Development of tolerance to antinociceptive effects of an intrathecal morphine/clonidine combination in rats

Previous animal studies have shown the antinociceptive effects of intrathecal clonidine and intrathecal morphine to be synergistic. This study investigated the intrathecal administration of multiple doses of this drug combination to examine the rate of development of tolerance and to determine whether there was any toxic effect on the spinal cord.Rats with indwelling intrathecal catheters were given saline, morphine (2.5–7.5 g), clonidine (17.5 g), or clonidine (17.5 g) plus morphine (1 g) intrathecally twice daily for 41/2 days (total of 9 doses). Hot plate and tail flick tests were conducted after the first, fifth and ninth doses. After the ninth dose animals were killed and their spinal cords were removed for histological examination.Tolerance developed to the antinociceptive effects of the drug combination, but at a slower rate than to morphine alone. No evidence of toxicity or injury to the spinal cord was observed other than changes which could be ascribed to the presence of the catheter.     

Central components in the antinociceptive activity of dipyrone in mice

Dipyrone is classified as a nonsteroidal anti-inflammatory drug. It has analgesic, antipyretic and anti-inflammatory properties and exerts its analgesic effect via both peripheral and central action. Dipyrone at the dose of 250 and 500 mg/kg showed dose-dependent antinociceptive activity in the hot plate, tail flick tests to radiant heat and tail clip test and the writhing test induced by acetic acid in mice. The antinociceptive effects of dipyrone (500 mg/kg) were antagonized by naloxone (1, 2, 5 mg/kg) in the tail flick test to radiant heat and tail clip test and hot plate tests but not in the writhing test. Cyproheptadine (100 g/kg) decreased the antinociceptive effect of dipyrone. There was an increase in the antinociceptive effects of dipyrone (500 mg/kg) when combined with buspiron (0.5 mg/kg) in the tail flick test to radiant heat and tail clip test. The results provide evidence for a central antinociceptive effect of dipyrone antagonized by naloxone which suggests that its activity may also involve the serotoninergic system.     

Lack of effect of baclofen on substance P and somatostatin release from the spinal cord in vitro

Summary High concentrations of K⁺ increase the release of substance P (SP) and somatostatin (SRIF) from superfused slices of rat spinal cord. This increase is Ca-dependent. Baclofen (100–500 M) does not significantly alter the K⁺-evoked release of SP or SRIF. Stereoisomers of baclofen and GABA, similarly, are without effect. The spinal analgesic action of baclofen does not appear to be due to alterations in the release of SP or SRIF.     

Evidence for a spinal origin of the effect of baclofen on the myocardial oxygen demand indexes

In a previous study in anaesthetized rabbits we observed that electrical stimulation of the hypothalamic paraventricular nucleus (PVN) elicited substantial rises in the maximum rate of change of left ventricular pressure (dP/dt _max) and in myocardial oxygen demand indexes (rate-pressure product and triple product), similar to the changes observed during stress or physical effort. Baclofen, a selective GABA_B receptor agonist, injected intravenously prevented these responses. In the present study, we show that low doses of baclofen (0.1, 0.3 and 1 g/kg), injected intrathecally (i.t.) at the T9 level, reduced the myocardial oxygen demand during PVN stimulation. After 0.3 g/kg baclofen i.t., the peak value of the triple product during stimulation was 140 ± 20 compared with 193 ± 20 before treatment. An i.t. injection (500 g/kg), of saclofen, a selective GABA_B receptor antagonist, did not modify the resting haemodynamics significantly but attenuated the inhibitory effects of baclofen (3 mg/kg i.v.). These results suggest that the main site of the effects of baclofen is located within the spinal cord and that GABAB receptors probably mediate these effects by modulating the central control of cardiac function. In conclusion, baclofen might be a useful tool to prevent the centrally evoked increases of myocardial oxygen demand.     

