Injection Of N-methyl-D-aspartate Research Articles

Systemic NMDA treatment increases plasma ACTH in the neonatal female and male rat

The present study was designed to examine the adrenocorticotropic hormone (ACTH) response to N- methyl- d-aspartate (NMDA) in neonatal rats. Subcutaneous injection of NMDA (30 mg/kg) was found to increase plasma ACTH concentrations two-fold after 15 min in 9–10 and 20–21 day-old female and male rats. Pretreatment with the competitive NMDA receptor antagonist CPP (10mg/kg) failed to attenuate the ACTH response to NMDA in the younger rats, yet reduced the response in older ones. These findings indicate that NMDA can elevate plasma ACTH in both female and male neonatal rats, however this response is not sensitive to CPP antagonism until the end of the neonatal period.

The involvement of NMDA receptors in acute and chronic effects of ethanol.

Recent evidence indicates involvement of excitatory amino acid receptors sensitive to N-methyl-d-aspartate (NMDA) in the action of ethanol (EtOH). Pronounced inhibition of NMDA receptor function is seen in vitro with concentrations of EtOH corresponding to those present during alcohol intoxication in humans. The present study was devoted to investigate the role of NMDA receptors in the action of EtOH in rats. Acute experiments showed antagonism by EtOH of convulsions induced by intracerebroventricular injection of NMDA. A similar effect was seen with a high dose of diazepam. Convulsions induced by an agonist of another excitatory amino acid receptor subtype, kainate, were also inhibited by EtOH. An uncompetitive antagonist of NMDA receptors, 5-methyl-10,11-dihydro-5H-dibenzocyclohepten-5,10-imine maleate (MK-801), potentiated EtOH-induced loss of righting, but attenuated the hypothermic action of EtOH. Moreover, MK-801 inhibited audiogenic convulsions in EtOH withdrawn rats. At the same time the effect of a proconvulsive dose of NMDA was not enhanced. Tolerance to the myorelaxant action of both EtOH and MK-801 upon repetitive administration was seen. Also some degree of cross-tolerance was observed. Moreover, MK-801 failed to modify EtOH preference in rats. The present results support involvement of NMDA receptors in expression of some acute and subchronic actions of EtOH and in expression of EtOH withdrawal.

Participation of arginine vasopressin-mediated and adrenergic system-mediated mechanisms in the hypertension induced by intracerebroventricular administration of NMDA in freely moving rats.

Effects of intracerebroventricular (third ventricle) injection of N-methyl-D-aspartate (NMDA) on arterial blood pressure, on heart rate, on arginine vasopressin (AVP) and levels of catecholamines in plasma and on the behaviour of normotensive freely-moving rats have been evaluated. N-Methyl-D-aspartate significantly (P less than 0.01) increased arterial blood pressure and levels of catecholamines and AVP in plasma. With 0.1-1.0 micrograms/rat all animals presented psychomotor agitation, stereotyped movements, hyperexcitability, exophthalmus, dyspnoea, jumping, rearing and teething. The selective antagonist for NMDA receptors, 2-APV injected in the third ventricle, significantly (P less than 0.01) antagonized the hypertension, the increase in levels of catecholamines and AVP in plasma and behavioural effects. An antagonist of alpha 1 adrenergic receptors, prazosin (i.v.), an agonist of alpha 2 adrenergic receptors, clonidine (i.c.v.) and a relatively selective antagonist of V1 subtype of receptor of AVP, CGP 25838 (i.c.v. and i.v.), 15 min before NMDA, significantly (P less than 0.01) decreased the effects induced by the injections of NMDA. On the contrary, an antagonist of opiate receptors, naloxone (i.v.), 15 min before NMDA, significantly (P less than 0.01) increased the NMDA-induced modifications. Pretreatment with the antagonists at these doses, did not significantly modify the basal values of arterial blood pressure and behaviour. Only 2-APV sometimes induced ataxia, lasting about 5 min. This study points out an increase in the central sympathetic efferent activity and in release of AVP involved in the NMDA-induced cardiovascular and behavioural effects.

