail by recording renal nerve activity in response to PVN stimulation. We were able to demonstrate that P V N stimulation evoked bimodal increases in renal nerve activity; introduction of the AVP antagonist into the intrathecal space blocked the second peak in renal activity while leaving the first peak relatively unscathed (Fig. 2; Riphagen and Pittman, 1989b). We interpreted these findings to indicate that the second peak of renal nerve activity was activated by release of AVP at a synapse within the thoraco-lumbar spinal cord. The lack of effect of the antagonist on the first evoked peak of renal nerve activity argued strongly against an action of the antagonist as a non-specific depressant. It can certainly be argued that activation of cardiovascular responses and release of AVP following
Fig. 2 . Post-stimulus time histograms illustrating multi-unit, renal nerve responses to electrical stimulation of the PVN (arrows: three stimuli at 200 Hz,60 nA) during perfusion of the spinal subarachnoid space with vehicle (top trace) and with the AVP antagonist d(CH2),Tyr(Me)AVP (lower trace) in the same rat. (From Riphagen and Pittman, 1989b. by permission of Oxford University Press.)
electrical stimulation of the brain is not equivalent to a physiological stimulation for vasopressin release. There is evidence, however, that interventions which activate cardiovascular afferents cause release of AVP into the brain. For example, Kasting et al. (1981) found that hemorrhage of the sheep reduced fever in these animals, an effect in keeping with the known antipyretic role for this peptide within the brain (cf., Pittman and Thornhill, 1991). Similarly, Burnard et al. (1983) utilized the known ability of AVP to “sensitize” its own receptor by determining that hemorrhage of the rat mimicked the effect of a central injection of AVP in causing enhanced motor responses upon subsequent AVP injection. These results were interpreted to indicate a release of AVP into the brain by the hemorrhagic stimulus. Demotes-Mainard et al. (1986) were also able to detect increased levels of immunoreactive AVP in push-pull perfusates of the lateral septum following a hemorrhage stimulus. It is apparent that a considerable amount of data now exist that stimuli relevant to cardiovascular control cause release of AVP into the brain but it has been difficult to demonstrate that this endogenously released AVP is important in the control of blood pressure. Injection of a specific AVP antagonist into the lateral cerebral ventricles is without effect on resting blood pressure or heart rate in normotensive rats (Lawrence et al., 1984; King et al., 1985) and in water deprived rats (Rockhold et al., 1984). However, there is evidence that central AVP receptors may mediate the pressor responses to intracerebroventricular injection of hypertonic saline (Gruber and Eskridge, 1986; Morris et al., 1986), angiotensin I1 (Gruber and Eskridge, 1986) and bradykinin (Brooks et al., 1986). More recently, Callahan et al. (1989) observed that injection of the AVP antagonist into the lateral cerebral ventricles abolishes the elevated heart rate responses to acute foot shock. While it would appear that AVP is involved at some level of the neuraxis in the elicitation of the appropriate cardiovascular responses, the actual mechanisms by which AVP activates peripheral sympathetic tone are not known. Unger et al. (1986) were able to sensitize the baroreflexes, however,
73
with intracerebroventricular injection of an AVP antagonist, thereby suggesting that endogenous AVP tonically inhibits baroreceptor inputs. However, other investigators (Rohmeiss et al., 1986) have failed to demonstrate such an influence of AVP on baroreflex mechanisms. We have also attempted to demonstrate a role for central AVP in central pressor actions by testing the hypothesis that AVP, released within the brain, may play a role in maintenance of blood pressure. We therefore gave an AVP antagonist intracerebroventricularly to determine if such blockade of central AVP receptors alters either the magnitude of hypotension due to hemorrhage, or the ability of the rat to restore its blood pressure to near normal values. Because of the redundancy in cardiovascular control mechanisms, we elected to treat the animals intravenously with saralasin and the AVP antagonist (d(CH2)STyr(Me)AVP) to block the pressor actions of circulating peptides angiotensin I1 and vasopressin. In animals treated in this manner, we found that injection of 50 pmol of the antagonist d(CH2)STyr(Me)AVP into the