Involvement of central GABA receptors in the regulation of the urinary bladder function of anaesthetized rats

Summary Cystometric recordings were performed in pentobaribitone anaesthetized rats and the effects of gammaaminobutyric acid (GABA) mechanisms on urinary bladder function were evaluated as their influence on a bladder hyperactivity induced by 1-dihydroxyphenylalanine (l-DOPA) after peripheral decarboxylase inhibition. The bladder response was inhibited by intracerebroventricular (i.c.v., 4th ventricle) injections of GABA (250 g), muscimol (0.2 g) and glycine (1,000 g) as well as by systemically administered muscimol (4 mg/kg) and diazepam (2 mg/kg). Intravenous (i.v.) bicuculline, but not i.v. strychnine, antagonized the inhibitory actions of intraperitoneal (i.p.) and i.c.v. muscimol and i.v. diazepam while the opposite was true for the inhibitory action of i.c.v. glycine. In rats not pretreated with l-DOPA, i.p. administration of bicuculline (4 mg/kg) after 15 min caused prominent detrusor contractions that were prevented by an infracollicular brain transection.It is suggested that GABA synapses in the pontinemesencephalic brain region may be involved in the modulation of urinary bladder function.     

Antagonism of the renal vasodilator activity of dopamine by metoclopramide

Summary The interaction of metoclopramide with renal dopamine receptors has been characterized in anesthetized dogs surgically prepared with arterial blood pressure catheters and renal artery blood flowprobes.In normal dogs, i. v. dopamine (3 g/kg) produced consistent and selective decrements in renal vascular resistance (RVR) and increments in renal blood flow over a 220 min test period; mean arterial blood pressure and cardiac rate were minimally affected. Pretreatment with metoclopramide, 1 and 10 mg/kg i. v., resulted in dose-related inhibition (maximum inhibition 44% and 94%, respectively) of the renal vasodilator activity of dopamine without altering baseline parameters. The duration of antagonism produced by 1 mg/kg of metoclopramide was approximately 30 min, while 10 mg/kg resulted in significant attenuation for the entire test period. Decreases in RVR produced by prostaglandin A₁ (0.03 and 0.3 g/kg, i. v.) and bradykinin (3 and 15 g/kg, i. v.) that were comparable to those of dopamine were unaltered by metoclopramide. Furthermore, metoclopramide did not affect the diastolic blood pressure responses to noradrenaline (0.1–3 g/kg, i. v.) or isoproterenol (0.03–0.3 g/kg, i. v.), nor did it alter dopamine-induced vasoconstriction of the iliac vasculature.In phenoxybenzamine (3 mg/kg, i. v.) treated dogs, dopamine (0.3–30 g/kg, i. v.) produced dose-related reductions in RVR. Administration of metoclopramide (10 mg/kg, i. v.) resulted in a 10-fold parallel displacement, to the right, of the RVR dose-response curve to dopamine.These findings demonstrate that metoclopramide is an effective antagonist of renal dopamine receptors following systemic administration in the dog. The results are not consistent with the classification of metoclopramide as a selective antagonist of D-2 receptors.     