Selective potentiation of NMDA-induced activity and release of excitatory amino acids by dynorphin: possible roles in paralysis and neurotoxicity

Selective antagonists of N-methyl-D-aspartate (NMDA) excitatory amino acid (EAA) receptors have been shown to protect against dynorphin-A (DYN)-induced paralysis and neurotoxicity in the spinal cord. To test the hypothesis that either DYN-induced paralysis or neurotoxicity involves an enhanced release of EAAs, we used microdialysis to monitor aspartate (Asp) and glutamate (Glu) release in both the lumbar spinal cord extracellular fluid (ECF) and the spinal cord cerebral spinal fluid (CSF) of conscious rats in response to DYN (1-13). Injection of 5 nmol of DYN produced temporary paralysis in 8 of 10 animals, but did not significantly change Asp or Glu release in either the ECF or the CSF. Injection of 20 nmol of DYN caused permanent paralysis and neuronal cell loss in all animals tested as well as a significant increase of Asp and Glu in both the ECF and the CSF, and a decrease in glutamine (Gln) release only in the ECF. Pretreatment with 1 mg/kg of the NMDA antagonist MK-801 blocked both paralysis and amino acid changes in the ECF. Pretreatment of animals with 5 mg/kg naloxone inhibited glutamate release in the ECF, but did not block paralysis, Asp release or inhibition of Gln release. As MK-801 sensitive paralysis by DYN was not mediated through enhanced EAA release, we examined whether DYN could act through postsynaptic facilitation of NMDA receptors by testing the ability of DYN to alter the magnitude of a behavioral response produced by intrathecal injection of NMDA in mice.(ABSTRACT TRUNCATED AT 250 WORDS)

The polyamine synthesis inhibitor α-difluoromethylornithine is neuroprotective against N-methyl-D-aspartate-induced brain damage in vivo

We examined the ability of α-difluoromethylornithine (DFMO), an inhibitor of ornithine decarboxylase, the rate limiting enzyme for polyamine biosynthesis, to protect the brain of the perinatal rat from N-methyl-D-aspartate (NMDA)-induced brain damage. Treatment of the rat pups with DFMO administered either by i.p. injection (500 mg/kg × 2) or through the milk of the mother (2% solution in mother's drinking water) significantly reduced, by 48 and 62%, respectively, the brain damage produced by intrastriatal NMDA injection. We conclude that activation of polyamine synthesis may mediate part of the neurotoxic action of NMDA.

Cerebral ischemia decreases the behavioral effects and mortality rate elicited by activation of nmda receptors in mice

The purpose of this study was to determine whether prior transient cerebral ischemia, in conscious mice, would alter the biological responses resulting from excessive activation of N- methyl- d-aspartate (NMDA) receptors, in an early stage. The responses to the activation of NMDA receptors by an intracerebroventricular injection of NMDA, such as wild running, tonic and clonic convulsions, absence of the visual placing reflex, loss of the righting reflex, impaired motor function and a high mortality rate, were to a large extent prevented if 30 min before treatment, either a 10-min period of global cerebral ischemia was induced or a 1 nmol intraventricular injection of NMDA was given but not if either of the above procedures was done one day before the test dose of NMDA. In contrast, behavioral symptoms, in response to activation of non-NMDA-type glutamate receptors elicited by intraventricular injection of either kainic acid or AMPA, were not clearly affected. Transient systemic hypercapnic anoxia (22-sec exposure to 100% CO 2 gas), before treatment with NMDA did not significantly reduce the NMDA-induced behavior. The severity of these behavioral responses and high mortality rate observed after intraventricular injection of pentylenetetrazole (PTZ, 30 μmol) were not altered by either prior global ischemie insult or by a preexposure to NMDA given intraventricularly. The NMDA antagonist, MK801 (0.1 and 0.3 mg/kg i.p.) greatly reduced the behavioral effects and mortality rate, resulting from the intraventricular injection of NMDA and somewhat reduced the effects of the intraventricular injection of PTZ. The reduced response to NMDA, following a prior activation of NMDA receptors through either injection of NMDA or transient cerebral ischemia, may be due to a decreased number of NMDA receptors or sensitivity and/or to depression of pathways in the NMDA receptor-mediated biochemical cascade. The physiological significance of this finding lies in the implication that repeated strong activation of NMDA receptors, within a short period, results in a diminution of the NMDA-induced behavioral effect, and mortality rate, in very early stage.

