21 Shoffner, J. M. et aL (1990) Cell 61,931-937 22 Zeviani, M. etaL (1990)Am. J. Hum. Genet 47, 904-914 23 Goto, Y., Nonaka, I. and Horai, S. (1990) Nature 348, 651-653 24 Morgan-Hughes, J. A., Schapira, A. H. V., Cooper, J. M. and Clark, J. B. (1988) J. Bioenerg. Biomembranes 20, 365-382 25 Zeviani, M. et aL (1989) Nature 339, 309--311 26 Nikoskelainen, E. (1984) Neurology 34, 1482-1484 27 Wallace, D. C. etaL (1988)Science 242, 1427-1430 28 Holt, I. J., Miller, D. H. and Harding, A. E. (1989) J. Med. Genet. 26, 739-743 29 Vilkki, J., Savontaus, M-L. and Nikoskelainen, E. K. (1989) Am. J. Hum. Genet. 45, 206-211 30 Vilkki, J., Ott, J., Savontaus, M-L., Aula, P. and Nikoskelainen, E. K. Am. J. Hum. Genet. (in press) 31 Parker, W. D., Oley, C. A. and Parks, J. K. (1989) New Engl. J. Med. 320, 1331-1333 32 Howell, N. and McCullough, D. (1990) Am. J. Hum. Genet. 47, 629-634
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to the
33 Holt, I. J., Harding, A. E., Petty, R. K. H. and Morgan-Hughes, J. A. (1990) Am. J. Hum. Genet. 46, 428-433 34 Hauswirth, W. W. and Laipis, P. J. (1985) in Achievements and Perspectives of Mitochondrial Research (Bio&enesis, Vol. II) (Quagliariello, E. et al., eds), pp. 49-60, Elsevier 35 Schapira, A. H. V. eta/. (1990)J. Neurochem. 54, 823-827 36 Parker, W. D., Boyson, S. J. and Parks, J. K. (1989) Ann. Neurol. 26, 719-723 37 Bindoff, L., Birch-Machin, M., Cartlidge, N. E. F., Parker, W. D. and Turnbull, D. M. (1989) Lancet ii, 49 38 Schapira, A. H. V. et aL (1990) Movement Disord. 5, 294-297 39 Ikebe, S. et al. (1990) Biochem. Biophys. Res, Commun. 170, 1044-1048 40 Linnane, A. W., Marzuki, S., Ozawa, T. and Tanaka, M. (1989) Lancet i, 642-645 41 King, M. P. and Attardi, G. (1988) Ce1152, 811-819 42 King, M. P. and Attardi, G. (1989) Science 246, 500-503 43 Lander, E. S. and Lodish, H. (1990) Cell 61,925-926
editor References
comprises the hippocampalseptal pathway, namely hippocampus to DLSN to MSN-NDB to SIR: hippocampus 5. In fact, recent In the article by Stewart and Fox~ lesion studies with ibotenic acid° 'Do septal neurons pace the concluded that the DLSN was hippocampal theta rhythm?' the critically involved in the generauthors show a certain degree of ation of hippocampal theta acuncertainty about the approtivity. Moreover, using an in vitro priateness of applying data obseptal slice preparation, we 7 have tained from non-primate mamobserved that DLSN neurons mals to humans. This reluctance is exhibit both bursting and singledue to the 'apparent absence of spike activity with a rhythmicity hippocampal theta rhythm from similar to the theta frequency. primates'. This statement is not This activity might serve as a correct. Hippocampal rhythmic direct 'pace-maker' or an indirect slow activity (RSA) or theta 'gate' in the processes that drive rhythm was incidentally observed rhythmic bursting in MSN-NDB in man by Giaquinto 2. Using Hippocampal RSA and neurons. Such an input would spectral analysis and radio-tel- DLSN neurons add an important relationship to emetry in order to record from SIR: the models proposed by Stewart freely behaving, epileptic patients Stewart and Fox ~ raised an im- and Fox, since others 8-I° have in whom chronically indwelling portant question in addition to demonstrated that the majority of serni-microelectrodes were surgi- that of their article's title. They MSN-NDB neurons do not cally implanted, we were able to stated that 'without additional exhibit intrinsic rhythmic bursting demonstrate RSA in humans 3. details of the interaction among activity. This hippocampal RSA indicated a septal cells, it is difficult to answer Thus, we suggest that cells in dominant low frequency (3-4 Hz) the question o f . . . how atropine- the DLSN be included as an inthat was modulated with move- sensitive cells are cholinergic tegral part of the septal oscillatory