J.Bradley Powers

Stria terminalis lesions alter the temporal pattern of copulatory behavior in the male golden hamster

The mating behavior of a group of male golden hamsters was observed before and after bilateral electrolytic lesions or knife cuts interrupting the stria terminalis (ST). Whereas males with bilateral lesions of the medial nucleus of the amygdala had previously been observed to stop mating, a majority of hamsters with bilateral ST destruction, whether by electrolytic lesions or knife cuts, continued to display mounts, intromissions, and ejaculations during tests over a two month postoperative period. ST-lesioned males did, however, display a distinctly altered pattern of copulation over the course of postoperative testing, consisting of an increase in mount latency during the first week, an increase in the inter-intromission interval during the second week, and an increase in the number of intromissions preceding ejaculation during the third and subsequent weekly tests. Males with bilateral lesions of the caudal amygdala, which damaged the amygdaloid exit of the ST, displayed alterations in copulatory behavior similar to those seen after bilateral ST destruction at a more rostral level.

Brain Aromatization of Testosterone in the Male Syrian Hamster: Effects of Androgen and Photoperiod

Estrogen formed by aromatization of testosterone (T) is involved in the activation of sexual behavior and control of the neuroendocrine system in the male Syrian hamster. Our study examined whether daylength influences formation of estrogen in the preoptic area (POA) and other neuroendocrine areas (anterior hypothalamus, AHT, and medial amygdala, MA) which are targets for T in behaviorally active males. Estrogen formation in individual brain samples was assayed in vitro using the stereospecific production of 3H2O from (1 beta-3H) T as a measurement of aromatase activity. Serum levels of PRL, LH, FSH and T were compared with brain aromatase activity. Groups of intact, castrated and T-treated (chronic silastic T implants) male hamsters, previously selected on behavioral criteria as being sexually active, were maintained on long (16:8LD) or short (8:16LD) daylength for 16 weeks. Two further groups of males either intact or castrated and T-treated were shifted after 7 weeks from the long photoperiod to 12:12LD. POA, AHT and MA areas of sexually active males contained active aromatase systems which converted 3H-T to estrogens. Enzyme activity differed between the areas (POA, MA greater than AHT). Aromatase activity was low in medial septum and cerebral samples. Castration, which reduced serum T to undetectable levels and elevated LH and FSH, had no effect on preoptic aromatase activity. Although estrogen formation in POA did not differ between 8:16LD and 16:8LD males, castration reduced aromatase activity in AHT of both short- and long-day groups. Aromatase activity in AHT was also significantly reduced in photo-inhibited 12:12LD intact males. There was no analogous decrease in 5 alpha-reductase or 17 beta-hydroxysteroid dehydrogenase (HSD) activity, indicating a specific effect on the aromatase. The effect of photoperiod on aromatase activity was not reversed by T treatment. Therefore, photoinhibition acts in part through the effects of reduced T levels on the anterior hypothalamus, but it also acts independently of circulating T. Our results suggest that both androgen and photoperiod may regulate the AHT aromatase system and that this occurs by different mechanisms. The more active aromatase system in POA is insensitive to both castration and photoperiod. Behavioral deficits in short-day males are not due to changes in the preoptic aromatase system, but may be related to changes in aromatase activity within AHT. We conclude that there is a difference in the regulation of two locally active aromatase systems within the preoptic-anterior hypothalamic complex of the male hamster.

Development of olfactory bulb organization in precocial and altricial rodents

The structural organization of the olfactory bulbs of spiny mice, Norway rats and Mongolian gerbils was followed over the course of their development. The pups of all 3 species normally begin to approach the odor of their dams at a time when their olfactory bulbs are at a similar stage of development. The data suggest that there may be a common aspect of olfactory bulb development that underlies the onset of olfactory guided approach behavior in rodents.

Photoperiodic and pineal influences on estrogen-stimulated behaviors in female Syrian hamsters.

Three experiments investigated the effects of short photoperiod exposure on the estrogenic facilitation of locomotor activity and lordosis. In Experiment 1, ovariectomized female hamsters were administered exogenous estrogen to stimulate locomotor activity in running wheels. Estrogen was effective in the long photoperiod group but did not stimulate running-wheel activity in the short photoperiod group. In Experiment 2, the role of the pineal gland in mediating photoperiodic influences on female hamster behavior was examined. Both estrogen-induced locomotor activity and estrogen+progesterone-stimulated lordosis behavior were significantly reduced in short photoperiod females. Both these photoperiodic effects were absent in pinealectomized hamsters. Sham-pinealectomized, short photoperiod females expressed behavioral deficits; pinealectomized hamsters in the short photoperiod did not. Experiment 3 investigated lordosis only and used hormone injections rather than silastic implants to administer estrogen. The photoperiodic and pineal effects observed in Experiment 2 were replicated in Experiment 3. Additionally, the suppression of lordosis responsiveness by short photoperiod exposure was estrogen dose dependent. Photoperiodic effects were present when 2 micrograms estradiol cypionate was used but absent when higher estrogen doses were used. These findings are discussed in the context of other results that suggested photoperiodic effects on hamster lordosis were pineal independent.

