Functional studies with the opioid peptide dynorphin: acute effects of injections into the substantia nigra reticulata of naive rats

Effect of acute and chronic administration of U50,488, a kappa opioid receptor agonist, in 6-OHDA-lesioned rats chronically treated with levodopa

To evaluate the possible involvement of kappa opioid receptor-mediated mechanisms in levodopa-induced motor fluctuations, we have investigated the effects of U50,488, a selective kappa opioid agonist, on levodopa-induced motor alterations in rats with unilateral 6-OHDA lesion. Acute and chronic administration of U50,488 has been studied to evaluate the possible reversion or prevention of these levodopa effects. In a first set of experiments, rats were treated with levodopa (25 mg/kg with benserazide, twice daily, ip) for 22 days and, on Day 23 U50,488 (0.5, 1, or 3 mg/kg, i.p.) was administered immediately before levodopa. In a second set of experiments, rats were treated daily for 22 days with levodopa and U50,488 (1 or 3 mg/kg/day, i.p.). The duration of the rotational behavior induced by chronic levodopa decreased after 22 days (P < 0.05). Acute administration of U50,488 on Day 23 reversed this effect when low doses were administered (P < 0.05). Chronic U50,488 administration did not prevent the shortening in response duration induced by levodopa. Our results demonstrate that the kappa opioid receptor agonist U50,488 reverses but does not prevents levodopa-induced motor alterations in parkinsonian rats. These results suggest a role for kappa opioid receptor-mediated mechanisms in the pathophysiology of levodopa-induced motor response complications. These findings suggest that the stimulation of kappa opioid receptors might confer clinical benefit to parkinsonian patients under levodopa therapy suffering from motor complication syndrome.

Paradoxical locomotor activating effects of kappa-opioid receptor stimulation in the preweanling rat: role of the ventromedial thalamus and superior colliculus

Stimulating kappa-opioid receptors in the substantia nigra pars reticulata robustly increases the locomotor activity of preweanling rats. To determine whether nigrothalamic and nigrotectal connections are necessary for this kappa-opioid-mediated locomotor activity, preweanling rats were given a systemic injection of saline or 5 mg/kg U50,488 (a kappa-opioid receptor agonist) 2 days after receiving sham or bilateral electrolytic lesions of the ventromedial thalamus (VMT) or superior colliculus (SC). Results showed that lesions of the VMT and SC attenuated the U50,488-induced locomotor activity of preweanling rats, indicating that the locomotor activating effects of kappa-opioid receptor stimulation require that nigrothalamic and nigrotectal connections be intact.

kappa-Opioid receptors in the substantia nigra pars reticulata mediate the U-50,488-induced locomotor activity of preweanling rats

The purpose of the present study was to determine the neuroanatomical location where kappa-opioid receptor stimulation induces locomotor activity in the preweanling rat. To confirm that the U-50,488-induced locomotor activity of preweanling rats is mediated by kappa-opioid receptors, 18-day-old rats were initially injected with vehicle or the kappa-opioid receptor agonist U-50,488 (5 mg/kg, s.c.) followed, 15 min later, by an injection of the kappa-opioid receptor antagonist nor-binaltorphimine (nor-BNI; 0, 2, 4, 8, or 12 mg/kg, s.c.). In subsequent experiments, 18-day-old rats were injected with vehicle or U-50,488 (5 mg/kg, s.c.) 15 min prior to bilateral administration (0.25 or 0.5 microl per side) of nor-BNI (0, 5, 10, or 20 microg) into the substantia nigra pars reticulata (SNR) or medial dorsal striatum (MDS). In the final experiment, 18-day-old rats received bilateral administration (0.25 microl per side) of vehicle or U-50,488 (0.0, 0.8, 1.6, or 3.2 microg) into the SNR. Results showed that systemically administered nor-BNI (0-12 mg/kg, s.c.) produced a dose-dependent reduction in the U-50, 488-induced locomotor activity of preweanling rats. The site of action for U-50,488's locomotor-activating effects appeared to be the SNR, because (a) bilateral administration of nor-BNI (5, 10, or 20 microg) into the SNR caused a complete attenuation of U-50, 488-induced locomotion, and (b) bilateral administration of U-50,488 into the SNR caused a dose-dependent increase in the locomotor activity of preweanling rats. Striatal injections of nor-BNI did not affect U-50,488-induced locomotor activity. When these findings are considered together it is apparent that stimulation of kappa-opioid receptors in the SNR is both necessary and sufficient for the occurrence of U-50,488-induced locomotor activity in the preweanling rat.

