Sexually naïve female prairie voles require exposure to a novel male to activate the neural circuits involved in the formation of stable pair bonds and to stimulate sexual receptivity. The objective of our study was to investigate the neural pathways involved in the formation of pair bonds. Cushing et al. (2003) studied neuronal activation in selected brain regions, as expressed by c-Fos immunoreactivity (ir), during the first hour of cohabitation in prairie voles. In the present study, we extend the findings of this study to examine neuronal activation associated with cohabitation to include 2, 6 and 12 hr. In addition, we examined the potential colocalization of luteininzing hormone releasing hormone (LHRH) and c-Fos to determine if this is a system activated during the initial stages of pair bond formation. The selected time periods include the initiation of sexual activation of the female and pair bond formation. Expression of c-Fos as analyzed in three regions that play a role in early social encounters. These areas included: (1) the sociosexual behavior circuit, (2) the reward pathway, and (3) nuclei whose peptides regulate the actions of those networks.

Based on the previous data, increased c-Fos expression was predicted in the social behavior circuit, including the medial amygdala, bed nucleus of the stria terminalis, medial preoptic nucleus, and ventromedial nucleus of the hypothalamus. The lateral septum was examined due to its role in the social behavior circuit and in the process of pair bond formation.

Next, we predicted that increased c-Fos activity would be observed over time in regions that did not show increases during initial contact, but are involved in pair bond formation. These regions include two components of the reward pathway: the nucleus accumbens and the ventral pallidum. Finally, nuclei known to regulate both the social behavior and reward circuits were examined and included the supraoptic nucleus and the pariventricular nucleus, which produce oxytocin and vasopressin. These neuropeptides are critical in social behavior and the formation of pair bonds.

Immediately following the period of cohabitation (0, 1, 2, 6 or 12 hour in length) animals were separated and brains fixed. Fixed brains were sectioned at 30μm and stained with c-Fos and LHRH antibodies using double label immunocytochemistry (ICC) (Berghorn et al., 1994). Significant colocalization of LHRH and c-Fos was not observed in the first twelve hours in either sex of cohabitated prairie voles. Additionally there was no difference in number of LHRH ir neurons between sex or treatments. LHRH ir neurons in male and female prairie voles were predominantly located in the diagonal band of broca, preoptic area, lateral hypothalamus and supraoptic decussation. Individual LHRH neurons that did express c-Fos were predominantly located in the POA and LH.

We observed a sexually dimorphic temporal pattern in c-Fos ir in the circuitry involved in pair bond formation in prairie voles. This pattern suggests that incoming

information is first sorted through the social-sexual circuit and continues to be processed as information is received by nuclei of the reward pathway a short time later in both sexes. An increase in immediate early gene (ieg) immunoreactivity in the social behavior network is reported concurrently with peak activation of the reward circuit. During the same time period, an increase in c-Fos ir is reported in the nuclei involved in the endocrine control of partner preference and pair bond formation. Together these data suggest that in prairie voles, both the social and reward circuits interact early in cohabitation prior to reproductive activation to establish a heterosexual bond and that both circuits may be regulated by neuropeptides produced by the PVN and SON. This research was conducted by funding from the following: NSF IBN-9817024 (MAO), NIH HD 38490 (CSC, GEH, BSC, MAO) and MH 01992 (BSC).

Subject Category: Neuronendocrine Activation following cohabitation

Keywords: LHRH, social behavior, reward pathway, c-Fos, neuronal activation