[3H]MK-801 binding in immature and mature chicken forebrain

Excitotoxic stimulation of brain microslices as an in vitro model of stroke

Examining molecular mechanisms involved in neuropathological conditions, such as ischemic stroke, can be difficult when using whole animal systems. As such, primary or 'neuronal-like' cell culture systems are commonly utilized. While these systems are relatively easy to work with, and are useful model systems in which various functional outcomes (such as cell death) can be readily quantified, the examined outcomes and pathways in cultured immature neurons (such as excitotoxicity-mediated cell death pathways) are not necessarily the same as those observed in mature brain, or in intact tissue. Therefore, there is the need to develop models in which cellular mechanisms in mature neural tissue can be examined. We have developed an in vitro technique that can be used to investigate a variety of molecular pathways in intact nervous tissue. The technique described herein utilizes rat cortical tissue, but this technique can be adapted to use tissue from a variety of species (such as mouse, rabbit, guinea pig, and chicken) or brain regions (for example, hippocampus, striatum, etc.). Additionally, a variety of stimulations/treatments can be used (for example, excitotoxic, administration of inhibitors, etc.). In conclusion, the brain slice model described herein can be used to examine a variety of molecular mechanisms involved in excitotoxicity-mediated brain injury.

Biochemical, behavioural and electrophysiological investigations of brain maturation in chickens

It is convenient to divide the development of synaptic networks into two phases: synapse formation during which synaptic contacts are established, and a subsequent maturation phase during which synaptic circuits are fine tuned and the properties of individual synapses are modified. Understanding the complex factors that control the protracted maturation process in humans is likely to be important for understanding a variety of neurological and psychiatric disorders. Chickens provide an ideal experimental model in which maturation specific changes can be identified and the mechanisms controlling them can be elucidated because the maturation phase is protracted and temporally separated from the formation phase. This paper reviews the knowledge about the biological mechanisms involved in the maturation phase of brain development in chickens and presents some new data. Studies of synaptic physiology suggest that maturation may alter the basal set point for stimulus induced synaptic plasticity. Biochemical and pharmacological studies of N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and metabotropic glutamate receptors (mGluRs) revealed major changes in receptor regulation and the intracellular signalling pathways linked to receptor activation. Not surprisingly, therefore, when immature or mature chickens learn the same behavioural task the learning induced molecular events at the synapse are different. Changes in the features of auditory event related potentials and the basal EEG provide non-invasive techniques for monitoring maturation changes in chicken brain but prepulse inhibition (PPI) is too small and variable in chickens to be useful. Experimentally induced mild late-onset hypothyroidism retards some aspects of brain maturation and may help identify some of the mechanisms controlling maturation.

Maturational Changes in the Subunit Composition of AMPA Receptors and the Functional Consequences of Their Activation in Chicken Forebrain

AMPA receptors play a critical role in synaptic plasticity and brain development. Here we show that Ca(2+) uptake in response to AMPA receptor activation decreases dramatically during maturation in chicken brain microslices without a change in tissue AMPA receptor content. We found that during maturation the relative concentration of GluR2 subunits increased, the concentration of the AMPA receptor-associated scaffold proteins SAP97 and GRIP decreased and that depolarization increased GluR1 phosphorylation at Ser831 in subcellular fractions enriched in postsynaptic densities at 2 weeks but not at 10 weeks. These changes are all consistent with a decreased Ca(2+) entry through AMPA receptor channels in response to receptor activation and may account for the changes in the functional properties of the receptor, which are thought to underlie, at least in part, the physiological changes that occur with maturation.

Spermine modulation of the glutamate(NMDA) receptor is differentially responsive to conantokins in normal and Alzheimer's disease human cerebral cortex

The pharmacology of the N -methyl-d-aspartate (NMDA) receptor site was examined in pathologically affected and relatively spared regions of cerebral cortex tissue obtained at autopsy from Alzheimer's disease cases and matched controls. The affinity and density of the [(3)H]MK-801 binding site were delineated along with the enhancement of [(3)H]MK-801 binding by glutamate and spermine. Maximal enhancement induced by either ligand was regionally variable; glutamate-mediated maximal enhancement was higher in controls than in Alzheimer's cases in pathologically spared regions, whereas spermine-mediated maximal enhancement was higher in controls in areas susceptible to pathological damage. These and other data suggest that the subunit composition of NMDA receptors may be locally variable. Studies with modified conantokin-G (con-G) peptides showed that Ala(7)-con-G had higher affinity than Lys(7)-con-G, and also defined two distinct binding sites in controls. Nevertheless, the affinity for Lys(7)-con-G was higher overall in Alzheimer's brain than in control brain, whereas the reverse was true for Ala(7)-con-G. Over-excitation mediated by specific NMDA receptors might contribute to localized brain damage in Alzheimer's disease. Modified conantokins are useful for identifying the NMDA receptors involved, and may have potential as protective agents.

A quantitative autoradiographic comparison of binding to glutamate receptor sub-types in hippocampus and forebrain regions of a food-storing and a non-food-storing bird

In two species of birds, food-storing marsh tits, P. palustris, and non-storing blue tits, P. caeruleus, quantitative receptor autoradiography was used to localize NMDA (N-methyl-D-aspartate)-sensitive [3H]glutamate, [3H]MK801, and [3H]AMPA binding sites, in six regions of the forebrain: hippocampus and parahippocampus, hyperstriatum accessorium (vision) and ventrale (sensory integration), neostriatum (auditory), and lobus parolfactorius (basal ganglia). In both species high levels of labelling to both NMDA and AMPA receptors were observed throughout the forebrain. However, a marked difference in receptor labelling was apparent between the two species, with levels of binding to NMDA ion channel sites being significantly lower (20%) in both the hippocampus and parahippocampus, in food storers compared to non-food storers. The levels of binding to other forebrain regions were remarkably similar in the two species. No differences were seen in the binding to AMPA receptors in forebrain regions of either species.

