Evidence for the existence of galanin receptors on cultured astrocytes of rat CNS: colocalization with cholinergic receptors

The cellular localization of binding sites for [¹²⁵I]galanin was studied in explant cultures of rat neocortex, cerebellum, locus coeruleus and spinal cord by means of autoradiography. Binding sites for the peptide were observed on a great number of astrocytes in all CNS regions studied. In addition to astrocytes, many neurones were intensely labelled by [¹²⁵I]galanin. Binding of [¹²⁵I]galanin (10⁻⁸ M) to both astrocytes and neurones was markedly reduced or inhibited by the unlabelled peptide at high concentration (10⁻⁶ M), suggesting `specific' binding of the radioligand. Evidence for the colocalization of galanin and cholinergic receptors on astrocytes was provided by combined autoradiographic and immunohistochemical studies. Many astrocytes were labelled by [¹²⁵I]galanin and immunostained with antibodies to either muscarinic or nicotinic receptors. Electrophysiological studies revealed that addition of galanin (10⁻⁹ to 10⁻⁷ M) to the bathing fluid caused a dose-dependent hyperpolarization of the majority of astrocytes studied. When galanin (10⁻⁸ M) and the cholinergic agonists muscarine and nicotine (10⁻⁶ M) were tested on the same astrocyte, all three compounds induced a hyperpolarization, suggesting a colocalization of functional galanin and cholinergic receptors on the glial membrane.