Effects of septal and hippocampal stimuli on paraventricular nucleus neurons

The electrical activity of 125 neurons within the hypothalamic paraventricular nucleus was recorded in urethan-anaesthetized male rats. Spontaneous activity of the cells and their responses following electrical stimuli delivered to the ipsilateral lateral septum and dorsal hippocampus were recorded. The mean firing rate of all the cells recorded was3.5 ± 0.4 Hz and the majority were located within the dorsal and medial components of the paraventricular nucleus. Forty-six percent of the cells were inhibited following stimulation of the lateral septum (onset,22.8 ± 6.7 ms; offset,195.1 ± 28.5 ms). Inhibitory responses to dorsal hippocampus stimulation were recorded from 44% of all cells (onset,28.1 ± 4.7ms; offset,180.7 ± 28.7ms). Stimulation of both sites caused excitation of equal proportions (26%) of the cells tested (lateral septum onset, 47.7 ± 4.5 ms; offset, 64.8 ± 6.6 ms; dorsal hippocampus onset,48.7 ± 5.6 ms; offset, 72.3 ± 8.8 ms). Of the sub-population of cells identified as projecting to the median eminence, inhibition was recorded from 50% following lateral septum stimulation and 43% following dorsal hippocampus stimulation, excitatory responses being recorded from only 9% of cells tested. The excitatory responses were only recorded from phasically firing, vasopressin-secreting cells identified as projecting to the median eminence, and also to the neurohypophysis. Following stimulation of either site, more phasic cells were excited whilst only few were inhibited. Continuously active cells, identified as projecting to the neurohypophysis, showed more mixed responses following stimulation.

The specificity of the response categories, according to cell-type, and the high degree of convergence of responses (79%;P < 0.05) appear to correlate with a variety of neuroendocrine studies concerning the regulation of anterior and posterior pituitary hormone secretion and the results are discussed in relation to such studies.