J. Bradley Powers

OLFACTORY AND VOMERONASAL DEAFFERENTATION OF MALE HAMSTERS: HISTOLOGICAL AND BEHAVIORAL ANALYSES

Deafferentation of the vomeronasal system by cutting the vomeronasal nerves severely impaired mating behavior in 44% of male hamsters over a 1--2 month period of postoperative testing, but the remaining males mated normally after the surgery. Damage to the main olfactory bulbs, concomitant to vomeronasal nerve cuts, did not account for this behavioral difference. Subsequent deafferentation of olfactory system by intranasal infusion of zinc sulfate solution (5 g ZnSO4--7H2O in 95 ml 0.5% NaCl) had no effect on intromission or ejaculation latencies of sham vomeronasal cut males but eliminated mating behavior 2 days after treatment in males with bilateral vomeronasal nerve cuts. Some of these males recovered the behavior in 1--3 weeks of post zinc sulfate testing. Histological analyses of the olfactory mucosa in 7 males on day 2 after zinc sulfate showed that 89--97% of the mucosa had been destroyed in 6 out of 7 of the males and 78% in the seventh. We conclude that destruction of the vomeronasal system irreparably reduces arousal necessary for mating in some hamsters but in other males sufficient arousal for this behavior to occur is mediated through the olfactory system, presumably in conjunction with other sensory inputs. Subsequent removal of the olfactory input in these animals eliminates the behavior.

Facilitation of lordosis in ovariectomized rats by intracerebral progesterone implants

Abstract Ovariectomized female rats were estrogen-primed by subcutaneous injection and two days later were tested for sexual receptivity 6 h following bilateral implants of crystalline progesterone or cholesterol combined with a subthreshold injection of progesterone administered systemically. Cannulae were located in either the medial basal hypothalamus (MBH) or the mesencephalic reticular formation (MRF) in different groups of females. Significant facilitation of estrous behavior was induced by brain progesterone in females whose cannulae were located in the MBH. Intracerebral progesterone was not effective in females with MRF cannulae. Within the MBH the ventromedial and arcuate nuclei appeared most responsive to the estrus-facilitating actions of progesterone. The possibility that the behavioral effects of implanted progesterone were mediated by entry of hormone into the ventricular system or general circulation is considered unlikely. It is suggested that progesterone facilitates lordosis behavior via a facilitatory rather than an inhibitory effect on MBH neurons.

Sexual behavior in peripherally anosmic male hamsters.

Abstract Sexually experienced and inexperienced male hamsters mated normally after they had been rendered temporarily anosmic by intranasal zinc sulfate treatment (Experiment 1), in contrast to the complete absence of sexual behavior following removal of the olfactory bulbs. The efficacy of our zinc sulfate technique was demonstrated by use of an olfactory discrimination training procedure. Hamsters lost the ability to smell both amyl acetate (Experiment 2) and the female hamster post-estrus vaginal discharge (Experiment 3) following zinc sulfate induced anosmia but at the same time retained normal sexual behavior.

Neonatal and two-stage olfactory bulbectomy: Effects on male hamster sexual behavior

The copulatory behavior of sexually mature male hamsters was investigated following removal of the olfactory bulbs in ways which might be expected to minimize postoperative behavioral effects. Neither two-stage bulbectomy in adults nor a one-stage operation in neonates prevented the total abolition of sexual behavior consequent to bilateral olfactory bulb removal in most of the animals. However, two bulbectomized hamsters (one, one-stage; one, two-stage) displayed an atypical persistence of mating postoperatively. Their lesions, which included total bilateral destruction of the main and accessory bulbs and damage to adjacent olfactory tissue, were histologically comparable to the lesions in two other bulbectomized hamsters (from different studies) which also copulated postoperatively. The behavior of these atypical animals supports the hypothesis that male hamsters can display normal sexual behavior in the absence of olfactory stimulation.

