Joanna Siwanowicz

The role of nitric oxide in chemically- and electrically-induced seizures in mice

The effect of the nitric oxide synthase (NOS) inhibitors N-nitro-L-arginine methyl ester (L-NAME) and 7-nitroindazole (7-NI) on seizures induced by N-methyl-D-aspartate (NMDA), pilocarpine (PIL) and pentylenetetrazol (PTZ), as well as on the electroconvulsive threshold was studied in mice. It was found that L-NAME and 7-NI decreased the dose of NMDA necessary to produce clonic convulsions in 50% of animals (CD50). Such a proconvulsant effect was not observed in mice pretreated with N-nitro-D-arginine methyl ester (D-NAME), an inactive isomer of L-NAME. Neither L-NAME nor 7-NI affected the convulsions induced by PIL (clonic seizures) or PTZ (clonic and tonic seizures), having no effect on their CD50 values. Similarly, neither NOS inhibitor affected the electroshock threshold. These results, together with some literature data, indicate that nitric oxide (NO) may be regarded as an anticonvulsant substance in relation to seizures induced by NMDA and other excitatory amino acids, but not by other agents, in mice.

The role of nitric oxide in the kainate-induced seizures in mice

The effect of L-arginine (L-Arg), D-arginine (D-Arg), N-nitro-L-arginine methyl ester (L-NAME) and N-monomethyl-L-arginine (L-NMMA) on the kainate-induced seizures was studied in mice. It was found that the precursor of nitric oxide (NO) L-Arg (150-600 mg/kg i.p.) increased dose-dependently the dose of kinate necessary to produce clonic convulsions in 50% of the animals (CD50). Such an anticonvulsant effect was not observed in mice pretreated with D-Arg (150-600 mg/kg i.p.), the latter drug not being a substrate for NO formation. The inhibitors of NO synthase L-NAME and L-NMMA, both administered in doses of 30-30 mg/kg i.p., reduced the convulsive threshold by decreasing the CD50 of kainate. Moreover, L-NAME (3 mg/kg) antagonized the anticonvulsant effect of L-Arg (300 mg/kg). These results indicate that NO may play a role of an endogenous anticonvulsant substance in mice.

Effect of antidepressant drugs in mice lacking the norepinephrine transporter.

One of the main theories concerning the mechanism of action of antidepressant drugs (ADs) is based on the notion that the neurochemical background of depression involves an impairment of central noradrenergic transmission with a concomitant decrease of the norepinephrine (NE) in the synaptic gap. Many ADs increase synaptic NE availability by inhibition of the reuptake of NE. Using mice lacking NE transporter (NET−/−) we examined their baseline phenotype as well as the response in the forced swim test (FST) and in the tail suspension test (TST) upon treatment with ADs that display different pharmacological profiles. In both tests, the NET−/− mice behaved like wild-type (WT) mice acutely treated with ADs. Autoradiographic studies showed decreased binding of the β-adrenergic ligand [3H]CGP12177 in the cerebral cortex of NET−/− mice, indicating the changes at the level of β-adrenergic receptors similar to those obtained with ADs treatment. The binding of [3H]prazosin to α1-adrenergic receptors in the cerebral cortex of NET−/− mice was also decreased, most probably as an adaptive response to the sustained elevation of extracellular NE levels observed in these mice. A pronounced NET knockout-induced shortening of the immobility time in the TST (by ca 50%) compared to WT mice was not reduced any further by NET-inhibiting ADs such as reboxetine, desipramine, and imipramine. Citalopram, which is devoid of affinity for the NET, exerted a significant reduction of immobility time in the NET−/− mice. In the FST, reboxetine, desipramine, imipramine, and citalopram administered acutely did not reduce any further the immobility time shortened by NET knockout itself (ca 25%); however, antidepressant-like action of repeatedly (7 days) administered desipramine was observed in NET−/− mice, indicating that the chronic presence of this drug may also affect other neurochemical targets involved in the behavioral reactions monitored by this test. From the present study, it may be concluded that mice lacking the NET may represent a good model of some aspects of depression-resistant behavior, paralleled with alterations in the expression of adrenergic receptors, which result as an adaptation to elevated levels of extracellular NE.

