M-current modulators alter rat spinal nociceptive transmission: an electrophysiological study in vitro

M-currents constitute a unique effector system to control neuronal excitability due to their voltage and ligand sensitivities. Here we have used retigabine, an M-current agonist, and XE-991, an M-current antagonist, to study the possible involvement of these currents in the processing of spinal sensory and motor processing of nociceptive information in normal, untreated rats. Experiments were performed in a hemisected spinal cord preparation from rat pups using extracellular recordings. Responses to activation of nociceptive and non-nociceptive afferent fibres were recorded. M-current modulators were bath applied to the entire cord or applied locally by pressure ejection. Retigabine and XE-991 produced long-lasting and concentration-dependent effects on nociceptive reflexes showing only minor effects on non-nociceptive reflexes. Retigabine depressed responses to repetitive stimulation of the dorsal root recorded from motor neurones and dorsal horn neurones, whereas XE-991 showed the opposite potentiatory effect and reversed effects of retigabine. Local application of the modulators close by motor nuclei produced changes in reflex responses similar to those caused by bath application. These results constitute a clear indication of the existence of functional M-currents in dorsal and ventral horn elements of the mammalian spinal cord where they may serve to regulate early sensory and motor processing of nociceptive information. The weak effect of modulators on non-nociceptive reflexes suggest that M-currents constitute a promising novel target for analgesics.     

Long-term benzodiazepine treatment reduces neuronal responsiveness to cholecystokinin: an electrophysiological study in the rat

Acute benzodiazepine administration has been reported to antagonize the effect of cholecystokinin both in the periphery and in the central nervous system. A two-week treatment with either diazepam (5 mg/kg per day) or flurazepam (15 mg/kg per day) markedly reduced the excitatory effect of microiontophoretically applied sulphated cholecystokinin octapeptide-(26-33) on rat CA3 hippocampal pyramidal neurons but not that of acetylcholine. In view of the sustained anxiolytic activity of benzodiazepines that contrasts with their transient sedative effect, the present results suggest that the reduction of neuronal responsiveness to cholecystokinin by these drugs might be related to their anxiolytic property.     

Serotonin-GABA interactions in the modulation of mu- and kappa-opioid analgesia

In the present study, we studied the interaction between serotonergic (5-HTergic) and gamma-aminobutyric acid (GABA)-ergic systems in the modulation of analgesia from morphine, a mu-opioid agonist, and U50,488, a kappa-opioid agonist. All experiments were performed in mice using the 49 degrees C tail-withdrawal assay. The benzodiazepine receptor agonist, diazepam, the serotonin synthesis inhibitor, para-chlorophenylalanine (p-CPA), and the 5-HT(1A) receptor agonist, 8-OH-DPAT, were all found to attenuate morphine and U50,488 analgesia. In each case, the attenuation was itself blocked by treatment with L-5-HTP, a serotonin precursor, bicuculline, a GABA(A) receptor antagonist or picrotoxin, a GABA(A)-gated chloride channel blocker. Neither L-5-HTP nor the GABA(A) receptor antagonists were found to affect morphine or U50,488 analgesia per se. Thus, these findings indicate that a benzodiazepine-GABAergic agent (diazepam) attenuates opioid analgesia through the serotonergic system, and antiserotonergic agents (8-OH-DPAT, p-CPA) attenuate opioid analgesia through the GABAergic system. The intimate interactions between GABA and serotonin in the present study further suggest that these neurotransmitters work in complex ways together rather than alone in the modulation of opioid analgesia.     

