Effects of GABA uptake inhibitors on posthypoxic myoclonus in rats.

Male Sprague-Dawley rats developed posthypoxic myoclonus following 10-min cardiac arrest and resuscitation. Previous results showed that dysfunction of central GABAergic neurotransmission may contribute to the disease. In current studies, effects of GABA uptake inhibitors, guvacine hydrochloride (1,2,5,6-tetrahydro-3-pyridine carboxylic acid hydrochloride) and (+/-)-cis-4-hydroxynipecotic acid ([+/-]-cis-4-hydroxy-3-piperidine carboxylic acid), in the pathophysiology of posthypoxic myoclonus were investigated. Administration of guvacine (1 or 10 mg/kg, IP) or nipecotic acid (0.5 or 5 mg/kg, IP) significantly attenuated myoclonus scores of the animals. Tolerance to antimyoclonus effects of these two compounds did not develop after chronic administration (twice a day for 14 days) of guvacine (10 mg/kg, IP) or nipecotic acid (5 mg/kg, IP). On the other hand, tolerance was noticed with clonazepam (2.5 mg/kg, IP twice a day for 7 days). The results indicate that guvacine or nipecotic acid may be used in combination with (at reduced doses) or as alternatives to clonazepam to treat patients with the disease so as to reduce tolerance phenomenon usually associated with clonazepam.

Effects of glutamate receptor antagonists on posthypoxic myoclonus in rats.

Male Sprague-Dawley rats developed posthypoxic myoclonus following 10-min cardiac arrest and resuscitation. In current studies, roles of N-methyl-D-aspartate (NMDA), non-NMDA (a-amino-3-hydroxy-5-methylisoxazole-4-propionate, AMPA, and kainate), and metabotropic glutamate receptors in the pathophysiology of posthypoxic myoclonus were investigated. Treatments with the competitive or noncompetitive NMDA receptor antagonist, D(-)-2-amino-5-phosphonopentanoic acid (D[-]-AP-5) (ED50: 12.5 mg/kg, i.p.) or MK-801 maleate (ED50: 0.034 mg/kg, i.p.), and competitive or noncompetitive non-NMDA (AMPA/kainate) receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX) (ED50: 9.25 nM/5 microliters, i.c.v.) or 1-(4-ami -nophenyl)-4-methyl-7,8-methylenedioxy -5H-2,3-benzodiazepine hydrochloride (GYKI 52466) (ED50: 0.67 mg/kg, IP), significantly decreased myoclonus episodes in rats. On the other hand, treatment with the metabotropic glutamate receptor antagonist, L(+)-2-amino-3-phosphonopropionic acid (L[+]-AP-3) (50 or 500 nM/5 microliters, i.c.v., exerted no significant effect on myoclonus scores in posthypoxic rats. These results indicate that activation of NMDA and non-NMDA receptors receptors may mediate posthypoxic myoclonus in rats, whereas, involvement of metabotropic glutamate receptors needs to be studied further.

Astrocyte-derived growth factor (S100 beta) and motor function in rats following cardiac arrest.

Following 10-min cardiac arrest and resuscitation, the central serotonergic system and motor function of rats were found to be affected and later on restored. Astrocyte-derived growth factor (S100 beta) is known to promote survival and neurite outgrowth of serotonergic neurons. In the present study, brain levels of S100 beta were investigated with quantitative immunoblot analysis at various time points following cardiac arrest. Significant reductions of S100 beta were found in the cerebral cortex (30%), midbrain (35%), and cerebellum (46%) of rats 3 days postcardiac arrest. In contrast, at 14 and > 45 days, significant increases of S100 beta were detected in the cerebral cortex (57%; 81%), midbrain (70%; 97%), and cerebellum (84%; 157%). The results indicate that reactive astrocytosis and elevated levels of S100 beta may participate in the recovery processes following hypoxic-ischemic insults to the brain.

Chronic treatments with 5-HT1A agonists attenuate posthypoxic myoclonus in rats.

