Immunocytochemical evidence for in vitro release of glutamate and GABA from separate nerve terminal populations in the rat pontine nuclei

• Published in: Experimental brain research 1992, Vol.89 (3), p.540-548
• Main Authors: AAS, J.-E, LAAKE, J. H, BRODAL, P, OTTERSEN, O. P
• Format: Article
• Language: English
• Subjects: Animals; Axons - drug effects; Axons - physiology; Axons - ultrastructure; Biochemistry and metabolism; Biological and medical sciences; Calcium - pharmacology; Central nervous system; Fundamental and applied biological sciences. Psychology; gamma-Aminobutyric Acid - analysis; gamma-Aminobutyric Acid - metabolism; Glutamates - analysis; Glutamates - metabolism; Glutamic Acid; In Vitro Techniques; Magnesium - pharmacology; Male; Microscopy, Immunoelectron - methods; Nerve Endings - physiology; Nerve Endings - ultrastructure; Pons - physiology; Pons - ultrastructure; Potassium - pharmacology; Rats; Rats, Inbred Strains; Synapses - drug effects; Synapses - physiology; Synapses - ultrastructure; Vertebrates: nervous system and sense organs
• ISSN: 0014-4819; 1432-1106
• DOI: 10.1007/BF00229879

A quantitative electron microscopic immunocytochemical method was used to study the synaptic handling of glutamate and GABA in slice preparations from the rat pontine nuclei. Slices were subjected to a depolarizing stimulus (55 mM K+, 20 min) in the presence of a physiological or low Ca(2+)-concentration. Depolarization at physiological [Ca2+] evoked a depletion of glutamate-like immunoreactivity from nerve terminals that contain round vesicles and establish asymmetric synaptic contacts. When depolarization was induced in the presence of only 0.1 mM Ca2+ (10 mM Mg2+ added), the loss of glutamate was significantly reduced or abolished, indicative of a Ca(2+)-dependent component of glutamate release. By means of a double-labeling immunocytochemical method we could identify a population of nerve terminals that displayed strong GABA-like immunoreactivity, and a level of glutamate-like immunoreactivity that was low but yet clearly above background level. This type of terminal contains elongated or pleomorphic vesicles and establishes symmetric synaptic contacts. In these terminals, depolarization evoked a Ca(2+)-dependent depletion of GABA-like immunoreactivity, but failed to change the level of glutamate-like immunoreactivity. The present study demonstrates that two different types of nerve terminal in the rat pontine nuclei contain releasable pools of glutamate and GABA, respectively, and that the GABA-releasing terminals also contain a non-releasable pool of glutamate. The glutamate of the latter pool could act as precursor of GABA.