Reversal of pargyline-induced inhibition of sexual behaviour in male rats by p-chlorophenylalanine

Dopamine, Erectile Function and Male Sexual Behavior from the Past to the Present: A Review

Early and recent studies show that dopamine through its neuronal systems and receptor subtypes plays different roles in the control of male sexual behavior. These studies show that (i) the mesolimbic/mesocortical dopaminergic system plays a key role in the preparatory phase of sexual behavior, e.g., in sexual arousal, motivation and reward, whereas the nigrostriatal system controls the sensory-motor coordination necessary for copulation, (ii) the incertohypothalamic system is involved in the consummatory aspects of sexual behavior (penile erection and copulation), but evidence for its role in sexual motivation is also available, (iii) the pro-sexual effects of dopamine occur in concert with neural systems interconnecting the hypothalamus and preoptic area with the spinal cord, ventral tegmental area and other limbic brain areas and (iv) D₂ and D₄ receptors play a major role in the pro-sexual effects of dopamine. Despite some controversy, increases or decreases, respectively, of brain dopamine activity induced by drugs or that occur physiologically, usually improves or worsens, respectively, sexual activity. These findings suggest that an altered central dopaminergic tone plays a role in mental pathologies characterized by aberrant sexual behavior, and that pro-erectile D₄ receptor agonists may be considered a new strategy for the treatment of erectile dysfunction in men.

Dopamine and sexual behavior

Among central neurotransmitters involved in the control of sexual behavior, dopamine is certainly one of the most extensively studied. Our attempt to review old and recent neuropharmacological, biochemical, electrophysiological, and psychobiological studies performed so far only in rats, monkeys, and humans, provides evidence that dopamine through its different neuronal systems and receptor subtypes plays different roles in the control of several aspects of sexual behavior. In fact, while the nigrostriatal system is necessary for the control of the sensory-motor coordination required for copulation, the mesolimbic-mesocortical system plays a key role in the preparatory phase of the behavior, mainly in sexual arousal, motivation and possibly reward. Conversely, the incertohypothalamic system plays a major role in the consummation of the behavior, mainly in seminal emission and erectile performance, but evidence for its involvement in sexual motivation also exists. The dopaminergic receptors playing the major role in the control of male sexual behavior belong to the D2 receptor subtype. However a D1/D2 receptor interaction is well established and an opposite role for D1 and D2 receptors in the preoptic area suggested. Despite some differences, most studies show that treatments that increase or decrease, respectively, brain dopaminergic activity improve or worsen, respectively, several parameters of copulatory activity, supporting a facilitatory role of dopamine in male sexual behavior. In contrast, no conclusion can be deduced from the available studies on the role of central dopaminergic systems in the control of proceptivity and receptivity, the two main components of female sexual behavior.

Genitourinary and sexual adverse effects of psychotropic medication

OBJECTIVE

We review the adverse effects on genitourinary and sexual function associated with antidepressants, neuroleptics, lithium, and benzodiazepines, and suggest treatment strategies that may be used for their management.

METHOD

This article is based on systematic review of the existing literature, including more than 130 relevant articles on genitourinary and sexual effects of psychotropic medications.

RESULTS

We find that genitourinary function, including effects on continence and flow, and sexual function, including libido, erection, ejaculation and orgasm, may be altered by psychotropic administration. Many of these effects may be consequent to the impact of these medications on neurophysiologic systems.

CONCLUSIONS

Genitourinary and sexual adverse effects associated with psychotropic therapy are important areas of study and clinical concern that may affect patient comfort and compliance with treatment.

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.

Opioids and sexual behavior

Opioids have long been known to inhibit sexual behavior. However, it is only within the last decade that the effects of opioids on sexual behavior have been studied extensively and a number of hormonal and neurochemical correlates established. In this review, the experimental literature on opioids and sexual behavior in humans and laboratory animals is examined. Clinical and anecdotal accounts of opioid use are also discussed, in addition to the pharmacology, neuroendocrinology, and biochemistry of opioid administration, to provide a synthesis of critical information. New research directions involving the study of endogenous opioid systems, opioid receptor subtypes, and the opioid modulation of neurotransmitter systems are outlined. Finally, a comprehensive bibliography of the human and animal literature is included.