Exposure to light prior to hatching induces asymmetry of receptor binding in specific regions of the chick forebrain

This paper describes neurochemical asymmetries present in forebrain regions of the newly hatched chick that result from environmental conditions; specifically from asymmetrical exposure of the chick embryo to light prior to hatching. Quantitative autoradiography was used to determine GABA and glutamate receptor subtype binding in a number of regions of the left and right forebrain hemispheres of chicks that had either the left (LES), or the right (RES), eye system exposed to light prior to hatching. On day 19 of incubation the embryo's head was withdrawn from the egg and the left or the right eye was occulded until hatching. [3H]MK-801, [3H]AMPA and [3H]muscimol binding assays were performed on frozen sections from 2 different coronal regions of the forebrain, sampled on day-1 posthatching. Significant [3H]MK-801, [3H]AMPA and [3H]muscimol binding asymmetries were determined in forebrain regions from chicks that had their RES exposed to light prior to hatching, particularly in forebrain regions which are known to receive afferent visual input. The reverse pattern of asymmetry was found for all 3 ligands in regions such as the ectostriatum of chicks that had their LES exposed to light, while asymmetry of muscimol and AMPA binding, present in many regions in right eye system chicks was not present in chicks that had the left eye system exposed to light during incubation. Thus, the presence and pattern of experience-dependent neurochemical asymmetries in the chick forebrain are specific to both region and receptor type.

CGP 39653 binding in the chick CNS after NMDA receptor antagonist treatment

To analyze the affect of blocking neuronal activity on NMDA receptor levels during development, we have injected chick embryos with the competitive NMDA receptor antagonist NPC 12626 from E17 to E19. Brains from drug-treated (n = 7) and control (n = 6) embryos (E20) were processed for receptor autoradiography using the NMDA competitive antagonist [3H]CGP 39653. NPC 12626 treatment caused a significant 24 to 46% increase in [3H]CGP 39653 binding in both the forebrain and cerebellar cortex. The results support the hypothesis that NMDA receptor levels are regulated by activity-dependent mechanisms.

[3H]MK-801 binding asymmetry in the IMHV region of dark-reared chicks is reversed by imprinting

Glutamate NMDA-type receptor binding in the intermediate medial hyperstriatum ventrale (IMHV) of dark-hatched chicks is lateralized. This lateralization was found to be markedly influenced by imprinting. In dark-reared chicks the binding of the selective ligand [3H]MK-801 was two-fold higher in the right IMHV than in the left IMHV. In contrast, imprinted chicks have significantly higher levels of [3H]MK-801 binding in the left IMHV region than in the right IMHV. Imprinting results in 2a learning-related increase in NMDA-type receptor binding levels in the left IMHV, whereas [3H]MK-801 binding levels in the right IMHV remain unchanged by imprinting. Thus, the plasticity present in the NMDA-type receptor system and associated with imprinting appears to occur in the left hemisphere only.

Long-term potentiation in the avian hippocampus does not require activation of the N-methyl-D-aspartate (NMDA) receptor

Evoked potentials recorded from slices of pigeon hippocampus were reversibly attenuated by 2 mM kynurenic acid. High frequency stimulation (3 x 200 Hz for 1 sec, with 1 sec intervals) evoked a persistent increase in the evoked potential, lasting in a nondecremental form for at least 2 hr. This increase in the magnitude of the potential was not blocked by antagonists of the NMDA receptor (APV and MK-801). These data suggest that the synaptic facilitation observed in the pigeon hippocampus, which appears to be a form of long-term potentiation, does not depend on the activation of NMDA receptors.

Mechanisms of synaptic plasticity. Changes in postsynaptic densities and glutamate receptors in chicken forebrain during maturation

We have shown that the synapse maturation phase of synaptogenesis is a model for synaptic plasticity that can be particularly well-studied in chicken forebrain because for most forebrain synapses, the maturation changes occur slowly and are temporally well-separated from the synapse formation phase. We have used the synapse maturation phase of neuronal development in chicken forebrain to investigate the possible link between changes in the morphology and biochemical composition of the postsynaptic density (PSD) and the functional properties of glutamate receptors overlying the PSD. Morphometric studies of PSDs in forebrains and superior cervical ganglia of chickens and rats have shown that the morphological features of synapse maturation are characteristic of a synaptic type, but that the rate at which these changes occur can vary between types of synapses within one animal and between synapses of the same type in different species. We have investigated, during maturation in the chicken forebrain, the properties of the N-methyl-D-aspartate (NMDA) subtype of the glutamate receptors, which are concentrated in the junctional membranes overlying thick PSDs in the adult. There was no change in the number of NMDA receptors during maturation, but there was an increase in the rate of NMDA-stimulated uptake of 45Ca2+ into brain prisms. This functional change was not seen with the other ionotropic subtypes of the glutamate receptor and was NMDA receptor-mediated. The functional change also correlated with the increase in thickness of the PSD during maturation that has previously been shown to be due to an increase in the amount of PSD associated Ca(2+)-calmodulin stimulated protein kinase II (CaM-PK II). Our results provide strong circumstantial evidence for the regulation of NMDA receptors by the PSD and implicate changing local concentrations of CaM-PK II in this process. The results also indicate some of the ways in which properties of existing synapses can be modified by changes at the molecular level.