The orexinergic synaptic innervation of serotonin- and orexin 1-receptor-containing neurons in the dorsal raphe nucleus

Orexin/hypocretin has been well demonstrated to excite the serotonergic neurons in the dorsal raphe nucleus (DRN). We studied the morphological relationships between orexin-containing axon terminals and serotonin- as well as orexin-receptor-containing neurons in the dorsal raphe nucleus. Using immunohistochemical techniques at the light microscopic level, orexin A (OX-A)-like immunoreactive neuronal fibers in the DRN were found to make close contact with serotonergic neurons. while some of the serotonergic neurons also expressed the orexin 1 receptor (OX1R). At the electron microscopic level, double-immunostaining experiments showed that the orexin A-like immunoreactive fibers were present mostly as axon terminals that made synapses on the serotonin- and orexin 1-receptor-containing neurons. While only axodendritic synapses between orexin A-containing axon terminals and serotonergic neurons were detected. the synapses made by orexin A-containing axon terminals oil the orexin 1-receptor-containing neurons were both axodendritic and axosomatic. The present study suggests that excitation effect of orexin A oil dorsal raphe serotonergic neurons is via synaptic communication through orexin 1 receptor. (C) 2004 Elsevier B.V. All rights reserved.