Effect of dynorphin-(1-13) and related peptides on respiratory rate and morphine-induced respiratory rate depression

Interaction between amino acid neurotransmitters and opioid receptors in hyperthermia-induced brain pathology

This review is focused on the possible interaction between amino acid neurotransmitters and opioid receptors in hyperthermia-induced brain dysfunction. A balance between excitatory and inhibitory amino acids appears to be necessary for normal brain function. Increased excitotoxicity and a decrease in inhibitory amino acid neurotransmission in hyperthermia are associated with brain pathology and cognitive impairment. This is supported by recent data from our laboratory that show a marked increase in glutamate and aspartate and a decrease in GABA and glycine in several brain areas following heat stress at the time of brain pathology. Blockade of multiple opioid receptors with naloxone restored the heat stress-induced decline in GABA and glycine and thwarted the elevation of glutamate and aspartate in the CNS. In naloxone-treated stressed animals, cognitive dysfunction and brain pathology are largely absent. Taken together, these new findings suggest that an intricate balance between excitatory and inhibitory amino acids is important for brain function in heat stress. In addition, opioid receptors play neuromodulatory roles in amino acid neurotransmission in hyperthermia.

Dynorphin-containing axons directly innervate noradrenergic neurons in the rat nucleus locus coeruleus

Stress causes increased dynorphin (DYN) expression in limbic brain regions and antagonism of kappa-opioid receptors may offer therapeutic potential for the treatment of depression. A potential site of DYN action relevant to stress and related neuropsychiatric disorders is the locus coeruleus (LC), the primary source of forebrain norepinephrine. Therefore, using immunofluorescence and immunoelectron microscopic analyses, we characterized the cellular substrates for interactions between DYN and tyrosine hydroxylase (TH), a catecholamine synthesizing enzyme in single sections through the rat LC. Light microscopic analysis of DYN immunoreactivity indicated that DYN fibers are distributed within the core and pericoerulear subregions of the LC. Using electron microscopy, immunoperoxidase labeling for DYN was primarily found in axon terminals, although in some cases was diffusely localized to somatodendritic processes. When DYN-containing axons formed synaptic contacts, they typically (89%) exhibited an asymmetric morphology. Almost a third (28%) of the postsynaptic targets of DYN-containing axons contained immunogold labeling for TH. These findings reveal some diversity as to the localization of DYN in the LC within axons that contact both TH and non-TH containing dendrites. However, the present data provide the first ultrastructural evidence that DYN-containing axon terminals directly innervate catecholaminergic LC dendrites. Moreover, DYN axon terminals targeting catecholaminergic LC dendrites via asymmetric synapses are consistent with localization within excitatory type afferents to the LC. Therefore, direct modulation of catacholaminergic LC neurons maybe an important site of action for DYN relevant to stress and stress-related disorders.

Dynorphin-A(1–17) decreases nitric oxide release and cytotoxicity induced with lipopolysaccharide plus interferon-γ in murine macrophage cell line J774

Nitric oxide (NO) is an important mediator of cytotoxicity caused by macrophages or by their resident counterpart in brain-glial cells. Modulation of NO release by both activated macrophages and glial cells has been reported in the presence of endogenous (peptide) and synthetic (non-peptide) agonists with kappa opioid-receptors (KOR) selectivity. The data obtained with macrophages and glial cells are contradictory: enhanced NO release by mouse macrophages was reported in the presence of synthetic agonist of KOR selectivity (Neuropeptides 32 (1998) 287), and decreased NO release by glial cells, in the presence of dynorphin-A((1-8)), endogenous opioid peptide with KOR selectivity (J. Biomed. Sci. 7 (2000) 241). In this study, we used a murine cell line J774 of macrophage origin and examined the effect of dynorphin-A((1-17)), endogenous opioid peptide with selectivity for KOR, on NO release induced with lipopolysaccharide (LPS) plus interferon-gamma (IFN-gamma). Dynorphin-A((1-17)) was chosen since in comparison to dynorphin-A((1-13)), it is more resistant to biodegradation (Peptides 17 (1996) 983), and its effects during prolonged treatment of cells could be more pronounced. The effect of dynorphin-A((1-17)) on NO release was compared to its effect on cytotoxicity, induced with LPS plus IFN-gamma. The data obtained have shown that activation-induced NO release by J774 cells is decreased in the presence of dynorphin-A((1-17)). This was associated with deceased LPS and IFN-gamma-induced cytotoxicity of J774 cells, suggesting their causal relationship. Neither of the observed effects of dynorphin-A((1-17)) could be prevented with the KOR selective antagonist, norbinaltorphimine, suggesting that they are mediated via non-opioid mechanism. By diminishing NO release dynorphin-A((1-17)) may affect cytotoxic ability of macrophages, but may also beneficially influence inflammation-induced damage of local tissue.

