Components of mating dissociated by lateral olfactory tract transection in male hamsters

Brain Research, 64 (1973) 437-441

437

© Elsevier ScientificPublishing Company, Amsterdam - Printed in The Netherlands

Components of mating dissociated by lateral olfactory tract transectioa in male hamsters MARSHALL DEVOR Department of Psychology ElO-114, Massachusetts Institute of Technology, Cambridge, Mass. 02139 (U.S.A.)

(Accepted August 30th, 1973)

Inability to detect odors severely disrupts reproductive behavior in the male of only a few species (hamster, monkey, and perhaps mouse) 2,13, but in most mammals limbic structures in close synaptic contact with the olfactory bulb are intimately associated with mating behavior. For example, lesions involving the prepyriform cortex or amygdala, temporal lobe structures only 2 synapses removed from the olfactory receptors, produce bizarre perturbations in the sexual behavior of male rats, monkeys, cats, and men6,7,n, 16. Previous studies have shown that mating in the Syrian Golden Hamster is eliminated by either bilateral olfactory bulbectomy or peripheral techniques of producing anosmia4,12. The present study takes advantage of this fact in exploring some modes by which central olfactory structures may act on the rest of the limbic lobe to facilitate reproduction. The sole efferent fibers of the olfactory bulb are the axons of the mitral and tufted cells 9. They run caudally in the lateral olfactory tract (LOT) and send off collaterals which terminate in the outer part of the plexiform layer of the various allocortical structures lying sequentially along their path (see Fig. 1A). Together these structures comprise what may be called primary olfactory projection cortex. The strategy adopted here was to investigate the effects on mating behavior of transection of the LOT at various levels along its caudal traverse. Provided they are complete, such transections will abolish all direct olfactory bulb connection with areas of olfactory projection cortex caudal to the cut. Since the vomeronasal organ is probably not involved in the behaviors under consideration 4, the accessory olfactory system will not be discussed. The behavior of 55 sexually vigorous adult male Golden Hamsters, Mesoericetus auratus Waterhouse, was observed. Under pentobarbital sodium anesthesia, 49 of these animals underwent one of the following 6 surgical treatments: (1) bilateral transection of the LOT (N = 17), (2) unilateral LOT transection combined with removal of the contralateral olfactory bulb (N -- 5), (3) bilateral piercing of olfactory projection cortex surrounding the LOT without severing its fibers (N = 11), (4) unilateral piercing with or without removal of the contralateral olfactory bulb

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Fig. 1. A: diagrammatic representation of the axons of mitral and tufted cells of the olfactory bulb projecting, via the LOT, to the anterior olfactory nuclei, tenea tecta olfactory tubercle, prepiriform and periamygdaloid cortices, nucleus of LOT, cortico-medial amygdaloid complex, and rostral entorhinal cortex. The scale on the left shows anterior-posterior levels charted from histological material. The white scale on the far left is marked in millimeters. B and C: examples of experimental cases with bilateral LOT transections at rostral (B) and more caudal (C) levels.

(N = 4), (5) unilateral transection of the L O T (N = 8) or (6) bilateral removal of the "olfactory bulbs (N = 4). The remaining 6 hamsters were left intact. The sexual behavior of each hamster was observed beginning 1 week after surgery and continuing for up to 3 months postoperatively. In these tests, a female hamster, rendered sexually receptive by subcutaneous injections of estradiol benzoate followed by progesterone4,12, was placed in the individual home cage of each male for a 10 min observation period, and the occurrences of 9 mutually exclusive behavior categories were registered using a continuous event recorder. Two of these were pre-copulatory or courting behaviors: (1) investigation or licking of female's head or body and (2) licking the anal-vaginal area; four involved overt copulation: (3) mounting, (4) intromission, (5) licking of penis, and (6) ejaculation; and three were classified as non-copulatory: (7) eating, (8) attacking female and (9) grooming. Shortly before being sacrificed many of the hamsters underwent a simple test of olfactory acuity 1. Seventy-two hours after all food had been removed from the cage a Purina food pellet was buried under the home cage bedding and the latencies to find and begin eating the pellet were measured during a 5 min observation period. U p o n completion of testing the hamsters were perfused transcardially with physiological saline followed by 10 ~ formalin in saline. Brains were photographed, embedded in ~ilbumen gelatine, frozen and cut in 30/~m serial sections. Alternate sections separated by 180/~m were stained with cresylecht violet to visualize cortical damage, and by the first method of Fink and Helmet 5 to visualize the extent of de-