lateral ventricle did not alter the magnitude of the fall in blood pressure or of the reflex tachycardia in conscious rats following a 2-ml hemorrhage. However, injection of 500 pmol of the antagonist resulted in a hypotension after hemorrhage which was significantly greater than that seen in the same animals given the vehicle only. In addition, the reflex tachycardia was reduced from an increase of 47 10 bpm to 7 k 16 bpm. Thus, we interpret these data to indicate that the central AVP antagonist blocked vasopressin receptors which are important in activating compensatory mechanisms designed to bring blood pressure back to normal. In conclusion, the location of AVP receptors in the brain (cf., Jard et al., 1987), as well as that of AVP immunoreactive projections throughout the brain (Sofroniew, 1985) argue strongly that this peptide may be involved in central control of blood pressure. Evidence is now accumulating that the peptide is released within the brain in response to appropriate stimulation and that this released peptide may be important in blood pressure control. Future
*
experiments must be directed towards dissecting the fine circuitry of the innervation of central autonomic nuclei by AVP fibers, and towards elucidation of the mechanism of this peptide in altering neuronal electrical responses. Acknowledgements This work was supported by the Medical Research Council of Canada and Alberta Heart Foundation. Q.J.P. is an AHFMR Scientist and B.B. is a Canadian Heart Foundation and AHFMR student. Thanks to Mrs. D. Shaw for typing the manuscript. References Brooks, D.P., Share, L., Crofton, J.T. andNasjletter, A. (1986) Interrelationship between central bradykinin and vasopressin in conscious rats. Brain Res., 371: 42-48. Burnard, D.M., Pittman, Q.J. and Veale, W.L. (1983) Increased motor disturbances in response to arginine vasopressin following hemorrhage or hypertonic saline: evidence for central AVP release in rats. Brain Res., 273: 59-65. Callahan, M.F., Kirby, R.F., Cunningham, J.T., EskridgeSloop, S.L., Johnson, A.K., McCarty, R. and Gruber, K.A. (1 989) Central oxytocin systems may mediate a cardiovascular response to acute stress in rats. Am. J. Physiol., 256: H1369- H1377. Ciriello, J . and Calaresu, F.R. (1980) Role of paraventricular and supraoptic nuclei in central cardiovascular regulation in the cat. Am. J. Physiol., 239: R137-RI42. Dampney, R. (1990) The subretrofacial nucleus: its pivotal role in cardiovascular regulation. NIPS, 5: 63 -68. Demotes-Mainard, J., Chauveau, J . , Rodriguez, F., Vincent, J.D. and Poulain, D.A. (1986) Septa1release of vasopressin in response to osmotic, hypovolemic and electrical stimulation in rats. Brain Res., 381: 314-321. Feuerstein, G., Zerbe, R.L. and Faden, A.I. (1984) Central cardiovascular effects of vasotocin, oxytocin and vasopressin in conscious rats. J. Pharmacol. Exp. Ther., 228: 348 - 353. Gelsema, A.J., Roe, M.J. and Calaresu, F.R. (1989) Neurally mediated cardiovascular responses to stimulation of cell bodies in the hypothalamus of the rat. Brain Res., 482: 67 - 77. Gruber, K.A. and Eskridge, S.L. (1986) Activation of thecentral vasopressin system: a common pathway for several centrally acting pressor agents. Am. J. Physiol., 251: R476- R480. Jard, S., Barberis, C., Audigier, S . and Tribollet, E. (1987) Neurohypophyseal hormone receptor systems in brain and periphery. Prog. Brain Res., 72: 173- 187. Howe, P.R.C. (1985) Blood pressure control by neurotransmitters in the medulla oblongata and spinal cord. J. Auton. Nerv. syst., 12: 95 - 115.
74 Kannan, H. and Yamashita, H. (1983) Electrophysiological study of paraventricular nucleus neurons projecting to the dorsomedial medulla and their response to baroreceptor stimulation in rats. Brain Res., 279: 31 -40. Kannan, H., Hayashida, Y. and Yamashita, H. (1989) Increase in sympathetic outflow by paraventricular nucleus stimulation in awake rats. A m . J. Physiol., 256: R1325 - R1330. Kasting, N.W., Veale, W.L., Cooper, K.E. and Lederis, K. (1981) Effect of hemorrhage on fever: the putative role of vasopressin. Can. J. Physiol. Pharmacol., 5 9 324- 328. Katafuchi, T., Oomura, Y. and Kurosawa, M. (1988) Effects of chemical stimulation of paraventricular nucleus on adrenal and renal nerveactivity in rats. Neurosci. Letr., 86: 195 - 200. King, K.A. and Pang, C.C.Y. (1986) Central effects of vasopressin antagonist in normotensive and hypotensive conscious rats. Proc. West. Pharmacol. SOC.,29: 223 - 225. King, K.A., Mackie, G., Pang, C.C.Y. and Wall, R.A. (1985) Central