Effects of the potent analgesic enkephalin-catabolizing enzyme inhibitors RB101 and kelatorphan on respiration

We investigated whether the enkephalin-catabolizing enzyme inhibitors RB101 and kelatorphan, which have been shown to be potent analgesics, depress respiration as do opioid analgesics. Ventilation was measured in cats and rodents by the barometric method, in the awake state and during anesthesia. Tissue distribution of the inhibitors was either generalized (RB101, 40-160 mg/kg i.p.), largely restricted by the blood-brain barrier to the periphery (kelatorphan, 0.7-20 mg/kg i.v.), or restricted to the brainstem (i.c.v. injection of RB101 in the fourth ventricle). RB101 did not affect ventilation in any condition tested, and large doses of kelatorphan produced a naloxone-reversible increase in ventilation and breathing frequency. Thus endogenous opioids released during conditions of normal ventilation do not exert any depressant neuromodulatory effect on this function, even when their extracellular concentrations are increased by peptidase inhibitors. The differential effect of these inhibitors on ventilation and nociception is discussed. We conclude that kelatorphan and RB101 are devoid of respiratory-depressant effects and might be interesting pharmacological alternatives to morphine and other opioid agonists.

Reduction of lipopolysaccharide-induced neurotoxicity in mouse mixed cortical neuron/glia cultures by ultralow concentrations of dynorphins

Previously we reported that ultralow concentrations of dynorphins (10(-16) to 10(-12) M) inhibited lipopolysaccharide (LPS)-induced production of nitric oxide (NO) and proinflammatory cytokines in mouse glia without the participation of kappa-opioid receptors. In the current study using mouse cortical neuron-glia cocultures, we examined the possibility that inhibition of glia inflammatory response by dynorphins might be neuroprotective for neurons. LPS, in a concentration-dependent manner, markedly increased the release of lactate dehydrogenase (LDH), an indicator of cellular injury. Ultralow concentrations (10(-14) to 10(-12) M) of dynorphin (dyn) A-(1-8) significantly prevented the LPS-induced release of LDH, loss of neurons, and changes in cell morphology, in addition to inhibition of LPS-induced nitrite production. Meanwhile, ultralow concentrations (10(-15) to 10(-13) M) of des-[Tyr(1)]-dyn A-(2-17), a nonopioid peptide which does not bind to kappa-opioid receptors, exhibited the same inhibitory effect as dyn A-(1-17). These results suggest that dynorphins at ultralow concentrations are capable of reducing LPS-induced neuronal injury and these neuroprotective effects of dynorphins are not mediated by classical opioid receptors.

Pharmacokinetics of intravenous dynorphin A(1-13) in opioid-naive and opioid-treated human volunteers

BACKGROUND

Dynorphin A(1-13) is a fragment of the endogenous opioid neuropeptide dynorphin A. Previous research suggested that intravenously administered dynorphin A(1-13) has the ability to modulate morphine-induced analgesia. We designed this study to characterize the disposition of intravenous dynorphin immunoreactivity in humans and to determine whether concomitant long-term opioid therapy influenced the pharmacokinetics or side-effects profile of dynorphin A(1-13).

