IDENTIFICATION OF THE CHOLINERGIC NEURONS IN GUINEA-PIG SPHINCTER OF ODDI-GANGLIA

The muscular tone of the sphincter of Oddi (SO) can be up- or down-reg ulated by neurons that lie within ganglia in the wall of the tissue. P revious studies have demonstrated that neurons in the ganglia of the g uinea-pig SO can be classified into two major populations, one of whic h expresses tachykinins and enkephalin and another which expresses nit ric oxide synthase. Although results of previous pharmacological studi es indicate that acetylcholine is released in the SO, the neurons that express this neurotransmitter have not previously been identified. Th is study was conducted to establish which neurons in the ganglia of th e guinea-pig SO are cholinergic by examining the distribution of choli ne acetyltransferase (ChAT) immunoreactivity, since the enzyme, ChAT i s necessary for acetylcholine synthesis. Choline acetyltransferase imm unoreactivity was intense and widespread in the ganglionated plexus of the SO. ChAT-immunoreactive nerve fibers were present in ganglia, int erganglionic fiber bundles and in the circular muscle layer. Neurons t hat were immunoreactive for ChAT comprised about 69% of the population and most of these neurons were also tachykinin-immunoreactive. Go-exp ression of ChAT and nitric oxide synthase was not observed in nerve ce ll bodies or nerve fibers. Data from this study support the concept th at SO ganglia are largely made up of two populations of neurons, one e xcitatory and the other inhibitory, on the basis of their chemical cod ing. The excitatory neurons are cholinergic and co-express tachykinin and opiate peptides and the inhibitory neurons are ChAT-negative and e xpress nitric oxide synthase.