vasopressin in the modulation of catecholamine release in conscious rats. Can. J. Physiol. Pharmacol., 63: 1501 - 1505. Landgraf, R., Malkinson, T., Horn, T., Veale, W.L., Lederis, K. and Pittman, Q.J. (1990) Release of vasopressin and oxytocin by paraventricular stimulation in rats. A m . J. Physiol., 258: R155 - R159. Lawrence, D. and Pittman, Q.J. (1985) Response of rat paraventricular neurones with central projections to suckling, hemorrhage or osmotic stimuli. Brain Res., 341: 176- 183. Lawrence, D., Ciriello, J . , Pittman, Q.J. and Lederis, K. (1984) The effect of the vasopressin antagonist d(CH&dTyrVAVP on the cardiovascular responses to stimulation of the paraventricular nucleus. Proc. West. Pharmacol. SOC.,27: 15 - 17. Martin, S.M., Malkinson, T. J., Bauce, L.L., Veale, W.L. and Pittman, Q.J. (1988) Plasma catecholamines in conscious rabbits after central administration of vasopressin. Brain Res., 457: 192- 195. Morris, M., Sain, L.E. and Schumacher, S.J. (1986) Involvement of central vasopressin receptors in the control of blood pressure. Neuroendocrinology, 43: 625 - 628. Neumann, I., Schwarzberg, H. and Landgraf, R. (1988) Measurement of septa1 release of vasopressin and oxytocin by the push-pull technique following electrical stimulation of the paraventricular nucleus of rats. Brain Res., 462: 181 - 184. Pittman, Q.J. and Franklin, L.G. (1985) Vasopressin antagonist in nucleus tractus solitarius/vagal area reduces pressor and tachycardia responses to paraventricular nucleus stimulation in rats. Neurosci. Lett., 56: 155- 160. Pittman, Q.J. and Thornhill, J . (1990) Neuropeptide mechanisms affecting temperature control. In: D. Ganten and D. Pfaff (Eds.), Current Topics in Neuroendocrinology, Vol. 10. Behavioral Aspects of Neuroendocrinology, Springer, New York, pp. 223-241. Pittman, Q . J . ,Riphagen, C.L. and Lederis, K. (1984) Release of immunoassayable neurohypophyseal peptides from rat spinal cord, in vivo. Brain Res., 300: 321 - 326.
Pittman, Q.J., Riphagen, C.L. and Martin, S.M. (1987) Arginine vasopressin: new roles for an old peptide. In: J . Ciriello, F.R. Calaresu, L.P. Renaud and C. Polosa (Eds.), Organization of the Autonomic Nervous System: Central and Peripheral Mechanisms, Alan R. Liss, New York, pp. 327 - 336. Porter, J.P. and Brody, M.J. (1986a) A V , vasopressin receptor antagonist has non-specific neurodepressant actions in the spinal cord. Neuroendocrinology, 43: 75 - 78. Porter, J.P. and Brody, M.J. (1986b) A comparison of the hemodynamic effects produced by electrical stimulation of subnuclei of the paraventricular nucleus. Brain Res., 375: 20-29. Poulain, D.A. and Wakerley, J.B. (1982) Electrophysiology of hypothalamic magnocellular neurones secreting oxytocin and vasopressin. Neuroscience, 7: 773 - 808. Riphagen, C.L. and Pittman, Q.J. (1989a) Mechanisms underlying the cardiovascular responses to intrathecal vasopressin administration in rats. Can. J. Physiol. Pharmacol., 67: 269 - 275. Riphagen, C.L. and Pittman, Q.J. (1989b) Spinal arginine vasopressin elevates renal nerve activity in the rat. J. Neuroendocrinol., l(5): 339- 344. Rockhold, R.W., Share, L., Crofton, J.T. and Brooks, D.P. (1984) Cardiovascular response to vasopressin vasopressor antagonist administration during water deprivation in the rat. Neuroendocrinology, 38: 139 - 144. Rohmeiss, P., Becker, H.,Dietrich, R., Luft, F. and Unger, T. (1986) Vasopressin: mechanisms of central cardiovascular action in conscious rats. . I . Cardiovasc. Pharmacol., 8 : 689 - 696. Sawyer, W.H. and Manning, M. (1984) The development of vasopressin antagonists. Fed. Proc., 43: 87 -90. Sofroniew, M.V. (1985) Vasopressin, oxytocin and their related neurophysins. In: A. Bjorklund and T. Hokfelt (Eds.), Handbook of Chemical Activity, Vol. 4, Elsevier, Amsterdam, pp. 93 - 165. Sofroniew, M.V. and Schrell, V. (1981)Evidence for adirect projection from oxytocin and vasopressin neurons in the hypothalamic paraventricular nucleus to the medulla oblongata: immunohistochemical visualization of both the horseradish peroxidase transported and the peptide produced by the same neurons. Neurosci. Lett., 22: 211 -217. Unger, T., Rohmeiss, P., Demmert, G., Ganten, D., Lang, R.E. and Luft, F.C. (1986) Differential modulation of the baroreceptor reflex by brain and plasma vasopressin. Hypertension, 8: 157 - 162. Yamashita, H., Kannan, H., Kasai, M. and Osaka, T. (1987) Decrease in blood pressure by stimulation of the rat hypothalamic paraventricular nucleus with L-glutamate or weak current. J. Auton. Nerv. Syst., 19: 229-234.