Functional characterization of substance P receptors in the rabbit ear artery

Summary Rabbit isolated ear arteries were perfused at a constant flow and stimulated with field pulses (5 Hz, 5 impulses). Different tachykinins and capsaicin depressed stimulation-induced vasoconstriction, substance P (SP) being the most potent inhibitor. The rank order of potency of the tachykinins was, SP physalaemin eledoisin>SP-methylester; that of SP and its C-terminal fragments, SPSP-(2–11)SP-(4–11)>SP-(6–11). SP-(1–9) was inactive. The SP antagonist (Arg⁵,d-Trp^7,9,Nle¹¹)SP-(5–11) 10 mol/l shifted the concentration-response curve of SP to the right (pA₂=5.43), whereas it did not reduce the action of capsaicin. Another SP antagonist (d-Pro⁴,d-Trp^7,9,10)SP-(4–11) 10 mol/l failed to affect the SP depression. Neither antagonist changed vasoconstriction by itself. Pretreatment of the arteries with a mixture of yohimbine, propranol, atropine, diphenhydramine, burimamide, methysergide and indomethacin, all 1 mol/l, did not influence the effect of SP or capsaicin. Only the inhibition by SP, but not that by capsaicin was abolished after mechanical destruction of the endothelium. SP, physalaemin and eledoisin, all 3 mol/l, reduced vasoconstriction by noradrenaline or histamine; capsaicin 30 mol/l depressed noradrenaline-induced vasoconstriction. In arteries preincubated with³H-noradrenaline, electrical stimulation (1 Hz, 120 pulses) triggered an increase in the outflow of tritium and evoked vasoconstriction. SP 1 mol/l did not change either basal or stimulation-evoked tritium outflow, whereas it reduced vasoconstriction. In conclusion, SP depresses nerve stimulation-induced vasoconstriction solely by changing smooth muscle contractility; the receptor activated seems to belong to the SP-P type. Under the conditions of these experiments there is no indication for a similar effect of endogenous SP.     

Effect of selective inhibition of cyclooxygenase-2 on lipopolysaccharide-induced hyperalgesia

Lipopolysaccharide (LPS) is known to increase the expression and release of various pro-inflammatory mediators, including cyclooxygenase-2 (COX-2) and produce hyperalgesia. It is also well known that prostaglandins (PGs), synthesised both in the periphery and centrally by COX isoforms, play a key role in sensitisation of nociceptors and nociceptive processing. To investigate the role of COX-2 in LPS-induced hyperalgesia, parecoxib, a selective COX-2-inhibiting pro-drug, was injected intravenously 30 min before assessing hyperalgesia induced by intraperitoneal or subcutaneous administration of LPS (50 g/mouse or 25 g/paw of rat, respectively). Acetic acid-induced writhing and tail immersion assay in mice and paw withdrawal response to thermal and mechanical stimuli in rats were used to assess the effect of inhibition of COX-2 on LPSinduced hyperalgesia. Animals showed significant hyperalgesic behavior 8 h after LPS injection. Parecoxib (up to 20 mg/kg, i.v.) had no effect in the two acute nociceptive assays but showed marked antinociceptive activity in writhing and tail immersion assay in LPS-pretreated mice. Similarly, parecoxib reversed the hyperalgesia in the LPS-injected paw but not in the contralateral paw of rats. Pre-treatment with dexamethasone, an inhibitor of COX-2 expression before LPS injection significantly affected the development of hyperalgesia in both mice and rats. These findings suggest that inducible COX-2 derived PGs are involved in central nociceptive processing, which resulted in hyperalgesic behavior following LPS administration and inhibition of COX-2 or its expression attenuated LPS-induced hyperalgesia.     