Serotonin antagonist pirenperone inhibits sexual behavior in the male rat: attenuation by quipazine

Peripheral administration of the serotonin (5-HT) antagonist pirenperone produced a dose dependent inhibition of sexual behavior in sexually naive and experienced male rats. In Experiment 1, both 75 micrograms/kg and 150 micrograms/kg pirenperone significantly reduced the proportion of naive males mounting, while 150 micrograms/kg also reduced the proportion of naive males intromitting and ejaculating. In Experiment 2, both 75 micrograms/kg and 150 micrograms/kg pirenperone significantly increased mount and intromission latencies in sexually experienced males, as well as decreased intromission frequency, with 150 micrograms/kg more potent in each regard. The 150 micrograms/kg dose also increased the post-ejaculatory interval, and decreased both mount frequency and copulatory efficiency. In Experiment 3, both 150 micrograms/kg pirenperone and 3 mg/kg of the 5-HT agonist quipazine produced significant inhibition of male sexual behavior; however, when co-administered, inhibitory effects of each drug were significantly attenuated. The mutual attenuation of effects by a 5-HT agonist and a 5-HT antagonist suggests that the observed effects of both of these drugs were serotonergically mediated. In the final experiment, the 5-HT antagonist ketanserin was shown to inhibit sexual behavior in a manner similar to that of pirenperone. Results suggest a facilitatory, as well as an inhibitory role for 5-HT in male sexual behavior.

The influence of testosterone in the brain of the male rat on levels of serotonin (5-HT) and hydroxyindole-acetic acid (5-HIAA)

There were two groups of rats: one was injected with testosterone propionate (10 mg/kg) every 7 days starting from weaning (23 days old); the other group had gonadectomy on the same day. The levels of 5-HT and 5-HIAA were measured by spectrofluorometry. The concentrations of 5-HT in the diencephalon of the testosterone propionate injected rats decreased significantly at 45 days, tending to become reestablished at 60 days; the rest of the brain followed the same pattern, but was less pronounced. The concentrations of 5-HIAA in the diencephalon and the rest of the brain decrease throughout postnatal development, although the differences are not significant. The castrated rats showed a marked increase at 45 days and later decreased at 60 days without recovering their initial values, in both brain areas. 5-HIAA concentrations were similar to those found in the injected animals. These facts can have various interpretations: early modifications in the brain, feed-back regulation mechanisms at the level of the hypothalamus, decrease in the release of the amine or reduction of its catabolism.

Facilitatory effects of monoamine synthesis inhibitors on lysine-vasopressin induced changes in the exploratory behaviour pattern of male rats

The possible interaction between vasopressin and monoamines in the regulation of exploratory behaviour was investigated. Treatment with lysine-vasopressin (LVP) influenced this behaviour. In particular, the investigative activity was enhanced. The dose-response relationship of LVP was non-linear. The catecholamine synthesis inhibitor alpha-methyl-p-tyrosine (alpha-MT, 100 mg/kg) also increased the investigative activity, indicating a suppressive influence of catecholamines on this behaviour. alpha-MT pretreatment potentiated the effect of LVP within certain dose levels. The dose-response relationship for the peptide effect on the investigative activity was shifted towards the left. Treatment with the serotonin biosynthesis inhibitor para-chloro-phenylalanine (PCPA) (100 + 50 + 50 mg/kg) also enhanced the investigative behaviour and potentiated the action of LVP similarly to alpha-MT. It is suggested that the action of LVP on the exploratory behaviour in rats can be modified by monoamines.

Different roles of catecholaminergic and serotoninergic neurons of the medial forebrain bundle on male rat sexual behavior

The medial forebrain bundle (MFB) plays an important role in the central elaboration of male sexual behavior. However the role of its ascending monoaminergic systems is not well understood. By selective lesions of catecholaminergic and serotoninergic axons of the MFB, produced with 6-OHDA and 5-7-DHT respectively, its possible involvement in male sexual behavior was studied. The evaluation of the behavioral patterns was studied by means of a microcomputer which also permitted the quantification of sequences in the sexual behavior patterns. Lesions with 5-7 DHT increased the intromission-ejaculation sequences and the percentage of ejaculatory males, without modifying other behavioral tests. Lesions with 6-OHDA diminished the number of "active" sexual patterns and motor activity. In order to check the specificity of the chemical lesions, the in vitro uptake of labeled norepinephrine and serotonin was measured, 5-7 DHT lesioned rats show a reduction in the uptake of 5-HT-C14 in the MPOA while 6-OHDA group show a reduction of NA-H3 in the striatum.

