Morphine decreases neuronal excitability in mouse enteric neurons via alterations in Na+ channel kinetics

Abstract

A major side‐effect of morphine and other opioids is severe constipation. Morphine decreases neurotransmitter release from myenteric neurons resulting in reduced peristalsis. In this study we used whole‐cell patch clamp techniques to directly study the effects of morphine on cell excitability and ionic currents in mouse enteric neurons. Primary cultures of mouse ileum myenteric neurons were prepared by enzymatic digestion from longitudinal muscle/myenteric plexus (LMMP) preparations and plated in modified Neurobasal A media. Two classes of neurons were identified based on the absence (12/33) and presence (21/33) of an after‐hyperpolarization (AHP) following evoked action potentials. In neurons with AHP, morphine (3 ÂμM) significantly reduced neuronal excitability via an increase in rheobase, a reduction in action potential height, and by inhibiting the neuron from firing multiple action potentials. The resting membrane potential (−51.8 mV) was not affected. The peak amplitude of Na+ currents were significantly attenuated by morphine. The availability of Na+ channels was markedly reduced by morphine as demonstrated by a shift in the steady‐state voltage‐dependent inactivation kinetics. These data demonstrate that inhibition of Na+ current results in reduced excitability of enteric neurons and contributes to the severe constipation concurrent with opioid administration. Supported by NIH DA024009.