Serotonin transporter localization in the hamster suprachiasmatic nucleus

Pacemaker cells within the hamster suprachiasmatic nucleus generate circadian rhythms. The suprachiasmatic nucleus is heavily innervated by serotonin axons originating in the median raphe nuclei. Consequently, serotonergic agonists and antagonists or agents that alter levels of serotonin in the synapse following transmission can modulate many aspects of circadian rhythmicity. Examples of the latter are some antidepressants and the stimulant amphetamine that bind to the serotonin transporter and block serotonin reuptake. It has been hypothesized that circadian rhythm dysfunction may be involved in depression, and that the efficacy of certain antidepressants in treating depression may involve an alteration of serotonin levels and certain circadian rhythm parameters. However, although the hamster is the behavioral model of choice for the study of circadian rhythms, the identification of serotonin transporters in this species has not been reported. Therefore, in this report we describe the distribution of the serotonin transporter in the hamster suprachiasmatic nucleus using immunohistochemical techniques. Our results demonstrate a dense labeling of the serotonin transporter throughout the ventral and medial regions of the suprachiasmatic nucleus, a pattern that overlaps the distribution of serotonergic afferents in this nucleus. Amphetamines and certain antidepressants may serve as substrates for this transporter and elicit chronopharmacological activity by elevating serotonin levels in the suprachiasmatic nucleus.