Extracellular somatostatin measured by microdialysis in the hippocampus of freely moving rats: evidence for neuronal release.
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 microliters/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 mM EGTA added to the Krebs-Ringer solution and by 65.5 +/- 3.2% (p < 0.05) in the presence of 1 microM tetrodotoxin. Depolarizing concentrations of the Na+ channel opener veratridine (6.25, 25, 100 microM) 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 microM veratridine was blocked by coperfusion with 5 microM 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 mM 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.
Vezzani
A
A
Laboratory of Neuropharmacology, Mario Negri Institute for Pharmacological Research, Milan, Italy.
Ruiz
R
R
Monno
A
A
Rizzi
M
M
Lindefors
N
N
Samanin
R
R
Brodin
E
E
D000818:Animals; D002118:Calcium / Q000494:pharmacology; D003956:Dialysis / Q000379:methods; D004533:Egtazic Acid / Q000494:pharmacology; D006624:Hippocampus / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D008832:Microchemistry; D009474:Neurons / Q000187:drug effects / Q000378:metabolism*; D010042:Ouabain / Q000494:pharmacology; D011189:Potassium Chloride / Q000494:pharmacology; D011863:Radioimmunoassay; D051381:Rats; D017207:Rats, Sprague-Dawley; D013004:Somatostatin / Q000378:metabolism*; D013779:Tetrodotoxin / Q000494:pharmacology; D014701:Veratridine / Q000494:pharmacology
D013779:Tetrodotoxin; D013004:Somatostatin; D004533:Egtazic Acid; D010042:Ouabain; D011189:Potassium Chloride; D014701:Veratridine; D002118:Calcium