69 CHAPTER 11
Vasopressin involvement in central control of blood pressure Q.J. Pittman and B. Bagdan Neuroscience Research Group and Department of Medical Physiology, University of Calgary, Calgary, Alberta T2N 4NI, Canada
Introduction There is now considerable evidence that arginine vasopressin (AVP) is involved in the central control of blood pressure. Firstly, it is found in regions of the brain known to be involved in cardiovascular regulation (Sofroniew, 1985). Secondly, an injection of the peptide into the lateral ventricles, intrat hecal space, or directly into circumscribed brain tissue sites causes elevation of blood pressure and heart rate in both anesthetized and conscious rats (reviewed in Pittman et al., 1987). These responses appear to be sympathetically mediated, as they are accompanied by increases in plasma catecholamines (King et al., 1985; Martinet al., 1988). They can be abolished by intravenous injection of a-adrenergic antagonist or ganglionic blocking agents (Riphagen and Pittman, 1989a) and they are accompanied by increases in renal nerve activity (Riphagen and Pittman, 1989b). A role for endogenous AVP in control of blood pressure has been more difficult to demonstrate. This chapter will focus on some of the recent evidence demonstrating a role for endogenous AVP as a neurotransmitter involved in neuronal pathways involved in blood pressure control. We will first of all show evidence that putative vasopressinergic neurons that project to central autonomic nuclei receive baroreceptor information. We will then examine the evidence that activation of central nuclei containing vasopressin neurons
elevates blood pressure. We will furthermore discuss the evidence that activation of such nuclei causes AVP release in appropriate post-synaptic areas and that this released AVP is responsible, in part, for the elevations in blood pressure. Finally, we will review the evidence that other physiological and pathophysiological stimuli which alter blood pressure, may do so through neuronal pathways involving AVP. Cardiovascular influences neuronal activity
on paraventricular
The paraventricular and supraoptic nuclei of the hypothalamus contain vasopressin and oxytocin synthesizing neurons which project to the posterior pituitary where their peptide products are secreted into the blood stream. These nuclei have been particularly amenable for electrophysiological investigation and such studies have provided definitive evidence that vasopressinergic, neurohypophyseal neurons respond to elevations in blood pressure with a reduction in neuronal activity (reviewed in Poulain and Wakerley, 1982). Electrophysiological studies have also been useful for determining whether centrally projecting PVN neurons respond in a similar manner to cardiovascular perturbations. A substantial number of descending neurons in the PVN project to the nucleus tractus solitarius/dorsal vagal complex (NTS/DVC). Kannan and Yamashita (1983) examined the responses
70
of 5 1 such cells to an increase in pressure of the “isolated” carotid sinus and found that such stimulus excited two neurons and inhibited seven. Lawrence and Pittman (1985) also examined the responses of this population of neurons to a number of stimuli and observed one unit which was activated by hemorrhage. In view of the fact that the synaptic interactions within the NTS of these descending neurons are not known, it is difficult to interpret these responses with respect to their potential participation in cardiovascular control. Also, it must be cautioned that extracellular recordings do not permit one to identify the peptidergic nature of the cell under study; indeed, it is calculated that only approximately 10% of the descending PVN neurons are immunoreactive for vasopressin and/or oxytocin (Sofroniew and Schrell, 1981). More recently, we have examined the responses to baroreceptor stimulation of another population of descending PVN neurons, that which projects t o the rostra1 ventrolateral medulla. This is an area which is believed to play a pivotal role in cardiovascular regulation as it is a major site of convergence of inputs arising from peripheral receptors and higher brain centers for transmittal to the pre-ganglionic sympathetics (Dampney, 1990). To date, of forty neurons tested in this pathway, to increases in blood pressure caused by intravenous injection of the alpha agonist, methoxamine, or decreases in blood pressure caused by intravenous injection of sodium nitoprusside, five neurons have responded with reproducible alterations in electrical activity in response to these stimuli. As was previously observed in studies on the PVN-NTS/DVC projection, a substantial number of neurons was unresponsive to cardiovascular stimuli. In contrast, in these same animals, it was possible to reproducibly obtain inhibitory responses in neurohypophyseal neurons following elevations in blood pressure. Examples of such neurons are shown in Fig. 1.
