| The aim of these experiments was to examine a role for the ventromedial nucleus of the hypothalamus in pseudopregnancy, a neuroendocrine reflex in female rats (Rattus norvegicus). A further goal was to determine whether noradrenergic input to the ventromedial hypothalamus is required for mating-induced pseudopregnancy. In this species, the genitosensory pathway by which the mating stimulation travels during the initiation stages of pseudopregnancy is not clearly understood. Experiments were designed to examine mating-induced noradrenergic activation within the ventromedial nucleus and determine the significant role of norepinephrine in the induction of pseudopregnancy. In all experiments, animals received different amounts of mating stimulation in combination with different pharmacological drugs targeting noradrenergic receptors and their axonal projections within the ventromedial hypothalamus.; Infusion of an alpha-1 noradrenergic antagonist into the ventromedial nucleus prevented mating-induced pseudopregnancy. In contrast, pseudopregnancy was induced by local infusions of the noradrenergic agonist phenylephrine, in combination with sub-threshold mating stimulation. Noradrenergic cell populations in the A1 and A2 medullary cell groups were lesioned with a retrograde neurotoxin infused into the ventromedial hypothalamus and medial amygdala. Numbers of noradrenergic neurons projecting to these two areas were reduced by 60%, as examined by immunocytochemistry of dopamine-beta-hydroxylase. Neurotoxic lesions of noradrenergic afferents into the ventromedial nucleus, not the amygdala, significantly reduced pseudopregnancy. These data suggest that the ventromedial hypothalamus is important for transduction of mating stimulation into pseudopregnancy through noradrenergic inputs, whereas the medial amygdala shows no noradrenergic involvement in the initiation of pseudopregnancy. A final experiment examined the effects of oxytocin receptor activation within the ventromedial hypothalamus on mating-induced pseudopregnancy. Attainment of the threshold stimulus capable of inducing pseudopregnancy was prevented by infusing an oxytocin receptor antagonist prior to mating. Overall, these data suggest the ventromedial nucleus is required for the establishment of pseudopregnancy at the time of mating under norepinephrine and oxytocin control. |