ment in a similar way to that of when they are the cells affected network proposed by Stewart and lower mammals. The relative dif- by atropine'. In fact, there are Foxl; these neurons could serve ficulty in demonstrating RSA in additional cholinoceptive cells the 'pace-maker' function for the human hippocampus 4 might within the septum other than theta rhythm within the hippobe due to the decrease in RSA those of the medial septal nucleus campal-septal pathway. amplitude and regularity in higher and the nucleus of the diagonal J. P. Gallagher primates 5. It is therefore necess- band (MSN-NDB) that appear to /H. J. Twery ary to use computer analysis play a central role in the oscil- The Universityof TexasMedical Branch, Room methods in recording from freely latory network envisioned by 1.101, PharmacologyBuilding, Galveston, TX behaving subjects in order to Stewart and Fox. 77550-2782, USA. detect hippocarnpal RSA in These cells lie within the dorsohumans. We must conclude that lateral septal nucleus (DLSN), are the human hippocampus is no most likely GABAergic 2, and ex- References exception among mammals with hibit both atropine-sensitive 3 and 1 Stewart, M. and Fox, S. E. (1990) regard to the ability to display -resistant 4 responses to cholinTrends Neurosci. 13, 163-168 RSA. 20nteniente, B., Tago, H., Kirnura, H. ergic receptor stimulation. The and Maeda, T. (1986) J. Comp. F. H. Lopes da Silva extensive projection of DLSN NeuroL 248, 422--430 neurons to the MSN-NDB has Dept Experimental Zoology, University of 3 Hasuo, H. Gallagher, J. P. and Amsterdam, Kruislaan320, 1098 SM Amster- long been considered a key link in Shinnick-Gallagher, P. (1988) Brain dam The Netherlands. the three-neuron circuit that Res. 438, 323-327
Hippocampal RSA in humans
138
I Stewart, M. and Fox, S. E. (1990) Trends Neurosci. 13, 163-168 2 Giaquinto, S. (1973) Confin. Neurol. 35, 285-303 3 Arnolds, D. E. A. T., Lopes da Silva, F. G., Aitink, W., Kamp, A. and Boeijinga, P. (1980) ElectroencephaIogr. C/in. Neurophysiol. 50, 324-328 4 Halgren, E., Babb, T. L. and Crandall, P. H. (1978) Electroencephalogr. Clin. Neurophysiol. 44, 778-781 5 Crowne, D. P. and Radcliffe, D. D. (1975) in The Hippocampus (Vol. 2) (Isaacson, R. L. and Pribrarn, K. H., eds), pp. 185-203, Plenum Press
TINS, VoL 14, No. 4, 1991
......................................... 4 Wong, L. A. and Gallagher, J. P. (1989) Nature 341,439-411 5 DeFrance, J. F. (1976) in The 5eptal Nuclei (DeFrance, J. F., ed.), pp. 185-227, Plenum 6 Stewart, D. J. and Vanderwolf, C. H. (1987) Brain Res. 423, 88-100 7 Twery, M. J., Phelan, K. D. and Gallagher, J. P. (1990) Soc. Neurosci. Abstr. 16, 58 8 Segal, M. (1986) J. Physiol. 379, 309-330 9 Alvarez de Toledo, G. and Lopez8arneo, J. (1988) J. Physiol. 396, 399-415 10 Griffith, W. H. (1988) J. Neurophysiol. 59, 1590-1612
Reply SIR:
We agree with Professor Lopes da Silva that the human hippocampus can possibly display hippocampal rhythmic slow activity (RSA), or theta rhythm. Two issues, however, have made us uncomfortable about applying data derived from experiments on rats to primates. Prof. Lopes da Silva has commented on our statements that were related to these issues. First, the behavioral representation of and even the existence of primate hippocampal theta rhythm are controversial. For the purposes of argument, we described an extreme position, albeit one that many would actually defend, in which the theta rhythm itself might be an epiphenomenon in non-primates that is absent from monkey or human brains. Numerous behavioral studies in rodents suggest that the brain 'state' associated with the theta rhythm contributes to the normal functioning of the hippocampus (reviewed in Ref. 1). On the basis of our understanding of the behavioral representation of hippocampal theta activity in non-primates, it seems fair to expect some predictability in the behavioral representation of such activity in primates. Several studies of monkeys 2'3 and humans4'5 have indicated that under conditions where hippocampal theta activity might have been expected to be found (e.g. translational movements) the hippocampal EEG actually desynchronized. While a number of TINS, VoL 14, No. 4, 1991
