Mu-Opioid receptors modulate noradrenaline release from the rat hippocampus as measured by brain microdialysis

Abstract

The modulation of noradrenaline (NA) release via presynaptic opioid receptors in the hippocampus of freely moving rats was studied by the use of brain microdialysis. Extracellular levels of NA were estimated by assaying its concentrations in the perfusion fluid using high-performance liquid chromatography (HPLC) with electrochemical detection (ECD). Spontaneous NA levels were reduced by tetrodotoxin (1 μM) co-perfusion and were increased by peripheral administration of desipramine (5 and 10 mg/kg, i.p.). Addition of potassium (K +, 60 and 120 mM) to the perfusion fluid evoked a concentration-dependent release of NA. K + (120 mM)-evoked NA release was markedly reduced by removal of calcium (Ca 2+) from the perfusion fluid. These results indicate that both the spontaneous and the K +-evoked NA release measured by the use of brain microdialysis coupled with HPLC-ECD can be used as indices of neuronal release from the noradrenergic nerve terminals. A μ-opioid receptor agonist, morphine (0.01–10 μM), when co-perfused with K + (120 mM), produced a reduction of K +-evoked NA release in a concentration-dependent manner. Neither co-perfusion with a high concentration of [ d-Pen 2, d-Pen 5]-enkephalin (DPDPE) (10 μM), an agonist selective for δ-opioid receptors, nor with U-69593 (10 μM), an agonist selective for κ-opioid receptors, modified the K + (120 mM)-evoked release of NA. Morphine-induced (1 μM) inhibition of NA release was blocked by a μ-opioid receptor antagonist, naltrexone (3 and 9 mg/kg, i.p). Naltrexone by itself did not alter the spontaneous NA levels or the K +-evoked NA release. The morphine-induced (1 μM) inhibition of NA release was abolished by pretreatment with pertussis toxin (PTX) that was perfused into the hippocampus by push-pull cannulation. These findings suggest that NA release is functionally modulated via the μ-opioid receptors located on noradrenergic nerve terminals in the rat hippocampus. It indicates, furthermore, the possibility that the inhibition of NA release via the μ-opioid receptors is linked to G-proteins which are substrates of PTX.