Effects of paraventricular stimulation If neurons in such pathways are involved in cardiovascular control mechanisms, and given that in-
Y
m P
E
‘I
A
o
2 200r
4
w 0 - 01
-m 0
- 2 m i4n
5 min
-
4
5 min
Fig. 1. Ratemeter records from single PVN neurons identified using antidromic invasion criteria on neurohypophyseal ( A )or projecting to the ventral lateral medulla ( E , C ) . Lower records in each panel display blood pressure responses of the rat in response to i.v. injection of the a-agonist methoxamine, at the times indicated by arrows. The PVN neuron in A displayed the typical and abrupt cessation of activity in response to the increase in blood pressure. The PVN neuron in B , which projects to the VLM displays the typical lack of response to blood pressure alteration seen in this population of neurons. Cdisplays response typical of a small number of VLM projecting PVN neurons which reduce activity during abrupt increases in blood pressure.
traventricularly injected AVP has pressor actions, activation of the paraventricular nucleus with electrical or chemical stimulation should elevate blood pressure. This was first shown in cats by Ciriello and Calaresu (1980) when they observed that electrical stimulation of the paraventricular nucleus elicited increases in arterial pressure and heart rate and also inhibited the reflex vagal bradycardia elicited by the stimulation of the carotid sinus nerve. These results were subsequently replicated in rats using both electrical stimulation (Lawrence et al., 1984; Pittman and Franklin, 1985) and chemical stimulation
71
(Gelsema et al., 1989) with an excitatory amino acid to rule out the possibility that these effects were due to activation of fibers of passage. It must be noted that, despite the preponderance of evidence that stimulation of the paraventricular nucleus elevates blood pressure, isolated reports have occurred indicating that such stimulation may cause depressor responses (Yamashita et al., 1987; Katafuchi et al., 1988).While individual laboratories have attributed these differences to type and level of anesthesia (e.g., Kannan et al., 1989), it is also possible that location of the stimulating electrode in the magnocellular part of the PVN rather than the parvicellular, autonomic areas may account for these differences (cf., Porter and Brody, 1986a,b). Release of AVP As the PVN synthesizes a number of different transmitters, among which are a number which have been shown to be active in producing centrally mediated cardiovascular effects (cf., Howe, 1985), it must be shown that such stimulation causes AVP release in terminal areas. The first such demonstration came from our perfusion studies of the intact spinal cord in anesthetized rats, when we were able to demonstrate that electrical stimulation of the paraventricular nucleus caused increased quantities of immunoreactive AVP (and oxytocin) to appear in spinal cord perfusates (Pittman et al., 1984). Subsequently, Neumann et al. (1988) observed increased quantities of immunoreactive AVP in push-pull perfusates of the septa1 area following PVN stimulation; similar findings were obtained by Landgraf et al. (1990) during push-pull perfusion of the NTS/DVC. It is important to point out that, in interpreting such data, electrical stimulation of the PVN is expected to release AVP into the circulation concurrent with its possible release into brain areas. However, it was possible to demonstrate that elevation in the level of blood-borne peptide following intravenous injection did not cause the appearance of an elevated level of peptide recovery in the push-pull perfusate, a finding in accord with the generally accepted view that peptides in the circulation do not
cross the blood-brain barrier (this, of course, does not rule out a possible penetration of the brain at one of the circumventricular organs lacking a bloodbrain barrier). Actions of AVP antagonist While the above studies provide unequivocal evidence in favor of a role for AVP as a neurotransmitter in appropriate areas of the brain involved in cardiovascular control, they do not prove that the released peptide is responsible for the increases in blood pressure that occur subsequent to PVN stimulation. To achieve this end, it is necessary to demonstrate that one can interfere with the action of the released peptide and thereby alter the magnitude of the pressor response. With respect to AVP, we are fortunate in that a relatively specific antagonist of the V, receptor has been synthesized (Sawyer and Manning, 1984). Using the antagonist d(CH2)5Tyr(Me)AVP, Pittman and Franklin (1985) were able to reduce the magnitude of the pressor response and tachycardia to PVN stimulation by introducing the antagonist directly into the NTWDVM area. It was of interest that it was impossible to completely block the induced pressor responses; we interpreted these findings to mean that a portion of the pressor response was due to release of AVP into the NTS/ DVC with the remaining pressor activity due either to release of another peptide in this area or to release of AVP in other parts of the brain (in addition to the NTS/DVC) involved in cardiovascular control. Porter and Brody (1986a) attempted similar experiments by introducing the antagonist into the intrathecal space and were able to demonstrate a blockade of the cardiovascular effects of PVN stimulation. However, since it also reduced the effects produced by stimulation of other brain sites which do not contain vasopressin cell bodies, they suggested that the antagonist has non-specific neurodepressant actions in the spinal cord. However, they apparently did not consider the possibility that stimulation of areas outside the PVN could produce cardiovascular effects in part through activation of afferent pathways to the PVN or to release of vaso-
12
pressin from descending fibers in terminal areas. More recently, we have examined this problem in more detail by recording renal nerve activity in response to PVN stimulation. We were able to demonstrate that P V N stimulation evoked bimodal increases in renal nerve activity; introduction of the AVP antagonist into the intrathecal space blocked the second peak in renal activity while leaving the first peak relatively unscathed (Fig. 2; Riphagen and Pittman, 1989b). We interpreted these findings to indicate that the second peak of renal nerve activity was activated by release of AVP at a synapse within the thoraco-lumbar spinal cord. The lack of effect of the antagonist on the first evoked peak of renal nerve activity argued strongly against an action of the antagonist as a non-specific depressant. It can certainly be argued that activation of cardiovascular responses and release of AVP following
Fig. 2 . Post-stimulus time histograms illustrating multi-unit, renal nerve responses to electrical stimulation of the PVN (arrows: three stimuli at 200 Hz,60 nA) during perfusion of the spinal subarachnoid space with vehicle (top trace) and with the AVP antagonist d(CH2),Tyr(Me)AVP (lower trace) in the same rat. (From Riphagen and Pittman, 1989b. by permission of Oxford University Press.)