The adrenergic receptor agonist, clonidine, potentiates the anti-parkinsonian action of the selective kappa-opioid receptor agonist, enadoline, in the monoamine-depleted rat

1. The treatment of Parkinson's disease relies predominantly upon dopamine replacement therapy, usually with l-dihydroxyphenylalanine (L-DOPA). However, side-effects of long-term treatment, such as L-DOPA-induced dyskinesias can be more debilitating than the disease itself. Non-dopaminergic treatment strategies might therefore be advantageous. 2. The aim of this study was to investigate the potential anti-parkinsonian efficacy of the kappa-opioid receptor agonist, enadoline, and the alpha-adrenoreceptor agonist, clonidine, both alone or in combination, in the reserpine-treated rat model of Parkinson's disease. 3. Rats were treated with reserpine (3 mg kg-1), and experiments carried out 18 h later, at which time they exhibited profound akinesia (normal animals 1251+/-228 mobile counts h-1, reserpine-treated animals 9+/-2 mobile counts h-1). Both enadoline and clonidine increased locomotion in reserpine-treated rats in a dose-dependent manner. The maximum locomotor-stimulating effect of enadoline alone was seen at a dose of 0.2 mg kg-1 (208+/-63 mobile counts h-1). The maximum effect of clonidine was seen at a dose of 2 mg kg-1 (536+/-184 mobile counts h-1). 4. Co-administration of enadoline (0.1 mg kg-1) and clonidine (0.01 - 0.1 mg kg-1) at sub-threshold doses, synergistically increased locomotion. 5. The synergistic stimulation of locomotion in the reserpine-treated rat involved activation of kappa-opioid receptors and a combination of both alpha1 and alpha2-adrenoreceptors. 6. The results presented suggest a need for further studies on the potential of stimulating kappa-opioid and/or alpha-adrenoreceptors as a therapy for Parkinson's disease. Furthermore, the studies may offer potential mechanistic explanations of the ability of alpha2-adrenergic receptor antagonist to reduce L-DOPA-induced dyskinesia in Parkinson's disease.

Indirect dopamine agonists augment the locomotor activating effects of the kappa-opioid receptor agonist U-50,488 in preweanling rats

kappa-Opioid receptor agonists (e.g., enadoline or U-50,488) increase the locomotor activity of preweanling rats, while the same drugs depress the locomotor activity of adults. Curiously, direct stimulation of dopamine (DA) D2-like receptors fully attenuates the U-50,488-induced locomotor activity of preweanling rats. The purpose of the present study was to determine whether indirect DA agonists (i.e., cocaine, methylphenidate, and amphetamine) would also attenuate U-50,488's behavioral effects. In two experiments, 17-day-old rats were injected with saline or U-50,488 (5 mg/kg, sc) and locomotor activity and stereotyped sniffing were assessed. After 20 min, the saline- and U-50,488-pretreated rats were injected with saline, cocaine (5, 10, or 20 mg/kg, i.p.), methylphenidate (10 or 20 mg/kg, i.p.), amphetamine (2.5 or 5 mg/kg, i.p.), or the direct D2-like receptor agonist NPA (1 mg/kg, i.p.). As expected, U-50,488 dramatically enhanced the locomotor activity of 17-day-old rats, while attenuating the stereotyped sniffing caused by indirect and direct DA agonists. All three indirect DA agonists augmented U-50,488's locomotor activating effects across the initial 10 min of testing and then activity declined to U-50,488 control values. Direct stimulation of DA receptors produced nearly opposite effects because NPA attenuated U-50,488-induced locomotor activity across the entire testing session. It is uncertain why direct and indirect DA agonists affected U-50,488-induced locomotor activity differently, but the relative amount of DA D1-like receptor activation is probably not responsible.