Activation of AMPA/kainic acid glutamate receptors in the zona incerta stimulates locomotor activity

Direct injections of dl-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), an AMPA/quisqualic acid receptor agonist, into the medial or posterior zona incerta (ZI) produced a marked stimulation of locomotor activity accompanied by a postural change. Similar responses were obtained by injection of kainic acid (KA) into the same areas. The behavioral effects of AMPA and KA were antagonized by coinjection of 6,7-dinitroquinoxaline-2,3-dione (DNQX), and non- N-methyl- d-aspartate (NMDA) glutamate receptor antagonist. In contrast, injections of NMDA or ibotenic acid failed to significantly stimulate locomotor activity. These results suggest that the AMPA/kainate glutamate receptor subtypes in the zona incerta may have a functional role in regulating locomotor activity.

Different stereoselectivity of the enantiomers of HA-966 (3-amino-1-hydroxy-2-pyrrolidinone) for neuroprotective and anticonvulsant actions in vivo

HA-966 is an antagonist at the glycine modulatory site of N- methyl- d-aspartate (NMDA) receptors. Neuroprotective effects of HA-966 were determined in postnatal day (PND) 7 rats that received intrastriatal injections of NMDA (15 nmol) and then were administered either racemic HA-966, or the purified ( R) or ( S) enantiomer 15 min later. The ( R)-enantiomer dose-dependently attenuated NMDA-induced brain injury, whereas the ( S)-enantiomer was ineffective. When given intravenously to mice, racemic HA-966 and the ( S)-enantiomer prevented tonic extensor seizures from low-intensity electroshock (ED 50 = 13.2 and 8.8 mg/kg, respectively). Anticonvulsant effects of the ( R)-enantiomer were much less potent (ED 50 = 105.9 mg/kg). Ataxia measured by inverted screen fall-off was 17 times more potent with the ( S)-enantiomer than with the ( R)-enantiomer. Since previous published work indicates that glycine-site NMDA antagonist activity is primarily due to the ( R)-enantiomer, our results suggest neuroprotective action for the ( R)-enantiomer and anticonvulsant action, not related to a glycine antagonist mechanism for the ( S)-enantiomer.

Pulsatile Luteinizing Hormone Responses to Intermittent iV-Methyl-D,L- Aspartate Administration in Hamsters Exposed to Long- and Short-Day Photoperiods*

