The effects of stimulation of the cerebellar fastigial nucleus (FN) on the activity of neurosecretory neurones in the hypothalamic paraventricular nucleus (PVN) of rats, anaesthetized with urethane and alpha-chloralose, were investigated by extracellular and intracellular recordings. 2. With extracellular recording, 139 PVN neurosecretory neurones were identified by antidromic activation following stimulation of the pituitary stalk, of which 120 were spontaneously firing and 19 were silent. Three types of responses to FN stimulation (1 or 2 pulses at 333 Hz) were observed in 43% of spontaneously firing PVN neurosecretory neurones: inhibition (type I) with a latency of 4.3 +/- 4.1 ms (mean +/- S.D., 32 of 120 neurones, 27%); excitation (type E) with 22.3 +/- 8.1 ms latency (14 of 120, 11%); and inhibition-excitation type (I-E) with 5.5 +/- 3.4 ms latency (6 of 120, 5%). Silent neurosecretory neurones did not respond to FN stimulation. Twelve per cent of non-neurosecretory cells (three out of twenty-six tested) responded to FN stimulation (one was inhibited, and two were excited). 3. Repetitive stimulation (60 Hz, 10 s) of the FN, which evoked a stimulus-locked pressor response, suppressed on-going activity of PVN neurosecretory neurones in 43% (twenty-nine of sixty-eight) of neurones tested. In 40% of these neurones, the activity was also inhibited by intravenous injection of phenylephrine (3 micrograms in 0.3 ml Ringer solution), while in other neurones the injection had no effect. Rebound excitation of neurone activity lasting for 1-5 min after termination of repetitive stimulation was observed in 28% of the neurones. Ten neurones (14%) were excited by repetitive stimulation. 4. Successful intracellular recordings were made from seventy-two PVN neurones, of which thirty-seven were neurosecretory cells. The mean resting membrane potential was -51 mV (n = 72; range from -40 to -75 mV). The input resistance of neurosecretory cells was 117 +/- 21 M omega (range 93-153 M omega; n = 8). This value was higher than that for non-neurosecretory cells which was found to be 53 +/- 10 M omega (range 35-72 M omega; n = 9). The difference was statistically significant (P less than 0.01, Student's t test). 5. In response to FN stimulation, sixteen (43%) of the thirty-seven neurosecretory neurones showed IPSPs with latencies of 7.4 +/- 2.8 ms and three (8%) exhibited EPSPs with latencies of 13.3 +/- 4.2 ms.(ABSTRACT TRUNCATED AT 400 WORDS)
