Neurons in the paraventricular nucleus projecting to the median eminence: a study of their afferent connections from peripheral baroreceptors, and from the A1-catecholaminergic area in the ventrolateral medulla.

In male rats anesthetized with urethane-chloralose, extracellular recordings were made from tuberoinfundibular (TI) neurons in the paraventricular nucleus (PVN) identified by antidromic stimulation of the median eminence. Activation of baroreceptors induced by intravenous administration of phenylephrine inhibited approximately two-thirds of the 32 TI-neurons tested. Electrical stimulation of the A1-catecholaminergic area in the ventrolateral medulla produced an excitation in one half of the 28 neurons. Microinjection of L-glutamate (40 nl of 0.5 M solution of pH 7.4-7.6) to the same A1-area evoked an excitation in 7 out of 8 TI-neurons tested. The results show that some TI-neurons in the PVN receive inhibitory synaptic inputs from the baroreceptors and excitatory inputs from neurons in the A1-catecholaminergic area.