Distribution of serotonin 2A, 2C and 3 receptor mRNA in spinal cord and medulla oblongata.

It is known that 5-HT receptors have significant roles in nociceptive and motor functions. We have compared the cellular localization of the mRNAs encoding serotonin 5-HT(2A,) 5-HT(2C,) 5-HT(3) receptor subtypes within different levels of the rat spinal cord and medulla. In the spinal cord, 5-HT(2C) receptor mRNA is expressed at high levels in most of the gray matter, except for lamina II. In contrast, 5-HT(2A) receptor mRNA is expressed exclusively in lamina IX. 5-HT(3) receptor mRNA has a low level and diffuse pattern of expression increasing towards the ventral horn. In both gray and white matter, there is a characteristic presence of a few highly stained cells. For each subtype, the expression pattern is similar in all four levels of the spinal cord. In the medulla, 5-HT(2C) receptor mRNA is at high levels in many nuclei including the hypoglossal nucleus, the gigantocellular reticular nucleus alpha and the parvocellular reticular nucleus alpha, the spinal nucleus of the trigeminal tract, the facial, and the dorsal medullary reticular field. Moderate to low levels of expression are seen in the spinal vestibular nucleus, the vagus, the solitary nuclei and the raphe. 5-HT(2A) receptor is expressed at high levels in some nuclei such as the hypoglossal nucleus, the intercalate nucleus, the inferior olive and the lateral reticular nucleus. Moderate to low levels of expression are seen in the facial, the medial vestibular nuclei, the nucleus ambiguous, the vagus, and the gigantocellular reticular nucleus. 5-HT(3) receptor mRNA is present at low levels in most of the nuclei examined, with a few scattered strongly labeled cells. The results show a distinct distribution of the three subtypes of receptors supporting their physiological roles and will help to understand the mechanisms of nociception and motor function.