Activation of protein kinase C in the hypothalamic ventromedial nucleus or the midbrain central gray facilitates lordosis

Many neurotransmitters and neuropeptides can act through the hypothalamic ventromedial nucleus (VMN) or midbrain central gray (MCG) to facilitate lordosis. Since these lordosis-facilitating agents can also stimulate the phosphoinositide (PI) second-messenger pathway, it was hypothesized that direct activation of this pathway can also potentiate the behavior. To evaluate this possibility, a phorbol ester, TPA (12-O-tetradecanoyl phorbol 13-acetate), was used to activate a key enzyme, protein kinase C (PKC), of the PI pathway in ovariectomized (OVX) rats either primed or not primed with estrogen. These female rats were paired with males for mating tests before and after an intracerebral infusion of TPA, and both the lordosis quotient (LQ) and the lordosis strength (LS) were measured. Bilateral infusion of TPA (5 μg/0.5 μl or 0.2 μg/0.2 μl, but not 0.1 μg/0.2 μl/side) into the VMN or MCG of estrogen-primed subjects facilitated both LQ and LS in 30 min, peaked at 60–90 min, and the facilitation lasted for more than 180 min. This facilitatory effect of TPA was: (1) not observed in OVX rats not primed with estrogen; (2) not observed if the infused TPA did not reach both sides of the VMN or MCG; (3) not mimicked by 4-phorbol 12,13-didecanoate, which does not activate PKC; (4) blocked by PKC inhibitors (H7 10 mM or staurosporine 1 μM, 0.2 μl/ side), which by themeselves did not facilitate lordosis; and (5) was not affected by pretreatment of the progestin antagonist RU486. These observations indicate that TPA facilitates lordosis in a dose-dependent fashion by activating, and not by depleting, PKC in the VMN or MCG, and that the TPA effect requires estrogen priming but not the activation of progestin receptors. Thus, the PI pathway or the activation of PKC may be a common mediator for lordosis facilitation in these two brain regions; and the requirement of estrogen priming further raises the possibility that this second-messenger system or its substrates in the VMN and MCG are modulated by estrogen.