Subcutaneous formalin enhances outflow of met-enkephalin- and cholecystokinin-like materials in the rat nucleus accumbens

Abstract

Using the microdialysis technique, the present study investigated the effects of a noxious stimulation on the extracellular levels of met-enkephalin and (sulfated octapeptide) cholecystokinin-like materials in the nucleus accumbens of freely moving rats. Injection of 50 microl of 5% formalin into the forepaw produced pain-related behaviours associated with an immediate and sustained (for approximately 2 h) increase (+27%) in the outflow of met-enkephalin-like material within the nucleus accumbens. This treatment also progressively enhanced the local outflow of cholecystokinin-like material that reached 200%-250% of the basal level at the end of the experiment, i.e. 4.5 h after formalin administration. Because naloxone (1.5 mg/kg i.p., 10 min prior to formalin injection) prevented the latter effect, it can be inferred that noxious stimulation-induced activation of cholecystokininergic neurotransmission in the nucleus accumbens probably resulted from the preceding activation of opioidergic systems. These data suggest that the nucleus accumbens may be another structure where interactions between opioids and cholecystokinin play a key role in the control of pain-processing mechanisms.

The Involvement of Descending Pain Inhibitory System in Electroacupuncture-Induced Analgesia

“…The peptide CCK-8 exerts anti-analgesic actions site- and dose-dependently in the both peripheral and central nervous systems (Rehfeld, 1980; Moran and Schwartz, 1994). It is generally accepted that CCK-8 can reverse the antinociceptive effect of morphine and therefore suppress the process of descending inhibitory initiated by opioids (Rehfeld, 1980; Lapeyre et al, 2001). Given the fact that opioids play a vital role in EA-induced analgesia, it is possible to assume the involvement of CCK-8 in reversing and prevention of EA antinociceptive effects.…”