Effects of dopaminergic modulation on electrophysiological brain response to affective stimuli

INTRODUCTION: Several theoretical accounts of the role of dopamine suggest that dopamine has an influence on the processing of affective stimuli. There is some indirect evidence for this from studies showing an association between the treatment with dopaminergic agents and self-reported affect. MATERIALS AND METHODS: We addressed this issue directly by examining the electrophysiological correlates of affective picture processing during a single-dose treatment with a dopamine D2 agonist (bromocriptine), a dopamine D2 antagonist (haloperidol), and a placebo. We compared early and late event-related brain potentials (ERPs) that have been associated with affective processing in the three medication treatment conditions in a randomized double-blind crossover design amongst healthy males. In each treatment condition, subjects attentively watched neutral, pleasant, and unpleasant pictures while ERPs were recorded. RESULTS: Results indicate that neither bromocriptine nor haloperidol has a selective effect on electrophysiological indices of affective processing. In concordance with this, no effects of dopaminergic modulation on self-reported positive or negative affect was observed. In contrast, bromocriptine decreased overall processing of all stimulus categories regardless of their affective content. DISCUSSION: The results indicate that dopaminergic D2 receptors do not seem to play a crucial role in the selective processing of affective visual stimuli.     

Differing actions of convulsant and nonconvulsant barbiturates: an electrophysiological study in the isolated spinal cord of the rat

The effects of various pairs of convulsant and nonconvulsant barbiturates on mono- and polysynaptic activity were studied in the isolated spinal cord of the immature rat, using extracellular recording. The convulsant barbiturates, 5-ethyl-5-(3-methylbut-2'-enyl) barbituric acid (3M2B), 5-ethyl-5-(1,3-dimethylbut-1'-enyl) barbituric acid (1,3M1B) and (+)-5-(1,3-dimethylbutyl)-5-ethyl barbituric acid [(+) DMBB] all increased the monosynaptic reflex at concentrations between 5 and 50 microM with no change in polysynaptic activity. When the concentration was raised to between 100 and 300 microM, however, the convulsants all reduced the monosynaptic reflex, thus producing a biphasic dose-response relationship. The nonconvulsant barbiturates phenobarbital, 5-ethyl-5-(3-methylbut-1'-enyl) barbituric acid (3M1B), amylobarbital (3MB) and (-)-5-(1,3-dimethylbutyl)-5-ethyl barbituric acid [(-)DMBB] produced only a decrease in mono- and polysynaptic reflexes. At concentrations which enhanced the monosynaptic reflex, the responses of motoneurones to glycine and eledoisin-related peptide (an analogue of substance P) were reduced by (+)DMBB, while 1,3M1B and 3M2B had no significant effects upon any of the neurotransmitters tested. At concentrations which depressed the monosynaptic reflex, the convulsants all reduced the response to glycine whereas the nonconvulsant barbiturates all increased the response to GABA. With the exception of phenobarbital, both convulsant and nonconvulsant barbiturates produced a direct depolarisation of the presynaptic terminal membrane, with only the convulsants producing a depolarisation of the membrane of the motoneurone. Using another convulsant barbiturate, 5-(2-cyclohexylideneethyl)-5-ethyl barbituric acid (CHEB), this direct depolarising action was found to be calcium-dependent.     

Activation of midbrain presumed dopaminergic neurones by muscarinic cholinergic receptors: an in vivo electrophysiological study in the rat

1. Extracellular single-unit recording and iontophoresis were used to examine the effects of different cholinoceptor agonists and antagonists on the firing rate and firing pattern of A9 and A10 presumed dopaminergic neurones in the anaesthetized rat.
2. Administration of low currents (1–5 nA) of the selective muscarinic agonists oxotremorine M (Oxo M) and muscarine and of the non-selective muscarinic/nicotinic agonist carbamylcholine (CCh) produced a dose-dependent increase in firing rate in most of the A9 and A10 presumed dopaminergic neurones tested. Oxo M-induced activation could be completely blocked by iontophoretic application of the muscarinic antagonist butyl-scopolamine or systemic administration of the muscarinic antagonist scopolamine (300 μg kg⁻¹, i.v.).
3. Iontophoretic application of the selective nicotinic agonist methylcarbamylcholine (MCCh), but not nicotine, induced a consistent increase in firing rate. Surprisingly, the excitatory effect of MCCh was significantly reduced by the selective muscarinic antagonist scopolamine (300 μg kg⁻¹, i.v.), but not by the selective nicotinic antagonist mecamylamine (2.2 mg kg⁻¹, i.v.). Mecamylamine (3 mg kg⁻¹, i.v.) was also ineffective in reducing the CCh-induced activation of presumed dopamine neurones, suggesting that both CCh and MCCh increased the activity of dopamine neurones via an interaction with muscarinic receptors.
4. Iontophoretic application of the endogenous agonist acetylcholine (ACh) had no or little effect on the firing activity of A10 presumed dopaminergic neurones. However, concomitant application of neostigmine, a potent cholinesterase inhibitor, with acetylcholine induced a substantial activation of these neurones. This activation consisted of two components; one, which was prevalent, was scopolamine (300 μg kg⁻¹, i.v.)-sensitive, and the other was mecamylamine (2 mg kg⁻¹, i.v.)-sensitive.
5. In addition to their effect on firing activity, Oxo M, muscarine and concomitant neostigmine/ACh caused a significant increase in burst firing of A10 neurones, but not of A9 neurones.
6. These data suggest that dopamine cells, both in the A9 and A10 regions, possess functional muscarinic receptors, the activation of which can increase their firing rate and, for A10 neurones, their amount of burst activity. These cholinoceptors would be able to influence the activity of the midbrain dopamine system greatly and may play a role in, and/or be a therapeutic target for, brain disorders in which dopamine is involved (e.g., Parkinson''s disease, drug addiction and schizophrenia).

