Modulation of ionic currents in bovine adrenal chromaffin cells by opioid peptides

Wayne Alan Twitchell, Purdue University

Abstract

Many physiological phenomena are mediated by the activation of opioid peptide receptors in the nervous system. Opioid receptors are postulated to mediate their effects via the inhibition of neuroexcitability and neurosecretion resulting from the modulation of ionic currents. To begin to address this issue, I used the patch clamp to investigate the effects of opioid peptides on ionic currents in bovine adrenal medullary chromaffin cells (BAMCCs). BAMCCs are an ideal model for the study of opioid modulation of ionic currents and their impact on secretion for the following reasons: BAMCCs possess opioid receptors, they cosecrete opioids and catecholamines, their preganglionic input is in part opioid peptidergic, opioids inhibit catecholamine release in vitro, and they are amenable to simultaneous single-cell electrophysiology and secretory measurement. Only two currents were found to be modulated by opioids: a Ca$\sp{2+}$-activated K$\sp+$ current (I$\sb{\rm BK}$) and a voltage sensitive Ca$\sp{2+}$ current (I$\sb{\rm Ca}$). I$\sb{\rm BK}$ was potentiation by opioids acting via a $\mu$-type opioid receptor while I$\sb{\rm Ca}$ was inhibited by opioids via either a $\kappa$ or $\delta$ opioid receptor. Opioid receptors are generally believed to act through G$\sb{\rm i/o}$ proteins. However, all attempts to disrupt or alter G-protein function failed to disrupt $\mu$ opioid receptor modulation of I$\sb{\rm BK}$. However, dopamine D$\sb2$ receptor potentiation of I$\sb{\rm BK}$ in these same cells was affected by PTX, GDP-$\beta$-S and GTP-$\gamma$-S in predictable fashion. These results suggest that the $\mu$ receptor modulates BK channel activity via a GTP-independent mechanism. This may represent a new mechanism by which 7-transmembrane spanning region receptors couple to their effetors. I have shown that opioid receptors modulate two currents in BAMCCs that are believed to be critical to controlling secretion. The causal relationship between opioid modulation of I$\sb{\rm BK}$ and I$\sb{Ca}$ and opioid inhibition of catecholamine secretion can now be directly studied using single cell electrochemical detection techniques. A mechanistic link between opioid modulation of ionic currents and opioid effects on neurosecretion will suggest a cellular basis for understanding the systemic effects of opioids.

Degree

Ph.D.

Advisors

Rane, Purdue University.

Subject Area

Neurology|Anatomy & physiology|Animals

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