Hamsters exposed to short-day photoperiods (6-h light, 18-h darkness) develop a reversible inhibition of the reproductive axis which includes decreased LH pulse frequency, decreased FSH and testosterone levels, testicular regression, and presumably, decreased luteinizing hormone-releasing hormone (LHRH) release. The decrease in LHRH release could reflect a decrease in the ability of LHRH neurons to release the decapeptide in response to intermittent neuronal excitation. To analyze this possibility, the LHRH secretagogue, N-methyl-D-aspartate (NMA) was used to estimate relative releasability of endogenous LHRH pulses in hamsters exposed to long- and short-day photoperiods. Long-day (LD) or short-day (SD) hamsters were fitted with indwelling atrial catheters 2 days before experimentation. During 5-h sampling sessions (1300-1800 h), blood was withdrawn at 10-min intervals. In initial experiments, a NMA dose-response analysis revealed that 10 and 20 mg/kg NMA but not 2.5 and 5 mg/kg NMA, produced significant, dose-related LH responses in LD hamsters. Treatments with LHRH antagonist 4 h prior to NMA treatment completely blocked LH responses. The 10 mg/kg NMA dose was then used to directly compare LHRH responsiveness in LD and SD hamsters. NMA injections were administered to groups of LD and SD hamsters after hours 1, 2, 3, and 4 of sampling. In LD hamsters, LH responses to the first NMA pulse (2.65 +/- 0.09) were followed by diminishing responses to the second, third, and fourth NMA pulses (1.22 +/- 0.50, 1.22 +/- 0.43, and 1.15 +/- 0.42). By contrast, initial LH responses in SD hamsters (1.61 +/- 0.31) were followed by even more robust LH responses to the second, third, and fourth NMA challenges (2.89 +/- 0.34, 3.08 +/- 0.59, 2.65 +/- 0.32). Although LH responses to the first NMA pulse were slightly less in SD vs. LD animals, responses to all subsequent injections were actually greater in SD hamsters vs. their LD counterparts. Injections of 20 ng/kg LHRH in SD hamsters produced LH responses similar to those evoked by NMA. The same LHRH dose also produced continued, robust LH responses in LD hamsters, indicating that decrements in response to NMA were not due to diminished pituitary responsiveness. These results suggest that 1) the stimulatory actions of NMA are mediated by LHRH, 2) exposure to short days does not decrease the responsiveness of LHRH neurons to NMA, and 3) successive NMA challenges produce diminishing LH responses in LD hamsters, but not in SD hamsters.(ABSTRACT TRUNCATED AT 400 WORDS)

Rapid alteration of synaptic number and postsynaptic thickening length by NMDA: An electorn microscopic study in the occipital cortex of postnatral rats

The N-methyl-D-aspartate (NMDA) receptor has been widely implicated in numerous activity-dependent models of neural plasticity, learning, and memory. The formation of new synapses is a major assumption of the neural basis of learning. The current research was conducted to determine whether NMDA receptor activation could induce synaptic formation and, if so, whether this ability would mirror developmental changes in NMDA receptors. Rats at various developmental ages were given a single intraperitoneal injection of NMDA and sacrificed at various brief postinjection intervals (0.5-2 hr). The rats showed an age-dependent decline in the behavioral response to NMDA, as evidenced by reduced seizure activity and duration. Quantitative electron microscopic observations on the molecular layer of the occipital cortex, an area rich in NMDA receptors, revealed a transient increase in the length of postsynaptic thickenings in 17- and 35-day-old animals, appearing within 0.5 hr of injection. At 1 and 2 hr postinjection, an increase in synaptic density (number of synapses) was observed in 8-day-old animals. These results provide evidence that NMDA administration alone is capable of rapidly inducing alterations in synaptic structure and the formation of new synapses, underscoring the importance of the NMDA receptor in synaptogenesis and synaptic structural plasticity.

Excitotoxic brain injury suppresses striatal high-affinity glutamate uptake in perinatal rats.

In immature rodent brain, the glutamate receptor agonist N-methyl-D-aspartate (NMDA) is a potent neurotoxin. In postnatal day (PND)-7 rats, intrastriatal injection of 25 nmol of NMDA results in extensive ipsilateral forebrain injury. In this study, we examined alterations in high-affinity [3H]glutamate uptake (HAGU) in NMDA-lesioned striatum. HAGU was assayed in synaptosomes, prepared from lesioned striatum, the corresponding contralateral striatum, or unlesioned controls. Twenty-four hours after NMDA injection (25 nmol), HAGU declined 44 +/- 8% in lesioned tissue, compared with the contralateral striatum (mean +/- SEM, n = 6 assays, p less than 0.006, paired t test). Doses of 5-25 nmol of NMDA resulted in increasing suppression of HAGU (5 nmol, n = 3; 12.5 nmol, n = 3; and 25 nmol, n = 5 assays; p less than 0.01, regression analysis). The temporal evolution of HAGU suppression was biphasic. There was an early transient suppression of HAGU (-28 +/- 4% at 1 h; p less than 0.03, analysis of variance, comparing changes at 0.5, 1, 2, and 3 h after lesioning); 1 or 5 days postinjury there was sustained loss of HAGU (at 5 days, -56 +/- 11%, n = 3, p less than 0.03, paired t test, lesioned versus contralateral striata).(ABSTRACT TRUNCATED AT 250 WORDS)

