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.