Reversal of the reserpine-induced ptosis by l-threo-3,4-dihydroxy-phenylserine (l-threo-DOPS), a (−)-norepinephrine precursor,and its potentiation by imipramine or nialamide

Summary The effect of l-threo-DOPS on the reserpine-induced ptosis in mice and its modification by imipramine, a norepinephrine (NE) uptake inhibitor, or nialamide, a monoamineoxidase inhibitor, were studied.Intraperitoneal (i.p.) injection of l-threo-DOPS (800 mg/kg) significantly reduced the severity of the ptosis. This reversal of the ptosis by l-threo-DOPS was markedly potentiated by i.p. injection of either imipramine (2.5 mg/kg) or nialamide (30 mg/kg). Response to l-threo-DOPS was also significantly potentiated by intracerebroventricular (i.c.v.) injection of imipramine (10 g). On the other hand, this treatment with imipramine (10 g, i.c.v.) also significantly potentiated the reversal of the ptosis by NE (20 g, i.c.v.), but the reversal by the subcutaneous (s.c.) injection of NE (1 and 3 mg/kg) was not affected.Reserpine (5 mg/kg, i.p.) markedly decreased the brain content of NE in mice, whereas l-threo-DOPS (400 mg/kg, i.p.) slightly restored it. Moreover, by the pretreatment with nialamide (30 mg/kg, i.p.), l-threo-DOPS produced a significant increase in the brain content of NE in reserpinetreated mice.These results suggested that l-threo-DOPS was capable of reversing the reserpine-induced ptosis due to the formation, at least in part of (–)-NE at the synaptic sites of central noradrenergic neurons.     

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     

Cardiovascular effects of intracerebroventricular GABA,glycine and muscimol in the rat

Summary Intracerebroventricular (i.c.v.) injections of gammaaminobutyric acid (GABA, 125–2,000 g), glycine (250–2,000 g) and muscimol (0.5–4 g) caused dose-dependent reductions of blood pressure and heart rate in conscious rats. Low doses of muscimol (<0.12 g) were hypertensive. The cardiovascular depression induced by GABA was antagonized by bicuculline (3.5 mg/kg i.p.), while that induced by glycine was counteracted more effectively by strychnine (0.7 mg/kg i.v.) than by bicuculline. The cardiovascular response to GABA, but not to glycine, was potentiated by pretreatment with pentobarbitone (50 mg/kg i.p.). It was prolonged by diazepam (5 mg/kg i.p.) and aminooxyacetic acid (AOAA, 25 mg/kg i.p., 4h) but not by AOAA (1 h) or nipecotic acid (1–100 g i.c.v.). Following treatment with d-amphetamine (5 mg/kg i.p.) or reserpine (10 mg/kg i.p., 6h) the cardiovascular depression induced by GABA was attenuated. Propranolol (2.5+2.5 mg/kg i.v. + s.c.) or atropine (2 mg/kg i.v.) alone partially counteracted and in combination completely prevented the negative chronotropic response to GABA. Neither central noradrenaline depletion by means of -MMT (after carbidopa) and -MT nor pretreatment with phenoxybenzamine (20 g i.c.v.) influenced the response to GABA. The arterial hypotension induced by GABA was prolonged after a high dose of atropine (10 mg/kg i.v.) but appeared attenuated by a low dose (2 mg/kg i.v.) or by physostigmine (0.1 mg/kg i.v.). Central serotonergic activation by means of 5-hydroxytryptophan after benzerazid antagonized the cardiovascular actions of GABA. Spiroperidol (0.05 mg/kg i.v.) and apomorphine (1 mg/kg i.v.) did not affect the blood pressure reduction in response to GABA.It is concluded that i.c.v. GABA causes cardiovascular depression by activation of GABA receptors and that the response is mediated by the sympathetic and vagal systems. There were no indications for an involvement of central noradrenergic of dopaminergic mechanisms while a decreased central cholinergic and an increased serotonergic activity possibly contribute to the cardiovascular effects of GABA.     

