Physiological and morphological properties of neurons in sphincter of Oddi region of the guinea pig.

Intracellular recordings and dye injections were used to investigate neurons located in ganglia of the sphincter of Oddi (SO) region in guinea pigs. Four types of neurons were encountered based on physiological properties. The two most abundant types, tonic and phasic, had similar membrane characteristics and morphologies but yet could be differentiated by their spiking characteristics. Tonic cells spiked throughout a depolarizing current pulse and were sometimes spontaneously active. Phasic cells fired only a single action potential at the onset of a current pulse regardless of stimulus amplitude or duration. Both tonic and phasic cells had Dogiel type I morphologies. They typically had a single long process and several very short processes emanating from the soma. NADPH diaphorase activity was demonstrated in cells with similar morphologies, indicating that nitric oxide may be an intrinsic transmitter in some of these cells. Cells with a prolonged afterhyperpolarization (AH cells), similar to the type 1/AH cells of the gut, were rarely encountered. This finding was consistent with the observation that very few calbindin D-immunoreactive neurons exist in this region. Action potentials could not be generated in the fourth type of neuron, called nonspiking neurons, even though they did receive synaptic input. Most tonic and phasic cells received at least one nicotinic fast excitatory postsynaptic potential (EPSP). In addition, both slow EPSPs and inhibitory postsynaptic potentials were observed. Most AH cells received only slow excitatory synaptic input.