Central,naloxone-reversible antinociception by diclofenac in the rat

Summary The antinociceptive effect of subcutaneously (s. c.), intracerebroventricularly (i.c.v.) or intrathecally (i.t.) administered diclofenac was studied in a series of experiments employing the tail-flick (0.01–10.0 mg/kg body weight i. p., 1–50 g i.c.v., 1–10 g i.t.) and hotplate (0.01–50 mg/kg body weight i. p., 1– 50 g i. c. v., 1–10 g i. t.) models representing somatosensory stimuli and the writhing test (0.001 mg–10 mg s. c., 0.1–200 g i.c.v., 0.1–100 g i. t.) and colorectal distension (1–200 g i.c.v.) models representing noxious visceral stimuli. Diclofenac did not exert any antinociceptive effects in the tail-flick or hot-plate models. In the writhing test, diclofenac dose-dependently inhibited the number of writhings after s. c. administration (0.001–10.0 mg/kg body weight) with an ED₅₀ of 1 mg/kg. A similar dose-dependent action of diclofenac was seen after i.c.v. (0.1–200 g) and i.t. (0.1–100 g) administration with an ED₅₀ of 3 in both cases. The antinociceptive effect of diclofenac administered s. c., i.c.v. or i.t. was almost completely reversed by pretreatment with naloxone, (1 mg/kg body weight s. c.).In the colorectal distension model, the i.c.v. administration of diclofenac (1–200 g), which attenuated the cardiovascular response to colorectal distension, was reversed by naloxone. The pressor and tachycardia response to a 20 s, 80 mmHg colorectal distension was inhibited by diclofenac 100 g i.c.v. (16.1 ± 1.7 mmHg or 58% and 39.4 ± 0.4 bpm or 70% versus control, respectively).It is concluded that diclofenac exerts a central, naloxone-reversible antinociceptive action in experimental animals after noxious visceral stimuli but not after somatosensory stimuli.Send offprint requests to R. Bjorkman at the above address     

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.     

Cardiovascular effects of the novel histamine H2 receptor agonist amthamine: interaction with the adrenergic system

The cardiovascular effects of the new histamine H₂ receptor agonist amthamine were studied in the anaesthetized rat, with particular reference to a possible interaction with the adrenergic system. Amthamine (0.03–3 mol/kg i.v.) caused vasodepressor responses which were antagonized by famotidine (3 mol/kg i.v.). At higher doses (30–100 mol/kg i.v.), amthamine induced a modest increase in the mean arterial pressure, which was significantly enhanced by the blockade of H₂ receptors and significantly reduced by the ₂ adrenoceptor antagonist yohimbine (1 mol/kg i.v.). The vasopressor response to amthamine was not modified in rats pre-treated with reserpine or 6-hydroxydopamine, and was only minimally modified in adrenalectomized animals, thus suggesting a predominant interaction with postjunctional ₂ adrenoceptors in the vascular muscle. The H₂ receptor agonist dimaprit (0.3–100 mol/kg i.v.) caused a reduction in arterial pressure, which was antagonized by famotidine, no pressor response being unmasked.Dimaprit (0.1–30 mol/kg i.v.) did not modify heart rate but caused a modest bradycardia at 100 mol/kg i.v. Amthamine (1-100 mol/kg i.v.) induced a dose-dependent tachycardia, which was only partially (approximately 20%) reduced by famotidine and was totally blocked by propranolol (0.3 mg/kg i.v.). This effect was significantly reduced in rats pre-treated with reserpine or 6-hydroxydopamine and was further reduced by cocaine, thus suggesting a tyramine-like action of amthamine.In conclusion, these data demonstrate that the H₂ receptor agonist amthamine can also interact with the adrenergic system when used at doses higher than those necessary to activate H2 receptors. Whereas the increase in blood pressure induced by amthamine seems to be mainly mediated by a direct activation of postjunctional ₂ adrenoceptors, the increase in heart rate is predominantly due to neuronal release of catecholamines. These effects should be considered when using amthamine in cardiovascular or other studies when high doses are employed.     

β-methyl-digoxin

Summary The tissue/plasma ratio of -methyl-digoxin for cardiac muscle in cats was about the same 24 h after a single dose of 30 g/kg as after a loading dose of 30 g/kg followed by 3 maintenance doses of 7.5 g/kg at 24 h intervals. The ratio for the brain increased 2-fold during that time.After the i.v. injection of a toxic loading dose of 70 g/kg -methyl-digoxin or digoxin, maintenance doses of as little as 15 g/kg at 48 h intervals sufficed to maintain the minimum plasma glycoside concentrations determined by RIA at about 3 ng/ml. There was no difference in the plasma concentrations or in the severity of intoxication produced by both glycosides.Cats vomited within 3 h after i.v. injection of 100 g/kg -methyl-digoxin, whereas a loading dose of 30 g/kg followed by 3 injections of 7.5 g/kg at 24 h intervals were well tolerated. The concentration of radioactivity in the brain 3 h after 100 g/kg was less than 24 h after the last injection of 7.5 g/kg in the experiments with repeated dosage.Cerebral side-effects such as vomiting, loss of appetite and weight were better correlated with the glycoside concentrations in the plasma than with those in the brain.     

