Basal release of Met-enkephalin and neurotensin in the ventrolateral periaqueductal gray matter of the rat: a microdialysis study of antinociceptive circuits

Abstract

The periaqueductal gray (PAG) contains neural circuits that participate in descending antinociception. Anatomical and electrophysiological evidence suggests that these circuits might employ opioid peptides and GABA in series to remove a tonic inhibition of descending PAG output neurons. The present studies examined the release of the antinociceptive peptides Met-enkephalin and neurotensin in the ventrolateral PAG, and investigated the interaction between GABA and Met-enkephalin release. In awake and freely moving rats the ventrolateral PAG was dialysed using 25 ga. concentric probes. Basal release of peptide in 12 min or 40 min fractions was determined using radioimmunoassays. To establish how the ventrolateral PAG responds to nociception, dialysis was performed following unilateral hindpaw inflammation using Complete Freund's Adjuvant. Twenty-four hours after inflammation was induced, neurotensin release was increased 133% and Met-enkephalin release was increased 353% compared to control animals. Seven days after inflammation was induced, neurotensin release declined precipitously, while basal Met-enkephalin release remained elevated 313% above controls. Thus, unlike enkephalin, increased basal neurotensin release is not sustained with persistent tonic nociception. In addition, we confirmed in normal animals that the ventrolateral PAG is induced to release Met-enkephalin by systemic morphine. A 43% increase in basal Met-enkephalin release was observed immediately following a 12 mg/kg i.p. morphine injection. Morphine should have the opposite effect (inhibit peptide release) if it acts directly on the enkephalinergic neurons. Thus, we examined the hypothesis that GABAergic interneurons in the PAG mediated morphine-stimulated enkephalin release. When the GABAantagonist bicuculline (0.25 μM to 25 μM) was co-infused with the dialysis medium, Met-enkephalin release increased in a dose-dependent fashion and peaked 68% above pre-infusion levels. These data elucidate the reciprocal inhibitory relationship between GABA and enkephalin in the ventrolateral PAG. We hypothesize that, when nociception induces Met-enkephalin release within this region, the tonic GABAergic inhibition is overcome, resulting in greater sensitivity of PAG enkephalinergic neurons. Ultimately, this enhanced enkephalin release should result in greater excitability of the descending PAG output neurons that are responsible for antinociception.