Significance of the plasma membrane for the nerve cell function, development and plasticity.

Lipoid character of plasma membrane namely the presence of polyenic fatty acids enables to interact with membrane proteins and in certain extent also to modulate their function. During the development, molecules of membrane fatty acids become more and more complex, and the ratio of polyenic fatty acids/saturated fatty acids in the brain rises, while the concentration of monoenic fatty acids remained relatively stable. This phenomenon is apparent also in the ratio of unsaturated fatty acids OMEGA-3 in plasma of newborns which correlates with the birth weight. Plasma membrane reflects local specializations of nerve cells. Its composition varies in functionally specialized regions called domains. Specialized domains of nerve cells determine the function of dendrites, soma, axon, axon hillock ect. Premature weaning of laboratory rats results in structural changes and in the increase of excitability of neuronal circuits in hypothalamus, septum and hippocampus which indicate the possibility of membrane composition changes. In synapses, transport proteins of synaptic vesicles, act together with the specific proteins of the presynaptic membrane. Membrane proteins determine the release of neurotransmitter at different conditions of synaptic activity, and they can contribute to the recovery of neurotransmitter content after the repeated hyperactivity. In the model of experimental kindling, repeated seizures bring about decreases and distribution changes of synaptic vesicles.

Neonatal administration of N-acetyl-L-aspartyl-L-glutamate induces early neurodegeneration in hippocampus and alters behaviour in young adult rats.

N-acetyl-L-aspartyl-L-glutamate (NAAG) is a dipeptide that could be considered a sequestered form of L-glutamate. As much as 25% of L-glutamate in brain may be present in the form of NAAG. NAAG is also one of the most abundant neuroactive small molecules in the CNS: it is an agonist at Group II metabotropic glutamate receptors (mGluR II) and, at higher concentrations, at the N-methyl-D-aspartate (NMDA) type of ionotropic glutamate receptors. As such, NAAG can be either neuroprotective or neurotoxic and, in fact, both characteristics have been discussed and described in the literature. In the present studies, 250 nmol NAAG was infused into each lateral cerebral ventricle of 12-day-old rat pups and, using Nissl-stained sections, neurodegeneration in the hippocampus was evaluated 24 or 96 h after the infusion. In several experiments, the neuronal death was also visualised by Fluoro-Jade B staining and studied by TUNEL technique. Some of the NAAG-treated animals were allowed to survive until 50 days post partum and subjected to behavioural (open field) tests. The administration of NAAG to 12-day-old rats resulted in extensive death of neurons particularly in the dentate gyrus of the hippocampus. The neurodegeneration was, in part, prevented by administration of an NMDA receptor antagonist MK-801 (0.1 mg/kg). The nuclear DNA-fragmentation demonstrated by TUNEL technique pointed to the presence of non-specific single-strand DNA cleavage. The NAAG-associated neonatal neuronal damage may have perturbed development of synaptic circuitry during adolescence as indicated by an altered performance of the experimental animals in the open field testing (changes in grooming activity) at postnatal day 50. The results underscore the potential neurotoxicity of NAAG in neonatal rat brain and implicate neonatally induced, NMDA receptor-mediated neuronal loss in the development of abnormal behaviour in young adult rats.

[Kainic acid and neurobiology]. / Kainát a neurobiologie.

Kainic acid, the analog of excitatory amino acid L-glutamate, interacts with specific receptors in the central nervous system. During last 25 years it has become a tool for studying many human brain disorders, for example human temporal lobe epilepsy, Huntington's chorea etc. Systemic administration of kainic acid results in neuronal death in experimental animals. The mechanism, by which kainic acid produces neuronal damage is still unclear, as well as physiological function of kainate receptors remain to be elucidate. This review attempts to survey the major achievements reached in the studies, which were publicized throw the last three decades.

Neuronal cell death in hippocampus induced by homocysteic acid in immature rats.

PURPOSE: To examine the morphologic alterations in the cerebral cortex and hippocampus of immature rats 6 days after the generalized clonic-tonic seizures induced by homocysteic acid (HCA). METHODS: Seizures were induced by bilateral intracerebroventricular infusion of HCA (600 nmol per each side) in 12-day-old rats. After 6 days, rat pups were transcardially perfused under deep ether anesthesia with heparinized normal saline and subsequently with the fixation solution (4% paraformaldehyde in phosphate buffer, pH 7.4, for light microscopy) or with Karnovsky's solution (4% paraformaldehyde and 2% glutaraldehyde in phosphate buffer, pH 7.4, for electron microscopic analysis). Nissl stain and the DNA-specific dye bis-benzimide (Hoechst 33342) were used. RESULTS: No pathologic changes were found in the cerebral cortex, whereas serious alterations occurred in the hippocampus. A total loss of CA3 pyramidal cells was observed, with marked changes in the CA1 region and dentate gyrus. A prominent glial reaction was seen in many regions of the hippocampal formation. A slight dilatation of the cerebral ventricles was noticed in some experimental as well as control animals. In the granule cell layer of the dentate gyrus, neurons with segmented or fragmented nuclei in various stages of degeneration were detected, displaying the features of apoptotic death. CONCLUSIONS: These findings demonstrate the vulnerability of the immature rat brain, which most likely reflects both the direct neurotoxic effect of HCA and prolonged seizure activity. The relative contribution of these two factors still remains to be assessed.

Nonconvulsive Seizures Result in Behavioral but Not Electrophysiological Changes in Developing Rats.

It is not known if nonconvulsive seizures lead to functional or morphological changes in immature rats. Therefore we studied consequences of such seizures induced by kainic acid (KA) on Postnatal Day (PD) 12 (2 mg/kg ip). The animals were examined electrophysiologically (cortical epileptic afterdischarges (ADs) were elicited in rats with implanted electrodes on PD 14, 18 or 25) and behaviorally (open field was studied in another group of animals on PDs 18 and 25). Hippocampal and cortical morphology was checked by light microscopy (Nissl staining) on PDs 18 and/or 25. Another group of rats was injected with a 6 mg/kg dose of KA on PD 18 and examined on PD 25. The dose of KA used induced only nonconvulsive seizures characterized by automatisms (scratching on PD 12, wet dog shakes on PD 18). Cortical ADs in animals stimulated on PD 14, 18, or 25 did not differ from those in control rats. KA-Treated rats exposed to open field two times (on PDs 18 and 25) exhibited more exploratory activities during the second exposure than control animals. A similar difference was noted in PD 25 rats injected with KA on PD 18. Qualitative histology did not reveal any obvious neuronal damage in hippocampus and cortex. These results demonstrate that nonconvulsive seizures induced at early developmental stages that do not result in observable electrophysiological and morphological changes may have delayed behavioral consequences.