ELECTRICAL-ACTIVITY OF NEURONS IN THE VENTROLATERAL SEPTUM AND BED NUCLEI OF THE STRIA TERMINALIS IN SUCKLED RATS - STATISTICAL-ANALYSIS GIVES EVIDENCE FOR SENSITIVITY TO OXYTOCIN AND FOR RELATION TO THE MILK-EJECTION REFLEX

Our previous results obtained by lesioning or stimulating the ventrola teral part of the lateral septum and the bed nuclei of the stria termi nalis suggested that this area is involved in the control of milk ejec tion pattern in rats. The present study was undertaken with the aim of testing ventrolateral part of the lateral septum-bed nuclei of the st ria terminalis neurons as a putative link of the neuronal network cont rolling the bursting activity of oxytocin neurons in suckled lactating rats (anaesthetized with urethane). Ventrolateral part of the lateral septum-bed nuclei of the stria terminalis neurons were recorded simul taneously with hypothalamic oxytocin neurons in either the paraventric ular or supraoptic nucleus in rats with (n = 26) or without (n = 29) p eriodic milk ejections. Analysis of their firing pattern enabled diffe rentiation of two subgroups: type I, characterized by numerous high fr equency spikes, often grouped in clusters; and type II with very few o r no high frequency clusters of spikes. The probability density functi on of the interspike intervals of both patterns could be modelled usin g a mixture of two log-normal distributions, the parameters of which d iffered significantly. The presence or absence of milk ejections did n ot influence the overall mean level of activity (2.0 +/- 0.5 and 1.9 /- 0.4 spikes/s, respectively). However, the characteristics of the ty pe I firing pattern were affected by the presence of the milk-ejection reflex. The average level of activity was not always constant and 16/ 55 ventrolateral part of the lateral septum-bed nuclei of the stria te rminalis neurons displayed cyclical activity (from 0.6 +/- 0.2 to 4.0 +/- 0.5 spikes/s) both in the presence (n = 8) and absence (n = 8) of the milk-ejection reflex. In five of eight neurons recorded during mil k-ejection reflex, the cycles in firing were clearly correlated with t he bursting of oxytocin neurons. These five neurons exhibited the type I firing pattern. The three remaining neurons and the eight neurons r ecorded in the absence of milk-ejection reflex displayed the type II f iring pattern. Oxytocin (1-2 ng = 0.45-0.9 mU) was injected into the t hird ventricle (i.c.v.) in order to examine the possible involvement o f ventrolateral part of the lateral septum-bed nuclei of the stria ter minalis neurons in the facilitatory effect of oxytocin on the reflex. Oxytocin caused an excitation of 13/47 ventrolateral part of the later al septum-bed nuclei of the stria terminalis neurons independent of wh ether the reflex was facilitated by oxytocin (n = 6) or not (n = 7). I n the former case, the increase in firing rate occurred before facilit ation of the reflex. Most of the oxytocin-sensitive neurons (10/13) ha d the type I firing pattern, the characteristics of which were variabl y affected by oxytocin. The positions of 18 neurons were histologicall y localized in the ventrolateral part of the lateral septum-bed nuclei of the stria terminalis by iontophoretic deposition of a dye. The loc ation of 10 of these neurons was further correlated with oxytocin-bind ing sites using autoradiography. The majority of these neurons (includ ing both oxytocin-sensitive and -insensitive neurons) were located in regions expressing oxytocin-binding sites. In conclusion, there exists a population of ventrolateral part of the lateral septum-bed nuclei o f the stria terminalis neurons displaying numerous high frequency spik es (type I firing pattern) which exhibits a strong correlation with mi lk ejection-related bursting activity of hypothalamic oxytocin neurons , and/or which are sensitive to oxytocin. It is suggested that these n eurons might be involved as an element of a neuronal network mediating the central effects of oxytocin on the milk-ejection reflex.