European Journal o f Pharmacology, 34 (1975) 1 5 1 - - 1 5 6 © North-Holland Publishing C o m p a n y , A m s t e r d a m -- Printed in The Netherlands
151
ROLE OF THE V E N T R A L SURFACE OF THE BRAIN STEM IN THE HYPOTENSIVE ACTION OF CLONIDINE P A S C A L B O U S Q U E T , J O S I A N E F E L D M A N , J E A N N E V E L L Y and R O G E R BLOCH
Institul de Pharmacologic el de M~decine Exp~rimentale, Equipe de Recherche Associde au CNRS No. 142, Facull~ de M~decine, Strasbourg, France Received 12 March 1975, revised MS received 21 May 1975, accepted 11 J u n e 1975
P. B O U S Q U E T , J. F E L D M A N , J. V E L L Y and R. BLOCH, Role o f the ventral surface o f the brain stem in the hypotensive action o f clonidine, E u r o p e a n J. Pharmacol. 34 ( 1975) 151--156. The areas S of the ventral surface of the brain stem and the immediately surrounding zone were superficially destroyed by the means o f electro-coagulation, in 14 cats. This destruction p r o d u c e d a drop in blood pressure, which was transient in 9 and definitive in 4 animals; in one cat only the arterial pressure did not change after the destruction. In 6 animals which have been sham-operated, clonidine (15 /gg/kg, i.v.) always induced a marked fall in b l o o d pressure whereas in 10 animals which had maintained or recovered a normal blood pressure after the destruction of the areas S, clonidine (15 pg/kg) injected intravenously no longer p r o d u c e d any decrease of the arterial pressure. These results suggest that the integrity of the areas S is necessary for the d e v e l o p m e n t of the hypotensive action of clonidine. This hypotensive drug may act, at least at the level of the ventral surface of the brain stem, through inhibition o f a vasopressive structure. Medullary chemosensitive areas
Clonidine
1. Introduction It is now a well known fact that clonidine has a centrally mediated hypotensive action (Hoefke and Kobinger, 1967; Hukuhara et at., 1968; Kobinger and Walland, 1967; Sattler and Van Zwieten, 1967); the medullary localization of this action has been demonstrated by Schmitt and Schmitt (1969); several authors have even suggested that structures like the nucleus tractus solitarii might be activated by clonidine (Schmitt et al., 1967; Haeusler, 1973). Bousquet and Guertzenstein {1973) have recently shown in cats that clonidine causes a drop in blood pressure when very small amounts are applied topically on a definite area of the ventral surface of the brain stem. This area, which is lateral to the pyramids and is limited rostrally by the pons and caudally by
Hypotensive action
the hypoglossal roots, has been studied by Schliifke and Loeschcke (1967) and Schl/ifke et al. (1970) and has therefore been called 'area S'. In this area, procaine and cooling (Schl~ifke et al., 1967, 1969), as well as pentobarbitone sodium, physostigmine, carbachol and GABA induce a vasodepressive effect, whereas strychnine and leptazol have the opposite effect (Feldberg and Guertzenstein, 1972; Guertzenstein, 1973). We have more recently reported that the topical application of large amounts of dopamine to this region usually produces a decrease of blood pressure, and that norepinephrine has either no effect, or a vasopressive one (Bloch et al., 1973). These considerations led us to study the effect of the destruction of the areas S. The aim of these experiments was to examine the role of this region in the regulation of vasomotor tone
152
and its importance as the trigger for the vasodepressive action of clonidine, thus whether or not its destruction could prevent the vasodepressive effect of the systemically injected drug.
2. Materials and methods
P. BOUSQUET ET AL.
The destruction was performed by passing a 10 mA high frequency current for 20 sec. At the end of the experiments the brain stem was removed and fixed in 10% formalin. Histological control was subsequently performed to determine the limits of the lesions. In the sham-operated animals, the ventral surface of the brain stem was also exposed, but no destruction was performed.
