N-methylaspartate: an effective tool for lesioning basal forebrain cholinergic neurons of the rat

Functional Convergence of Motor and Social Processes in Lobule IV/V of the Mouse Cerebellum

Topographic organization of the cerebellum is largely segregated into the anterior and posterior lobes that represent its "motor" and "non-motor" functions, respectively. Although patients with damage to the anterior cerebellum often exhibit motor deficits, it remains unclear whether and how such an injury affects cognitive and social behaviors. To address this, we perturbed the activity of major anterior lobule IV/V in mice by either neurotoxic lesion or chemogenetic excitation of Purkinje cells in the cerebellar cortex. We found that both of the manipulations impaired motor coordination, but not general locomotion or anxiety-related behavior. The lesioned animals showed memory deficits in object recognition and social-associative recognition tests, which were confounded by a lack of exploration. Chemogenetic excitation of Purkinje cells disrupted the animals' social approach in a less-preferred context and social memory, without affecting their overall exploration and object-based memory. In a free social interaction test, the two groups exhibited less interaction with a stranger conspecific. Subsequent c-Fos imaging indicated that decreased neuronal activities in the medial prefrontal cortex, hippocampal dentate gyrus, parahippocampal cortices, and basolateral amygdala, as well as disorganized modular structures of the brain networks might underlie the reduced social interaction. These findings suggest that the anterior cerebellum plays an intricate role in processing motor, cognitive, and social functions.

Vestibular Modulation of Long-Term Potentiation and NMDA Receptor Expression in the Hippocampus

Loss of vestibular function is known to cause spatial memory deficits and hippocampal dysfunction, in terms of impaired place cell firing and abnormal theta rhythm. Based on these results, it has been of interest to determine whether vestibular loss also affects the development and maintenance of long-term potentiation (LTP) in the hippocampus. This article summarizes and critically reviews the studies of hippocampal LTP following a vestibular loss and its relationship to NMDA receptor expression, that have been published to date. Although the available in vitro studies indicate that unilateral vestibular loss (UVL) results in reduced hippocampal field potentials in CA1 and the dentate gyrus (DG), the in vivo studies involving bilateral vestibular loss (BVL) do not. This may be due to the differences between UVL and BVL or it could be a result of in vitro/in vivo differences. One in vitro study reported a decrease in LTP in hippocampal slices following UVL; however, the two available in vivo studies have reported different results: either no effect or an increase in EPSP/Population Spike (ES) potentiation. This discrepancy may be due to the different high-frequency stimulation (HFS) paradigms used to induce LTP. The increased ES potentiation following BVL may be related to an increase in synaptic NMDA receptors, possibly increasing the flow of vestibular input coming into CA1, with a loss of selectivity. This might cause increased excitability and synaptic noise, which might lead to a degradation of spatial learning and memory.

Using MRI to predict the fate of excitotoxic lesions in rats

Excitotoxic lesions are frequently used to assess the role of cerebral structures in cognitive processes in rodents. However, the precise site and extent of these lesions remain unknown without histological verifications. Using a 7-Teslas MRI system and a T2-weighted turbo-RARE sequence, MR images were acquired at several time points following NMDA lesions (1h, 6h, 24h, 48h, 1 week and 2 weeks). NMDA infusions into the parenchyma induced a clear and delineable hyperintense signal from 1h up to 1-week post-surgery. Hyperintensity volumes were compared with NeuN and Cresyl violet histological quantifications of the lesion magnitude. NMDA-induced hypersignal is observed as soon as 1h post-injection and is a reliable estimate of the presence (or absence) of a lesion. Compared to NeuN, Cresyl violet staining underestimates the extent of the lesion in significant proportions. The MRI hyperintensity generated by NMDA instillation into the parenchyma can be used as a powerful tool to confirm the diffusion of the drug into the cerebral tissue, to ascertain the locus of injection and predict with a high success rate the fate of NMDA lesions as soon as 1h post-surgery. This approach could be very useful in a large variety of lesion studies in rodents.

Neurons of the lateral preoptic area/rostral anterior hypothalamic area are required for photoperiodic inhibition of estrous cyclicity in sheep

Photoperiod determines the timing of reproductive activity in many species, yet the neural pathways whereby day length is transduced to a signal influencing gonadotropin-releasing hormone (GnRH) release are not fully understood. Physical lesions of the lateral preoptic area (lPOA)/rostral anterior hypothalamic area (rAHA) in female sheep extend the period of estrous cyclicity during inhibitory photoperiods. In the present study we sought to determine whether destroying only neurons and not fibers of passage in this area would lead to similar resistance to photosuppression. Additionally, neural tract-tracing was used to map connectivity between the lPOA/rAHA and other hypothalamic areas implicated in photoperiodic regulation of reproduction. Progesterone secretion was monitored in six sheep to determine estrous cycles for 90 days during a short-day (permissive) photoperiod. Three sheep then received bilateral injections of the excitotoxic glutamate analog, n-methyl-aspartic acid, directed toward the lPOA/rAHA, whereas three others served as controls. All were then exposed to a long-day (suppressive) photoperiod for 120 days. Control sheep ceased cycling at 40 ± 10 days (mean ± SEM), whereas lesioned sheep continued cycling through the end of the study. The results of the tract-tracing study revealed both afferent and efferent projections to the medial POA, retrochiasmatic area, arcuate nucleus, and premammillary region. Furthermore, close proximal associations with GnRH neurons from efferent projections were observed. We conclude that neurons located within the lPOA/rAHA are important for timing cessation of estrous cycles during photosuppression and that this area communicates directly with GnRH neurons and other hypothalamic areas involved in the photoperiodic regulation of reproduction.

