Paul L. Wood

Multiple opiate receptors : Differential binding of μ, κ and δ agonists

Abstract Much data has accumulated suggesting that μ and δ opiate receptors represent distinct topographical units within the CNS. In addition, in vitro binding studies by the present authors also suggest that a unique κ receptor may be present in the brain. In this regard, [3H]ethylketocyclazocine binding to membrane preparations of rat brain was found to be potently displaced by κ partial μ and agonist/antagonist analgesics. However, μ, δ, σ and ϵ receptor agonists were much less active at this binding site, suggesting that the κ site possesses a different pharmacology from μ and δ sites. These data therefore support previous in vivo experiments which demonstrated a unique behavioral pharmacology and CNS distribution for κ-mediated analgesia.

Enkephalinase: selective peptide inhibitors.

A unique, CNS membrane bound enkephalinase is described with greatest activities being measured in the striatum of the mouse. This enzyme was resistant to inhibition by puromycin and bestatin which are potent aminopeptidase inhibitors and to the angiotensin converting enzyme inhibitors, captopril and the free acid of MK-421, which were also very weak inhibitors of aminopeptidase. However, the glycopeptide, phosphoramidon, and the hydroxamic acids, HO-NHCOCH(CH2CH(CH3)2)-CO-Ala-Gly-NH2 and HO-NHCOCH(CH2C6H5)-CO-Ala-Gly-NH2, were potent enkephalinase inhibitors with IC50s (nM) of 39, 3.1 and 8.4, respectively. These peptides remain to be tested invivo.

Spinal analgesia: comparison of the mu agonist morphine and the kappa agonist ethylketazocine.

Abstract The sites of analgesic action of the mu agonist morphine and the purported kappa agonist ethylketazocine (EKC) were compared. Using local drug injections and parenteral administration of drugs to spinalized rats, our data support a predominantly spinal site of action for EKC and a major supraspinal action for morphine in antinociceptive tests. This spinal analgesic action of EKC was dose dependent and naloxone reversible indicating opiate receptor involvement. The possibility that EKC activates a spinal kappa receptor population is under further study.

Antagonists of excitatory amino acids and cyclic guanosine monophosphate in cerebellum

The excitatory amino acid analogues kainate, quisqualate, domoic acid, 4-fluoroglutamate, homocysteic acid and N-methylaspartate as well as the tremor-inducing drugs harmaline and oxotremorine all induced significant elevations in cyclic guanosine monophosphate (cGMP) levels in the cerebellum in vivo. The putative antagonists of excitatory amino acids, 2-amino-5-phosphonovalerate (APV) and piperidine dicarboxylate (PDA) both blocked the actions of the tremorogens. Piperidine dicarboxylate also blocked the in vivo activity of all the amino acid analogues except homocysteic acid and N-methylaspartate. 2-Amino-5-phosphonovalerate (APV) was inactive against kainate, quisqualate and homocysteic acid. It therefore appears that PDA and APV are useful tools for the further study of the function of glutamate and asparatate receptors.

Morphine and nigrostriatal function in the rat and mouse: The role of nigral and striatal opiate receptors

Using local injections of drugs and hemisection experiments, a comparison was made of the actions of morphine on the dopaminergic nigrostriatal system of the rat and mouse. In the rat, morphine appeared to act exclusively at presynaptic opiate receptors on dopaminergic nerve endings in the striatum. Activation of these receptors resulted in enhanced dopamine synthesis but with no associated increase in dopamine release. In the mouse, morphine acted at the level of the substantia nigra to enhance both striatal dopamine synthesis and release. The exact localization of these receptors in the substantia nigra remains to be determined.

Cortical lesions modulate turnover rates of acetylcholine and γ-aminobutyric acid

Abstract Interruption of the cortico-striatal glutamatergic pathway by decoratication results in significant decreases in the turnover rates of acetylcholine TRACh and γ-aminobutyric acid TRGABA of striatum. These data support the hypothesis that the cortical input to the striatum is excitatory and acts to modulate cholinergic and GABAergic function in this nucleus. Studies of kainate-induced lesions of the striatum also indicate that most striatal cholinergic interneurons receive a glutamatergic input and that multiple injections of kainate are required to destroy this large population of interneurons.

Mu opiate isoreceptors: Differentiation with kappa agonists

In vivo pharmacological data support the concept of mu (mu) isoreceptors in the rat CNS. The mu 1 receptor mediates analgesia and regulation of cholinergic neurons. Naloxazone may be a specific antagonist of this receptor type. In contrast, the mu 2 receptor is responsible for respiratory depression and regulation of nigrostriatal dopaminergic neurons. In this case, the kappa agonists EKC and MR-2034 appear to be specific mu 2 antagonists.

Kyotorphin (tyrosine-arginine): Further evidence for indirect opiate receptor activation

Analgesia, opiate receptor binding, and neurochemical effects of kyotorphin (tyrosine-arginine) were studied in the rat. It was found that while kyotorphin, in vivo, causes naloxone reversible analgesia, and affects dopamine metabolism and acetylcholine turnover in the same manner as do morphine and other opiate agents, the dipeptide does not bind to mu, delta or kappa opiate receptors in vitro. Taken together, these data support the concept that there is an indirect action of kyotorphin on opiate receptors.

Actions of kappa, sigma and partial mu narcotic receptor agonists on rat brain acetylcholine turnover.

Cholinergic neurons which possess mu and delta receptors and/or which are regulated by neurons possessing these receptors, do not appear to have kappa receptors. Only at doses far exceeding (16--32 x) the analgesic ED50 of the prototype kappa antagonist ethylketazocine is acetylcholine turnover depressed in the cortex and hippocampus. This suggests that such activity may involve occupation of mu and/or delta receptors at high doses. The lack of activity of the sigma agonist SKF 10047 also indicates a possible absence of sigma receptors on cholinergic neurons. The partial mu antagonist buprenorphine was found to be a unique narcotic agent in that it behaved as a classical mu agonist at low doses, but as an antagonist at high doses.

Increases in choline levels in rat brain elicited by meclofenoxate

Meclofenoxate, the p-cholorophenoxyacetic acid ester of deanol, was found to dramatically elevate choline (Ch) levels in the rat CNS. In the hippocampus, this elevation in choline was accompanied by a new elevated steady state level in acetylcholine (ACh). No such coupling was observed in the striatum or parietal cortex. Deanol also elevated choline levels in the CNS but was about half as potent as meclofenoxate; p-chlorophenoxyacetic acid was inactive in this respect. Lesions of striatal neurons with kainic acid and of hippocampal cholinergic nerve endings with surgical section of the fimbria indicated that the changes in choline levels were mainly extraneuronal. In spite of the changes in choline and ACh levels, no consistant alterations in ACh turnover were measured. In summary, meclofenoxate induced dramatic alterations in CNS choline metabolism and may, therefore, be a useful therapeutic tool for potentiating depressed cholinergic neurons.