Forskolin enhancement of opioid currents in rat locus coeruleus neurons

1. Opioids are known to hyperpolarize all neurons in the nucleus locus coeruleus (LC) and to inhibit adenylyl cyclase. Recent work has shown that activation of adenylyl cyclase with forskolin in creased the amplitude of the opioid hyperpolarization in LC cells. The aim of the present study was to determine the mechanism of this augmented hyperpolarization. 2. Agonist- induced currents were studied in LC cells in brain slices using both intracellular and whole cell recordings. Forskolin increased the amplitude of μ-opioid- and α₂-adrenoceptor-mediated currents by ~30% of control measured at -60 mV. This effect of forskolin was dependent on the concentration having a threshold of ~1 μM and a peak effect at ~30 μM. Dideoxyforskolin (30 μM) caused a small reduction (-52 ± 28 pA, mean ± SE) in the amplitude of the opioid current. 3. Forskolin increased the agonist current in the outward direction over the entire potential range between - 140 and -50 mV when recordings were made from neurons in cells recorded from slices cut in the horizontal plane. This augmented current produced a shift of the apparent reversal potential to more negative values. 4. Both the forskolin augmentation of the opioid current and the opioid current itself were reduced when the space clamp was improved by cutting the slice in the coronal plane, increasing the extracellular potassium concentration, and treating the slice with carbenoxolone. In addition, forskolin did not change the reversal potential of the opioid current. When expressed as a percentage change from control, forskolin had no significant effect on the opioid current in carbenoxolone (-13 ± 13%) but produced a small augmentation in high extracellular potassium (15 ± 4%) and coronoal slices (31 ± 12%). 5. Two models were tested to explain the action of forskolin, one where cells are coupled electrotonically by a forskolin-sensitive conductance (coupled- cell model) and a second where opioids mediate an inhibition of a forskolin- induced cation conductance (2-conductance model). The experimental results were fit well only by the coupled-cell model, which predicted that the opioid/forskolin interaction is indirect and occurs primarily in response to forskolin increasing the degree of electrotonic coupling between LC neurons. The consequence of increased coupling would be to augment synchronous activity within the nucleus.