Delta-opioid receptor immunoreactivity on astrocytes is upregulated duringmitosis

Endogenous opioid peptides and opioid receptors are expressed by brain cell s early during normal development, and exogenous opiate exposure in this pe riod is known to affect brain cell proliferation and maturation. Despite th e abundant evidence that opioids affect brain development, little is known about the mechanisms involved. In this study cortical astrocytes in primary culture were examined immunohistochemically by using antibodies against th e opioid receptors. The immunoreactivity for delta-opioid receptors was str ongly upregulated during mitosis with an increase in immunostaining that st arted in early prophase and lasted through the M-phase to cytokinesis. Simi lar effects could not be observed when antibodies against the mu- or kappa- opioid receptor subtypes were used. Cultured neurons and microglia presente d a strong and homogenous immunostaining for the delta-opioid receptor and no further upregulation of immunoreactivity could be detected in these cell s. The presence of functional delta-opioid receptors on the mitotic astrocy tes was verified by using microspectrofluorometry for detection of delta-op ioid agonist induced changes in intracellular free calcium concentrations ( [Ca2+](i)). In these experiments fluo-3/AM incubated cells showed a rapidly induced delta-opioid agonist (DPDPE, 10(-6) M) evoked increase in [Ca2+](i ). These results suggest an upregulation of the delta-opioid receptors that could represent a mechanism involved in the response to opioids in the dev eloping brain. (C), 1999 Wiley-Liss, Inc.