Increased expression of carbon monoxide-producing enzymes in the MPOA after sexual experience in male rats.

The hypothalamus contains numerous nuclei involved in the regulation of reproductive, stress, circadian, and homeostatic behaviors, with many of these nuclei concentrated within the preoptic and anterior regions. The gaseous neurotransmitter, nitric oxide (NO), has already been shown to have an important regulatory role within the medial preoptic area (MPOA) of the anterior hypothalamus, where it facilitates sexual behaviors. However, little is known about the role of other gaseous neurotransmitters in this area. Here, we report that the carbon monoxide (CO) producing enzymes HO-1 and HO-2 are present in the MPOA and are differentially influenced by sexual experience in a manner similar to that previously reported for NO enzymes. Immunohistochemical staining of brains collected after 0, 1, or 7 sexual experiences reveals that HO-1 is expressed transiently after the first sexual experience, while HO-2 increases only with repeated experience. This increase appears to be specific to the MPOA, as nearby brain areas do not exhibit this degree or pattern of expression. We observed a transient increase in HO-2 colocalization with neuronal nitric oxide synthase (nNOS) after a single sexual experience, but these cells appear to be otherwise disparate, despite the fact that both express within the central nucleus of the MPOA. Together, these findings suggest that endogenous CO may be behaviorally relevant within the MPOA and that CO and NO may be differentially regulated there.