Morphological features and electrophysiological properties of serotonergic and non-serotonergic projection neurons in the dorsal raphe nucleus. An intracellular recording and labeling study in rat brain slices.

The morphology and electrophysiological properties of serotonergic and non-serotonergic projection neurons in the dorsal raphe nucleus (DRN) of the rat were examined in frontal brain slices. Biocytin was injected intracellularly into the intracellularly recorded neurons. Then the morphology of the recorded neurons was observed after histochemical visualization of biocytin. The recorded neurons extending their main axons outside the DRN were considered as projection neurons. Subsequently, serotonergic nature of the neurons was examined by serotonin (5-HT) immunohistochemistry. The general form of the dendritic trees is radiant and poorly branching in both 5-HT- and non-5-HT neurons. However, the dendrites of the 5-HT neurons were spiny, whereas those of the non-5-HT neurons were aspiny. The main axons of both 5-HT- and non-5-HT neurons were observed to send richly branching axon collaterals to the DRN, ventrolateral part of the periaqueductal gray and the midbrain tegmentum. In response to weak, long depolarizing current pulses, the 5-HT neurons displayed a slow and regular firing activity. The non-5-HT neurons fired at higher frequencies even when stronger current was injected. Some other differences in electrophysiological properties were also observed between the 5-HT-immunoreactive spiny projection neurons and the 5-HT-immunonegative aspiny projection neurons.