Endogenous excitatory amino acids and glutamate receptor subtypes involved in the control of hypothalamic luteinizing hormone-releasing hormone secretion.

These studies were designed to evaluate the actions and relative potencies of different endogenous and excitatory amino acid (EAA) selective analogs on EAA-induced neuropeptide secretion as well as to analyze the receptor subtypes involved. For this purpose, different glutamate agonists were tested for their ability to evoke release of the hypothalamic neuropeptide LHRH from arcuate nucleus-median eminence (AN-ME) fragments incubated in vitro. Different glutamate agonists, i.e. 3-amino-3-hydroxy-5-methyl-isoxazole-4-propionic (AMPA), kainic, quisqualic, homocysteic (HCA), quinolinic (QUIN), N-methyl-D-aspartic (NMDA), and pyroglutamic (PYR) acids, elicited LHRH release from AN-ME fragments in vitro. Further evaluation of the range of activity of several of these compounds, both in terms of the dose inducing a half-maximal response and the LHRH-releasing effect at that particular dose, indicated that AMPA greater than HCA greater than QUIN greater than PYR, suggesting that non-NMDA receptors are primarily involved in EAA-induced LHRH release at the level of the AN-ME. Evaluation of the receptor types involved using two specific antagonists for NMDA and non-NMDA receptors, D,L-2-amino-7-phosphoheptanoic acid and 6,7-cyanoquinoxaline-2,3-dione, respectively, showed that the effects of AMPA and HCA on LHRH release can be completely blocked by 6,7-cyanoquinoxaline-2,3-dione, whereas QUIN activity was blocked by D,L-2-amino-7-phosphoheptanoic acid. The effects of PYR on LHRH release were abolished by both receptor blockers. The metabotropic receptor agonist trans-1-amino-cyclopentyl-1,1,3-dicarboxylic acid was not active in eliciting LHRH secretion. The data indicate that endogenous substances active at EAA receptor sites, such as HCA, QUIN, and PYR, can significantly increase the secretion of the neuropeptide LHRH and, thus, may participate in the physiological regulation of the activity of this important neuroendocrine neuronal system. In addition, the results suggest that non-NMDA receptor sites may be preferentially activated at lower ligand concentrations, although NMDA receptors may also be involved in the response to certain endogenous agonists.

Complementary roles for the amygdala and hippocampus in aversive conditioning to explicit and contextual cues.

Experiment 1 investigated the effects of catecholaminergic deafferentation or cell body lesions of the amygdala on fear conditioning to explicit and contextual cues. Bilateral infusions of quinolinic acid mainly damaged neurons within the basolateral region of the amygdala. 6-Hydroxydopamine infusions at the same coordinates resulted in an 86% depletion of noradrenaline and a 63% depletion of dopamine from the amygdala, but had no effect on the concentration of 5-hydroxytryptamine. After recovery from surgery, lesioned rats and controls were exposed to pairings of an auditory (clicker) conditioned stimulus and (foot shock) unconditioned stimulus in a distinctive environment. During testing, rats with both 6-hydroxydopamine and cell body lesions showed severely impaired conditioning to explicit cues, compared with controls, indicated by their reduced suppression of drinking when the conditioned stimulus was introduced into a separate, lick-operant chamber. Neither lesion affected fear conditioning to contextual cues, measured as preference for a "safe" environment over the one in which they were shocked. In Experiment 2, rats received bilateral, ibotenic acid-induced lesions of the hippocampal formation. Lesioned rats and controls were again tested for aversive conditioning to explicit and contextual cues. Rats with cell body lesions of the hippocampus showed normal suppression of drinking in the presence of the conditioned stimulus, but were severely impaired in choosing the safe environment based on contextual cues alone. These results suggest a double dissociation of the effects of amygdala and hippocampal damage on fear conditioning to explicit and contextual cues.

Influence of quinolinic acid, an endogenous neurotoxin, on a passive avoidance conditioned reflex in rats.

Quinolinic acid, an endogenous neurotoxin, was introduced into the cerebral ventricles of a rat in doses of 0.5 microgram and 30 micrograms. The influence of the substance on the reproduction of a passive avoidance conditioned reflex (CR) was evaluated. It was found that a small dose of the preparation decreases extinction, while a large dose disrupts the reproduction of a passive avoidance CR. The disruption of the reproduction of the skill was also observed after bilateral electrolytic destruction of the dorsal hippocampus. Quinolinic degeneration of this structure exerted a substantial influence on the reproduction of a passive avoidance CR.

Localization of immunoreactive GABA and enkephalin and NADPH-diaphorase-positive neurons in fetal striatal grafts in the quinolinic-acid-lesioned rat neostriatum.

