We used two, 3-min field stimulation cycles 30 min apart (S1, S2) in 3H-norepinephrine-loaded, superfused rat nucleus tractus solitarii-dorsal motor vagal nucleus (NTS-DVN) slices. The stimulation-induced release was expressed as the area above the baseline. Drugs were introduced 12 min before S2 and drug actions were characterized in terms of alterations of S2/S1 ratios. The S2/S1 ratio was 1.047 (0.946-1.159, n=4, geometric mean and 95% confidence interval) in controls and 0.336 (0.230-0.490, n=3), 0.726 (0.590-0.892, n=4), 0.613 (0.594-0.683, n=4) and 0.665 (0.500-0.886, n=4) in the presence of 10-6 M clonidine, D-Ala2,MePhe4,Gly5-ol-enkephalin (DAMGO), endomorphin-1 (Tyr-Pro-Trp-Phe-NH2, EM-1) and -2 (Tyr-Pro-Phe-Phe-NH2, EM-2) [the latter two in the presence of 10-4 M diprotin A, an inhibitor of dipeptidyl-aminopeptidase IV (DAP-IV) enzyme]. The effect of DAMGO at 10-5 M was significantly higher than at 10-6 M, whereas the effect of endomorphins did not differ at the two concentration levels. Diprotin A potentiated only very modestly the action of endomorphins. These data (a) confirm the presence of functional μ-opioid receptors in the vagal complex, (b) render it likely that the enzymic degradation of endomorphins is not a highly effective process in brain slices and (c) may suggest that the apparent ceiling in the effect of endomorphins might be related to their partial agonist property.
- D-Ala,MePhe,Gly-ol-enkephalin (DAMGO)
- Diprotin A
ASJC Scopus subject areas
- Clinical Biochemistry
- Cellular and Molecular Neuroscience