A demonstration that 5-hydroxytryptamine administered peripherally can affect sexual behavior in male rats

In the male rat, subcutaneous injections for 7 days of 20 mg/Kg B.W./day of 5-hydroxytryptamine creatinin sulphate (5-HT), caused remarkable inhibitory effects on sexual behavior. The mount and intromission latencies were increased in rats treated with 5-HT, whereas ejaculation latency in the few rats treated with 5-HT that it achieved, was similar to that obtained in control rats. The mount and intromission frequencies were decreased in the rats treated with 5-HT. The mean inter-intromission interval (MIII) and post-ejaculatory interval were prolonged in rats treated with 5-HT. These data provide evidence for the role of peripheral 5-HT in regulating sexual behavior of male rats.

5-hydroxytryptamine depletion with para-chlorophenylalanine: effects on eating, drinking, irritability, muricide, and copulation

Forty-four male rats were tested for eating, drinking, irritability, and copulation before and after intraperitonial para-chlorophenylalanine (PCPA) or control injections. Eleven of these rats were tested for muricide before and after PCPA injections (Group 1), while 18 others were tested only after PCPA injections (Groups 2). Group 1 rats received four 350 mg/kg PCPA injections spaced 6 days apart and showed hyperdipsia, weight loss, and a 24% increase in muricide. Group 2 rats received five daily 100 mg/kg PCPA injections repeated 11 days later and showed hyperdipsia and weight loss; in addition, 78% of them killed mice. Neither group showed significant changes in copulation. At the end of the experiment, t6 rats from Group 2 that were irritable and killed mice were injected intraperitonially with 5-hydroxytryptophan (80 mg/kg). Five of these rats lost their irritability and four stopped killing. The various behavioral changes were not corrleated significantly either with each other or with the degree of 5-hydroxytryptamine depletion. This tentatively suggests that PCPA may produce its effects on behavior by other means in addition to 5-hydroxytryptamine depletion.

Monoaminergic mediation of masculine and feminine copulatory behavior in female rats

Ovariectomized rats treated with testosterone propionate (TP; 100 mug/kg X 6 days) and para-chlorophenylalanine (pCPA; 100 mg/kg X 3 days), a serotonin synthesis inhibitor, showed more masculine copulatory behavior (including the ejaculatory pattern) than did females receiving either TP or pCPA alone. The facilitatory effect of pCPA on the masculine copulatory behavior in females was not potentiated by pargyline (50 or 100 mg/kg), a monoamine oxidase inhibitor; instead, pargyline antagonized the effect of pCPA. Apomorphine (100 mug/kg), a dopamine receptor stimulant, did not increase masculine copulatory behavior in TP treated females. Adopaminergic facilitatory effect was therefore not demonstrated. These results suggest a serotonin-mediated inhibition of masculine copulatory behavior in female rats. When feminine copulatory behavior was tested, females receiving TP plus pCPA plus pargyline, TP plus pargyline, or TP plus apomorphine displayed lordosis in response to mounting by male rats. Lordosis did not occur after administration of TP, PCPA, or pargyline, individually or in any other combination. The responses in pargyline groups are consistent with the hypothesis of a noradrenergic facilitatory system for lordotic behavior. The responses in the apomorphine group are discussed in terms of a possible role for low level dopaminergic stimulation in facilitating lordosis.

The significance of dopamine, versus other catecholamines, for L-dopa induced facilitation of sexual behavior in the castrated male rat

The effects of a wide dose range of L-DOPA on male rat sexual behavior were investigated. The animals were castrated as adults and supplied with small amounts of testosterone propionate. It was found that doses of L-DOPA up to 2.5 mg/kg facilitated, while higher doses inhibited, sexual behavior in animals pretreated with pargyline, 20 mg/kg, + MK486, 50 mg/kg. The effects of L-DOPA on sexual behavior were not restricted to the copulatory act, but included elements preceding the copulatory act as well. Most of the facilitatory effects of L-DOPA 2.5 mg/kg were prevented by the dopamine receptor blocker pimozide; 0.10 mg/kg. It is concluded that dopamine is the catecholamine of major importance in mediating the L-DOPA induced facilitation of sexual behavior in the castrated male rat. However, some elements of the copulatory act appear to be modified by noradrenaline and/or adrenaline as well.

