Distribution of gamma-aminobutyric acid receptors in cultured adrenergic and noradrenergic bovine chromaffin cells.

Fluorescence imaging techniques for recording cytosolic Ca(2+) from single chromaffin cells were used to characterize and discriminate between cell subpopulations containing gamma-aminobutyric acid (GABA)(A) and GABA(B) receptor subtypes. By combining this methodology with the immunoidentification of individual chromaffin cells using specific antibodies against tyrosine hydroxylase (TH), phenyl-etanolamine-N-methyl transferase (PNMT), and glutamic acid decarboxylase (GAD) linked to different fluorescent probes, we have been able to ascribe single-cell calcium responses to identified adrenergic and noradrenergic chromaffin cells. GAD enzyme is present in 30% of the chromaffin cell population, located primarily in adrenergic cells; 86% of GAD(+) cells were also PNMT(+). GAD expression was not correlated with the presence of GABA receptors. GABA-responsive cells were found with equal frequency in the GAD(+) and GAD(-) groups. However, the expression of GABA receptors was correlated with the adrenergic phenotype. Ca(2+) responses to GABA were found more frequently in adrenergic than in noradrenergic cells. GABA(A) receptors are more evenly distributed; about 90% of GABA-responsive cells have them. GABA(B) receptors have a more restricted distribution (present in 45% of responding cells). The coexpression of both GABA(A) and GABA(B) subtypes is the rule; only a minor subpopulation (about 12%) displays exclusively GABA(B) receptors. GABA receptor subtypes are distributed in a similar way when chromaffin cells are separated according to GAD(+)/GAD(-) or PNMT(+)/PNMT(-) classifications, with only minor differences. These data indicate that the intrinsic GABAergic system in the adrenal medulla is not designed as a paracrine model in which a group of cells specializes in transmitter synthesis and a different group serves as a specific target.