A Role forN-Methyl-D-Aspartate Receptors in the Control of Seasonal Breeding*

The neuroendocrine basis for short-day induced testicular regression was studied in Syrian hamsters of the LSH/Ss Lak strain. Adult animals were maintained either under long or short days (14:10D or 6L:18D, respectively) and given single, daily ip injections of N-methyl-D-aspartate (NMDA) (25 mg/kg BW); control animals received injections of saline. As expected, the testes of the short-day controls had completely regressed to a prepubertal condition within 6 weeks, a change that was associated with significantly reduced mean plasma gonadotropin levels. In contrast, the NMDA-treated hamsters from both the long-day and short-day groups, as well as the long-day controls, all maintained large testes and elevated plasma gonadotropin levels, although plasma luteinizing hormone (LH) was partially suppressed in the short-day group. In a second experiment which lasted 2 weeks, short-day hamsters with completely regressed testes were either transferred to long days, maintained further on short days, or maintained on short days and given a daily ip injection of NMDA (25 mg/kg BW). The short-term exposure to long days caused an expected increase in plasma and pituitary concentrations of follicle-stimulating hormone, pituitary LH, and testicular weight. Similar, but even more marked, changes were observed in the short-day hamsters that were treated with NMDA, including significant increases in plasma LH and serum testosterone concentrations. Moreover, histological examination revealed that the recrudescing testes from this latter group already contained mature spermatocytes and in some individuals even spermatozoa. These results demonstrate that NMDA receptors may play a pivotal role in both the termination and onset of the breeding season in photoperiodic species.

NADPH‐diaphorase reactive neurons of the rabbit retina: Differential sensitivity to excitotoxins and unusual morphologic features

Various populations of retinal and brain neurons display different degrees of sensitivity to the toxic effects of excitatory amino acid agonists. To further define this phenomenon, the effects of intravitreal injections of kainic acid, N-methyl-D-aspartate (NMDA), quinolinic acid, and ibotenic acid on NADPH-diaphorase (NADPHd) reactive cells of the rabbit retina were examined. In normal rabbit retinas, NADPHd histochemistry selectively stains two populations of cells at the inner margin of the inner plexiform layer: large densely staining (Type I) and more numerous, smaller and lightly staining (Type II) cells. All of the toxins studied destroy both cell types at high doses. Kainic acid, however, at low doses destroys Type I cells but selectively spares Type II cells. NMDA injections result in a more complex and less consistent pattern of cell loss; overall, the Type I cells are perhaps slightly resistant to this agent as compared to Type II cells. The Type I neurons in this respect are analogous to striatal NADPHd-reactive neurons, which have, at most, a slight relative resistance to NMDA neurotoxicity. The excitotoxin lesions reduce the complexity of the fiber plexus in the inner plexiform layer and thereby reveal anatomic features of surviving NADPHd reactive neurons that are not apparent in normal retinas. In particular, each Type I cell gives off several fine, straight, beaded processes that emanate from dendrites and run several millimeters in the inner plexiform layer. These processes are presumably involved in long-distance interactions within the retina. Excitotoxin lesions may provide a generally useful anatomic tool for elucidating morphological features of neurons stained by histochemical and immunohistochemical methods.

Protection Against N-methyl-D-aspartate Receptor-Mediated Neuronal Degeneration In Rat Brain by 7-chlorokynurenate and 3-amino-1-hydroxypyrrolid-2-one, Antagonists at The Allosteric Site for Glycine.

