Clonidine inhibits and phorbol acetate activates glutamate release from rat spinal synaptoneurosomes

Glutamate is a major neural transmitter of noxious stimulation in the spina l cord. We measured glutamate release from rat spinal synaptoneurosomes by using an enzyme-linked fluorimetric assay. Glutamate was released from spin al cord synaptoneurosomes in response to the addition of 30 mM potassium ch loride, 1 mM 4-aminopyridine, or 1 mu M ionomycin in the presence of extern al calcium. There was less release of glutamate in the absence, versus the presence, of external calcium. Clonidine significantly reduced the level of glutamate released from the spinal cord synaptoneurosomes. Tetradecanoyl p horbol acetate, an activator of protein kinase C, enhanced glutamate releas e. Forskolin, a protein kinase A activator, had no effect on the glutamate efflux. Our data indicate that glutamate released in the spinal cord is dep endent on protein kinase C but is independent of the protein kinase A pathw ay. They also suggest that the inhibition of glutamate release may be the u nderlying mechanism of antinociception by clonidine at the spinal cord leve l. Implications: We demonstrated that synaptoneurosomes from rat spinal cor d could release glutamate in response to depolarization. We showed that an activator of protein kinase C increased glutamate released from spinal cord synaptoneurosomes but that clonidine decreased it. Glutamate release may b e one of the mechanisms of antinociception at the spinal cord level.