kappa-Opioid agonist modulation of [3H]thymidine incorporation into DNA: evidence for the involvement of pertussis toxin-sensitive G protein-coupled phosphoinositide turnover

Substantial evidence exists to support the notion that maternal drug abuse elicits, in addition to fetal growth delay, physiological and behavioral deficits in offspring (McDowell and Kitchen, 1987; Hayford et al., 1988; Farrar and Blumer, 1991). Among the drugs abused are opioid agonists, which appear to modify phenotypic expression of opioid receptors during brain ontogeny (Zagon and McLaughlin, 1987; Tempel et al., 1988; Barg et al., 1989a). In addition, opioids act as DNA synthesis modulators during brain development (Vertes et al., 1982; Bartolome et al., 1986, 1991; Kornblum et al., 1987; Schmahl et al., 1989). Opioid regulation of thymidine incorporation in rat brain has proven transient and dependent on developmental age (Kornblum et al., 1987; Barg et al., 1990). Opioid agonists such as morphine, β-endorphin, [d-Ala2,Me-Phe4,Gly-ol5]enkephalin (DAMGE), and Met-enkephalin decrease thymidine incorporation into DNA in vivo and their actions are reversed by the opioid antagonist naloxone, indicating opioid receptor mediation (Bartolome et al., 1986, 1991; Kornblum et al., 1987; Schmahl et al., 1989). Other reports emphasize the effect of opioids on DNA synthesis in vitro (Davila-Garcia and Azmitia, 1989; Eccleston et al., 1989; Barg et al., 1990; Coscia et al., 1991). Alteration of DNA synthesis by opioids in vitro is not restricted to neural systems, as it occurs in nonneural cells present in human lung and breast cancers and in the immune system as well (Gilmore and Weiner, 1989; Kusnecov et al., 1989; Maneckjee and Minna, 1990; Maneckjee et al., 1990; Taub et al., 1991). Protein kinase C (PKC) is included among the signal generators that trigger gene activation, leading to DNA synthesis and cell proliferation (Berridge, 1987; Kikkawa et al., 1989; Huang, 1990; Gschwendt et al., 1991). Moreover, modulators of this enzyme, such as phorbol esters, can decrease or increase DNA synthesis. PKC activators facilitate DNA synthesis and differentiation of neurons. Among the signal transduction systems that activate PKC are the phosphoinositide (PtdIns) systems. Implication of PKC in opioid modulation of thymidine incorporation into DNA has yet to be addressed. Furthermore, it is not known whether opioid abatement of thymidine incorporation involves activation of other receptor systems and/or intercellular signaling. Also unresolved are the target cells and opioid receptor types that mediate thymidine incorporation. Previously we have examined the ability of μ opioids to influence DNA synthesis (Coscia et al., 1991; Barg et al., 1992). In this study, the role of κ opioids is addressed and evidence is gained to suggest the intermediacy of κ-opioid receptors in not only neural (neuronal and glial) but also mixed glial cultures. A pertussis toxin-sensitive GTP-binding regulatory protein (G protein) and PtdIns turnover are also implicated.