Chronic partial sleep deprivation reduces brain sensitivity to glutamate N-methyl-D-aspartate receptor-mediated neurotoxicity

It has been hypothesized that insufficient sleep may compromise neuronal function and contribute to neurodegenerative processes. While sleep loss by itself may not lead to cell death directly, it may affect the sensitivity to a subsequent neurodegenerative insult. Here we examined the effects of chronic sleep restriction (SR) on the vulnerability of the brain to N-methyl-d-aspartate (NMDA)-induced excitotoxicity. Animals were kept awake 20 h per day and were only allowed to rest during the first 4 h of the light phase, i.e. their normal circadian resting phase. After 30 days of SR all rats received a unilateral injection with a neurotoxic dose of NMDA into the nucleus basalis magnocellularis (NBM). Brains were collected for assessment of damage. In the intact non-injected hemisphere, the number of cholinergic cells in the NBM and the density of their projections in the cortex were not affected by SR. In the injected hemisphere, NMDA caused a significant loss of cholinergic NBM cells and cortical fibres in all animals. However, the loss of cholinergic cells was attenuated in the SR group as compared with the controls. These data suggest that, if anything, SR reduces the sensitivity to a subsequent excitotoxic insult. Chronic SR may constitute a mild threat to the brain that does not lead to neurodegeneration by itself but prepares the brain for subsequent neurotoxic challenges. These results do not support the hypothesis that sleep loss increases the sensitivity to neurodegenerative processes.

Rhythmic cFos expression in the ventral subparaventricular zone influences general activity rhythms in the Nile grass rat, Arvicanthis niloticus

Circadian rhythms in behavior and physiology are very different in diurnal and nocturnal rodents. A pacemaker located in the suprachiasmatic nucleus (SCN) of the hypothalamus is responsible for generating and maintaining circadian rhythms in mammals, and cellular and molecular rhythms within the SCN of diurnal and nocturnal rodents are very similar. The neural substrates determining whether an animal has a diurnal or nocturnal phase preference are thus likely to reside downstream of the SCN. The ventral subparaventricular zone (vSPVZ), a major target of the SCN that is important for the expression of circadian rhythmicity in nocturnal lab rats (Rattus norvegicus), exhibits different rhythms in cFos expression in diurnal Nile grass rats compared to lab rats. We examined the effects of chemotoxic lesions of the cFos-expressing cells of the vSPVZ on activity rhythms of grass rats to evaluate the hypothesis that these cells support diurnality in this species. Male grass rats housed in a 12:12 light:dark (LD) cycle were given bilateral injections of the neurotoxin n-methyl-D-L-aspartic acid (NMA) or vehicle aimed at the vSPVZ; cells in the SCN are resistant to NMA, which kills neurons in other brain regions, but leaves fibers of passage intact. vSPVZ-damaged grass rats exhibited highly unstable patterns of activity in constant darkness (DD) and in the LD cycle that followed. However, crepuscular bouts of activity could be seen in all animals with vSPVZ lesions. Damage to the vSPVZ reduced cFos expression in this area but not in the SCN. Using correlational analyses, we found that the number of cFos-ir cells in the vSPVZ was unrelated to several parameters of the activity rhythms during the initial post-surgical period, when animals were in LD. However, the number of cells expressing cFos in the vSPVZ was positively correlated with general activity during the subjective day relative to the subjective night when the animals were switched to DD, and this pattern persisted when a LD cycle was reinstated. Also, the number of cFos-ir cells in the vSPVZ was negatively correlated with the strength of rhythmicity in DD and the number of days required to re-entrain to a LD cycle following several weeks in DD. These data suggest that the vSPVZ emits signals important for the expression of stable diurnal activity patterns in grass rats, and that species differences in these signals may contribute to differences in behavioral and physiological rhythms of diurnal and nocturnal mammals. (Author correspondence: mschw009@umaryland.edu ).

Delayed neurodegeneration and early astrogliosis after excitotoxicity to the aged brain

Excitotoxicity is well recognised as a mechanism underlying neuronal cell death in several brain injuries. To investigate age-dependent differences in neurodegeneration, edema formation and astrogliosis, intrastriatal N-methyl-d-aspartate injections were performed in young (3 months) and aged (22-24 months) male Wistar rats. Animals were sacrificed at different times between 12h and 14 days post-lesion (DPL) and cryostat sections were processed for Toluidine blue, Fluoro-Jade B staining, NeuN and GFAP immunohistochemistry. Our results show that both size of tissue injury and edema were reduced in the old subjects only up to 1DPL, correlating with a slower progression of neurodegeneration with peak numbers of degenerating neurons at 3DPL in the aged, contrasting with maximum neurodegeneration at 1DPL in the young. However, old animals showed an earlier onset of astroglial response, seen at 1DPL, and a larger area of astrogliosis at all time-points studied, including a greater glial scar. In conclusion, after excitotoxic striatal damage, progression of neurodegeneration is delayed in the aged but the astroglial response is earlier and exacerbated. Our results emphasize the importance of using aged animals and several survival times for the study of acute age-related brain insults.