Spinal antinociception by morphine in rats is antagonised by galanin receptor antagonists

Galanin, a 29 amino acid peptide, has been reported to possess antinociceptive properties at the spinal site and to potentiate opioid-induced antinociception. Our aim was to investigate whether also endogenous galanin interacts with an exogenously administered opioid, morphine, in the rat spinal cord. This question was investigated by use of the recently developed galanin receptor antagonists galantide [M-15, galanin(1–13)-substance P-(5 -11) amide] and M-35 [galanin-(1–13)-bradykinin-(2–9) amide].Nociception was assessed in the rat tail-flick test using radiant heat and the rat Randall-Selitto model of inflammatory pain using vocalization as the nociceptive criterion. Intrathecal (i.t.) injections were performed in rats under ether anaesthesia. Morphine was administered either i.t. or intraperitoneally (i.p.), and the antagonists were injected i.t. [¹²⁵I]Galanin binding experiments were performed on crude synaptosomal membranes of the rat spinal cord.In the rat tail-flick test, i.t. injection of 3 g morphine evoked antinociception of about 75% of the maximal possible effect (% MPE). Co-injection of either 2 g galantide or 2 g M-35 with morphine almost completely abolished the antinociceptive effect of morphine. I.p. injection of 2.15 mg/kg morphine elicited about 80% MPE when given 10 min prior to i.t. saline injection. Injection of the antagonists instead of saline antagonised the antinociceptive effect of morphine partially thus showing the spinal proportion of the overall antinociceptive effect. In the rat Randall-Selitto test, 3 g morphine, injected i.t., produced antinociception of almost 100% MPE. Coinjection of the antagonists reduced the maximum effect partially by about 25–35%. I.p. injection of 7.5 mg/kg morphine 10 min prior to Lt. injection of saline elicited an antinociceptive effect of 90–100% MPE; injection of the antagonists instead of saline reduced the peak effect to a similar degree as after i.t. injection of 3 g morphine. To exclude a direct interference by morphine with the galanin receptor, in vitro binding of [¹²⁵I]galanin to a spinal synaptosomal fraction was assessed. Morphine, 10 M, did not interfere with the specific [¹²⁵I]galanin binding. These results provide further evidence that galanin is involved in spinal nociceptive processing. It seems to be involved in the mediation of the effects of morphine at this site, either as a co-transmitter, or subsequent to µ-receptor activation on nerve terminals or on interneurones. Correspondence to: W. Reimann at the above address     

Baclofen analgesia in mice: a GABAB-mediated response.

The effect of baclofen, a GABAB agonist, has been studied in three antinociceptive tests (tail flick latency, hot plate method and acetic acid-induced writhing) in mice. In all three models, baclofen was found to elicit a dose-dependent antinociceptive effect. The observed antinociceptive effect was stereospecific, as the levo isomer of baclofen was found to be more potent than the racemic mixture. Baclofen also potentiated morphine analgesia. The antinociceptive effect of baclofen was reversed by both CGP 35348, a GABAB antagonist, and naloxone, an opioid antagonist, but not by bicuculline or picrotoxin, GABAA antagonists. However, in acetic acid-induced writhing, naloxone failed to reverse baclofen analgesia. The data suggest that the antinociceptive effect of baclofen is GABAB receptor-mediated and that there may be a GABAergic and opiate/or non-opiate interaction in eliciting the analgesic effect.     

Cyclic nucleotides and aminophylline produce different effects on nociceptive motor and sensory responses in the rat spinal cord