Effect of the phosphodiesterase inhibitor 4-(3-cyclopentyloxy-4-methoxyphenyl)-2-pyrrolidone (ZK 62711) on gastric secretion and gastric mucosal cyclic AMP

Summary The effect of the phosphodiesterase inhibitor 4-(3-cyclopentyloxy-4-methoxyphenyl)-2-pyrrolidone (ZK 62711) on gastric secretion and the cyclic AMP system of the gastric mucosa was studied in rats and guinea pigs. In rats, 0.03–0.3 moles/kg ZK 62711 i.v. stimulated acid and pepsin secretion in a dose-dependent manner and 0.03 moles/kg i.v. enhanced the effect of histamine. In guinea pigs no reproducible stimulation was found after intravenous injections of ZK 62711. The stimulation of gastric secretion in the rat by 0.3 moles/kg ZK 62711 i.v. was not affected by vagotomy but was totally inhibited by 10 moles/kg cimetidine i.v. The structurally related phosphodiesterase inhibitor, 4-(3-butoxy-4-methoxybenzyl)-2-imidazolidine (Ro 20-1724), at the dose of 3.3 moles/kg i.v. stimulated gastric secretion in anaesthetised rats to a similar extent as 0.3 moles/kg ZK 62711 i.v. The content of cyclic AMP in the rat gastric mucosa in vivo was slightly increased by 10 moles/kg ZK 62711 i.v, whereas lower doses did not change it. Accumulation of cyclic AMP in the rat gastric mucosa by 50 moles/kg histamine i.v. was enhanced by simultaneous injections of 3.3 moles/kg ZK 62711 i.v. In rat gastric tissue slices in vitro 1–50 M ZK 62711 increased the level of cyclic AMP but 100 M histamine had no effect in the absence or presence of ZK 62711. In gastric mucosal slices of the guinea pig 10 and 50 M ZK 62711 increased the cyclic AMP content which effect was inhibited by 100 M cimetidine and enhanced the stimulatory effect of 100 M histamine on cyclic AMP. The activity of soluble cyclic AMP phosphodiesterase was inhibited by ZK 62711 in the rat (IC₅₀=18 M) and guinea pig gastric mucosa (IC₅₀=1.5 M). Adenylate cyclase was not affected in the homogenate of the guinea pig gastric mucosa. The results indicate that the phosphodiesterase inhibitor ZK 62711 which increases cyclic AMP levels in the gastric mucosa in vivo and in vitro, is a potent stimulator of gastric acid secretion.     

Localization in the CNS of adrenoceptors which facilitate a cardioinhibitory reflex

Summary In dogs (pentobarbitone, 25 mg/kg) the brain was removed rostrally to the pons, leaving the cerebellum intact (decerebrate animals). In other animals the cerebellum was additionally removed (bulbar animals). In all animals -adrenoceptors were blocked by toliprolol (5 mg/kg s.c.). Angiotensin (0.025–0.3g/kg) was repeatedly injected i.v. and the resulting maximal reflex bradycardia was recorded. Intracisternal (i.ci.) injection of clonidine, 0.5 or 1 g/kg, in decerebrate or i.v. injection of 10 or 30 g/kg in bulbar animals significantly facilitated the reflex bradycardia. This effect was antagonised by a subsequent injection of piperoxan, 50 g/kg i.ci. in decerebrate or 1 mg/kg i.v. in bulbar animals. It is concluded that the facilitatory action of clonidine is mediated by -adrenoceptors within the medulla oblongata.Presented in part at the 15th Spring Meeting of the German Pharmacological Society, Mainz, March 1974.     

