Growth cone calcium elevation by GABA.

Abstract

Cytoplasmic calcium plays a key role in neurite growth. In contrast to previous work suggesting that gamma aminobutyrate's (GABA) role in regulating growth cone calcium is primarily to antagonize the effects of glutamate, we report that GABA can act in an excitatory manner on developing hypothalamic neurites, independently raising calcium in growing neurites and their growth cones. Time-lapse digital video and confocal laser microscopy with the calcium-sensitive dyes fluo-3 and fura-2 were used to study the influence of GABA on neurite calcium levels. GABA (10 microM) evoked a calcium rise in both bicarbonate- and Hepes-based buffers. The calcium rise was greatly reduced after chloride transport was blocked. GABA raised calcium by stimulating the cell body, resulting in an increase in calcium throughout the neuronal cell body and dendritic arbor. GABA also acted locally, stimulating a neuritic calcium rise only in a single dendrite or growth cone. In some neurites and growth cones during early development, GABA generated a greater calcium rise than did glutamate. Bicuculline, a GABAA receptor antagonist, reduced calcium levels in neurites of young synaptically coupled neurons, indicating that ongoing synaptic release of GABA raised neuritic calcium. These data suggest that during early development, GABA may play a significant role in regulating process growth and modulating the formation of early connections in the hypothalamus. Our data support the hypothesis that GABA receptors are functionally active and may play a calcium regulating role similar to that of glutamate in neuronal development. This is particularly true in early development, as later in development GABA's role becomes more inhibitory, and glutamate plays the primary excitatory role.