Extracellular Somatostatin Measured by Microdialysis in the Hippocampus of Freely Moving Rats: Evidence for Neuronal Release |
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Authors: | A Vezzani R Ruiz A Monno M Rizzi N Lindefors R Samanin E Brodin |
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Institution: | Laboratory of Neuropharmacology, Mario Negri Institute for Pharmacological Research, Milan, Italy;Department of Pharmacology, Karolinska Institute, Stockholm, Sweden |
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Abstract: | Abstract: Intracerebral microdialysis combined with a sensitive and specific radioimmunoassay was used to monitor the neuronal release of somatostatin (somatostatin-like immunoreactivity, SLI) in the dorsal hippocampus of freely moving rats. The sensitivity of the radioimmunoassay was optimized to detect <1 fmol/ml. The basal concentration of SLI in 20-min dialysate fractions (5 μl/min) collected 24 h after probe implantation was stable over at least 200 min. The spontaneous efflux dropped by 54 ± 6.4% ( p < 0.05) when Ca2+ was omitted and 1 m M EGTA added to the Krebs-Ringer solution and by 65.5 ± 3.2% ( p < 0.05) in the presence of 1 μ M tetrodotoxin. Depolarizing concentrations of the Na+ channel opener veratridine (6.25, 25, 100 μ M ) induced 11 ± 2 ( p < 0.05), 17 ± 2 ( p < 0.05), and 21 ± 5 ( p < 0.01) fold increase in SLI concentration, respectively, during the first 20 min of perfusion. The effect of 100 μ M veratridine was blocked by coperfusion with 5 μ M tetrodotoxin ( p < 0.01) and reduced by 79% ( p < 0.01) in the virtual absence of Ca2+. Neuronal depolarization by 20 min of perfusion with Krebs-Ringer solution containing 25 and 50 m M KCl and proportionally lowered Na+ increased the dialysate SLI 4.4 ± 1 ( p < 0.05) and 17 ± 3 ( p < 0.01) fold baseline, respectively. Ten micromolar ouabain, a blocker of Na+,K+-ATPase, increased the dialysate SLI 15-fold baseline, on average ( p < 0.05), during 80 min of perfusion. The results demonstrate the suitability of brain microdialysis for monitoring the neuronal release of SLI and for studying its role in synaptic transmission. |
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Keywords: | Neuronal somatostatin release Hippocampus In vivo microdialysis Extracellular space Freely moving rats Radioimmunoassay |
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