The effect of repeated combined treatment with nifedipine and antidepressant drugs or electroconvulsive shock on the hippocampal corticosteroid receptors in rats

The effect of nifedipine, a calcium channel antagonist, on changes in the density of glucocorticoid (GR) and/or mineralocorticoid receptors (MR), induced by long-term treatment with antidepressant drugs (imipramine and amitriptyline) or electroconvulsive shock (ECS) was investigated in the rat hippocampus. Long-term treatment with imipramine or amitriptyline significantly increased the density of GR, while chronic ECS significantly elevated the density of both GR and MR. Nifedipine administered repeatedly had no effect on the basal GR and MR levels, however when the rats were pretreated with nifedipine, the antidepressant drugs and ECS were unable to increase the density of GR, or GR and MR, respectively. These results indicate that, unlike in other models, nifedipine blocks the effects of antidepressant drugs and ECS on GR and/or MR. They also show, that antidepressant drugs and ECS differ in their effect on the density of GR and MR.

Activation of serotonin (5-HT)1A receptors inhibits amphetamine sensitization in mice

The effects of serotonin (5-HT)1A drugs on the development and expression of sensitization to the locomotor effect of amphetamine (AMPH) were studied in mice. 8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT1A agonist, dose-dependently reduced the expression of AMPH (2.5 mg/kg)-induced sensitization. The latter inhibitory effect of 8-OH-DPAT was reversed by (S)-N-tert-butyl-3-(4-(2-methoxyphenyl)piperazin-1-yl)-2-phenyl propamine (WAY 100135), a 5-HT1A antagonist. WAY 100135 given alone did not affect expression of AMPH sensitization. Combined injections of 8-OH-DPAT, but not WAY 100135, with AMPH (2.5 mg/kg) during the development of sensitization, protected against the expression of sensitization to a challenge dose of AMPH (2.5 mg/kg) 3 days after withdrawal. The above inhibitory effect of 8-OH-DPAT on the development of AMPH sensitization was blocked by pretreatment with WAY 100135. The AMPH-induced conditioned locomotion was unaffected by pretreatment with 8-OH-DPAT. These results indicate that 5-HT1A receptors are not involved in AMPH-induced sensitization per-se, whereas their pharmacological activation leads to the inhibition of both the development and the expression of AMPH-induced sensitization.

The effect of chronic treatment with antidepressant drugs on salbutamol-induced hypoactivity in rats.

Several lines of evidence (binding studies, reduced responsiveness of brain adenylate cyclase to noradrenergic stimulation, electrophysiological data) indicate that chronic treatment with antidepressant drugs induces subsensitivity of central β-adrenergic receptors. We studied the effect of acute (single dose) and chronic (14 days, twice daily) treatment with imipramine, desmethylimipramine, amitriptyline, fluvoxamine and citalopram (10 mg/kg, orally) on salbutamol-induced suppression of exploratory activity in rats. This effect of salbutamol was antagonized by chronic, but not acute treatment with antidepressants. Chronic treatment with antidepressants as a rule did not significantly affect exploratory activity. Our results may be regarded as functional evidence at the behavioural level for the subsensitivity of β-adrenergic receptors.

Protective effects of neurosteroids against NMDA-induced seizures and lethality in mice.

The effects of some neurosteroids on N-methyl-D-aspartic acid (NMDA)-induced seizures were examined in mice. Intraperitoneal (i.p.) administration of 5 alpha-pregnan-3 alpha-ol-20-one (5, 10 and 20 mg/kg). 5 beta-pregnan-3 alpha-ol-20-one (10 and 20 mg/kg), 5 alpha-pregnan-3 alpha-ol-11,20-dione (15 mg/kg), 5 alpha-androstan-3 alpha-ol-17-one (10 mg/kg) and dehydroepiandrosterone sulfate (25 mg/kg) significantly increased the dose of NMDA necessary to induce clonic convulsions in 50% of the tested animals (CD50). Furthermore, 5 alpha-pregnan-3 alpha-ol-20-one, 5 beta-pregnan-3 alpha-ol-20-one, 5 alpha-pregnan-3 alpha-ol-11,20-dione and 5 alpha-androstan-3 alpha-ol-17-one also protected the mice against NMDA-induced mortality. Importantly, it is only at the highest doses that neurosteroids impair motor performance of the animals, as estimated by a rotorod equilibrium procedure. The other neurosteroids tested, such as 5 alpha-pregnan-3 beta-ol-20-one (5-20 mg/kg), 5 alpha-pregnan-3 alpha,21-diol-20-one (10 and 15 mg/kg), 5 alpha-pregnan-3,20-dione (15 mg/kg) and pregnenolone sulfate (12.5-100 mg/kg) had no significant effects on the measured parameters. In another set of experiments, we evaluated the effects of neurosteroids on D-[3H]-aspartate release from rat hippocampal slices. None of the neurosteroids tested exerted a significant effect on basal D-[3H]-aspartate release. On the other hand, K(+)-stimulated D-[3H]-aspartate release was significantly attenuated by 5 alpha-pregnan-3 alpha-ol-20-one, 5 beta-pregnan-3 alpha-ol-20-one, alphaxalone, pregnenolone sulfate and dehydroepiandrosterone sulfate. The effect of 5 alpha-pregnan-3 alpha-ol-20-one was the most potent and was distinctly concentration-dependent, whereas the other compounds were effective only at the highest concentrations used. The above results indicate that some neurosteroids administered in non-sedative doses can protect mice against NMDA-induced seizures and mortality; furthermore, they inhibit D-[3H]-aspartate release in rat hippocampal slices.

