Gastric effects of μ-, δ- and κ-opioid receptor agonists on brainstem unitary responses in the neonatal rat

Abstract

Single units in the medial subnucleus of the nucleus tractus solitarii, responding to electrical stimulation of subdiaphragmatic vagal fibers, were recorded extracellularly in an in vitro neonatal rat brainstem-gastric preparation. Selective opioid receptor agonists were applied only to the gastric compartment of the bath chamber and therefore, the brainstem functions of the preparation were not affected. The peripheral gastric effects of the μ-opioid receptor agonist, [ d-Ala 2, N-MePhe 4,Gly 5-ol]enkephalin (DAMGO) and κ-opioid receptor agonist, { trans-3,4-dichloro- N-methyl- N-[2-(1-pyrrolidinyl)cyclohexyl]benzeneacetamide methanesulfonate hydrate} (U-50,488H), were evaluated on 69 tonic units that received the subdiaphragmatic vagal input. For approximately 75% of the units observed, DAMGO (1.0 μM; IC 70: 80 nM) and U-50,488H (1.0 μM; IC 70: 200 nM) induced a concentration-dependent inhibition of 62.7 ± 8.9% (mean ± S.D.) and 50.6 ± 6.2% of the control level of the brainstem neuronal activity, respectively. The μ-opioid selective receptor antagonist, naltrexone and non-selective opioid receptor antagonist, naloxone, respectively, blocked the inhibitory effects by DAMGO and U-50,488H. The δ-opioid receptor agonist, [ d-Pen 2, d-Pen 5]enkephalin (DPDPE) (10 μM; IC 70: 400 nM) produced a lesser extent of inhibition of 21.9 ± 8.0% in only 10 out of 51 (20%) neurons tested, and this effect was blocked by naloxone. The area of the stomach where gastric opioid receptors contributed most to brainstem unitary activity was also examined. This was achieved by comparing the opioid effects on a whole-stomach preparation to its effects on a partial-stomach preparation. Our data indicated that the distal stomach containing the pylorus played a key role in the gastric effects of μ- and κ-opioid receptors on brainstem neuronal activity. These results suggest that the μ- and κ-opioid receptors of the distal stomach are important in modulation of brainstem neuronal activity and may play a role in regulating the digestive process.