Effects of RU 52583, an alpha 2-antagonist, on memory in rats with excitotoxic damage to the septal area

The anti-amnesic action of RU 52583, an alpha 2-adrenergic receptor antagonist, was evaluated through performance of spatial tasks in a radial maze by rats with N-methyl-D-aspartic acid (NMDA) lesion of the medial septal (MS) nuclei. Memory performance of lesioned or sham-operated rats was evaluated by measuring reference memory as long-term maintenance of an acquired performance and working memory or memory for recent events. The lesion: a produced significant impairments of the animals' memory performance, b) significantly reduced the sodium-dependent high-affinity choline uptake in the hippocampal formation, and c) deeply disrupted cholinergic hippocampal theta waves. Oral administration of RU 52583 at 1 and 2 mg/kg (tested doses: 1-5 mg/kg) prior to performance of the task markedly reduced memory impairments, whereas idazoxan, another alpha 2-adrenergic receptor antagonist, had no effect at tested doses (2-5 mg/kg). Cholinergic drugs--arecoline at 0.1 and 1 mg/kg (tested doses: 0.05-1 mg/kg) and physostigmine at 0.02 and 0.1 mg/kg (tested doses: 1, 2, and 5 mg/kg)-administered intraperitoneally showed a tendency to alleviate memory deficits. The present results show that the alpha 2-adrenergic antagonist RU 52583 possesses cognition-enhancing properties in rats with damage to the septohippocampal system.

Roles of the basolateral amygdala and hippocampus in social recognition in rats

Lesions of the amygdala or hippocampus have a large impact on social behavior of rats. In this study we investigated whether a social recognition test was also affected by those lesions. An NMDA-induced lesion of the basolateral amygdala did not impair the ability to distinguish a familiar from an unfamiliar juvenile rat. It was argued that the cortico-medial amygdala may be more important for social recognition than the basolateral amygdala. Fimbria-transected rats could no longer distinguish a familiar from an unfamiliar juvenile. Moreover, during all encounters they spent less time investigating the juvenile. The precise nature of this deficit, especially the reason for the overall reduced social investigation time, could not be specified with the classical procedure of the social recognition test.

Microglial response to N-methyl-D-aspartate-mediated excitotoxicity in the immature rat brain

The intracerebral injection of N-methyl-D-aspartate (NMDA) has been proposed as a model for hypoxic-ischemic insult in the immature brain. In this light, the aim of this study was to describe the time course of the microglial reaction in the areas undergoing primary degeneration at the site of intracortical NMDA injection as well as in areas undergoing secondary anterograde and/or retrograde degeneration. Fifty nanomoles of NMDA were injected in the sensorimotor cortex of 6-day-old rats. After survival times ranging from 10 hours to 28 days, cryostat sections were stained for routine histology and for the demonstration of microglial cells by means of tomato lectin histochemistry. The areas affected by primary degeneration caused by the intracortical injection of NMDA were the neocortex, the hippocampus, and the rostral thalamus. Secondary degeneration (retrograde and anterograde) was observed in the ventrobasal complex of the thalamus. The cortical lesion also caused Wallerian degeneration of the cortical descending efferents as observed in the basilar pons. Microglial reactivity in all these areas was present at 10 hours postinjection and was restricted to the areas undergoing neuronal or axonal degeneration. Reactive microglial cells were stained intensely and showed a round or pseudopodic morphology. At 3 days, an apparent increase in the number of tomato lectin-positive cells was observed in the areas undergoing neuronal death. By 7 days after the injection, the lesion became nonprogressive, and by 14 and 28 days, microglial cells showed moderate lectin binding and a more ramified morphology.

Differential effects of ventral striatal lesions on the conditioned place preference induced by morphine or amphetamine

The present experiment examined the role of the ventral striatum in the rewarding effect of morphine and amphetamine by testing whether lesions of cell bodies within this region disrupt the development of a conditioned place preference to either drug. Bilateral, N-methyl-D-aspartate- or kainic acid-induced lesions of the ventral striatum block a conditioned place preference to amphetamine (1.5 mg/kg x 3 pairings) but not to morphine (2 mg/kg x 3 pairings). Because both lesions spared anterior portions of the ventral striatum, we examined the effect of larger or more selective ventral striatal lesions on a conditioned place preference induced by morphine. Destruction of the entire ventral striatum reduced, but did not eliminate, a conditioned place preference to morphine, whereas selective lesions of the anterior ventral striatum were ineffective. These results indicate that the ventral striatum is not critically involved in morphine's rewarding effect and support the suggestion that the rewarding effects of opiates and stimulants do not involve identical neural substrates.