2.1. General 2. 3. Drugs The experiments were carried out in cats of either sex (weighing 2--3 kg) anesthetized with pentobarbitone sodium (30 mg/kg i.p.). A polyethylene catheter filled with heparinized saline was inserted into the left femoral artery; this catheter was connected to two strain gauge transducers (Statham P23 Db), which were themselves connected to a Gilson Minipolygraph. The blood pressure was recorded by a I CS3 potentiometer servomodule and the mean blood pressure was monitored by means of a ICMP integrator module. The animals were placed under artificial ventilation after cannulation of the trachea. The left femoral vein was also cannulated for intravenous injections.
Pentobarbitone (Nembutal, Abbott); clonidine • HC1 (Catapressan, B o e h r i n g e r - Ingelheim). For i.v. injections, clonidine was diluted in 0.9% NaC1 solution. Means -+ S.E.M. are given throughout this paper. Significance of differences were calculated with the Student's t-test; no significant difference: p > 0.05; significant difference: p is indicated in the text.
3. Results 3.1. Effects o f medullary destruction on the mean blood pressure
2. 2. Medullary lesions The cat's head was fixed to the ear bars and to the mouthpiece of a stereotaxic instrument (La precision cin~matographique fran~aise) and the cat was then placed on its back. The method used to expose the ventral surface of the brain stem was the same as that originally described by Feldberg and Guertzenstein (1972). The basal plate of the occipital bone and the dura mater covering the medulla oblongata were carefully removed without injuring the basilar artery or the nerves passing over the bullae tympanae. The destructions were produced by electrocoagulation under direct visual observation. A curved stainless hypodermic needle was used as an anode and was applied tangentially to each surface, perpendicularly to the midline. The cathode was placed on the back of the animal.
The destruction of both the areas S was performed as superficially as possible. In fact, the histological controls showed that the depth of the lesion did not exceed 2 mm. In 14 animals the average value of the mean blood pressure was 118 + 5 m m Hg before and 84 + 8 mm Hg after the destruction (thisvalue was obtained by taking into account the maximal drop in blood pressure observed in each case); these values are significantly different (p -4o.
CLONIDINE
15 ~uglkg MINUTES
:)
'4
6
8
4. Discussion
10 1'21~, 1 6 1 8 2 0 2 2 2 4 2 ' 6 2 8 3 0
Fig. 1. V a r i a t i o n o f t h e m e a n b l o o d pressure in shamo p e r a t e d a n i m a l s a f t e r i.v. i n j e c t i o n o f c l o n i d i n e ( m e a n values -+ S.E.M.; n = 6).
The present study reports new evidences for the involvement of the brain stem, more precisely the areas S, in the cardiovascular control
154
and in the development of the hypotensive action of clonidine. Petrovicky (1968) described, in this region, groups of nerve cells which, because of the exceptional thinness of the glia marginalis, lie immediately under the pia mater. These pools of neurons could be the morphological structures corresponding to the areas S. Our results show that superficial destruction of the brain stem at the level of the areas S generally induces a marked fall in systemic blood pressure (decreased in 13 out of 14 experiments and unchanged in only one case). On the basis of these data, we can consider that the areas S play a part in the regulation of the vasomotor tone, although it is not y e t possible to define clearly the connections between these areas and the other centres and pathways involved in the regulation ofvasomotricity. Previous experiments have already given rise to such a hypothesis: indeed, Trouth et al. (1973) have reported that the electrical stimulation of several points situated superficially at the level of the areas S mainly induces a vasopressive response. Furthermore, Bousquet and Guertzenstein (1973) have previously reported that clonidine topically applied to this zone causes a drop in blood pressure. Thus, in order to establish the importance of this region as a trigger for the hypotensive action of clonidine, we injected the drug into 10 cats which had recovered or maintained a normal blood pressure after superficial destruction of the areas S; in these animals, clonidine never induced any decrease of arterial pressure at a dose which caused a marked vasodepressive effect in each sham-operated animal. These data suggest that the integrity of the region of the areas S is necessary for the development of the hypotensive action of clonidine. A very superficial destruction of the brain tissue of this ventral zone was sufficient to prevent the hypotensive action of i.v. clonidine. These experiments thus confirm that, when the drug is topically applied to the ventral surface of the brain stem, its vasodepressive effect does not seem to be due to a diffusion of the drug into