The central action of pizotifen

The central action of the potential antidepressant drug pizotifen (Sandomigran) was studied in mice, rats and rabbits. Pizotifen in doses up to 10 mg/kg i.p. was ineffective in classic tests for antidepressant activity. It neither antagonized the effects of reserpine in rats (hypothermia, ptosis) nor potentiated the effects of amphetamine (in mice and rats), nialamide or L-dopa (in mice) on locomotor activity. However, its antidepressant activity was found in the ‘despair test’ in rats.On the other hand, pizotifen inhibited the head twitch reaction induced by L-5-hydroxytryptophan in mice (ED50=0.009 mg/kg, i.p.) and by 5-methoxytryptamine (+tranylcypromine) in rats (ED50=0.45 mg/kg, i.p.). It also antagonized tryptamine-induced clonic convulsions of fore-paws in rats (ED50=0.35 mg/kg, i.p.), and in doses of 5–10 mg/kg s.c. inhibited hyperthermia produced by LSD in rabbits. Finally, pizotifen (0.1–0.3 mg/kg, i.v.) inhibited or abolished LSD- or quipazine-induced stimulation of the hind limb flexor reflex of spinal rats; the above effect was not due to noradrenolytic action of the drug. These results suggest that pizotifen strongly blocks the central postsynaptic serotonin receptors.

Effects of tramadol on α2-adrenergic receptors in the rat brain

Abstract In recent years, it has been postulated that tramadol, used mainly for the treatment of moderate to severe pain, might display a potential as an antidepressant drug. The present study investigated the effects of acute and repeated tramadol administration on the binding of [3H]RX 821002, a selective α2-adrenergic receptor ligand, in the rat brain. Male Wistar rats were used. Tramadol (20 mg/kg, i.p.) administered acutely (single dose), at 24 h after dosing, induced a significant decrease in the α2-adrenergic receptors in all brain regions studied. The most pronounced effects were observed in all subregions of the olfactory system, nucleus accumbens and septum, thalamus, hypothalamus, amygdala, and cerebral cortex. Repeated treatment with tramadol (20 mg/kg, i.p., once daily for 21 days) also induced statistically significant downregulation of [3H]RX 821002 binding sites in the rat brain. However, the effect—although statistically significant—was less pronounced than in the group treated acutely with the drug. Since drugs such as mianserin and mirtazapine are potent antagonists of central α2-adrenergic receptors and are effective antidepressants, it is tempting to suggest that, in addition to other alterations induced by tramadol, downregulation of these receptors may represent a potential antidepressant efficacy. On the other hand, one should be careful to avoid the treatment of chronic pain with tramadol in patients already receiving antidepressant drugs. Tramadol-induced downregulation of α2-adrenergic receptors—when combined with ongoing antidepressant therapy with drugs, which themselves inhibit serotonin reuptake or are antagonists of α2-adrenergic receptors—might cause threatening complications.

Some behavioral effects of repeated administration of calcium channel antagonists

The effect of the calcium channel antagonists nifedipine, nimodipine, and diltiazem (10 mg/kg PO) was studied after single and repeated administration to rats. All the compounds administered repeatedly reduced significantly the duration of immobility in the forced swimming test. At the same time the locomotor activity of rats was reduced (nifedipine, nimodipine) or unchanged (diltiazem). All the calcium channel antagonists studied did not modify the behavior of normal or phenylephrine-stimulated rats in the open field test. Only nimodipine, given repeatedly, was able to antagonize the clonidine-induced behavioral inhibition in the latter test. The results indicate that, like antidepressants, calcium channel antagonists given repeatedly to rats reduce the immobility time in the forced swimming test, but do not change the responsiveness of alpha 1- and alpha 2-adrenoceptors to their agonists.