Olfactory neurogenesis: genetic or environmental controls?

37 Rosner, W. (1990) Endocr. Rev. 11, 80-91 38 Kelly, M. J., Moss, R. L. and Dudley, C. A. (1978) Neuroendocrinology 25, 204-211 39 Havens, M. D. and Rose, J. D. (1988) Neuroendocrinology 48, 120-129 40 Kim, K. and Ramirez, V. D. (1986) Neuroendocrinology 42, 392-398 41 Hampson, E. and Kimura, D. (1988)

Behav. Neurosci. 102,456-459 42 Haskett, R. F. (1987) in Handbook of Clinical Psychoneuro-endocrinology (Nemeroff, C. B. and Loosen, P. T., eds), pp~ 449-459, John Wiley & Sons 43 Backstr6m, T., Bixo, M. and Hammarback, S. (1985) Acta Obstet. GynecoL Scand. (SuppL) 130, 19-24 44 Laduron, P. M. (1984) Biochem. Pharmacol. 33,897-903

45 Levin, B. E. (1982) Science 217, 555-557 46 Steward, O. et al. (1988)44ol. NeurobioL 2,227-261 47 Boyle, M. B., MacLusky, N. J., Naftolin, F. and Kaczmarek, L. K. (1987) Nature 330, 373-375 48 McEwen, B. S. (1988) J. Steroid Biochem. 30, 179-183

Olfactory neurogenesis:genetic or environmental controls? Albert I. Farbman A/bert l. Farbmanis at the Departmentof Neurobiologyand Physiology, Northwestern University, Evanston, IL 60208, USA.

Vertebrate olfactory neurons are unique among neulons in that they are continually replaced throughout the life of the animal. The rate of neurogenesis can be regulated by manipulating the system to abbreviate or prolong the average life of a sensoryneuron. Moreover, the neuron may die before or after reaching full maturity. When compared with other neurons, the fully mature olfactory neuron retains juvenile characteristics, it is probable that genetic controls operate to maintain this relatively immature state. Replacement of neurons is, with very few exceptions, essentially unknown in the postnatal vertebrate. Most neurons that are generated postnatally, such as granule cells in the cerebellum and olfactory bulb, are formed during infancy as a part of normal growth and development. In poikilothermic animals, such as goldfish, which exhibit continuous growth throughout life, continuing proliferation of neurons in the CNS is a reflection not of cell turnover but of growth in cell numbers to accommodate the increase in body size. However, neuron replacement is known to occur in the vocalization centers in the brain of the male songbird 1.2 and in the olfactory and vomeronasal systems of adult vertebrates in general (reviewed in Ref. 3). Neurogenesis in the avian brain is usually tied to reproductive cycles. In contrast, in olfactory and vomeronasal epithelium it is continuous throughout life, thus endowing the animal with the ability to replace neurons that might be lost as a consequence of physical, chemical, or infectious trauma. Certain sensory cells, such as taste receptor cells, are continually replaced throughout the life of adult vertebrates, both aqueous and terrestrial (cf. Refs 4-6). These cells apparently have a finite life span - i n mammals it is about 9 or 10 days for some, perhaps somewhat longer for others 6 - and they are constantly replaced by new cells. However, taste sensory cells have neither axons nor dendrites, and are not considered true neurons, although they do exhibit electrical activity when appropriately stimulated, and they are synaptically connected with a nerve ending. Hair cells, the auditory receptors, can be replaced in the avian cochlea after injury 7,8, but there is no evidence of continued genesis and turnover of these sensory cells. Olfactory sensory cells, in contrast to taste and auditory receptor cells, have a true axon and form axodendritic synapses in the CNS, so that the replacement of these cells is

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possibly a more complex phenomenon. The question is whether the control of neurogenesis in this population of cells is genetic, environmental, or both.

