31 Thornhill, R. A. (1970) Z Zellforsch. 109, 147-157 32 Barber, P. C. and Raisman, G. (1978) Brain Res. 141, 57-66 33 Wilson, K. C. P. and Raisman, G. (1980) Brain Res. 185, 103-113 34 Leblond, C. P. and Walker, B. E. (1956) Physiol. Rev. 36, 255-275 35 Hinds, J. W., Hinds, P. L. and McNelly, N. A. (1984) Anat. Rec. 210, 375-383 36 Breipohl, W., Mackay-Sim, A., Grandt, D., Rehn, B. and Darrelmann, C. (1986) in Ontogeny of Olfaction (Breipohl, W., ed.), pp. 21-33, Springer-Verlag 37 Farbman, A. I., Brunjes, P. C., Rentfro, L., Michas, J. and Ritz, S. (1988) J. Neurosci. 8, 3290-3295 38 Cuschieri, A. and Bannister, L. H. (1975) J. Anat. 119, 471-498 39 Chuah, M. I., Farbman, A. I. and Menco, B. P. M. (1985) Brain Res. 338, 259-266
letters Comparisons of neocortex and hippocampus SIR:
Upon reading the review by Connors and Gutnick ~ concerning the firing patterns of neocortical neurons, a striking parallel between neocortex and hippocampus becomes obvious. This is interesting in light of the ongoing interest in the idea that hippocampus represents 'allocortex', a simplified form of neocortex. The view of hippocampus as a simplified neocortex has been suggested by comparative anatomical studies of these structures; each is composed of distinct layers of pyramidal cells, with numerous non-pyramidal, local-circuit neurons ('interneurons') scattered throughout all layers. Despite the anatomical similarities, it has not been obvious how to compare the physiology of hippocampus with that of neocortex. Connors and Gutnick provide a classification of neocortical neurons that offers a means to compare neocortex with hippocampus on physiological grounds, because hippocampal neurons fall into the same general classification scheme as that proposed by Connors and Gutnick for neocortical neurons. Thus, the pyramidal cells of area CA1 are similar to the neocortical pyramidal cells that are 'regularspiking', and the pyramidal cells of area CA3 are similar to the neocortical pyramidal cells of layers IV and V that have intrinsic burst firing behavior. Finally, the third class of neocortical neurons, which are 'fast-spiking' and appear to correspond to the TINS, VoL 13, No. 9, 1990
Acknowledgements
40 Simmons, P. A. and Getchell, T. V. (1981) J. NeurophysioL 45, 529-549 41 Gasser, H. S. (1956) J. Gen. Physiol. 39, 473-496 42 Schwob, J. E., Farber, N. B. and Gottlieb, D. I. (1986) J. Neurosci. 6, 208-217 43 Yamagishi, M., Hasegawa, S,, Nakano, Y., Takahashi, S. and Iwanaga, T. (1989)Ann. Otol. Rhinol. Laryngol. 98, 384-388 44 Talamo, B. eta/. (1989) Nature 337, 736-739 45 Viereck, C., Tucker, R. P. and Matus, A. (1989) J. Neurosci. 9, 3547-3557 46 Carr, V. M. and Farbman, A. I. (1989) Soc. Neurosci. Abstr. 15,444 47 Mackay-Sim, A. and Beard, M. D. (1987) Dev. Brain Res. 36, 190-198 48 Mackay-Sim, A. and Patel, U. (1984) Exp. Brain Res. 57, 99-106 49 Mackay-Sim, A., Breipohl, W. and Kremer, M. (1988) Exp. Brain Res. 71, 189-198
non-pyramidal GABA-containing neurons, are similar electrophysiologically to the hippocampal non-pyramidal GABAergic neurons. Detailed studies of hippocampal neurons have described exceptions to this categorization for pyramidal 2'3, as well as non-pyramidal cells4, but in general hippocampal neurons can be categorized in the way that Connors and Gutnick described. Therefore, the hippocampus could be considered as actually quite similar to neocortex, except that the two types of pyramidal cells are segregated into areas in hippocampus as opposed to layers in neocortex. Hippocampal pyramidal neurons with similar physiology to the neocortical regular-spiking cells appear to congregate in area CA1 of hippocampus, and hippocampal pyramidal cells similar to the bursting neurons of neocortical layers IV and V cluster in hippocampal area CA3. The organization of intrinsic bursting cells into one region in hippocampus (CA3) may be one of the reasons that area CA3 is prone to synchronous burst discharges, and may be relevant to the seizure susceptibility of hippocampus. Such classifications and comparisons are interesting in their ability to provide insights into the functional organization of both neocortex and hippocampus, in normal situations as well as under pathological conditions such as epilepsy. The comparison described above suggests that one of the major differences between hippocampus and neocortex may be the different pattern of mi-
to the
Theauthorgratefully acknowledgesthe helpful comments and criticismsof Drs BernardP. M. Menco and VirginiaMcM. Carr. Thiswork was supportedby NIH grants #DC 00080 and a program projectgrant, #DC00347.