Fetal striatal tissue grafts have been shown to partially reverse the biochemical and behavioral deficits induced by excitotoxic lesions. To determine if grafted striatal neurons contain neurochemical markers similar to those in neurons in the caudate nucleus and to establish the morphological characteristics and relative frequency of labeled neurons in the grafts, the localization of immunoreactive GABA and leucine-enkephalin (ENK) and of NADPH-diaphorase (NADPH-d) activity was examined in fetal striatal grafts at the light and electron microscopic levels. Striatal tissue from 17-day fetuses was grafted into the caudate nucleus of adult rats 1 week after intracaudate injections of either a low or high dose of quinolinic acid. At the light microscopic level, immunoreactive GABA and ENK and NADPH-d-positive neurons, processes, and punctate structures were present within adjacent sections of the same grafts. The frequency and morphological features of these labeled cell populations were similar in grafts placed into either minimally or extensively lesioned striata. Immunoreactive GABA and ENK neurons in the grafts constituted 28% and 13.5%, respectively, of the neuronal population of the graft and their mean diameters were 22 and 14% larger, respectively, than neostriatal neurons that contained the same chemical markers. NADPH-d-positive neurons in the grafts formed 3.5% of total grafted neurons and exhibited characteristics of neostriatal NADPH-d-containing aspiny cells, including medium-sized somata, indented nuclei, and varicose dendrites. At the electron microscopic level most GABA-positive neurons in the grafts contained indented nuclei and most immunoreactive ENK somata had unindented nuclei. Dendrites and dendritic spines with GABA or ENK immunoreactivity were present in the grafts where they were postsynaptic to unlabeled axons. Immunoreactive GABA and ENK axon terminals formed synapses with unlabeled neuronal profiles in the grafts. These findings demonstrate that fetal striatal grafts contain chemically defined neuronal populations that form synaptic connections within the graft and share some features with corresponding cell groups in the neostriatum. These results provide an anatomical basis for the graft-induced recovery from behavioral and biochemical deficits caused by instrastriatal lesions reported in other studies.

[Effect of serotoninergic and antiserotoninergic preparations on convulsions induced by the endogenous convulsants kynurenine and quinolinic acid]. / Vliianie serotoninergicheskikh i antiserotoninergicheskikh preparatov na sudorogi, vyzvannye éndogennymi konvul'santami kinureninom i khinolinovoi kislotoi.

Serotoninergic drugs DL-5-hydroxytryptophan, 5-methoxytryptamine (mexamine) and 5-hydroxytryptamine (serotonin) were found to reduce clonic convulsions induced by intracerebroventricular administration of DL-kynurenine sulfate in mice. Serotonin antagonist deseril weakened the antikynurenine effect of DL-5-hydroxytryptophan and mexamine. Quinolinic acid-induced convulsions could be attenuated only by DL-5-hydroxytryptophan. Deseril and metergoline and destruction of the cerebral serotoninergic neurons by 5,6-dihydroxytryptamine potentiated the convulsant effect of quinolinic acid but not kynurenine. The serotoninergic protective anticonvulsant mechanism is probably better developed against kynurenine rather than quinolinic acid.

Acute tachycardia and pressor effects following injections of kainic acid into the antero-dorsal medial hypothalamus.

The responses of systemic arterial blood pressure and heart rate to intra-cerebral injections of the excitatory neurotoxin, kainic acid, were examined in urethane-anesthetized rats. Injections of kainic acid into the antero-dorsal medial hypothalamus produced dose-related increases in both blood pressure and heart rate over a range of 30-1000 ng. Exophthalmos, mydriasis, increased respiratory rate and movements of the vibrissae were also noted. Injections of 1000 ng of kainic acid into the antero-dorsal medial hypothalamus produced significantly greater increases in blood pressure and heart rate than comparable injections of equimolar doses of the excitatory neurotoxins, quisqualic acid, N-methyl-D-aspartic acid or quinolinic acid. No differences in the magnitude of the cardiovascular responses to 1000 ng of kainic acid were detected between injections directed towards the paraventricular nucleus of the hypothalamus (PVN), lateral hypothalamus or lateral cerebral ventricle. In contrast, at doses of 30 ng, injections directed towards the paraventricular nucleus produced significantly greater responses than comparable injections into the lateral hypothalamus, medial thalamus or lateral cerebral ventricle. The distribution of radiolabelled kainic acid at this dose was found to extend ipsilaterally in the medial hypothalamus as far as 1 mm rostral and caudal to the injection site. The results suggest that excitation of neuronal cell bodies within the medial hypothalamus by excitatory neurotoxins produces acute increases in blood pressure and heart rate. However, widespread diffusion of kainic acid, in particular, was documented and caution in interpretation of the results produced by local intra-cerebral injections of this agent is recommended.

Short- and long-term consequences of intracranial injections of the excitotoxin, quinolinic acid, as evidenced by GFA immunohistochemistry of astrocytes.