METHODS

The study subjects comprised 20 volunteers divided into two groups of 10 each, stratified by dose (low dose, 250 micrograms/kg; high dose, 1000 micrograms/kg). There were four volunteers receiving long-term opioid therapy and six opioid-naive volunteers (nonopioid group) within each dosing group. Dynorphin A(1-13) was infused over 10 minutes, and arterial blood samples were drawn and assayed for dynorphin immunoreactivity. A population modeling approach was used to characterize the pharmacokinetics. Dynorphin effects on heart rate and arterial blood pressure were also studied.

RESULTS

The pharmacokinetics of dynorphin immunoreactivity were linear over the dose range studied and were best described by a three-compartment mammillary model whose parameters were volume 1, 5.0 L; volume 2, 0.80 L; volume 3, 12 L; clearance 1, 6.0 L/min; clearance 2, 0.054 L/min; and clearance 3, 0.044 L/min. Concomitant opioid medication did not affect the disposition of dynorphin immunoreactivity. Tachycardia and flushing were commonly observed side effects. The incidence of side effects was dose dependent and was not influenced by long-term opioid use.

CONCLUSIONS

Intravenously administered dynorphin A(1-13) is very rapidly metabolized, on the basis of the time course of immunoreactivity in the blood. Long-term opioid therapy did not influence either the pharmacokinetics or incidence of side effects.

RB 101, a purported pro drug inhibitor of enkephalin metabolism, is antinociceptive in pregnant mice

In an earlier study, we demonstrated the enhancement of pregnancy-induced analgesia with an inhibitor of endogenous enkephalin metabolism. The purpose of the present study was to evaluate the antinociceptive effect of another inhibitor of enkephalin metabolism, RB 101, on pregnant mice. Further, since other studies have shown RB 101 to be free of opioid side effects, we examined its effect on respiratory rate. Analgesia was assessed using the hot plate test, and respiratory rate was measured by recording the output from an end-tidal carbon dioxide detector. In pregnant mice, experiments were conducted on Day 17 or Day 18 of pregnancy; mice usually deliver on Day 19. For the hot plate test, animals were tested in the following groups: Group 1, RB 101 150 mg/kg (n = 15); Group 2, RB 101 50 mg/kg (n = 15); Group 3, RB 101 vehicle (n = 15); Group 4, morphine 5 mg/kg (n = 14); and Group 5, RB 101 150 mg/kg + naloxone 5 mg/kg (n = 10). The test was repeated on the second day after delivery in animals in Groups 1 and 3 (given RB 101 150 mg/kg and RB 101 vehicle, respectively). RB 101 150 mg/kg and morphine 5 mg/kg were significantly different (mean percentage of maximum possible effect 30.0 and 37.7, respectively, at 30 min and 41.6 and 32.6, respectively, at 60 min) in their antinociceptive effect in pregnant animals from all other groups. Naloxone, when coadministered with RB 101, prevented the development of antinociception. RB 101 150 mg/kg was not antinociceptive after delivery. Depression of respiratory rate was tested in a separate set of animals in the following groups: Group 1, RB 101 150 mg/kg (n = 16); Group 2, morphine 5 mg/kg (n = 16); Group 3, RB 101 vehicle (n = 15). Morphine 5 mg/kg produced significant depression of respiratory rate at 30 min postinjection when compared with RB 101 150 mg/kg and RB 101 vehicle (mean percent change in respiratory rate was 78.5% compared with 87.7% and 92.4%, respectively, where 100% = no change). These results suggest that drugs such as RB 101 may produce antinociception with minimal effects on respiration.

