Concomitant changes in formaldehyde-induced fluorescence of dopamine interneurones and in slow inhibitory post-synaptic potentials of the rabbit superior cervical ganglion, induced by stimulation of the preganglionic nerve or by a muscarinic agent.

1. Dopamine was identified by formaldehyde histochemistry and cytospectrofluorometry in the rabbit's superior cervical ganglion. Dopamine was localized to the intraganglionic ;small intensely fluorescent' cells, and also to the characteristically beaded fibres forming a network in close contact with virtually all ganglion cell bodies. The extensive beaded fibres are therefore presumed to be processes of the small intensely fluorescent cells.2. Changes in the dopamine content of these interneurones were studied by recording alterations in their relative fluorescence intensity in conjunction with changes in the slow inhibitory post-synaptic potential (s.-i.p.s.p.) response of the ganglion to orthodromic nerve input.3. Dopamine content was lower after several hours in vitro even without special stimulation; this was in accord with a regularly observed spontaneous reduction of the s.-i.p.s.p. response.4. After a period of conditioning stimulation of the preganglionic nerve, in the presence of an anticholinesterase agent (eserine) and an inhibitor of catecholamine synthesis (alpha-methyl-p-tyrosine), the s.-i.p.s.p. was selectively and markedly reduced. The dopamine fluorescence in the small intensely fluorescent cell interneurones was also significantly reduced, to a mean value of about 55 or 60% of the fluorescence in the dopamine interneurones of the paired but unstimulated control ganglion. A significant reduction in dopamine fluorescence was always accompanied by a marked loss of s.-i.p.s.p. response; the reverse was not always true.5. Treatment with the muscarinic agent bethanechol for 30 min, with no alpha-methyl-p-tyrosine or eserine present, similarly resulted in reductions in the s.-i.p.s.p. response of the ganglia and in the formaldehyde-induced fluorescence of the dopamine interneurones.6. A functional uptake of extrinsic dopamine by the dopamine interneurones was also demonstrated: temporary exposure to dopamine restored a large fraction of both the s.-i.p.s.p. response and the dopamine fluorescence of the small intensely fluorescent cells, in ganglia already subjected either to the conditioning stimulation of the preganglionic nerve or to the action of bethanechol.7. It is concluded that (a) preganglionic impulses, by a cholinergic muscarinic synaptic action, can induce a release of dopamine from dopamine interneurones (small intensely fluorescent cells) in the superior cervical ganglion, (b) the ability of the ganglion to respond with a s.-i.p.s.p. to orthodromic input may be viewed as being dependent on the supply of functionally releasable dopamine in these interneurones, (c) the functionally releasable transmitter in vitro appears to comprise roughly 50% of the total dopamine content of the interneurones, and (d) the results fulfil some of the requirements of the hypothesis that a dopamine interneurone is activated muscarinically by preganglionic nerve impulses and mediates the production of s.-i.p.s.p. in sympathetic ganglion cells.