Copulation in noncopulators: effect of PCPA in male rats

Twenty eight virgin male rats who failed to mount spontaneously on four successive mating tests were randomly assigned to two equal groups. One group received four daily injections of PCPA (100 mg/kg-IP), and the other, four vehicle injections. About 24 hr after the last injection all rats were subjected to a mating test. Eight rats of the PCPA group and only two of the vehicle, started to copulate. The difference is significant at the 0.05 level (two-tail). The 18 rats that did not start to copulate, failed to do so also on a repeated test a week later, and were then given four additional daily injections of PCPA. Sixteen of them started to copulate on the following mating test. Most of the rats copulated successfully on additional mating tests that took place three to eight weeks later, in the absence of further PCPA treatment.

Ontogeny of monoamine neurons in the locus coeruleus, raphe nuclei and substantia nigra of the rat. II. Synaptogenesis

Synaptogenesis was studied in the monoamine (MA) cell groups locus coeruleus (LC), dorsal and medial raphe nuclei (RN) and substantia nigra, zona compacta (SN) between day 18 of gestation and postnatal day 60 using ethanolic phosphotungstic acid (E-PTA) to visualize synaptic profiles. Nuclear area, and cellular packing density (inversely proportional to area of neuropil) were also determined. As determined using the E-PTA method, synaptogenesis begins in the neuropil of the SN first, on or before 18 days of gestation, and in the LC and RN at 19 days. Synaptogenesis on MA cell perikarya is first observed in the SN, on or before 18 days, and in the LC and RN at 20 days. The onset of somatic synaptogenesis coincides with the beginning of nuclear growth and development of the neuropil (decrease in cellular packing density) in all MA cell areas, raising the possibility of common factors in the initiation of these processes. Nonsynaptic contacts precede the appearance of synaptic profiles both in the neuropil and on the somata of the MA cells of the LC, RN and SN, and may represent precursors of mature synapses or desmosome-like contacts. Somatosomatic nonsynaptic contacts occur only prenatally between adjacent MA neurons in the LC, RN and SN. Although some synaptogenesis occurs prenatally in these MA cell groups (indiciating that these parts of the MA circuitry may be functional before birth), most of this synaptogenesis occurs postnatally and continues into adulthood. Since such synaptogenesis does not begin until 2-4 days prior to birth, whereas these neurons and their processes exhibit MA fluorescence as early as 12-14 days of gestation, they apparently are capable of synthesizing transmitter and proliferating terminals before they themselves are innervated.

Lack of copulatory behaviour in male castrated rats after p-chlorophenylalanine

1 The effect of p-chlorophenylalanine (PCPA) on the copulatory behaviour of normal and castrated male rats with females in oestrus was studied.2 Castration 2 months before the experiment completely prevented the increased copulatory behaviour produced by PCPA in normal rats.3 The administration of testosterone restored the copulatory behaviour in the castrated rats indicating that testosterone is essential for this behaviour.

An animal model for psychopharmacological research with relevance to psychiatry

The concept is presented that animal models for psychopharmacological research should take into account clinical methods if they are to yield information which is relevant to psychiatry. Animal species with appropriate behavioural characteristics should be selected, drugs should be administered chronically and groups of animals should be used if social interaction is to be observed. Some of the relevant pharmacological, behavioural and environmental factors are presented and illustrated with examples taken from a study on groups of squirrel monkeys who had received parachlorophenylalanine or alpha-methylparatyrosine to lower specific brain amines. It is noted that social interaction may affect the expression of the drug effect.

Decreased conversion of tyrosine to catecholamines in the brain of rats treated with p-chlorophenylalanine

Repeated treatment with p-chlorophenylalanine (100 mg kg−1) daily for 5 days impairs the transport of amino-acids from plasma into the brain and decreases the rate of conversion of [3H]tyrosine to [3H] catecholamines in the rat brain. The possible mechanisms involved are discussed.