The cortico-nigral projection: reduced glutamate content in the substantia nigra following frontal cortex ablation in the rat

Modulation of dendritic release of dopamine by N-methyl-D-aspartate receptors in rat substantia nigra

A superfusion system was used to study the effects of excitatory amino acids (EAA) on release of [3H]dopamine ([3H]DA) previously taken up by rat substantia nigra (SN) slices. The EAA tested (20-250 microM), with the exception of quisqualate and kainate, markedly evoked [3H]DA release from nigral slices when Mg2+ ions were omitted from the superfusion medium. The EAA receptor agonists exhibited the following relative potency in stimulating [3H]DA release: L-glutamate (L-Glu) greater than N-methyl-D-aspartate (NMDA) greater than NM(D,L)A greater than D-Glu much greater than quisqualate = kainate. D-2-Amino-5-phosphonovalerate (100-200 microM), an antagonist for NMDA receptors, substantially reduced [3H]DA release evoked by L-Glu or NMDA. In contrast, L-Glu diethyl ester (100-200 microM) produced a lesser blocking effect on [3H]DA release evoked by the EAA. Further experiments showed that the NMDA-mediated release of [3H]DA was totally suppressed by the omission of Ca2+ or by the addition of tetrodotoxin (0.1 microM) to the superfusion medium. In addition, strychnine, an antagonist for glycine (Gly) receptors, significantly decreased NMDA (100 microM)-evoked as well as glycine (100 microM)-evoked release of [3H]DA from nigral slices. The results shown support the idea that activation of NMDA subtype receptors in SN may trigger a Ca2+-dependent release of DA from dendrites of nigro-striatal DA-containing neurons. Furthermore, a transsynaptic mechanism that may partially involve Gly-containing interneurons is proposed to account for some of the events mediating NMDA receptor activation and DA release in SN.

Compartmental distribution of endogenous amino acids in the substantia nigra of the rat

Using a push-pull cannula we have monitored the spontaneous efflux of 8 endogenous amino acids into perfusates of the substantia nigra of the rat. The extracellular concentrations of the amino acids were estimated and compared to their respective tissue levels. High intra-/extracellular concentration ratios were found for gamma-aminobutyric acid (GABA), aspartate, glutamate and taurine. Much lower values were found for glycine, alanine, serine and glutamine. These results provide evidence for possible neurotransmitter roles for aspartate, glutamate and taurine in the substantia nigra in addition to that, long recognized, for GABA.

Corticonigral projections from area 6 in the raccoon

The corticonigral projections from area 6 in the raccoon were investigated using the autoradiographic tracing method. Injections of tritiated proline and leucine were made into either medial or lateral area 6 subdivisions. Uniformly distributed silver grains were observed overlying the ipsilateral substantia nigra pars compacta (SNc) while more restricted foci of label indicative of fiber labeling were present in the substantia nigra pars reticulata (SNr). Autoradiographic label was also present in the substantia nigra pars lateralis (SNl), the retrorubral area and the ventral tegmental area of Tsai. The existence of corticonigral projections from area 6 may serve to modulate SNc activity as a whole and provide an important substrate for the cerebral control of movement.

Kindling does not cause persistent changes in firing rates or transmitter sensitivities of substantia nigra pars reticulata neurons

These studies were undertaken to determine whether the kindling process induces persistent alterations in the functional status of neurons of the substantia nigra pars reticulata, a brain area identified previously as a site important in regulating the expression of generalized motor seizures. Extracellular, single-unit recordings of pars reticulata neurons were made in chloral hydrate-anesthetized, fully kindled rats (2-3 weeks after the last seizure), or unkindled control rats of the same age and weight. Kindling caused no alterations in several electrophysiological parameters examined. For instance, neither the number of active pars reticulata cells encountered, nor their firing rates, were significantly different between kindled and control groups. In addition, kindling failed to alter the sensitivities of pars reticulata neurons to iontophoretic application of two inhibitory transmitters, gamma-aminobutyric acid and glycine, and two transmitters that excite these cells, glutamate and acetylcholine. These results suggest that while kindling produces enduring increases in seizure susceptibility, it causes no persistent interictal changes in either basal activity or several measures of transmitter sensitivity of substantia nigra pars reticulata neurons.

