Behavioral, and neural mechanisms of conditioned partner preference in the female rat

We assessed the development of a conditioned partner preference in female rats towards males associated with paced copulation (PC), relative to nonpaced copulation (NPC). Chapter 1 assessed the behavioral mechanisms. Ovariectomized, hormone-primed females were conditioned to associate an odor or a strain of male with PC. A partner preference test occurred in an open field with two tethered males in opposite corners, one associated with PC and the other with NPC. Paired females (relative to unpaired or random-paired groups) developed a partner preference for PC-related males. They displayed more solicitations and hops and darts, and were more likely to choose the PC-related male for their first ejaculation. Similarly, strain conditioning produced partner preference towards the PC-related male, regardless of the strain; although females were more likely to receive their first ejaculation from the PC-related male only if it was of their own strain. The experiments in Chapter 2 assessed neuroanatomical correlates. The brains of females in both paired and unpaired conditions were processed for Fos-immunoreactivity (Fos-IR) following exposure only to the conditioned stimulus (CS) associated either with PC or with NPC. The CS paired with PC induced more Fos-IR in the piriform cortex, the medial preoptic area, and the ventral tegmental area, compared to the induction by the same CS associated with NPC. The experiments in Chapter 3 assessed neurochemical mechanisms. In Part 1, females were treated with the general opioid antagonist naloxone (4 mg/kg) before every conditioning trial, but were drug-free during the final partner preference test. Naloxone treated females did not develop a conditioned partner preference. In Part 2, females were treated with the dopamine antagonist flupenthixol (.25 mg/kg) before conditioning trials. Only Long-Evans females failed to develop an olfactory conditioned partner preference, just like naloxone-treated females. However, flupenthixol had no effect on conditioned preference for a strain of male in Wistar females. The experiments in Chapter 4 depict preliminary data on the activation of oxytocin, vasopressin and gonadotrophin-releasing hormone (GnRH) neurons following exposure to the CS associated with PC. These data show trends toward a significant activation of Fos within oxytocin neurons in the paraventricular nucleus of the hypothalamus, and GnRH neurons in the anterior hypothalamus, in paired females exposed to the CS. These results demonstrate that the sexual reward induced by PC can be associated with cues on a partner to develop a conditioned preference. When this occurs via olfactory conditioning of a neutral odor, the mechanisms depend on the interaction of opioids and dopamine in mesolimbic areas. However, the conditioning process for a strain of male appears to depend only on opioids