H__ l e t t e r s reports of prominent EEG activity in the theta frequency band in monkeys and humans have appeared in the literature (reviewed in Ref. 6), the frequency of the EEG does not, by itself, prove functional similarity with the theta-related neuronal activity of rats and rabbits. Second, in non-primates the amplitude of the hippocampal theta rhythm is enormous. By comparison, only poor quality signals have been recorded from the primate brain, in spite of the efforts of numerous investigators. The reasons for this difference have not been established. These concerns have been sufficient to prevent a widespread acceptance of the existence of a primate theta activity identical to that of non-primates. Many questions wait to be specifically addressed so that the functional similarities of, and differences between, the hippocampal EEG of primates and non-primates can be identified. To this end we have recently begun investigating the hippocampal EEG in urethaneanesthetized monkeys. Early data from depth profiles, pharmacological manipulations and frequency analysis indicate that monkeys (and, we expect, humans) generate hippocampal theta activity that is homologous, but not identical, to that of rats6. However, the differences are sufficiently substantial that an understanding of the functional role of theta activity in the primate brain and its potential use as a non-invasive indicator of the functional integrity of the septo-hippocampal system remains in the future. Drs Gallagher and Twery recommend inclusion of DLSN neurons in the model of the pacemaker mechanism for the hippocampal theta rhythm. They view this nucleus as entraining MSNNDB cells or as a link in a septohippocampo-septal 'loop'. Reversible blockage of septo-hippocampal projections and the hippocampo-septal feedback has been shown to eliminate the hippocampal theta rhythm without effect on the rhythmic bursting activity of MSN-NDB cells1,7. Thus, a septo-hippocampo-septal
to
the
editor
loop is unnecessary (see also Ref. 8). The report that septal ibotenic acid lesions (primarily affecting the dorso-medial LSN)9 disrupt urethane-induced theta activity does not support such a loop hypothesis. In these LSN-lesioned animals, hippocampal theta rhythm was present during walking, probably because of activity of undamaged MSN-NDB cells. The lesion data do, however, leave open the possibility that DLSN cells are a rhythmic input to MSN-NDB cells. To our knowledge there has never been a report of rhythmic theta-related firing in lateral septal neurons. RanckI° reported data from 164 lateral septal cells (151 of which he considered to be within the DLSN) during hippocampal theta rhythm. None fired rhythmically, as the MSN-NDB cells did, and no phase relationship to the hippocampal theta rhythm could be detected. We reported data from 68 cells (intermediate part of LSN) and detected a weak phase relationship to the hippocampal theta rhythms in a third of the cells only after averaging across many cycles of the theta rhythm I~. This degree of phase locking could be entirely accounted for on the basis of feedback from hippocampal complex-spike cells that show pronounced phase-locked (but not rhythmic) firing. We view the fact that there is no rhythmic bursting of MSNNDB cells in vitro as evidence that these cells are dependent upon certain (apparently tonic) inputs to 'gate' their oscillation. This gating effect might be produced by projections from several brain regions, including the brainstem and LSN, and its source might vary with behavior. The importance of these gating inputs is highlighted by the point that neither the hippocampal theta rhythm nor the rhythmic bursting of MSN-NDB cells is continuously present. We maintain that the theta rhythm begins within the network of MSN-NDB cells, which transforms a variety of inputs to a rhythmic output that can synchronize cells throughout the hippocampal formation. Our model was intended to detail the 139
