Low-affinity binding of [3H]imipramine to primary astrocyte cultures

High-affinity uptake of serotonin (5-HT) by primary cultures of rat cortical astrocytes has been shown recently to be potently inhibited by tricyclic antidepressants in a manner similar to that described for brain synaptosomes [Katz and Kimelberg, J. Neurosci. 5, 1901 (1985)]. Since the high-affinity binding of [3H]imipramine (IMI) to brain membranes has been well correlated with the inhibition of synaptosomal 5-HT uptake, the binding of [3H]IMI to these astrocyte cultures was examined. No evidence for the existence of a high-affinity binding site was detected in either intact astrocytes or membranes prepared from astrocyte cultures. However, a very dense population of low-affinity binding sites was observed using both methods. This site was similar in affinity (0.606 microM for membranes and 0.959 microM for intact cells) to a low-affinity site observed with rat brain membranes (1.79 microM) but was present at a much greater density in astrocytes (1610 pmoles/mg protein for membranes and 672 for intact cells versus 53 pmoles/mg protein in brain), and may have prevented detection of the high-affinity site. Low-affinity binding to astrocytes was sodium independent, as was low-affinity binding to brain membranes. There was a poor correlation between the inhibitory potencies of the drugs tested against imipramine binding and 5-HT uptake. The binding of 15 nM [3H]IMI was nearly equipotently inhibited by all of the antidepressants tested with IC50 values ranging from 0.56 to 2.6 microM. Other receptor ligands such as 5-HT, chlorpheniramine, quipazine, atropine and benztropine were relatively weak inhibitors of [3H]IMI binding, whereas chlorpromazine was more potent than the tricycle antidepressants.