The cardiovascular effects of intraventricularly administered histamine in the anaesthetised rat

Summary In urethane-anaesthetised rats intraventricular (i.c.v.) injections of histamine (0.1–10.0 g) elicited dose-related rises in both the resting blood pressure and heart rate. These cardiovascular effects of histamine were antagonised in a dose-dependent manner by i.c.v. pretreatments with the histamine H₁-receptor antagonists mepyramine (10, 50 and 100 g) and diphenylpyraline (100 and 200 g). Pretreatment with the histamine H₂-receptor antagonist metiamide (100 and 200 g i.c.v.) failed to modify either of the responses. A dose-related antagonism of the hypertensive response to histamine i.c.v. was elicited by phentolamine (100 and 200 g i.c.v.) but the positive chronotropic effect was not modified by this pretreatment. The cardiovascular responses to histamine i.c.v. were abolished by mecamylamine (5.0 mg/kg i.v.) and greatly reduced by 6-hydroxydopamine (3×250 g i.c.v.), but only the tachycardia was significantly modified by atropine (100 g i.c.v.) and propranolol (1 mg/kg i.v.). Propranolol (100 g i.c.v.), bilateral vagotomy, or acute bilateral adrenal demedullation failed to modify the cardiovascular responses to histamine i.c.v. The results suggest that histamine is able to modify the resting blood pressure and heart rate by independent central modes of action, which involve central adrenergic and cholinergic mechanisms.Preliminary findings of this study were presented at the Autumn meeting of the British Pharmacological Society (Finch and Hicks, 1975).     

Pharmacological analysis of the central cardiovascular effects of four GABA analogues

Summary The central cardiovascular effects of 4 structural analogues of GABA were investigated. The drugs were injected intracerebroventricularly (i.c.v.) in cumulative doses into pentobarbital-anaesthetized normotensive rats. Muscimol (0.01–10 g/kg), THIP (0.01–100g/kg), kojic amine (0.1–100 g/kg) and isoguvacine (0.1–100g/kg) produced dose-dependent hypotension and bradycardia. The maximal fall in the mean blood pressure was of about 35% of the initial values. These effects appears to be of central origin since the intravenous (i.v.) injection of the same doses of the drugs did not produce any similar cardiovascular modifications. The hypotensive effects of muscimol and kojic amine were antagonized partly by i.c.v. bicuculline. The combination of bicuculline and kainic acid almost completely prevented the blood pressure lowering effects of muscimol, kojic amine and isoguvacine. THIP however was only slightly antagonized by bicuculline and kainic acid. Atropine i.v. also prevented partly the cardiovascular effects of all these drugs.Thus, the mechanisms of the central cardiovascular actions of GABA analogues appear to be more complex than expected and variable from one drug to another.The involvement of GABA receptors of the A and B types and of cholinergic mechanisms in the hypotensive effect of the drugs is discussed.     

Assessment of the muscarinic receptor subtype involved in the mediation of pilocarpine-induced purposeless chewing behaviour

Purposeless chewing behaviour in rats was enhanced by intraperitoneal administration of the muscarinic agonists pilocarpine (1.0–8.0 mg/kg). RS 86 (0.5–0.8 mg/kg), oxotremorine (1–2 mg/kg) and arecoline (2–32 mg/kg), but not by nicotine (0.1–3.2 mg/kg). Chewing behaviour was also induced by the ICV administration of the muscarinic agonists carbachol (12.5–100 g) and pilocarpine (50–200 g), but not by the putative M-1 receptor agonist McN-A-343 (50–200 g) or AH 6405 (100–200 g). The muscarinic receptor antagonists scopolamine (0.01–0.1 mg/kg SC), benzhexol (0.075–2.5 mg/kg SC), secoverine (1–10 mg/kg SC), and dicyclomine (1.25–10 mg/kg SC) antagonised purposeless chewing behaviour induced by pilocarpine (4 mg/kg IP). AF-DX 116 (2.5–100 mg/kg SC), an M-2 antagonist, partially inhibited the actions of pilocarpine (4 mg/kg IP). Based on ED₄₀ values the rank order of potency following IP administration was scopolamine > benzhexol > secoverine > dicyclomine > AF-DX 116. The ICV administration of the muscarinic antagonists N-methylscopolamine (2.5–10 g) and oxyphenonium (10–40 g) antagonised chewing behaviour induced by pilocarpine (4 mg/kg IP) in a dose-related manner. The M-2 antagonist 4-DAMP (40–160 g ICV), as well as AF-DX 116 (40–160 g ICV), also inhibited the effects of pilocarpine (40–160 g ICV). The putative M-1 receptor antagonist pirenzepine (80–320 g ICV) did not antagonise chewing behaviour induced by pilocarpine (4 mg/kg IP). Based on ED₄₀ values, the rank order of potency of muscarinic antagonists administered ICV was N-methylscopolamine > oxyphenonium > 4-DAMP > AF-DX 116 > pirenzepine. Comparisons of the actions of muscarinic antagonists in vivo, with their published actions in vitro suggest that pilocarpine-induced chewing behaviour is mediated through central M-2 receptors rather than via central M-1 sites.     