letters
to the
33 Holt, I. J., Harding, A. E., Petty, R. K. H. and Morgan-Hughes, J. A. (1990) Am. J. Hum. Genet. 46, 428-433 34 Hauswirth, W. W. and Laipis, P. J. (1985) in Achievements and Perspectives of Mitochondrial Research (Bio&enesis, Vol. II) (Quagliariello, E. et al., eds), pp. 49-60, Elsevier 35 Schapira, A. H. V. eta/. (1990)J. Neurochem. 54, 823-827 36 Parker, W. D., Boyson, S. J. and Parks, J. K. (1989) Ann. Neurol. 26, 719-723 37 Bindoff, L., Birch-Machin, M., Cartlidge, N. E. F., Parker, W. D. and Turnbull, D. M. (1989) Lancet ii, 49 38 Schapira, A. H. V. et aL (1990) Movement Disord. 5, 294-297 39 Ikebe, S. et al. (1990) Biochem. Biophys. Res, Commun. 170, 1044-1048 40 Linnane, A. W., Marzuki, S., Ozawa, T. and Tanaka, M. (1989) Lancet i, 642-645 41 King, M. P. and Attardi, G. (1988) Ce1152, 811-819 42 King, M. P. and Attardi, G. (1989) Science 246, 500-503 43 Lander, E. S. and Lodish, H. (1990) Cell 61,925-926
editor References
comprises the hippocampalseptal pathway, namely hippocampus to DLSN to MSN-NDB to SIR: hippocampus 5. In fact, recent In the article by Stewart and Fox~ lesion studies with ibotenic acid° 'Do septal neurons pace the concluded that the DLSN was hippocampal theta rhythm?' the critically involved in the generauthors show a certain degree of ation of hippocampal theta acuncertainty about the approtivity. Moreover, using an in vitro priateness of applying data obseptal slice preparation, we 7 have tained from non-primate mamobserved that DLSN neurons mals to humans. This reluctance is exhibit both bursting and singledue to the 'apparent absence of spike activity with a rhythmicity hippocampal theta rhythm from similar to the theta frequency. primates'. This statement is not This activity might serve as a correct. Hippocampal rhythmic direct 'pace-maker' or an indirect slow activity (RSA) or theta 'gate' in the processes that drive rhythm was incidentally observed rhythmic bursting in MSN-NDB in man by Giaquinto 2. Using Hippocampal RSA and neurons. Such an input would spectral analysis and radio-tel- DLSN neurons add an important relationship to emetry in order to record from SIR: the models proposed by Stewart freely behaving, epileptic patients Stewart and Fox ~ raised an im- and Fox, since others 8-I° have in whom chronically indwelling portant question in addition to demonstrated that the majority of serni-microelectrodes were surgi- that of their article's title. They MSN-NDB neurons do not cally implanted, we were able to stated that 'without additional exhibit intrinsic rhythmic bursting demonstrate RSA in humans 3. details of the interaction among activity. This hippocampal RSA indicated a septal cells, it is difficult to answer Thus, we suggest that cells in dominant low frequency (3-4 Hz) the question o f . . . how atropine- the DLSN be included as an inthat was modulated with move- sensitive cells are cholinergic tegral part of the septal oscillatory ment in a similar way to that of when they are the cells affected network proposed by Stewart and lower mammals. The relative dif- by atropine'. In fact, there are Foxl; these neurons could serve ficulty in demonstrating RSA in additional cholinoceptive cells the 'pace-maker' function for the human hippocampus 4 might within the septum other than theta rhythm within the hippobe due to the decrease in RSA those of the medial septal nucleus campal-septal