Investigation of peripeduncular-hypothalamic pathways involved in the control of lordosis in the rat.

Single shock stimuli applied to the peripeduncular nucleus (PPN) elicited a complex evoked response in the hypothalamic ventromedial nucleus (VMN). An early component and a late component could be distinguished in the evoked response on the basis of their different latency, threshold, site of maximal amplitude, frequency-response characteristics and also because restricted lesions eliminated specifically the short-or the long-latency components. Transection of the dorsal supraoptic pathway immediately in front of the PPN suppressed all the VMN-evoked responses. The same lesion eliminated lordotic responses in ovariectomized rats treated with estradiol benzoate and progesterone. Similar lesions placed more dorsally had no effect on either the sexual behavior or the evoked response. Evidence was obtained suggesting that the short-latency component is generated by activity that reaches the VMN directly through the ventral supraoptic commissure, while the long-latency response involves substations in the amygdala and the bed nucleus of the stria terminalis. The effect of lesions on the performance of lordosis may be attributed to the disruption of ascending and/or descending neural impulses circulating between the PPN and the VMN, with relay stations in the amygdala and the bed nucleus of the stria terminalis.