Effects of ceruletide and cholecystokinin octapeptide on eating in mice

Summary Subcutaneous (SC) injections of ceruletide (caerulein diethylammonium hydrate, CER) and the octapeptide of cholecystokinin (CCK-8) reduced the intake of liquid food in male NMRI mice starved for 18 h. The corresponding ED₅₀ values were 2 g/kg for CER and 24 g/kg for CCK-8; hence, on a molar basis, CER was 14 times more potent than CCK-8. Naloxone (0.2 and 1 mg/kg, SC) inhibited eating. (d-Ala)² (MePhe)⁴-(Met(O)-ol)⁵-enkephalin (FK 33-824; 0.3 and 1 mg/kg) was only stimulatory. Naloxone enhanced the effect of CER, whereas FK 33-824 antagonized it. It is concluded that concerning the inhibition of food intake, opioid peptides can be antagonists of CCK-like peptides. This is consistent with the current view of the regulation of appetitive behaviour.     

The competitive NMDA receptor antagonists CGP 37849 and CGP 39551 are potent,orally-active anticonvulsants in rodents

Summary Anticonvulsant properties of CGP 37849 and CGP 39551, two novel phosphono-amino acids which are competitive NMDA receptor antagonists, were examined in rodents. At optimal pretreatment times CGP 37849 suppressed electroshock-induced seizures in mice and rats with ED₅₀ ^s ranging from 8 to 22 mg/kg after oral administration, and 0.4 to 2.4 mg/kg after i. v. and i. p. injection. Relative to CGP 37849, CGP 39551 was more potent after p. o. (ED₅₀ 3.7–8.1 mg/kg), and less potent after i.v. or i.p. treatment (ED₅₀ 2.7–8.7 mg/kg). Following oral treatment, the duration of action of CGP 37849 was about 8 h, while CGP 39551 still showed good activity after 24 h (ED₅₀ 8.7 mg/kg, mouse; 21 mg/kg, rat). Both compounds were anticonvulsant at doses below those at which overt behavioural side effects were apparent. CGP 39551 delayed the development of kindling in rats at doses of 10 mg/kg p. o. and above, and showed weak anticonvulsant activity against pentylenetetrazolevoked seizures. CGP 37849 and CGP 39551 are the first competitive NMDA antagonists to show oral anticonvulsant properties in a therapeutically-useful dose-range, and hence are interesting candidates for novel antiepileptic therapy in man. Send offprint requests to M. Schmutz at the above address     

Antidepressant-like effect of novel 5-HT3 receptor antagonist N-n-butyl-3-ethoxyquinoxalin-2-carboxamide (6p): An approach using rodent behavioral antidepressant tests

Objective:

The present study was designed to investigate the antidepressant potential of N-n-butyl-3-ethoxyquinoxalin-2-carboxamide (6p), a novel 5-HT₃ receptor antagonist in rodent behavioral models of depression.

Materials and Methods:

The compound 6p was examined in various behavioral models like forced swim test (FST), tail suspension test (TST), mechanistic models [5-hydroxytryptophan (5-HTP)-induced head twitch and reserpine-induced hypothermia (RIH)], and in chronic surgery model-olfactory bulbectomy in rats.

Results:

Compound 6p (1, 2, and 4 mg/kg, i.p.) exhibited antidepressant-like effect in FST and TST after acute treatment without having an effect on baseline locomotor activity. Moreover, 6p (2 mg/kg, i.p.), potentiated the 5-HTP–induced head twitch responses in mice and inhibited the RIH in rats. Chronic treatment (14 days) with 6p (1 and 2 mg/kg, p.o.) and paroxetine (10 mg/kg, p.o.) in rats significantly reversed the behavioral anomalies induced by bilateral olfactory bulbectomy using open field exploration.