Effects of high intravenous doses of dynorphin A(1-13) on tail flick latency and central nervous system histology in rats

Dynorphin A(1-13) blocks opiate withdrawal in rats without producing dependence, and enhances analgesia in morphine-tolerant animals. Its potential use in humans is therefore of interest. Dynorphin A(1-13) has little toxicity when administered at modest doses IV but has been reported to cause hindlimb paralysis and necrosis of the spinal cord in rats, at the catheter tip, when administered intrathecally. To further evaluate its potential neurotoxicity, we administered dynorphin A(1-13) to rats at very high doses IV. Rats (n = 6-10 per group) received dynorphin A(1-13) as bolus IV doses of 5 mg/kg, or as continuous IV infusions of 40 mg/kg/day for 1 day, with saline controls. The appearance and behavior of all animals was normal. Tail flick latencies remained unchanged (p > 0.5). There were no histologic abnormalities of the spinal cord or brain when examined by light microscopy. Two additional groups received bolus injections of dynorphin A(1-13) 50 or 100 mg/kg IV. Animals receiving 50 mg/kg showed cutaneous flushing, labored respirations, and decreased spontaneous movement, which resolved within 10 min. Histology at 1 week was normal. All six animals receiving 100 mg/kg convulsed and died within minutes. Three animals that received dynorphin A(1-13) 40 mg/kg/day for 7 days had normal behavior and histology. We conclude that the previously observed neurotoxicity of intrathecally administered dynorphin A(1-13) is a local effect that does not occur when dynorphin A(1-13) is administered IV, even at very high doses.

Involvement of mesolimbic kappa-opioid systems in the discriminative stimulus effects of morphine

The neuroanatomical basis of opiate addiction has been studied using a variety of behavioural techniques. The aim of the present study was to investigate the role of mesolimbic opioid systems, in particular kappa-opioid systems, in the expression of the discriminative stimulus effects of abused drugs. Rats were trained to discriminate morphine (3.0 mg/kg s.c.) from saline under a fixed ratio schedule of food reinforcement. Once rats had acquired the discrimination, a randomized sequence of different doses of the highly selective kappa-opioid receptor agonist U69593 (0.02-0.16 mg/kg s.c.) was given 20 min prior to a systemic morphine injection. U69593 dose-dependently blocked the morphine discrimination. It is important to note that U69593 at these doses failed to generalize to the systemic morphine cue. The site of action by U69593 (0.02-0.16 microgram) was examined by microinjecting discrete amounts into target brain regions. Intra-nucleus accumbens injections of U69593 dose-dependently blocked the systemic morphine cue, whereas, U69593 failed to generalize to the discriminative stimulus. The same doses did not affect morphine discrimination after intra-ventral tegmental area or striatum injections. Besides the rewarding effects of drugs of abuse, the discriminative stimulus properties of these agents are seen as a major factor in drug seeking behaviours. The present study shows that the discriminative effects of morphine, a measure of the subjective effects of this drug can be blocked by the activation of kappa-opioid receptors located in the nucleus accumbens. In view of these findings which show that the activity of endogenous potassium-opioid systems (dynorphin) may serve as physiological antagonists to counteract the effects of morphine, potassium-agonists therefore may be useful in the treatment of opioid addictions.

Long-term observations in gerbil brain following transient cerebral ischemia: autoradiographic and histological study

We investigated the long-term changes that occur in the gerbil brain following transient cerebral ischemia using histology and receptor autoradiography. Transient ischemia was induced for 3 and 10 min, and animals were allowed to survive for 8 months. A histological study showed that 3-min ischemia caused neuronal damage and mild atrophy only in the hippocampal CA1 sector, and that 10-min ischemia produced severe neuronal damage and marked shrinkage in the hippocampal CA1 and CA3 sectors. Furthermore, severe neuronal damage was seen in the striatum after 10-min ischemia. Autoradiography study revealed that 3-min ischemia caused a significant reduction in [3H] naloxone binding in the frontal cortex, striatum, dentate gyrus, and thalamus, whereas [3H]SCH 23390 and [3H] forskolin binding was not significantly altered in all regions. In contrast, 10-min ischemia produced marked alteration in these binding sites in the striatum, hippocampus, thalamus, and substantia nigra. The alteration was especially notable in the hippocampal region and substantia nigra. These results indicate that hippocampal damage after transient ischemia, compared with that in other regions, is not static, but particularly progressive. Furthermore, they demonstrate a reduction in adenylate cyclase system in the striatum and substantia nigra after transient ischemia. Moreover, our results suggest that long-term survival after ischemia may induce synaptic modification of neurotransmitter and adenylate cyclase system in the hippocampus.

