Cortistatin increase of a potassium conductance in rat locus coeruleus in vitro

1 In this study we examined the effects of cortistatin, a putative endogenous ligand for somatostatin (SRIF) receptors, on the membrane properties of rat locus coeruleus (LC) neurones in vitro, by use of intracellular and whole cell patch clamp recording. We have compared the actions of cortistatin with those of SRIF and the SRIF analogue D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP). 2 When LC neurones were voltage clamped to -60 mV, application of cortistatin caused an outward current in all cells examined (n=44), with a pEC(50) of 6.62. SRIF also caused an outward current in all cells examined (n=43), with a PEC50 Of 6.93. 3 The outward currents caused by cortistatin in 2.5 mM extracellular K+ reversed polarity at -106 mV, very close to the predicted K+ reversal potential of -105 mV. Increasing extracellular K+ to 10.5 mM resulted in a shift of the reversal potential of +38 mV, a shift consistent with a K+ conductance. The conductance activated by cortistatin showed mild inward rectification. 4 Continuous application of a high concentration of SRIF (1 mu M) resulted in a decrease of the outward current to a steady level of 49% of the maximum response, with a t(1/2) of 131 s. Application of a high concentration of cortistatin (3 mu M) during the desensitized portion of the SRIF response did not result in any further outward current. Continuous application of a high concentration of cortistatin (10 mu M) resulted in a decrease of the outward current to a steady level of 42% of the maximum response with a t(1/2) of 114 s. Application of a high concentration of SRIF (3 mu M) during the desensitized portion of the cortistatin response produced only a small outward current. 5 Continuous application of cortistatin (3 mu M) also resulted in a decrease of the outward current (by 43%, t(1/2) of 136 s) and application of a high concentration of CTOP (10 mu M) during the desensitized portion of the cortistatin response did not produce any outward current. Continuous application of a high concentration of CTOP (10 mu M) resulted in a decrease of the outward current to a steady level of 70% of the maximum response with a t(1/2) of 143 s. Application of a high concentration of cortistatin (3 mu M) during the desensitized portion of the CTOP response did not result in any further outward current. 6 The actions of cortistatin (300 nM-10 mu M) were not affected by the opioid antagonist naloxone (10 mu M). Application of met-enkephalin during the desensitized portion of the response to a high concentration of cortistatin (3 mu M) produced an outward current similar to that produced by metenkephalin application alone. 7 Thus cortistatin efficaciously activates an inwardly rectifying K+ conductance in LC neurones. These actions appear to be mediated by a population of SRIF receptors, at which CTOP is also an agonist. Cortistatin does not appear to be a ligand for mu-opioid receptors in rat LC neurons.