IN-VIVO RELEASE OF NEUROACTIVE AMINO-ACIDS FROM THE INFERIOR COLLICULUS OF THE GUINEA-PIG USING BRAIN MICRODIALYSIS

Microdialysis techniques were used to measure in vivo release of neuroactive amino acids from the central nucleus of the inferior colliculus (ICC) in anesthetized guinea pigs. Concentric dialysis probes were implanted in the ICC and perfused with Ringer solution of various compositions at a flow rate of 2.0 mu l/min. Consecutive 10-min fractions of the dialysate were collected for up to 3 h under different experimental conditions, frozen and assayed for amino acid content by high performance liquid chromatography (HPLC). There was an initial high outflow of amino acids which declined to stable baseline levels,after 2 h. Following this stabilization period, perfusion with a medium containing 100 mM KCl produced an increase in the extracellular levels of aspartate (Asp), glutamate (Glu), gamma-aminobutyric acid (GABA) and glycine (Gly). Only the increases in GABA and Gly were statistically significant. None of the increases occurred in the presence of 2.0 mM cobalt suggesting the release of amino acids is calcium dependent. Histological examination revealed that tissue damage was minimal and largely confined to the immediate vicinity of the probes. We were also able to show that the blood brain barrier (BBB) appeared to heal 2 h after probe implantation. Thus, following intravenous injection of [H-3]alpha-aminoisobutyric acid (AIB), which does not cross the intact BBB, no isotope was recovered in the dailysate. These results demonstrate that microdialysis is a unique and suitable method to monitor changes in the extracellular levels of amino acid neurotransmitters in a central auditory structure. They also provide in vivo data that supports the notion that inhibitory amino acids play a functional role in processing acoustic information in the inferior colliculus.