Sex, Drugs and Gluttony: How the Brain Controls Motivated Behaviors (2011)

2.1. 5-HT effects in the anterior LH

My former student Dan Lorrain used microdialysis to show that 5-HT is released in the anterior LH at the time of ejaculation [8] (see Fig. 1), just as Bart Hoebel had reported 5-HT release there with feeding [9]. Furthermore, microinjection of a selective 5-HT reuptake inhibitor (SSRI) antidepressant into the LH inhibited copulation, similar to post-ejaculatory quiescence and similar to the inhibitory sexual side effects of SSRI’s used to treat depression. Thus, the Hoebel lab showed that systemic increases in 5-HT impaired female sexual behavior [10], and the Hull lab located at least one brain area, the anterior LH, where local 5-HT increases inhibited male sexual behavior [8]. In a later article, we reported that reverse-dialysis of 5-HT into the anterior (perifornical) LH decreased DA release in the NAc [11]. Therefore, 5-HT release in the LH at the time of ejaculation may contribute to post-ejaculatory quiescence, at least in part, by inhibiting the mesocorticolimbic DA pathway.

     

Fig. 1

 

Temporal changes in extracellular serotonin (5-HT) collected from the lateral hypothalamus of male rats before and during copulation. Each data point is the mean (±SEM) for 6-min dialysate samples collected during baseline (B), in the presence of an estrous female (F), during copulation (C), during the post-ejaculatory interval (P), and after the female was removed (expressed as % of mean baseline levels). 5-HT levels increased during the second (P2) and third (P3) postejaculatory intervals, compared to the final baseline. 5-HT during P3 was also higher than in the fourth copulatory interval. Samples collected during the second and third copulation series were not analyzed, because most males ejaculated before a full 6-min sample could be collected. The summary graph (inset) shows the mean (± SEM) for data for the 15 sample periods collapsed into five groups, based on behavioral condition. Samples collected during post-ejaculatory intervals showed higher 5-HT levels than all other conditions. (Figure from [8] with permission.)

 

2.2. OX/HCRT in the anterior (perifornical) hypothalamus

We have more recently provided a sequel to the lateral hypothalamic 5-HT story. A group of neurons in the LH produces the peptide orexin (OX, also known as hypocretin, HCRT). Furthermore, 5-HT was previously reported to inhibit those neurons (12). OX/HCRT is primarily known for its stimulation of feeding behavior [13,14] and control of sleep-wake cycles [15, 16]. OX/HCRT-containing neurons had previously been reported to project to the ventral tegmental area (VTA) [17], the source of the mesocorticolimibc DA tract. Furthermore, intra-VTA administration of OX/HCRT was reported to increase DA release in the NAc [18]. My former student John Muschamp hypothesized that the lateral hypothalamic neurons that were inhibited by post-ejaculatory 5-HT might be those OX/HCRT-containing cells. We showed that mating increased c-Fos-immunoreactivity in OX/HCRT-containing cells [19]. In addition, castration decreased the number of OX/HCRT-immunoreactive neurons, which were mostly restored by systemic injections of estradiol. OX/HCRT is behaviorally relevant, as systemic administration of an OX/HCRT antagonist impaired copulation [19]. In addition, microinjection of OX/HCRT into the VTA produced dose-dependent effects on dopaminergic cell firing. The two lower doses increased cell firing and population responses, although the highest dose apparently resulted in depolarization block of VTA dopaminergic neurons, which was reversed by stimulating DA autoreceptors with the DA agonist apomorphine. Finally, triple-label immunohistochemistry revealed that mating increased c-Fosimmunoreactivity in dopaminergic neurons in the VTA that were apposed to OX/HCRT fibers. Therefore, OX/HCRT neurons appear to act in a steroid-dependent manner to activate the mesocorticolimbic DA pathway, thereby promoting sexual behavior and other natural and drug-induced rewards.

2.3. DA release in the medial preoptic area (MPOA)

In addition to the LH and mesocorticolimbic DA system, my lab has investigated the role of the MPOA, at the anterior end of the hypothalamus, in the control of male sexual behavior. MPOA lesions disrupt male sexual behavior in all vertebrate species that have been studied (reviewed in [20]). Electrical or chemical stimulation of the MPOA enhances copulation and ex copula genital reflexes. Local A14 periventricular DA neurons innervate the MPOA, as do DA neurons from several other sites [21].

There is a close correlation between male rat sexual behavior and extracellular DA levels in the MPOA. DA is released in the MPOA of male rats in response to an estrous female and during copulation [22] (see Fig. 2). The recent presence of testosterone was necessary for both DA release and copulation. Intact males, testosterone-treated castrates, and oil-treated castrates that copulated showed a pre-copulatory DA increase, which was maintained or increased further during mating [22, 23]. Oil-treated castrates that did not copulate did not show the increase. There was both behavioral and anatomical specificity for the DA response. Furthermore, the fact that DA increased before mating began suggests that the increase was not caused by copulation, but was probably associated with sexual motivation. Two-, five-, and ten-day regimens of testosterone treatment of castrates resulted in increasing copulatory ability that correlated closely with the restoration of DA release [24]. Testosterone treatment for two days did not restore mating or the DA response. Most of the five-day testosterone-treated castrates were able to copulate and showed a DA response, with half of them able to ejaculate. All of the castrates treated with testosterone for 10 days copulated to ejaculation, and all showed the DA response. There were again numerous correlations between copulatory measures and DA levels. Therefore, both the loss of copulation following castration and its restoration by testosterone are closely associated with the MPOA DA response to an estrous female.

