Characterization of high affinity opioid binding sites in rat periaqueductal gray P2 membrane

Enhanced brain opioid receptor activity precedes blood-brain barrier disruption

We studied the effects of transient postischemic increased opioid receptors (OPR) binding (mu, delta, kappa) on blood-brain barrier (BBB), brain water content and brain mitochondrial oxidative enzymes system. Cats were exposed to temporary middle cerebral artery occlusion (MCAO). The significant increased OPR bindings observed 10 min after the release of MCAO (ischemic rCBF = 7 +/- 1 to 11 +/- 2 ml/100 g/min) preceded the early and late BBB disruptions, brain edema and postischemic impaired mitochondrial oxidative enzymes functions. Further, the study suggests indirectly that the latter process was irreversible and hence associated with subsequent ischemic cerebral infarction. In addition, the results revealed a possible viable therapeutic window in the early postischemic recirculation period, before the onset of impaired mitochondrial oxidative function.

Pharmacology and mechanisms of opioid analgesic activity

Opiates by an action at specific receptors can induce a highly selective alteration in the response of humans and animals to strong and otherwise aversive chemical, mechanical or thermal stimuli. Specific investigations in a variety of species from rodent to primate using microinjection techniques to examine the pharmacology of local drug action have shown potent antinociceptive actions to be mediated by a receptor specific action at a number of sites within the brain, including the periaqueductal gray (PAG: mu receptor), the rostral ventral medulla (mu/delta receptor) and the substantia nigra (mu receptor) and within the spinal dorsal horn (mu/delta/kappa receptor). Mechanistic studies have shown these actions in the different sites to be mediated by several discrete mechanisms. For example, in the PAG, the local opiate effect is likely mediated by the indirect activation of bulbospinal pathways, rostral projections to forebrain sites and by a local alteration in afferent input into the brainstem core. In the spinal cord, this effect is mediated by an action presynaptic to the primary afferent and by a post-synaptic effect to hyperpolarize projection neurons. In addition, it is now appreciated that mu and kappa receptors in the periphery can modulate the sensitized state of the small afferent terminal innervating inflamed tissue and exert an anti-hyperalgesic action. After systemic delivery of an opiate, it is thus clear that a wide array of central and peripheral systems serve to explain the powerful analgesic effect exerted by this class of agents.

Immunoelectron microscopy of enkephalinergic innervation of GABAergic neurons in the periaqueductal gray

The pre-embedding double immunoreaction method was used to study synaptic relations of enkephalinergic and GABAergic neuronal elements in the ventrolateral part of the periaqueductal gray of the Wistar albino rat. The enkephalin-like neuronal elements were immunoreacted by the silver-gold intensified peroxidase-antiperoxidase method and the GABA-like immunoreactive neurons were immunoreacted by the unintensified peroxidase-antiperoxidase method. GABA-like immunoreactive neuronal somata were post-synaptic to both the enkephalin-like immunoreactive and the non-immunoreactive axon terminals. Enkephalin-like immunoreactive axon terminals were found to make synapses with GABA-like immunoreactive and non-immunoreactive dendrites. The synapses between the two kinds of chemically characterized neurons appeared to be both asymmetrical and symmetrical. Possible functional activity related to pain modulation, and synaptic relations between the enkephalinergic and GABAergic neurons in the periaqueductal gray and the dorsal raphe nucleus, are discussed.

Endogenous opioid system activity following temporary focal cerebral ischemia

We studied changes in opioid receptors (mu, delta, kappa) concentrations during temporary middle cerebral artery occlusion (MCAO) in cats by sequential displacement of unselective opioid antagonist, [3H]-diprenorphine with highly selective ligands for mu, delta and kappa, subsites. Following threshold cerebral ischemia (rCBF < 10 ml/100 g/min) there was a 2 to 3 fold increase in the 3 opioid receptor subtype concentrations at 10 min following the release of MCAO. Further, 56% of the cats depicted early postischemic hyperemia BBB opening, at 1 h and 3 h following the release of occlusion, with significant subsequent progression of brain edema. We believe that the enhanced brain opioid activity may be relevant to the neuronal damage caused by the early postischemic BBB opening.

Opioid peptides (DAGO-enkephalin, dynorphin A(1-13), BAM 22P) microinjected into the rat brainstem: comparison of their antinociceptive effect and their effect on neuronal firing in the rostral ventromedial medulla

The highly mu-selective agonist Tyr-D-Ala-Gly-MePhe-Gly-ol-enkephalin (DAGO) produces potent, dose-dependent naloxone-reversible antinociception when microinjected into the ventrolateral periaqueductal gray (PAG) (ED50 = 0.72 nmol) or rostral ventromedial medulla (RVM) (ED50 = 0.05 nmol) as measured on the rat tail flick (TF) assay. In single-unit recording experiments, DAGO microinjected into the PAG also affected On- and Off-Cell firing in the RVM in the same way as previously demonstrated by our group for morphine. PAG-microinjected DAGO inhibits spontaneous and noxious-evoked On-Cell firing (attenuating the characteristic On-Cell burst) (n = 19), and excites spontaneous Off-Cell firing, preventing the characteristic Off-Cell pause (n = 12) at doses which suppress the TF. These results support a major role for the mu receptor in PAG and RVM mechanisms of opiate antinociception. In our experiments using BAM22P, an endogenous weakly mu-selective opioid peptide, we could not demonstrate a dose-dependent antinociceptive effect, whether the peptide was microinjected supraspinally into the PAG (n = 9) or RVM (n = 11), or intrathecally at the lumbar cord (n = 4). In two animals, a naloxone-reversible antinociceptive effect was observed following the microinjection of 10 nmol BAM 22P into the RVM; however, no effect was seen in 3 animals microinjected with 20 nmol. Dyn A(1-13), a putative endogenous ligand for the kappa receptor, had no antinociceptive effect when microinjected into the ventrolateral PAG, and no effect on the firing (spontaneous or noxious-evoked) of RVM On (n = 3)- or Off (n = 2)-Cells.

Mu type opioid receptors in rat periaqueductal gray-enriched P2 membrane are coupled to G-protein-mediated inhibition of adenylyl cyclase

The periaqueductal gray (PAG) region of the midbrain has been implicated in both stimulation-produced and opioid-induced analgesia. High-affinity mu-selective opioid-binding sites associated with mu type opioid receptors have been detected in rat PAG-enriched P2 membranes, and these receptors have been shown to be coupled to guanine nucleotide-binding proteins (G-proteins). In the present study the potential G-protein-mediated coupling of mu type opioid receptors to the inhibition of adenylyl cyclase was examined utilizing in vitro adenylyl cyclase assays. In the presence of Na+, opioid agonists inhibited adenylyl cyclase in a mu selective, naloxone reversible, dose dependent, and pertussis toxin sensitive manner. Overall the data suggests that mu type opioid receptors in the rat PAG are coupled to G-protein-mediated inhibition of adenylyl cyclase.