GABA and picrotoxinin receptors in clonal nerve cells

Abstract

γAminobutyric acid (GABA) is a neurotransmitter which seems to mediate inhibitory synaptic transmission in many regions of the central nervous system (CNS) of vertebrates and invertebrates¹. GABA has been shown to act by increasing membrane chloride permeability in invertebrate muscle^2,3, and a similar mechanism has been suggested for the vertebrate CNS⁴. The response to GABA is inhibited noncompetitively by the convulsant picrotoxinin^1–5. Binding sites for radioactive GAB A^6–8 and picrotoxinin^9–11 (the active component of picrotoxin) have been described in homogenates of invertebrate muscle and mammalian CNS; these binding sites show many properties expected of postsynaptic membrane macromolecules involved in the receptor-ionophore function of GABA. Nevertheless, unambiguous correlation between biochemical assays and in vivo function is difficult to obtain, especially in heterogeneous tissue such as mammalian CNS. One approach to this problem involves cloned cell lines of neuronal origin which retain in culture the differentiated function to be studied (see, for example, ref. 12). Certain cell lines cloned from rat¹³ and mouse¹⁴ nervous system possess neuronal properties such as electrical excitability, neurotransmitter synthetic enzymes¹², neurotransmitter uptake^15,16, binding of defined ligands to their appropriate neurotransmitter receptors^12,17,18, and receptor-coupled responses to neurotransmitters^17,19,20. Several cell lines of apparent glial origin have also been described^13,21,22. To determine whether binding sites for GABA and picrotoxinin are localised to nerve cells, a variety of clonal cell lines were assayed for their ability to bind those ligands. We report here the presence of GABA and picrotoxinin binding sites with receptor-like properties in several clonal nerve cell lines obtained from the rat CNS, and the absence of such binding sites in many other cell lines of non-neuronal origin.