Anticonvulsant action of a kainate antagonist gamma-D-glutamyl aminomethylsulphonic acid injected focally into the substantia nigra and entopeduncular nucleus

Focal injection of gamma-D-glutamylaminomethylsulphonic acid (GAMS) into basal ganglia areas, the substantia nigra and entopeduncular nucleus, protects against electroshock (50 mA, 60 Hz, 0.2 s) and pilocarpine (380 mg/kg i.p.) induced seizures in rats. GAMS infusion into substantia nigra pars reticulata significantly protects against generalised (5, 10 nmol) and limbic (10, 50 nmol) seizures. The highest dose of GAMS results in similar suppression of seizure activity following injection into substantia nigra pars compacta against generalised (10 nmol) and limbic (50 nmol) seizures. The anticonvulsant action of GAMS is less marked after infusion into the entopeduncular nucleus. High doses of GAMS are required to provide protection against electroshock test (10 nmol) and limbic (40 nmol) seizures. Protection against limbic and electroshock seizures is more pronounced after infusion of GAMS into substantia nigra than into the entopeduncular nucleus. The substantia nigra pars reticulata is particularly sensitive to the anticonvulsant action of GAMS suggesting that antagonism at the kainate receptor is functionally important.

Excitability changes induced in the striatal dopamine-containing terminals following frontal cortex stimulation

The excitability of the dopamine-containing terminal field in the striatum (St) following frontal cortex (FC) stimulation was investigated in halothane-anesthetized rats. Either glutamic acid (GLU, 6.2 mM) or square pulses (a train of 25 pulses, 500-800 microA/0.3 ms: 1 Hz/20 s) were used to stimulate FC. To stimulate St monophasic square wave pulses (10-4000 microA/0.5 ms/1 Hz) were delivered. Excitability was measured by determining the threshold for antidromic activation of substantia nigra cells. Threshold was defined as the minimum current required for antidromic invasion of the cell on 100% of non-collision trials. The mean threshold current was 1029 +/- 167 microA. Following FC stimulation a significant decrease (30%) in excitability was observed in most cases (80%). No correlation between firing rate and threshold fluctuations was observed. It is concluded that FC activity decreases the excitability of the dopaminergic nigrostriatal terminal field. Whether this is a direct or an indirect effect is discussed.

Excitatory neurotransmission within substantia nigra pars reticulata regulates threshold for seizures produced by pilocarpine in rats: effects of intranigral 2-amino-7-phosphonoheptanoate and N-methyl-D-aspartate

