gamma-Aminobutyric acid and 5-hydroxytryptamine interrelationship in the rat nucleus raphe dorsalis: combination of radioautographic and immunocytochemical techniques at light and electron microscopy levels.

Serotonin and gamma-aminobutyric acid (GABA) neurons in the nucleus raphe dorsalis were identified by immunocytochemistry using antibodies to 5-hydroxytryptamine or GABA. The pattern of the 5-hydroxytryptamine and GABA immunostaining presented similar features: 5-hydroxytryptamine or GABA immunoreactive somata were fusiform or ovoid (15-20 micron) and positive dendritic profiles were found either without any connection with other nerve elements or in contact with one or several terminals. In addition, some 5-hydroxytryptamine nerve endings were apposed to 5-hydroxytryptamine immunoreactive cell bodies or dendrites; also some GABA-immunopositive terminals were in contact with GABA-immunopositive nerve cell bodies. On the other hand, GABA and 5-hydroxytryptamine patterns may be differentiated in several respects: the 5-hydroxytryptamine-reactive nerve cell bodies were more numerous than the GABA ones. Some small, round (8-10 micron) nerve cell bodies were reactive with GABA antiserum, but no neurons of this type were reactive with a 5-hydroxytryptamine antiserum; finally, GABA nerve terminals were more numerous than 5-hydroxytryptamine ones. In order to understand the relationship between GABA and 5-hydroxytryptamine neurons, radioautographic and immunocytochemical procedures were combined: 5-hydroxytryptamine and GABA immunocytochemistry was combined with radioautography of [3H]GABA and [3H]5-hydroxytryptamine uptake, respectively. Some nerve cell bodies, dendrites or terminals, which were 5-hydroxytryptamine-immunopositive, were also capable of accumulating [3H]GABA and, conversely, some GABA-immunopositive elements were capable of accumulating [3H]5-hydroxytryptamine. Moreover, several nerve elements were reactive with both glutamate decarboxylase and 5-hydroxytryptamine antisera. These data confirm in electron microscopy previous studies suggesting the coexistence of both GABA and 5-hydroxytryptamine in the same neurons. The presence of uptake mechanisms for GABA and 5-hydroxytryptamine may indicate the action of both neurotransmitters in the same neuron. On the other hand, the [3H]GABA-labelled nerve endings in contact with 5-hydroxytryptamine-positive dendrites or nerve cell bodies indicate the possibility of a GABAergic control of the activity of some 5-hydroxytryptamine neurons; this corroborates biochemical and electrophysiological studies whereby a trans-synaptic control of the 5-hydroxytryptamine neurons by GABA may be envisaged.