THE symptoms of Huntington's chorea, an hereditary movement disorder, result from degeneration of neurones primarily in the basal ganglia1. Several neurochemical abnormalities have been identified in the brains of patients dying with this disorder2–5, but no animal system with similar neuropathological changes has been described. We now report that the injection of kainic acid into the rat striatum causes neuronal degeneration, neurochemical alterations and behavioural responses resembling Huntington's chorea. This procedure could provide an animal model for the study of the disease.

Lesion of striatal neurones with kainic acid provides a model for Huntington's chorea.

The existence of a receptor for γ-aminobutyric acid (GABA) on neurones of the mammalian central nervous system (CNS) is now firmly established1–3. It is generally accepted that bicuculline (and its methohalide salts) is an antagonist of the actions of GABA4,5, although resistance to bicuculline has been described6,7. The view that bicuculline prevents GABA from interacting with a membrane recognition site is supported by results obtained in radiolabelled ligand binding studies8,9. Bicuculline-sensitive GABA receptors are not confined to neurones in the CNS but are also present on neurones and axons of the peripheral nervous system10,11. Their existence on neurones in sympathetic ganglia made us consider the possibility that they are also present on the terminals of such neurones. This has recently been tested12,13 by assuming that if GABA depolarises the terminals in a manner similar to the cell bodies, evoked transmitter output might be decreased (see ref. 14). GABA (ED50, 4µM) clearly decreased the evoked release of accumulated 3H-noradrenaline from rat atria in vitro and 3H-acetylcholine from preganglionic terminals in the rat superior cervical ganglion in vitro without affecting the basal release of tritium. In neither system was the effect of GABA antagonised by bicuculline methobromide, even though the ganglion terminal depolarisation was13. This suggested that the two phenomena, depolarisation and inhibition of transmitter release, were separate. The decrease in transmitter release not surprisingly leads to a decrease in the postsynaptic response15. Again, the decrease in response was not prevented by bicuculline or other GABA antagonists. We believe that these results indicate the presence of a novel GABA receptor on nerve terminals, a theory supported by results obtained with a variety of GABA analogues. For example, 3-aminopropane sulphonic acid (3-APS) which is at least as active as GABA at bicuculline sensitive sites10,16,17 is inactive at the terminal receptor. By contrast, the analogue baclofen (β-chlorophenyl GABA) is inactive at bicuculline-sensitive sites18,19 but is as active as GABA in reducing evoked transmitter output15. This effect of baclofen is stereospecific, the (−) isomer being > 100-fold more active than the (+) isomer15. We now report the presence of the novel GABA receptor within the mammalian CNS.

Baclofen decreases neurotransmitter release in the mammalian CNS by an action at a novel GABA receptor

Immunocytochemical methods for peptides and serotonin have greatly advanced the study of neurones in which these substances are likely to be transmitters1,2. Such direct techniques have not so far been available for the amino acid transmitter candidates. We report here the selective immunocytochemical visualization of the putative transmitters glutamate (Glu) and γ-aminobutyrate (GABA) by the use of antibodies raised against the amino acids coupled to bovine serum albumin (BSA) with glutaraldehyde (GA). The tissue localizations of Glu-like and GABA-like immunoreactivities (Glu-LI and GABA-LI) matched those of specific uptake sites for Glu and GABA, and, in the case of GABA-LI, also that of the specific marker enzyme glutamic acid decarboxylase (GAD). Thus, GABA-LI was located in what are believed to be GABAergic inhibitory neurones, whereas Glu-LI was concentrated in excitatory, possibly glutamatergic neurones. Preliminary electron microscopic observations suggest that the transmitter amino acids are significantly concentrated in synaptic vesicles.

First visualization of glutamate and GABA in neurones by immunocytochemistry

In addition to its role as an inhibitory neurotransmitter, γ-aminobutyric acid (GABA) is presumed to be involved in the development and plasticity of the nervous system. GABA is synthesized by glutamic acid decarboxylase (GAD), but the respective roles of its two isoforms (GAD65 and 67) have not been determined. The selective elimination of each GAD isoform by gene targeting is expected to clarify these issues. Recently we have produced GAD65 −/− mice and demonstrated that lack of GAD65 does not change brain GABA contents or animal behavior, except for a slight increase in susceptibility to seizures. Here we report the production of GAD67 −/− mice. These mice were born at the expected frequency but died of severe cleft palate during the first morning after birth. GAD activities and GABA contents were reduced to 20% and 7%, respectively, in the cerebral cortex of the newborn GAD67 −/− mice. Their brain, however, did not show any discernible defects. Previous pharmacological and genetic investigations have suggested the involvement of GABA in palate formation, but this is the first demonstration of a role for GAD67-derived GABA in the development of nonneural tissue.

https://www.pnas.org/content/pnas/94/12/6496.full.pdf

Cleft palate and decreased brain gamma -aminobutyric acid in mice lacking the 67-kda isoform of glutamic acid decarboxylase

Publisher Summary This chapter discusses the important roles of glutamate and gamma-aminobutyric acid (GABA) in brain metabolism. The chapter shows the amino acids as excitatory and inhibitory transmitters in the CNS. The inactivation of these transmitters is generally brought about by sodium-dependent high-affinity uptake systems rather than enrymatic degradation. The importance of the different cell types in these processes and the characteristics of the different transport systems are discussed. The quantitative importance of the high-affinity uptake of glutamate and GABA into glial cells is investigated in primary cultures of astrocytes that seem to constitute a valid model for such cells in vivo. From these studies, it can be concluded that astrocytes are quantitatively important for uptake of putative transmitter amino acids. Whether this uptake has to do primarily with the transmitter pools of these amino acids, and hence their inactivation, or the uptake is involved in the metabolic processes remains to be finally elucidated. The fact that the processes of astrocytes control a large surface area around synapses seems to favor a function of such cells in the inactivation of these amino acids.On the other hand, it is undoubtedly correct that these amino acids can be taken up into presynaptic nerve endings through high-affinity transport processes. In the case of GABA, it is even possible to distinguish between these transport mechanisms employing inhibitors that have been shown to be selective for the transport carriers.

Communicative Disorders

Papers

GABA in nervous system function