Aspects of the neuronal and endocrine components of reflex milk ejection in conscious rabbits

Abstract

Nursing behaviour and reflex milk ejection were studied over 257 suckling periods in 26 Californian rabbits. Detailed plasma profiles of oxytocin were obtained for 161 suckling periods in 16 animals. Plasma oxytocin was detected by radioimmunoassay in serial samples of 0.2 ml blood collected during nursing. Oxytocin titres were below the lower limit of the assay (5 pmol/l) before suckling, and started to rise at a rate of 2.8 +/- 0.8 (S.D.) pmol/l per s 10-30 s after the onset of suckling. Peak levels of hormone were 345 +/- 113 pmol/l and were attained towards the end of nursing. In 33 of these experiments simultaneous records of activity from oxytocin neurones were taken whilst chronically sampling the blood. Each neurone gave an average of seven bursts of neurosecretory activity in suckling. Onset of this bursting pattern of discharge began 5-24 s before the rise in plasma oxytocin was detected. Oxytocin neurone activity alone was monitored in a further 55 suckling periods in eight rabbits. A marked relationship between the duration of suckling period, milk yield, peak oxytocin levels and the length of neurosecretory bursts was demonstrated over the course of lactation. All four parameters increased in parallel from day 1 to reach maximal values on days 15-20 of lactation then started to wane until the doe ceased nursing her litter on days 25-27 of lactation.

Relationship between oxytocin release and amplitude of oxytocin cell neurosecretory bursts during suckling in the rat

“…Relationship between plasma concentration of oxytocin and amplitude of oxytocin cell neurosecretory bursts The sole attempt to establish a relationship between plasma concentrations of oxytocin and oxytocin cell firing during the milk ejection reflex was carried out in the rabbit (Summerlee et al 1986). In this species the suckling period is brief (3-5 min) and oxytocin cell neurosecretory bursts occur frequently (interval between bursts of less than 1 min (Paisley & Summerlee, 1984): this interval being less than the oxytocin half-life).…”

“…The plasma concentration of oxyto¬ cin, therefore, increases progressively, which gives an appearance of sustained release. Nevertheless, Summerlee et al (1986) established that plasma concentrations of oxytocin increase simultaneously with the duration of the neurosecretory bursts from day 1 to day 15 of lactation, the maximum concen¬ tration being reached at the end of the suckling period.…”

“…The first hypothesis is probably valid in women, as suggested by the results of Lucas, Drewett & Mitchell (1980) and McNeilly, Robinson, Houston & Howie (1983). The second is clearly valid in the rabbit; during suckling, oxytocin cell neuro¬ secretory bursts (brief acceleration of firing) occur at brief intervals (five to nine bursts during 3-5 min of suckling) (Paisley & Summerlee, 1984) and are associated with a sustained increase in plasma concentrations of oxytocin (Summerlee et al 1986). …”

“…In the guinea-pig (Robinson, Woolf & Parsons, 1981) and the pig (Ellendorff, Forsling & Poulain, 1982), pulsatile and brief increases in plasma concentrations of oxytocin have been clearly demonstrated during suckling. In con¬ trast, in cows (Schams, Mayer, Prokopp & Worstorff, 1984), women (Johnston & Amico, 1986) and rabbits (Summerlee, Paisley, O'Byrne et al 1986) the increase in the plasma concentration of oxytocin generally appears sustained, lasting for several minutes. In these three species the pulsatility of oxytocin release might not have been observed either because the timing of blood sampling was inadequate or because the pulsa¬ tile releases occurred at such short intervals as to give the appearance of a continuous and sustained increase in oxytocin release.…”

Relaxin Inhibits the Pulsatile Release of Oxytocin but Increases Basal Concentrations of Hormone in Lactating Rats1

“…In all five animals, milk ~milk ejection in the rat has not been confirmed previously. The pattern of release of oxytocin during milk ejection has been studied in detail in the rabbit [21,22]. Oxytocin increases steadily over the 3-to 5-min sucking period and reaches a maximum level at or shortly after the mother leaves the nest.…”