P. BOUSQUET ET AL.
the brain stem, but to a direct action on superficial brain structures at that level. Moreover following the destruction of the areas S, clonidine injected i.v. elicited in most experiments a much more pronounced vasopressive effect than in intact animals. Thus, one can assume that in intact animals the central hypotensive effect of the drug could mask, at least partially, the pressor effect induced by clonidine. This rise in arterial pressure can be considered as a result either of activation by the drug of other brain structures where clonidine can produce a pressor effect (Bousquet and Guertzenstein, 1973) or of the peripheral vasoconstrictor effect of the drug or still more probably of both actions. However, the data reported in the present study indicate that even following this hypothesis, the region of the areas S must be intact for the development of the hypotensive action of clonidine. According to Schmitt et al. (1967, 1973); Haeusler (1973, 1974a,b) and Philippu et al. (1973, 1974), the hypotension and bradycardia induced by clonidine could be due to an activation of medullary structures involved in the depressor baroreceptor reflex; the nucleus tractus solitarii which contains the first synaptic relay of the afferent pathways of this reflex could be one of the triggers for the action of clonidine in the C.N.S. As a matter of fact, Philippu (1974) has observed in the cat that clonidine superfused on the nucleus tractus solitarii decreases the pressor response to the electrical stimulation of the posterior hypothalamus but this superfusion produced by itself no hypotensive effect. Furthermore, De Jong (1974; unpublished observations) has also shown that clonidine directly injected in the nucleus tractus solitarii of the rat is usually ineffective on the blood pressure whereas noradrenaline and a-methylnoradrenaline elicit a decrease of blood pressure when injected into this structure. Nevertheless, it should be recalled here that Schmitt and Schmitt (1969) observed a vasodepressive effect of clonidine applied to the floor of the fourth ventricle by means of pieces
CLONIDINE AND VENTRAL SURFACE OF THE BRAIN STEM
of blotting paper; but Bousquet and Guertzenstein (1973) have shown that, even in this experiment the possibility remains that the real trigger for the action of clonidine lies in the ventral surface of the brain stem. Finally, in the rat, Struycker Boudier and Van Rossum (1972) observed a vasodepressive action of clonidine when injected in the posterior hypothalamus; however, the doses they used were very large and a diffusion to other brain structures cannot therefore be excluded. Our results do not exclude the possibility that the destruction of the areas S could interrupt some pathways originating in some more rostrally situated hypothetical site of the hypotensive action of clonidine. But the region of the areas S is up to n o w the only one into the brain where it has been possible to demonstrate simultaneously that clonidine induces a drop in blood pressure when locally applied to this area, and that the integrity of this region is necessary for the development of the hypotensive action of the drug. Besides, many other drugs as well as clonidine actually induce a vasodepressive action when superfused or topically applied at the level of the areas S; among these drugs are membrane stabilizers such a procaine and pentobarbitone, whose depressive action fits well with the hypothesis of an inhibitory effect on underlying neurons. To summarize, the experiments reported here show that a superficial destruction of the areas S decreased blood pressure, and that, following such lesions, clonidine injected i.v. no longer induced a fall in blood pressure. These results suggest that the integrity of the areas S is necessary for the development of the hypotensive action of clonidine. This hypotensive drug may act, at least at that level of the ventral surface of the brain stem, through inhibition of a structure playing a role in the central regulation of blood pressure.
