Electrophysiological characterization of reciprocal connections between the parabrachial nucleus and the area postrema in the rat.

Neuroanatomical studies have demonstrated reciprocal connections between the parabrachial nucleus (PBN) and both the area postrema (AP) and the nucleus tractus solitarius (NTS). To functionally characterize these projections, antidromic identification of AP and NTS neurons projecting to the PBN was attempted. Orthodromic influences on these cells, resulting from PBN stimulation, were also examined. Four percent of AP neurons tested (n = 74) were antidromically identified as projecting to the PBN [latency (L) = 26 +/- 4 msec, threshold current (T) = 79 +/- 11 microA]. Parabrachial stimulation orthodromically influenced 24% of AP cells. Equal numbers of these neurons (12%) were excited [L = 25 +/- 9 msec, duration (D) = 29 +/- 14 msec] and inhibited (L = 28 +/- 8 msec, D = 107 +/- 40 msec). Of 46 NTS neurons tested, 11% were antidromically identified as projecting to the PBN (L = 12 +/- 4 msec, T = 61 +/- 18 microA), while orthodromic influences were seen in 41% of these neurons. Initial responses of 30% of the cells were excitatory (L = 34 +/- 14 msec, D = 63 +/- 24 msec), PBN stimulation inhibited the remaining 11% of NTS neurons (L = 30 +/- 10 msec, D = 108 +/- 32 msec). These findings suggest that a functional heterogeneity exists in the PBN efferents to the AP and NTS. However, the small proportion of antidromically identified AP and NTS efferents to the PBN disagrees with neuroanatomical studies suggesting a denser projection.