Morphology and distribution of neurons expressing serotonin 5-HT1A receptors in the rat hypothalamus and the surrounding diencephalic and telencephalic areas

Disorders of serotonergic neurotransmission are involved in disturbances of numerous hypothalamic functions including circadian rhythm, mood, neuroendocrine functions, sleep and feeding. Among the serotonin receptors currently recognized, 5-HT_1A receptors have received considerable attention due to their importance in the etiology of mood disorders. While previous studies have shown the presence of 5-HT_1A receptors in several regions of the rat brain, there is no detailed map of the cellular distribution of 5-HT_1A receptors in the rat diencephalon.In order to characterize the distribution and morphology of the neurons containing 5-HT_1A receptors in the diencephalon and the adjacent telencephalic areas, single label immunohistochemistry was utilized. Large, multipolar, 5-HT_1A-immunoreactive (IR) neurons were mainly detected in the magnocellular preoptic nucleus and in the nucleus of diagonal band of Broca, while the supraoptic nucleus contained mainly fusiform neurons. Medium-sized 5-HT_1A-IR neurons with triangular or round-shaped somata were widely distributed in the diencephalon, populating the zona incerta, lateral hypothalamic area, anterior hypothalamic nucleus, substantia innominata, dorsomedial and premamillary nuclei, paraventricular nucleus and bed nucleus of stria terminalis.The present study provides schematic mapping of 5-HT_1A-IR neurons in the rat diencephalon. In addition, the morphology of the detected 5-HT_1A-IR neural elements is also described. Since rat is a widely used laboratory animal in pharmacological models of altered serotoninergic neurotransmission, detailed mapping of 5-HT_1A-IR structures is pivotal for the neurochemical characterization of the neurons containing 5-HT_1A receptors.