Following 10 min cardiac arrest and resuscitation, male Sprague-Dawley rats developed posthypoxic myoclonus. This phenomenon peaked at 14 days and disappeared by 60 days after cardiac arrest. From previous results, the 5-hydroxytryptamine (5-HT) system was implicated in the pathogenesis of the disease. In the present study, we investigated the involvement of 5-HT1A receptors in posthypoxic myoclonus in rats. Single injections of 5-HT1A agonists, buspirone (5 and 10 mg/kg body wt.) or 8-OH-DPAT (1, 2, and 4 mg/kg), had no effect on either the intensity or time course of the disease. In contrast, multiple injections (twice a day for 7 or more days) of buspirone (10 mg/kg) or 8-OH-DPAT (4 mg/kg) significantly attenuated the myoclonus scores of animals (p < 0.05). The results indicate that chronic stimulation of 5-HT1A receptors in the brain may accelerate endogenous compensatory mechanisms and shorten the time course of the disease.

Regulation of tyrosine protein kinase receptor Trk-B and motor function in rats following cardiac arrest.

Following 10 min cardiac arrest and resuscitation, male Sprague-Dawley rats developed posthypoxic myoclonus. Sixty days later, the motor function of the animals was restored. In the present study, we investigated brain levels of tyrosine protein kinase receptor Trk-B with quantitative immunoblot analysis at various time points following cardiac arrest. In the frontal cortex, a significant reduction of Trk-B was found in rats 3 days (53%) after cardiac arrest, whereas significant increases were detected in rats 14 (124%) and an average 60 days (98%) after cardiac arrest. In the striatum, significant increases were found in rats 3 (389%), 14 (483%), and 60 days (521%) after resuscitation. In contrast, significant reductions of Trk-B were detected in the cerebellum of rats 3 (46%), 14 (22%), and 60 days (18%) after cardiac arrest. The results indicate that regulation of Trk-B may vary in different brain regions and have important roles in recovery processes following hypoxic-ischemic insults to the brain.

Alterations of brain levels of phosphoinositidase-C-linked Gq alpha/G11 alpha proteins and motor function in rats after cardiac arrest.

BACKGROUND AND PURPOSE: Phosphoinositidase-C-linked Gq alpha and G11 alpha proteins have only recently been characterized. Second messenger systems are known to be affected by hypoxia-ischemia. However, the effects of hypoxia-ischemia on the brain levels of Gq alpha and G11 alpha proteins are not known. Therefore, in the present studies, the effects of hypoxia-ischemia on Gq alpha and G11 alpha proteins in rats were investigated with quantitative immunoblot analysis. METHODS: Cardiac arrest was induced in male Sprague-Dawley rats by an intracardial injection of KCl. Resuscitation began 10 minutes afterwards. At various time points after resuscitation, animals were killed and the cerebral cortex, striatum, and cerebellum were dissected. Levels of Gq alpha and G11 alpha proteins were investigated by quantitative immunoblot analysis. RESULTS: At 1, 2, 4, and 6 hours after resuscitation, Gq alpha and G11 alpha protein levels remained unaltered. However, a significant reduction of these proteins was seen in the cerebral cortex and cerebellum of rats 3 and 14 days after cardiac arrest, with partial recovery by an average of 60 days. In contrast, no significant change was detected in the striatum. CONCLUSIONS: These observations indicate that phosphoinositidase-C-linked signal transduction pathways may be attenuated after hypoxic-ischemic insults to the brain, and that this phenomenon, together with many other factors, may contribute to the expression of motor dysfunction in rats after cardiac arrest.

Expression of Bcl-2 and Bax in the frontoparietal cortex of the rat following cardiac arrest.

Following 10-min cardiac arrest and resuscitation, male Sprague-Dawley rats developed posthypoxic myoclonus. This phenomenon peaked at 14 days and disappeared by 45 days after cardiac arrest. The mechanisms for the initial dysfunction and later restoration of motor function are not completely known. In the present study, involvement of Bcl-2 and Bax in these phenomena was investigated. In the frontoparietal cortex, both bcl-2 and bax mRNA levels were significantly increased 1, 3, 7, 14, and 28 days postresuscitation. bax mRNA levels continued to be high 45 days postcardiac arrest, whereas bcl-2 mRNA levels were returned to control levels. The apoptotic cells were found in layers IV to VI of the frontoparietal cortex of rats 3 days postcardiac arrest. These results indicate that after cardiac arrest, the initial rise of Bax levels may mediate apoptosis and neurodegeneration in the rat brain. At later time points, increased levels of Bcl-2 may contribute to recovery of motor function in posthypoxic rats.

