Physiological characteristics of the solitario-parabrachial relay neurons with tongue afferent inputs in rats.

Among 180 units in the solitary tract nucleus (NTS) of rats, 34 solitario-parabrachial relay neurons (SP neurons), were identified by antidromic activation from the parabrachial nucleus. The SP neurons were classified into two groups, fast and slow, according to their antidromic latencies. The responsiveness of the SP in comparison with non-SP neurons was studied by electrical stimulation of three tongue nerves: the lingual (L), chorda tympani (CT) and glossopharyngeal (G) nerves. About half the SP neurons produced a single spike with an orthodromic latency of 2-5 ms, while about one third of them discharged more than two spikes. A few neurons gave rise to a long-lasting discharge consisting of five or six spikes. Some SP neurons were excited by stimulation of the tongue afferents with a low stimulus intensity, but other SP neurons produced spikes at only very high voltages. Fast SP cells were not differentiated from slow SP cells, except that latency of orthodromic responses to CT stimulation was significantly shorter in the former than in the latter (P less than 0.05, Mann-Whitney U-test). Locations of the SP and non-SP neurons, reconstructed histologically, indicate that they do not distribute evenly throughout the mediolateral extent at the rostral pole of the NTS, but clustered in its medial half.