Mineralization of the globus pallidus following excitotoxic lesions of the basal forebrain

The excitotoxin N-methyl aspartic acid was injected into the rat nucleus basalis to destroy basal forebrain cholinergic (BFC) neurons. In long-term survival experiments (up to 11 months post-lesion), conspicuous mineralized deposits were found in the globus pallidus and to a lesser extent in the thalamus. Deposits stained in a manner consistent with a composition of calcium and iron. Typically, deposits were absent from the center of the injection site, where BFC cell loss was most severe, but were present within the ventral and lateral globus pallidus where there was substantial sparing of BFC neurons. The similarity of this pathology to basal ganglia calcification and its relationship to Alzheimer's Disease and Down's syndrome is discussed.

Neuroprotection against NMDA induced cell death in rat nucleus basalis by Ca2+ antagonist nimodipine, influence of aging and developmental drug treatment

In the current study the neuroprotective effect of the L-type calcium channel antagonist nimodipine in rat brain was investigated in N-methyl-D-aspartate-induced neuronal degeneration in vivo. In the present model NMDA was unilaterally injected in the magnocellular nucleus basalis and the neurotoxic impact assessed by measuring cortical cholinergic fibre loss as a percentage of fibre density of the intact control hemisphere. This procedure proved to be a reproducible model in which the degree of damage was almost linearly proportional to the NMDA dose. Neuroprotection by nimodipine was determined in a number of conditions. First, the effect of nimodipine treatment in adult animals starting two weeks prior to neurotoxic injury was compared with neuroprotection provided by perinatal treatment of the mother animals with the calcium antagonist. Surprisingly, the degree of protection was in both cases similar, yielding almost 30% reduction of fibre loss. The neuroprotective effect in adulthood of perinatal nimodipine treatment may be explained by developmentally enhanced calcium binding proteins or persistent developmental changes in calcium channel characteristics. Protection by nimodipine was also investigated in aged, 26 month old rats. Compared to young adult cases, aged animals proved to be less vulnerable to NMDA exposure, while nimodipine application was more potent, thus yielding a reduction of nearly 50% in nerve fibre damage induced by NMDA infusions. Possible mechanisms of differential calcium influx in the various experimental conditions will be discussed.

Ganglioside GM1 prevents N-methyl-D-aspartate neurotoxicity in rabbit hippocampus in vivo. Effects on calcium homeostasis

Microdialysis was used to apply 1 mM N-methyl-D-aspartate (NMDA) for 20 min to the hippocampus of rabbits, control and pre-treated with GM1 ganglioside (im injections of 30 mg/kg for 3 d, twice a day). Concentrations of ionized Ca2+ and 6-keto prostaglandin F1 alpha (6-keto PGF1 alpha)-immunoreactive material in the dialyzates and 45Ca and [14C]sucrose efflux from the prelabeled hippocampus were determined. After 24 h, the morphology of the hippocampal neurons was examined. In control animals, the application of NMDA resulted in 25% decrease in Ca2+ concentration and in 1000% increase in 6-keto PGF 1 alpha concentration in the dialyzates. A 30% decrease in 45Ca efflux was accompanied by 20% increase in [14C]sucrose efflux, reflecting a corresponding reduction of the extracellular space volume. A degeneration of CA1 pyramidal neurons in the vicinity of a microdialysis probe was observed. In GM1-treated rabbits the NMDA-induced decrease in Ca2+ concentrations in the dialyzates was not reduced significantly, whereas a 70% stimulation of 45Ca efflux was noted, with a concomitant 40% reduction of 6-keto-PG F1 alpha release. NMDA-evoked increase in [14C]sucrose efflux did not differ from the control. In these animals CA1 neurons were well preserved. These results indicate that the pretreatment with GM1 results in activation of calcium extrusion from the NMDA-stimulated rabbit hippocampal neurons that alleviates destabilization of calcium homeostasis and reduces NMDA-evoked neuronal injury.

Neurochemical, histopathological and mnemonic effects of combined lesions of the medial septal and serotonin afferents to the hippocampus

Male Long-Evans rats received micro-injections of either N-methyl-D-aspartate (NMDA) in the medial septum/vertical diagonal band (MS/DB), 5,7-dihyroxytryptamine (5,7-DHT) in the fimbria/fornix and cingulate bundle or combined NMDA/5,7-DHT micro-injections. NMDA administration caused considerable damage to the MS and enlarged the lateral ventricles. It reduced the activity of choline acetyltransferase as well as the intensity of acetylcholinesterase staining in the hippocampus. 5,7-DHT selectively reduced the concentration of hippocampal serotonin. The rats were assessed for spatial memory in the Morris water maze and the radial arm maze (reference and working memory version). The 5,7-DHT-induced lesion of hippocampal serotonin had no effect by itself on either task. However, it augmented the reference memory impairment caused by the NMDA-induced lesion and delayed the recovery from NMDA-induced impairment of working memory on the radial maze. Combined damage of hippocampal cholinergic and serotonergic afferents did not severely affect spatial memory.

