Oxotremorine-induced Hypothermia Research Articles

Effects of calcium channel inhibitors on the hypothermic response to oxotremorine in normo and hypercholinergic rats

The Flinders Sensitive Line of rats (FSL) has been selectively bred to have increased sensitivity to cholinergic drugs. Typically, these rats react with twice as great a hypothermic effect to muscarinic agonists such as oxotremorine, as do similarly bred Flinders Resistant Line rats (FRL). We compared the effects of three chemically different calcium channel inhibitors (diltiazem, nicardipine and verapamil) on the hypothermia induced in FRL and FSL rats by oxotremorine (0.2 mg kg-1 s.c.). Each drug was injected i.p. in a dose of 20 mumol kg-1 30 min before oxotremorine. Methylatropine (2 mg kg-1 s.c.) was administered 15 min before oxotremorine to block the peripheral effects of the agonist. The hypothermic effect of oxotremorine in FSL rats was antagonized by nicardipine and diltiazem. In contrast, verapamil failed to influence the hypothermic response in FSL rats. Verapamil significantly (P less than 0.05) augmented oxotremorine hypothermia in FRL rats. Diltiazem and nicardipine were without effect on oxotremorine-induced hypothermia in FRL rats. There were no significant changes in temperature in separate groups of FRL and FSL rats treated with calcium channel inhibitors alone.

Oxotremorine-induced hypothermia as a method for evaluating long-term neuronal changes following poisoning by cholinesterase inhibitors in rats

Severe poisoning by inhibitors of cholinesterase (ChE) enzymes is often associated with prolonged central or peripheral neuronal damage. Oxotremorine is a cholinergic agonist known to induce acute hypothermia. Central and peripheral cholinergic signaling is involved in the induction of hypothermia as well as in its recovery. These processes were used in the present study to reveal prolonged neuronal abnormalities in poisoned rats, using oxotremorine with and without concomitant administration of the peripheral muscarinic antagonist methyl scopolamine. In non-poisoned naïve rats, the hypothermic effect of oxotremorine appeared faster while its recovery was delayed following co-administration of methyl scopolamine, suggesting predominantly peripheral processes in counteracting the hypothermia. One month after exposure to approximately 1LD 50 of the carbamates aldicarb and oxamyl, the hypothermic effect of oxotremorine was similar to that found in saline-treated control group. However, the effect of methyl scopolamine on the recovery process was significantly diminished, indicating that the impaired cholinergic mechanisms were predominantly peripheral. In contrast, 1 month following organophosphate (OP) poisoning by the nerve agents sarin and VX, oxotremorine-induced hypothermia was reduced, indicating mainly impaired central cholinergic mechanisms. The development of severe convulsions during nerve agent poisoning may explain the central neuronal damage in OP-poisoned rats, displayed as reduced hypothermia. As convulsions were not part of the poisoning symptoms with the carbamates tested, their long-term damage was displayed at the recovery stage. This method might be used as a relatively simple means for detecting differential long-term central and peripheral cholinergic injuries, long after toxicity signs have receded.

Contribution of the cholinergic basal forebrain to proactive interference from stored odor memories during associative learning in rats.

E. De Rosa and M. E. Hasselmo (2000) demonstrated that 0.25 mg/kg scopolamine (SCOP) selectively increased proactive interference (PI) from stored odor memories during learning. In the present study, rats with bilateral cholinergic lesions limited to the horizontal limb of the diagonal band of Broca, made with 192 IgG-saporin, were not impaired in acquiring the same olfactory discrimination task relative to control rats. Rats with bilateral 192 IgG-saporin lesions to all basal forebrain cholinergic nuclei (BF) also showed no impairment in acquisition of this task. However, the BF-saporin rats were hypersensitive to oxotremorine-induced hypothermia and demonstrated an increased sensitivity to PI following a low dose of SCOP (0.125 mg/kg) relative to control rats. The results suggest that weaker cholinergic modulation after cholinergic BF lesions makes the system more sensitive to PI during blockade of the remaining cholinergic elements.

In vivo muscarinic cholinergic mediated effects of Lu 25-109, a M1 agonist and M2/M3 antagonist in vitro.

