INDIRECT EFFECTS OF AMINO‐ACIDS ON SYMPATHETIC GANGLION CELLS MEDIATED THROUGH THE RELEASE OF γ‐AMINOBUTYRIC ACID FROM GLIAL CELLS

Abstract

1 All experiments were performed on rat isolated desheathed superior cervical ganglia maintained in Krebs solution containing amino-oxyacetic acid (10 μm) at 25°C. 2 Influx rates of γ-amino-n-butyric acid (GABA) were measured by incubating ganglia in 0.5 μm [³H]-GABA for 30 minutes. Influx was inhibited by 50% on adding 14.3 μm unlabelled GABA, 59.2 μm β-alanine (BALA) or 424 μm β-amino-n-butyric acid (BABA). 3 Efflux of [³H]-GABA into non-radioactive solution superfused over ganglia previously incubated for 60 min in 1 μm [³H]-GABA was measured. The mean resting efflux rate coefficient (k) was 0.64 ±0.05 × 10⁻³ min⁻¹. Addition of high concentrations of unlabelled GABA, BABA or BALA to the superfusing solution increased k by (maximally) 3.6–4.3 times; half-maximal increases occurred at the following concentrations: GABA, 16 μm; BALA, 85 μm; BABA, 606 μm. Replacement of external Na⁺ with Li⁺ or TRIS increased the resting value of k and inhibited acceleration by external amino acids. Prior incubation in 1 μm [³H]-GABA with 1 mm unlabelled GABA increased resting k 1.5 times;, but did not alter the peak rate coefficient produced by external amino acids. 4 Neuronal depolarization produced by the amino acids was measured with surface electrodes. Preincubation in 1 mm GABA for 60 min potentiated low-amplitude responses to BALA or BABA but not those to GABA or 3-aminopropanesulphonic acid (a potent agonist with low affinity for the GABA carrier). Omission of external Na⁺ reduced responses to BABA but increased those to GABA. 5 Incubation in 1 mm GABA for 60 min (as required to potentiate BABA or BALA actions) increased the amount of GABA in the tissue from 0.21 to 0.73 mmol/kg wet weight. Autoradiographs in which labelled GABA was used indicated that uptake into neuroglial cells was responsible for this accumulation. 6 It is suggested that: (i) BALA and BABA are substrates for the inward GABA carrier responsible for GABA entry into ganglionic glial cells; (ii) they accelerate efflux by inhibiting carrier-mediated re-accumulation of effluent GABA by the glial cells; (iii) interstitial GABA concentrations are thereby increased to a level capable of depolarizing adjacent neurones; and (iv) this, rather than direct GABA-receptor activation, accounts for the depolarization produced by low concentrations of BALA and BABA. Potentiation of their depolarizing action after pre-incubation in 1 mm GABA is suggested to result from the increased amount of intracellular GABA available for release, and is quantitatively compatible with this increase; inhibition in Na⁺-free solution is due to their inability to inhibit re-accumulation of GABA under these conditions. 7 A model for the action of carrier substrates is described in an Appendix. Calculations based thereon yield increments in interstitial GABA concentration in the presence of carrier substrates compatible with those determined experimentally (up to 1 μm at rest or 3.4 μm after pre-incubation in GABA).