electrical stimulation of the brain is not equivalent to a physiological stimulation for vasopressin release. There is evidence, however, that interventions which activate cardiovascular afferents cause release of AVP into the brain. For example, Kasting et al. (1981) found that hemorrhage of the sheep reduced fever in these animals, an effect in keeping with the known antipyretic role for this peptide within the brain (cf., Pittman and Thornhill, 1991). Similarly, Burnard et al. (1983) utilized the known ability of AVP to “sensitize” its own receptor by determining that hemorrhage of the rat mimicked the effect of a central injection of AVP in causing enhanced motor responses upon subsequent AVP injection. These results were interpreted to indicate a release of AVP into the brain by the hemorrhagic stimulus. Demotes-Mainard et al. (1986) were also able to detect increased levels of immunoreactive AVP in push-pull perfusates of the lateral septum following a hemorrhage stimulus. It is apparent that a considerable amount of data now exist that stimuli relevant to cardiovascular control cause release of AVP into the brain but it has been difficult to demonstrate that this endogenously released AVP is important in the control of blood pressure. Injection of a specific AVP antagonist into the lateral cerebral ventricles is without effect on resting blood pressure or heart rate in normotensive rats (Lawrence et al., 1984; King et al., 1985) and in water deprived rats (Rockhold et al., 1984). However, there is evidence that central AVP receptors may mediate the pressor responses to intracerebroventricular injection of hypertonic saline (Gruber and Eskridge, 1986; Morris et al., 1986), angiotensin I1 (Gruber and Eskridge, 1986) and bradykinin (Brooks et al., 1986). More recently, Callahan et al. (1989) observed that injection of the AVP antagonist into the lateral cerebral ventricles abolishes the elevated heart rate responses to acute foot shock. While it would appear that AVP is involved at some level of the neuraxis in the elicitation of the appropriate cardiovascular responses, the actual mechanisms by which AVP activates peripheral sympathetic tone are not known. Unger et al. (1986) were able to sensitize the baroreflexes, however,
73
with intracerebroventricular injection of an AVP antagonist, thereby suggesting that endogenous AVP tonically inhibits baroreceptor inputs. However, other investigators (Rohmeiss et al., 1986) have failed to demonstrate such an influence of AVP on baroreflex mechanisms. We have also attempted to demonstrate a role for central AVP in central pressor actions by testing the hypothesis that AVP, released within the brain, may play a role in maintenance of blood pressure. We therefore gave an AVP antagonist intracerebroventricularly to determine if such blockade of central AVP receptors alters either the magnitude of hypotension due to hemorrhage, or the ability of the rat to restore its blood pressure to near normal values. Because of the redundancy in cardiovascular control mechanisms, we elected to treat the animals intravenously with saralasin and the AVP antagonist (d(CH2)STyr(Me)AVP) to block the pressor actions of circulating peptides angiotensin I1 and vasopressin. In animals treated in this manner, we found that injection of 50 pmol of the antagonist d(CH2)STyr(Me)AVP into the lateral ventricle did not alter the magnitude of the fall in blood pressure or of the reflex tachycardia in conscious rats following a 2-ml hemorrhage. However, injection of 500 pmol of the antagonist resulted in a hypotension after hemorrhage which was significantly greater than that seen in the same animals given the vehicle only. In addition, the reflex tachycardia was reduced from an increase of 47 10 bpm to 7 k 16 bpm. Thus, we interpret these data to indicate that the central AVP antagonist blocked vasopressin receptors which are important in activating compensatory mechanisms designed to bring blood pressure back to normal. In conclusion, the location of AVP receptors in the brain (cf., Jard et al., 1987), as well as that of AVP immunoreactive projections throughout the brain (Sofroniew, 1985) argue strongly that this peptide may be involved in central control of blood pressure. Evidence is now accumulating that the peptide is released within the brain in response to appropriate stimulation and that this released peptide may be important in blood pressure control. Future
