The effect of serotonin on GABA synthesis in cultured rat spinal dorsal horn neurons.

The spinal dorsal horn (SDH) is the first step in the integration of primary nociceptive information, which is controlled by the descending serotonin (5-HT) system as well as the principal inhibitory neurotransmitter gamma-aminobutyric acid (GABA). However, the influence exerted by 5-HT on GABA synthesis remains poorly understood. The major pathway for GABA synthesis is the enzymatic decarboxylation of glutamate by glutamic acid decarboxylase (GAD) 65 and 67. In the present research, western blotting results show a time- and dose-dependent enhancement of GAD65 and GAD67 expression induced by 5-HT treatment and a concentration of 100nM 5-HT applied for 3 days is shown to be the optimal condition for maximal expression of GAD67 and a significant expression of GAD65. Under the stimulation of such 5-HT application the phosphorylation of Akt and p42/p44 mitogen-activated protein (MAP) kinase is activated and specifically blocked by inhibitors of phosphatidylinositol 3-kinase (PI3-K) (LY294002) or the p42/p44 MAP kinase (PD98059 and U0126) pathways. Moreover, LY294002, or PD98059, or U0126 partially inhibit 5-HT-stimulated increases in GAD67 or GAD65 expression. Further, 5-HT application has no effect on the number of GAD65/GAD67-immunopositive neuronal cells; but it can induce an increase in the total area, process length and number of primary neurites of GAD65/67-positive neurons, an increase that appears to involve LY294002 and PD98059. The results of this study provide an in vitro model of the regulation of 5-HT on synthesis of GABA in the SDH that is putatively thought to occur in vivo as a result of excitatory neural activity.