Tolerance development to butorphanol: comparison with morphine.

In order to evaluate and to compare the time course, dose response, and the degree of tolerance development to butorphanol and morphine, rats were continuously intracerebroventricularly (ICV) infused with saline vehicle (1 microliter/h), butorphanol (6.5, 13, 26, and 52 nmol/microliters/h), or morphine (1.6, 6.5, and 26 nmol/microliters/h) through osmotic minipumps for 1 to 3 days. The tail-flick responses were determined pre-, during, and postinfusion. Tolerance to morphine developed faster than that to butorphanol. The antinociceptive response to the ICV challenge dose (6 h after the termination of drug infusion) of butorphanol or morphine was decreased significantly and there was a negative correlation between the dose of the drug infused and the observed antinociceptive response. In terms of butorphanol and morphine tolerance, a parallel rightward shift in the dose-response curve was produced with the degree of shift proportional to the log of the infusion dose. In tail-flick tests, the shifts of the dose-response curves for butorphanol and morphine in tolerant animals were 11.8- and 46.3-fold, respectively. However, in the acetic acid writhing test, the shifts of the dose-response curves for butorphanol and morphine in tolerant animals were 11.3- and 11.7-fold, respectively. These results suggest that there is a greater degree of tolerance to morphine than there is to butorphanol, but the degree of butorphanol tolerance is still substantial. In addition, two pain assays (tail flick vs. writhing) yielded different estimations of tolerance in a comparison of morphine and butorphanol.

Involvement of 5-HT2 receptors in posthypoxic stimulus-sensitive myoclonus in rats.

We have previously reported that rats exhibited audiogenic myoclonus at 3 days after cardiac arrest. This phenomenon peaked at 14 days, gradually tapered off at older ages, and disappeared in most rats by 60 days following cardiac arrest. Because treatment with the 5-HT2-selective agonist, (+/-)-1-2,5-dimethoxy-4-iodophenyl-2-aminopropane (DOI) significantly attenuated audiogenic myoclonus in these postcardiac-arrest rats, the involvement of 5-HT2 receptors in posthypoxic stimulus-sensitive myoclonus was suggested. In the current study, we, therefore, examined the binding properties of 5-HT2 receptors in the rat bain at various time points following cardiac arrest. The affinity constant of [3H]ketanserin binding to 5-HT2 receptors in cortical membranes of rats did not change. In contrast, Bmax values were found to be reduced at 3 and 14 days after cardiac arrest with some recovery after 60 days. Taken together with previous results, these results indicate that hypoactivity of central 5-HT2 neurotransmission may underlie the development of posthypoxic stimulus-sensitive myoclonus in rats.

Involvement of kappa-opioid receptors in opioid dependence/withdrawal: studies using butorphanol.

The dependence liability of a class of opioid agonist/antagonist analgesics, e.g. pentazocine, butorphanol, and buprenorphine, is widely recognized. However, the relative involvement of mu-, delta-, and kappa-opioid receptors mediating physical dependence on these compounds is not completely known. In the present study, butorphanol dependence was produced by continuous intracerebroventricular (i.c.v.) infusion of butorphanol (26 nmol/h) for 3 days in male Sprague-Dawley rats. Nor-binaltorphimine, a long-acting kappa-opioid receptor antagonist, and naloxone, a nonspecific antagonist, were administered i.c.v. to precipitate withdrawal in butorphanol-dependent animals, so as to investigate the involvement of central kappa-opioid receptors in opioid dependence. ED50 ratios (naloxone/nor-binaltorphimine) for eliciting withdrawal signs were: teeth-chattering (1.25), yawning (2.13), and ejaculation (0.72). Our data indicate that nor-binaltorphimine precipitated withdrawal behaviors similar to those precipitated by naloxone. It appears that central kappa-opioid receptors may play a major role in the development of butorphanol dependence in rats.

Involvement of delta-opioid receptors in physical dependence on butorphanol.

