Effects on penile reflexes and plasma hormones of hyperprolactinemia induced by MtTW15 tumors

Sexual function in altered physiological states: comparison of effects of hypertension, diabetes, hyperprolactinemia, and others to "normal" aging in male rats

In this review, we examine the changes in sexual function that accompany deviations from "normal" physiological states. We propose that the changes one observes in many altered physiological states should not be viewed in isolation. We describe our paradigms for assessing sexual function, and proceed to evaluate how sexual function changes with hormonal deprivation and aging, in rat models for hypertension, in severe hyperprolactinemia, in streptozotocin-induced diabetes, after chronic alcohol intake, after chronic morphine administration, and after exposure to the heavy metal, cadmium. We will provide evidence for the involvement of adrenergic transmitters and two neuropeptides, neuropeptide Y and somatostatin, in the neuroendocrine regulation of sexual behavior. Finally, we compare and contrast the changes observed relative to the changes seen in "normal" aging in rats. The sequence of age-related changes in sexual function is distinct. The first change observed is a decrement in ex copula erectile reflexes. Next are decreases in ejaculatory threshold, followed shortly by increases in initiation and reinitiation of copulation after ejaculation. This is followed by a decrement in the number of males copulating to ejaculation. Finally, there is a failure to initiate the copulatory process. This sequelae is relatively common, being evident after castration, with hyperprolactinemia, and after exposure to cadmium. The data available for sexual function in hypertension is incomplete and modified by the etiology, but a suggestion for this sequelae is seen in SHR. In contrast, sexual dysfunction associated with chronic morphine administration appears to be due to an initial deficit in motivational aspects. Testosterone reverses sexual dysfunction associated with castration, but not with idiopathic sexual inactivity, nor with sexual dysfunction associated with aging, diabetes, or chronic morphine administration. Comparing sexual function in rat models for hypertension, diabetes and chronic ethanol leads to the conclusion that increases in blood pressure, like decreases in testosterone, cannot be the primary causal factor for sexual dysfunction. Age, hormonal history of the subject, and the age at castration influence changes in sexual function. Age-related sexual dysfunction appears to be contributed to by changes in adrenergic-neuropeptidergic, to include sympathetic, systems. Site-specific administration of NPY induces alterations in parameters of copulatory behavior which mimic those seen in aging and the retention of ejaculatory behavior with aging is associated with site-selective attenuation (or reversal) of age-associated changes in NPY content. Yohimbine enhances copulatory activity in castrated and aging rats, and attenuates or reverses the antisexual effects of clonidine, epinephrine and somatostatin.(ABSTRACT TRUNCATED AT 400 WORDS)

Cadmium-induced sexual dysfunction does not involve increased hepatic metabolism of testosterone nor increased circulating levels of corticosterone

Sexually experienced male rats were injected IP with 0, 0.3, 1.5, 3.0, or 6.0 mg/kg cadmium chloride. The highest dose was fatal within 48 h of injection. A dose-related deficit in erectile function was observed in ex copula tests 48 h after injection. Copulatory dysfunction was evident in mating tests 72 h after injection. Hepatic mixed function oxidase activity after 0.3 mg/kg injections was not different from controls, but was diminished by 50% in rats treated with 3.0 mg/kg. The higher cadmium doses (1.5 and 3.0 mg/kg) were associated with equivalent and marked reductions in circulating testosterone levels, and lesser decrements in circulating corticosterone levels. The rats treated with 1.5 or 3.0 mg/kg cadmium chloride lost weight (32 +/- 12 and 39 +/- 9 g, respectively). Relative decapsulated adrenal gland weights were markedly increased in rats treated with 3.0 mg/kg. These data indicate that exposure to cadmium is associated with alterations in sexual, hepatic, and adrenal function, with erectile dysfunction occurring at the lowest dose. The effects on copulatory behavior are especially striking because it requires some weeks for copulatory behavior to wane following castration.

Extra-hypothalamic dopamine is not involved in the effects of hyperprolactinemia on male copulatory behavior

Experiments were performed to determine if the inhibition of copulatory behavior observed in male rats with chronically elevated serum prolactin levels (hyperprolactinemia) is associated with changes in central dopaminergic function in the nigrostriatal and mesolimbic systems. Chronic hyperprolactinemia, induced by ectopic pituitary grafts, inhibited sexual activity but was not associated with changes in locomotor activity, serotyped behavior in response to various doses of apomorphine, or 3H-spiroperidol binding to striatal homogenates. However, open-field defecation was reduced in the pituitary grafted animals. The results of the present study show that changes in nigrostriatal dopamine receptor sensitivity do not contribute to the inhibition of sexual behavior in hyperprolactinemic male rats. In addition, these results also demonstrate that the effects of hyperprolactinemia are relatively specific to copulatory behavior and appear not to involve general behavioral suppression.