155
References Bloch, R.B., P. Bousquet, J. Feldman and J. Schwartz, 1973, The mechanism of action of clonidine, in Frontiers in Catecholamine Research (Pergamon Press, Oxford) p. 853. Bousquet, P. and P.G. Guertzenstein, 1973, Localization of the central cardio-vascular action of clonidine, Brit. J. Pharmacol. 49, 573. De Jong, W., 1974, Noradrenaline: central inhibitory control of blood pressure and heart rate, European J. Pharmacol., 29, 179. Feldberg, W. and P.G. Guertzenstein, 1972, A vasodepressor effect of pentobarbitone sodium, J. Physiol. 224, 83. Guertzenstein, P.G., 1973, Blood pressure effects obtained by drugs applied to the ventral surface of the brain stem, J. Physiol. 229,395. Haeusler, G., 1973, Activation of the central pathway of the baroreceptor reflex, a possible mechanism of the hypotensive action of clonidine, NaunynSchmiedeb. Arch. Pharmacol. 278,231. Haeusler, G., 1974a, Sympathetic nerve activity after noradrenaline depletion and its alteration by clonidine, Naunyn-Schmiedeb. Arch. Pharmacol. 282, R29. Haeusler, G., 1974b, Further similarities between the action of clonidine and a central activation of the depressor baroreceptor reflex, Naunyn-Schmiedeb. Arch. Pharmacol. 285, 1. Hoefke, W. and W. Kobinger, 1967, Pharmakologische Wirkungen eines neuen Antihypertensivums mit Imidazolinstruktur, Naunyn-Schmiedeb. Arch. Pharmakol. 257, 28. Hukuhara, T., Y. Otsuka, R. Takeda and F. Sakai, 1968, Die zentralen Wirkungen des 2-(2,6-Dichlorophenylamino)-2-Imidazolin Hydrochlorids, Arzneim. Forsch. 18, 1147. Kobinger, W. and A. Walland, 1967, Investigation into the mechanism of the hypotensive effect of 2(2,6-dichlorophenylamino)-2-imidazoline HCI, European J. Pharmacol. 2, 155. Petrovicky, P., 1968, Ueber die Glia marginalis und oberfl~ichliche Nervenzellen im Hirnstamm der Katze, Z. Anat. Entw. Gesch. 127,221. Philippu, A., W. Roensberg and H. Przuntek, 1973, Effects of adrenergic drugs on pressor responses to hypothalamic stimulation, Naunyn-Schmiedeb. Arch. Pharmacol. 273, 373. Philippu, A., R. Demmeler and G. Roensberg, 1974, Effects of centrally applied drugs on pressor responses to hypothalamus stimulation, NaunynSchmiedeb. Arch. Pharmacol. 282,389. Sattler, R.W. and P.A. van Zwieten, ]967, Acute hypotensive action of 2o(2,6-dichlorophenylamino)-2-imidazoline (ST 155) after infusion into the cats vertebral artery, European J. Pharmacol. 2, 9.
156 Schl~fke, M. and H.H. Loeschcke, 1967, Lokalisation eines an der Regulation von Atmung und Kreislauf beteiligten Gebietes an der ventralen Oberfl~iche der Medulla oblongata durch K~ilteblockade, Pflfigers Arch. Ges. Physiol: 297,201. Schl~fke, M., W.R. See and H.H. Loeschcke, 1970, Ventilatory response to alterations of H* ion concentration in small areas of the ventral medullary surface, Resp. Physiol. 10, 198. Schl~ifke, M., W.R. See, W.H. Massion and H.H. Loeschcke, 1969, Die Rolle, spezifischer und unspezifischer Afferenzen flit den Antrieb der Atmung, untersucht durch Reizung und Blockade yon Afferenzen und der decerebrierten Katze, Pfli]gers Arch. Ges. Physiol. 312, 189. Schmitt, H. and H. Schmitt, 1969, Localization of the hypotensive effect of 2-(2,6-dichlorophenylamino)-2-imidazoline hydrochloride (ST 155) (Catapresan), European J. Pharmacol. 6, 8.
