Hypothalamic Integration: Organization of the Paraventricular and Supraoptic Nuclei

Anatomical evidence of direct projections from the nucleus of the solitary tract to the hypothalamus, amygdala, and other forebrain structures in the rat

The organization of noradrenergic pathways from the brainstem to the paraventricular and supraoptic nuclei in the rat

Electrophysiology of hypothalamic magnocellular neurones secreting oxytocin and vasopressin

Physiological stimulation of the hypothalamo-neurohypophysial system by salt loading of rats resulted in a dramatically increased glucose utilization in the posterior pituitary but not in the paraventricular or supraoptic nuclei The good correlation between glucose utilization and neural activity in the posterior pituitary (that is, nerve terminals) contrasted with the lack of correlation in the paraventricular and supraoptic nuclei (that is, the sites of the cell bodies of the same neurons) This difference in the metabolic response to functional activity between the two regions of these neurons can be explained by the differences in surface-to-volume ratios of these regions

http://science.sciencemag.org/content/sci/205/4407/723.full.pdf

Metabolic mapping of functional activity in the hypothalamo-neurohypophysial system of the rat

Antidromically identified neurosecretory cells of the paraventricular (p.v.) and supraoptic (s.o.) nuclei of the hypothalamus were recorded in lactating rats under urethane anaesthesia during reflex milk ejection (m.e.) and haemorrhage. Eighty one p.v. and s.o. neurones were studied. Their background firing rates ranged from < 0.1 to 6.3 spikes/s and three distinct patterns of activity were encountered: slow irregular (73%), fast continuous (10%) and phasic (17%). Forty units (49%) displayed a brief (2-4 s) high-frequency discharge (30-60 spikes/s) correlated with suckling-induced m.e., and these were classified as m.e. (oxytocin-secreting) neurones. The remainder of the cells showed no activation at this time and were classified as non-m.e. neurones. Ten m.e. neurones tested through haemorrhage (5 ml of blood) showed a gradual acceleration of firing rates, reaching a maximum of 3.7 $\pm $ 0.7 spikes/s (mean $\pm $ s.e.) about 20 min after blood withdrawal. The firing pattern of the m.e. neurones therefore changed from a slow irregular to a fast continuous type. By contrast, 11 non-m.e. neurones tested with the same procedure showed a rapid activation reaching a maximum of 6.4 $\pm $ 0.6 spikes/s by the fourth minute. Non-m.e. neurones which were initially of the slow irregular type, first became fast continuous and later evolved into a highly characteristic phasic pattern of activity which was never induced in the m.e. neurones. After the blood was replaced, all the cells returned to their original firing pattern. In a parallel series of experiments, plasma samples taken 5 and 25 min after haemorrhage showed a ten-fold elevation in antidiuretic activity. A slight but non-significant increase in m.e. activity was also observed. Thus p.v. and s.o. neurosecretory cells may be electrophysically differentiated into two functionally distinct populations: (1) oxytocin releasing neurones which show a high-frequency discharge before m.e. induced by suckling, and (2) vasopressin-releasing neurones which adopt a phasic pattern of firing during vasopressin release induced by haemorrhage. We suggest that the rate of vasopressin secretion into the circulation largely depends on the proportion of vasopressin neurones firing phasically, their firing rates within the phases and the duration and degree of synchronization of the phases.

Electrophysiological differentiation of oxytocin-and vasopressin-secreting neurones

APPROXIMATELY 25% of the neurones projecting to the neurohypophysis from the paraventricular (PV) and supraoptic (SO) nuclei of the hypothalamus fire phasically and such a pattern is seldom seen in other hypothalamic cells1. Phasic neurones respond to osmotic stimuli2 but are rarely activated by suckling1,3. It is tempting therefore, to associate these cells with the secretion of vasopressin rather than oxytocin. The functional relationship of their phasic pattern to hormone release, however, remains obscure. We have studied phasic PV neurones during haemorrhage, which releases vasopressin, sometimes with small amounts of oxytocin4. Haemorrhage evoked changes in phasic units over a period long enough for a detailed analysis of their behaviour. Our results show the importance of pattern modulation in phasic neurones and suggest that the mechanism for vasopressin release differs from that described for oxytocin release during suckling1,3.

R. E. J. Dyball

Papers

Electrophysiological differentiation of oxytocin-and vasopressin-secreting neurones

Activity of phasic neurosecretory cells during haemorrhage.

The importance of short interpulse intervals in determining vasopressin release from the isolated neurohypophysis [proceedings].

Electrophysiological Determinants of Neurosecretion