Genesis and longevity of olfactory sensory neurons It is now well known that olfactory sensory neurons are replaced after olfactory axotomy 9-16, after experimental injury to the m u c o s a 17-24, and even under physiological conditions 3,25-31. Mitosis occurs in the basal layer of the epithelium, and the daughter cells differentiate into new sensory neurons that grow axons to the olfactory bulb and establish synapses (Fig. 1). Similarly, in the vomeronasal organ, neurogenesis of olfactory cells continues in adult mice under physiological conditions32, 33.

These data gave rise to the notion that sensory neurons in the olfactory system were unique because they were 'disposable', i.e. new neurons were made continuously to replace the old ones, and they had a finite life span of approximately one month in mammals 3,1s,29 and somewhat longer in amphibians (cf. Ref. 16). In terms of their disposability, then, olfactory sensory neurons were thought to be similar to epidermal cells, intestinal lining (epithelial) cells and red blood cells, all of which have a limited life span and are continuously replaced. The notion of disposable neurons upset the pre-existing dogma that all neurons, once dead, were irreplaceable 34. Graziadei and Monti Graziadei 3 suggested that the regenerative capacity of the olfactory system was a genetic characteristic. This idea was challenged in the 1980s. Hinds et al. 3~ found several long-lived (12 months)olfactory cells in mice raised in the filtered air environment of a laminar flow hood. They suggested that in the absence of disease-related destruction of the olfactory epithelium, most or all sensory cell death was in the population of newly formed or not fully mature cells that failed to establish synapses with the olfactory bulb. This would mean that a functionally mature, synaptically connected cell could remain alive for an indeterminate (i.e. not genetically programmed) length of time, and that its life span is determined not by genetic factors but extrinsically, by factors related to nutrition, disease, age, hor-

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TINS, VOI. 13, NO. 9, 1990

perspectives monal state, or injury (see also Ref. 36). Related to this is work showing that unilateral occlusion of a naris can reduce the number of mitotic divisions in the progenitor cell population without changing the number of mature cells; this treatment presumably protects the ipsilateral olfactory epithelium from potentially harmful environmental influences and results in the reduction of the rate of genesis of new cells in the olfactory epithelium 37. Implicit in this model is the suggestion that an olfactory sensory neuron cannot become fully mature unless it forms an appropriate synaptic connection with its target. In other words, without this contact, the cell may be deprived of a factor it requires for the final stages of maturation; as a consequence, its development is arrested in this 'almost mature' state, and it will die for lack of the factor required for its full maturation. If the presence of a full complement of cilia in the olfactory dendrite can be used as a criterion for maturity, there are suggestions from developmental 38 and organ culture studies 39, as well as from studies on degeneration/reconstitution 4°, that ciliogenesis is incomplete in the absence of direct contact with the bulb, and that the bulb may provide elements required for full maturation, and perhaps continued survival, of sensory cells (cf. Refs 35, 36). While the new model is attractive from an evolutionary standpoint, particularly in animals that are dependent on olfaction for survival, it does not refute the evidence suggesting that the sensory cell is genetically programmed to live a relatively short life. Consistent with the latter idea is the fact that the anatomical relationship between olfactory axons and their sheath cells never progresses beyond what might be considered the embryonic state 41. Moreover, only a very small fraction of olfactory sensory cells synthesize proteins typically found in intermediate filaments (neurofilaments) of mature neurons 42~4. Further, the mature olfactory neuron continues to express juvenile forms of microtubuleassociated proteins 45. Finally, while the rate of cell proliferation in olfactory epithelium can be modulated 37, there is no evidence, to date, that mitosis can be completely shut off, short of killing the animal. Environmental influences It is clear, however, that environmental influences can modulate the survival times of olfactory neurons. We now have preliminary data suggesting that, under physiological conditions, some olfactory cells die before reaching maturity, and that the relative proportion of cells that die prematurely is increased during the reconstitution of olfactory epithelium following bulbectomy 46. In hypothyroid animals, the average longevity of recently formed sensory cells is also reduced but the rate of cell genesis is unchanged 47. Given that both the rate of genesis 37 and the life span 46,47 of olfactory neurons can be modulated, it is highly likely that regulatory mechanisms exist, probably within the epithelium itself, that modulate both cell division and cell death. Maintenance of a constant number of sensory cells TINS, VoL 13, No. 9, 1990