     

Peripheral and central antinociceptive actions of ethylketocyclazocine in the formalin test

The antinociceptive actions of ethylketocyclazocine and morphine were examined in rats in a thermal nociceptive test (tail-immersion) and a test involving minor tissue injury (formalin). In the formalin test, the antinociceptive effects of high doses of ethylketocyclazocine, but not morphine, were attenuated by the peripherally acting antagonist naloxone methylbromide. Naloxone methylbromide had no effect on antinociception produced by ethylketocyclazocine in the tail-immersion test. When ethylketocyclazocine was injected intraventricularly, only partial antinociception was observed in the formalin test. Conversely, naloxone given intraventricularly only partially attenuated the antinociception produced by ethylketocyclazocine given systemically. The data indicate that the antinociceptive effects of ethylketocyclazocine in the tissue injury-induced nociception are a result of summation of central and peripheral actions. Morphine antinociception reaches ceiling at doses that are devoid of such peripheral actions. The data imply that it may be possible to develop a new class of peripherally acting analgesics that are effective in acute inflammatory pain.     

Effect of BHT 920 on monoaminergic neurons of the rat brain: an electrophysiological in vivo and in vitro study

Summary BHT 920 was originally described as a dopamine autoreceptor agonist. In this study, the effect of this compound on the firing rate of noradrenergic locus coeruleus, serotonergic dorsal raphe and dopaminergic ventral tegmental area neurons was examined both in the anaesthetized rat and in rat brain slices. Extracellular recordings were performed in cells whose identity was determined by electrophysiological, pharmacological and histological criteria. In vivo, BHT 920 inhibited the firing of locus coeruleus neurons (ID 50: 14.5 ± 4.7 g/kg, mean ± SEM) and ventral tegmental area neurons (ID50 7 ± 3 g/kg) at very low doses. As a comparison, the ID50 of clonidine on locus coeruleus cells was 5.5 ± 0.6 g/kg and the ID50 of apomorphine on ventral tegmental area neurons was 13 ± 3 g/kg. BHT 920 also decreased the firing of dorsal raphe cells, but this effect was obtained at higher doses (ID50: 57 ± 11 g/kg).The in vitro study confirmed the results obtained in vivo. BHT 920 potently inhibited the firing of locus coeruleus cells (IC50: 71 ± 28 nM) and was less potent than clonidine (IC50: 5.3 ± 0.98 nM). The compound also inhibited the firing of ventral tegmental area neurons at very low concentrations (IC50: 21 ± 3.3 nM), being more potent than apomorphine (IC50: 56 ± 29 nM).BHT 920 only slightly decreased the firing rate of dorsal raphe neurons at 50 gM, showing that the drug has little direct effect on these cells.A pharmacological analysis performed in vitro showed that the effect of BHT 920 was specifically inhibited by the D₂ antagonist sulpiride (1 M) in the ventral tegmental area and by the alpha₂ antagonist idazoxan (1 M) in the locus coeruleus.This electrophysiological study shows that BHT 920 is a potent D₂ and alpha₂ agonist in the rat brain.Send offprint requests to Prof. A. Dresse at the above address     