pathway. amplitude and regularity in higher and the nucleus of the diagonal J. P. Gallagher primates 5. It is therefore necess- band (MSN-NDB) that appear to /H. J. Twery ary to use computer analysis play a central role in the oscil- The Universityof TexasMedical Branch, Room methods in recording from freely latory network envisioned by 1.101, PharmacologyBuilding, Galveston, TX behaving subjects in order to Stewart and Fox. 77550-2782, USA. detect hippocarnpal RSA in These cells lie within the dorsohumans. We must conclude that lateral septal nucleus (DLSN), are the human hippocampus is no most likely GABAergic 2, and ex- References exception among mammals with hibit both atropine-sensitive 3 and 1 Stewart, M. and Fox, S. E. (1990) regard to the ability to display -resistant 4 responses to cholinTrends Neurosci. 13, 163-168 RSA. 20nteniente, B., Tago, H., Kirnura, H. ergic receptor stimulation. The and Maeda, T. (1986) J. Comp. F. H. Lopes da Silva extensive projection of DLSN NeuroL 248, 422--430 neurons to the MSN-NDB has Dept Experimental Zoology, University of 3 Hasuo, H. Gallagher, J. P. and Amsterdam, Kruislaan320, 1098 SM Amster- long been considered a key link in Shinnick-Gallagher, P. (1988) Brain dam The Netherlands. the three-neuron circuit that Res. 438, 323-327
Hippocampal RSA in humans
138
I Stewart, M. and Fox, S. E. (1990) Trends Neurosci. 13, 163-168 2 Giaquinto, S. (1973) Confin. Neurol. 35, 285-303 3 Arnolds, D. E. A. T., Lopes da Silva, F. G., Aitink, W., Kamp, A. and Boeijinga, P. (1980) ElectroencephaIogr. C/in. Neurophysiol. 50, 324-328 4 Halgren, E., Babb, T. L. and Crandall, P. H. (1978) Electroencephalogr. Clin. Neurophysiol. 44, 778-781 5 Crowne, D. P. and Radcliffe, D. D. (1975) in The Hippocampus (Vol. 2) (Isaacson, R. L. and Pribrarn, K. H., eds), pp. 185-203, Plenum Press
TINS, VoL 14, No. 4, 1991
......................................... 4 Wong, L. A. and Gallagher, J. P. (1989) Nature 341,439-411 5 DeFrance, J. F. (1976) in The 5eptal Nuclei (DeFrance, J. F., ed.), pp. 185-227, Plenum 6 Stewart, D. J. and Vanderwolf, C. H. (1987) Brain Res. 423, 88-100 7 Twery, M. J., Phelan, K. D. and Gallagher, J. P. (1990) Soc. Neurosci. Abstr. 16, 58 8 Segal, M. (1986) J. Physiol. 379, 309-330 9 Alvarez de Toledo, G. and Lopez8arneo, J. (1988) J. Physiol. 396, 399-415 10 Griffith, W. H. (1988) J. Neurophysiol. 59, 1590-1612
Reply SIR:
We agree with Professor Lopes da Silva that the human hippocampus can possibly display hippocampal rhythmic slow activity (RSA), or theta rhythm. Two issues, however, have made us uncomfortable about applying data derived from experiments on rats to primates. Prof. Lopes da Silva has commented on our statements that were related to these issues. First, the behavioral representation of and even the existence of primate hippocampal theta rhythm are controversial. For the purposes of argument, we described an extreme position, albeit one that many would actually defend, in which the theta rhythm itself might be an epiphenomenon in non-primates that is absent from monkey or human brains. Numerous behavioral studies in rodents suggest that the brain 'state' associated with the theta rhythm contributes to the normal functioning of the hippocampus (reviewed in Ref. 1). On the basis of our understanding of the behavioral representation of hippocampal theta activity in non-primates, it seems fair to expect some predictability in the behavioral representation of such activity in primates. Several studies of monkeys 2'3 and humans4'5 have indicated that under conditions where hippocampal theta activity might have been expected to be found (e.g. translational movements) the hippocampal EEG actually desynchronized. While a number of TINS, VoL 14, No. 4, 1991