Neuropeptides and male sexual behavior

Evidence is rapidly accumulating that a number of neuropeptides are involved in the central control of male sexual behavior. This is consistent with their neuroanatomical distribution, i.e., in CNS loci previously implicated in the control of this behavior such as the medial preoptic area, and with recent findings that the peptide content of some of these regions is regulated by testosterone or its metabolites. Most of the work has been done using rats, but relevant human studies have been included whenever such material has been available. At this point there are relatively few studies which directly demonstrate the involvement of peptides in this behavior. Inhibitory and facilitatory actions, however, have been demonstrated following injections of peptides, peptide antisera, or antagonists into the CNS of male rats. Significant new developments include demonstrations that injections of substance P and A-MSH directly into the medial preoptic area can facilitate this behavior, while ventricular injection of an oxytocin antagonist can produce a powerful inhibition. The emerging picture is that GnRH, oxytocin, A-MSH and substance P stimulate, while CRF, beta-endorphin, prolactin, and neuropeptide Y are inhibitory. The inhibitory peptides CRF, beta-endorphin and prolactin are related, as they are released in response to stress. This may be relevant to the low level of sexual motivation in some depressed men. Questions concerning sites of action and mechanisms of action which mediate the behavioral effects which have been demonstrated remain largely unanswered.

The effect of luteinizing hormone releasing hormone on the copulatory behavior of hyperprolactinemic male rats

The present study was undertaken to test the hypothesis that the deficits in copulatory behavior observed in hyperprolactinemic male rats may be related to a reduction in hypothalamic release of luteinizing hormone releasing hormone (LHRH). Adult male Fischer 344 rats were made hyperprolactinemic by ectopic pituitary grafts or were sham operated and 30 min prior to being tested for copulatory performance received a single subcutaneous injection of 500 ng LHRH, 100 ng LHRH, or saline. On different occasions, testosterone (T) levels were measured in plasma collected 30 min following identical treatments. Plasma prolactin (PRL) levels were determined in samples collected 30 min after injection of 500 ng LHRH. Pituitary grafting produced the expected, significant increase in plasma PRL levels and significant deficits in copulatory behavior. Treatment of hyperprolactinemic subjects with 500 ng LHRH significantly reduced both the time to first intromission and the time to ejaculation to times comparable with those of sham-operated subjects. The 100-ng dose produced a significant reduction in mount frequency. Plasma T levels were significantly elevated following either dose of LHRH. These results demonstrate that exogenous LHRH can restore normal copulatory performance in hyperprolactinemic male rats and support the hypothesis that a reduction in hypothalamic LHRH release is responsible for the behavioral deficits observed in those animals.

Pharmacological analysis of male rat sexual behavior

Pharmacological influences on male rat sexual behavior are reviewed in an attempt to identify neurotransmitters and their respective receptor types that regulate various factors comprising the behavioral pattern. Evidence is presented that: (1) serotonergic influence is generally inhibitory to sexual behavior, although two receptor subtypes may lower ejaculation threshold; (2) dopaminergic agonists facilitate several aspects of copulatory behavior and ex copula genital responses; (3) noradrenergic activity appears to increase sexual arousal; (4) cholinergic agonists facilitate ejaculation, or in some cases, delay or prevent initiation of copulation; (5) GABA agonists inhibit sexual responses both in and ex copula; (6) opiate agonists appear to inhibit copulation and penile reflexes, although antagonists have mixed effects; (7) ACTH and MSH peptides promote copulatory behavior and genital responses; (8) oxytocin facilitates ex copula penile responses, but may contribute to postejaculatory refractoriness; and (9) long-term exposure to prolactin inhibits sexual behavior and penile responses. Although some progress has been made in identifying neurotransmitter-receptor effects on behavioral components, copulatory behavior is complex and no drug has been found to affect only a single component. Furthermore, drug specificity is only relative.

Effects of a selective alpha 1-adrenoceptor agonist, methoxamine, on sexual behavior and penile reflexes

Methoxamine, an adrenergic agonist with selectivity for the alpha 1-adrenoceptor, when administered intraperitoneally 10 minutes prior to mating tests (1 to 5 mg/kg), effected reductions in the ejaculatory threshold, evidenced by a decrease in the number of intromissions preceding ejaculation. In mounting tests after penile anesthetization, a test which specifically assesses sexual motivation, 3 mg/kg methoxamine was without a stimulatory effect. Further, in penile reflex tests (ex copula) 1 mg/kg methoxamine was without effect, whereas 5 mg/kg decreased the number of erections, cups and flips per test, and increased the incidence of seminal emission. These data indicate a facilitation of the ejaculatory mechanism, both in and ex copula, coupled with an inhibition of erectile responses for moderate doses of methoxamine. Treatment of male rats with the alpha 2-adrenoceptor agonist clonidine (0.25 mg/kg, IP, five minutes pretest) drastically reduced the number of animals exhibiting intromissive and ejaculatory behavior in mating tests. This suppressive effect of clonidine was not prevented by prior treatment with methoxamine (3 mg/kg, 10 minutes pretest and five minutes preclonidine). Further, ST-91, a polar analog of clonidine which does not readily enter the central nervous system, was without effect on male sexual behavior. Since (1) the effects of methoxamine administration are not of similar quality or magnitude to those reported earlier after yohimbine, an alpha 2-adrenoceptor antagonist, (2) since concurrent stimulation of alpha 1- (by methoxamine) and alpha 2- (by clonidine) adrenoceptors is followed by a suppression of sexual behavior similar to that seen after clonidine alone.(ABSTRACT TRUNCATED AT 250 WORDS)