Morphological and electrophysiological properties of neurones in the dorsal vagal complex of the rat activated by arterial baroreceptors

This study physiologically identifies and anatomically describes arterial baroreceptive neurones in the nucleus tractus solitarii of the rat. Neurones were recorded using neurobiotin-containing whole cell patch electrodes in a working heart-brainstem preparation and characterized physiologically as arterial baroreceptive in response to stimulation of the aortic arch and/or ipsilateral carotid sinus. Fifteen of 84 neurones tested were arterial baroreceptive, 7 of 8 were morphologically identified as located in the solitary tract nucleus (NTS), and 1 of 8 was located in the dorsal vagal nucleus. The seven NTS neurones had a resting membrane potential of -52 +/- 3.6 mV and a membrane input resistance of 233 +/- 38 M omega. Action potential height was 62 +/- 4.2 mV, width at half amplitude 1.46 +/- 0.38 ms, and duration of after-hyperpolarization 1.7 +/- 2.33 ms. In six of eight neurones labelled there was an invariant excitatory synaptic input (latency 3.95 +/- 0.3 ms) to stimulation of the solitary tract. Labelled somata were dorsomedial or medial to the solitary tract from -0.3 mm to +1.5 mm with regard to obex. Neurones had three to eight primary dendrites, and branches often entered the solitary tract and also extended across the ipsilateral NTS. Axons, which were mostly unmyelinated with boutons of the en passant variety, could ramify within the NTS while the main axon exited the NTS (n = 4/6), in the direction of the ipsilateral ventral medulla (n = 5/6). This is the first morphological and localisation data of physiologically characterised arterial baroreceptive NTS neurones in the rat. By comparing labelled cells with each other as well as with other unidentified cells, we conclude that NTS arterially baroreceptive neurones are morphologically and physiologically heterogenous.