Glutamate-dependent mechanisms in the induction of a calcium long-term potentiation-like phenomenon

Long-term potentiation in the optic tectum of rainbow trout

We examined synaptic plasticity in the optic tectum of rainbow trout by extracellular recordings. We found that the field-excitatory postsynaptic potential in the retinotectal synapses was potentiated by repetitive stimuli of 1.0 Hz for 20 s to the retinotectal afferents. The long-term potentiation (LTP) developed slowly, and was maintained for at least 2 h. Applications of an antagonist for N-methyl-D-aspartic acid (NMDA) receptors or Mg2+ -free saline showed that activation of NMDA receptors was required to form the LTP beyond the induction period. The present findings indicate that presynaptic stimulation in the retinotectal synapses causes LTP mediated by NMDA receptors in the optic tectum of rainbow trout.

Chemically-induced long-term potentiation in rat motor cortex involves activation of extracellular signal-regulated kinase cascade

The involvement of the extracellular signal-regulated kinase 1/2 (ERK1/2) cascade in long-lasting potentiation of synaptic transmission, induced by tetraethylammonium (TEA) or by elevated extracellular calcium concentration, was investigated in layer V horizontal connections within motor cortex in rat brain slices. Brief application of TEA (25 mM) resulted in a long-lasting potentiation of field potentials by 54+/-12%. A transient exposure of slices to elevated extracellular calcium (5 mM) induced long-lasting potentiation of responses reaching 30+/-8%. The induction of both forms of potentiation was prevented by the exposure of slices to inhibitors of the upstream activator of ERK 1/2, MEK (ERK kinase), U0126 (20 microM) and PD 98059 (50 microM). PhosphoERK2 immunoreactivity was transiently increased above baseline levels 15 min after termination of the exposure of slices to either TEA or elevated calcium concentration. Both forms of potentiation were partially occluded by Sp-adenosine 3',5'-cyclic monophosphorothioate triethylammonium salt (Sp-cAMPS; 100 microM), an activator of cAMP-dependent protein kinase (PKA), and they were blocked after preincubation with Rp-adenosine 3',5'-cyclic monophosphorothioate triethylammonium salt (Rp-cAMPS; 100 microM), a specific inhibitor of PKA activation by cAMP. It has previously been shown that TEA-induced potentiation represents a N-methyl-d-aspartate (NMDA) receptor-independent form of persistent synaptic enhancement, and, on the contrary, calcium-induced potentiation depends on NMDA receptors. Thus, the activation of PKA and the ERK1/2 cascade are required for two forms of chemically induced long-lasting increases of synaptic efficacy in slices of rat motor cortex.

Calcium-induced long-term potentiation in horizontal connections of rat motor cortex

A transient (10 min) exposure of brain slices of young adult rats to elevated extracellular calcium (5 mM) resulted in a long-lasting potentiation of field potentials evoked in layer II/III and layer V horizontal connections of the primary motor cortex. This form of synaptic plasticity was blocked by D,L-2-amino-5-phosphonovalerate (APV, 100 micro M), an antagonist of NMDA receptors.

The neurochemical basis of learning and neurocomputation: the redox theory

This paper presents a new theory of the biochemical basis of learning and neurocomputation. It has now been determined that excitatory synapses on dendritic spines in the brain are continually being formed and removed. This requires a neurochemical mechanism. There is evidence to suggest that the redox state of the glutamate synapse plays an important role in determining the growth or deletion of that synapse. This redox state is controlled by the balance between the pro-oxidants hydrogen peroxide and the nitric acid radical, and the antioxidants ascorbate, carnosine, the nitrosium ion and catecholamines. Key enzymes involved are prostaglandin H synthase and nitric acid synthase. Mediation of signals of reinforcing stimuli by the catecholamines may be mediated in part by their antioxidant effect on glutamate synapses. Some experiments to test the theory are suggested.

Transient global brain hypoxia-ischemia in adult rats: neuronal damage, glial proliferation, and alterations in inositol phospholipid hydrolysis

A model of ischemic-hypoxic brain injury which combines bilateral occlusion of common carotid arteries for 10 min and mild hypoxia (15% O2 for 10 min before and during occlusion) was developed. Global ischemia was assessed by a simplified EEG recording indicating isoelectric line, i.e. full arrest of cortical electrical activity. Histological examination of brain 7 days after ischemic insult showed from moderate to severe damage, mainly in the cerebral cortex (layers III, V and VI) and hippocampus (mainly CA1 subfield). The injury consisted of neuronal degeneration and necrosis with nuclear pyknosis and karyorrhexis. Immunohistochemical staining for gliofibrillar acidic protein showed a marked glial proliferation in the cerebral cortex and hippocampus. In the cortical slices, inositol phosphates accumulation stimulated by excitatory amino acid agonists (ACPD, ibotenate and quisqualate), as well as by norepinephrine and carbachol, was enhanced significantly (p < 0.01) with respect to sham-operated rats 7 days, but not 24 h, after the ischemic insult. The overall data show that the relatively simple transient brain hypoxia/ischemia rat model produces full arrest of cortical EEG, histopathological alterations and those relative to post-receptor neurochemical mechanisms characteristic of four-vessel occlusion model.