Electrophysiological identification of LHRH neurons that are involved in the LH surge induced by positive feedback action of estrogen.

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We have raised the hypothesis that the LHRH neurons that are involved in the phasic release of LH (LH surge) induced by positive feedback action of estrogen are different from those that are involved in the pulsatile release of LH occurring in the ovariectomized conditions. The present study was undertaken to investigate this hypothesis further using the multiunit activity (MUA)recording technique. The results are as follows.1. The LHRH neurons showing MUA which increases intermittently in association with LH pulses are involved in the basal release of LH controlled by negative feedback effect of estrogen. We have called these LHRH neurons Type I. The negative feedback effect of estrogen to these neurons are mediated by opioid peptidergic neurons. Further, the intermittent, cyclic firing of these Type I neurons is pentobarbital sensitive.2. We called those LHRH neurons that are involved in phasic LH release (LH surge) Type II. Those neurons are different from Type I in some aspects as follows : (1) These LHRH neurons do not show MUA which increases intermittently in association with LH pulses in ovariectomized rats. (2) The mechanism that involves Type II LHRH neurons and acts to control LH surge in response to positive feedback effects of estrogen dose not involve opioid peptidergic neurons, but (3) is sensitive to pentobarbital.In conclusion, the results have suggested that LHRH neurons in the CNS have two functional subtypes, arbitrarily called Type I and Type II. The electrical activity of Type II neurons could not be recorded, even by the MUA technique. It seems necessary to place the electrodes at the more frontal site.