Conclusion:

The preliminary studies reveal that compound 6p exhibits antidepressant-like effect in behavioral rodent models of depression.KEY WORDS: 5-HT₃ receptor antagonists, antidepressant, forced swim test, quinoxaline, serotonin     

Modulation by antidepressant drugs of CNS postsynaptic α2-adrenoceptors mediating mydriasis in the rat

Summary The modulation of central postsynaptic ₂-adrenoceptors mediating mydriasis in the pentobarbitoneanaesthetized rat was studied after the acute and short/longterm administration of antidepressant treatments (drugs, electroshock). The acute administration of cyclic antidepressant drugs (2.5 mg/kg, i.v.) resulted in different mydriatic effects (amitriptyline > protriptyline imipramine > clomipramine > nortriptyline > desipramine maprotiline) which were attenuated (17–55%) by idazoxan (1 mg/kg, i.v., 5 min) and reserpine (5 mg/kg, s.c., 18 h) indicating that, besides the well-known anticholinergic properties of some of these drugs, their mydriatic effects are due in part to activation of ₂-adrenoceptors (through endogenous noradrenaline). In contrast, the long-term (7–21 days) but not the short-term (1–4 days) administration of cyclic antidepressant drugs (2.5–10 mg/kg, i. p.), MAO inhibitors (1 mg/kg, i.p.), lithium (20 mg/kg, i.p.) and electroshock (150 mA, 63 Hz, 8 ms during 300 ms) resulted in dose- and time-dependent reductions of the dose-pupillary response curve for clonidine (ED₅₀ increased 1.2–2.0-fold; E _max decreased by 9–29%) which indicated desensitization of postsynaptic ₂-adrenoceptors. In line with these findings, treatment for 7 days with clonidine (0.1–1 mg/kg, i.p.) or idazoxan (3–10 mg/kg, i.p.) led to an opposite modulation (down- and up-regulation) of the dose-pupillary response curve for clonidine. The main results demonstrate that cyclic antidepressant drugs, through indirect mechanisms which involve endogenous noradrenaline, can modulate the sensitivity of brain postsynaptic ₂-adrenoceptors mediating mydriasis in the rat. Send offprint requests to A. Menargues at the above address     

Effects of the antidepressant compound nefazodone on central monoaminergic neuronal discharge in rats

The effects of i.v. administration of the antidepressant compound nefazodone were assessed on the firing rates of spontaneously active noradrenergic neurons in the locus coeruleus (LC), serotonergic neurons in the dorsal raphe (DR), and dopaminergic neurons in the substantia nigra (SN) of chloral hydrate anesthetized male albino adult rats, utilizing extracellular single-unit recording methods. Nefazodone, tested in doses of 0.1–10.0 mg/kg, had variable effects on LC neurons, but the predominant effect was a mild excitation of firing (ED₂₅ = 1.953 mg/kg). This may in part be caused by the 5-HT₂ antagonist properties of the compound, since ritanserin also produced a mild excitation of LC neurons. By comparison, desipramine very reliably inhibited LC neurons (ED₅₀ = 0.333 mg/kg, i.v.). For DR neurons, i.v. nefazodone (0.1–3.2 mg/kg) produced variable effects, with inhibition being the most common (63% of cases tested). The ED₂₅ for inhibition was 1.230 mg/kg, and in no case was inhibition of 50% or greater observed. By comparison, clomipramine very reliably inhibited DR neurons (ED₅₀ = 0.501 mg/kg, i.v.). For SN neurons, the effects of i.v. nefazodone were also variable, with no consistent effects observed. These results indicate that acute nefazodone produces relatively weak effects on monoaminergic neuronal impulse flow in anesthetized rats, and suggest that major modifications of monoaminergic neuronal impulse flow probably do not play an important role in any neuroadaptive changes that may contribute to the clinical antidepressant actions of nefazodone.     

