Desensitization of somatostatin-induced inhibition of low extracellular magnesium concentration-induced calcium spikes in cultured rat hippocampal neurons

Neuronal excitability is inhibited by somatostatin, which might play important roles in seizure and neuroprotection. The possibility of whether the effect of somatostatin on neurotransmission is susceptible to desensitization was investigated. We tested the effects of prolonged exposure to somatostatin on 0.1 mM extracellular Mg²⁺ concentration ([Mg²⁺]_o)-induced intracellular free Ca²⁺ concentration ([Ca²⁺]_i) spikes in cultured rat hippocampal neurons using fura-2-based microfluorimetry. Reducing [Mg²⁺]_o to 0.1 mM elicited repetitive [Ca²⁺]_i spikes. These [Ca²⁺]_i spikes were inhibited by exposure to somatostatin-14. The inhibitory effects of somatostatin were blocked by pretreatment with pertussis toxin (PTX, 100 ng/ml) for 18-24 h. Prolonged exposure to somatostatin induced a desensitization of the somatostatin-induced inhibition of [Ca²⁺]_i spikes in a concentration-dependent manner. The somatostatin-induced desensitization was retarded by the nonspecific protein kinase C (PKC) inhibitor staurosporin (100 nM) or chronic treatment with phorbol dibutyrate (1 μM) for 24 h, but not by the protein kinase A inhibitor KT5720. The desensitization was significantly retarded by the novel PKCε  translocation inhibitor peptide (1 μM). In addition, suramin (3 μM), an inhibitor of G-protein-coupled receptor kinase 2 (GRK2), caused a reduction in the desensitization. After tetrodotoxin (TTX, 1 μM) completely blocked the low [Mg²⁺]_o-induced [Ca²⁺]_i spikes, glutamate-induced [Ca²⁺]_i transients were slightly inhibited by somatostatin and the inhibition was desensitized by prolonged exposure to somatostatin. These results indicate that the prolonged activation of somatostatin receptors induces the desensitization of somatostatin-induced inhibition on low [Mg²⁺]_o-induced [Ca²⁺]_i spikes through the activation of GRK2 and partly a novel PKCε in cultured rat hippocampal neurons.