Permissive effect of centrally administered oxytocin on the excitatory response of oxytocin neurones to ventral tegmental stimulation in the suckled lactating rat

The mesencephalic ventral tegmentum has been implicated in the milk-ejectio n reflex and modulation of inputs from this region could provide a mechanis m whereby central oxytocin facilitates synchronous bursting of oxytocin neu rones during suckling. Experiments were therefore undertaken to investigate the effect of intracerebroventricular (i.c.v.) oxytocin on the response of oxytocin neurones to ventral tegmental stimulation. Oxytocin neurones were recorded in the supraoptic nucleus of urethane-anaesthetized lactating rat s during suckling, and their response to single shock stimulation of the ve ntral tegmentum was monitored using peri-stimulus time-interval histograms. Before i.c.v. oxytocin, oxytocin neurones were either unresponsive to vent ral tegmental stimulation, or displayed a small inhibition. However, after administration of oxytocin (2.2 ng i.c.v.), seven out of eight neurones tes ted displayed a pronounced excitatory response (onset latency 78.4 +/- 4.8 ms, duration 73.4 +/- 8.3 ms). This permissive effect on the excitatory res ponse was only observed in the presence of suckling, and followed the same time course as facilitation of the milk-ejection reflex, being maximal imme diately before each facilitated bursting response in the oxytocin neurones. The response to ventral tegmental stimulation remained unaltered after int raperitoneal administration of hypertonic saline to cause a generalized inc rease in the excitability of the oxytocin neurones. Moreover, i.c.v. oxytoc in had no effect on the response of oxytocin neurones to stimulation of a d escending input from the medial septum. In conclusion, administration of i. c.v. oxytocin has a selective permissive effect on the excitation of oxytoc in neurones from the ventral tegmentum, and this supports previous in vitro studies suggesting that centrally released oxytocin may act as a modulator of afferent transmission to the magnocellular nuclei. This effect on the a fferent excitation of oxytocin neurones may provide a mechanism whereby i.c .v. oxytocin facilitates suckling-evoked bursting activity.