Factors governing the periodic activation of supraoptic and paraventricular neurosecretory cells during suckling in the rat.

Abstract

SUMMARY1. One hundred and thirty-two antidromically identified paraventricular (PV) and supraoptic (SO) neurones were studied during milk ejection evoked by the suckling of the young, in thirty-four lactating rats anaesthetized with urethane (1.1 g/kg i.P.). The spike activity of these neurones was analysed to determine some of the factors governing their orthodromic activation. Changes in spike activity were compared with the amount of oxytocin released from the neurohypophysis, as measured by the rise in intramammary pressure at milk ejection.2. Sixty-five units, from both the PV and SO nuclei, displayed a characteristic burst of accelerated activity 12-18 sec before milk ejection. With a suckling stimulus of nine or ten pups, these neurosecretory responses were both regular in their occurrence (recurring every 4-8 min) and uniform in character, i.e. all responsive cells accelerated to 30-80 spikes/sec for 1-4 sec. Two units deviated from this pattern and both were recorded in animals which failed to display a detectable milk ejection. Both cells displayed diminutive responses (10-20 spikes/sec, for 0 5 sec) at regular intervals.3. The number of suckling pups was the most important factor controlling both the occurrence and expression of the neurosecretory response. When successive pups were removed from the nipples there was a progressive and linearly related fall in the magnitude of the unit responses, i.e. the peak firing rate and the number of spikes in each response declined. Neurosecretory activation was normally not observed in anaesthetized and stereotaxically restrained animals when six pups or less were placed to suckle. D. W. LINCOLN AND J. B. WAKERLEY neurosecretory activation had an important influence on the expression of the unit response. On a population basis and ignoring variations created by differences in the number of suckling young, units with background firing rates of > 1*0 spikes/sec displayed responses that were significantly (P < 0.05) greater (peak rate x number of spikes) than those units firing at < 1-0 spikes/sec. This correlation was also observed with single units, particularly where large fluctuations in background firing rate were encountered.5. When five to seven pups were placed to suckle -a threshold number, two factors were observed to occasionally trigger a neurosecretory response. These were the injection of 0 05 ml. saline into one of the cannulated mammary glands, and the addition or removal of a pup. In either case the response was immediate and occurred with both ipsilateral and contralateral stimulation.6. The amount of oxytocin released in response to each period of neurosecretory activation was always proportional to the magnitude of the response displayed by the individual unit, thus a larger pulse of oxytocin was released in response to the suckling of nine pups than seven pups. All suckling-induced milk ejections were the result of oxytocin being released in pulses of 0-2-1'5 m-u.7. We found no conclusive evidence to suggest that the number of pups or the suck...

Electrophysiological evidence for facilitatory control of oxytocin neurones by oxytocin during suckling in the rat.

“…In fact, injecting oxytocin into the 3rd ventricle induced neither neurosecretory burst nor milk ejection in lactating non-suckled rats and, in suckled rats, removing the young from the nipples immediately stopped an oxytocin-activated pattern in the same way as described by Lincoln & Wakerley (1975) with a normal pattern. The increase in background activity observed on slow firing oxytocin neurones was not sufficient to induce oxytocin release detectable from intramammary pressure.…”

“…The periodic activation of oxytocin neurones is induced by suckling and depends on a sustained stimulus applied to the nipples: removing the young immediately stops milk ejection (Lincoln & Wakerley, 1975). The activation of neurosecretory cells is controlled by a facilitatory dopaminergic system (Moos & Richard, 1982), by a noradrenergic one, via ac-receptors (Moos & Richard, 1980), but also by a serotoninergic inhibitory system ) and a noradrenergic one via fl-receptors (Moos & Richard, 1980).…”

Effects of Unilateral Electrolytic Lesion of the Dorsomedial Nucleus of the Hypothalamus on Milk-ejection Reflex in the Rat

“…56: [98][99][100][101][102] 2010) uring suckling, oxytocin (OT) is released intermittently from the neurohypophysial terminals [1]. Each OT release, shown by an increase of intramammary pressure, is preceded by a synchronized short high-frequency burst of action potentials in most OT cells of the paraventricular (PVN) and supraoptic nucleus (SON) [2]. In spite of many studies on the neural mechanisms controlling such periodic and synchronized activation of OT cells during milk-ejection reflex, the exact mechanisms are still not fully understood.…”

Factors affecting milk ejection and removal during milking and suckling of dairy cows

“…However, it cannot be excluded that there is an indirect influence on oxytocin release and milk ejection. The reduction of the suckling stimulus by removing two or three pups from nipples produced a proportional fall in the magnitude of the burst of firing in rats (Lincoln and Wakerley, 1975). The burst is known as one of the two different modes burst and tonic, as thalamic cells respond to excitatory inputs (Sherman, 2001).…”