Lateralized effects on hamster lordosis of unilateral hormonal and somatosensory stimuli

To better describe the hypothalamic efferents that mediate estrogenic effects on lordosis, each of 18 ovariectomized hamsters received a 30 gauge implant of estradiol aimed at the right or left ventromedial hypothalamus (VMN). Six days later, subjects were treated with progesterone and observed for their durations of lordosis during brief periods with sexually-active males. Subsequently, the male was removed, as light manual stimulation was applied to the flank ipsi- or contralateral to the implant in attempts to (a) maintain as yet unbroken lordosis responses, or (b) reinstate lordosis in females showing limb movements indicative of their emergence from the male-elicited posture. The results failed to show any difference in the behavioral effects of implants aimed at the right versus left VMN. However, implants did interact selectively with manual stimulation of the ipsi- and contralateral flanks. Specifically, lordosis responses were more likely, more rapid, and better maintained to stimulation of the contralateral flank. This lateralization of the priming effect derived from a VMN estradiol implant is impressive for its persistence, and for its consistency with the trajectories of VMN efferents to the midbrain and of midbrain afferents from the flanks. The consistency of these behavioral and neuroanatomical patterns suggests that lordosis depends on direct interactions of the VMN and dorsal midbrain. In turn, it implicates the dorsal midbrain in the mediation on VMN-initiated hormonal effects on lordosis, and in the eventual integration of hormonal and somatosensory influences on sexual receptivity.