P. BOUSQUET ET AL. Schmitt, H., H. Schmitt, J.R. Boissier and J.F. Giudicelli, 1967, Centrally mediated decrease in sympathetic tone induced by 2-(2,6-dichlorophenylamino)-2-imidazoline (ST 155, Catapresan), European J. Pharmacol. 2, 147. Schmitt, H., H. Schmitt and S. Fenard, 1973, Action of (~-adrenergic blocking drugs on the sympathetic centres and their interactions with the central sympatho-inhibitory effect of clonidine, Arzneim. Forsch. 23, 40. Struyker Boudier, H.A.K. and J.M. van Rossum, 1972, Clonidine induced cardiovascular effects after stereotaxic application in the hypothalamus of rats, J. Pharm. Pharmacol. 24,410. Trouth, C.O., H.H. Loeschcke and J. Berndt, 1973, Topography of the circulatory responses to electrical stimulation in the medulla oblongata, Pfliigers Arch. Ges. Physiol. 339,185.
151
ROLE OF THE V E N T R A L SURFACE OF THE BRAIN STEM IN THE HYPOTENSIVE ACTION OF CLONIDINE P A S C A L B O U S Q U E T , J O S I A N E F E L D M A N , J E A N N E V E L L Y and R O G E R BLOCH
Institul de Pharmacologic el de M~decine Exp~rimentale, Equipe de Recherche Associde au CNRS No. 142, Facull~ de M~decine, Strasbourg, France Received 12 March 1975, revised MS received 21 May 1975, accepted 11 J u n e 1975
P. B O U S Q U E T , J. F E L D M A N , J. V E L L Y and R. BLOCH, Role o f the ventral surface o f the brain stem in the hypotensive action o f clonidine, E u r o p e a n J. Pharmacol. 34 ( 1975) 151--156. The areas S of the ventral surface of the brain stem and the immediately surrounding zone were superficially destroyed by the means o f electro-coagulation, in 14 cats. This destruction p r o d u c e d a drop in blood pressure, which was transient in 9 and definitive in 4 animals; in one cat only the arterial pressure did not change after the destruction. In 6 animals which have been sham-operated, clonidine (15 /gg/kg, i.v.) always induced a marked fall in b l o o d pressure whereas in 10 animals which had maintained or recovered a normal blood pressure after the destruction of the areas S, clonidine (15 pg/kg) injected intravenously no longer p r o d u c e d any decrease of the arterial pressure. These results suggest that the integrity of the areas S is necessary for the d e v e l o p m e n t of the hypotensive action of clonidine. This hypotensive drug may act, at least at the level of the ventral surface of the brain stem, through inhibition o f a vasopressive structure. Medullary chemosensitive areas
Clonidine
1. Introduction It is now a well known fact that clonidine has a centrally mediated hypotensive action (Hoefke and Kobinger, 1967; Hukuhara et at., 1968; Kobinger and Walland, 1967; Sattler and Van Zwieten, 1967); the medullary localization of this action has been demonstrated by Schmitt and Schmitt (1969); several authors have even suggested that structures like the nucleus tractus solitarii might be activated by clonidine (Schmitt et al., 1967; Haeusler, 1973). Bousquet and Guertzenstein {1973) have recently shown in cats that clonidine causes a drop in blood pressure when very small amounts are applied topically on a definite area of the ventral surface of the brain stem. This area, which is lateral to the pyramids and is limited rostrally by the pons and caudally by
Hypotensive action
the hypoglossal roots, has been studied by Schliifke and Loeschcke (1967) and Schl/ifke et al. (1970) and has therefore been called 'area S'. In this area, procaine and cooling (Schl~ifke et al., 1967, 1969), as well as pentobarbitone sodium, physostigmine, carbachol and GABA induce a vasodepressive effect, whereas strychnine and leptazol have the opposite effect (Feldberg and Guertzenstein, 1972; Guertzenstein, 1973). We have more recently reported that the topical application of large amounts of dopamine to this region usually produces a decrease of blood pressure, and that norepinephrine has either no effect, or a vasopressive one (Bloch et al., 1973). These considerations led us to study the effect of the destruction of the areas S. The aim of these experiments was to examine the role of this region in the regulation of vasomotor tone
152
and its importance as the trigger for the vasodepressive action of clonidine, thus whether or not its destruction could prevent the vasodepressive effect of the systemically injected drug.