Summary The effect of intrathecal (i.t.) and systemic (i.p. and i.v.) administration of morphine, aminophylline, dibutyryl cyclic adenosine monophosphate (DBcAMP) and dibutyryl cyclic guanosine monophosphate (DBcGMP) on motor and sensory responses of the spinal nociceptive system was studied in rats. Motor responses were assessed in the tail-flick test performed on rats with an intact spinal cord, or as flexor reflex activity elicited in the electromyogram of the tibialis anterior muscle by supramaximal electrical stimulation of the sural nerve in rats in which the spinal cord was transected at the lower thoracic level. The sensory response consisted of activity in single ascending axons of the spinal cord evoked by electrical stimulation of afferent C fibres in spinal rats.Morphine (20 g i.t. or 2 mg/kg i.p.) prolonged the tail-flick latency and aminophylline (25 mg/kg i.p. or 50 g i.t.) prevented the antinociceptive effect of morphine. Aminophylline alone, administered by i.t. injection, reduced the tail-flick latency in a dose-dependent way. Morphine (2 mg/kg i.v. or 10 g i.t.) reduced flexor reflex activity, and this reduction was abolished by aminophylline (25 mg/kg i.v. or 50 g i.t.). Morphine (2 mg/kg i.v.) depressed spontaneous and evoked activity in single ascending axons responding to stimulation of afferent C fibres. This depressant effect of morphine was not abolished by aminophylline (50 g i.t.); the depression was antagonized by naloxone (10 g i.t.).DBcAMP (5 to 100 ng i.t.) dose-dependently prolonged the tail-flick latency. The antinociceptive effect of DBcAMP (50 ng i.t.) was prevented by aminophylline (50 g i.t.) or naloxone (5 g i.t.). DBcAMP (100 ng i.t.) reduced flexor reflex activity but facilitated activity in ascending axons responding to stimulation of afferent C fibres. DBcAMP (100 ng i.t.) did not affect ascending activity evoked by stimulation of afferent A or A fibres.DBcGMP (150 ng i.t.) depressed spontaneous and evoked activity in ascending axons responding to afferent A and C fibre stimulation. It produced no effect on activity in ascending axons evoked by afferent A fibre stimulation.These results demonstrate that (1) the depression by morphine of nociceptive motor and sensory responses of the rat spinal cord is based on different mechanisms in which nucleotides could (but need not) be involved, and (2) the two responses may divergently be influenced by drugs.Supported by a grant of the Sonderforschungsbereich 38 Membranforschung     

The involvement of central cholinergic system in the pressor effect of intracerebroventricularly injected U-46619, a thromboxane A2 analog, in conscious normotensive rats

The aim of this study was to determine the involvement of the central cholinergic system in the rise in blood pressure evoked by the thromboxane A2 (TxA2) analog, U-46619, given centrally. Intracerebroventricular (i.c.v.) injections of U-46619 (0.5, 1.0 and 2.0 g) caused dose- and time-related increases in blood pressure and decreased heart rate in awake rats. U-46619 (1 g; i.c.v.) also produced an approximately 65% increase in posterior hypothalamic extracellular acetylcholine and choline levels. Pretreatment with SQ-29548 (8 g; i.c.v.), selective TxA2 receptor antagonist, completely inhibited both the cardiovascular responses and the increase in acetylcholine and choline levels to subsequent injection of U-46619 (1 g; i.c.v.). Atropine (10 g; i.c.v.), nonselective muscarinic receptor antagonist, pretreatment did not affect the cardiovascular responses observed after U-46619 (1 g; i.c.v.). Pretreatment with the nonselective nicotinic receptor antagonist, mecamylamine (50 g; i.c.v.) attenuated the pressor effect of U-46619 (1 g; i.c.v.). Higher doses of mecamylamine (75 and 100 g; i.c.v.) pretreatments did not change the magnitude of the blockade of pressor response to U-46619; however, they abolished the bradycardic effect of U-46619 dose-dependently. Interestingly, pretreatment of rats with methyllycaconitine (10 g; i.c.v.) or -bungarotoxin (10 g; i.c.v.), selective antagonists of 7 subtype of nicotinic acetylcholine receptors (7nAChRs), partially abolished the pressor response to i.c.v. injection of U-46619 (1 g). Similar to the mecamylamine data, the use of higher doses of methyllycaconitine (25 and 50 g; i.c.v.) produced the same magnitude of blockade that was observed after the 10 g methyllycaconitine pretreatment, but it completely abolished the bradycardic effect of U-46619 (1 g; i.c.v.) at the dose of 25 g. The present results show that central administration of U-46619 produces pressor and bradycardic effect and increase in hypothalamic acetylcholine and choline levels by activating central TxA2 receptors. The activation of central nicotinic receptors, predominantly 7nAChRs, partially mediates the cardiovascular responses to i.c.v. injection of U-46619.     