Involvement of kappa-opioid receptors in peripheral response to nerve stimulation in kappa-opioid receptor knockout mice

1 The present study aimed to evaluate the role of kappa-opioid receptors at two peripheral sites, the vas deferens and the proximal colon, in kappa-opioid receptor knockout mice. We investigated the role of the kappa-opioid receptor in the vas deferens twitch response and in the colonic "off-contraction", a rebound contractile response which follows the inhibitory response to low frequencies stimulation (10, 20, 30 Hz) and which has been suggested to "locally" reproduce the contractile component of the peristaltic reflex. 2 Transmural stimulation of the vas deferens at lower frequencies (10 Hz, 10 V, 1 ms pulse trains lasting 0.5 s) evoked a contractile response that was significantly higher in the preparations from knockout mice because of lack of kappa-opioid receptors than in wild type mice. A selective kappa-opioid receptor agonist, U-50,488H, induced a dose-dependent inhibition of the electrically stimulated contraction in vas deferens. The percentages of reduction of the twitch response were significantly lower in knockout mice than in wild type mice after treatment with U-50,488H. The reduction of twitch response caused by U-50,488H was not reversed by administration of nor-binaltorphimine (nor-BNI) (5 x 10-6 m), a selective kappa-opioid receptor antagonist, in preparations from both knockout mice and wild type mice. U-50,488H has no effect on postsynaptic adrenergic receptors, as its administration did not affect the direct contractile response to noradrenaline. 3 Transmural stimulation (5 Hz, 20 V, 2 ms pulse trains lasting 30 s) induced inhibition of spontaneous activity of colonic strips during the period of stimulation, followed by an "off-contraction" after the cessation of stimulation. The statistical evaluation of the "off-contraction" responses between the two strains showed no significant difference. The off-contraction, measured in specimens from knockout mice, was inhibited concentration-dependently by U-50,488H (P < 0.01) and significantly less than from wild type mice. 4 The effect of U-50,488H was not reversed by administration of nor-BNI (5 x 10-6 m), either in preparations from knockout mice or from wild type mice. 5 Our data may suggest that kappa-opioid receptors are involved in some peripheral responses to the nerve stimulation, as indicated by the effect of U-50,488H, a selective kappa-opioid receptor agonist. However, the involvement of kappa-opioid receptor was also present, although less apparent, in kappa -opioid receptor knockout mice, suggesting either that this drug acts not only on kappa-opioid receptors but also on other receptor sites, such as kappa-like receptors. An alternative interpretation can be related to a sodium channel blocking action of U-50,488H, which could explain the inhibitory effects of twitch response still present but less evident in knockout strain and the lack of effect of the antagonist nor-BNI.     

Local modulation of the 5-HT release in the dorsal striatum of the rat: an in vivo microdialysis study

Using in vivo microdialysis in freely moving rats, we examined the involvement of major striatal transmitters on the local modulation of the 5-HT release. Tetrodotoxin reduced the striatal 5-HT output to 15–20% of baseline. The selective 5-HT_1B receptor agonist CP 93129 (50 μM) reduced (50%) and the 5-HT_2A/2C receptor agonist DOI (1–100 μM) increased (220%) the 5-HT output. Neither GABA nor baclofen (100 nM–100 μM) altered the 5-HT output. The glutamate reuptake inhibitor -trans-PDC (1–4 mM) raised 5-HT to 280% of baseline. This effect was not antagonized by the NMDA receptor antagonist MK-801 (0.5 mg/kg i.p.). Local MK-801 (10–100 μM) did not significantly alter the 5-HT output. Finally, neither carbachol (10–100 μM) nor quipirole (10 μM–1 mM) affected 5-HT. These data suggest that the striatal 5-HT release is influenced by local serotonergic and glutamatergic (but not GABAergic) inputs.     

