The influence of steroid hormones on competing sexual and ingestive behavior in the male rat

On the intricate relationship between sexual motivation and arousal

Sexual motivation and sexual arousal are widely used concepts. While there seem to be considerable agreement as to the meaning of sexual motivation, there is certain confusion about the exact meaning of sexual arousal. Some use it as a synonym to sexual motivation and others make it equivalent to erection or vaginal lubrication. An unresolved question is the relationship between sexual arousal and general arousal as well as that between arousal and motivation. I present arguments for the view that arousal refers to the general state of alertness of the organism. Consequently, there is no such thing as a specific sexual arousal. I suggest that this term should be abandoned, or if that is not feasible, to make it a synonym to enhanced genital blood flow. The notion of a subjective sexual arousal, some kind of vaguely described mental state, seems to lack all explanatory value. I then show that general arousal is an important determinant of sexual motivation, and that the execution of copulatory acts leads to increased general arousal. This increase leads to enhanced sexual motivation, making the activation of sexual reflexes requiring high levels of motivation possible. Examples of such reflexes may be ejaculation in males of many species, and perhaps the psychic state of orgasm in women.

p-Chloroamphetamine (PCA) suppresses ingestive behavior in male rats

Ingestive behavior was activated in male rats by intraoral intake and intake from a bottle of 1-M solution of sucrose. Intraperitoneal injection of p-chloroamphetamine (PCA), releasing central 5-hydroxytryptamine (5-HT) from serotonergic nerve terminals, inhibited ingestion of the sucrose solution. Significant inhibition of sucrose intake by PCA was observed at 1.25 and 2.5 mg/kg dose in a bottle intake test, and at 5.0 mg/kg dose in an intraoral intake test. These findings suggested that 1.25 and 5.0 mg/kg of PCA suppressed appetitive ingestive behavior and consummatory ingestive behavior in male rats, respectively.

Simultaneous observation of ingestive and copulatory behavior of the male rat

In a preliminary test male rats were allowed to ingest a 1 M solution of sucrose from a drinking spout. After daily intake of sucrose became stabilized, the males were given a sexually receptive or non-receptive female and the bottle filled with sucrose solution simultaneously. The ingestive and copulatory behavior was observed for 60 min under illumination by a red lamp. The data obtained from this study showed that the ingestive behavior of males was suppressed by the presence of sexually receptive females and, conversely, the sexual behavior of males was not affected by the presence of a bottle of sucrose. These results suggest that the presence of a sexual partner inhibits appetitive ingestive behavior, i.e., the responses used by male rats to obtain food.

Sex differences in EEG in adult gonadectomized rats before and after hormonal treatment

EEG activity was recorded from the left and right parietal cortex in adult male and female Wistar rats that were gonadectomized (GNX) after puberty during 2 days without and 3 days with hormonal treatment (either testosterone propionate, 5 alpha-DHT or vehicle in males and progesterone, estradiol benzoate or vehicle in females). In contrast to EEG characteristics reported for intact rats, GNX abolished right over left parietal activation in both sexes and, sex differences in EEG interhemispheric correlation and in theta and delta relative power in the right parietal; additionally GNX males showed higher absolute power than females. Hormonal treatment reestablished interparietal asymmetry in both sexes and a lack of sex differences in absolute power, however, it was not enough to reestablish sex differences in delta and theta proportion in the right parietal nor in interhemispheric correlation. Differential effects were obtained with testosterone propionate and 5 alpha-DHT in males suggesting that activational effects of testosterone on EEG are probably exerted through testosterone or its aromatized metabolites. The results of our study indicate that the activational effects of gonadal steroids after puberty are necessary for maintaining sex differences in the EEG of the adult rat.

Simultaneous display of sexual and ingestive behaviour by rats

Intraoral infusion of sucrose activates consummatory ingestive behaviour in rats selectively, i.e. the rat only emits the responses used to ingest food. Activation of consummatory ingestive behaviour in this way had no effect on the subsequent display of sexual behaviour by male or female rats and vice versa. Rats infused intraorally with sucrose and presented with a sexual partner showed ingestive and sexual behaviour simultaneously. Pretreatment with cholecystokinin octapeptide inhibited the ingestion of sucrose in both males and females but had no effect on the simultaneous display of sexual behaviour. Ingestion of sucrose from a drinking spout, a test in which the rat has to emit responses to obtain food, i.e. show appetitive ingestive behaviour, was inhibited by the presentation of a sexual partner in rats of both sexes. These results show that the mechanisms controlling consummatory sexual and ingestive behaviour operate independently and that the presentation of a sexual partner inhibits appetitive ingestive behaviour. Daily intraoral infusion of sucrose reduced pellet intake in ovariectomized rats while the rats maintained their body weight. Implantation of an oestradiol-filled implant reduced body weight and inhibited daily intake of pellets but had no effect on the intake of intraorally administered sucrose. Subsequent removal of the oestradiol implant increased sucrose intake and body weight but did not have a marked effect on pellet intake. Thus, rats respond to a lowering of the set point for body weight by decreasing their intake of the least preferable kind of food and increase their intake of the most preferable kind of food in response to an elevation of the set point for body weight. Ovariectomized rats infused intraorally once daily with a highly nutritive milk diet in the absence of food pellets ingested very large amounts and reduced their intake in response to oestradiol implantation. Thus, although oestradiol can inhibit consummatory ingestive behaviour, its suppressive effect on ingestion cannot be described in terms of selective effects on appetitive and/or consummatory aspects of the behaviour nor in terms of an alteration in the preference for a sweet solution. Inhibition of ingestive behaviour occurred within 24 h after oestrogen treatment as opposed to stimulation of sexual behaviour which had a longer latency, suggesting that oestradiol affects ingestive and sexual behaviour via different mechanisms. While the mechanisms controlling consummatory ingestive and sexual behaviour must be different, there is evidence for a common mechanism mediating the incentive motivation and reward aspects of these behaviours. The mechanisms which enable rats to select between two, possibly equally rewarding courses of action, i.e. display of sexual or ingestive responses, however, are unknown.