7-Chlorokynurenate (7-Cl KYNA) and 3-amino-1-hydroxypyrrolid-2-one (HA-966), two selective antagonists of the glycine site on the N-methyl-D-aspartate (NMDA) receptor, have been used to assess the involvement of this site in the neurodegeneration resulting from injection of excitotoxins in the rat brain. In the rat striatum, reductions in the enzymes choline acetyltransferase (CAT) and glutamate decarboxylase (GAD), occurring 7 days after a unilateral, intrastriatal injection of quinolinate (200 nmol), were prevented in a dose-dependent manner by intrastriatal administration of 7-Cl KYNA (10 - 50 nmol) and HA-966 (200 - 500 nmol) 1 h after the excitotoxin. In the rat hippocampus, degeneration of pyramidal and granule neurons caused by direct injection of quinolinate (60 nmol) was completely prevented by 7-Cl KYNA (50 nmol) and partially by HA-966 (500 nmol) injected intrahippocampally 1 h after the excitotoxin. In the rat striatum, 7-Cl KYNA (50 nmol) and HA-966 (500 nmol) also reduced neurotoxicity caused by intrastriatal injection of NMDA (200 nmol), but not that caused by the 'non-NMDA' receptor agonists DL-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) or kainate. The time course of protective effects of 7-Cl KYNA and HA-966 in the striatum was similar to that previously observed with the uncompetitive NMDA receptor antagonist MK-801, indicating that activation of the glycine site contributes to the delayed degeneration of neurons which occurs over the first 5 h following quinolinate injection. The neuroprotective effects of both 7-Cl KYNA and HA-966 in the rat striatum appear to be mediated via the glycine site on the NMDA receptor as they were completely reversed by D-serine, but not L-serine. These results indicate that activation of the glycine site is essential for the expression of the delayed degeneration of neurons resulting from intracerebral injection of an NMDA receptor agonist, a process which bears similarities to the delayed neurodegeneration which results from a period of cerebral ischaemia.

A quantitative analysis of the effects of excitatory neurotoxins on retinal ganglion cells in the chick

The present study examines the differential effects of three excitotoxins, kainic acid (KA), N-methyl-D-aspartate (NMDA), and alpha-amino-2,3-dihydro-5-methyl-3-oxo-4-isoxazolepropanoic acid (AMPA) on neurons within the ganglion cell layer (GCL) of the chick retina. Two-day-old chicks were given a single, 5 microliters, intravitreal injection of KA, NMDA, or AMPA at a range of doses. Following treatment with 40 nmol KA, there was a 21% loss of neurons in the GCL. At 200 nmol KA, the loss increased to 46%. Exposure to KA eliminated mainly small neurons of soma area 5-15 microns2, and medium-sized ganglion cells of soma area 15-25 microns2. Large ganglion cells (greater than 25 microns2) remained unaffected. The vast majority of small cells were probably displaced amacrine cells. Exposure to 400 nmol NMDA resulted in a 14% loss of neurons, predominantly involving the large ganglion cells. At a dose of 3000 nmol NMDA, no further loss of cells was evident. Exposure to 200 nmol AMPA resulted in a 30% loss of large and some medium-sized ganglion cells. In a further series of experiments, exposure to excitotoxin was followed by a retinal scratch, which eliminated retinal ganglion cells within the axotomized region. The results indicate that only a small proportion of displaced amacrine cells are destroyed by NMDA and AMPA, whereas virtually all displaced amacrine cells are sensitive to KA. The findings of this study indicate the existence of subclasses of ganglion cells with specificity towards different types of excitatory amino acids (EAA).

