Extent of colocalization of serotonin and GABA in the neurons of the rat raphe nuclei

Previous investigations of the distribution of neurons containing both serotonin and GABA in the brainstem raphe nuclei have yielded discrepant results amongst different authors. This study attempted to clarify the distribution as well as the proportions of raphe and other brainstem neurons that contain both neurotransmitters. All the nine serotonergic cell groups known to be present in the brainstem were examined with an indirect immunofluorescence method using antibodies against serotonin and glutamic acid decarboxylase in colchicine-treated rats. Sections were incubated either simultaneously or sequentially for the two immunolabels. Brainstem neurons that were labelled for both markers were generally infrequent. Of all the serotonin cell groups in the brainstem, the nucleus raphe magnus contained the most double-labelled cells (a mean of 3.6% of a total of 625–1155 serotonin-immunoreactive cells counted in this nucleus), followed by the nucleus raphe obscurus (1.5% of a total of 220–550 serotonin-immunoreactive neurons counted). The dorsal, median and pontine raphe nuclei as well as the supralemniscal nucleus (the B9 group) contained very few double-labelled cells, which comprised a mean of 0.1–0.7% of all serotonin-immunoreactive cells in each of these nuclei. No double labelled cells were present in the caudal linear raphe nucleus or the nucleus raphe pallidus, nor in the B4 group. These results suggest that only a very small percentage of serotonergic neurons in the medullary raphe nuclei (raphe magnus and raphe obscurus) also contain GABA, whereas such cells are virtually absent in the midbrain raphe nuclei or in the non-raphe serotonergic cell groups in the brainstem.