Opioid and nonopioid cardiovascular effects of dynorphins

Dynorphin A (2-13) improves mecamylamine-induced learning impairment accompanied by reversal of reductions in acetylcholine release in rats

Accumulating evidence indicates that the endogenous opioid peptides dynorphin A (1-17) and synthetic dynorphin A (1-13) interact not only with opioid receptors but also with as yet poorly characterized non-opioid binding sites. Dynorphin A (1-13) improved impairments of learning and memory via not only kappa-opioid receptor-mediated, but also 'non-opioid' mechanisms. In the present study, the effects of des-tyrosine(1) dynorphin A (2-13) as a non-opioid metabolite of dynorphin A, and dynorphin A (1-13) on mecamylamine-induced impairment of the acquisition of learning in rats were investigated using a step-through type passive avoidance task. Further, hippocampal acetylcholine release was examined using in vivo microdialysis. Mecamylamine significantly shortened the step-through latency when given 30 min before the acquisition trial. Not only dynorphin A (1-13) but also dynorphin A (2-13) attenuated the mecamylamine-induced impairment of the acquisition of learning. The effect of dynorphin A (2-13) was not blocked by pre-treatment with nor-binaltorphimine (nor-BNI), a selective kappa-opioid receptor antagonist. Dynorphin A (2-13) completely abolished the decrease in the extracellular acetylcholine concentration induced by mecamylamine and this effect was not blocked by nor-BNI. Taken together with our previous findings, the present results may indicate that dynorphin A (2-13) improves impairment of learning and/or memory in 'non-opioid' mechanisms and dynorphin A (1-13) ameliorates impairment of the acquisition of learning via not only kappa-opioid receptor-mediated mechanisms but also 'non-opioid' mechanisms, by regulating the release of extracellular acetylcholine.

A direct chemical interaction between dynorphin and excitatory amino acids

The endogenous opioid peptide dynorphin A elicits non-opioid receptor-mediated neurotoxic effects. These effects are blocked by pretreatment with N-methyl-D-aspartate (NMDA) receptor antagonists. Herein, the mechanism for the non-opioid effects of dynorphin and related peptides was studied by matrix-assisted laser desorption ionization (MALDI) mass-spectrometry. We observed that both glutamate or aspartate bind non-covalently to dynorphin A and dynorphin 2-17. However, when dynorphin A or dynorphin 2-17 were added to an equimolar mixture of Glutamate and Aspartate, they both complexed preferentially with glutamate. These data may explain the non-opioid physiological effects of dynorphin A and related peptides and indicate that the direct chemical interaction between neurotransmitters should be monitored when studying interactions between different neurochemical systems.

Interactions of dynorphin A-(1-13) and nociceptin with cardiac D2 binding sites: inhibition of ischemia-evoked release of noradrenaline from synaptosomal-mitochondrial fractions

The effect of dynorphin A (Dyn A)-related peptides and nociceptin on the binding of the D2 receptor antagonist, [(3)H]raclopride, was examined in membrane preparations of rat heart. Non-linear regression saturation binding analysis of [(3)H]raclopride binding revealed the presence of a single high-affinity binding site with a K(d)of 4.1 n M and a B(max)of 220 fmol/mg protein. The D2 stereospecificity of [(3)H]raclopride binding was demonstrated by competition experiments using two enantiomer pairs of antagonists. (+)-Butaclamol (IC(50): 8.0 n M) and (-)-sulpiride (IC(50): 112.3 n M) were 27 000 and 24 times more potent than (-)-butaclamol (IC(50): >100 microm) and (+)-sulpiride (IC(50): 2666 n M), respectively. Nociceptin and Dyn A-(1-13) were also potent inhibitors of the binding of [3H]raclopride with shallow inhibition curves that fitted best with two sites model. Their order of potency on the low affinity site [alpha -Neo-endorphin>nociceptin>Dyn A-(2-13)>Dyn A-(1-13)>Dyn B>Dyn A-(6-10)] correlated well with their ability to inhibit the binding of [3H]nociceptin (r=0.82). The indirect nature of the inhibitory effects of the peptides on the D2 receptor was demonstrated by their inability to inhibit [(3)H]raclopride binding to a membrane preparation (Sf9 cells transfected with the human D2(long)receptor) that does not contain the ORL(1)receptor and the lack of effect of raclopride (0.1 n M-10 microm) on both [(3)H]nociceptin and [(3)H]Dyn A-(1-13) binding. Isolated cardiac mitochondrial-synaptosomal fractions submitted to ischemic conditions (1 m M iodoacetate +2 m M NaCN, 5 min at 37 degrees C) released 10.9% of their content in preloaded [(3)H]noradrenaline ([(3)H]NA). Dyn A-(1-13) (10 microm), nociceptin (10 microm) and the selective D2 receptor agonist, quinpirole (10 microm) were potent blockers of the release of [(3)H]NA evoked by the ischemic conditions. The inhibitory effect of Dyn A-(1-13), nociceptin and quinpirole were antagonized by the selective D2 receptor antagonist, raclopride (10 microm); whereas naloxone, at a concentration (1 microm) known to affect the ORL(1)receptor, blocked the effects of the peptides but not those of quinpirole. The results demonstrate the presence of D2 receptors in rat heart and suggest that Dyn A-(1-13) and nociceptin modulate ischemia-induced NA release by a mechanism that involves the participation of both ORL(1)and D2 receptors.

Chronic blockade of hippocampal kappa receptors increases arterial pressure in conscious spontaneously hypertensive rats but not in normotensive Wistar Kyoto rats

Previous studies in this laboratory have shown that dynorphins acting on hippocampal kappa opioid receptors in rat brain exert a restraining influence on arterial blood pressure and that a relative deficiency of their production in the spontaneously hypertensive rat (SHR) may be involved in SHR hypertension. Kappa receptor blockade should therefore exacerbate hypertension in SHR. We explored the latter possibility by chronic unilateral infusion of nor-binaltorphimine dihydrochloride (nor-BNI), a selective kappa receptor antagonist, into the right dorsal hippocampus of conscious SHR and normotensive Wistar-Kyoto (WKY) controls over a 14 day period. Additional controls consisted of similar hippocampal infusions of equivalent volumes of drug vehicle in both rat strains. Mean arterial pressure (MAP) and heart rates (HR) were determined in each animal once daily by the tail cuff method during this period. Nor-BNI induced a sustained increase in the basal high level of MAP in SHR from 132 +/- 8 to 150 +/- 10 mmHg throughout the 14 days and an increased HR from 427 +/- 17 to 477 +/- 30 on day 3 to 5 following the drug. By contrast, nor-BNI had no significant effects on either MAP or HR in WKY rats and control infusions of drug vehicle were similarly without effect in both strains. The results support our previous suggestion that the kappa opioid system of the hippocampus ordinarily restrains arterial blood pressure in SHR since prolonged hippocampal kappa receptor blockade results in augmented hypertension in this strain.

Endogenous opiates: 1997

This paper is the twentieth installment of our annual review of research concerning the opiate system. It summarizes papers published during 1997 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; eating and drinking; alcohol; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunologic responses; and other behaviors.

Structure-activity relationship studies of dynorphin A and related peptides

An up-to-date review is presented covering all the available information concerning the isolation, discovery, synthesis, conformation, receptor binding characteristics, pharmacological properties and SAR studies of dynorphin A and related peptides. The potential of dynorphin A and its analogs has yet to be fully realized.