A slow EPSP in mammalian inferior mesenteric ganglion persists after in vivo capsaicin.

The non-cholinergic slow excitatory postsynaptic potential (EPSP) in the guinea pig inferior mesenteric ganglion (IMG) is believed to be mediated by substance P (SP). Yet, in ganglia of guinea pigs treated with capsaicin (50, 100, or 350 mg/kg, s.c.), a compound that depletes SP from sensory neurons, slow EPSPs were only 50% smaller in amplitude than those in vehicle-treated controls; this was true at all dosages. When ganglia from capsaicin-treated animals were desensitized to SP, slow EPSP amplitude was decreased by 50% from predesensitization values. These data indicate that capsaicin does not eliminate the slow EPSP in the IMG and suggest that both SP and another transmitter are involved in co-mediating this response.