NEUROPROTECTIVE EFFECT OF SOMATOSTATIN ON NONAPOPTOTIC NMDA-INDUCED NEURONAL DEATH - ROLE OF CYCLIC-GMP

Somatostatin (SRIF) exerts a modulatory function on neuronal transmiss ion in the CNS. It has been proposed that a reduction of calcium curre nts is the major determinant of the inhibitory activity of this peptid e on synaptic transmission. Because the neurotoxicity induced by activ ation of the NMDA subtype of glutamate receptor is mediated through ex cessive Ca2+ influx, we investigated whether SRIF counteracted NMDA-in duced neuronal cell death. Neurons from embryonic rat cerebral cortex were cultured for 7-10 days and then exposed to 0.5 and 1 mM NMDA for 24 h. The neuronal viability, as assessed by the colorimetric method, decreased by 40 and 60%, respectively, compared with the control condi tion. Morphological and biochemical evidence indicated that cell death occurred by necrosis and not through an apoptotic mechanism. SRIF (0. 5-10 mu M), simultaneously applied with excitatory amino acid, signifi cantly reduced in a dose-dependent manner the neurotoxic effect of NMD A but not that of KA (0.25-0.5 mM). GABA (10 mu M) partially protected neurons to a similar extent from NMDA- or KA-induced toxicity. SRIF t ype 2 receptor agonists, octreotide (SMS 201-995; 10 mu M) and vapreot ide (RC 160; 10 mu M), did not influence the NMDA-dependent neurotoxic ity. The intracellular mechanism involved in SRIF neuroprotection was investigated. Pertussin toxin (300 ng/ml), a G protein blocker, antago nized the protective effect of SRIF on NMDA neurotoxicity. Furthermore , the neuroprotective effect of SRIF was mimicked by dibutyryl-cyclic GMP (10 mu M), a cyclic GMP analogue, whereas 8-(4-chlorphenylthio)-cy clic AMP (10 mu M), a cyclic AMP analogue, was ineffective. The cyclic GMP content was increased in a dose-dependent manner by SRIF (2.5-10 mu M). Finally, both specific (Rp-8-bromoguanosine 3',5'-monophosphate , 10 mu M) and nonspecific [1-(5 isoquinolinylsulfonyl)-2-methylpipera zine (H7), 10 mu M] cyclic GMP-dependent protein kinase (cGMP-PK) inhi bitors did not interfere with NMDA toxicity but substantially reduced SRIF neuroprotection. Our data suggest a selective neuroprotective rol e of SRIF versus NMDA-induced nonapoptotic neuronal death in cortical cells. This effect is likely mediated by cGMP-PK presumably by regulat ion of the intracellular Ca2+ level.