The analgesic effect of opioids varies widely among vertebrate species. This may be due to the distribution, number and type of opioid receptors within the brain. In general, distribution of opioid receptor types is conserved across species in brainstem and spinal cord areas but varies significantly in the forebrain [2]. In the pigeon forebrain 76% of the total opioid receptors are kappa [2]. E...

Zhu M, Cho YK, Li C-S. Activation of -opioid receptors reduces excitatory input to putative gustatory cells within the nucleus of the solitary tract. J Neurophysiol 101: 258–268, 2009. First published November 19, 2008; doi:10.1152/jn.90648.2008. The rostral nucleus of the solitary tract (NST) is the first central relay in the gustatory pathway and plays a key role in processing and modulation ...

delta and micro opioid receptors are homologous G protein-coupled receptors that are differentially sorted between divergent degradative and recycling membrane pathways following agonist-induced endocytosis. Whereas delta opioid receptors are selectively sorted to lysosomes, micro opioid receptors recycle rapidly to the plasma membrane by a process that has been proposed to occur via bulk membr...

One of the main components of the stress system is hypothalamus- pituitary-adrenal (HPA) axis. Acute activation of µ-opioid receptors increases the activity of the HPA axis, leading to release of ACTH and corticosterone. Glucocorticoids can change behaviors, depend on age but there were no evidences about the interaction between age, opioid system and glucocorticoids. In this experiment, ...

Chronic morphine treatment has been shown to produce constitutive activation of mu-opioid receptors, and this transition might contribute to the development of tolerance and dependence. The apparent ability of chronic morphine to increase the spontaneous, agonist-independent activation of mu-opioid receptors may be unique, due to its distinct partial agonist properties of possessing a relativel...

The antinociception induced by i.t. or i.c.v. administration of endomorphins is mediated via mu-opioid receptors. However, although endomorphins do not have an appreciable affinity for kappa-opioid receptors, pretreatment with the kappa-opioid receptor antagonist norbinaltorphimine markedly reduces the antinociceptive response to i.c.v. or i.t. administered endomorphin-2 but not endomorphin-1. ...

Four experiments studied the role of opioid receptors in the midbrain periaqueductal gray matter (PAG), an important structure eliciting conditioned fear responses, in the extinction of Pavlovian fear. Rats received pairings of an auditory conditioned stimulus (CS) with a foot shock unconditioned stimulus (US). The freezing conditioned response (CR) elicited by the CS was then extinguished via ...

Opioids have played a critical role in achieving pain relief in both modern and ancient medicine. Yet, their clinical use can be limited secondary to unwanted side effects such as tolerance, dependence, reward, and behavioral changes. Identification of glial-mediated mechanisms inducing opioid side effects include cytokine receptors, kappa-opioid receptors, N-methyl-D-aspartate receptors, and t...

1. When administered subcutaneously HS-599, a new didehydroderivative of buprenorphine (18,19-dehydrobuprenorphine), produced a long-lasting antinociceptive response in rats. Its potency exceeded twice that of buprenorphine. In the tail-flick test it acted as a full agonist but in the plantar test only as a partial agonist. Whereas the mu-opioid antagonists naloxone and naltrexone antagonized H...

Neuropeptide FF (NPFF) is known to be an endogenous opioid-modulating peptide. Nevertheless, very few researches focused on the interaction between NPFF and endogenous opioid peptides. In the present study, we have investigated the effects of NPFF system on the supraspinal antinociceptive effects induced by the endogenous µ-opioid receptor agonists, endomorphin-1 (EM-1) and endomorphin-2 (EM-2)...