GABA-IMMUNOREACTIVE AND GLUTAMATE-IMMUNOREACTIVE SYNAPSES ON SYMPATHETIC PREGANGLIONIC NEURONS PROJECTING TO THE SUPERIOR CERVICAL-GANGLION

Our previous work suggests that virtually all of the synapses on sympa thetic preganglionic neurons projecting to the rat adrenal medulla are immunoreactive for either the inhibitory amino acid, gamma-aminobutyr ic acid (GABA) or the excitatory amino acid, L-glutamate. To investiga te whether or not this is true for other groups of sympathetic pregang lionic neurons, and to determine whether or not the proportion of inpu ts containing each type of amino acid neurotransmitter is the same for different groups of sympathetic preganglionic neurons, we retrogradel y labelled rat and rabbit sympathetic preganglionic neurons projecting to the superior cervical ganglion and used post-embedding immunogold on ultrathin sections to localise GABA- and glutamate-immunoreactivity . The cell bodies and dendrites of both rat and rabbit sympathetic pre ganglionic neurons projecting to the superior cervical ganglion receiv ed synapses and direct contacts from nerve fibres immunoreactive for G ABA and from nerve fibres immunoreactive for glutamate. In the rat, GA BA was present in 48.9% of the inputs to sympathetic preganglionic neu rons projecting to the superior cervical ganglion, and glutamate was p resent in 51.7% of inputs. Double immunogold labelling for glutamate a nd GABA on the same section, as well as labelling of consecutive seria l sections for the two antigens, indicated that GABA and glutamate occ ur in separate populations of nerve fibres that provide input to rat s ympathetic preganglionic neurons projecting to the superior cervical g anglion. We now have shown that GABA or glutamate is present in virtua lly all of the inputs to sympathetic preganglionic neurons projecting to the superior cervical ganglion and in essentially all of the inputs to sympathetic preganglionic neurons supplying the adrenal medulla. T hese findings are consistent with the hypothesis that all fast synapti c transmission in central autonomic pathways may be mediated by either excitatory or inhibitory amino acids. Furthermore, we showed a statis tically significant difference in the proportion of glutamate-immunore active inputs between sympathetic preganglionic neurons projecting to the superior cervical ganglion and sympathoadrenal neurons (data from Llewellyn-Smith et al. [Llewellyn-Smith, I.J., Phend, K.D., Minson, J. B., Pilowsky, P.M., Chalmers, J.P., 1992. Glutamate immunoreactive syn apses on retrogradely labelled sympathetic neurons in rat thoracic spi nal cord. Brain Res. 581, 67-80]), with preganglionics supplying the a drenal medulla receiving more excitatory inputs than those supplying t he superior cervical ganglion. This increased excitatory input to symp athoadrenal neurons may explain the predominant activation of these ne urons following baroreceptor unloading. (C) 1998 Elsevier Science B.V. All rights reserved.