Tetanic stimulation-induced changes in [3H]glutamate binding and uptake in rat hippocampus

Plasticity of excitatory amino acid transporters in experimental epilepsy

PURPOSE

To examine the relationship between seizures and excitatory amino acid transporter (EAAT) activity and whether up-regulation of EAAT activity alters epileptogenicity.

METHODS

In this study, we exposed rat hippocampal slices to different convulsants before measuring EAAT activity. Rats were exposed to the EAAT inhibitor pyrrolidine-2,4-dicarboxylic acid (PDC) before entorhinal cortex/hippocampal slices were obtained. These slices were exposed to low-Mg2+ buffer while electrophysiological recordings were obtained from the entorhinal cortex. mGluR III acting agents were used to study whether activation of mGluR III could regulate EAAT activity and if this regulation could overcome the effects on EAAT activity induced by the convulsants.

RESULTS

Veratridine, kainic acid (KA), and pilocarpine reduced EAAT activity in rat hippocampal slices. L-2-Amino-4-phosphonobutyric acid (an mGluR III agonist) restored EAAT activity and reduced epileptiform activity to near control levels. The saturation curve for glutamate uptake in slices from KA-seized rats killed 2 hours after the first forelimb clonus was displaced to the left, suggesting a compensatory change for the enhanced excitation. On the other hand, rats injected with the EAAT inhibitor PDC (by intracerebroventricular injection) had more severe KA-induced seizures and N-methyl-D-aspartate epileptiform activity than control rats. Furthermore, hippocampal slices from KA- or KA+PDC-treated rats exposed to low Mg2+ reduced their firing rate to nearly zero once they returned to normal solution, whereas their control counterparts continued to fire, although at a lower rate.

CONCLUSIONS

These results suggest a significant contribution of EAATs in some experimental epilepsy models and point to their short-term regulation by mGluR III as a possible source of their plasticity.

Excitatory amino acid transporters: a family in flux

As the most predominant excitatory neurotransmitter, glutamate has the potential to influence the function of most neuronal circuits in the central nervous system. To limit receptor activation during signaling and prevent the overstimulation of glutamate receptors that can trigger excitotoxic mechanisms and cell death, extracellular concentrations of excitatory amino acids are tightly controlled by transport systems on both neurons and glial cells. L-Glutamate is a potent neurotoxin, and the inadequate clearance of excitatory amino acids may contribute to the neurodegeneration seen in a variety of conditions, including epilepsy, ischemia, and amyotrophic lateral sclerosis. To establish the contributions of carrier systems to the etiology of neurological disorders, and to consider their potential utility as therapeutic targets, a detailed understanding of transporter function and pharmacology is required. This review summarizes current knowledge of the structural and functional diversity of excitatory amino acid transporters and explores how they might serve as targets for drug design.

Statistical analysis of long-term potentiation of large excitatory postsynaptic potentials recorded in guinea pig hippocampal slices: binomial model

Excitatory postsynaptic potentials (EPSPs) were recorded in guinea pig hippocampal slices (area CA1) from 15 neurons after stimulation of stratum radiatum (str. rad.) and stratum oriens. EPSP amplitudes increased in 8 neurones (10 post-tetanic regions) recorded 15 to 45 min after tetanic stimulation of str. rad. The increase was considered to represent long-term potentiation (LTP). Quantal analysis was performed by two methods assuming binomial statistics: the histogram method using deconvolution of noise and the variance method. According to both methods, LTP was associated with an increase in mean quantal content (m) which correlated with LTP magnitude. A statistically significant increase in quantal size (v) was found only by the histogram method and the increase was not correlated with LTP magnitude. A separate analysis of EPSPs with small LTP magnitude demonstrated that with the histogram method only v was increased but not m. A smaller increase in m for the pooled data of both methods did not correlate with LTP magnitude for this EPSP subset. The increase in m for the whole EPSP set corresponds to previous results on the quantal analysis of LTP in in vivo preparations and favours a presynaptic location of major mechanisms underlying LTP maintenance. The increase in v indicates the existence of another mechanism responsible for the maintenance of a small part of LTP. This mechanism might involve either pre- or postsynaptic changes or both.

