Melatonin binding sites in sciurid and hystricomorph rodents: studies on ground squirrels and guinea pigs

Little is known about the distribution of binding sites for the pineal hormone melatonin in non-myomorph rodents. We used 2-[125I]iodomelatonin (IMEL) to analyze the distribution, affinity, and specificity of binding sites in the golden-mantled ground squirrel, a sciurid rodent that reportedly lacks IMEL binding sites in the brain. Specific binding was found not only in the pars tuberalis, but also in several telencephalic and diencephalic areas including the hypothalamic suprachiasmatic region. The affinity and specificity of IMEL binding are comparable to those reported in other rodents. IMEL binding studies in a hystricomorph rodent, the guinea pig, revealed high concentrations of receptor in the nucleus accumbens and dorsolateral thalamus. Central melatonin binding sites have now been demonstrated in species of all three rodent families. The heterogeneous distribution of melatonin receptors appears similar in the species studied, and no evidence is found to link IMEL binding sites at any particular locus to photoperiodic, circannual, or non-seasonal breeding patterns.