Astroglial reactions to intrastriatal and intrahypothalamic injections of the endogenous excitotoxin quinolinic acid (50 micrograms in 1 microliter) were studied in adult rats, using immunohistochemistry with antiserum to glial fibrillary acidic protein. Animals were sacrificed 6 h, 24 h, 3, 7 and 30 days or 1 year after the injection. Six and 24 h after quinolinic acid, the amount of glial fibrillary acidic protein-like immunoreactivity in the injected striatum was lower than in controls but returned to a normal level at 3 days. Not until 7 days was a clear striatal gliosis apparent, as evidenced by an increased density of glial fibrillary acidic protein-positive structures and brightly fluorescent, clearly hypertrophic cells. This gliosis was even more developed in animals sacrificed 30 days postoperatively. A weak astrocytic reaction was also observed in the ipsilateral corpus callosum at 6 h after quinolinic acid. By 3 days, a marked gliosis restricted to the injected hemisphere was present throughout corpus callosum and cortex cerebri. In animals sacrificed 30 days after quinolinic acid the extrastriatal astrocytic reaction was clearly diminished, although the striatal gliosis was still prominent. One year postinjection, no obvious gliosis could be observed in cortex cerebri or corpus callosum while striatal tissue, now markedly reduced in volume, was clearly gliotic. Using neurofilament antiserum, increased fluorescence intensity was noted in striatal nerve bundles during the first day after an intrastriatal quinolinic acid injection and persisted 1 year postoperatively. Controls were similarly injected with an equimolar amount of nicotinic acid, the non-excitatory, non-neurotoxic decarboxylation product of quinolinic acid. No changes in immunoreactivity of glial fibrillary acidic protein or neurofilament were found in these animals. In animals treated intrahypothalamically, a spherical central area almost devoid of glial fibrillary acidic protein-immunoreactivity was noted around the injection site 7 days after quinolinic acid administration. Around this area, gliosis was observed. Apart from a very restricted gliotic reaction around the needle tract, no astrocytic reaction was observed in nicotinic acid-injected control animals. We conclude that quinolinic acid causes both reversible and long-lasting gliosis when injected into the rat striatum. As a natural brain metabolite, quinolinic acid may constitute a particularly valuable tool for the elucidation of a possible role of glia in neurodegenerative disorders.

Effects of intrahypothalamic injection of quinolinic acid on anterior pituitary hormone secretion in the unanesthetized rat.

Bilateral intrahypothalamic injections of the brain metabolite quinolinic acid (QUIN) were made in an attempt to examine its effects on the secretion of LH, PRL, GH and TSH. Quin, a neuroexcitatory amino acid with close structural similarities to glutamate, kainate and N-methylaspartate, was infused into unanesthetized male rats, the animals sacrificed 7.5 min later, and serum hormone concentrations determined by radioimmunoassay. QUIN caused surges in LH, PRL and GH release (316, 607 and 1,134% of control, respectively, at 50 micrograms QUIN) without affecting the serum concentrations of TSH. At lower doses, a preferential effect of QUIN on PRL release was observed. All QUIN-induced hormonal changes were inhibited by concomitant administration of the specific antagonist (-)-2-amino-7-phosphonoheptanoic acid, indicating the presence of QUIN-sensitive receptors on neurons which are intimately associated with endocrine regulation. Moreover, because QUIN-treated animals exhibited behavioral signs of seizure activity and neuroendocrine dysfunction has been reported to occur in human convulsive disorders, the data are also of interest in view of a possible mechanistic link between epileptic phenomena and hormone secretion.

[Convulsions induced by kynurenine and quinolinic acid as a sensitive test for assessing the anticonvulsant activity of GABA-ergic preparations]. / Sudorogi, vyzyvaemye kinureninom i khinolinovoi kislotoi, kak chuvstvitel'nyi test dlia otsenki protivosudorozhnoi aktivnosti GAMK-ergicheskikh preparatov.

Phenibut, sodium hydroxybutyrate and baclofen are selectively effective against seizures induced in mice by the endogenous metabolites of tryptophan, L-kynurenine and quinolinic acid. The seizures were not affected by the drugs in doses under study. Depakine and aminooxyacetic acid as well as diazepam and phenobarbital appeared the most effective against pentylenetetrazole seizures. GABA and muscimol administered intracerebroventricularly merely prolonged the latency of seizures. Dissimilarities in the GABA-ergic mechanisms of the anticonvulsant effects of the drugs under consideration are discussed.

Excitatory effects of kynurenine and its metabolites, amino acids and convulsants administered into brain ventricles: differences between rats and mice.

When introduced intracerebroventricularly, quinolinic acid appeared to be the only kynurenine metabolite among those tested (L- and DL-kynurenine sulfate, kynurenic and nicotinic acids, nicotinamide) which induced locomotor excitement and clonic seizures in rats; in high dosage all exhibited convulsant action in mice. L-Kynurenine sulfate (500 micrograms) induced continuous rotation in rats around a longitudinal axis in one or other direction. It also potentiated the convulsant effect of strychnine sulfate and caffeine. Neither the excitatory amino acids, L-glutamic and L-aspartic acids nor the inhibitory amino acids, GABA, glycine and taurine induced excitement or seizures in rats but did in mice. In rats, GABA, glycine and taurine induced sedation, side position and discoordination. The convulsants, strychnine sulfate and pentylenetetrazole, induced seizures both in rats and mice. Differences between species may derive from the better access of intracerebroventricularly administered drugs to mouse hippocampus. Thus mice may be preferable for studies of this type on excitatory amino acids (including kynurenine pathway metabolites) and rats for those on inhibitory amino acids.