John T. Clark

Evidence for the modulation of sexual behavior by α-adrenoceptors in male rats

Clonidine, a commonly used antihypertensive agent believed to act by stimulation of central α- adrenoceptors, produced a dose-related suppression of ejaculatory behavior in sexually vigorous male rats

Neuropeptide Y (NPY)-induced feeding behavior in female rats: comparison with human NPY ([Met17]NPY), NPY analog ([norLeu4]NPY) and peptide YY

Porcine neuropeptide Y (pNPY) administered into the third ventricle of the brain is known to elicit a powerful feeding response in steroid-treated ovariectomized and intact male rats. The present study compared the effects of pNPY and 3 structurally related peptides, human NPY (hNPY), an analog of NPY (NPY-A, [norLeu4]NPY) and peptide YY (PYY) on feeding behavior in intact female rats. Intraventricular administration of pNPY, hNPY, NPY-A and PYY over a dose range of 0.5 to 10 micrograms evoked feeding behavior to a varying extent. Cumulative food intake during 60 and 120 min was increased in a dose-related fashion at 0.5 and 2.0 microgram for the 4 peptides. Whereas the 10-micrograms dose of pNPY evoked a feeding response smaller than that seen after 2 micrograms, the responses to either 10 micrograms hNPY or 10 micrograms PYY were similar to that seen after 2 micrograms. The effects of these peptides on the time spent eating were quite different: while pNPY increased the time spent eating, this effect was not dose-related, whereas hNPY, NPY-A and PYY produced dose-related increments in the time spent eating. The most dramatic increment in local eating rate was observed after 2.0 micrograms pNPY, with lesser increments seen after 2.0 microgram hNPY and NPY-A. This increased local eating was apparently responsible for the highest cumulative food intake observed. These results demonstrate that (a) 2 micrograms pNPY is equally effective in stimulating feeding behavior in intact female rats as it is in steroid-primed ovariectomized female and intact male rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Testosterone is not required for the enhancement of sexual motivation by yohimbine

Yohimbine HCL (2 mg/kg, 20 min prior to testing) administration was followed by significant decreases in the latencies to initial mount, intromission and ejaculation in castrated male rats bearing 2 mm testosterone-containing Silastic capsules 51 days after castration. Further, yohimbine stimulated copulatory activity in castrated, nonhormone-treated male rats up to 91 days after castration. Finally, yohimbine induced mounting in intact, nonreceptive female rats. These observations indicate that testosterone is not required for the enhancement of sexual motivation by yohimbine and support the suggestion that alpha 2-adrenoceptors are involved in the modulation of sexual arousal.

Hormones and sexual behavior in relationship to aging in male rats

To determine if the age-related decline in male sex behavior is correlated with hormonal factors, a longitudinal study was conducted. Sexually experienced males were given mating tests every 2 months from 7 through 27 months of age. To study possible relationships between changes in behavior and alterations in hormone levels, blood samples were taken before and after these bimonthly tests. At 23 months, cross-sectional studies were also conducted comparing results to those obtained in 5-month-old males. Significant changes in mating behavior first appeared at 11 months; mount latency, intromission latency, ejaculation latency, postejaculatory interval, and intercopulatory interval were increased. Similarly, detectable decreases in testosterone (T) also occurred at this age. A significant decline in luteinizing hormone (LH) was not seen until 19 months. Correlational analyses revealed small (r less than or equal to -0.29) but significant negative correlations between T and parameters of mating behavior with age. When each age was examined separately, no significant correlations appeared. Plasma T was not predictive of behavioral performance. At 23 months, cross-sectional studies revealed deficits in mounting and penile reflex behavior but ejaculatory reflex capacity was unimpaired. At 28 months, males were decapitated. Only T levels showed a significant effect of age; estradiol, prolactin, and LH were unaffected when compared to 5-month-old males. The data suggest that although there are small and significant negative correlations between circulating testosterone and parameters of mating behavior with advancing age, it is unlikely that the observed decline in testosterone is the primary cause of the age-induced behavioral deficits. It is likely that the major causal factor(s) involves non-hormone-dependent changes within the CNS.

