The Detection, Isolation and Properties of Sub-Populations of Mammalian Brain Synaptosomes of Defined Neurotransmitter Type

Abstract

Mammalian brain synaptosomes are derived from many populations of neurons each employing a different neurotransmitter. The isolation of highly purified subpopulations of mammalian synaptosomes of specific neurotransmitter type would greatly enhance the value of synaptosome preparations, particularly for studies of presynaptic control of neurotransmitter release, secretion-synthesis coupling, and the co-existence and co-release of neurotransmitters and/or neuropeptides. The detection of specific outer surface markers for sub-categories of synaptosomes would form a basis for a method of separation. Our own recent studies of the ability of antisera recognising biosynthetic enzymes to cause complement-mediated immunolysis of the appropriate synaptosome subpopulation have revealed the presence of such specific surface marker antigens (Docherty et al 1985, 1985a, 1986). These appear to be closely related to a particular key biosynthetic enzyme for the neurotransmitter system involved, and are likely to be the particular enzyme itself in a membrane bound-form (Badamchian et al 1986; Barochovsky et al 1986; Benishin and Carroll 1983; Docherty and Bradford 1987). Pure cholinergic synaptosomes have been isolated from electric organ of Torpedo (Morel et al 1987) and greatly enriched cholinergic synaptosomes have been prepared from mammalian brain (Richardson et al 1984), but no successful method has previously been described which can separate other neurotransmitter-related subtypes of synaptosome. However, using magnetic microspheres (Magnogel) coupled a Protein A we have recently found it possible to prepare highly purified and metabolically viable GABAergic, cholinergic and serotonergic synaptosomes from mamalian cerebral cortex, using immunoglobulins recognising glutamate decarboxylase (GAD), choline acetyltransferase (ChAT), and tryptophan hydroxylase (TPH) respectively. These synaptosomes display Ca-dependent neurotransmitter release and are proving excellent preparations for studying the dynamics of neurotransmitter synthesis, release, coexistence and co-release in specific neuronal systems.