Baclofen and phaclofen modulate GABA release from slices of rat cerebral cortex and spinal cord but not from retina

Abstract

The effects of (-)-baclofen, muscimol and phaclofen on endogenous γ-aminobutyric acid (GABA) release from rat cortical slices, spinal cord slices and entire retinas were studied. The spontaneous resting release of GABA from the three tissues was 3 to 6 pmol mg-1 wet wt 10 min-1. Depolarization of cortical slices with KCl (50 mM)(high-K) produced an 8 fold increase in GABA release but high-K did not evoke an increased release of GABA from spinal slices or retinas. When rats were injected with γ-vinyl-GABA (250 mg kg-1 i.p.)(GVG) 18 h before death, the tissue GABA stores were increased 3 to 6 fold and high-K then evoked striking Ca-dependent releases of GABA from all three tissues. Thus, in subsequent experiments, unless otherwise stated, the nervous tissues were taken from GVG-treated rats. (-)-Baclofen (10 μM) significantly reduced the K-evoked release of GABA from cortical and spinal slices but retinal release was not affected, even at a concentration of (±)-baclofen of 1 mM. For cortical slices, the IC50 for baclofen was approximately 5.2 μM. The inhibitory effect of baclofen on GABA release from cortical slices also occurred in slices prepared from saline-injected rats, indicating that GVG treatment did not qualitatively affect the results. The inhibitory effect of (-)-baclofen on the K-evoked release of GABA from cortical and spinal slices was antagonised by phaclofen (500 μM), confirming that baclofen was producing its effects by acting at the GABA(B)-receptor. Phaclofen (500 μM) increased the spontaneous resting release of GABA from cortical slices taken from GVG-treated rats but not from saline-injected rats. Phaclofen did not increase GABA release from spinal slices or retinas taken from GVG-treated rats. Baclofen (10 μM) significantly reduced the K-evoked release from cortical slices of glutamate, aspartate, glycine and taurine. Muscimol (10 μM) and δ-aminolaevulinic acid (10 μM) had no effect on either the resting or K-evoked release of GABA from cortical slices prepared from saline-injected or GVG-treated rats. The results obtained with cortical and spinal slices are consistent with the presence of inhibitory GABA(B)-autoreceptors. The phaclofen-induced increase in GABA release from cortical slices taken from GVG-treated rats, but not from saline-injected rats, implies that under conditions of high GABA release, considerable feedback inhibition is occurring via activation of the GABA(B) inhibitory autoreceptors. No evidence was found for GABA(B)-autoreceptors on retinal GABAergic amacrine cells or for GABA(A)-autoreceptors in cortical slices or spinal cord slices.