Antagonism by γ-aminobutyric acid of the stimulant effect of angiotensin II on cardiac sympathetic ganglia in spinal dogs

Summary The effects of angiotensin II and neuro-aminoacids administered through the right subclavian artery (i. a.) to the cardiac sympathetic ganglia were investigated in spinal dogs. Angiotensin II (1–8 g) elicited a dose-dependent positive chronotropic effect which was reduced after i. a. injection of saralasin (100g). The effect of angiotensin II was not reduced after combined treatment with either hexamethonium (10 mg/kg) plus atropine (0.1 mg/kg) or hemicholinium-3 (5 mg/kg) plus preganglionic stimulation. The dosedependent response to angiotensin II of heart rate was inhibited by GABA (50, 500g), GABOB (500g) and muscimol (50, 100g). The inhibition of the response to angiotensin II by a small dose of GABA (50g), but not by a high one (500g), was antagonized by i. a. injection of picrotoxin (2 mg). The positive chronotropism induced by bethanechol (25, 50g) and a small dose of acetylcholine (25g) were significantly inhibited by a high dose (500g) but not by a low dose (50g) of GABA. These results confirm that angiotensin II stimulates cardiac chronotropism by acting on the angiotensin II receptor located at the cardiac ganglia and show that this stimulant effect is antagonized by GABA.     

Behavioral analysis of the effect of substance P injected into the ventral mesencephalon on investigatory and spontaneous motor behavior in the rat

In the present experiments the behavioral response to substance P (SP) microinfusion into the ventral tegmental area (VTA), substantia nigra (SN), and sensorimotor cortex (CX) was investigated in detail. The experiments were carried out using an eight-hole box to measure exploratory behavior and a video monitor for the analysis of spontaneous motor behavior. When infused into the VTA, SP (0.125, 0.5, 3.0 g) augmented the frequency and total duration of hole-pokes, and tended to diminish the mean duration of hole-pokes. The strategy and organization of responses, as measured by the order of hole-visits and hole-switching, were unchanged by SP and there was no indication of stereotypy, measured by the number of hole-pokes per hole-visit. The open-field analysis revealed a marked increase in locomotion and rearing, both in the periphery and center of the arena; grooming was decreased by SP. The behavioral profile following SN infusions of SP (3.0 g) was similar to that elicited by VTA infusions, with the exception that center rearing was not enhanced. SP administration into cortex (3 g) had no significant effect on any behavioral measures. It is hypothesized that SP infused into the ventral mesencephalon results in an enhancement of approach response tendencies, suggesting that endogenous SP in this region may regulate spontaneous behavior. The possibility of an interaction between SP and meso-telencephalic dopamine neurons is discussed.     

Hyperalgesia produced by intrathecal substance P and related peptides: desensitization and cross desensitization.

The hyperalgesic effect of intrathecally administered substance P (SP), physalaemin, eledoisin and eledoisin-related peptide (ERP) was investigated in the rat tail flick test. Hyperalgesia produced by SP (2.5-15 micrograms, 1.9-11 nmol) was maximal 10-20 min after injection, lasted 30 min and was dose-related. The effect was mimicked by all of the peptides examined. The rank order of potency was physalaemin greater than SP greater than eledoisin greater than ERP. Desensitization to the hyperalgesic effect of SP was produced by three repeated intrathecal injections. Rats desensitized to SP no longer responded to physalaemin or ERP, indicating cross-desensitization. Phentolamine continued to produce hyperalgesia following such desensitization. The demonstration of a hyperalgesic effect for SP provides further support for a role for SP in nociceptive transmission. The receptor mediating this effect appears to be a SP-P subtype. Cross-desensitization between peptides suggests an action on the same receptor.     

