Astrocytes In Situ Exhibit Functional Neurotransmitter Receptors

Abstract

Astrocytes constitute a major portion of brain cells and envelop most neuronal elements. The intimate association of astrocytes and neurons led early anatomists to speculate that astrocytes interact with neurons (1–4). Work beginning in the 1970s determined that astroglia (astrocytes in culture) exhibit a wide variety of neurotransmitter receptors that regulate both second messenger systems and ion channels. These findings suggested that astrocytes in vivo might have neurotransmitter receptors enabling them to respond to neuronal activity. The importance, however, of studying astrocytes without culturing them was underscored by reports indicating that the neuroligand responsiveness of astroglia changes in culture (5–7). That placing astroglia in culture altered their phenotype was not surprising given that these cells are generally isolated from their normal cellular and chemical milieu at an early developmental stage and placed into an artificial environment that almost certainly lacks critical developmental cues. Unfortunately, the complex morphology of astrocytes together with their inability to propagate action potentials makes it difficult to study these cells in vivo with methods that were so powerful in elucidating the neuronal signaling systems in vivo.