2. Materials and methods
P. BOUSQUET ET AL.
The destruction was performed by passing a 10 mA high frequency current for 20 sec. At the end of the experiments the brain stem was removed and fixed in 10% formalin. Histological control was subsequently performed to determine the limits of the lesions. In the sham-operated animals, the ventral surface of the brain stem was also exposed, but no destruction was performed.
2.1. General 2. 3. Drugs The experiments were carried out in cats of either sex (weighing 2--3 kg) anesthetized with pentobarbitone sodium (30 mg/kg i.p.). A polyethylene catheter filled with heparinized saline was inserted into the left femoral artery; this catheter was connected to two strain gauge transducers (Statham P23 Db), which were themselves connected to a Gilson Minipolygraph. The blood pressure was recorded by a I CS3 potentiometer servomodule and the mean blood pressure was monitored by means of a ICMP integrator module. The animals were placed under artificial ventilation after cannulation of the trachea. The left femoral vein was also cannulated for intravenous injections.
Pentobarbitone (Nembutal, Abbott); clonidine • HC1 (Catapressan, B o e h r i n g e r - Ingelheim). For i.v. injections, clonidine was diluted in 0.9% NaC1 solution. Means -+ S.E.M. are given throughout this paper. Significance of differences were calculated with the Student's t-test; no significant difference: p > 0.05; significant difference: p is indicated in the text.
3. Results 3.1. Effects o f medullary destruction on the mean blood pressure
2. 2. Medullary lesions The cat's head was fixed to the ear bars and to the mouthpiece of a stereotaxic instrument (La precision cin~matographique fran~aise) and the cat was then placed on its back. The method used to expose the ventral surface of the brain stem was the same as that originally described by Feldberg and Guertzenstein (1972). The basal plate of the occipital bone and the dura mater covering the medulla oblongata were carefully removed without injuring the basilar artery or the nerves passing over the bullae tympanae. The destructions were produced by electrocoagulation under direct visual observation. A curved stainless hypodermic needle was used as an anode and was applied tangentially to each surface, perpendicularly to the midline. The cathode was placed on the back of the animal.
The destruction of both the areas S was performed as superficially as possible. In fact, the histological controls showed that the depth of the lesion did not exceed 2 mm. In 14 animals the average value of the mean blood pressure was 118 + 5 m m Hg before and 84 + 8 mm Hg after the destruction (thisvalue was obtained by taking into account the maximal drop in blood pressure observed in each case); these values are significantly different (p -4o.
CLONIDINE
15 ~uglkg MINUTES
:)
'4
6
8
4. Discussion
10 1'21~, 1 6 1 8 2 0 2 2 2 4 2 ' 6 2 8 3 0
Fig. 1. V a r i a t i o n o f t h e m e a n b l o o d pressure in shamo p e r a t e d a n i m a l s a f t e r i.v. i n j e c t i o n o f c l o n i d i n e ( m e a n values -+ S.E.M.; n = 6).
