Introduction to synaptic circuits, Gordon M.Shepherd and Christof Koch membrane properties and neurotransmitter actions, David A.McCormick peripheral ganglia, Paul R.Adams and Christof Koch spinal cord - ventral horn, Robert E.Burke olfactory bulb, Gordon M.Shepherd, and Charles A.Greer retina, Peter Sterling cerebellum, Rodolfo R.Llinas and Kerry D.Walton thalamus, S.Murray Sherman and Christof Koch basal ganglia, Charles J.Wilson olfactory cortex, Lewis B.Haberly hippocampus, Thomas H.Brown and Anthony M.Zador neocortex, Rodney J.Douglas and Kevan A.C.Martin Gordon M.Shepherd. Appendix: Dendretic electrotonus and synaptic integration.

The synaptic organization of the brain

Transmitters mediating 'fast' synaptic processes in the vertebrate central nervous system are commonly placed in two separate categories that are believed to exhibit no interaction at the receptor level. The 'inhibitory transmitters' (such as glycine and GABA) are considered to act only on receptors mediating a chloride conductance increase, whereas 'excitatory transmitters' (such as L-glutamate) are considered to activate receptors mediating a cationic conductance increase. The best known excitatory receptor is that specifically activated by N-methyl-D-aspartate (NMDA) which has recently been characterized at the single channel level. The response activated by NMDA agonists is unique in that it exhibits a voltage-dependent Mg block. We report here that this response exhibits another remarkable property: it is dramatically potentiated by glycine. This potentiation is not mediated by the inhibitory strychnine-sensitive glycine receptor, and is detected at a glycine concentration as low as 10 nM. The potentiation can be observed in outside-out patches as an increase in the frequency of opening of the channels activated by NMDA agonists. Thus, in addition to its role as an inhibitory transmitter, glycine may facilitate excitatory transmission in the brain through an allosteric activation of the NMDA receptor.

Glycine potentiates the NMDA response in cultured mouse brain neurons

Excitatory amino acid transmitters

Glutamate is ubiquitously distributed in brain tissue, where it is present in a higher concentration than any other amino acid. During the last 50 years glutamate in brain has been the subject of numerous studies, and several different functions have been ascribed to it. Early studies by Krebs (1935) suggested that glutamate played a central metabolic role in brain. The complex compartmentation of glutamate metabolism in brain was first noted by Waelsch and coworkers (Berl et al., 1961). These studies were precipitated by the claim that glutamate improved mental behaviour and was beneficial in several neurological disorders including epilepsy and mental retardation. Other scientists pointed out its function in the detoxification of ammonia in brain (Weil-Malherbe, 1950). Glutamate is also an important building block in the synthesis of proteins and peptides, including glutathione (Meister, 1979). The toxic effect of administered glutamate and its analogues kainic acid, ibotenic acid, and N-methyl aspartic acid on CNS neurones has become a large and independent line of research (Lucas and Newhouse, 1957; Olney et al., 1974; Lund-Karlsen and Fonnum, 1976; Coyle, 1983). Attention has also been focused on the role of glutamate as a precursor for the inhibitory neurotransmitter y-aminobutyric acid (GABA) (Roberts and Frankel, 1950). Electrophysiological studies (Curtis and Watkins, 1961) focused early on the powerful and excitatory action of glutamate on spinal cord neurones. Since the action was widespread and effected by both the Dand Lforms, it was at first difficult to believe that glutamate could be a neurotransmitter. During the last 15 years, however, several studies have provided support for the concept that glutamate is a transmitter in brain (for review see Curtis and Johnston, 1974; Fonnum, 1978; 1981; Roberts et al., 1981; DiChiara and Gessa, 1981). Glutamate satisfies today to a large extent the four main criteria for classification as a neurotransmitter: (1) it is presynaptically localized in specific neurones; (2) it is specifically released by physiological stimuli in concentrations high enough to elicit postsynaptic response; (3) it demonstrates identity of action with the naturally occurring transmitter, including response to antagonists; and (4) mechanisms exist that will terminate transmitter action rapidly. The evidence for glutamate as a transmitter at the locust neuromuscular junction has recently been carefully evaluated by Usherwood (1981). In that case the identity of action of glutamate with the naturally occurring transmitter on the neuromuscular receptor, the release from nerve terminals, and its similarity to acetylcholine at the mammalian neuromuscular junction with regard to presynaptic pharmacology and denervation supersensitivity are compelling evidence for glutamate as a neurotransmitter. The main methods used to identify glutamergic pathways in brain will be critically reviewed and discussed. The effect of lesions on high-affinity uptake and release are particularly important, but immunohistochemical methods to study enzymes and glutamate itself are becoming more interesting. The release of glutamate has been demonstrated by several different procedures using both in vivo and in vitro preparations. The synthesis of large groups of specific agonists and antagonists has been important both for identification and characterization of the glutamate receptor by electrophysiological techniques and for the isolation of glutamate receptors. High and perhaps low-affinity uptake into nerve terminals and glial cells is important for the termination of transmitter action. Particular attention is given in this review to the complex compartmentation of glutamate synthesis and the possibility of identifying the transmitter pool of glutamate.