Seizures produced by pilocarpine given i.p. to rats provide an animal model for studying the initiation, spread and generalisation of convulsive activity within the forebrain. Pilocarpine, 380 mg/kg, produces a sequence of behavioural and electroencephalographic alterations indicative of motor limbic seizures and status epilepticus, which is followed by widespread damage to the limbic forebrain resembling that occurring subsequent to prolonged intractable seizures. Microinjections of a selective antagonist at the N-methyl-D-aspartate receptor, (+/-)-2-amino-7-phosphonoheptanoate, into the substantia nigra pars reticulata, bilaterally, protects against the behavioural, electrographic and morphological features of seizures produced by pilocarpine, 380 mg/kg, with an ED50 of 0.0007 mumol (0.0004-0.0011). Microinjections of (+/-)-2-amino-7-phosphonoheptanoate, 0.005 or 0.01 mumol, into the substantia nigra pars compacta or into the dorsal part of mid-anterior striatum do not modify the electrographic and morphological sequelae of pilocarpine, 380 mg/kg. In rats pretreated with microinjections of N-methyl-D-aspartate into the substantia nigra pars reticulata, a non-convulsive dose of pilocarpine, 100 mg/kg, results in recurrent motor limbic seizures and status epilepticus. The ED50 of N-methyl-D-aspartate for the generation of seizures after pilocarpine, 100 mg/kg, is 0.0014 mumol (0.001-0.0019). Electrographic monitoring shows a pattern and sequence of evolution of convulsant activity within the hippocampus and cortex similar to that produced with pilocarpine, 380 mg/kg, alone. Morphological examination of brains from rats treated with N-methyl-D-aspartate in the substantia nigra pars reticulata and subsequently given pilocarpine, 100 mg/kg, which underwent status epilepticus, reveals widespread damage to the amygdala, thalamus, olfactory cortex, substantia nigra, neocortex, and hippocampus. Microinjections of N-methyl-D-aspartate, 0.002 mumol, into either the substantia nigra pars compacta or dorsal striatum, bilaterally, do not augment seizures produced by pilocarpine, 100 mg/kg. The results indicate that the threshold for pilocarpine-induced seizures in rats is modulated by excitatory amino acid neurotransmission within the substantia nigra pars reticulata.

Cerebral blood flow and metabolic rate during seizures. Relationship to epileptic brain damage

After long periods of status epilepticus, selective neuronal necrosis is incurred in the neocortex (layer III-IV), in the hippocampus (CA1 and CA4), and in the thalamus (VPL-VPM). In these areas the cerebral metabolic rate for glucose is increased to between 200-300% of control, indicating a correlation between neuronal damage and enhanced neuronal activity. Measurements of local cerebral blood flow indicate that the damage is not due to insufficient supply of oxygen. In most rats with status epilepticus lasting longer than 30 minutes, an infarction develops in the substantia nigra pars reticulata. In this region the metabolic rate is first increased but later during the seizure activity falls to very low values indicating cell necrosis.

CONCLUSION

prolonged neuronal hyperactivity with a concomitant increase in the metabolic rate for glucose is a prerequisite for the development of neuronal damage. However, the necrosis of the SNPR demonstrates that other factors determine the vulnerability of neurons to hyperexcitation, e.g., the type of agonist acting on the neuron.

Specific binding of L-[3H]-glutamic acid to rat substantia nigra synaptic membranes

The specific binding of L-[3H]-glutamic acid (GLU) was investigated in synaptic membranes from rat substantia nigra. L-[3H]-GLU binding to the membrane preparations occurred in a reversible and saturable way. The specific binding was stimulated by the presence of CaCl2 and was reduced by freezing and thawing the membranes. Scatchard analysis of the saturation isotherms yielded a non-linear plot suggesting that the binding reaction does not occur through a simple bimolecular association. Assuming non-interacting binding sites, a high (KD1, 139 nM; Bmax1, 3.5 pmoles/mg protein) and a low (KD2, 667 nM; Bmax2, 15.1 pmoles/mg protein) affinity L-[3H]-GLU binding site were obtained. The kinetics of dissociation of bound L-[3H]-GLU was biphasic; the respective dissociation rate constant (k-1) being 0.20 min-1 and 0.013 min-1. A series of amino acid receptor agonists and antagonists were tested as inhibitors of L-[3H]-GLU specific binding. Quisqualic acid, L-GLU and D-alpha-aminoadipate (D-alpha-AA) were the most potent inhibitors. DL-2-amino-4-phosphonobutyrate (APB), N-Methyl-D-aspartate (NMDA) and D-GLU were moderate inhibitors, whereas diaminopimelic acid (DAPA) and glutamate diethyl ester (GDEE) exhibited the lowest relative potency. Kainic acid (KA), gamma-aminobutyric acid (GABA) and bicuculline were not able to modify at any concentration used the specific binding of L-[3H]-GLU. These data demonstrate the presence of specific GLU binding sites in synaptic structures at substantia nigra level and support the idea that excitatory amino acids may play a role in synaptic transmission in this brain region.