Re-expression of glia-derived nexin/protease nexin 1 depends on mode of lesion-induction or terminal degeneration: observations after excitotoxin or 6-hydroxydopamine lesions of rat substantia nigra

The serine protease inhibitor and neurite outgrowth promoter glia derived nexin (GDN) is expressed in the rat CNS during embryogenesis and persists in the olfactory system of the adult where receptor neurons are replaced throughout life. We investigated whether GDN-immunoreactivity also appears in the adult at sites of synaptic rearrangement following nerve cell death and anterograde terminal degeneration in experimental models for Parkinson's disease. Rat substantia nigra was unilaterally lesioned by stereotaxic application of different toxins: 6-hydroxydopamine, which selectively destroys dopaminergic neurons, the excitotoxic glutamate analog ibotenic acid, or the glutamate receptor agonists N-methyl-D-aspartate and quisqualate, which cause circumscript lesions of the whole substantia nigra. Nerve cell death and astroglial reactivity were monitored by parallel cresyl staining and immunocytochemistry for glial fibrillary acidic protein, at survival times ranging from 2 to 100 days. Sustained de novo synthesis of GDN occurred in the dopamine depleted caudate putamen following excitotoxin or 6-hydroxydopamine induced degeneration of the substantia nigra and of the nigrostriatal pathway provided that the lesions were nearly complete. This is consistent with compensatory changes occurring in deafferented caudate putamen and suggests a permissive role of GDN in neuronal plasticity. In the substantia nigra astroglia exhibited GDN-immunoreactivity following excitotoxin injection but not after application of 6-hydroxydopamine. Thus differences in action mechanisms of neurotoxins may have distinct consequences on the astrocyte mediated response of the same affected brain region.

Visualization of dendritic morphology of cortical projection neurons by retrograde axonal tracing

Currently there is no reliable retrograde tracing technique for visualization of dendritic morphologies of projection neurons. Here we describe a simple and efficient method that can be used to label neurons in Golgi-like fashion. The approach relies on activity-dependent uptake of tracer. For this purpose we inject the glutamate receptor agonist N-methyl-D,L-aspartic acid (NMDA) at the tracer injection site to massively stimulate neurons and to thereby promote uptake of biocytin or biotinylated dextran amine (BDA) by axon terminals. The results show that co-injections of NMDA/biocytin and NMDA/BDA into the extrastriate lateromedial area (LM) of rat visual cortex labels large numbers of neurons in area 17 in Golgi-like fashion. Similarly injections of the lateral geniculate nucleus (LGN) lead to Golgi-like labeling of corticogeniculate neurons in area 17. The distribution of labeled neurons is highly topographic. In addition the method allows excellent preservation of ultrastructure, indicating that this approach is useful for determining the organization of neuronal circuits within the central nervous system.

Monosialoganglioside GM1 reduces NMDA neurotoxicity in neonatal rat brain

Monosialoganglioside GM1 prevents excitatory amino acid (EAA)-related neuronal death in cultured central nervous system (CNS) neurons and reduces the severity of acute brain damage in different experimental models of cerebral ischemia. Using a model of brain damage induced by intracerebroventricular administration of N-methyl-D-aspartate (NMDA) in neonate rats, we evaluated whether GM1 is capable of exerting antiexcitotoxic effects following its systemic administration in vivo. Newborn rats subjected to brain damage by NMDA and contemporaneously treated subcutaneously with GM1 showed significantly reduced (i) loss in hemispheric weight, (ii) loss in tissue choline acetyltransferase activity, and (iii) morphological damage in various brain areas. These results indicate that systemic GM1 treatment is efficacious in reducing EAA-related neuronal damage in vivo and suggest that such a phenomenon may underlie its capability to ameliorate neurological outcome following cerebral ischemia.

Sources of presumptive glutamatergic/aspartatergic afferents to the magnocellular basal forebrain in the rat

The distribution of presumptive glutamatergic and/or aspartatergic neurons retrogradely labeled following injections of [3H]-D-aspartate into the magnocellular basal forebrain of the rat was compared with the distribution of neurons labeled by comparable injections of the nonspecific retrograde axonal tracer wheat germ agglutinin conjugated to horseradish peroxidase. Cells retrogradely labeled by wheat germ agglutinin-horseradish peroxidase were found in a wide range of limbic and limbic-related structures in the forebrain and brainstem. In the telencephalon, labeled neurons were seen in the orbital, medial prefrontal, and agranular insular cortical areas, the amygdaloid complex, and the hippocampal formation. Labeled cells were also seen in the olfactory cortex, the lateral septum, the ventral striatopallidal region, and the magnocellular basal forebrain itself. In the diencephalon, neurons were labeled in the midline nuclear complex of the thalamus, the lateral habenular nucleus, and the hypothalamus. In the brainstem, labeled cells were found bilaterally in the ventral midbrain, the central gray, the reticular formation, the parabrachial nuclei, the raphe nuclei, the laterodorsal tegmental nucleus, and the locus coeruleus. A significant fraction of the afferents to the magnocellular basal forebrain appear to be glutamatergic and/or aspartatergic. Only a few of the regions labeled with wheat germ agglutinin-horseradish peroxidase were not also labeled with [3H]-D-aspartate in the comparable experiments. Most prominent among the non-glutamatergic/aspartatergic projections were those from fields CA1 and CA3 of the hippocampus, the hilus of the dentate gyrus, the dorsal subiculum, the tuberomammillary nucleus, and the ventral pallidum. In addition, most of the lateral hypothalamic and brainstem projections to the magnocellular basal forebrain were not significantly labeled with [3H]-D-aspartate. In addition to these inputs, a commissural projection from the region of the contralateral nucleus of the horizontal limb of the diagonal band was confirmed with both wheat germ agglutinin-horseradish peroxidase and the anterograde axonal tracer Phaseolus vulgaris leucoagglutinin. This projection did not label with [3H]-D-aspartate or [3H]-GABA, suggesting that it is not glutamatergic/aspartatergic or GABAergic. Furthermore, double labeling experiments with the fluorescent retrograde tracer True Blue and antibodies against choline acetyltransferase indicate that the projection is not cholinergic.