The kappa-opioid receptor: evidence for the different subtypes

Classification of drugs acting on the kappa-opioid receptors seems to be difficult, since some of these ligands are also sigma agonists and/or display non-opioid actions as well. Furthermore, certain benzomorphans having kappa-agonistic character, are shown to be mu-antagonists too. Therefore the classification of the kappa-opioid receptor has to be presently restricted to two subclasses that also have physiological meaning. Dynorphin and Met-enkephalin-Arg6-Phe7 are proposed as endogenous peptide ligands for kappa-receptors. Nonpeptide agonists are benzeneacetamides interacting with the kappa1 receptor. Benzomorphans bind to both subtypes of kappa-receptors. No selective nonpeptide ligand for the kappa2 receptor exists as yet. Nor-binaltorphimine, a specific kappa-antagonist also inhibits both kappa-subtypes. Further research for kappa2 selective drugs is necessary for clear distinction between the two kappa-opioid binding sites. Molecular cloning of opioid receptors including their subtypes are expected to provide direct proof of their existence.

Effects of acute and chronic ketocyclazocine and its modulation by oxytocin or vasopressin on food intake in rats

The effects of acute and chronic ketocyclazocine (KCZ, a kappa receptor agonist) and its interactions with oxytocin (OXY) or vasopressin (AVP) were investigated on food intake in free-fed rats. Acute treatment with KCZ (1 mg/kg) produced a generalized hyperphagia during the light phase (0-6 h) without influencing dark phase (6-24 h) food intake. On chronic administration, tolerance developed to hyperphagic effect during light phase, whereas an enhancement in the food intake was seen during dark phase. OXY or AVP (both at 10 micrograms/kg) per se, did not affect the food intake response during either the light or the dark phase, after acute as well as chronic treatment. In the interaction studies, acute AVP or OXY attenuated the hyperphagia of KCZ during the light phase. On chronic treatment, both AVP and OXY blocked (a) the tolerance, and (b) the "reverse tolerance" to the food intake response to KCZ during light and dark phases, respectively. These results are discussed in light of complex opioid-OXY/AVP interactions during food intake in rats.

Relationship between regulation of morphine-induced EEG effects and changes in naloxone sensitivity

The present data indicate that pretreatment with i.c.v. injection of dynorphin, morphine and dynorphin/morphine resulted in quantitative and qualitative changes in EEG power spectra in rats given i.c.v. morphine 24 h later. Correlated changes in sensitivity to antagonism of these EEG effects by naloxone were also found. Rats were implanted with cortical EEG electrodes and i.c.v. and i.v. cannulas. I.c.v. injections of morphine (20 micrograms/rat) produced high-voltage, slow-wave EEG bursts (1-10 Hz) associated with behavioral stupor which lasted about 2 h. Injections of i.c.v. morphine in rats pretreated with i.c.v. dynorphin (20 micrograms/rat), morphine (20 micrograms/rat) or dynorphin/morphine 24 h earlier, produced quantitative increases in absolute EEG spectral power. Injections of i.c.v. morphine in rats pretreated with i.c.v. dynorphin/morphine 24 h earlier, also produced qualitatively different EEG power spectra with a predominant peak in the 4-6 Hz band, similar to the EEG power spectra seen after acute administration of kappa opioids. After 20 min of morphine-induced high voltage EEG bursts, i.v. naloxone was given in sequential doses (0.0025, 0.0125, 0.025, 0.050 mg/kg) every 3 min until the EEG bursts were suppressed for 20 min. Relatively low doses of naloxone suppressed morphine-induced EEG bursts in rats that received i.c.v. H2O/H2O pretreatment. Slightly higher, but significant, doses of naloxone suppressed morphine-induced EEG bursts in rats that received i.c.v. H2O/morphine or dynorphin/H2O pretreatment. Moreover, a 10-fold increase in naloxone dose was needed to suppress EEG bursts in rats that received dynorphin/morphine pretreatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Dynorphin A(1-13) modulates apomorphine-induced behaviors using multidimensional behavioral analyses in the mouse