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generated axons in the LOT. Additional verification of the completeness of LOT transection was obtained by removing one olfactory bulb 3 days before the day of perfusion in 3 animals with bilateral L O T cuts. Failure to find degenerating axon terminals caudal to the cut, on the side of the bulbectomy, confirmed that virtually all L O T fibers had been transected in the original surgery. Based on these histological data, animals with complete L O T transections either bilaterally or unilaterally in combination with complete contralateral bulbectomies were assigned to the experimental group (N = 22) and the others to either bulbectomized (N = 4), operated (N = 23), or intact (N = 6) control groups. Data from the experimental and operated control animals were further analyzed with reference to the anterior-posterior plane in which complete L O T transection or maximal damage (in the control group) appeared (Fig. 1). In the cases of asymmetric bilateral intervention, animals were categorized according to the level of the more caudal of the two cuts. Performance scores Were averaged over the first 2 postoperative mating tests for all but 4 of the experimental hamsters (Fig. 2). In these cases some erratic mounting was observed shortly after surgery and so testing was continued until the animals' behavior had stabilized. In these casesresults of the final 2 tests were included in Fig. 2. All of the operated control animals mated normally and vigorously, like the unoperated animals, and most were able to find the buried food in the allotted 300 sec. The 4 bilaterally bulbectomized animals showed very little interest in the females, did not m a t O 2, and were unable to find buried food (Fig. 2). Among the hamsters

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Fig. 2. Mating performance and success at finding buried food within a 300 sec test in hamsters with bilateral olfactory bulb removal (BOB), experimental or control lesions at rostral (8-10) or more caudal (11-15) levels along the LOT, and intact animals.

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with LOT transections, reproductive behavior was invariably impaired, but the nature of the deficit depended on the level of the transection. The 14 animals with cuts at levels rostral to the olfactory tubercle (levels 8, 9 and 10 in Fig. 1A, e.g. see the cut illustrated in Fig. 1B) behaved like bulbectomized hamsters inasmuch as they showed little courting behavior, no copulation and were unable to find buried food (Fig. 2). In each of these measures they differed significantly from the 10 animals in their matched control group (2-tailed t-test, P < 0.01), but not from the bulbectomized group (P > 0.2). It should not be concluded that these hamsters were completely anosmic, however, as rats with similar LOT transection are able to relearn olfactory discriminations and have relatively normal odor detection thresholds la. The 8 animals with cuts at more caudal levels (level 11 and beyond in Fig. 1A, e.g. Fig. 1C), in contrast, became quite aroused and showed considerable interest in the female. But although they spent a great deal of their test time investigating and licking the female, they did not copulate with her (Fig. 2). This arousal without follow-through accounts for the fact that these hamsters spent significantly more time (P < 0.01) than their 13 matched control animals in precopulatory investigation, although they spent somewhat less time in vaginal licking (P < 0.05). Unlike the animals with rostral LOT cuts, most of these animals had little difficulty finding the buried food pellet (Fig. 2). In no case did mating reappear. Instead, many of the hamsters showed a gradual decline of sexual interest with time. These behavioral deficits probably reflect impaired responsiveness to a spectrum of cues associated with the female 3 rather than disorders of neuro-endocrine regulation as (i) testicular weight of the experimental group did not differ significantly from that of the control group (P > 0.2) and (ii) treatment of 4 of the experimental hamsters with 7 daily injections of 5 mg testosterone propionate did not restore mating as it does in castrates 12. There appears to be an abrupt rather than a gradual transition between the syndromes characteristic of rostral and caudal LOT transections. The 7 experimental hamsters with cuts at level 10, for example, spent an average of 35 sec investigating the female while the 3 with cuts at level 11 averaged 110 sec. The fact that the rostral edge of the olfactory tubercle lies between levels 10 and 11 (see Fig. 1) suggests that this structure may have a major influence on sexual arousal and courting perhaps by its relationship to the nearby hypothalamic structuresg, 14 that have been implicated in the control of reproductive behavior even in animals not as dependent on the sense of smell as is the hamster6-S, 10. Extending this notion of topographic functional specialization, it is possible that the cortical amygdaloid nucleus is an essential link between olfactory sensors and the mechanisms controlling the copulatory phase of mating. On the other hand, it remains possible that olfactory information may be distributed evenly over the olfactory cortex without much regional specialization. Following this line of reasoning one might contend that the various parts of the olfactory projection cortex exert their influence via the centrifugal or centripetal connections that converge upon other diencephalic structuresg, 14. In this way, the size of the functional olfactory projection cortex rather than specific components of it would determine the extent to which the influences of particular biologically meaningful odors are expressed. Further knowledge of the relative importance of

