GABA agonist induced changes in ultrastructure and GABA receptor expression in cerebellar granule cells is linked to hyperpolarization of the neurons

GABA has been shown to exert a neurotrophic like activity by enhancing the morphological and functional maturation of neurons. Mechanisms involved in this effect of GABA are largely unknown but since GABA has been shown to mediate a hyperpolarizing action on neurons it can be assumed that this action might be important. In order to investigate this possibility, the ability to mimic the trophic actions of GABA of different agents known to influence the membrane potential or the GABA gated chloride channels was studied. Hence, GABA receptor expression as well as the ultrastructure of cerebellar granule cells were monitored after exposure of the cells in culture to either bromide, valinomycin or picrotoxin. It was found that cells which at early developmental stages (4 days in culture) were exposed to bromide or valinomycin expressed low affinity GABA receptors similar to cells treated with the GABA agonist 4,5,6,7‐tetrahydroisoxazolo[5,4‐c]pyridin‐3‐ol (THIP). This was in contrast to untreated cultures or cultures treated with both THIP and picrotoxin which expressed only high affinity GABA receptors and no low affinity receptors. In cultures at more mature stages (14 days in culture) bromide treatment did not lead to formation of low affinity GABA receptors. Studies of the ultrastructure of the cells (4‐day‐old cultures) showed that exposure to bromide or valinomycin mimicked the ability of THIP to enhance the cytoplasmic density of rough endoplasmic reticulum, Golgi apparatus, vesicles and coated vesicles. Again, in 14‐day‐old cultures treatment with bromide had no effect on the ultrastructure. Exposure of the cells to picrotoxin significantly reduced the ability of THIP to induce the increase in the density of the organelles mentioned above. Altogether the results strongly suggest that the hyper‐polarizing action of GABA via the chloride channels coupled to high affinity GABA receptors is the permissive signal for initiation of further reactions involved in the trophic and differentiating actions of GABA.