ELECTRICAL-STIMULATION OF THE RAT PERIVENTRICULAR NUCLEUS INFLUENCES THE ACTIVITY OF HYPOTHALAMIC ARCUATE NEURONS

In rats, the release of growth hormone (GH) is inhibited during electrical stimulation of the periventricular nucleus but after the end of stimulation, there is a rebound 'hypersecretion' of GH. We examined the responses of arcuate neurones in pentobarbitone-anaesthetized male rats, following electrical stimulation of the periventricular nucleus to test the hypothesis that the effects of periventricular nucleus stimulation on GH secretion are mediated via effects upon GH-releasing hormone (GRF) neurones in the arcuate nucleus. The electrical activity of 2 groups of arcuate neurones were analysed before, during and after periventricular nucleus stimulation (10 Hz, 5 min, 0.5 mA biphasic, 0.5/1.0 ms): a) putative neurosecretory cells which were antidromically identified (AD) as projecting to the median eminence (n = 53) and b) non-neurosecretory cells, identified by their spontaneous 'bursting' pattern of activity (n = 29), During stimulation predominantly inhibitory responses were observed in both AD and bursting cell groups. Of the 39 AD cells which were spontaneously active, 25 were inhibited during the periventricular nucleus stimulation, and 10 of these showed a rebound hyperactivation following the end of stimulation. Fifteen bursting cells were inhibited during stimulation and 4 of these displayed a rebound hyperactivation following the end of stimulation. Additional evidence was sought for the identity of these cells by testing their response to electrical stimulation of the basolateral amygdala (which has previously been shown to increase plasma GH concentration without influencing the release of other pituitary hormones). Six of the 10 AD cells which displayed the inhibition/rebound response to periventricular nucleus stimulation were also excited following electrical stimulation of the basolateral amygdala. We conclude that 1) electrical stimulation of the periventricular nucleus and the basolateral amygdala exert predominantly inhibitory and excitatory effects respectively upon the activity of arcuate neurones but for neither site were the effects of stimulation exclusively upon GRF neurones, and 2) the rebound hypersecretion of GH following PeN stimulation is likely to involve the rebound activation of arcuate neurones.