OXYTOCIN-CONTAINING PATHWAY TO THE BED NUCLEI OF THE STRIA TERMINALIS OF THE LACTATING RAT-BRAIN - IMMUNOCYTOCHEMICAL AND INVITRO ELECTROPHYSIOLOGICAL EVIDENCE

Immunocytochemical staining within the forebrain of lactating rats revealed oxytocin-immunoreactive perikarya in a continuum running from the anterior parvocellular hypothalamic paraventricular nucleus through the anterior commissural nucleus and perifornical region. Beaded axons could be seen arising from these perikarya to enter the bed nuclei of the stria terminalis. In sections cut at a 45-degrees angle to the parasagittal plane, much of this pathway could be maintained intact, and in vitro tissue slices prepared in this orientation were used for electrophysiological studies of oxytocinergic innervation of the bed nuclei. By extracellular recording, neurons of the bed nuclei of the stria terminalis were tested for their response to exogenous oxytocin and to stimulation of the paraventricular hypothalamus. Both short latency (3-40 ms) orthodromic excitation (26/78 neurons) and longer latency (> 100 ms) excitation (12/78 neurons) were observed following paraventricular hypothalamic stimulation, possibly representing mono- and polysynaptic inputs, respectively. Removal of extracellular Ca2+ blocked these orthodromic responses (n = 6). Antidromic invasion was seen in a further 11/78 neurons with characteristics of constant latency (mean = 5.9 +/- 0.7 ms), high frequency following (40-80 Hz) and persistence in Ca2+-free medium. When tested for the effect of oxytocin (10(-7) M), none (0/11) of the antidromically activated neurons were excited, but nine of 34 of the orthodromically excited neurons (both short and long latency) responded with a marked increase in activity. In three of eight cases, the orthodromic synaptic excitation following hypothalamic stimulation could be reversibly attenuated by the receptor antagonist [d(CH2)5,D-Tyr(OEt)2, Val4,Cit8]-vasopressin (0.5 or 2.5 x 10(-6) M), further substantiating the involvement of oxytocin. These data provide anatomical and electrophysiological evidence for an oxytocinergic innervation of the bed nuclei of the stria terminalis. This pathway is discussed in terms of possible involvement in mediating the facilitatory effect of oxytocin on the milk-ejection reflex of lactating rats which has been suggested to act through this part of the limbic system.