     

Fig. 2

 

Testosterone-mediated enhancement of sexual activity may occur in part through increased DA release in the MPOA. Gonadally intact male rats showed an increase in extracellular DA during precopulatory exposure to an inaccessible estrous female, and all intact males then copulated when the female was placed in their cage. Males castrated 2 weeks before showed no DA release in response to the female, and none copulated. Two thirds of 1-week castrates copulated and showed the DA increase, whereas the remaining third did not copulate and did not show a DA increase. *P<.05, compared to baseline for testosterone-treated castrates; **P<.01, compared to final baseline for intact males or for one-week vehicle-treated castrates that copulated; +P<.05, compared to final baseline for vehicle-treated castrates that failed to copulate. (Reprinted from Ref. [22] with permission.)

 

Testosterone’s metabolites were differentially effective in restoring DA release in long-term castrates [25]. Estradiol restored normal basal levels of DA, but not the increase in response to a female. Estradiol-treated castrates intromitted, but none showed an ejaculatory behavior pattern. Neither dihydrotestosterone nor oil vehicle maintained copulation or basal or female-stimulated DA release. However, when dihydrotestosterone was administered with estradiol, the combination restored both copulation and basal and female-stimulated DA release [25].

Although extracellular levels of MPOA DA are lower in castrates than in gonadally intact males, intracellular levels are actually higher than in intact males [26]. Indeed, there was a negative correlation between tissue (stored) DA levels and the ability to copulate [27]. Non-copulating animals (dihydrotestosterone- and oil-treated castrates) had higher levels of tissue DA than did the groups that did copulate (estradiol−, estradiol+dihydrotestosterone-, and testosterone-treated castrates). Therefore, synthesis and storage of DA in the MPOA is at least as great in castrates as in intact males; the deficiency in castrates is not in their ability to synthesize and store DA, but in their ability to release their abundant stores.

2.4. The role of NO in MPOA DA release

Earlier studies had reported that DA release in the striatum was facilitated by NO [28, 29]. Therefore, we tested whether NO would have similar effects in the MPOA. Indeed, the precursor of NO, L-arginine, increased basal MPOA DA release, and the NO synthase (NOS) antagonist L-NMMA decreased release [30]. A different NOS inhibitor, L-NAME, inhibited copulation-induced DA release [31], an effect that was mediated by cGMP [32]. Furthermore, neuronal NOS (nNOS) immunoreactivity was decreased after castration and was restored by testosterone administration [33]. Therefore, one means by which testosterone facilitates copulation is by increasing nNOS in the MPOA, which in turn increases both basal and female-stimulated DA release in intact males and testosterone-treated castrates.

2.5. The effects of sexual experience

Our lab has also investigated the effects of sexual experience. Experienced males copulate with greater “efficiency.” They have shorter latencies to mount, intromit, and ejaculate and are able to ejaculate with fewer mounts and intromissions (reviewed in [20]). Merely exposing a male rat repeatedly to an estrous female is sufficient to enhance his copulatory ability and to increase c-Fos immunoreactivity in the MPOA elicited by one ejaculation [34]. NO may mediate some of the cellular effects of experience. The NOS inhibitor L-NAME, microinjected into the MPOA, prevented copulation in sexually naïve males and decreased the numbers of intromissions and ejaculations in sexually experienced males [35]. When administered into the MPOA before each of seven exposures to an estrous female, it blocked the facilitative effects of those exposures. Furthermore, nNOS immunoreactivity in the MPOA is increased by previous sexual experience [36]. Therefore, increases in NO production in the MPOA, and its consequent increase in DA release, may mediate some of the beneficial effects of sexual experience.

2.6. Input from the medial amygdala to the MPOA

A major stimulus for the MPOA DA response to a female is input from the medial amygdala (MeA). Juan Dominguez made large excitotoxic lesions of the amygdala, which abolished copulation in male rats [37]. However, microinjections of the DA agonist apomorphine into the MPOA completely restored copulation in those males. Smaller radiofreqency lesions of the MeA impaired, but did not abolish copulation. Basal MPOA DA levels were not affected, but the DA increase in response to the female was blocked [37] (see Fig 3). Therefore, as with estradiol restoration of copulation in castrates [25], basal MPOA DA levels were sufficient for inefficient mating, but an additional female-stimulated increase was required for optimal copulation. In anesthetized animals, chemical stimulation of the MeA, using glutamate plus a glutamate reuptake inhibitor, increased extracellular DA levels in the MPOA, mimicking the effect of copulation [38] (see Fig. 4). Therefore, one way in which the MeA promotes copulation is by increasing DA release in the MPOA.

    

Fig. 3

 

Lesions of the medial amygdala inhibit the release of DA in the MPOA resulting from exposure to an estrous female and copulation. Levels represent % changes from baseline (BL) in response to precopulatory exposure to an estrous female (PRE), during copulation (C1 – C3) and after copulation (POST). Extracellular DA significantly increased during the precopulatory and copulatory stages of testing for animals with sham lesions but not for animals with MeA lesions. Values are expressed as mean ± SEM. *P<.05; **P<.01. (Reprinted from [37] with permission.)

 

    

Fig. 4

 

Levels of DA in dialysate from the MPOA of animals receiving MeA stimulation or vehicle microinjection. Levels represent % change from baseline (BL) in response to MeA-stimulation or vehicle microinjection; samples collected after microinjections into the MeA are post-injection samples 1 – 6 (P1 – P6). Levels of extracellular DA significantly increased after MeA microinjections for animals receiving MeA stimulation but not for animals receiving vehicle. Values are expressed as mean ± SEM. (*P<.05) (Reprinted from [38] with permission.)

 

Research reported here was supported by NIH grant MH040826 to E.M. Hull.