Individual differences in exploratory behavior can predictably influence psychostimulant selfadministration behavior. Male rats that exhibit a high degree of locomotor activity in a novel environment (High Responders, HR) will self-administer cocaine more readily than males exhibiting low levels of novelty-induced locomotion (Low Responders, LR). The present experiment investigates the combined influences of the sex of an individual and individual phenotypes in noveltyinduced locomotion to predispose animals to acquire cocaine self-administration behavior, in male and female rats selectively bred for the HR-LR phenotypes. We first established that HR females, like their male counterparts, exhibit a dramatically greater locomotor response to novelty and less anxiety-like behavior than do LR females. While locomotor behavior was subtly influenced by estrous stage, with both HR and LR females showing increased activity during metestrus and diestrus compared to proestrus and estrus, the effect did not obscure HR-LR differences. When male and female HR-LR animals were trained to self-administer cocaine (2 h/day, 5 days/wk × 3 wk, 0.2 mg cocaine/kg/infusion), HR males and females acquired cocaine self-administration significantly faster than their LR counterparts. Furthermore, HR females self-administered significantly more cocaine than all other groups. In conclusion, female rats, like males, exhibit HR-LR phenotypes that predict rapidity of acquiring cocaine self-administration. Moreover, HR females self-administer more cocaine than HR males and both LR groups.
Studies have emphasized the role of the medial preoptic area (MPOA) as an important site for the regulation of male sexual behavior. Indeed, ablations of the MPOA impair sexual behavior, whereas stimulation of the MPOA enhances behavior. Furthermore, neural activity in the MPOA increases with mating. The current study tested the hypothesis that activation of N-methyl-D-aspartate (NMDA) receptors occurs in MPOA neurons and is essential for the expression of male sexual behavior in rats. Results indicate that nearly all MPOA neurons that expressed Fos following mating also contained the NR1 subunit of NMDA receptors. Furthermore, mating increased phosphorylation, thus activation, of NR1 in the MPOA. Additionally, blocking NMDA receptors significantly decreased mating-induced Fos expression and mating-induced phosphorylation of NMDA receptors and impaired male sexual behavior. These results provide evidence that mating activates NMDA receptors in the MPOA and that this activation is important for the expression of male sexual behavior.
Exogenous opioids influence male rat sexual behavior, suggesting that endogenous opioid peptides are released during mating. Supporting this hypothesis, the authors recently showed that mating induced activation of mu opioid receptors. However, it is unknown which ligand(s) is acting on these receptors during mating. The current set of experiments tested the hypothesis that beta-endorphin-producing neurons, that is, proopiomelanocortin (POMC) neurons, are activated during sexual behavior. Mating-induced activation of POMC neurons was investigated during either the dark phase or the light phase, following different components of male rat sexual behavior or following control manipulations that resulted in general arousal. Results show activation of POMC neurons in the mediobasal hypothalamus following general arousal but not specifically related to sexual behavior per se. In addition, mating did not activate the subpopulation of POMC neurons that project to the medial preoptic nucleus. These results suggest that it is unlikely that POMC neurons contribute to the action of endogenous opioids in the brain area during sexual behavior but instead may contribute to the change in arousal state essential for the expression of sexual behavior.
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