Hemodynamic alterations produced by N,N-di-n-propyldopamine in anesthetized dogs

Summary Intravenous infusion of N,N-di-n-propyldopamine (DPDA) (100–900 g/kg) produced dose-related arterial hypotension which was accompanied by bradycardia at higher doses. Increments in arterial blood pressure induced by carotid artery occlusion were attenuated during administration of DPDA (600 g/kg, i.v.), whereas cardiovascular compensation to postural change remained unaltered. DPDA-induced hypotension and its attenuation of carotid occlusion responses were prevented by pretreatment with sulpiride (0.5 mg/kg, i.v.), indicating involvement of dopamine receptors in these activities.Hemodynamic studies demonstrated that DPDA (600 g/kg, i.v.) lowered mean arterial blood pressure by dilating the systemic vasculature; mean aortic blood flow increased in the presence of bradycardia due to an increment in stroke volume. Left ventricular minute work, stroke work and myocardial oxygen consumption were decreased as a consequence of the hypotension and bradycardia produced by DPDA. The results of other experiments illustrated that propranolol (0.5 mg/kg, i.v.) and nitroglycerin (40 g/kg, i.v.) administered in combination, but not individually, resulted in hemodynamic alterations similar to those of DPDA (600 g/kg, i.v.). However, at equivalent reductions in mean arterial blood pressure, cardiac rate and myocardial oxygen consumption only DPDA increased mean aortic blood flow and lowered left ventricular end-diastolic pressure.The hemodynamic effects produced by DPDA are interpreted to be a reflection of its known ability to inhibit neurogenic release of noradrenaline by stimulation of neuronal dopamine receptors. The composite data suggest that orally effective dopamine receptor agonists may have clinical utility as antihypertensive agents with minimal liability for producing orthostasis. Furthermore, a potential use in ischemic heart disease is suggested by data indicating that DPDA maintained systemic perfusion at reduced levels of myocardial work and oxygen consumption.     

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.     

Inhibition of furosemide-induced renin release by vasoconstrictors

Summary The effect of i. v. infused (asp¹--amid, val⁵)-angiotensin II (1.0 g/kg min), octapressin (phe², lys⁸-vasopressin) (10.0 mU/kg min) and of the -sympathomimetic amine phenylephrine (40.0 g/kg min) on the stimulation of renin secretion by furosemide (10.0 mg/kg i.v.) was investigated. The vasoconstrictors abolished the renin release induced by furosemide. Studies on the clearance of p-aminohippuric acid (PAH) (i.e. renal plasma flow) showed that the action of the vasoconstrictors cannot be explained by a decrease in access of furosemide to its intrarenal sites of action.The mechanism of the suppressive action of the vasoconstrictors on renin release is discussed.Supported by DFG grant Me 541/1.     

