Distribution of delta opioid receptor immunoreactivity in the hamster suprachiasmatic nucleus and intergeniculate leaflet

The hamster suprachiasmatic nucleus (SCN) is innervated by a dense plexus of enkephalin-containing axons originating from cells in the intergeniculate leaflet (IGL) of the thalamus. However, the distribution of opioid receptors within the hamster SCN has not been reported. Opioid receptors consist of three primary subtypes: mu, delta and kappa opioid receptors. Enkephalins have the highest affinity for delta opioid receptors. Therefore, in the present study, we examined the distribution of delta opioid receptor immunoreactivity in the hamster SCN and the IGL of the thalamus. Coronal sections of the hamster hypothalamus inclusive of the SCN or thalamic regions containing the IGL were prepared at specific times of the day and labeled with anti-delta opioid receptor polyclonal antisera using standard immunohistochemical techniques. delta opioid receptors were heavily distributed within rostral-caudal regions of the SCN, with the densest labeling located in the ventral and medial regions of the mid-SCN. Similar patterns of labeling were observed for tissue prepared during mid-day or mid-night times. In contrast, delta opioid receptor immunoreactivity only sparsely labeled cells in the IGL. Cellular staining in all regions appeared as dark punctate labeling surrounding cells, indicative of terminal boutons. Therefore, it is suggested that delta opioid receptors are located presynaptically on axon terminals within the hamster SCN and IGL. These results suggest that delta opioid receptors may play a role in modulating circadian rhythms generated within the SCN, possibly by regulating transmitter release within the nucleus.