The present study reports new evidences for the involvement of the brain stem, more precisely the areas S, in the cardiovascular control
154
and in the development of the hypotensive action of clonidine. Petrovicky (1968) described, in this region, groups of nerve cells which, because of the exceptional thinness of the glia marginalis, lie immediately under the pia mater. These pools of neurons could be the morphological structures corresponding to the areas S. Our results show that superficial destruction of the brain stem at the level of the areas S generally induces a marked fall in systemic blood pressure (decreased in 13 out of 14 experiments and unchanged in only one case). On the basis of these data, we can consider that the areas S play a part in the regulation of the vasomotor tone, although it is not y e t possible to define clearly the connections between these areas and the other centres and pathways involved in the regulation ofvasomotricity. Previous experiments have already given rise to such a hypothesis: indeed, Trouth et al. (1973) have reported that the electrical stimulation of several points situated superficially at the level of the areas S mainly induces a vasopressive response. Furthermore, Bousquet and Guertzenstein (1973) have previously reported that clonidine topically applied to this zone causes a drop in blood pressure. Thus, in order to establish the importance of this region as a trigger for the hypotensive action of clonidine, we injected the drug into 10 cats which had recovered or maintained a normal blood pressure after superficial destruction of the areas S; in these animals, clonidine never induced any decrease of arterial pressure at a dose which caused a marked vasodepressive effect in each sham-operated animal. These data suggest that the integrity of the region of the areas S is necessary for the development of the hypotensive action of clonidine. A very superficial destruction of the brain tissue of this ventral zone was sufficient to prevent the hypotensive action of i.v. clonidine. These experiments thus confirm that, when the drug is topically applied to the ventral surface of the brain stem, its vasodepressive effect does not seem to be due to a diffusion of the drug into
P. BOUSQUET ET AL.
the brain stem, but to a direct action on superficial brain structures at that level. Moreover following the destruction of the areas S, clonidine injected i.v. elicited in most experiments a much more pronounced vasopressive effect than in intact animals. Thus, one can assume that in intact animals the central hypotensive effect of the drug could mask, at least partially, the pressor effect induced by clonidine. This rise in arterial pressure can be considered as a result either of activation by the drug of other brain structures where clonidine can produce a pressor effect (Bousquet and Guertzenstein, 1973) or of the peripheral vasoconstrictor effect of the drug or still more probably of both actions. However, the data reported in the present study indicate that even following this hypothesis, the region of the areas S must be intact for the development of the hypotensive action of clonidine. According to Schmitt et al. (1967, 1973); Haeusler (1973, 1974a,b) and Philippu et al. (1973, 1974), the hypotension and bradycardia induced by clonidine could be due to an activation of medullary structures involved in the depressor baroreceptor reflex; the nucleus tractus solitarii which contains the first synaptic relay of the afferent pathways of this reflex could be one of the triggers for the action of clonidine in the C.N.S. As a matter of fact, Philippu (1974) has observed in the cat that clonidine superfused on the nucleus tractus solitarii decreases the pressor response to the electrical stimulation of the posterior hypothalamus but this superfusion produced by itself no hypotensive effect. Furthermore, De Jong (1974; unpublished observations) has also shown that clonidine directly injected in the nucleus tractus solitarii of the rat is usually ineffective on the blood pressure whereas noradrenaline and a-methylnoradrenaline elicit a decrease of blood pressure when injected into this structure. Nevertheless, it should be recalled here that Schmitt and Schmitt (1969) observed a vasodepressive effect of clonidine applied to the floor of the fourth ventricle by means of pieces
CLONIDINE AND VENTRAL SURFACE OF THE BRAIN STEM
of blotting paper; but Bousquet and Guertzenstein (1973) have shown that, even in this experiment the possibility remains that the real trigger for the action of clonidine lies in the ventral surface of the brain stem. Finally, in the rat, Struycker Boudier and Van Rossum (1972) observed a vasodepressive action of clonidine when injected in the posterior hypothalamus; however, the doses they used were very large and a diffusion to other brain structures cannot therefore be excluded. Our results do not exclude the possibility that the destruction of the areas S could interrupt some pathways originating in some more rostrally situated hypothetical site of the hypotensive action of clonidine. But the region of the areas S is up to n o w the only one into the brain where it has been possible to demonstrate simultaneously that clonidine induces a drop in blood pressure when locally applied to this area, and that the integrity of this region is necessary for the development of the hypotensive action of the drug. Besides, many other drugs as well as clonidine actually induce a vasodepressive action when superfused or topically applied at the level of the areas S; among these drugs are membrane stabilizers such a procaine and pentobarbitone, whose depressive action fits well with the hypothesis of an inhibitory effect on underlying neurons. To summarize, the experiments reported here show that a superficial destruction of the areas S decreased blood pressure, and that, following such lesions, clonidine injected i.v. no longer induced a fall in blood pressure. These results suggest that the integrity of the areas S is necessary for the development of the hypotensive action of clonidine. This hypotensive drug may act, at least at that level of the ventral surface of the brain stem, through inhibition of a structure playing a role in the central regulation of blood pressure.
