Alpha-melanocyte stimulating hormone desensitizes the responsiveness of carbon-fibres within seconds

We monitored electrooxidation of noradrenaline and alpha-melanocyte stimulating hormone (alpha-MSH) at a carbon-fibre microelectrode (CFME). The solution of noradrenaline (1 mM) or alpha-MSH (1 mM) was applied by a pressure pulse (2 s) from a micropipette to a voltage-clamped (850 mV) CFME immersed into bathing solution of an inverted microscope chamber. The distance between the CFME and micropipette was 2 to 12 microm. The maximal currents recorded for these two agents were 8.0 +/- 0.5 pA (N = 9) and 3.0 +/- 1.1 pA (N = 9), respectively. Pressure application of control solution did not affect the measured current. The noradrenaline-evoked anodic current was characterized by a monotonic increase that attained the maximum at the end of the pressure pulse. In contrast, the time-course of the alpha-MSH-evoked current was biphasic. The maximum amplitude of this current was attained in 0.59 +/- 0.15 s (N = 9) and then it declined with a time constant of 7.5 +/- 4.0 s (N = 9) until the pressure pulse was terminated. We explain this phenomenon to be due to an interaction between the peptide oxidation products and the CFME which results in its desensitization.