Effect of atypical antipsychotic drugs on 5-HT2 receptors in the rat orbito-frontal cortex: an in vivo electrophysiological study

Low doses of the atypical antipsychotic drug risperidone are effective in patients with obsessive compulsive disorder (OCD) not responding to serotonin (5-HT) reuptake inhibitors, although higher doses have been reported to induce OCD symptoms in psychotic patients. Since such atypical antipsychotics exert, in addition to dopamine, 5-HT₂ receptor antagonistic properties, it was deemed essential to investigate the electrophysiological effect of these agents on 5-HT₂ receptors in the rat orbito-frontal cortex (OFc), a brain region implicated in OCD. Microiontophoretic application of the GABA_A receptor antagonist bicuculline had no effect on the suppressant effect of neuronal activity in the OFc induced by microiontophoretic application of the preferential 5-HT_2A and 5-HT_2C receptor agonists (+)-1-(4-iodo-2, 5-dimethoxyphenyl)-2-aminopropane (DOI) and m-chlorophenyl-piperazine (mCPP), respectively, but it antagonized the effect of GABA on the same neurons. These results indicate a lack of involvement of GABA interneurons in the suppressant effect of DOI and mCPP. While the 5-HT₂ receptor antagonist ritanserin (2 mg/kg, IV) attenuated the inhibitory effect of DOI and mCPP in the medial prefrontal cortex (mPFc), the inhibition was unaffected in the OFc. In the mPFc, the effect of DOI and mCPP was blocked by both clozapine (1.0 and 10 mg/kg, IV) and risperidone (0.1 and 1.0 mg/kg, IV). In the OFc, only the suppressant effect of mCPP was attenuated by both doses of clozapine but only by the high dose of risperidone. These results suggest that the 5-HT₂ response in the OFc is more akin to the 5-HT_2C subtype and that the deleterious effect sometimes observed with high doses of risperidone and clozapine may be due to a decrease in 5-HT neurotransmission. In contrast, the beneficial effect of low doses of risperidone may be due, in part, to the antagonism of dopamine receptors. Received: 22 June 1998/Final version: 8 October 1998     

Effect of prolonged administration of tianeptine on 5-HT neurotransmission: an electrophysiological study in the rat hippocampus and dorsal raphe

Extracellular unitary recordings of dorsal hippocampus CA₃ pyramidal neurons and of dorsal raphe 5-hydroxytryptamine (5-HT) neurons were used to assess the effect of tianeptine, a putative antidepressant, on the efficacy of 5-HT neurotransmission. Sustained tianeptine administration (20 mg/kg/day, s.c. × 14 days) did not modify the firing activity of 5-HT neurons in the dorsal raphe. Their responsiveness to the intravenous injection of LSD, an agonist of the somatodendritic 5-HT autoreceptor, and of 8-OH-DPAT, a selective 5-HT_1A agonist, was also unaffected by this treatment. The responsiveness of CA₃ pyramidal neurons to microiontophoretic application of 5-HT remained unchanged after sustained tianeptine administration, but it was markedly enhanced in rats treated with repeated electroconvulsive shocks. Finally, the duration of suppression of firing activity of CA₃ pyramidal neurons produced by electrical stimulation of the ascending 5-HT pathway, delivered at 1 Hz and 5 Hz, was not modified in rats treated with tianeptine. Methiothepin, an antagonist of the terminal autoreceptor enhanced the effectiveness of 5-HT pathway stimulation to the same extent in control and tianeptinetreated rats. The present results indicate that, administered at a dose known to stimulate 5-HT reuptake (20 mg/kg/day, s.c.; by minipump), and for a period of time (14 days) for which other antidepressant treatments have been shown to enhance 5-HT function, tianeptine does not modify the efficacy of 5-HT synaptic transmission in the rat hippocampus.     