Effect of LP-805, a releaser of endothelium-derived nitric oxide,on systemic vasodilatation in vivo

Summary We have investigated relations between hypotensive responses to LP-805, a newly synthesized vasodilator, and the production of nitric oxide (NO), in anesthetized rats. LP-805 (0.1–0.5 mg/kg, i.v.) or acetylcholine (ACh) (0.3 – 3.0 g/kg, i.v.) caused a dose-dependent transient decrease in diastolic blood pressure. The decrease induced by 0.3 mg/kg LP-805 (i.v.) was partially inhibited by pretreatment with N^G-nitro-l-arginine (LNNA), a specific inhibitor of endothelial NO synthase, but the responses to lower or higher doses of LP-805 (0.1 or 0.5 mg/kg, i.v.) were not affected. The dose-dependent decrease in diastolic blood pressure, caused by LP-805, was not affected by pretreatment with l- or d-arginine. The dose-dependent decrease in diastolic blood pressure caused by ACh was not affected by pretreatment with L-NNA or with l- or d-arginine. The hypotensive response to 20-min infusions of LP-805 (100 g/kg per min) wassignificantly inhibited by pretreatment with L-NNA (10 mg/kg, i.v.). The half-recovery times (T_1/2) of LP-805 or ACh-induced depressor responses were shortened by pretreatment with L-NNA. They were prolonged by l-arginine, but not by d-arginine. This shortening, by L-NNA, of the half-recovery time after LP-805 or ACh was reversed by l-arginine, but not by d-arginine. The T_/2 of the LP-805-induced hypotensive response was not affected by pretreatment with indomethacin (1 mg/kg, i.v.). In the presence of L-NNA (10 mg/kg, i.v.), the T_/2 of the LP-805-induced hypotensive response was not affected by pretreatment with indomethacin. The results suggest that the LP-805-induced hypotensive response may be related to direct or indirect activation of NO synthase in vascular endothelial cells, and to release of endothelium-derived NO. Correspondence to M. Inazu at the above address     

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.     

The stimulatory effect of clonidine through imidazoline receptors on locus coeruleus noradrenergic neurones is mediated by excitatory amino acids and modulated by serotonin

Clonidine and other imidazoline/oxazoline drugs, such as cirazoline and rilmenidine, have been shown to stimulate the activity of noradrenergic neurones in the locus coeruleus (NA-LC) by an ₂-adrenoceptor-independent mechanism through the activation of I-imidazoline receptors. The endogenous modulation of the stimulatory effect of clonidine on NA-LC neurones was further investigated after inactivation of ₂-adrenoceptors with N-ethoxycarbonyl-2-ethoxy-1, 2-dihydroquinoline (EEDQ). In EEDQ-pretreated rats (6 mg/kg, i.p., 6h), clonidine caused a rapid and dose-dependent (320–5120 g/kg, i.v.) increase in the firing rate of NA-LC neurones (ED₅₀ = 809 g/kg, E_max = 90%). The stimulatory effect of clonidine on NA-LC neurones was completely blocked by pretreatment of rats with the excitatory amino acid receptor antagonist kynurenic acid (1–3 mol in 10–30 mol i.c.v., 2–5 min before clonidine). In contrast, the stimulatory effect of clonidine on NA-LC neurones was potentiated by pretreatment with reserpine (5 mg/kg, s.c., 18 h) (E_max increased by 63%). Pretreatment with -methyl-p-tyrosine (250 mg/kg, i.p., 24 h) did not alter the stimulatory effect of clonidine, but pretreatment with p-chloro-phenylalanine (400 mg/kg, i.p., 24 h) markedly enhanced the stimulatory effect of clonidine on NA-LC neurones (E_max increased by 139%). The present results indicate that the imidazoline receptor-mediated stimulatory effect of clonidine on NA-LC neurones is an indirect effect dependent on an excitatory amino acid pathway and modulated by an inhibitory serotonin mechanism.     

