Role of the ventrolateral part of the medulla oblongata in the control of gonadotropin secretion. I. Electrophysiological studies on the neural features between the limbic-preoptic structures and the ventrolateral part of the medulla oblongata in the female rat

Abstract

Electrophysiological studies were undertaken in order to investigate the neural features between the preoptic suprachiasmatic nucleus (POSC) and the ventrolateral part of the medulla oblongata (VLMO) and between the bed nucleus of stria terminalis (BST) and VLMO by eliciting antidromic responses and performing microionophoresis of estradiol. Unit activity of neurons in the VLMO was recorded in ovariectomized estradiol benzoate-primed Wistar female rats under urethane anesthesia (1.2g/kg body weight). Antidromic activations were performed by applying negative square wave pulses (intensity 1.0--1.5mA, duration 0.1msec, frequency 0.6Hz) which were delivered through a bipolar concentric stainless steel electrode to the POSC or the BST. In total, 116 neurons in the POSC and 118 neurons in the BST with ascending projection were identified in the VLMO. Both in the POSC and the BST, two types of antidromic spike potentials were distinguished on the basis of their waveforms. One type ("fast spikes") was characterized by a sharp and smooth rising phase. The other ("slow spikes") had a notch in the rising phase. In the POSC, mean antidromic spike latency for the fast spikes was 9.8msec, while the value for the slow spikes was 30.4msec and in the BST, mean antidromic spike latency for the fast spikes was 10.2msec, while the value for the slow spikes was 31.5msec. In the POSC, ionophoretic injection of estradiol hemisuccinate was accomplished on 57 of the 116 antidromically identified cells, of which 32 showed slow responses and 25 responded with fast spikes. In cells with slow spikes, estradiol hemisuccinate facilitated (n = 14) or suppressed (n = 5) their generation of action potentials. None of the cells with fast responses changed their activity in response to estradiol hemisuccinate. In the BST, ionophoretic injection of estradiol hemisuccinate was accomplished on 51 of 118 antidromically identified cells, of which 30 showed slow responses and 21 responded with fast spikes. In cells with slow spikes, estradiol hemisuccinate facilitated (n = 9) or suppressed (n = 8) their generation of action potentials. None of the cells with fast responses changed their activity in response to estradiol hemisuccinate. In addition, POSC or BST unit responses to the VLMO stimulation (intensity 1.0--1.5mA, duration 0.1msec, frequency 0.6Hz) was examined. Unit firing activity in the POSC or BST, elicited by the VLMO stimulation, was much more enhanced in proestrous rats than in diestrous rats. After the injection of phenoxybenzamine, an alpha-adrenergic receptor blocker, the rate of facilitatory responses in the POSC or in the BST neurons was significantly decreased in proestrous rats.(ABSTRACT TRUNCATED AT 400 WORDS)