Individual differences in sexual responsiveness to estrogen and progesterone in ovariectomized rats

Individual differences among 83 ovariectomized rats in behavioral responsiveness to estrogen were measured by scoring the quality of sexual receptivity induced by injections of estradiol benzoate (EB) and progesterone (P). The P dose remained constant but the quantity of EB administered was systematically reduced over successive weeks until lordosis behavior could no longer be elicited. This EB dose was considered threshold. This sequence of weekly hormone injections and receptivity tests was repeated to assess the reliability of our procedures. Animals had thresholds of either 2.0, 1.0 or 0.5 μg/kg EB on both tests; the correlation between threshold values on the two tests was high (r=0.66; p<0.001). Sixty-two females were used to determine the facilitating effects of various quantities of P following EB treatment. Subgroups were tested after the E alone and again after one of 6 P doses. Zero, 20, 50 and 100 μg P failed to elevate receptivity scores significantly; both 250 and 500 μg P had significant facilitating effects. The results demonstrated that individual differences in EB sensitivity can be measured reliably, and a further analysis also suggests similar individual differences in P responsiveness. Our threshold determination procedures provide a useful technique for measuring the effects of various experimental manipulations on the hormone sensitivity of brain mechanisms which regulate estrous behavior.

Progesterone: Examination of its postulated inhibitory actions on lordosis during the rat estrous cycle

POWERS, J. B. AND J. MOREINES. Progesterone: examination of its postulated inhibitory actions on lordosis during the rat estrous cycle. PHYSIOL. BEHAV. 17(3) 493-498, 1976. - Three experiments tested whether the inhibitory effects of progesterone could be of physiological significance in regulating the duration of behavioral estrus in female rats. In animals displaying 5 day estrous cycles, a second period of sexual receptivity, one day following the occurrence of spontaneous estrus, could be induced by exogenous hormone administration, regardless of whether the ovaries were intact or were removed during the period over which the exogenous hormones were acting. In a second experiment, acute ovariectomy at various times during the progesterone surge acted only to degrade the quality of receptive behavior subsequently observed, never to enhance it by removing a postulated inhibitory influence. In the final experiment there was some suggestion that progesterones facilitating effect on lordosis during the later portions of spontaneous estrus were attenuated by prior exposure to ovarian secretions during the early period of behavioral estrus. Our results are consistent with the hypothesis that the duration of receptive behavior under physiological conditions is not primarily regulated by inhibitory actions of progesterone, but rather by the quantity and duration of estrogen secretions during the conditioning period.

Hormonal induction of behavioral estrus modified by electrical stimulation of hypothalamus

Abstract Sexual receptivity was induced by sequential injections of estrogen and progesterone in ovariectomized female rats which had bilateral, monopolar electrodes permanently implanted into either the medial preoptic area (MPOA) or the medial basal hypothalamus (MBH). Intermittent electrical stimulation of the MPOA during the first 6 hours of estrogen priming significantly reduced the intensity of estrous behavior measured 2 days later. Comparable stimulation of the MBH produced a non-significant increase in receptivity. The use of electrical stimulation to study the ways in which hormones affect central sites of action is discussed.

Effects of acute ovariectomy on the lordosis response of female rats

The sexual receptivity of intact females with 4- or 5-day estrous cycles was compared to that of other females which had been ovariectomized at particular times during their cycles. The quality and frequency of lordosis responding were more degraded the earlier during the cycle ovariectomy was performed. This effect was more pronounced in 4-day than in 5-day cyclic females. Because exogenous progesterone was administered to all ovariectomized females, these behavioral deficits were attributed to removal of ovarian estradiol. Ovariectomy 6 hr before the critical period for luteinizing hormone release significantly shortened the duration of behavioral estrus, even though it had no effect when lordosis was tested at the time intact estrous females are maximally receptive. These findings are consistent with the hypothesis that the continual availability of estradiol throughout the 18--24 hr interval perior to the onset of behavioral estrus is essential for optimal conditioning of sexual receptivity to occur under physiological conditions. The relevance of triggering and maintenance functions of estradiol to these results is discussed.