MK-801 pretreatment enhances N-methyl-D-aspartate-mediated brain injury and increases brain N-methyl-D-aspartate recognition site binding in rats

Direct intracerebral administration of N-methyl-D-aspartate typically produces focal brain injury. (+)-5-Methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-immi ne maleate (MK-801), a non-competitive N-methyl-D-aspartate antagonist, can protect against N-methyl-D-aspartate-mediated brain injury when administered shortly before or after an intracerebral injection of N-methyl-D-aspartate. However, in this study we report that in perinatal rats if MK-801 (1 mg/kg) is administered intraperitoneally 24 h prior to a unilateral intrastriatal N-methyl-D-aspartate injection, N-methyl-D-aspartate-mediated brain injury is paradoxically enhanced. The severity of resulting brain injury is 15-25% greater in groups that received MK-801 in comparison with saline-treated controls (P less than 0.001, linear regression analysis). In contrast, the severity of brain injury resulting from intrastriatal injection of the glutamate agonist quisqualate is not altered by a similar 24 h MK-801 pretreatment. Furthermore, the enhanced toxicity of N-methyl-D-aspartate produced by a 24 h pretreatment with MK-801 is completely blocked if a second dose of MK-801 is administered 15 min after the intrastriatal injection of N-methyl-D-aspartate. To determine if MK-801 produced alterations in glutamate receptor pharmacology co-incident with the enhanced toxicity of N-methyl-D-aspartate, in vitro quantitative autoradiography for excitatory amino acid receptor subtypes was performed with [3H]glutamate and [3H]N-1-(2-thienyl)cyclohexyl-3,4-piperidine in seven-day-old rats killed 2 or 24 h after MK-801 (1 mg/kg) administration. A 2 h MK-801 pretreatment produced a 30-50% increase in [3H]glutamate binding at N-methyl-D-aspartate preferring recognition sites in all four brain regions examined (areas CA1 and CA3 of the hippocampus, corpus striatum, cingulate cortex) in comparison with saline-treated controls (P less than 0.05, ANOVA). [3H]N-1-(2-Thienyl)cyclohexyl-3,4-piperidine binding to the phencyclidine site associated with the N-methyl-D-aspartate receptor was reduced by 60-80% in all brain regions examined (P less than 0.001). Quisqualate-sensitive [3H]glutamate binding was not altered by a 2 h MK-801 pretreatment. In animals that received a 24 h MK-801 pretreatment.(ABSTRACT TRUNCATED AT 400 WORDS)

Systemic administration of MK-801 protects against N-methyl-D-aspartate- and quisqualate-mediated neurotoxicity in perinatal rats

MK-801, a non-competitive antagonist of N-methyl-D-aspartate-type glutamate receptors, was tested for its ability to antagonize excitotoxic actions of N-methyl-D-aspartate or quisqualic acid injected into the brains of seven-day-old rats. Stereotaxic injection of N-methyl-D-aspartate (25 nmol/0.5 microliters) or quisqualic acid (100 nmol/1.0 microliter) into the corpus striatum under ether anesthesia consistently produced severe unilateral neuronal necrosis in the basal ganglia, dorsal hippocampus and overlying neocortex. The distribution of the damage corresponded to the topography of glutamate receptors in the vulnerable regions demonstrated by previous autoradiographic studies. N-Methyl-D-aspartate produced severe, confluent neuronal destruction while quisqualic acid typically caused more selective neuronal necrosis. Intraperitoneal administration of MK-801 (0.1-1.0 mg/kg) 30 min before N-methyl-D-aspartate injection had a prominent dose-dependent neuroprotective effects as assessed morphometrically by comparison of bilateral striatal, hippocampal and cerebral hemisphere cross-sectional areas five days later. A 1 mg/kg dose of MK-801 given as pre-treatment completely protected the infant brain. The same dose of MK-801 was also completely protective when administered 30 or 40 min after N-methyl-D-aspartate and afforded partial protection when given 2 h later. MK-801 pre-treatment also prevented the electrically confirmed behavioral seizures induced by N-methyl-D-aspartate. The drug significantly reduced striatal but not hippocampal or neocortical injury when given as two doses (1 mg/kg) 30 min prior to and immediately following quisqualic acid injection. The data indicate that systemic administration of MK-801 can prevent N-methyl-D-aspartate-induced neuronal injury in perinatal rat brain even when administered after the initial insult. MK-801 also partially antagonized quisqualic acid-mediated neurotoxicity, suggesting that quisqualic acid-induced toxicity is, in part, mediated through N-methyl-D-aspartate receptor activation. The sensitivity of the developing brain to the toxicity of N-methyl-D-aspartate provides a sensitive and reproducible in vivo model for exploring these issues and for screening prospective neuroprotective drugs that act at the N-methyl-D-aspartate receptor-channel complex.

