In vivo voltammetric detection of neuropeptides with micro carbon fiber biosensors: possible selective detection of somatostatin.
The electrochemical activity of catechol- and indoleamines, measured by differential pulse voltammetry (DPV) with specifically electrically pretreated carbon fiber microelectrodes, has been utilized to develop sensitive assays for amine neurotransmitters and metabolites. So far, four oxidation peaks have been recorded in vivo between -200 and +500 mV and are well identified. We now report that by increasing the potential sweep range to +950 mV, a further peak, called Peak 5, was detected at +800 mV in vivo in the striatum of anesthetized rats. Neuropeptides containing tyrosine, tryptophan and/or cysteine appear to be electrochemically active between +600 and +900 mV in vitro in a buffered solution at pH 7.4. The present study investigates the chemical nature of Peak 5 and the possible contribution of electroactive neuropeptides to this in vivo voltammetric signal. Experiments performed in vitro and in vivo with amino acids, neuropeptides, or bacitracin (a potent peptidase inhibitor) support the view that Peak 5 is peptidergic. Furthermore, peripheral administration of cysteamine and intrastriatal injection of specific somatostatin antisera both cause the eventual disappearance of Peak 5, suggesting that somatostatin (which oxidases in vitro at approx +800 mV), or a structurally related peptide, could be the principal component of striatal Peak 5.