Inhibition of the lordosis reflex in rats by intrahypothalamic infusion of neural excitatory agents: evidence that the hypothalamus contains separate inhibitory and facilitatory elements.

In attempts to activate lordosis-facilitating neural mechanisms in the ventromedial hypothalamus (VMH), neural excitatory agents were infused into the medial hypothalamus, and the effects of the infusions on the lordosis reflex and on the electrical activity of VMH neurons were studied. Surprisingly, bilateral intrahypothalamic infusion of glutamate (10 mM, 1.0 microliter/side) into behaving, ovariectomized, estrogen-treated rats displaying moderate lordotic responsiveness did not facilitate lordosis, but instead, resulted in a rapid (within a few minutes) and transient (recovery in about 20 min) inhibition of lordosis. Further experiments showed that this lordosis-inhibiting effect of glutamate was dose-related, and was completely blocked by prior infusion of a local anesthetic, procaine. Infusion of KC1 (1.5 or 15%, 1.0 microliter/side) also induced a dose-related, rapid and transient inhibition of lordosis, that was essentially identical to that induced by glutamate. Kainic acid (0.25 micrograms/0.5 microliter/side) also caused a rapid inhibition of lordosis, but the effect was long-lasting (days). The inhibition of lordosis by these agents was dissociated in time course, presence, and/or severity from effects on non-lordotic behaviors. Electrophysiological studies showed that all three agents tested could excite multiunit activity of the VMH, and that the time courses of these excitations were closely comparable to those of the inhibition of lordosis induced by the respective agents. Altogether, these studies indicated that the excitation of certain medial hypothalamic neurons can inhibit the lordosis reflex. The implied lordosis-inhibiting neural mechanisms are separate from facilitatory mechanism(s), according to differences in latency, duration, and procaine-sensitivity of response.