Calcium channel modulation by dihydropyridines modifies sufentanil-induced antinociception in acute and tolerant conditions

Summary The study was aimed at elucidating the possible participation of l-type Ca²⁺ channel in the acute analgesic effect of an opiate and the development of tolerance to this action. Sufentanil, a selective p agonist, and two dihydropyridines, the Ca²⁺ antagonist nimodipine and the Ca²⁺ agonist Bay K 8644, were selected. The tail-flick test was used to assess the nociceptive threshold. In naive rats, nimodipine (200 g/kg) potentiated the analgesic effect of sufentanil reducing the ED₅₀ from 0.26 to 0.08 g/kg. Similar results were observed with its (–)-enantiomer Bay N 5248, while the (+) enantiomer Bay N 5247 was ineffective. Tolerance to the opiate was induced by chronic subcutaneous administration of sufentanil with minipumps (2 g/h, 7 days). In these conditions the dose-response curve to sufentanil was displaced to the right and the ED₅₀ was increased to 1.49 g/kg. In tolerant rats, nimodipine preserved its potentiating ability and prevented the displacement to the right of the sufentanil dose response-curve (ED₅₀ = 0.48 g/kg). When nimodipine was pumped (1 g/h, 7 days) concurrently with sufentanil, the development of tolerance to the opioid was not disturbed. However, the expression of tolerance was abolished and even the effect of acutely administered sufentanil was markedly potentiated (ED₅₀ = 0.03 g/kg). Similar experiments were performed with Bay K 8644. In naive rats, Bay K 8644 at a low dose (20 g/kg) that behaves as a calcium agonist, antagonized the analgesic effect of sufentanil (ED₅₀ = 0.58 g/kg), whereas at a high dose (200 g/kg) it potentiated this action (ED₅₀ = 0.15 g/kg). In tolerant rats, Bay K 8644 (20 g/kg) preserved its antagonizing ability inducing a displacement to the right of the sufentanildose-response curve (ED₅₀ = 4.2 g/kg). When Bay K 8644 was pumped (1 g/h, 7 days) concurrently with sufentanil, it enhanced the expression of tolerance to the opiate (ED₅₀ = 3.8 g/kg). These results suggest that the calcium fluxes through the l-type channel in neurones are functionally linked to the activation of the opiate receptor: the blockade of the channel increased the potency of sufentanil, whereas its activation reduced the potency of the opiate. In chronic experiments, DHPs concurrently administered with sufentanil did not affect the development of tolerance to the opiate. However, nimodipine prevented the expression of this phenomenon. Even more, the animals became hypersensitive to the opiate suggesting that the adaptative mechanisms induced by chronic opiate could be affected by chronic nimodipine.This work was supported by grants from Universidad de Cantabria-Caja Cantabria (1988) and Bayer AG, Wuppertal, FRGPredoctoral Fellow: Fondo de Investigaciones Sanitarias de la Seguridad Social.Send offprint requests to: M. A. Hurlé at the above address     

Interaction between clonidine and physostigmine in normal rats and in rats after sinoaortic denervation

Summary Clonidine (3–30 g · kg^–1, i.v.) induced a fall in mean arterial pressure in rats after sinoaortic denervation but not in sham-operated animals. Moreover, sinoaortic denervation reduced the bradycardic action of this antihypertensive drug. Pressor and tachycardic response to physostigmine (60 g · kg^–1, i.v.) were greater in denervated than in sham-operated rats. The increase of mean arterial pressure was 26.2 ± 2.2 mm Hg in sham-operated rats (n = 12) and 53.8 ± 2.0 mm Hg in denervated rats (n = 12, P < 0.005).Pretreatment with 3 g · kg^–1 (i. v.) of clonidine did not alter the pressor response to physostigmine (60 g · kg^–1) in either of the two groups; 10 and 30 g · kg^–1 of clonidine reduced the physostigmine-induced increase of mean arterial pressure in sham-operated rats but enhanced the pressor response in denervated animals. Furthermore, an ineffective dose of physostigmine (30 g - kg^–1 i.v.) induced a pressor response after pretreatment with clonidine (10 gg · kg^–1) in denervated rats.Clonidine (10 g · kg^–1) did not affect the pressor effect of 1,1 dimethyl-4-phenylpiperazinium iodide (DMPP: 50 g · kg^–1 i.v.) or phenylephrine (4 g · kg ^–1, i.v.) in either group.The anticholinergic effect of clonidine in sham-operated rats may be explained by an inhibitory action on the release of acetylcholine in several brain structures but the facilitatory effect of clonidine observed in denervated animals is not clear. The results did not suggest a peripheral involvement in this facilitatory effect. Send offprint requests to M. A. Enero at the above address     