Increased dopamine receptor sensitivity in the rat following acute administration of sufentanil,U50,488H and d-Ala2-d-Leu5-enkephalin

Summary The effects of acutely administered opioid receptor agonists sufentanil, U50,488H and [d-Ala², d-Leu⁵]-enkephalin (DADL) were observed upon dopamine D₁ and D₂ binding site density in the striatum of the rat. In addition, the functional implications of opioid-induced changes in dopamine receptor sensitivity were studied using the behavioural profile elicited by apomorphine in the rat. The -agonist sufentanil (1 or 20 Erg/kg, i. p.), the -agonist U50,488H (10 mg/kg, i. p.) and DADL (1 g/animal, i. c. v.) all significantly elevated D₂ but not D₁ binding site density in rat striatum. Pretreatment with sufentanil (1 g/kg, i. p.) induced an elevation in apomorphine-induced sterotyped behaviour, but attenuated locomotor activity. Following administration of U50,488H (10 mg/kg, i. p.), both the degree of stereotypy and the intensity of the locomotor activity were enhanced. Contralateral rotation was observed in animals pretreated with DADL (1 g/animal, i. c. v.) following challenge with apomorphine. It is concluded that the opioid agonists studied induce a significant elevation in functional D₂ sites to the exclusion of D₁ sites. However, the precise mechanism by which this effect is elicited remains to be established. Send offprint requests to R. D. E. Sewell at the above address     

Preclinical pharmacology of F-98214-TA, a novel potent serotonin and norepinephrine uptake inhibitor with antidepressant and anxiolytic properties

Rationale Serotonin (5-HT) and norepinephrine (NE) re-uptake inhibitors (SNRIs) have been proposed to have a higher efficacy and/or faster onset of action than previously available antidepressants. Objectives We examined in biochemical, electrophysiological and behavioural assays the antidepressant properties of (S)-(−)-4-[(3-fluorophenoxy)-phenyl]methyl-piperidine (F-98214-TA), a compound that displays very high affinity for 5-HT and NE transporters. Results F-98214-TA potently inhibited the uptake of both 5-HT and NE into rat brain synaptosomes (IC₅₀=1.9 and 11.2 nM, respectively) and decreased the electrical activity of dorsal raphe serotonergic neurones (ED₅₀=530.3 μg/kg i.v.), an effect completely abolished by the 5-HT_1A antagonist WAY100,635. In acute behavioural assays in mice, the orally administered compound potentiated the 5-hydroxy-tryptophan (5-HTP)-induced syndrome [minimal effective dose (MED)=10 mg/kg], antagonized the hypothermia induced by a high dose of apomorphine (ED₅₀=2 mg/kg) and reduced the immobility in the tail suspension test (MED=10 mg/kg). Moreover, it also decreased the immobility in the forced swimming test in mice and rats (30 mg/kg, p.o.). Chronic administration of F-98214-TA (14 days, 30 mg kg⁻¹ day⁻¹, p.o.) attenuated the hyperactivity induced by olfactory bulbectomy in rats, confirming its antidepressant-like properties. Interestingly, the same dosage regimen significantly increased the social interaction time in rats, suggesting an additional potential anxiolytic activity. In most assays the compound was more potent than fluoxetine, venlafaxine and desipramine. Conclusions F-98214-TA is a novel SNRI that displays greater potency than other reference antidepressants in animal models predictive of antidepressant and anxiolytic activities. A preliminary report of this work was presented at the 30th Annual Meeting of the Society for Neuroscience, New Orleans, LA, 2000.     