editor
gration of developing pyramidal neurons. If the different trophic 'factors' responsible for such migrations were to be identified, experimental manipulation of the general cytoarchitecture of the hippocampus or neocortex might be possible. Helen E. $charfman Howard Hughes Medical Institute, Department of Neurobiology and Behavior, SUNYat Stony Brook, StonyBrook, NY 11794, USA.
References 1 Connors, B. W. and Gutnick, M. J. (1990) Trends Neurosci. 13, 99-104 2 Masukawa, L. M., Benardo, L. S. and Prince, D. A. (1982) Brain Res. 242, 341-344 3 Bilkey, D. K. and Schwartzkroin, P. A. (1990) Brain Res. 514, 77-83 4 Kawaguchi, Y. and Hama, K. (1988) Exp. Brain Res. 72,494-502
Reply SIR:
We certainly agree that there are striking parallels between the intrinsic firing patterns of neurons in hippocampus and neocortex. But the parallels extend further, to include (at least) mammalian piriform cortex ~, as well as the dorsal cortex 2 and hippocampus 3 of reptiles. Each of these structures may have the equivalent of regular-spiking, intrinsically bursting and fast-spiking classes of neurons. Their ubiquity across species and across structures of the vertebrate cerebral cortex suggests that neurons within each basic class are homologous, i.e. they arose early in telencephalic evolution from the ancestral cell
© 199o.ElsevierSciencePublishersLtd,(UK) 0166- 2236/90/S0200
365
letters Comparisons of neocortex and hippocampus SIR:
Upon reading the review by Connors and Gutnick ~ concerning the firing patterns of neocortical neurons, a striking parallel between neocortex and hippocampus becomes obvious. This is interesting in light of the ongoing interest in the idea that hippocampus represents 'allocortex', a simplified form of neocortex. The view of hippocampus as a simplified neocortex has been suggested by comparative anatomical studies of these structures; each is composed of distinct layers of pyramidal cells, with numerous non-pyramidal, local-circuit neurons ('interneurons') scattered throughout all layers. Despite the anatomical similarities, it has not been obvious how to compare the physiology of hippocampus with that of neocortex. Connors and Gutnick provide a classification of neocortical neurons that offers a means to compare neocortex with hippocampus on physiological grounds, because hippocampal neurons fall into the same general classification scheme as that proposed by Connors and Gutnick for neocortical neurons. Thus, the pyramidal cells of area CA1 are similar to the neocortical pyramidal cells that are 'regularspiking', and the pyramidal cells of area CA3 are similar to the neocortical pyramidal cells of layers IV and V that have intrinsic burst firing behavior. Finally, the third class of neocortical neurons, which are 'fast-spiking' and appear to correspond to the TINS, VoL 13, No. 9, 1990
Acknowledgements
40 Simmons, P. A. and Getchell, T. V. (1981) J. NeurophysioL 45, 529-549 41 Gasser, H. S. (1956) J. Gen. Physiol. 39, 473-496 42 Schwob, J. E., Farber, N. B. and Gottlieb, D. I. (1986) J. Neurosci. 6, 208-217 43 Yamagishi, M., Hasegawa, S,, Nakano, Y., Takahashi, S. and Iwanaga, T. (1989)Ann. Otol. Rhinol. Laryngol. 98, 384-388 44 Talamo, B. eta/. (1989) Nature 337, 736-739 45 Viereck, C., Tucker, R. P. and Matus, A. (1989) J. Neurosci. 9, 3547-3557 46 Carr, V. M. and Farbman, A. I. (1989) Soc. Neurosci. Abstr. 15,444 47 Mackay-Sim, A. and Beard, M. D. (1987) Dev. Brain Res. 36, 190-198 48 Mackay-Sim, A. and Patel, U. (1984) Exp. Brain Res. 57, 99-106 49 Mackay-Sim, A., Breipohl, W. and Kremer, M. (1988) Exp. Brain Res. 71, 189-198
