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D. A. Poulain and J. G. Tasker


In urethane-anaesthetized lactating rats, intramammary pressure occasionally displayed recurrent variations or oscillations having a slow rise time, low amplitude, long duration and a periodicity of 1–4 min. These oscillations differed from changes in intramammary pressure characteristic of reflex milk ejections induced by suckling, and were also observed in unsuckled rats. They were suppressed by lesions of the pituitary stalk or by stimulating the septum, a structure that inhibits the activity of the magnocellular system. They could be induced by long-term low frequency stimulation of the pituitary stalk, lesions of the septum or long-term infusions of oxytocin at a low rate of 0·05–0·3 mu./min. We suggest that the recurrent oscillations in intramammary pressure constitute a particular mode of response of the mammary gland to a tonic release of oxytocin resulting from a moderate but sustained increase in the basal level of electrical activity of the oxytocin-secreting neurones.

J. Endocr. (1985) 107, 89–96

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C. M. Montagnese, D. A. Poulain, J.-D. Vincent and D. T. Theodosis


In the supraoptic nucleus (SON) of parturient and lactating rats, large portions of the surface membranes of almost all oxytocinergic neurones are directly juxtaposed with no glial interposition. A significant number of the same neurones are also contacted by the same presynaptic terminal ('double' synapses).

Our present observations have revealed that direct appositions between adjacent neurones in the SON increase quite rapidly during the day before parturition. 'Double' synapses also become visible during late gestation, but they appear more progressively. Earlier studies have shown that 1 month after weaning, as in virgin rats, there are again very few appositions and 'double' synapses in the nucleus. We show here that the SON can remain structurally modified, and to the same degree, beyond normal weaning time so long as lactation is prolonged by renewing suckling litters. However, if the mothers are deprived of their pups immediately after birth, neuronal appositions disappear within 2 days and 'double' synapses by 10 days. In non-pregnant primiparous rats, continuous exposure to suckling litters leads to pseudogestation and eventually lactation (in 16–22 days). Examination of the SON in such animals revealed that the oxytocinergic system is already modified by day 12 of dioestrus; during suckling-induced lactation, the anatomical changes are identical to those seen during a normal post-partum lactation.

These observations indicate that neither gestation nor suckling alone are indispensable for the anatomical reorganization of the SON apparent at lactation. However, suckling is important in maintainingthe structural changes during prolonged lactation, and can lead to restructuring of the nucleus by inducing pseudogestation and lactation in non-pregnant animals. Taken together with earlier data, these results suggest that hormonal factors, especially oxytocin and gonadal steroids, may act singly or, more probably, in synergy to produce the structural plasticity of the adult oxytocinergic system.

J. Endocr. (1987) 115, 97–105