Synaptic transmission in the rabbit inferior mesenteric ganglion

Electrical properties, cholinergic neurotransmission and non-cholinergic neurotransmission in the rabbit inferior mesenteric ganglion (IMG) in vitro were examined with intracellular recording techniques. A single ganglionic neuron received an average of 42 nicotinic cholinergic synaptic inputs. An atropine-sensitive slow excitatory postsynaptic potential not followed by a non-cholinergic late slow excitatory postsynaptic potential (LS-EPSP) was observed in 7% of the cells. In 63% of the cells a LS-EPSP insensitive to antagonism of nicotinic and muscarinic receptors was observed following repetitive nerve stimulation. The involvement of substance P (SP) in the genesis of the LS-EPSP was tested by applications of SP, applications of SP antagonists and applications of capsaicin. Neither SP, SP antagonists nor capsaicin affected the LS-EPSP. These findings distinguish the LS-EPSP in the rabbit IMG from its counterpart in the guinea pig IMG where SP has been proposed as the mediator of the LS-EPSP. A late slow inhibitory postsynaptic potential was observed in 13% of the cells. This hyperpolarization followed repetitive nerve stimulation and was insensitive to blockade of cholinergic receptors. There is a marked convergence of subthreshold fast excitatory postsynaptic potentials (F-EPSPs) of both central and peripheral origin onto these cells. The LS-EPSP could provide a mechanism for increasing the likelihood of temporal and/or spatial summation of these fast synaptic inputs, thereby increasing the probability of action potential generation in the ganglion cells.