Effects of a potent dopamine receptor agonist, RDS-127, on penile reflexes and seminal emission in intact and spinally transected rats

Administration of RDS-127 (3.0 mg/kg) induced seminal emission within three minutes of IP injection and suppressed the display of penile reflexes in intact and spinally transected rats. In Experiment 1, RDS-127 was administered to intact, sexually experienced rats in a protocol previously demonstrated to selectively lower the ejaculatory threshold of copulating animals. The incidence of seminal emission was significantly elevated by RDS-127 but penile reflexes were present in only 8% of the drug-treated rats, compared to 59% of controls. In Experiment 2, seminal emission was induced 2.3 +/- 0.4 (S.E.) minutes from injection of RDS-127. Animals which responded to RDS-127 with multiple emissions had significantly lower ejaculation latencies during copulatory tests conducted prior to drug treatment than animals which had no or only single seminal emissions following RDS-127 injection. Spontaneous seminal emission in the 3 day period initiated 2 hours after RDS-127 injection was unaffected by the drug. Spontaneously produced plugs were approximately twice the weight of those induced by RDS-127. In Experiment 3, seminal emission was induced in spinally transected rats 1.7 +/- 0.4 minutes following RDS-127 administration, whereas drug treatment attenuated the enhancement of penile reflexes observed following midthoracic spinal transection. These experiments suggest that a spinally-mediated dopaminergic mechanism is capable of stimulating seminal emission acutely in the rat and inhibiting the display of penile reflexes by the supine animal.

Estrogen Depletion Increases Blood Pressure and Hypothalamic Norepinephrine in Middle-Aged Spontaneously Hypertensive Rats

Abstract—In male spontaneously hypertensive rats (SHR) a high NaCl diet increases arterial pressure via a reduction in anterior hypothalamic nucleus norepinephrine release. Young female SHR are relatively well protected from this NaCl-sensitive hypertension, but depletion of both endogenous and dietary estrogens greatly exacerbates NaCl-sensitive hypertension. This study tests the hypothesis that estrogen also protects late middle-aged female SHR from NaCl-sensitive hypertension and that this effect is mediated by an estrogen-related effect on hypothalamic norepinephrine release. Ten-month-old female SHR were ovariectomized and placed on a phytoestrogen-free diet containing either basal or high NaCl. Each rat was implanted with a silastic tube containing 17&bgr; estradiol or vehicle. Three months later, arterial pressure and hypothalamic norepinephrine metabolite levels (MOPEG) were measured. On the basal NaCl diet, estrogen-depleted rats displayed increased arterial pressure (12 mm Hg) and decreased anterior hypothalamic nucleus MOPEG (20%). Both effects were reversed by estrogen treatment. In all groups, the high NaCl diet increased arterial pressure by over 35 mm Hg and reduced anterior hypothalamic nucleus MOPEG by >60%. Across all groups, there was a significant inverse correlation between arterial pressure and anterior hypothalamic nucleus MOPEG. These data suggest that both dietary NaCl excess and estrogen depletion raise arterial pressure in middle-aged female SHR by a decreasing hypothalamic norepinephrine.

Neuropeptide Y (NPY) levels in alcoholic and food restricted male rats: implications for site selective function

Neuropeptide Y (NPY) has been implicated in the control of ingestive, cardiovascular, and reproductive function. Blood pressure and sexual function were examined in Long-Evans rats receiving 6% ethanol-containing or calorically matched liquid diets, or rat chow ad lib. After 12 weeks of exposure, rats were sacrificed and plasma hormone levels and NPY content of microdissected brain regions were determined. Neither long-term alcohol ingestion nor caloric restriction were associated with major decrements in copulatory behavior. Long-term alcohol ingestion was associated with decrements in erectile function ex copula. Long-term alcohol ingestion was also associated with: (i) a moderate degree of hypertension; (ii) a failure to gain weight; (iii) decrements in circulating levels of LH, testosterone, and ACTH (but not progesterone); and (iv) increased corticosterone levels. Long-term alcohol ingesting and calorically-restricted rats exhibited alterations in daily feeding patterns. These physiological changes in response to long-term alcohol ingestion or caloric restriction were associated with neural site-selective differences in NPY content. Elevated NPY in the paraventricular nucleus was associated with voluntary (as in alcohol ingestion) or involuntary (as in caloric restriction) reductions in food intake. Differences in NPY in the suprachiasmatic and ventromedial nuclei were associated with the differences in feeding patterns. The decrements in hormone levels were associated with higher levels of NPY in the median eminence and in the arcuate nucleus.