Physiological and pathophysiological roles of excitatory amino acids during central nervous system development

Recent studies suggest that excitatory amino acids (EAAs) have a wide variety of physiological and pathophysiological roles during central nervous system (CNS) development. In addition to participating in neuronal signal transduction, EAAs also exert trophic influences affecting neuronal survival, growth and differentiation during restricted developmental periods. EAAs also participate in the development and maintenance of neuronal circuitry and regulate several forms of activity-dependent synaptic plasticity such as LTP and segregation of converging retinal inputs to tectum and visual cortex. Pre- and post-synaptic markers of EAA pathways in brain undergo marked ontogenic changes. These markers are commonly overexpressed during development; periods of overproduction often coincide with times when synaptic plasticity is great and when appropriate neuronal connections are consolidated. The electrophysiological and biochemical properties of EAA receptors also undergo marked ontogenic changes. In addition to these physiological roles of EAAs, overactivation of EAA receptors may initiate a cascade of cellular events which produce neuronal injury and death. There is a unique developmental profile of susceptibility of the brain to excitotoxic injury mediated by activation of each of the EAA receptor subtypes. Overactivation of EAA receptors is implicated in the pathophysiology of brain injury in several clinical disorders to which the developing brain is susceptible, including hypoxia-ischemia, epilepsy, physical trauma and some rare genetic abnormalities of amino acid metabolism. Potential therapeutic approaches may be rationally devised based on recent information about the developmental regulation of EAA receptors and their involvement in the pathogenesis of these disorders.

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 (r2 = 0.97).(ABSTRACT TRUNCATED AT 250 WORDS)

The olfactory system and Alzheimer's disease

Alzheimer's disease (AD) is considered to be the number one health problem and seems to be reaching epidemic proportion in the USA. The cause of AD is not known, a reliable animal model of the disease has not been found and appropriate treatment of this dementia is wanting. The present review focuses on the possibility that a virus or exogenous toxic materials may gain access to the CNS using the olfactory mucosa as a portal of entry. Anterograde and retrograde transport of the virus/zeolites to olfactory forebrain regions, which receive primary and secondary projections from the main olfactory bulb (MOB) and which, in turn, project centrifugal axons to the MOB, may initiate cell degeneration at such loci. Pathological changes may, thus, be initially confined to projecting and intrinsic neurons localized in cortical and subcortical olfactory structures; arguments are advanced which favor the view that excitotoxic phenomena could be mainly responsible for the overall degenerative picture. Neurotoxic activity may follow infection by the virus itself, be facilitated by loss of GABAergic terminals in olfactory cortex, develop following repeated episodes of physiological long term potentiation (which unmasks NMDA receptors) or be due to excessive release, faculty re-uptake or altered glutamate receptor sensitivity. Furthermore, a reduction in central inhibitory inputs to the MOB might then result in disinhibition of mitral/tufted neurons and enhance the excitotoxic phenomena in the MOB projecting field. Within this context, and in line with recent studies, it is believed that pathology begins at cortical (mainly olfactory) regions, basal forebrain neurons being secondarily affected due to retrograde degeneration. In addition, failure to produce a critical level of neurotrophic factors by a damaged MOB and olfactory cortex, could adversely affect survival of basal cholinergic neurons which innervate both regions. Support for these hypothesis is provided, first, by recent reports on pathological findings in AD brains which seem to involve preferentially the olfactory and entorhinal cortices, the olfactory amygdala and the hippocampus, all of which receive primary or secondary projections from the MOB; secondly, by the presence of severe olfactory deficits in the early stages of the disease, mainly of a discriminatory nature, which points to a malfunction of central olfactory structures.

Infusion of NMDA into the nucleus basalis of Meynert, frontal cortex or lateral ventricle in rats: effect on memory and cholinergic brain neurochemistry

The present study's aim was to examine the behavioral and neurochemical effects of damage limited to intrinsic neurons of the frontal cortex in rats. Specifically, it was of interest to evaluate the effects of N-methyl-D-aspartic acid-induced lesions of discrete frontal cortical loci on passive avoidance memory and on cortical cholinergic neurochemical markers (choline acetyltransferase--CAT and acetylcholinesterase--ACHE). The present study also compared the behavioral and neurochemical effects produced by frontal cortical damage with those effects produced by lesions of the nucleus basalis of Meynert (nbM). Results indicated that nbM lesions and lesions to a rostral frontal cortical site produced severe passive avoidance memory impairments when subjects were tested 72 hours after training. Cortical, but not hippocampal, levels of CAT and ACHE were depleted in nbM animals only. These data were interpreted as providing support for the view that intrinsic frontal cortical neurons contribute to memory.

Basal forebrain lesions impair tactile discrimination and working memory

Rats received bilateral injections of the excitotoxin, N-methyl-D,L aspartate, which resulted in degeneration of basal forebrain cholinergic (BFC) neurons in the nucleus basalis magnocellularis. Most tests of general neurological function revealed no differences between control rats and those with BFC lesions and where differences were found they appeared to be due to hyperemotionality. Rats with BFC lesions demonstrated significant deficits in working memory, as evaluated in an 8-arm radial maze. In addition, these rats showed a severe impairment in tactile discrimination learning, an effect of BFC lesions not previously demonstrated. We propose that cholinergic deafferentation of the somatosensory cortex with consequent disruption in somatosensory information processing might account at least in part for this effect.