*
experiments must be directed towards dissecting the fine circuitry of the innervation of central autonomic nuclei by AVP fibers, and towards elucidation of the mechanism of this peptide in altering neuronal electrical responses. Acknowledgements This work was supported by the Medical Research Council of Canada and Alberta Heart Foundation. Q.J.P. is an AHFMR Scientist and B.B. is a Canadian Heart Foundation and AHFMR student. Thanks to Mrs. D. Shaw for typing the manuscript. References Brooks, D.P., Share, L., Crofton, J.T. andNasjletter, A. (1986) Interrelationship between central bradykinin and vasopressin in conscious rats. Brain Res., 371: 42-48. Burnard, D.M., Pittman, Q.J. and Veale, W.L. (1983) Increased motor disturbances in response to arginine vasopressin following hemorrhage or hypertonic saline: evidence for central AVP release in rats. Brain Res., 273: 59-65. Callahan, M.F., Kirby, R.F., Cunningham, J.T., EskridgeSloop, S.L., Johnson, A.K., McCarty, R. and Gruber, K.A. (1 989) Central oxytocin systems may mediate a cardiovascular response to acute stress in rats. Am. J. Physiol., 256: H1369- H1377. Ciriello, J . and Calaresu, F.R. (1980) Role of paraventricular and supraoptic nuclei in central cardiovascular regulation in the cat. Am. J. Physiol., 239: R137-RI42. Dampney, R. (1990) The subretrofacial nucleus: its pivotal role in cardiovascular regulation. NIPS, 5: 63 -68. Demotes-Mainard, J., Chauveau, J . , Rodriguez, F., Vincent, J.D. and Poulain, D.A. (1986) Septa1release of vasopressin in response to osmotic, hypovolemic and electrical stimulation in rats. Brain Res., 381: 314-321. Feuerstein, G., Zerbe, R.L. and Faden, A.I. (1984) Central cardiovascular effects of vasotocin, oxytocin and vasopressin in conscious rats. J. Pharmacol. Exp. Ther., 228: 348 - 353. Gelsema, A.J., Roe, M.J. and Calaresu, F.R. (1989) Neurally mediated cardiovascular responses to stimulation of cell bodies in the hypothalamus of the rat. Brain Res., 482: 67 - 77. Gruber, K.A. and Eskridge, S.L. (1986) Activation of thecentral vasopressin system: a common pathway for several centrally acting pressor agents. Am. J. Physiol., 251: R476- R480. Jard, S., Barberis, C., Audigier, S . and Tribollet, E. (1987) Neurohypophyseal hormone receptor systems in brain and periphery. Prog. Brain Res., 72: 173- 187. Howe, P.R.C. (1985) Blood pressure control by neurotransmitters in the medulla oblongata and spinal cord. J. Auton. Nerv. syst., 12: 95 - 115.
74 Kannan, H. and Yamashita, H. (1983) Electrophysiological study of paraventricular nucleus neurons projecting to the dorsomedial medulla and their response to baroreceptor stimulation in rats. Brain Res., 279: 31 -40. Kannan, H., Hayashida, Y. and Yamashita, H. (1989) Increase in sympathetic outflow by paraventricular nucleus stimulation in awake rats. A m . J. Physiol., 256: R1325 - R1330. Kasting, N.W., Veale, W.L., Cooper, K.E. and Lederis, K. (1981) Effect of hemorrhage on fever: the putative role of vasopressin. Can. J. Physiol. Pharmacol., 5 9 324- 328. Katafuchi, T., Oomura, Y. and Kurosawa, M. (1988) Effects of chemical stimulation of paraventricular nucleus on adrenal and renal nerveactivity in rats. Neurosci. Letr., 86: 195 - 200. King, K.A. and Pang, C.C.Y. (1986) Central effects of vasopressin antagonist in normotensive and hypotensive conscious rats. Proc. West. Pharmacol. SOC.,29: 223 - 225. King, K.A., Mackie, G., Pang, C.C.Y. and Wall, R.A. (1985) Central vasopressin in the modulation of catecholamine release in conscious rats. Can. J. Physiol. Pharmacol., 63: 1501 - 1505. Landgraf, R., Malkinson, T., Horn, T., Veale, W.L., Lederis, K. and Pittman, Q.J. (1990) Release of vasopressin and oxytocin by paraventricular stimulation in rats. A m . J. Physiol., 258: R155 - R159. Lawrence, D. and Pittman, Q.J. (1985) Response of rat paraventricular neurones with