Suppression of copulatory behavior in male rats following central administration of clonidine.

Adrenergic transmitters have been implicated in the regulation of male sexual behavior. In the present study the contribution of alpha 2-adrenoceptors, located within the central nervous system, was evaluated. Sexually experienced male Long-Evans rats were implanted with a permanent cannula in either the third cerebral ventricle or, unilaterally, in the medial preoptic area. Baseline mating tests were administered 4, 7 and 10 days after surgery and only males ejaculating (at least) in the 7 and 10 day tests were used. Clonidine-evoked dose-dependent decrements in the number of males mounting, intromitting and ejaculating, with administration into the medial preoptic area more effective than into the 3rd ventricle. In those animals mating, administration of 20 nmol into the 3rd ventricle was associated with decreases in the latency to ejaculation and intercopulatory interval, whereas administration of 2 nmol into the medial preoptic area was associated with increases in these parameters and decreases in the ejaculatory threshold. Administration of yohimbine into the medial preoptic area attenuated the effects of systemically-administered clonidine and the systemic administration of yohimbine completely prevented the copulatory suppression induced by administration of clonidine into the medial preoptic area. It is suggested that central alpha 2-adrenoceptors are important in the control of male sexual behavior and that alterations in adrenergic mechanisms in the medial preoptic area may underlie sexual dysfunction of various etiologies.

Chronic morphine and testosterone treatment: effects on sexual behavior and dopamine metabolism in male rats

The effects of sustained delivery of morphine and/or testosterone (T) on male rat copulatory behavior, penile reflexes and dopaminergic metabolism in selected brain regions were examined. Castration was followed by (1) a decrease in the number of male rats exhibiting intromissive and ejaculatory behavior in mating tests, (2) decreased erections in ex copula tests, and (3) increases in dopamine and dihydroxyphenylacetic acid (DOPAC) concentrations in the mediobasal hypothalamus (MBH) and the preoptic area-anterior hypothalamus (POA-AH). The decreased incidence of copulatory behavior and penile reflexes seen after castration was effectively prevented by a 4-day treatment with 5-mm T-containing Silastic capsules. Chronic morphine implants, conversely, accentuated the castration-induced decrements in copulatory behavior and prevented the 5-mm-T-induced facilitation, but did not alter the number of animals displaying erection (although the number of erections displayed by testosterone-treated rats was reduced) in ex copula tests. Treatment of castrated rats with 5 mm T, but not morphine alone, nor the combination of 5 mm T plus morphine, significantly reduced dopamine and DOPAC levels in the MBH. In the POA-AH, 5 mm T was without effect, whereas morphine, alone or in combination with 5 mm T, reduced the levels of dopamine and DOPAC. These data suggest that (1) the decline in sexual behavior induced by chronic morphine is primarily due to a failure of sexual arousal, and not of erectile ability, and (2) although the decline in sexual activity seen after castration is associated with alterations in dopaminergic metabolism, the effects of morphine and testosterone on sexual activity are opposite and dissociated from alterations in dopaminergic metabolism.

Alpha2-adrenoceptor blockade attenuates feeding behavior induced by neuropeptide Y and epinephrine

Neuropeptide Y (NPY, 0.47 nmol) and epinephrine (28.9 nmol) evoked robust, and quantitatively similar, increments in food intake and local eating rate following administration into the third cerebral ventricle (IIIV). Whereas IIIV pretreatment with phentolamine (71 nmol), a nonselective alpha-adrenoceptor antagonist, or prazosin (9.5 nmol), a selective alpha 1-adrenoceptor antagonist, was without effect on NPY-induced feeding behavior, pretreatment with the alpha 2-adrenoceptor antagonist yohimbine (15 nmol) dramatically attenuated the stimulatory effects of NPY or epinephrine on both food intake (by over 50%) and local eating rate. Additionally, yohimbine administered alone was associated with a stimulatory effect on food intake for the periods of 80-110, and 110-140 minutes posttreatment. These data demonstrate that feeding behavior induced by IIIV administration of NPY or epinephrine is attenuated by prior blockade of alpha 2-adrenoceptors and suggest that, as in other systems innervated by neurons displaying NPY and adrenergic transmitter colocalization, the effects of NPY on feeding behavior may, at least in part, be mediated via alpha 2-adrenoceptors.