Magnesium reduces N- (NMDA)-mediated brain injury in perinatal rats

We evaluated the neuroprotective effects of systemically administered magnesium against N-methyl-D-aspartate (NMDA)-mediated brain injury in perinatal rats. Postnatal day (PND) 7 rats received unilateral intrastriatal injections of 25 nmol NMDA followed 15 min later by single or multiple doses of magnesium intraperitoneally (i.p.). Animals were sacrificed five days later and the severity of brain injury was assessed by comparison of the weights of the injected and contralateral cerebral hemispheres. NMDA injection reduced the weight of the injected cerebral hemisphere by 31 +/- 3%. Single doses of magnesium reduced the severity of NMDA-induced brain injury in a dose-dependent fashion (2 mmol/kg, 29 +/- 11% protection; 3 mmol/kg, 52 +/- 12% protection; 4 mmol/kg, 62 +/- 7% protection). Multiple doses of magnesium reduced brain injury by 65 +/- 4%. These data demonstrate that systemically administered magnesium antagonizes the neurotoxic effects of NMDA in vivo in perinatal rats.

Peripubertal Treatment with N-Methyl-D-Aspartic Acid or Neonatally with Monosodium Glutamate Accelerates Sexual Maturation in Female Rats, an Effect Reversed by MK-801

Recent reports from several laboratories have implicated the excitatory neurotransmitter glutamate as a component in the neural regulation of sexual maturation. In the rat we have previously proposed that a hypothalamic opioid restraint mechanism may ultimately be overridden by maturation of an excitatory drive, culminating in first ovulation. We have now investigated whether glutamate may be the excitatory factor. Treatment of immature female rats with single, daily injections of two N-methyl-D-aspartate (NMDA) antagonists--dextrorphan (18 mg/kg) and MK-801 (0.1 mg/kg)--beginning on the 27th postnatal day, significantly delayed the timing of vaginal opening (VO). Interestingly, treated rats reached VO in spite of continued antagonist treatment. The antagonist effect was reversed by preinjection of NMDA, suggesting that endogenous glutamate exerts its effect via an NMDA-subtype glutamate receptor. Injection of NMDA alone (15 mg/kg; once daily) produced a striking synchronization of VO such that all treated rats showed VO over a 24-hour period compared to a normal distribution of several days for control rats. In a model of first ovulation, i.e., rats induced to ovulate by pregnant mare serum, MK-801 (1 mg/kg) arrested treated rats at proestrus. This was readily reversible after discontinuing injections. A lower dose of MK-801 (0.1 mg/kg/day) was ineffective in delaying ovulation. In a second series of experiments we studied the consequences of a neonatal hypothalamic lesion which destroys glutamate-sensitive neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

N-methyl D-aspartate (NMDA) evoked changes in blood pressure and heart rate from the rat superior colliculus

Electrical stimulation and microinjections of the GABA antagonist bicuculline methiodide into the superior colliculus (SC) of the anaesthetized rat can evoke changes in blood pressure and heart rate. The long latency of bicuculline evoked responses, however, raises the possibility that they may have been produced by the diffusion of bicuculline to surrounding tissue, in particular, the periaqueductal gray. Thus, the purpose of the present study was to investigate whether such changes could have been produced by the activation of extracollicular neural elements. This was achieved by assessing the relative regional sensitivity of the SC and underlying structures (periaqueductal gray and dorsal tegmentum) with microinjections of the excitatory amino acid N-methyl D-aspartate (NMDA) for the production of short latency pressor responses. Dorsal midbrain injections of NMDA (200 nl of 100 mM) in the Saffan anaesthetized rat evoked clear short latency (single or double phase) increases in blood pressure accompanied in most cases by a longer latency increase in heart rate and respiration. Two regions within the dorsal midbrain contained significantly higher proportions of active sites compared with surrounding tissue: i) the superficial and intermediate layers of rostromedial SC, and ii) the caudal periaqueductal gray. The distribution of active sites indicated that pressor responses elicited from the rostromedial SC could not be explained by the diffusion of NMDA to underlying tissue. The question of whether the rostromedial SC might represent an early link in the circuitry responsible for organizing defensive movements and appropriate physiological changes to potentially dangerous overhead stimuli is considered.