The effects of intracerebroventricular injection of dynorphin A(1-13) on apomorphine-induced behavioral changes were investigated in the mouse using multidimensional behavioral analyses based upon a capacitance system. Although lower doses (0.1 or 0.3 mg/kg) of apomorphine were without marked effects on behaviors, a 0.56 mg/kg dose of the drug evoked a significant increase in rearing behaviors. Furthermore 1.0 and 3.0 mg/kg doses of apomorphine produced a marked increment in linear locomotion, circling and rearing. Dynorphin A(1-13) (3.0 or 10.0 microgram) itself had no effects on behaviors. The apomorphine (0.56 and 1.0 mg/kg)-induced increase in rearing behaviors was clearly inhibited by treatment with dynorphin A(1-13) (3.0 and 10.0 microgram). Simultaneously, the marked increases in linear locomotion and circling were displayed by apomorphine (1.0 mg/kg) plus dynorphin A(1-13) (10.0 microgram). The effects of dynorphin A(1-13) (10.0 microgram) on the apomorphine (1.0 mg/kg)-induced increase in rearing were entirely reversed by the opioid antagonist Mr2266. These results suggest that the antagonistic effects of dynorphin A(1-13) on the apomorphine (1.0 mg/kg)-induced increase in rearing are mediated via opioid receptors, possibly K-sites.

Morphine-like discriminative stimulus effects of opioid peptides: possible modulatory role of D-Ala2-D-Leu5-enkephalin (DADL) and dynorphin A (1-13)

The role of the different opioid receptors was studied in rats trained to discriminate SC injections of 3.0 mg/kg morphine from saline by tests for generalization to graded doses of morphine and receptor-selective peptides administered into the lateral cerebral ventricle. Dose-dependent morphine-like stimulus effects were produced over a wide range of doses (0.001-30 micrograms), depending upon ligand and animal, by morphine, by the mu-selective peptides DAGO[D-Ala2-NMePhe4-Gly(ol)-enkephalin] and FK33824[D-Ala2,NMePhe4-Met(O)5-(ol)-enkephalin], and by the delta-selective peptide, DADL[D-Ala2,D-Leu5)enkephalin]. The order of relative potency of these substances was: FK33824 greater than DAGO greater than morphine greater than DADL. In contrast, DPLPE[D-Pen2,L-Pen5)enkephalin], which has much greater delta receptor selectivity than does DADL, and dynorphin A(1-13) (0.1-10 micrograms), a kappa-receptor agonist, engendered choice responding appropriate for saline. When 1.0 micrograms DADL, a dose lacking morphine-like discriminative effects, was administered concurrently with SC morphine, the stimulus effects of morphine were potentiated. Concurrent administration of 10 micrograms dynorphin A(1-13) and morphine attenuated the stimulus effects of morphine inconsistently. These results support previous findings that mu-opioid receptors are of primary importance in mediating the morphine-like discriminative effects of opioid peptides. They also suggest that morphine-like discriminative effects can be modulated by other types of opioid receptors.