Involvement of the nigrotectal and nigrothalamic pathways in kappa opioid-induced circling

The relationship between kappa opioid-induced movement and output stations of the substantia nigra pars reticulata (SNpr) was examined using the rodent circling model. Contralateral rotation produced by intranigral microinjection of the kappa opiate U50,488 was lower in animals with ibotenic acid lesions of either the ipsilateral ventromedial thalamus or superior colliculus than in control animals without lesions. These results suggest that endogenous kappa opioids in the SNpr may influence movement through actions on the nigrothalamic and nigrotectal pathways. In contrast, animals with ipsilateral lesions of the striatum showed an increase in circling relative to controls, possibly due to kappa receptor supersensitivity in the SNpr.

Inhibitory effects of the kappa opiate U50,488 in the substantia nigra pars reticulata

The effect of the selective kappa opiate agonist U50,488 on the firing rate of neurons of the substantia nigra pars reticulata (SNR) was investigated in a series of extracellular single unit recording experiments in rats. Intravenous administration of U50,488 produced a dose-related decrease in the spontaneous firing rate of SNR neurons, an effect that was reversed by naloxone in most cases. Iontophoretic application of the kappa agonist also inhibited the firing of SNR neurons, indicating a direct action of U50,488 in the SNR. U50,488 was particularly effective in inhibiting a local group of SNR neurons that exhibit increased firing in response to mechanical pressure. In separate experiments, it was determined that these pressure sensitive neurons (1) include nigrotectal and nigrothalamic cells, (2) are responsive to mild as well as painful levels of pressure, and (3) are not responsive to light flashes. This group of neurons may be a primary target of naturally occurring kappa selective opioids of the prodynorphinergic striatonigral pathway.

Rotational behaviour produced by intranigral injections of bovine and human beta-casomorphins in rats

The biological activity of beta-casein derived beta-casomorphin peptides was evaluated by injecting bovine beta-casomorphin-5 (Tyr-Pro-Phe-Pro-Gly), the homologous sequence in human beta-casein (Tyr-Pro-Phe-Val-Glu) and the corresponding N-terminal tetrapeptides into the left substantia nigra of rats. Their ability to produce rotational behaviour was compared to that produced by three reference compounds, morphine, D-ala2D-leu5 enkephalin and U50,488H, ligands for mu, delta and kappa types of opioid receptors, respectively. The relative potencies of beta-casomorphins and morphine were compared to those tested in two in vitro assays for opioid activity: (1) inhibition of the electrically induced contraction of the isolated myenteric plexus-longitudinal muscle of the guinea-pig ileum and (2) displacement of 3H-dihydromorphine binding to brain membranes. The same ranking order of potency was found in all three assays, the peptides from human beta-casein being about 10-fold less potent than those from bovine beta-casein. The effects of both morphine and bovine beta-casomorphin-5 in producing rotational behaviour were antagonized by naloxone; however, approximately 10-fold more naloxone was required to antagonize the beta-casomorphin-5 effect than that of morphine. The present data are discussed in the light of the recent observation that high concentrations of beta-casomorphin-like peptides are found in the cerebrospinal fluid and plasma of women with postpartum psychosis.

Characterization of methamphetamine effects on the striatal-nigral dynorphin system

Multiple high doses of methamphetamine (METH) induced 200-300% increases in the concentration of striatal and nigral dynorphin-like immunoreactivity (DLI). Increases in striatal and nigral DLI levels also were seen within 6 h following a single administration. The changes in the striatal-nigral dynorphin system had subsided 48 h after either acute or multiple treatments with METH. Selective lesioning of the nigral-striatal dopamine pathway blocked the effects; thus, the METH-induced changes appeared to be mediated by dopamine released from the nigral-striatal dopamine projection. Administration of the METH analog, 3,4-methylenedioxymethamphetamine, appeared to alter striatal and nigral DLI concentrations in a manner similar to that of METH.