Fig. 1. Diagrammatic representation of ce// maturation in the olfactory epithelium. The basal cells (B) divide, giving rise to differentiating neurons. Nuclei of young neurons (1) move up in the epithelium as they grow an axor, and begin to grow a dendrite. Nuclei of almost mature neurons (2) are located more superficially; these cells have grown an axon that has reached the olfactory bulb, but has not formed a synapse, and their dendrite has nearly reached the epithelial surface. The fully mature neuron (3) is citiated and has formed a synapse in the bulb. Abbreviations: S, supporting cells; N, olfactory nerve.

is probably locally controlled because both the thickness and the rate of proliferation of olfactory epithelium within an individual animal may vary. In the salamander, thicker epithelia at the rostral end of the nasal cavity have a lower rate of cell genesis and a higher number of almost mature cells than the thinner epithelium at the caudal end 40. One regulatory mechanism that may be related to cell death in the almost mature population is the availability of space at the epithelial surface for newly developing dendrites 49. In the everyday life of most or all vertebrate animals, olfactory sensory neurons rarely live for extended periods of time because their location renders them so vulnerable to various kinds of injury. Most vertebrates are heavily dependent on the olfactory sense for behaviors such as foraging, reproduction, and various social behaviors. Without 363

perspectivesll

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that the equilibrium will be restored. The answer to the question posed in the title of this article, then, seems to be that both gens etic and environmental influences s directly or indirectly regulate olfactory neurogenesis. Important questions remain. What is the nature of the neuroALMOST PROGENrrOR IMMATURE MATURE MATURE genic stimulus (and/or the recepCELLS CELLS CELLS CELLS tor apparatus) that regulates proliferation in the olfactory cell population? Does cell longevity in the Fig. 2. The boxes represent pools of olfactory sensory cells in four stages of development, olfactory population influence cell progressing from progenitor cells, to young, almost mature and mature neurons. It is probable that division, and, if so, how? These cell death can occur in at least three, and possibly all four stages (cf. Ref. 36). issues should now be confronted. the ability to make new olfactory neurons continually, the survival of these animals might be seriously jeopardized, creating a powerful selection pressure for the evolution of a means for replacement of these neurons. The preservation of a pool of progenitor cells in olfactory epithelium, and conservation of a means to stimulate mitosis and promote development and maturation of sensory neurons distinguishes this neuronal population from virtually all others in the nervous system. Concluding remarks In any population of cells, it can be expected that the life span of individual cells, if represented on a Gaussian curve, would have extremes. While the peak of the curve, representing the average life span, might be at approximately 30 days (in mammals) it has been shown that some olfactory cells can live much longer than average 35, whereas others die prematurely 36,46. Evidence is accumulating that, under physiological conditions, olfactory cells can die at any stage of their differentiation, whether young, almost mature or fully mature (Fig. 2). It is possible that an excess of sensory neurons may be generated continuously, even under physiological conditions. Equilibrium in the total number of sensory cells in the population might then be maintained by death of surplus (immature) neurons that fail to make synaptic connections. This would not necessarily require a change in the rate of cell proliferation. By perturbing the olfactory cell population in various ways it is possible to increase or decrease the mean life span of the cells. In order to maintain a reasonable equilibrium in the total size of the neuronal population, any change, beyond a certain threshold, in the mean life span of individual neurons must be coordinated with whatever mechanisms are involved in controlling the rate of proliferation. We can think of the system as follows. There is evidence favoring the suggestion that continued mitosis in olfactory epithelium is under genetic control. However, the longevity of an olfactory cell can be modified or manipulated by environmental factors, both in nature and in the laboratory. If the equilibrium between cell division and cell death is disturbed, the rate of neurogenesis will be altered so 364