In vivo pharmacological evaluation of newly synthesized nonpeptidic AT1 receptor antagonists in rats

This study was conducted to characterize thein vivo pharmacology of KR-30988, KR-30992 and losartan, new AT₁ antagonists, given as i.v. cumulative doses, in two animal models of high renin, conscious renal artery-ligated hypertensinve rats (RHRs) and normotensive rats anesthetized with urethane (900 mg/kg, i.p.) and α-chloralose (90 mg/kg, i.p.), with a special emphasis on the pharmacological characterization of the latter model. In conscious RHRs, KR-30988, KR-30992, losartan and captopril caused a dose-dependent decrease in blood pressure, their relative potencies (ED₂₀) being 0.057, 0.208, 0.164 and 0.018 mg/kg i.v., repectively. In anesthetized rats, 2 hours after anesthesia, plasma renin activity was increased from 7.31 to 34.07 ng/ml/h, the level approximately 1.5 times greater than the highest level in RHRs. In anesthtized rats, the ED₂₀S for all four compounds were 0.011, 0.046, 0.035 and 0.004 mg/kg i.v., repectively. By comparison, ED₂₀s from anesthetized rats were 4 to 5 times smaller than those from conscious RHRs, with a good correlation (r=0.999) noted between ED₂₀s from two groups of rats. The data on ED₂₀ may indicate the higher sensitivity of anesthetized rats to the hypotensive activity of the compounds and the same order of potencies in two models. These results suggest that, in addition to RHRs, the nomotensive rats anesthetized as above can serve as a suitable model for the rapid pharmacological evaluation of AT₁ receptor antagonists.     

Role of nitric oxide in penile erection and yawning induced by 5-HT1c receptor agonists in male rats

The effect of the intracerebroventricular (i.c.v.) administration of N^G-nitro-l,-arginine methyl ester and N^G-monomethyl-l.-arginine, two inhibitors of nitric oxide (NO) synthase, on penile erection and yawning induced by 1-(3-chlorophenyl)-piperazine (m-CPP)- and N-(3-trifluoromethylphenyl)-piperazine (TFMPP), two selective 5HT_1c receptor agonists, was studied in male rats. Both NO synthase inhibitors (50–500 g i.c.v.) prevented dose-dependently the behavioural responses induced by m-CPP (0.5 mg/kg s.c.) or by TFMPP (I mg/kg s.c.), but N^G-nitro-l-arginine methyl ester was about 4–5 times more potent than N^G-monomethyl-l,-arginine. The D-isomer of N_G-monomethyl-l-arginine, which does not inhibit nitric oxide synthase, was ineffective. The inhibitory effect of N^G-nitro-l-arginine methyl ester on m-CPP- and TFMPP-induced responses was prevented by the administration of l-arginine (1 mg i.c.v.). In contrast, N^G-nitro-l-arginine methyl ester (20 g) was ineffective when injected in the paraventricular nucleus of the hypothalamus, a brain area that plays a key role in the expression of these behavioural responses. m-CPP- and TFMPP-induced penile erection and yawning was prevented also by the i.c.v. administration of LY 83583 (50–200 g) or methylene blue (50–400 g), two inhibitors of guanylate cyclase but not by reduced hemoglobin (50–400 g), a NO scavenger. The results suggest that central nitric oxide is involved in the expression of penile erection and yawning induced by 5-HT_1c receptor agonists.     

Estradiol and testosterone modulate the anesthetic action of the GABA-A agonist THIP,but not of the neurosteroid 3α,5β-pregnanolone in the rat

