Response properties of the parabrachio-thalamic taste and mechanoreceptive neurons in rats

A total of 66 taste and 33 mechanoreceptive neurons were isolated from the parabrachial nucleus (PB) of rats. Among them, 39 taste and 8 mechanoreceptive neurons were identified as parabrachio-thalamic relay (P-T) neurons on the basis of antidromic activation from either or both sides of the thalamic taste areas (TTAs). On average, the P-T taste neurons produced larger response magnitudes than the non-P-T taste neurons, and whereas about half the P-T taste neurons were NaCl-best, only a small number of the non-P-T taste neurons were NaCl-best. Both the P-T and non-P-T taste neurons showed a similar breadth of responsiveness to four basic taste stimuli. The response magnitudes of the P-T taste neurons to all taste stimuli were ca. 3 times larger than those of the solitario-parabrachial relay (SP) neurons (afferents to the PB); in particular, the response magnitudes of the NaCl-best P-T neurons were 4-5 times larger than those of the NaCl-best SP neurons. The response magnitudes and breadth of taste responsiveness of the P-T taste neurons were reciprocally correlated with the antidromic latencies from either side of the TTAs. A histological examination revealed that the P-T taste neurons in the ventral part of the PB had a shorter antidromic latency from the ipsilateral TTA than those in the dorsal part of the nucleus. Mechanoreceptive neurons were excited by stroking the tissue in the oral cavity or perioral tissue, or by pinching them with non-serrated forceps. The mechanoreceptive P-T neurons were also activated from either or both sides of the TTAs. No particular relation was noticed between the antidromic latency of the mechanoreceptive P-T neurons and their response properties or locations in the nucleus.