Selected references 1 Nottebohm, F. (1987)in Handbook of Physiology (Sect. 1: The Nervous System; Pt I1: Motor Control) (Mountcastle, V. and Plum, F., eds), 85-108, American Physiological Society 2 Paton, J. A. and Nottebohm, F. N. (1984) Science 225, 1046-1048 3 Graziadei, P. P. C. and Monti Graziadei, G. A. (1978) in Handbook of Sensory Physiology (Vol. IX) (Jacobson, M., ed.), pp. 55-82, Springer-Verlag 4 Beidler, L. M. and Smallman, R. (1965) J. Ceil Biol. 27, 263-272 5 Conger, A. D. and Wells, M. A. (1969) Radiation Res. 37, 31-49 6 Farbman, A. I. (1980) Cell Tissue Kinet. 13, 349-357 7 Corwin, J. T. and Cotanche, D. A. (1988) Science, 240, 1772-1774 8 Ryals, B. M. and Rubel, E. W. (1988) Science 240, 1774-1776 9 Costanzo, R. M. (1984) Brain Res. 307, 295-301 10 Graziadei, P. P. C. and Monti Graziadei, G. A. (1980) J. Neurocytol. 9, 145-162 11 Graziadei, P. P. C. and Okano, M. (1979) Acta Anat. 104, 220-236 12 Graziadei, P. P. C., Karlan, M. S., Monti Graziadei, G. A. and Bernstein, J. J. (1980) Brain Res. 186, 289-300 13 Monti Graziadei, G. A. and Graziadei, P. P. C. (1979) J. Neurocytol. 8, 197-213 14 Nagahara, Y. (1940)Japn J. Med. Sci. (Pathol.)5, 165-199 15 Samanen, D. W. and Forbes, W. B. (1984) J. Comp. Neurol. 225, 201-211 16 Simmons, P. A. and Getchell, T. V. (1981) J. Neurophysiol. 45, 516-528 17 Matulionis, D. H. (1975) Am. J. Anat. 142, 67-90 18 Matulionis, D. H. (1976) Am. J. Anat. 145, 79-100 19 Mulvaney, B. D. and Heist, H. E. (1971) J. Ultrastruct. Res. 35, 274-281 20 Mulvaney, B. D. and Heist, H. E. (1971) Am. J. Anat. 131, 241-252 21 Schultz, E. W. (1941) Proc. Soc. Exp. Biol. Med. 46, 41-43 22 Schultz, E. W. (1960) Am. J. Pathol. 37, 1-19 23 Smith, C. G. (1951) Anat. Rec. 109, 661-671 24 Rehn, B., Breipohl, W., Schmidt, C., Schmidt, U. and Effenberger, F. (1981) Chem. Senses 6, 317-328 25 Graziadei, P. P. C. (1973) Tissue Cell 5, 113-131 26 Graziadei, P. P. C. (1973) in The Ultrastructure of Sensory Organs (Friedmann, I., ed.), pp. 267-305, North Holland 27 Graziadei, P. P. C. and Metcalf, J. F. (1971) Z. Zellforsch. 116, 305-318 28 Moulton, D. G. (1974) Ann. NYAcad. Sci. 237, 52-61 29 Moulton, D. G. (1975) in Olfaction and Taste (Vol. V) (Denton, D. A. and Coghlan, J. P., eds), pp. 111-114, Academic Press 30 Moulton, D. G., Celebi, G. and Fink, R. P. (1970) in Ciba Foundation Symposium on Taste and Smell in Vertebrates (Wolstenholme, G. E. W. and Knight, J., eds), pp. 227-250, Churchill TINS, Vol. 13, No. 9, 1990

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

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