On the mechanism of the decrease in cerebellar cyclic GMP content elicited by opiate receptor agonists

Summary Morphine, dextromoramide (4 mol/kg i.p.) and vimonol R₂ (17 mol/kg i.p.) in analgesic doses (28 to 112 mol/kg i.p.) decreased 3,5-cyclic guanosine monophosphate (cGMP) in rat cerebellar cortex; morphine also decreased the cGMP content in deep cerebellar nuclei. Intrastriatal but not intracerebellar injections of morphine (20 g) decreased cerebellar cGMP content. Naltrexone, an opiate receptor antagonist, but not apomorphine, a dopaminergic receptor agonist, blocked the effect of morphine on cerebellar cGMP. Pretreatment with 3-acetylpyridine (3-AP) which destroys the climbing fibers, failed to antagonize the effect of morphine on cerebellar cGMP. These results suggest that activation of opiate receptors in striatum decreases cerebellar cGMP content presumably by reducing activity in the mossy fiber excitatory input to cerebellum.     

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     

Methyldopa produces central inhibition of parasympathetic activity in the cat

Summary -Methyldopa (10–100 mg/kg i.v.) produced a dose-dependent pupillary dilation in anaesthetized cats which was antagonized by subsequent administration of yohimbine hydrochloride (0.5 mg/kg i.v.). The peak effects were observed approximately 2–3h after injection. This -methyldopa-induced mydriasis was present only when the parasympathetic innervation to the iris was intact. Prior treatment with yohimbine (0.5 mg/kg i.v.) 30 min before -methyldopa also antagonized the mydriatic effect, whereas pretreatment with phenoxybenzamine (2.5 mg/kg i.v.) did not. In contrast, phenoxybenzamine, but not yohimbine, effectively antagonized the pupillary dilation produced by adrenaline (0.3–10.0 g/kg i.v.). These results suggest that -methyldopa produces mydriasis in the cat by means of CNS inhibition of tonic outflow from the oculomotor nucleus and that an -adrenergic inhibitory mechanism may be involved. This conclusion is supported further by experiments in which direct measurements of ciliary nerve activity were made.     

The role of descending inhibition in the antinociceptive effects of the pyrazolone derivatives,metamizol (dipyrone) and aminophenazone (“Pyramidon”)

Summary The study was carried out to provide further evidence that the two pyrazolone derivatives, metamizol and aminophenazone, produce central antinociceptive effects by stimulating inhibition descending from the periaqueductal grey (PAG) to the spinal cord. Experiments were carried out on rats in which the tail-flick response to radiant heat, nociceptive activity in ascending axons of the spinal cord, and activity of neurones in the PAG and the substantia nigra were studied. Microinjection of procaine (10 g) into the PAG reduced the tail-flick latency and abolished the increase in latency caused by i.p. injection of metamizol (40 mg/kg) and aminophenazone (150 mg/kg); it did not significantly reduce the antinociceptive effect of i.p. injection of morphine (2 mg/kg). Threshold doses of morphine (1 and 2 g) administered by intrathecal (i.t.) injection potentiated the effect of threshold doses of metamizol injected i.p. (10 mg/ kg) or into the PAG (10 g) in the tail-flick test. Morphine (2 g) injected i.t. potentiated the effect of i.v. injection of metamizol (80 mg/kg) on nociceptive activity in ascending axons by eliminating the stimulant effect of metamizol on about one third of the axons. Threshold doses of morphine injected i.t. failed to potentiate the antinociceptive effect of aminophenazone (50 mg/kg) injected i.p. in the tail-flick test. The results support the view that metamizol and aminophenazone activate pathways descending from the PAG and exerting an inhibitory effect on nociceptive impulse transmission at the spinal level.Supported by the Schwerpunkt Nociception and Schmerz of the Deutsche Forschungsgemeinschaft Send offprint requests to I. Jurna at the above address