Rationale As sex steroids modify the number and distribution of brain -aminobutyric acid (GABA)_A receptor subunits, we investigated the potential modulation of anesthesia, induced by agents acting on the GABA_A receptor, by estrogen and androgen.Objectives To assess possible effects of sex and hormonal condition (i.e., intact vs castrate; estradiol vs testosterone treatment) on the anesthetic effect of a GABA_A agonist, THIP (4,5,6,7-tetrahydroisoxazolo[5,4,-c]pyridin-3-ol hydrochloride), and an allosteric modulator of the GABA_A receptor: 3-hydroxy-5-pregnan-20-one (epipregnanolone).Methods The potencies of THIP and epipregnanolone for inducing loss of the righting response were compared between: (a) female and male rats; (b) intact and castrated animals of each sex; (c) untreated castrates and castrates given estradiol or testosterone.Results Sex and endocrine condition influenced sensitivity to i.v. THIP for the induction of anesthesia. ED₅₀ values were: gonadectomized females, 80 mg/kg > intact males, 50 mg/kg > proestrous females, 35 mg/kg > gonadectomized males, 28 mg/kg. Estradiol benzoate (EB; 3 g/day for 5 days) significantly increased THIP sensitivity in gonadectomized females: THIP + EB: ED₅₀=26 mg/kg vs THIP + sesame oil: ED₅₀=94 mg/kg, while testosterone propionate (TP; 10 mg injected 24 h before THIP) decreased THIP sensitivity in orchidectomized males when compared with vehicle-injected animals (ED₅₀=72 mg/kg vs 22 mg/kg, respectively).Conclusions Results suggest that estrogen increases the density or availability of GABA_A receptor subtypes on which THIP acts, while testosterone exerts the opposite effect. Neither sex nor gonadal condition influenced the anesthetic action of epipregnanolone as a similar potency was found in intact and in gonadectomized males and females.     

Difenoxin and loperamide: studies on possible mechanisms of intestinal antisecretory action

Summary Experiments have been performed to determine whether the antisecretory (antidiarrhoeal) actions of difenoxin and loperamide are mediated by enteric neurones. An iso-osmotic perfusion solution was circulated around the lumen of the jejunum of anaesthetised rats. Vasoactive intestinal peptide was infused intra-arterially to induce net fluid secretion which was inhibited by difenoxin (ED₅₀, 0.23 mg/kg) and loperamide (ED₅₀, 0.5 mg/kg). However, neither were able to restore the fluid transport rate to the control level of absorption.The antisecretory effects of difenoxin (0.77 mg/kg) and loperamide (0.6 mg/kg) were blocked by the opiate receptor antagonist naloxone (2 mg/kg). Their effects were also abolished by pretreatment with the 5-HT synthesis inhibitor p-chlorophenylalanine (PCPA; 200 mg/kg; with desmethylimipramine given beforehand to protect noradrenergic nerves and enhance 5-HT depletion). The effect of difenoxin was blocked with methiothepin (1 mg/kg) and methysergide (30 g/kg) but not ketanserin (30 g/kg), ritanserin (30 mg/kg), ondansetron (10 g/kg) or ICS 205-930 (3 mg/kg). None of the above 5-HT receptor antagonists modified the antisecretory effect of loperamide. The antisecretory effect of difenoxin but not loperamide was prevented by phentolamine (2 mg/kg) and by pretreatment with 6-hydroxydopamine (150 mg/kg) total.It is concluded that both difenoxin and loperamide inhibit net fluid secretion by indirect mechanisms. It is proposed that the initial action is on enteric -opiate receptors and that this results in the release of 5-HT. In the case of difenoxin, the 5-HT may act on 5-HT₁-like receptors to release noradrenaline. However, the major difference in the mechanism of action of loperamide compared to difenoxin is that it does not utilize noradrenaline as the final mediator of its antisecretory action.Correspondence to A. De Luca at the above address     

Antidepressant-like activity of (4-phenylpiperazin-1-yl) (quinoxalin-2-yl) methanone (4a), a novel 5-HT3 receptor antagonist: An investigation in behaviour-based rodent models of depression

Aim:

The present study was designed to investigate the antidepressant potential of (4-phenylpiperazin-1-yl) (quinoxalin-3-yl) methanone (4a), a novel 5-HT₃ receptor antagonist, with an optimal log P (2.84) and pA₂ value (7.3) greater than ondansetron (6.9) using rodent behavioural models of depression.

Materials and Methods:

Swiss albino mice were used in actophotometer test, forced swim test (FST) and 5-hydroxytryptophan (5-HTP) induced head twitch response. Reserpine induced hypothermia (RIH) and olfactory bulbectomy were performed in male Wistar rats. Statistical analysis was carried out by using one-way analysis of variance followed by Tukey''s test.