155
References Bloch, R.B., P. Bousquet, J. Feldman and J. Schwartz, 1973, The mechanism of action of clonidine, in Frontiers in Catecholamine Research (Pergamon Press, Oxford) p. 853. Bousquet, P. and P.G. Guertzenstein, 1973, Localization of the central cardio-vascular action of clonidine, Brit. J. Pharmacol. 49, 573. De Jong, W., 1974, Noradrenaline: central inhibitory control of blood pressure and heart rate, European J. Pharmacol., 29, 179. Feldberg, W. and P.G. Guertzenstein, 1972, A vasodepressor effect of pentobarbitone sodium, J. Physiol. 224, 83. Guertzenstein, P.G., 1973, Blood pressure effects obtained by drugs applied to the ventral surface of the brain stem, J. Physiol. 229,395. Haeusler, G., 1973, Activation of the central pathway of the baroreceptor reflex, a possible mechanism of the hypotensive action of clonidine, NaunynSchmiedeb. Arch. Pharmacol. 278,231. Haeusler, G., 1974a, Sympathetic nerve activity after noradrenaline depletion and its alteration by clonidine, Naunyn-Schmiedeb. Arch. Pharmacol. 282, R29. Haeusler, G., 1974b, Further similarities between the action of clonidine and a central activation of the depressor baroreceptor reflex, Naunyn-Schmiedeb. Arch. Pharmacol. 285, 1. Hoefke, W. and W. Kobinger, 1967, Pharmakologische Wirkungen eines neuen Antihypertensivums mit Imidazolinstruktur, Naunyn-Schmiedeb. Arch. Pharmakol. 257, 28. Hukuhara, T., Y. Otsuka, R. Takeda and F. Sakai, 1968, Die zentralen Wirkungen des 2-(2,6-Dichlorophenylamino)-2-Imidazolin Hydrochlorids, Arzneim. Forsch. 18, 1147. Kobinger, W. and A. Walland, 1967, Investigation into the mechanism of the hypotensive effect of 2(2,6-dichlorophenylamino)-2-imidazoline HCI, European J. Pharmacol. 2, 155. Petrovicky, P., 1968, Ueber die Glia marginalis und oberfl~ichliche Nervenzellen im Hirnstamm der Katze, Z. Anat. Entw. Gesch. 127,221. Philippu, A., W. Roensberg and H. Przuntek, 1973, Effects of adrenergic drugs on pressor responses to hypothalamic stimulation, Naunyn-Schmiedeb. Arch. Pharmacol. 273, 373. Philippu, A., R. Demmeler and G. Roensberg, 1974, Effects of centrally applied drugs on pressor responses to hypothalamus stimulation, NaunynSchmiedeb. Arch. Pharmacol. 282,389. Sattler, R.W. and P.A. van Zwieten, ]967, Acute hypotensive action of 2o(2,6-dichlorophenylamino)-2-imidazoline (ST 155) after infusion into the cats vertebral artery, European J. Pharmacol. 2, 9.
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