Anti-nociceptive effect of morphine,opioid analgesics and haloperidol injected into the caudate nucleus of the rat

Summary The effect of intracaudate (i.c.) microinjections of morphine, opioid analgesics and haloperidol was determined on the tail-flick response evoked by radiant heat in rats. Bilateral injections (0.2 l on each side) into the caudate nuclei of morphine 5 g, pethidine 50 g, levorphanol 4 g, dextrorphan 10 g and haloperidol 5 g significantly increased the reaction time of the tail-flick response. The antinociceptive effect of an i.c. injection of morphine or levorphanol was abolished by an intraperitoneal (i.p.) injection of naloxone 0.2 mg/kg or apomorphine 2 mg/kg. The anti-nociceptive effect of pethidine, dextrorphan and haloperidol was reduced but not abolished by an i.p. injection of naloxone 0.2 mg/kg. An i.p. injection of apomorphine 2 mg/kg abolished the effect of an i.c. injection of haloperidol. A bilateral i.c. injection of naloxone 5 g or apomorphine 10 g reduced the anti-nociceptive effect of an i.p. injection of morphine 2 mg/kg or haloperidol 2 mg/kg, but did not abolish it. It is concluded that (1) an anti-nociceptive effect can be achieved by an action on the caudate nucleus of the drugs tested; (2) the anti-nociceptive effect exerted by morphine and levorphanol in the caudate nucleus is due to a specific action mediated by opiate receptors, whilst that produced by pethidine and dextrorphan is due to a specific and/or unspecific action; (3) the anti-nociceptive effect of haloperidol in the caudate nucleus is due to an impairment of dopaminergic impulse transmission, which is also involved in the effect of morphine and levorphanol.Supported by the Sonderforschungsbereich 38 Membranforschung     

Role of ascending and descending serotonergic pathways in the antinociceptive effect of baclofen

Summary The role of ascending and descending serotonergic pathways in the antinociceptive effect of baclofen was examined by lesioning specific pathways with the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT). Antinociception in rats was assessed using the tail flick and hot plate tests 3/4 and 10/11 days after lesioning and the placement of lesions verified by analysis of serotonin (5-HT) in brain and spinal cord. Lesions to the ventromedial tegmentum depleted 5-HT selectively in brain and inhibited the antinociceptive effect of baclofen in the tail flick test 3/4 but not 10/11 days after lesioning. Lesions to the nucleus raphe medianus produced a marked depletion of 5-HT in the hippocampus and produced the same effect on baclofen. Lesions to the nucleus raphe dorsalis were less selective, depleting 5-HT in a number of brain regions and in the spinal cord, and inhibited the antinociceptive effect of baclofen at the later but not the earlier time interval. Lesions to descending pathways by microinjection of 5,7-DHT into the ventral raphe or nucleus raphe magnus did not affect the action of baclofen significantly. Lesions to both ascending and descending pathways by intracerebroventricular 5,7-DHT increased the effect of baclofen. The hot plate test generally was less sensitive to these manipulations, although changes parallel to the tail flick test were observed in a number of instances. Both the destruction of 5-HT pathways and development of supersensitivity at 5-HT receptors may contribute to the interactions observed. Send offprint requests to J. Sawynok     