Nor-binaltorphimine: an ultra-long acting kappa-opioid antagonist in pigeons

This study assessed the ability of naltrexone and nor-binaltorphimine (NBNI) to antagonize the rate-decreasing effects of opioid agonists. Food-restricted pigeons were trained to peck a lit key under a fixed-ratio (FR) 20 schedule of food reinforcement. Bremazocine, a kappa-opioid agonist, decreased food-reinforced responding (ED(50) = 0.02mg/kg), and naltrexone (5.6mg/kg) reduced the potency of bremazocine six-fold. The effect of naltrexone lasted less than 24h. A single injection of NBNI (1mg/kg) was given to four pigeons, and the time course of antagonism of the rate-decreasing effects of bremazocine was measured. One hour after NBNI was given, it was ineffective. Eight days later, NBNI produced a five-fold reduction in the potency of bremazocine. Between 12 and 20 days after NBNI, it reduced the potency of bremazocine 14-fold. NBNI continued to antagonize bremazocine for 11 weeks. Smaller doses of NBNI (0.001-0.1mg/kg) were ineffective. The effect of NBNI was not due to tolerance to bremazocine, since tolerance failed to develop to bremazocine administered repeatedly. NBNI (1mg/kg) did not antagonize the response rate-decreasing effects of morphine, a mu-opioid agonist, or BW373U86, a delta-opioid agonist. NBNI was an effective and extremely long-lasting kappa-opioid antagonist in the pigeon. The duration of action of NBNI is among the longest yet described in any species.     

Norbinaltorphimine, a selective kappa-opioid receptor antagonist, induces an itch-associated response in mice

We examined the possibility that scratching induced by norbinaltorphimine, a selective kappa-opioid receptor antagonist, is due to an itch sensation, using compound 48/80 as control pruritogenic agent. When norbinaltorphimine was injected s.c. into the rostral back, mice scratched the skin around the injection site with their hind paws. Although the intensity of the scratching could not be compared because the dose and injection route were different, the character and time course of the scratching behavior induced by compound 48/80 injected i.d. were similar to those with norbinaltorphimine. The scratching behavior induced by norbinaltorphimine was dose-dependently and significantly inhibited by pretreatment with chlorpheniramine. Compound 48/80-induced scratching was also dose-dependently and significantly inhibited by p.o. pretreatment with chlorpheniramine. The scratching behavior induced by norbinaltorphimine was dose-dependently and significantly inhibited by pretreatment with U-50,488H (trans-(+/-)-2-(3,4-dichlorophenyl)-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl] acetamide methansulfonate), a kappa-opioid receptor agonist. Unexpectedly, the scratching behavior induced by compound 48/80 was also dose-dependently and significantly reduced by pretreatment with U-50,488H. These results suggest that the injection of norbinaltorphimine into the rostral back of the mouse elicited scratching, which may be an itch-associated response. Furthermore, the scratching behavior produced by norbinaltorphimine may be due in part to the release of histamine followed by antagonism of kappa-opioid receptors.     

Flupirtine antinociception in the rat orofacial formalin test: an analysis of combination therapies with morphine and tramadol

Combination therapy with two drugs is a straightforward strategy to improve the risk-benefit ratio of analgesic treatments. Flupirtine is a non-opioid analgesic drug acting via the enhancement of so-called M currents, associated to Kv7 potassium channels in the central nervous system. In this study we used the orofacial formalin test as a model of acute inflammatory pain in the rat; putative synergistic interactions between flupirtine and morphine or tramadol, given in various combinations, were investigated. We found that flupirtine exerts antinociception in the second phase of the test, whereas morphine and tramadol induced analgesia both in the first and in the second phase. An isobolographic analysis of data was carried out, showing a synergistic interaction between flupirtine and morphine, as well as between flupirtine and tramadol, in the second phase of the test. Conversely, in the first phase of the test only a single combination of morphine plus flupirtine, but not any of the combinations of tramadol and flupirtine, resulted in a synergistic interaction. Our data clearly indicate that flupirtine enhances in a synergistic manner the acute antinociceptive effects exerted by opioids in this paradigm.     