Comparative effects of preoptic area infusions of opioid peptides, lesions and castration on sexual behaviour in male rats: studies of instrumental behaviour, conditioned place preference and partner preference

The effects on the sexual behaviour of male rats of excitotoxic amino acid-induced lesions of the medial preoptic area-anterior hypothalamic area (mPOA/AHA), infusions of beta-endorphin, alpha-melanocyte stimulating hormone and naloxone into the mPOA/AHA, systemic naloxone and castration were compared using different behavioural paradigms. These included measures of unconditioned copulatory behaviour, instrumental responses for an oestrous female presented under a second-order schedule of reinforcement, conditioned place preference and partner preference. The results demonstrate that manipulations of the mPOA/AHA markedly affect consummatory aspects of sexual behaviour (mounting, intromitting and ejaculating) but tend not to affect appetitive or reward-related aspects of sexual behaviour, although intra-mPOA/AHA alpha MSH did result in a small increase in instrumental responses, while beta-endorphin infused into the mPOA/AHA also abolished preference for an oestrous over an anoestrous female. Systemic naloxone, on the other hand, reduced instrumental behaviour and a place preference conditioned by prior sexual interaction, while the same compound infused into the mPOA/AHA markedly facilitated copulatory responses but did not affect other measures of appetitive sexual responses. Castration caused an extremely rapid attenuation of conditioned place preference which was apparent before the males had experienced reductions in their copulatory performance. This treatment only slowly reduced partner preference. The results indicate that the use of several behavioural procedures can reveal discrete actions of neuroendocrine treatments on separable psychological processes which underly the integrated pattern of masculine sexual behaviour. In particular, they suggest that the mPOA/AHA is especially concerned with the copulatory responses of mounting and intromitting, but is much less important for a variety of appetitive sexual acts as well as sexual reward, as measured in the place preference procedure. The marked effects of castration on conditioned place preference taken together with the lack of effect of lesions of the mPOA/AHA on this measure indicate that testosterone affects sexual reward-related processes by an action at a site other than the mPOA/AHA. The implications of these findings are discussed.

Differential effects of beta-endorphin infused into the hypothalamic preoptic area at various phases of the male rat's sexual behaviour

Beta-Endorphin, infused into the pre-optic/anterior hypothalamus (40 pmoles bilaterally) of the male rat before he was placed in an arena containing an oestrous female, inhibited mounting, intromitting and ejaculation, but investigative behaviour continued at control levels. If the infusion was delayed until the male had made an intromission, then beta-endorphin no longer had any effect on sexual interaction, the male mounting and ejaculating as if he had received a control infusion of artificial cerebrospinal fluid. However, if the male was returned to a different female after the infusion had been completed, then the suppressive effects of beta-endorphin returned. Males infused during the refractory period following an ejaculation (and returned to the same female) showed unimpaired return of sexual activity. Imposing a delay of up to 2 h after an intromission and an infusion showed that the effect of beta-endorphin was still antagonized when the male was again paired with the same female; however, by 6 h, its inhibitory effects were beginning to return. Allowing the male to mount (but not intromit) a female whose vagina had been taped partially counteracted the behavioural effect of beta-endorphin. If the female was separated from the male by a small wire cage which allowed limited interaction with her, subsequently infusing the males with beta-endorphin suppressed their mounting behaviour. These results show that both investigative and mounting behaviour can occur after infusions of beta-endorphin into the pre-optic/anterior hypothalamus, but that the transition between them is prevented if infusions are made before a critical point in the behavioural sequence. This is the onset of following the female and mounting her. Analysis of the behavioural sequence after either artificial cerebrospinal fluid or beta-endorphin infusions confirmed this; beta-endorphin interrupted the sequence at the first transition between investigative and mounting behaviour. These results suggest that beta-endorphin acts on a neural mechanism in the medial preoptic area/anterior hypothalamus which allows matching of incentive stimulus to specific behavioural response, and this may be a general property of this part of the brain.