Biovector™ Nanoparticles Improve Antinociceptive Efficacy of Nasal Morphine

Purpose. We have studied the antinociceptive activity and blood andbrain delivery of nasal morphine with or without Biovectornanoparticles in mice. Methods. A tail flick assay was used to evaluate theantinociceptive activity. The kinetics of morphine were evaluated in blood andbrain, using tritiated morphine as tracer. Results. These nanoparticles were shown to increase the durationof the antinociceptive activity of morphine after nasal administration.This effect was not due to an increase of morphine in the blood; andthe analgesic activity of morphine in association with nanoparticleswas reversed by naloxone. The ED₅₀ value was 33.6 ±15.6 mg/kg for morphine alone and 14.4 ± 7.6 mg/kg in presenceof nanoparticles. They were only effective at low doses (1.5 to 2.5 g),a higher or a lower dose had no effect. No interaction was found betweennanoparticles and morphine. NaDOC, a permeation enhancer, was unable toimprove nasal morphine activity. Conclusions. These results show the presence of nanoparticles onlyat a very specific dose increases the antinociceptive activity of nasalmorphine in mice. The occurrence of a direct transport of morphinefrom the nasal mucosa to the brain is discussed.     

Theophylline-induced potentiation of the antinociceptive action of baclofen

1--Theophylline (35, 50 mg/kg) potentiated the antinociceptive action of intraperitoneally administered baclofen in the tail flick and hot plate tests. Potentiation was most marked when the pretreatment time was 1 h, but some potentiation was still apparent following a 2 h pretreatment. 2--Theophylline alone (50 mg/kg) produced only slight alterations in reaction latency in the two tests. 3--When baclofen was applied directly into the spinal subarachnoid space, a 1 h pretreatment with theophylline produced minimal effects, but a 2 h pretreatment produced an increase in the antinociceptive action of baclofen. 4--These results suggest that theophylline can potentiate the antinociceptive action of baclofen by actions at both supraspinal and spinal sites.     

Role of ATP-sensitive K+ channels in antinociception induced by R-PIA,an adenosine A1 receptor agonist

The influence of several K⁺ channel-acting drugs on antinociception induced by the adenosine A₁ receptor agonist (–)-N⁶-(2-phenylisopropyl)-adenosine (R-PIA) was evaluated with a tail flick test in mice. The subcutaneous administration of R-PIA (0.5–8 mg/kg) induced a dose-dependent antinociceptive effect. The ATP-sensitive K⁺ (K_ATP) channel blocker gliquidone (2–8 g/mouse, i.c.v.) produced a dose-dependent displacement to the right of the R-PIA dose-response line, whereas the K_ATP channel opener cromakalim (32 g/mouse, i.c.v.) shifted it to the left. Several K_ATP channel blockers dose-dependently antagonized the antinociceptive effect of R-PIA, the order of potency being gliquidone > glipizide > glibenclamide (i.e., the same order of potency shown by these drugs in blocking K_ATP channels in neurons). In contrast, the K⁺ channel blockers 4-aminopyridine and tetraethylammonium did not antagonize the effect of R-PIA. These data suggest that antinociception produced by adenosine A₁ receptor agonists is mediated by the opening of ATP-sensitive K⁺ channels. The present results, together with those of previous studies, further support a role for K⁺ channel opening in the antinociceptive effect of agonists of receptors coupled to G_i/G_o proteins. Correspondence to: José M. Baeyens at the above address     

Calcium agonists and antagonists of the dihydropyridine type: Antinociceptive effects,interference with opiate-μ-receptor agonists and neuropharmacological actions in rodents