Quantitative assessment of neuroprotection against NMDA-induced brain injury

In immature rodent brain, unilateral intrastriatal injections of selected excitatory amino acid (EAA) receptor agonists, such as N-methyl- d-aspartate (NMDA), produce prominent ipsilateral forebrain lesions. In Postnatal Day (PND) 7 rats that receive a right intrastriatal injection of NMDA (25 nmol) and are sacrificed 5 days later, there is a considerable and consistent reduction in the weight of the injected cerebral hemisphere relative to that of the contralateral side (−28.5 ± 1.9%, n = 6). In animals treated with specific NMDA receptor antagonists, the severity of NMDA-induced damage is markedly reduced. We have previously reported that the efficacy of potential neuroprotective drugs in limiting NMDA-induced lesions can be assessed quantitatively by comparison of hemisphere weights after a unilateral NMDA injection. In this study, we compared three quantitative methods to evaluate the severity of NMDA-induced brain injury and the degree of neuroprotection provided by NMDA receptor antagonists. We characterized the severity of brain injury resulting from intrastriatal injections of 1–50 nmol NMDA in PND 7 rats sacrificed on PND 12 by (i) comparison of cerebral hemisphere weights; (ii) assay of the activity of the cholinergic neuronal marker, choline acetyltransferase (ChAT) activity; and (iii) measurement of regional brain cross-sectional areas. The severity of the resulting brain injury as assessed by comparison of hemisphere weights increased linearly with the amount of NMDA injected into the striatum up to 25 nmol NMDA. The magnitude of injury was highly correlated with the degree of reduction in ChAT activity ( r 2 = 0.97). Quantification of neuroprotection against NMDA toxicity by measurement of cerebral hemisphere weight disparities was highly correlated with comparisons of hemisphere and striatal cross-sectional areas ( r 2 = 0.98). The potency and efficacy of the dissociative anesthetic ketamine was easily distinguished from those of two other NMDA receptor antagonists (MK-801, (+)-5-methyl-10,11-dihydro-5H-dibenzo[ a,d]cyclohepten-5,10-immine maleate and CPP, 3-((±)-2-carboxypiperazine-4-yl)-propyl-1-phosphonic acid) using this model. These data demonstrate that this model can be used to accurately quantitate NMDA-induced brain injury and evaluate neuro-protective properties of glutamate antagonists in vivo.

Hyperactivity induced by N-methyl-d-aspartate injections into nucleus accumbens: Lack of evidence for mediation by dopaminergic neurons

To test the hypothesis that the motor hyperactivity associated with intra-accumbens injections of N-methyl-d-aspartate (NMDA) results from stimulation (direct or indirect) of nucleus accumbens dopaminergic mechanisms, the behavioral effects of intra-accumbens and intraventricular NMDA were compared to those of the prototypic dopaminergic releasing agent, amphetamine, and the competitive NMDA receptor antagonist, 3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP). Drugs were injected into the right lateral ventricle, or bilaterally into the nucleus accumbens of rats. Locomotor activity was monitored electronically and by direct observation for 40 min prior to, and 1 hour after, drug treatment. Intra-accumbens injections of NMDA (0.4, 1.2 and 2.0 μg/side) produced dose-related increases in distance traveled, but had no significant effect on movement time or vertical movements. The NMDA-induced increase in distance traveled was temporally correlated with convulsive wild running, but not with exploratory behavior, suggesting that this increase may have been secondary to seizure-like activity. Intra-accumbens injections of amphetamine (10, 20 and 40 μg) or CPP (0.1 μg) produced dose-related increases in all three measures. By the intraventricular route, the effects of NMDA were similar to those of intra-accumbens administration, whereas intraventricularly administered d-amphetamine had no effect. The behavioral effects of intra-accumbens NMDA cannot be explained by an NMDA receptor-mediated facilitation of dopaminergic neurotransmission; rather, this type of facilitation may be associated with competitive NMDA receptor antagonism.