Glutamate: a neurotransmitter in mammalian brain.

Eighteen animal, models of depression are reviewed in relation to three sets of validating criteria. Of the 18 models, five could only be assessed for predictive, validity, seven could be assessed for predictive and face validity, and six could potentially have predictive, face and construct validity. Some traditional models (reserpine reversal, amphetamine potentiation) are rejected as invalid; the models with the highest overall validity are the intracranial self-stimulation, chronic stress and learned helplessness models in rats, and the primate separation model.

The validity of animal models of depression.

Inhibitory of glycine on spinal neurons in the cat.

IT IS now generally accepted that mediation of synaptic excitation and inhibition within the mammalian nervous system is chemical. The major excitatory or inhibitory transmitters acting on spinal motoneurons and interneurons have not been identified with certainty. For this reason, the iontophoretic studies of CURTIS and WATKINS (1960) are of great interest. They showed that certain amino acids cause excitation while others result in depression of neuronal activity when these compounds are applied in proximity to the external surfaces of spinal neurons. For instance, the excitatory effects of L-glutamic acid and L-aspartic acid and the inhibitory effects of GABA and glycine are 'transmitter-like' in their rapidity of action, quick reversibility, and the great scnsitivity of the neurons to small quantities of these agents. These observations have been confirmed in our laboratories. Similar actions of these amino acids were shown in other parts of the central nervous system (see KRNJEVIC, 1965, for references). These findings suggest a role for amino acids as synaptic transmitters, the point of departure of our research. In this paper, we present data on the distribution of glutamate, glutamine, GABA, aspartate, and glycine in dorsal and ventral roots and four spinal cord areas in the cat. On the basis of these findings, we discuss the relative merits of these compounds as possible spinal excitatory or inhibitory transmitters. Preliminary reports of several aspects of this study have been published (APRISON, GRAHAM, BAXTER and WERMAN, 1965a; APRISON, GRAHAM, LIVENGOOD and WERMAN, 196%; APRISON and WERMAN, 1965). E X P E R I M E N T A L Preparation of tissue. Tissue was obtained from mature cats (2-4 kg) as follows: under nernbutal anaesthesia, laminectomy of the upper sacral and lower lumbar spine was carried out. The spinal roots were identified by size and site of dural penetration. The dura was opened, spinal fluid drained, and lumbar 6 and 7 and sacral 1 dorsal and ventral roots were isolated and removed. The ventral roots were always excised first. The roots were rinsed in saline, blotted quickly on filter paper, and immediately frozen on dry ice. Thin sections (2-3 mm) of the cord were cut and frozen on dry ice. After wrapping in aluminum foil and labelling, the samples were stored at -23\" to -28\" until used, In some of the earlier experiments, the sections were taken from the freezer and placed on a slab

Distribution of some synaptic transmitter suspects in cat spinal cord: glutamic acid, aspartic acid, gamma-aminobutyric acid, glycine and glutamine.

The distribution of glycine in cat spinal cord and roots.

Present status and significance of the glutamine cycle in neural tissues.

IONTOPHORESIS of some amino acids such as glutamate and aspartate near spinal neurons produces excitation of these neurons, while iontophoresis of several other amino acids such as GABA and glycine results in depression of these cells (CURTIS and WATKINS, 1965; WERMAN, DAVIDOFF and APRISON, 1966u, 19668,1967). Several of these potent amino acids are present in high concentrations in discrete areas of the mammalian spinal cord (APRISON, GRAHAM, BAXTER and WERMAN, 1965~ ; APRISON, GRAHAM, LIVENGOOD and WERMAN, 19658; APRISON and WERMAN, 1965; WERMAN et al., 1966b; GRAHAM, SHANK, WERMAN and APRISON, 1967). On the basis of known pharmacological actions and biochemical distribution in the spinal cord, it has been hypothesized that glycine and GABA could be inhibitory transmitter compounds and glutamate and aspartate could be excitatory transmitters. Interneurons-whose somas are confined to gray matter-are interpolated in inhibitory and polysynaptic excitatory pathways from dorsal root fibres to spinal motoneurons (ECCLES, 1964). Since a transmitter should be found in those neurons from which it is released, it is of interest to ascertain which, if any, amino acids are predominantly associated with interneurons. Consequently, we attempted to produce a relatively selective loss of interneurons in the spinal cord of cats. A comparison was then made among the concentrations of glycine, GABA, glutamate, aspartate and glutamine in dorsal and ventral gray matter and white matter of spinal cords from experimental animals and normal animals (GRAHAM et al., 1967).

M.H. Aprison

Papers

Inhibitory of glycine on spinal neurons in the cat.

Distribution of some synaptic transmitter suspects in cat spinal cord: glutamic acid, aspartic acid, gamma-aminobutyric acid, glycine and glutamine.

The distribution of glycine in cat spinal cord and roots.

Present status and significance of the glutamine cycle in neural tissues.

Changes in amino acid concentrations associated with loss of spinal interneurons.