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 dynorphin (1-8) on movement: non-opiate effects and structure-activity relationship

Cell bodies in the head of the caudate nucleus that synthesize prodynorphin peptides form a substantial projection to the substantia nigra pars reticulata (SNR). The discovery of this pathway suggested an involvement of prodynorphin products in motor control. The effects of unilateral nigral microinjections of prodynorphin products were tested in an in vivo circling model. Dynorphin (1-8), dynorphin (1-7), dynorphin (1-6), dynorphin (2-17) (des-Tyr-dynorphin), and Leu-enkephalin induced spontaneous contralateral circling at 20 nmol doses. The effect of dynorphin (1-8) was dose dependent and was not blocked by pretreatment with naloxone or WIN 44,441-3. These findings clearly demonstrate the dynorphinergic involvement in nigral motor control which may consist of an opioid and a non-opioid component.

Striato-nigral dynorphin and substance P pathways in the rat. I. Biochemical and immunohistochemical studies

The effect of striatal ibotenic acid lesions on dynorphin-, substance P- and enkephalin-like immunoreactivities in the substantia nigra has been studied with immunohistochemistry as well as biochemistry. A comparison was made with the effects produced by intranigral ibotenic acid lesion and by 6-hydroxy-dopamine injection into the medial forebrain bundle. In addition, the effect of the striatal lesions on nigral glutamic acid decarboxylase (GAD)-positive structures was analysed with immunohistochemistry. The effect of the lesions was analysed functionally in the Ungerstedt rotational model, in order to obtain a preliminary evaluation of the extent of the lesions. The striatal lesions produced a parallel depletion of dynorphin and substance P levels in the substantia nigra, pars reticulata, ipsilateral to the treated side, which was dependent upon the extent and location of the lesion. Ibotenic acid lesions into the tail and the corpus of the striatum produced stronger nigral-peptide depletion than lesions in the head and the corpus of the striatum. Comparison of placement of lesions and localization of depleted area in the substantia nigra revealed a topographical relationship. Furthermore, the nigral depletion patterns of dynorphin and substance P were similar. The immunohistochemical analysis revealed that also GAD-positive fibers in the pars reticulata to a large extent disappeared after striatal lesions, in parallel to the dynorphin- and substance P-positive fibers. However, the depletion was less pronounced for GAD than for the peptides, probably related to presence of local GABA neurons in the zona reticulata of the substantia nigra. These results indicate that with the types of lesion used in this study it is not possible to provide evidence for a differential localization within the striatum of dynorphin-, substance P- and GABA-positive cell bodies projecting to the substantia nigra. The radioimmunoassay showed that (Leu)- but not (Met)-enkephalin was affected to the same extent as the dynorphin peptides, supporting the view that (Leu)-enkephalin in the pars reticulata of the substantia nigra is derived from proenkephalin B and not from proenkephalin A. In the immunohistochemical analysis (Met)-enkephalin-like immunoreactivity could only be detected in the pars compacta of the substantia nigra and did not seem to be affected by any of the lesions. The striatal lesions produced a behavioural asymmetry, which could be disclosed by stimulating the rats with apomorphine, which produced ipsilateral rotation.(ABSTRACT TRUNCATED AT 400 WORDS)

Gilles de la Tourette's syndrome. A postmortem neuropathological and immunohistochemical study

Immunocytochemical studies of the human forebrain have shown that enkephalin-like, dynorphin-like and substance-P-like immunoreactivity (respectively ELI, DLI, and SPI) normally present in unique pattern (now termed woolly fibers) in the globus pallidus and substantia nigra, in which their concentration is at its densest. Quantitative determinations moreover indicate that the levels of all 3 peptides are higher in the globus pallidus and substantia nigra than in any other region of the brain. We report here the distribution of immunoreactivity of these 3 peptides in the brain of a patient showing the typical clinical manifestations of Gilles de la Tourette's syndrome (TS); a disease for which no characteristic or consistent neuropathological features have been discerned. In the case described here neuropathological examination by means of the usual histopathological methods showed no abnormalities to which the patient's illness could be ascribed. ELI- and SPLI-positive woolly fibers were densely stained and of normal distribution. DLI-staining was, however, considerably less dense throughout the brain than normal. The most striking finding was the total absence of DLI-positive woolly fibers in the dorsal part of the external segment of the globus pallidus; the ventral pallidum showed very faint staining. These observations, which indicate a decrease of dynorphin in striatal fibers projecting to the globus pallidus, are, to our knowledge, the first evidence of a distinct pathological change in the brain in TS.