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topographic or convergent mechanisms may be obtained by extending the use of selective fiber transections to the various efferent connections of the olfactory projection cortex. This research was supported by a grant from the Alfred P. Sloan Foundation to the M.I.T. Psychology Department. The assistance of S. L. Chorover, M. R. Murphy and G. E. Schneider is gratefully acknowledged.

1 ALBERTS,J. R., AND GALEF, JR., B. G., Acute anosmia in the rat: a behavioral test of a peripherally-induced olfactary deficit, PhysioL Behav., 6 (1971) 619-621. 2 CHEAL,M. L., AND SPROTT, R. L., Social Olfaction: a review of the role of olfaction in a variety of animal behaviors, Psychol. Reports, 29 (1971) 195-243. 3 DEVOR, M., AND MURPHY, M. R., Social agnosia produced by peripheral olfactory blockage in hamsters, Amer. Zool., 12 (1972) 653. 4 DEVOR, M., AND MURPHY, M. R., The effect of peripheral olfactory blockade on the social behavior of the male Golden Hamster, Behav. Biol., 9 (1973) 31-42. 5 FINK, R. P., AND HEIMER,L., Two methods for selective silver impregnation of degenerating axons and their synaptic endings in the central nervous system, Brain Research, 4 (1967) 369-374. 6 GIANTONIO,G. W., LUND, N. L., AND GERALL, A. A., Effect of diencephalic and rhinencephalic lesions, on the male rat's sexual behavior, J. comp. physiol. Psychol., 73 (1970) 38-46. 7 GREEN, J. D., CLEMENTE,C. D., AND DEGROOT, J., Rhinencephalic lesions and behavior in cats, J. comp. Nenrol., 108 (1957) 505-545. 8 HART, B. L., HAUGEN, C. M., AND PETERSON,D. M., Effects of medial preoptic-anterior hypothalamic lesions on mating behavior of male cats, Brain Research, 54 (1973) 177-191. 9 HEIMER, L., Synaptic distribution of centripetal and centrifugal nerve fibers in the olfactory system of the rat. An experimental anatomical study, J. Anat. (Lond.), 103 (1968) 413-432. 10 HEIMER, L., AND LARSSON,K., Impairment of mating behavior in male rats following lesions in the preoptic-anterior hypothalamic continuum, Brain Research, 3 (1966) 248-263. 11 KLUVER,H., AND BUCY, P. C., Preliminary analysis of functions of the temporal lobes in monkeys, Arch. Neurol. Psychiat. (Chic.), 42 (1939) 979-1000. 12 MURPHY, M. R., AND SCHNEIDER,G. E., Olfactory bulb removal eliminates mating behavior in male hamsters, Science, 167 (1970) 302-303. 13 ROWE, F. A., AND EDWARDS, D. A., Olfactory bulb removal: Influences on the mating behavior of male mice, Physiol. Behav., 8 (1972) 37-41. 14 SCOTT, J. W., AND LEONARD, C. M., The olfactory connections of the lateral hypothalamus in the rat, mouse and hamster, J. comp. Neurol., 141 (1971) 331-344. 15 SLOTNICK,B. M., Odor detection and discrimination in rats following section of lateral olfactory tract, paper presented at the meeting of the Society for Neuroscience, Houston, Texas, Oct. 1972. 16 TERZIAN,H., AND DALLE ORE, G., Syndrome of Klfiver and Bucy reproduced in man by removal of the temporal lobes, Neurology, 5 (1955) 373-380.