Pinealectomy prevents short photoperiod inhibition of male hamster sexual behavior.

The role of the pineal gland in mediating photoperiodic influences on copulatory behavior (CB) of male hamsters (Mesocricetus auratus) was assessed in the presence and absence of testosterone (T). The results demonstrate that the pineal gland is necessary for short photoperiod exposure to alter CB. Sexually experienced males were exposed to either long (14L:10D; LP) or short (8L:16D; SP) photoperiods for 13 weeks; after the first 2 weeks of exposure, all animals were castrated and then either pinealectomized (PINX) or sham operated (SHAM PINX). CB tests over an 8-week period following surgery indicated that copulatory impairments developed in all animals, but deficits occurred more rapidly among short photoperiod males with intact pineal glands (SP-SHAM PINX), compared to pinealectomized males housed in either the long (LP-PINX) or short photoperiod (SP-PINX). LP-PINX and SP-PINX animals were not statistically different on any of the CB measures examined. Nine weeks after castration (11 weeks of photoperiod exposure), all hamsters were given a T-filled Silastic capsule to restore CB. Restoration of sexual behavior was less rapid and less complete among SP-SHAM PINX hamsters. Additionally, males in this group took longer to initiate copulation relative to the pinealectomized hamsters. These findings are compared to other reports suggesting that photoperiodic effects on the sexual behavior of female hamsters do not require an intact pineal gland.

Short photoperiods increase ultrasonic vocalization rates among male Syrian hamsters

Two experiments investigated the effects of daylength on the emission of 35 kHz ultrasonic (US) calls among male hamsters. In Experiment 1, castrated males received Silastic implants subcutaneously that contained either low doses of testosterone in oil or oil alone; US calls were recorded when these males were paired with receptive females. Males exposed to eight hours of light per day (short photoperiod) called more often than males exposed to fourteen hours of light per day (long photoperiod). This was true whether or not they received testosterone. In Experiment 2, a similar testing and photoperiod exposure paradigm was used, but the subjects were gonadally intact. Among males exposed to short photoperiods, US call rates increased while endogenous testosterone levels decreased. In contrast, hamsters exposed to long photoperiods maintained stable calling rates and testosterone levels. These findings are related to recent studies concerning the neural mechanisms that regulate ultrasonic vocalizations and to the possible role of photoperiod in modulating conspecific aggression.

Melatonin and the coding of day length in male Syrian hamsters

Abstract: Two experiments investigated the response of the pituitary‐gonadal axis of pinealectomized male Syrian hamsters to programmed systemic administration of melatonin. In the first experiment, castrated male Syrian hamsters were housed in a short photoperiod (8L:16D) and maintained on subcutaneous testosterone implants for 7 weeks. These males were then pinealectomized or sham‐pinealectomized and their testosterone capsules removed. Daily infusions of melatonin 250 ng/infusion) or its vehicle were administered for 3 weeks; infusion duration was long (11 or 12 hr) or short (6 hr). Measurement of serum luteinizing hormone (LH) following this 3‐week period indicated that long‐duration melatonin infusions mimicked short‐day conditions (LH levels were low), but short‐duration infusions did not (LH levels were significantly elevated). In the second experiment, pinealectomized, gonadally intact males were housed in a 12L:12D photoperiod and injected once daily with melatonin or its vehicle, either 3 or 5 hr after dark onset for 11 weeks. These times were chosen to coincide with the light: dark cycle phase that according to published reports is optimally responsive to exogenous melatonin for the induction of short‐photoperiodic effects. Melatonin injections did not induce gonadal regression in pinealectomized hamsters. Melatonin and vehicle‐treated males responded similarly; their testis widths and serum testosterone levels were not significantly different at the end of the experiment. These results support the hypothesis that the duration of melatonin secretion each night is an important variable in conveying photoperiodic information, but that the circadian phase during which melatonin is present is not.

INFLUENCE OF DAYLENGTH ON MALE HAMSTER SEXUAL BEHAVIOR : MASKING EFFECTS OF TESTOSTERONE

Exposure of male hamsters to short photoperiods for 6-8 weeks cause deficits in sexual behavior with receptive females. The present experiment tested the hypothesis that short photoperiodic effects on behavior could be masked in the presence of chronic and stable levels of testosterone. Males were castrated and administered Silastic capsules of testosterone while housed in long (16L:8D) or short (8L:16D) photoperiodic conditions for 7 weeks. Sexual behavior tests at this time indicated that the short photoperiod males copulated less well, but group differences were not robust. Testosterone capsules were then removed and half the animals in both 16L:8D and 8L:16D were transferred to the opposite photoperiod. Sexual behavior was tested 18 days later as the effects of this functional castration developed. These tests indicate that photoperiodic effects were much more obvious in the absence of testosterone than they were during week 7 tests when testosterone was still present. The behavior of the males that were transferred from one photoperiod to the other demonstrated that exposure to the short photoperiod for only 18 days was not sufficient to generate short photoperiod-like sexual behavior deficits. In contrast, exposure to the long photoperiod for 18 days was sufficient to reverse short photoperiodic effects that had already developed.