Comparison of behavioral effects of nucleus basalis magnocellularis lesions and somatosensory cortex ablation in the rat

Cholinergic neurons in the nucleus basalis region of the forebrain project to various portions of the cerebral cortex, including somatosensory cortex. Degeneration of these neurons and their cortical projections is a major feature of the neuropathology of Alzheimer's disease. Injecting an excitotoxin into the basal forebrain to destroy nucleus basalis neurons provides a potentially useful animal model for studying the role of these neurons in Alzheimer's disease. Previously, we demonstrated that rats with nucleus basalis excitotoxin lesions performed poorly on a tactile discrimination task and on a test of working memory. In an effort to clarify further the role of impaired memory versus other types of impairment (e.g. disrupted somatosensory processing due to cholinergic deafferentation of somatosensory cortex), we compared a group of rats with bilateral nucleus basalis excitotoxin lesions and a group with bilateral somatosensory cortical ablations on a variety of behavioral tasks. Rats with nucleus basalis lesions performed as well as controls on a battery of neurological tests but exhibited increased emotionality unlike rats with somatosensory cortical ablations which performed poorly on the battery but were not hyperemotional. The two lesion groups were impaired significantly and to a comparable degree in performing two-choice tactile discriminations in a T-maze. In contrast, only rats with nucleus basalis lesions showed deficits in working memory as tested in an eight-arm radial maze. Both lesion groups performed comparably to sham controls on a test of reference memory involving a black/white discrimination in a T-maze. The findings suggest that rats with nucleus basalis lesions manifest disturbances in several of the same spheres (emotionality, somatosensory information processing, memory) that are disrupted in Alzheimer's disease and further confirm the utility of the excitotoxin lesion approach for studying the pathophysiology of Alzheimer's disease.

Neurotoxicity of N-methyl-D-aspartate is markedly enhanced in developing rat central nervous system

The neurotoxic lesion produced by direct injection of 25 nmol of N-methyl-D-aspartate (NMDA) into the corpus striatum of 7-day-old rats was compared to the effects of injecting 75 nmol into the striatum or hippocampus of adults. The area of histopathology in the immature striatum was 21 X larger than the striatal lesion in adults. Damage from NMDA injected into the immature striatum also extended into the dorsal hippocampus and produced an area of destruction which was 16 X larger than observed after direct injection into the adult hippocampus. Several studies have implicated excessive N-methyl-D-aspartate receptor activation in the pathogenesis of hypoxic-ischemic and hypoglycemic injury and our results suggest that this neurotoxic mechanism is extremely active in the immature brain.

The role of basal forebrain in the primary cholinergic vasodilation in rat neocortex produced by systemic administration of cismethrin

The pyrethroid insecticide cismethrin (9 mumol/kg) causes a large blood flow increase in cerebral cortex, without a parallel increase in metabolism. A unilateral lesion of the basal forebrain attenuated the blood flow increase in the cortex ipsilateral to the lesion but augmented that in the contralateral cortex. Cortical choline acetyltransferase was similarly affected. Atropine sulphate substantially reduced the flow increase and was additive to the lesion effects. Systemic cismethrin is thus capable of activating a cholinergic vasodilation in the cortex and, in the parietal cortex at least, a substantial proportion of the flow increase is mediated by extrinsic projections from the basal forebrain.

Sources of presumptive glutamergic/aspartergic afferents to the rat ventral striatopallidal region

The distribution of presumptive glutamergic and/or aspartergic neurons retrogradely labeled following injections of 3H-D-aspartate (3H-D-Asp) into the ventral striatopallidal region was compared with the distribution of neurons labeled by comparable injections of wheat germ agglutinin-horseradish peroxidase (WGA-HRP). The afferents labeled by 3H-D-Asp were a subset of those labeled by WGA-HRP. The major sources of afferents to the nucleus accumbens and olfactory tubercle that could be labeled by 3H-D-Asp were in the medial frontal and insular cortices; the olfactory cortex; the lateral, basolateral, and basomedial amygdaloid nuclei; and the midline nuclear complex of the thalamus. The corresponding afferents to the ventral pallidum arose in the central, medial, and basomedial amygdaloid nuclei and the midline thalamic nuclei. In addition, the nucleus of the lateral olfactory tract was moderately or heavily labeled by 3H-D-Asp injections into all three areas, and cells were labeled in the subiculum following injection in the anteromedial part of the nucleus accumbens. Conversely the ventral striatopallidal structures themselves were, at best, sparsely labeled by any of the 3H-D-Asp injections. Neurons in the substantia nigra, ventral tegmental area, dorsal raphe, and locus coeruleus were labeled by WGA-HRP but not by 3H-D-Asp, except for an occasional cell in the raphe. The results indicate that 3H-D-Asp is a specific retrograde tracer and suggest that there are widespread, presumably excitatory, glutamergic and/or aspartergic inputs to the ventral striatum and pallidum.