Dynorphin-containing pathways in the substantia nigra and ventral tegmentum: a double labeling study using combined immunofluorescence and retrograde tracing

A rich plexus of dynorphin (DYN)-containing fibers and terminals is present throughout the substantia nigra-ventral tegmental area (SN-VTA). Lesion studies have indicated that these terminals originate, in part, from DYN-containing cell bodies in the caudate-putamen. We have utilized combined immunofluorescence and retrograde tracing techniques to confirm this finding and to demonstrate the presence of other potential DYN pathways in the region of SN-VTA. The location of DYN-containing cell bodies and fibers was determined by immunohistochemistry in untreated and colchicine-treated adult albino rats. In further double-labeling experiments, animals received injections of fluorescent retrograde tracers into SN-VTA, brainstem or amygdala, followed one to fourteen days later by colchicine treatment. Brains were subsequently processed for combined FITC immunofluorescence and fluorescent dye localization. These experiments provided neuroanatomical evidence for heterogeneous DYN-containing pathways in the SN-VTA. Topographically organized afferents arose from the caudate putamen, amygdala and hypothalamus, while efferents arose from only the most lateral sectors of the SN to the amygdala. While most of the observed descending projections terminated in the SN-VTA, hypothalamic and amygdala projections also continued en passage to the brainstem. These findings implicate DYN systems in striatal and limbic, as well as neuroendocrine, functions.

Lesions of the globus pallidus and striatum attenuate ketocyclazocine-induced feeding

A large body of evidence suggests that endogenous opioids are involved in the regulation of feeding. As the striatum and globus pallidus have relatively high concentrations of opioid receptors, these areas are possible sites of action for the stimulatory effects of opiates on feeding. To test these possibilities, male rats were lesioned bilaterally in the globus pallidus or striatum. Nocturnal food intake was then measured after the subcutaneous administration of the opiate antagonist, naloxone (0-10 mg/kg). Spontaneous daytime intake was measured after the subcutaneous administration of the kappa opiate agonist ketocyclazocine (0-10 mg/kg). Neither pallidal nor striatal lesions affected the sensitivity of naloxone in reducing food intake. On the other hand, both lesioned groups were 10-100 times less sensitive to the stimulatory effects of ketocyclazocine. These results suggest that the globus pallidus and striatum may be target areas for the stimulatory effects of exogenous opiates on food intake. Additionally, the relationship of these areas to the dopaminergic nigrostriatal tract suggests that feeding regulation may involve an interaction between dopaminergic and opioid systems.

Effect of intranigral injections of dynorphin, dynorphin fragments and alpha-neoendorphin on rotational behaviour in the rat

Peptides deriving from the proenkephalin B precursor were studied in the Ungerstedt rotational model after their unilateral injection into the substantia nigra. Dynorphin (DYN)-(1-17), DYN-(1-13) and DYN-(1-8) in 0.1-10 micrograms doses induced marked contralateral rotation. This effect was enhanced by subsequent systemic administration of D-amphetamine and blocked by previous treatment with naloxone. alpha-Neoendorphin produced similar effects although there was no evidence for dose-dependency. DYN-(6-17) which lacks opioid activity also produced contralateral rotation, which, however, was not naloxone reversible and D-amphetamine given subsequently did not induce asymmetric activation. Methionine enkephalin and leucine enkephalin, deriving from the proenkephalin A precursor were tested for comparison. Only the former produced weak contralateral rotation. GABA injected at the same site as DYN-(1-17) also induced contralateral rotation which was mimicked by nanogram doses of the gabaergic agonist muscimol. These findings suggest an interaction between peptides from the proenkephalin B precursor and nigro-striatal dopamine neurons as well as gabaergic striato-nigral efferents and/or interneurons.