WARM-SENSITIVE AFFERENT SPLANCHNIC C-FIBER UNITS IN-VITRO

Receptive fields of 41 slowly conducting sensory fibers were located u sing a thermal (warm) search stimulus in an in vitro splanchnic nerve- mesentery preparation. Warm-sensitive receptive fields were punctate a nd were densest in the region surrounding the prevertebral ganglia, an area with prominent deposits of brown adipose tissue, where the abdom inal aorta branches into the major trunks supplying the abdominal visc era. Impulse activity was recorded while applying a warm stimulus to i dentified receptive fields (RFs). The warm stimulus consisted of a war ming ramp (10-15 degrees C in 1-2 s to a 42-49 degrees C peak temperat ure) followed by a 10- to 30-s period during which the RF was maintain ed at this peak temperature (plateau phase). Eighty percent (33/41) of warm-sensitive units responded to warming with discharge comprising b oth a phasic and a tonic component (slowly adapting warm-sensitive, or SA-W, units). The remainder(8/41) responded with only phasic discharg e (rapidly adapting warm-sensitive, or RA-W, units). Units' adaptation characteristics were consistent from trial to trial and when applying stimuli from different positions. Fifty percent of SA-W units (8/16) and 17% of RA-W units (1/6)were activated by transient exposure to 9-9 0 nM bradykinin (BK). Twenty-seven percent (9/33) of SA-W units and 12 % (1/8) of RA-W units were activated by probing their RF with von Frey hairs with bending forces <10 mN (similar to 1 g equivalent mass). An additional five SA-W units tested were activated by strong mechanical stimuli (compression with a metal probe or firm stretching). No BK-re sponsive warm-sensitive units were activated by von Frey probing <10 m N, but two (both SA-W) responded to strong mechanical stimuli. In six SA-W units and one RA-W unit, the number of impulses evoked by warming similar to 5 min after exposure to BK was >2 SD greater than the mean pre-BK response, indicating sensitization. This sensitization was tra nsient, the response to warming returning to within one standard devia tion of the pretrial mean or less over the course of the next 5-10 min . Changes in background activity, mechanical sensitivity, BK sensitivi ty, and BK-induced sensitization were noted in various splanchnic unit s over the course of prolonged observations: suggesting that these ind ices may not reliably distinguish unit type, but instead may indicate the functional state of the sense organ. Splanchnic neurons responsive to the intense warming used in the present in vitro experiments may p articipate in the cardiovascular responses observed in vivo in heat-st ressed rats. The dense distribution of warm-receptive fields in the vi cinity of the celiac-superior mesenteric ganglionic complex is consist ent with the localization of splanchnic thermosensitive units previous ly noted in vivo in the rabbit.