Quantal analysis of long-term potentiation of "minimal" excitatory postsynaptic potentials in guinea pig hippocampal slices: binomial approach

"Minimal" excitatory postsynaptic potentials (EPSPs) were recorded from 13 neurones in area CA1 of guinea pig hippocampal slices after double-pulse stimulation of stratum radiatum (str. rad.) and stratum oriens (str. or.). Amplitudes of EPSPs significantly increased in 8 neurones 5 to 55 min after 9 tetanizations in str. rad.. The increase was considered to represent long-term potentiation (LTP). Altogether 26 EPSPs (42 post-tetanic regions) were statistically analysed by four methods of the quantum hypothesis assuming the binomial model of transmitter release: the deconvolution (histogram), the variance, the failures, and the combined (variance-failures) methods. The mean quantal content (m) significantly increased after LTP induction according to all methods used. Quantal size (v) also tended to increase but according to some methods, the increase was not statistically significant and it did not correlate with LTP magnitude. However, for an EPSP subset with a LTP magnitude of less than 1.55, the increase in v correlated with LTP magnitude, whereas the increase in m did not. The relative contribution of the increase in v to LTP magnitude was larger for cases with small LTP than for the whole EPSP set. In general, the increase in m corresponds to previous studies and favours the presynaptic location of major mechanisms of LTP maintenance, i.e. an increase in the average number of transmitter quanta released by each presynaptic volley. The post-tetanic increase in v might reflect some additional mechanisms which presumably include an increase in the amount of transmitter in one quantum.

Specific protein changes during memory acquisition and storage

Changes in several distinct types of neuronal proteins are now known to be associated with learning. In this review, we will summarize the properties of these proteins and relate these properties to prominent theories of the biochemical basis of memory.

Long-term potentiation at nicotinic synapses in the rat superior cervical ganglion

1. Nicotinic fast excitatory postsynaptic potentials (fast EPSPs) were recorded intracellularly from postganglionic neurones in the isolated rat superior cervical ganglion. 2. An hours-long potentiation of the fast EPSP could be induced by brief tetanic stimulation of the preganglionic nerve (5 Hz for 5 s to 20 Hz for 20 s). While long-term potentiation (LTP) can be detected in every ganglion by extracellular techniques, LTP was induced in only two-thirds of the nicotinic synaptic responses. 3. Muscarinic blockade with atropine did not prevent LTP of the fast EPSP. 4. LTP of the fast EPSP did not correlate with changes in input resistance nor cell potential, as recorded in the soma. 5. The formation of nicotinic LTP appeared to depend upon stimulation of the nerve terminals. Non-synaptic tetanic depolarization of the postganglionic neurone, effected by injecting depolarizing current pulses through the intracellular microelectrode, was not sufficient. LTP could be induced by synaptic tetani in two-thirds of the same neurones. 6. The response to exogenous 1,1-dimethyl-4-phenylpiperazinium (DMPP), a selective nicotinic agonist, was not increased during nicotinic synaptic LTP. This was true whether DMPP was applied by pressure-ejection from an extracellular micropipette during intracellular recording, or by brief superfusion during sucrose-gap recording of postganglionic responses. 7. Responses to exogenous acetylcholine and carbachol were increased during nicotinic LTP when these non-selective cholinergic agonists were applied by pressure-ejection during intracellular recording. However, the potentiation of the fast EPSP was always at least twofold greater than the potentiation of the response to these exogenous agonists. 8. Potentiation of the responses to acetylcholine and carbachol may have been due to long-term enhancement of muscarinic responses. Thus, no postsynaptic basis for nicotinic LTP was uncovered in these studies.