Striatal neurochemistry of dynorphin-(1-13): in vivo electrochemical semidifferential analyses

The striatal neurochemistry of dynorphin-(1-13) was studied by simultaneously measuring extracellular dopamine and serotonin voltammetrically and in vivo after the injection of dynorphin-(1-13) to male Sprague-Dawley rats. The subcutaneous administration of dynorphin-(1-13), at a dose (1.5 mg/kg), known to exert CNS mediated behavioral effects, caused a statistically significant decrease in extracellular dopamine and a statistically significant increase in extracellular serotonin from rat anterior striatum. These parallel and opposite effects of dynorphin-(1-13) on these biogenic amines occurred gradually during a three hour time course. Maximal effects on dopamine (55%) and on serotonin (62%) occurred at the end of the three hour period of study. Mean effects on dopamine and serotonin (35% and 42% respectively) were averaged from scan results over the three hour period of study; the results were significantly different from control values. Dose response studies showed that a lower dose of dynorphin-(1-13) (0.5 mg/kg sc) had little or no effect on the alteration of these biogenic amines from striatum. The highest dose of dynorphin-(1-13) studied, (3.0 mg/kg sc), predictably and significantly altered extracellular biogenic amines. The dose response, however, was not incremental. The results are consistent with the role of dynorphin-(1-13) as a neuromodulatory peptide. The results further support the concept that the neuromodulatory role of dynorphin-(1-13) may take place through neurotransmitter regulation. The data suggest that the function of dynorphin-(1-13) may be a presynaptic modulation of neurotransmission in striatum.

Effects of dynorphin1-13 on opiate binding and dopamine and GABA uptake in stroked cat brain

We previously reported that the opioid peptide dynorphin1-13 improves survival chances in stroked cats. Some evidence also suggests that changes in dopamine and gamma-aminobutyric acid (GABA) uptake may be associated with stroke. In the present study, therefore, we determined binding of the opiate [3H]ethylketocyclazocine (EKC), as well as dopamine and GABA uptake in various brain regions of control, stroked and dynorphin1-13-treated stroked cats. Cats were stroked by middle cerebral artery occlusion. In the EKC binding study, the Kd of the high-affinity site of the occluded cortex was significantly increased, relative to that of both the unoccluded side and control cortex. Dynorphin1-13 treatment reversed this effect, lowering the Kd to control level. In the dopamine uptake study, the Km was decreased and Vmax was increased significantly in unoccluded cortex, compared with that in the occluded cortex or in control cortex. Again, dynorphin1-13 reversed these effects, raising the Km and lowering the Vmax. However, the Km of occluded cortex was also increased so that it became significantly higher than that of control cortex. The Km of unoccluded subcortex in stroked cats treated with dynorphin1-13 was significantly reduced compared with control. In the GABA uptake study, there was no significant change in any parameter. The change in opioid binding observed here and its reversal by dynorphin1-13 are consistent with the notion that the peptide's beneficial effect on stroke is mediated through opiate receptors. Since opioid systems in the brain are known to have association with dopaminergic ones, the change in dopamine uptake could also be the result of an opioid effect.

Generalization tests with intraventricularly applied pro-enkephalin B-derived peptides in rats trained to discriminate the opioid kappa receptor agonist ethylketocyclazocine

Rats were trained in a two-lever food-reinforced procedure to discriminate between the effects of saline and the opioid kappa receptor agonist ethylketocyclazocine. After acquisition of this discrimination, generalization tests with opioid peptides such as beta-endorphin, alpha-neoendorphin, dynorphin A and some dynorphin-derived peptides were conducted. The rats dose-dependently generalized the effects of intracerebroventricularly injected ethylketocyclazocine but not beta-endorphin, alpha-neoendorphin, dynorphin A1-8, dynorphin A1-13, D-Cys2-L-Cys5-dynorphin A1-13 or dynorphin A. D-Cys2-L-Cys5-dynorphin A1-13, in contrast to dynorphin A itself, dose-dependently caused analgesia and catatonia that was reversible with naloxone. Studies into the receptor preference of this derivative, using the technique of "selective tolerance", revealed that this dynorphin derivative is almost devoid of kappa-receptor activity.