It is well known that opioids cause analgesia and feeding elicitation by action in the either periaqueductal gray (PAG) or ventral tegmental area (VTA).
We have investigated that determination of opioids receptor subtype on the lateral hypothalamic area, as feeding center, electrical stimulation induced feeding (LHA-ESIF) followed microinjection of μ-(morphine), δ-(DADLE) and κ-(U50, 488H) receptor selective agonists and physiological saline into the VTA and PAG of chronic Wistar male rats, weighing between 350 and 480g. with stimulation fixed at a modulate level, 50Hz. time to eat three-45mg pellets within 45sec. were studied.
Dose of 10 (but not 1) nmol of three agonists microinjected into the VTA significantly (p<0.01) reduced time of LHA-ESIF, these agonists were almost equally effective at this dose. Naloxone(NLX) reversed the effects of each agonists. NLX was slightly more effective agonist morphine than against DADLE or U50, 488H. This results suggest that all three receptor subtype may be contributed to the VTA fascilitation of the LHA-ESIF, and NLX is a selective antagonist of μ-receptor.
In the microinjection of PAG, morphine (20, but not 2nmol) showed increasing LHA-ESIF, while both δ-and κ-agonists were each without effects. These results indicated that the PAG inhibition of LHA-ESIF is mediated solely by μ-receptor.
It is, thus suggests that opioids receptor subtypes on the PAG, as site of the acupuncture analgesia, might be contributed, neither δ-nor κ-receptors, through only via μ-receptor.