Pre-synaptic modulation of cortical excitatory amino acid neurotransmitter release

Neville, Lewis Frederic (1988) Pre-synaptic modulation of cortical excitatory amino acid neurotransmitter release. University of Southampton, Doctoral Thesis.

Abstract

The release of exogenous D-[³H]-aspartate and endogenous glutamate as well as their pharmacological modulation was investigated using superfused rat cortical slices and purified guinea-pig cerebro-cortical synaptosomes. Protoveratrine A, a sodium channel activator was found to potently induce a calcium dependent release of pre-loaded D-[³H]-aspartate from rat cortical slices. This release was found to be subject to a receptor specific neuromodulation by the drugs carbachol and GABA. The cholinomimetic potentiated the actions of protoveratrine A, whereas GABA depressed this same evoked release, suggesting the presence and function of pre-synaptic heteroreceptors on glutamate/aspartate transmitter nerve terminals. Also the presence of a glutamate auto-receptor on these same nerve terminals may be postulated since the glutamate analogue L-APB inhibited the KC1 induced release of D-[³H]-aspartate. Using continuous fluorometry to measure the release of endogenous glutamate from purified guinea-pig cortical synaptosomes, the preparation failed to maintain a constant evoked release of transmitter glutamate upon incubation at 30⁰C. This phenomenon occurred despite the preservation of a stable metabolic activity but strikingly paralleled a rapid time dependent failure of the preparation to transport exogenous D-[³H]-aspartate and L-[¹⁴C]-glutamate as well as endogenous, extra-synaptosomal glutamate upon increasing synaptosomal incubation at 30⁰C. Hence, synaptosomes appear to be fragile structures during experiments performed at a physiological temperature. Contrastingly, synaptosomes incubated on ice, maintain a constant amount of evoked transmitter glutamate. Using the latter incubation protocol, depolarising concentrations of KC1 and kainic acid released glutamate from a synaptic transmitter pool which was susceptible to the inhibitory actions of 2-chloroadenosine. Quinolinic acid, induced a Mg++ insensitive release of glutamate which was attenuated by MK-801. The potassium channel antagonist 4-aminopyridine enhanced a totally Ca++ independent release of glutamate from synaptosomes. These effects on release are discussed in terms of pre-synaptic receptor interactions. The finding that 2-chloroadenosine inhibited the kainic acid release of glutamate compliments its neuroprotective action against kainic acid induced toxicity, suggesting a potential pharmaco-therapeutic action of adenosine analogues against excitatory amino acid induced neurodegeneration.

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