Peripheral kappa-opioid agonist, ICI 204448, evokes hypothermia in cold-exposed rats

ICI 204448, a selective kappa-opioid agonist with limited CNS access, can be used to discriminate central and peripheral opioid actions on physiological systems such as pain and thermoregulation. Therefore, we investigated the effect of ICI 204448 (2.5, 5, and 10 mg/kg, s.c.) on male Sprague-Dawley rats exposed to ambient temperatures of 5, 20, or 32 degrees C. ICI 204448 did not alter the body temperature of rats maintained at 20 or 32 degrees C. However, 5 and 10 mg/kg of ICI 204448 evoked significant hypothermia in rats exposed to 5 degrees C. The i.c.v. administration of nor-BNI, a kappa-opioid antagonist, did not affect the hypothermia produced by the systemic injection of ICI 204448. Thus, an involvement of brain kappa-opioid receptors in ICI 204448-evoked hypothermia is unlikely. The present data demonstrate for the first time that ICI 204448 produces hypothermia in cold-exposed rats and suggest that the role of peripheral kappa-opioid receptors in thermoregulation becomes more significant at cold ambient temperatures.     

Reduced kappa-opioid activity in a rat model of cholestasis

Increased endogenous opioid activity has been implicated in cholestatic pruritus. In the present study, we have further defined the involvement of opioids in cholestasis. Rats underwent either bile duct ligation or a sham operation. Five days after surgery, brains were removed and agonist-stimulated [35S]GTPgammaS binding was measured in ten brain regions. Serum endomorphin-2, leu-enkephalin and dynorphin A levels were measured using ELISA on day five. Microdialysis to the dorsal hypothalamic area was conducted in the same animal before and after cholestasis. Dialysate endomorphin-1, leu-enkephalin and dynorphin A levels also were measured. Delta- and kappa-stimulated binding was significantly decreased in cholestasic animals compared to controls in the dorsal hypothalamic area. The serum dynorphin A level was lower in the cholestasic group than in controls (2.56+/-0.09 and 3.29+/-0.22 ng/ml, respectively, P<0.01). We propose that pruritus in cholestasis may result from an impaired balance between mu- and kappa-opioid systems.     

Assessing the permeability of the rat sciatic nerve epineural sheath against compounds with local anesthetic activity: an ex vivo electrophysiological study

Abstract

Studies have shown that the sciatic nerve epineural sheath acts as a barrier and has a delaying effect on the diffusion of local anesthetics into the nerve fibers and endoneurium. The purpose of this work is to assess and to quantify the permeability of the epineural sheath. For this purpose, we isolated the rat sciatic nerve in a three-chamber recording bath that allowed us to monitor the constant in amplitude evoked nerve compound action potential (nCAP) for over 24?h. For nerves exposed to the compounds under investigation, we estimated the IT₅₀ the time required to inhibit the nCAP to 50% of its initial value. For desheathed nerves, the half-vitality time was denoted as IT₅₀(?) and for the ensheath normal nerves as IT₅₀(+). There was no significant difference between the IT₅₀ of desheathed and ensheathed nerves exposed to normal saline. The IT₅₀(?) for nerves exposed to 40?mM lidocaine was 12.1?±?0.95?s (n?=?14) and the IT₅₀(+) was 341.4?±?2.49?s (n?=?6). The permeability (P) coefficient of the epineural sheath was defined as the ratio IT₅₀(+)/IT₅₀(?). The P coefficient for 40?mM lidocaine and linalool was 28.2 and 3.48, correspondingly, and for 30?mM 2-heptanone was 4.87. This is an indication that the epineural sheath provided a stronger barrier against lidocaine, compared to natural local anesthetics, linalool and 2-heptanone. The methodology presented here is a useful tool for studying epineural sheath permeability to compounds with local anesthetic properties.     

Comparative electrophysiological response of young and old rat myocardium to pharmacological agents

1. Age-related changes in electrophysiological and pharmacological properties of rat ventricular cells have been investigated. 2. As compared to adults, transmembrane action potential (TAP) of aged rat myocardium exhibits a prolonged phase of repolarization. 3. For both young and aged groups tetrodotoxin reduces TAP duration. 4. Nifedipine and isoproterenol induce more pronounced modifications of the TAP in aged subjects. 5. Amiloride and tetraethylammonium ions prolong, for both groups, phase 2 of repolarization. This effect is more marked in the aged group. 6. Our results suggest that the prolonged TAP in aged myocardium could result from an age-related increase in calcium current.