central projections to suckling, hemorrhage or osmotic stimuli. Brain Res., 341: 176- 183. Lawrence, D., Ciriello, J . , Pittman, Q.J. and Lederis, K. (1984) The effect of the vasopressin antagonist d(CH&dTyrVAVP on the cardiovascular responses to stimulation of the paraventricular nucleus. Proc. West. Pharmacol. SOC.,27: 15 - 17. Martin, S.M., Malkinson, T. J., Bauce, L.L., Veale, W.L. and Pittman, Q.J. (1988) Plasma catecholamines in conscious rabbits after central administration of vasopressin. Brain Res., 457: 192- 195. Morris, M., Sain, L.E. and Schumacher, S.J. (1986) Involvement of central vasopressin receptors in the control of blood pressure. Neuroendocrinology, 43: 625 - 628. Neumann, I., Schwarzberg, H. and Landgraf, R. (1988) Measurement of septa1 release of vasopressin and oxytocin by the push-pull technique following electrical stimulation of the paraventricular nucleus of rats. Brain Res., 462: 181 - 184. Pittman, Q.J. and Franklin, L.G. (1985) Vasopressin antagonist in nucleus tractus solitarius/vagal area reduces pressor and tachycardia responses to paraventricular nucleus stimulation in rats. Neurosci. Lett., 56: 155- 160. Pittman, Q.J. and Thornhill, J . (1990) Neuropeptide mechanisms affecting temperature control. In: D. Ganten and D. Pfaff (Eds.), Current Topics in Neuroendocrinology, Vol. 10. Behavioral Aspects of Neuroendocrinology, Springer, New York, pp. 223-241. Pittman, Q . J . ,Riphagen, C.L. and Lederis, K. (1984) Release of immunoassayable neurohypophyseal peptides from rat spinal cord, in vivo. Brain Res., 300: 321 - 326.
Pittman, Q.J., Riphagen, C.L. and Martin, S.M. (1987) Arginine vasopressin: new roles for an old peptide. In: J . Ciriello, F.R. Calaresu, L.P. Renaud and C. Polosa (Eds.), Organization of the Autonomic Nervous System: Central and Peripheral Mechanisms, Alan R. Liss, New York, pp. 327 - 336. Porter, J.P. and Brody, M.J. (1986a) A V , vasopressin receptor antagonist has non-specific neurodepressant actions in the spinal cord. Neuroendocrinology, 43: 75 - 78. Porter, J.P. and Brody, M.J. (1986b) A comparison of the hemodynamic effects produced by electrical stimulation of subnuclei of the paraventricular nucleus. Brain Res., 375: 20-29. Poulain, D.A. and Wakerley, J.B. (1982) Electrophysiology of hypothalamic magnocellular neurones secreting oxytocin and vasopressin. Neuroscience, 7: 773 - 808. Riphagen, C.L. and Pittman, Q.J. (1989a) Mechanisms underlying the cardiovascular responses to intrathecal vasopressin administration in rats. Can. J. Physiol. Pharmacol., 67: 269 - 275. Riphagen, C.L. and Pittman, Q.J. (1989b) Spinal arginine vasopressin elevates renal nerve activity in the rat. J. Neuroendocrinol., l(5): 339- 344. Rockhold, R.W., Share, L., Crofton, J.T. and Brooks, D.P. (1984) Cardiovascular response to vasopressin vasopressor antagonist administration during water deprivation in the rat. Neuroendocrinology, 38: 139 - 144. Rohmeiss, P., Becker, H.,Dietrich, R., Luft, F. and Unger, T. (1986) Vasopressin: mechanisms of central cardiovascular action in conscious rats. . I . Cardiovasc. Pharmacol., 8 : 689 - 696. Sawyer, W.H. and Manning, M. (1984) The development of vasopressin antagonists. Fed. Proc., 43: 87 -90. Sofroniew, M.V. (1985) Vasopressin, oxytocin and their related neurophysins. In: A. Bjorklund and T. Hokfelt (Eds.), Handbook of Chemical Activity, Vol. 4, Elsevier, Amsterdam, pp. 93 - 165. Sofroniew, M.V. and Schrell, V. (1981)Evidence for adirect projection from oxytocin and vasopressin neurons in the hypothalamic paraventricular nucleus to the medulla oblongata: immunohistochemical visualization of both the horseradish peroxidase transported and the peptide produced by the same neurons. Neurosci. Lett., 22: 211 -217. Unger, T., Rohmeiss, P., Demmert, G., Ganten, D., Lang, R.E. and Luft, F.C. (1986) Differential modulation of the baroreceptor reflex by brain and plasma vasopressin. Hypertension, 8: 157 - 162. Yamashita, H., Kannan, H., Kasai, M. and Osaka, T. (1987) Decrease in blood pressure by stimulation of the rat hypothalamic paraventricular nucleus with L-glutamate or weak current. J. Auton. Nerv. Syst., 19: 229-234.