Results:

Acute treatment of 4a (1-4 mg/kg, i.p.) in mice produced antidepressant-like effects in FST without affecting the baseline locomotion in actophotometer test. Further, 4a (2-4 mg/kg, i.p.) potentiated the 5-HTP induced head twitches response in mice and also antagonized RIH in rats. Furthermore, sub-chronic (14 days) treatment with 4a (2-4 mg/ kg, p.o.) significantly attenuated the behavioural anomalies induced by bilateral olfactory bulbectomy in rats in modified open field exploration.

Conclusions:

These preliminary investigations confirm that 4a exhibits antidepressant-like activity in behaviour based rodent models of depression.KEY WORDS: (4-phenylpiperazin-1-yl) (quinoxalin-2-yl) methanone, 4a; 5-HT₃ receptor antagonist, depression, forced swim test, olfactory bulbectomy     

Metabolism and distribution in the rat of peak E substance,a constituent inl-tryptophan product implicated in eosinophilia-myalgia syndrome

Peak E substance, 1,1-ethylidenebis[tryptophan], a contaminant found inl-tryptophan tablets, has been suggested as a causative agent for eosinophilia-myalgia syndrome (EMS). Peak E substance (50 mg/kg) was administered perorally to Wistar rats to determine its metabolism and distribution. A purification procedure using Bond Elut C₈ cartridges followed by HPLC was developed for the determination of peak E substance. The plasma concentration of peak E substance was 136 ng/ml at 1 h, and urinary excretion was 717 ng at 5 h and 10342 ng for 5–24 h, showing slow excretion of peak E substance into urine. The amount of peak E substance in the contents of the large intestine at 5 h, however, was 3136 g, much greater than urinary excretion for 24 h, indicating considerable transfer of peak E substance to large intestine without decomposition by gastric fluid in the stomach. We have detected for the first time not only the occurrence of peak E substance in plasma and urine, but also 1-methyl-tetrahydro--carboline-3-carboxylic acid (MTCA) in blood and organs of rats treated with peak E substance, thereby suggesting MTCA as one of the the metabolites of peak E substance. The amount of MTCA in the contents of the large intestine as well as in urine of rats treated with peak E substance was significantly greater than inl-tryptophantreated rats (50 mg/kg p.o.), demonstrating that MTCA was more readily produced from peak E substance than froml-tryptophan. Finally, we propose acetaldehydeinduced production of MTCA from peak E substance.     

Stimulatory effects of clonidine,cirazoline and rilmenidine on locus coeruleus noradrenergic neurones: possible involvement of imidazoline-preferring receptors

Summary Clonidine and related drugs not only interact with ₂-adrenoceptors but also recognise non-adrenoceptor sites in the brain. The involvement of these imidazoline-preferring receptors in the regulation of the activity of locus coeruleus noradrenergic neurones (NA-LC) was investigated after inactivation of ₂-adrenoceptors with N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). In EEDQ-pretreated rats (6 mg/kg, i.p., 6 h), the characteristic inhibitory effect of low doses of clonidine on these neurones was abolished and a paradoxical, dose-dependent increase in firing rate was observed at higher doses (640–5120 g/kg, i.v.) (ED₅₀ = 702 g/kg, E_max = 83 %, n = 14). Guanfacine (0.3–20 mg/kg) did not modify neuronal activity but antagonised the stimulatory effect of clonidine. Cirazoline (80–640 g/kg) and rilmenidine (0.3–10 mg/kg) also stimulatedneuronal activity(ED₅₀ = 192 g/kg, E_max = 102%, n = 5; ED₅₀ = 1563 g/kg, E_max = 70%, n = 1–5, respectively) by an ₂-adrenoceptor-independent mechanism. The results suggest that these drugs can modulate the activity of locus coeruleus noradrenergic neurones through the activation of I₁-imidazoline-preferring receptors.