H__ l e t t e r s reports of prominent EEG activity in the theta frequency band in monkeys and humans have appeared in the literature (reviewed in Ref. 6), the frequency of the EEG does not, by itself, prove functional similarity with the theta-related neuronal activity of rats and rabbits. Second, in non-primates the amplitude of the hippocampal theta rhythm is enormous. By comparison, only poor quality signals have been recorded from the primate brain, in spite of the efforts of numerous investigators. The reasons for this difference have not been established. These concerns have been sufficient to prevent a widespread acceptance of the existence of a primate theta activity identical to that of non-primates. Many questions wait to be specifically addressed so that the functional similarities of, and differences between, the hippocampal EEG of primates and non-primates can be identified. To this end we have recently begun investigating the hippocampal EEG in urethaneanesthetized monkeys. Early data from depth profiles, pharmacological manipulations and frequency analysis indicate that monkeys (and, we expect, humans) generate hippocampal theta activity that is homologous, but not identical, to that of rats6. However, the differences are sufficiently substantial that an understanding of the functional role of theta activity in the primate brain and its potential use as a non-invasive indicator of the functional integrity of the septo-hippocampal system remains in the future. Drs Gallagher and Twery recommend inclusion of DLSN neurons in the model of the pacemaker mechanism for the hippocampal theta rhythm. They view this nucleus as entraining MSNNDB cells or as a link in a septohippocampo-septal 'loop'. Reversible blockage of septo-hippocampal projections and the hippocampo-septal feedback has been shown to eliminate the hippocampal theta rhythm without effect on the rhythmic bursting activity of MSN-NDB cells1,7. Thus, a septo-hippocampo-septal
to
the
editor
loop is unnecessary (see also Ref. 8). The report that septal ibotenic acid lesions (primarily affecting the dorso-medial LSN)9 disrupt urethane-induced theta activity does not support such a loop hypothesis. In these LSN-lesioned animals, hippocampal theta rhythm was present during walking, probably because of activity of undamaged MSN-NDB cells. The lesion data do, however, leave open the possibility that DLSN cells are a rhythmic input to MSN-NDB cells. To our knowledge there has never been a report of rhythmic theta-related firing in lateral septal neurons. RanckI° reported data from 164 lateral septal cells (151 of which he considered to be within the DLSN) during hippocampal theta rhythm. None fired rhythmically, as the MSN-NDB cells did, and no phase relationship to the hippocampal theta rhythm could be detected. We reported data from 68 cells (intermediate part of LSN) and detected a weak phase relationship to the hippocampal theta rhythms in a third of the cells only after averaging across many cycles of the theta rhythm I~. This degree of phase locking could be entirely accounted for on the basis of feedback from hippocampal complex-spike cells that show pronounced phase-locked (but not rhythmic) firing. We view the fact that there is no rhythmic bursting of MSNNDB cells in vitro as evidence that these cells are dependent upon certain (apparently tonic) inputs to 'gate' their oscillation. This gating effect might be produced by projections from several brain regions, including the brainstem and LSN, and its source might vary with behavior. The importance of these gating inputs is highlighted by the point that neither the hippocampal theta rhythm nor the rhythmic bursting of MSN-NDB cells is continuously present. We maintain that the theta rhythm begins within the network of MSN-NDB cells, which transforms a variety of inputs to a rhythmic output that can synchronize cells throughout the hippocampal formation. Our model was intended to detail the 139