non-pyramidal GABA-containing neurons, are similar electrophysiologically to the hippocampal non-pyramidal GABAergic neurons. Detailed studies of hippocampal neurons have described exceptions to this categorization for pyramidal 2'3, as well as non-pyramidal cells4, but in general hippocampal neurons can be categorized in the way that Connors and Gutnick described. Therefore, the hippocampus could be considered as actually quite similar to neocortex, except that the two types of pyramidal cells are segregated into areas in hippocampus as opposed to layers in neocortex. Hippocampal pyramidal neurons with similar physiology to the neocortical regular-spiking cells appear to congregate in area CA1 of hippocampus, and hippocampal pyramidal cells similar to the bursting neurons of neocortical layers IV and V cluster in hippocampal area CA3. The organization of intrinsic bursting cells into one region in hippocampus (CA3) may be one of the reasons that area CA3 is prone to synchronous burst discharges, and may be relevant to the seizure susceptibility of hippocampus. Such classifications and comparisons are interesting in their ability to provide insights into the functional organization of both neocortex and hippocampus, in normal situations as well as under pathological conditions such as epilepsy. The comparison described above suggests that one of the major differences between hippocampus and neocortex may be the different pattern of mi-
to the
Theauthorgratefully acknowledgesthe helpful comments and criticismsof Drs BernardP. M. Menco and VirginiaMcM. Carr. Thiswork was supportedby NIH grants #DC 00080 and a program projectgrant, #DC00347.
editor
gration of developing pyramidal neurons. If the different trophic 'factors' responsible for such migrations were to be identified, experimental manipulation of the general cytoarchitecture of the hippocampus or neocortex might be possible. Helen E. $charfman Howard Hughes Medical Institute, Department of Neurobiology and Behavior, SUNYat Stony Brook, StonyBrook, NY 11794, USA.
References 1 Connors, B. W. and Gutnick, M. J. (1990) Trends Neurosci. 13, 99-104 2 Masukawa, L. M., Benardo, L. S. and Prince, D. A. (1982) Brain Res. 242, 341-344 3 Bilkey, D. K. and Schwartzkroin, P. A. (1990) Brain Res. 514, 77-83 4 Kawaguchi, Y. and Hama, K. (1988) Exp. Brain Res. 72,494-502
Reply SIR:
We certainly agree that there are striking parallels between the intrinsic firing patterns of neurons in hippocampus and neocortex. But the parallels extend further, to include (at least) mammalian piriform cortex ~, as well as the dorsal cortex 2 and hippocampus 3 of reptiles. Each of these structures may have the equivalent of regular-spiking, intrinsically bursting and fast-spiking classes of neurons. Their ubiquity across species and across structures of the vertebrate cerebral cortex suggests that neurons within each basic class are homologous, i.e. they arose early in telencephalic evolution from the ancestral cell
© 199o.ElsevierSciencePublishersLtd,(UK) 0166- 2236/90/S0200
365
