The parabrachial area: electrophysiological evidence for an involvement in visceral nociceptive processes

1. Neurons (n = 142) were recorded with extracellular micropipettes in the parabrachial (PB) area, in the anesthetized rat, some of them being antidromically driven from the centralis nucleus of the amygdala (Ce). The spontaneous activity of these neurons was low, (10th percentile < median frequency < 90th percentile; 0.01 < 0.3 < 11 Hz), and the activity of a very high proportion of the PB neurons (89%, 127/142) were affected by mechanical or thermal cutaneous stimuli almost exclusively in the noxious range; most of them were activated and received inputs from A delta and/or C fibers. 2. A majority of the PB neurons (66%, 93/142) were affected by visceral stimuli (bradykinin intraperitoneal and/or colorectal distension), almost exclusively in the noxious range; all of them (except one) were also affected by cutaneous noxious stimuli. The remaining neurons not affected by these visceral stimuli were only activated by cutaneous noxious stimuli (n = 35) or completely unresponsive (n = 14; i.e., only 10% of the whole population). 3. Thirty-five percent (49/142) of the whole population of PB neurons responded to bradykinin intraperitoneal and/or to strong colorectal distension with an intense and sustained increase of discharge. The response to bradykinin and to colorectal distension was often dissociated, i.e., the activation was often produced by only one of the stimuli the other being ineffective or inhibitory. The intensity of responses to visceral noxious stimuli was between 5-45 Hz with a mean value of 20 +/- 2 (SE) Hz (n = 49). The visceral activated neurons exhibited a clear capacity to encode the colorectal distension in noxious range: 1) the stimulus-response function was almost always positive and monotonic; 2) as for the individual curves, the slope of the mean curve progressively increased up to the highest interval of pressure tested (100-125 mmHg); and 3) the threshold for neuronal response to colorectal distension was between 25-100 mmHg with a mean pressure threshold of 56 +/- 24 (SD) mmHg (n = 19). Viscerosomatic convergence was observed for 84% of these neurons: they were also activated by thermal and/or mechanical cutaneous noxious stimuli while only 16% of them were activated only by visceral noxious stimulation. 4. Thirty-one percent (44/142) of the whole population of PB neurons were inhibited by both strong colorectal distension and intraperitoneal bradykinin (82%) or only by one of the stimuli being unresponsive to the other (18%).(ABSTRACT TRUNCATED AT 400 WORDS)