Butorphanol, a synthetic agonist/antagonist, has been shown to act on mu-, delta- and kappa-opioid receptors. However, the relative involvement of opioid receptor subtypes in mediating butorphanol dependence is not known. In the present study, naltrindole, a delta-selective non-peptide antagonist, was administered intracerebroventricularly (i.c.v.) to mask supraspinal delta-opioid receptors before and during the induction of butorphanol dependence. Treatment with naltrindole (0.1, 1, or 10 nmol/5 microliters per rat) significantly blocked naloxone-, a nonspecific antagonist, precipitated butorphanol withdrawal behaviors (escape behavior, teeth-chattering, wet shakes, forepaw tremors, ptosis, diarrhea, body weight loss, and hypothermia) at all doses tested, and decreased ejaculation at 0.1 nmol in butorphanol-infused rats. In contrast, naltrindole treatment had no effect on yawning, nor urination. These results indicate that central delta-opioid receptors are involved in mediating butorphanol dependence in rats.

Effects of nor-binaltorphimine on butorphanol dependence.

Butorphanol has been shown to act on mu-, delta-, and kappa-opioid receptors. However, the relative involvement of different opioid receptor subtypes in butorphanol dependence is not known. In the present study, nor-binaltorphimine, a long-acting non-peptide kappa-opioid receptor antagonist, was employed to mask central kappa-opioid receptors before and during the induction of butorphanol dependence in rats, so that the involvement of kappa-opioid receptors could be elucidated. The results revealed that treatment with nor-binaltorphimine markedly blocked naloxone-precipitated withdrawal signs of escape behavior, teeth-chattering, wet shakes, ptosis, body weight loss, and hypothermia at all doses tested, and attenuated the withdrawal symptoms of forepaw tremors (24 nmol: P < 0.001) and diarrhea (12 nmol: P < 0.05; 24 nmol: P < 0.01). In contrast, nor-binaltorphimine had no effect on yawning, ejaculation, nor urination in butorphanol-infused rats undergoing withdrawal. Three days of butorphanol infusion significantly increased KD values (in the cortex and striatum), decreased Bmax (in the cortex only) of [3H]U-69,593 binding, and shifted Ki of nor-binaltorphimine against [3H]U-69,593 (4.5 nM) binding in the cortex by more than 10-fold. Treatment with nor-binaltorphimine blocked the effects of butorphanol on kappa-opioid receptors. It is therefore concluded that kappa-opioid receptors are involved in mediating escape behavior, teeth-chattering, wet shakes, forepaw tremors, ptosis, diarrhea, weight loss, and hypothermia in butorphanol-dependent rats undergoing withdrawal. Furthermore, kappa-opioid receptors become desensitized to agonists (in the cortex and striatum), down-regulated (in the cortex), and supersensitive to antagonists in butorphanol-dependent rats.

Opioid antagonists and butorphanol dependence.

Butorphanol has been known to act on mu-, delta-, and kappa-opioid receptors, mu- and possibly delta-receptors are thought to mediate morphine dependence. Relative to morphine, butorphanol has a higher affinity for mu- and delta-receptors. In the present study, beta-funaltrexamine (beta-FNA) and naltrindole (NTI) (nonequilibrium mu- and delta-antagonist, respectively) were used to precipitate withdrawal in butorphanol-dependent rats. It was found that beta-FNA (12, 24, 48, and 100 nM) did not elicit significant withdrawal behaviors, while NTI caused teeth-chattering (100 nM), wet shakes (100 nM), forepaw tremors (24 nM), yawning (48 and 100 nM), ejaculation (24 nM), and urination (100 nM). The present results indicate that delta-opioid receptors may be involved in mediating butorphanol dependence, while the involvement of mu-opioid receptors needs to be further investigated.

Effects of beta-funaltrexamine on butorphanol dependence.

The present experiments were performed to investigate the effects of the selective mu opioid receptor antagonist, beta-funaltrexamine (beta-FNA), on the physical dependence liability of butorphanol (a mixed agonist/antagonist opioid analgesic). Butorphanol (26 nmol/microliter/h) was continuously infused via osmotic minipumps into the lateral cerebral ventricle of male Sprague-Dawley rats for 72 h. beta-FNA (12, 24, and 48 nmol/5 microliter/rat) was administered ICV 3 h prior to and 48 h after initiation of the butorphanol infusion. Treatment with beta-FNA significantly diminished naloxone-induced escape behavior, hypothermia, and loss of body weight in a dose-dependent manner, while naloxone-induced teeth-chattering, forepaw tremors, and urination were also reduced, but in a dose-independent manner. These results suggest that the mu opioid receptor is partially involved in the development of physical dependence upon butorphanol.