The calcium antagonist dihydropyridine derivative nimodipine and its enantiomers BAY N 5247, BAY N 5248, as well as BAY R 4407 (calcium antagonist (+)-enantiomer of the calcium agonist dihydropyridine BAY K 8644) do not exert antinociceptive effects in the rat as measured by the vocalization test in doses up to 100 g/kg IV, and in the mouse as measured by the hot plate test in oral doses up to 100 mg/kg. The calcium agonists BAY K 8644 and BAY R 5417 ((–)-enantiomer of BAY K 8644) are also ineffective in the rat vocalization test but BAY K 8644 increases reaction time in the hot plate test (mouse) dose-dependently (1–10 mg/kg PO). -Receptor agonist (fentanyl) antinociceptive effects are potentiated by simultaneous IV administration of the calcium antagonists, the (–)-enantiomer of nimodipine BAY N 5248 being the most potent. This applies for the rat (vocalization test) and the mouse (hot plate test). The influence on fentanyl antinociception in the rat of the calcium agonist BAY K 8644 and its (–)-enantiomer BAY R 5417 is biphasic: low doses attenuate, high doses potentiate fentanyl antinociception. In the mouse (hot plate test) antinociceptive effects of BAY K 8644 plus fentanyl are less than additive, indicating that the calcium agonist decreases fentanyl effects. The relative potency of calcium antagonists in potentiation of fentanyl antinociception correlates with their relative potency as calcium antagonists as measured by receptor binding studies, effects on vascular and cardiac muscle, and with their neuropharmacological actions (anticonvulsive effects, inhibition of balance and spontaneous motility as well as tranquilizing effects in the mouse). It is concluded that calcium antagonism potentiates -receptor agonist antinociceptive effects, whereas calcium agonism antagonizes -receptor agonist antinociception.Dedicated to Prof. Dr. H. Coper (Berlin) on the occasion of his 60th anniversary     

Serotonin type-2 receptor mediated regulation of substance P release in the ventral spinal cord and the effects of chronic antidepressant treatment

Summary Regulation of the release of substance P (SP) by the coexisting neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) in the ventral spinal cord and the effects of chronic antidepressant treatment mediated changes in serotonin metabolism on the regulation, were examined.The K⁺ (40 mmol/l) evoked release of (SP) from slices of the ventral spinal cord of the rat was potentiated by (5-HT) applied to 100 mol/l concentration. This effect was blocked by the serotoninergic antagonists methysergide (10 mol/l), methiotepin (10 mol/l) and fully blocked by ketanserin (10 mol/l). Thus the 5-HT receptor which regulates the release of SP appears to belong to the type-2 5-HT receptors. Chronic treatment with the selective serotonin uptake inhibitor zimelidine (14 days, 2×10 mol/kg/day, p.o.) lowered the tissue levels of the 5-HT metabolite: 5-hydroxyindol acetic acid (5-HIAA) and elevated the tissue levels of SP in both the ventral and dorsal spinal cord as compared to that in the vehicle treated group (14 days, 2×5 ml saline/kg/day, p.o.). The decrease in the 5-HIAA levels after chronic zimelidine treatment was quantitatively similar in the dorsal (33%,p<0.01) and ventral (31%,p<0.05) spinal cord. The increase in SP levels after chronic zimelidine treatment was more pronounced in the ventral cord (80%,p<0.01) where the majority of the SP containing nerve endings also contain 5-HT, than in the dorsal spinal cord (22% increase in SP,p<0.05), where only a minor fraction of the SP-containing nerve endings shows a 5-HT/SP coexistence. Chronic treatment with imipramine (14 days, 2×10 mol/kg/day, p.o.) gave qualitatively similar results to those obtained by zimelidine treatment, but increases in SP levels, and decreases in 5-HT and 5-HIAA levels in ventral and dorsal spinal cord, were less pronounced. The K⁺ (40 mmol/l) evoked release of SP was studied in a slice preparation of the ventral spinal cord, from rats chronically treated with imipramine, zimelidine and saline. In the zimelidine treated group the amount of SP released (pmol/g tissue) and the fractional SP release upon K⁺ (40 mmol/l) stimulation was increased by 53% (p<0.01) and 42% (p<0.01) respectively, when compared to the control group. No significant changes in the amount of SP released or fractional SP release were observed when tissue preparations from rats treated with imipramine were used. Thus, it seems that treatment with specific serotoninergic or monoaminergic antidepressant drugs does not only change the tissue levels of the monoamine and its metabolite but also affects the coexisting peptidergic transmitter; SP in the ventral spinal cord. This change is also reflected in the size of the releaseable pool of SP.