[CITATION][C] GABA and spinal afferent terminal excitability in the cat

DR Curtis, D Lodge, SJ Brand - Brain Research, 1977 - Elsevier
DR Curtis, D Lodge, SJ Brand
Brain Research, 1977Elsevier
Depolarization by gamma-aminobutyric acid (GABA) at axo-axomc synapses upon the
terminals of group Ia afferent fibres has been proposed to underlie the reduced effectiveness
of la impulses to excite motoneurones ('presynaptic'inhibition) when preceded by impulses
in muscle and cutaneous afferent fibres 1. Few direct pharmacological studies have been
made in vivo in the cat spinal cord of this action of GABA, and although an earlier study
indicated that electrophoretic GABA depressed the excitability of spinal afferent terminals 5 …
Depolarization by gamma-aminobutyric acid (GABA) at axo-axomc synapses upon the terminals of group Ia afferent fibres has been proposed to underlie the reduced effectiveness of la impulses to excite motoneurones ('presynaptic'inhibition) when preceded by impulses in muscle and cutaneous afferent fibres 1. Few direct pharmacological studies have been made in vivo in the cat spinal cord of this action of GABA, and although an earlier study indicated that electrophoretic GABA depressed the excitability of spinal afferent terminals 5, more recent investigations report enhanced excitability in both the ventraP 7 and dorsaP 1 horns. The present report is concerned with changes in the excitability of single la terminals produced by GABA and related amino acids.
Within the gastrocnemius (G) motonuclei of lumbar segments of spinal cats anaesthetised with pentobarbitone, group Ia fibres and terminals were stimulated with 0.3 msec negative pulses (~ 2# A) via the 3.6 M NaCI containing centre barrel of 7-barrel micropipettes (4-6/zm). Antidromically conducted action potentials were recorded monophasically (1.7-2.9 msec latency) from either the lateral (LG) or medial gastrocnemius (MG) nerves peripherally. Amino acids were ejected as cations or anions from the other barrels of the micropipette, one of which contained 165 mM NaC1 and through which equalizing currents were passed so that the net current flow through all barrels remained at a constant level. Thus, for example, the ejection of GABA with a cationic current of 40 nA from a barrel in which the amino acid had been retained with an anionic current of 10 nA was balanced by ejecting chloride ions from the NaCI barrel with an anionic current of 50 nA. Changes in terminal excitability were not produced by balanced ejections of Na and chloride ions. The excitability of primary afferent structures was determined from the magnitude of the stimulating current which evoked antidromic action potentials i~ a 4 of 8 trials at 4 Hz for studying the effects of amino acids and at 1 Hz for studying the effect oftetanic stimulation (3:,~ threshold, 4 pulses at 320 Hz) of the posterior bicepssemitendinosus (PBST) nerve 50-60 msec previously. Such afferent volleys are known to produce prolonged inhibition of gastrocnemius monosynaptic reflexes and depolarization of gastrocnemius la terminals 1, 16. Considerable difficulty was experienced in maintaining stable stimulating conditions, movements of the micropipette over dis-
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