Lu 25-109 [5-(2-ethyl-2H-tetrazol-5-yl)-1,2,3,6-tetrahydro-1-methylpyridine] , has M agonistic and M2/M3 antagonistic effects at muscarinic receptors in vitro; a pharmacological profile that may be beneficial in treatment of Alzheimer's disease. In the present study, we compare functional in vivo effects of Lu 25-109 and reference compounds in animal models of muscarinic cholinergic function. Lu 25-109 substituted completely for the discriminative stimulus effects of (-)-7-methyl-3-(2-propynyloxy)-4,5,6,7-tetrahydroisothiazolo -[4, 5-c]pyridine (Lu 26-046), a partial M1/M2 agonist, but only weakly for the effects of the non-selective M1/M2/M3 agonist 3-methoxy-4,5,6,7-tetrahydro-isoxazolo[4, 5-c] pyridine (O-Me-THPO). Lu 25-109 did not reverse O-Me-THPO-induced discriminative stimulus. Tacrine did not substitute for any of the training drugs. Lu 25-109 did not substitute in (-)-nicotine trained rats. Lu 25-109 did not antagonize oxotremorine-induced hypothermia, tremor and salivation in mice and antagonized physostigmine-induced lethality with low potency. Unlike non-selective muscarinic agonists and acetylcholinesterase inhibitors, Lu 25-109 did not induce hypothermia, tremor or salivation in mice. Spontaneous locomotor activity and motor co-ordination were inhibited only at high doses. Lu 25-109 had no effect on mean blood pressure in anaesthetized rats. Lu 25-109 and O-Me-THPO produced a significant increase in heart rate. The maximum increase was 37%. In anaesthetized cats, increasing i.v. doses of Lu 25-109 were without effect on the mean blood pressure, except for a short lasting (<2 min) depressor effect following the IV injection. Furthermore, Lu 25-109 did not attenuate the reflex mechanisms restoring blood pressure following orthostasis in cats. In conclusion, the drug discrimination studies suggest a unique activity profile of Lu 25-109, and the in vivo profile suggests none or a very low frequency of unwanted cholinergic mediated effects.

Neurophysiological Effects ofEuphorbia hirta L. (Euphorbiaceae)

A lyophilized aqueous extract of Euphorbia hirta L. (Eh) has been evaluated for benzodiazepine-like properties and for hypnotic, neuroleptic and antidepressant properties, in order to complete the study of dose-dependent sedative and analgesic effects previously demonstrated. The plant extract did not protect mice against the convulsant effects of pentylenetetrazol, it did not cause muscle relaxant effects and it did not seem to possess any affinity for benzodiazepine receptors. In addition, Eh did not have its own hypnotic effects in mice, but it intensified those of barbiturates, and caused a direct action on the central nervous system. This aqueous extract did not possess neuroleptic activity, but slight antidepressant effects were obtained against reserpine-induced ptosis and oxotremorine-induced hypothermia.

The involvement of muscarinic receptor subtypes in the mediation of hypothermia, tremor, and salivation in male mice.

The potency of centrally administered non-selective (atropine and N-methyl scopolamine) and putatively selective muscarinic antagonists (pirenzepine, AF-DX 116 and 4-DAMP) in inhibition of oxotremorine-induced hypothermia, tremor and salivation in male mice has been compared with their potency in vitro in three functional systems, where muscarinic effects are mediated preferentially by M1 (i.e. superior cervical ganglion), M2 (i.e. atrium), and M3 (i.e. ileum) receptors. Atropine, N-methyl scopolamine and 4-DAMP potently abolished the effects of oxotremorine. Pirenzepine abolished tremor and salivation, whereas hypothermia was antagonized partially only. AF-DX 116 had but weak antagonistic effects. Atropine and N-methyl scopolamine were potent antagonists in all three in vitro test systems. High potency was also seen with 4-DAMP, in particular in the ileum preparation. Pirenzepine showed its highest potency in the ganglion preparation. AF-DX 116 was a weak and non-selective antagonist in all three in vitro preparations. Our studies indicate that the muscarinic induction of tremor and salivation may be preferentially mediated by M3 receptors whereas both M2 and M3 receptors may be involved in the mediation of hypothermia. However, the overall conclusion is that compounds with higher receptor subtype selectivity are needed.

Triiodothyronine (T3) modifies cholinergic-induced hypothermia and tremor in rats.

Hypothermia and tremor responses of oxotremorine and eserine were studied in rats after several T3 treatment regimens. The T3 antagonized oxotremorine-induced hypothermia and failed to antagonize eserine hypothermic effect, but potentiated oxotremorine- and eserine-induced tremors. Acetylcholinesterase activity was not altered in T3 rats. The hypothetical mechanisms to explain changes of central cholinergic responses caused by T3 are discussed.

SR 46559A: a novel and potent muscarinic compound with no cholinergic syndrome.

The cholinergic activities of SR 46559A, 3-[N-(2 diethyl-amino-2-methylpropyl)-6-phenyl-5-propyl] pyridazinamine sesquifumarate, have been investigated in vitro and in vivo, in rodents. Using rat brain cortical membranes, SR 46559A was a competitive ligand (Ki = 112 nM) at muscarinic M1 receptors, its affinity for muscarinic M2 (cardiac) and M3 (glandular) receptors being 6-7 times lower. SR 46559A did not interact with brain nicotinic receptors and high affinity choline uptake sites nor did it inhibit brain acetylcholinesterase activity. In contrast to reference muscarinic agonists, SR 46559A (1 mM) did not inhibit the forskolin-induced activation of cAMP synthesis nor did it stimulate phosphoinositides breakdown in various brain preparations. However, this compound enhanced (+67% at 1 mM) diacylglycerol formation in rat striatal miniprisms, an effect fully reversed by atropine. As shown with reference agonists, SR 46559A inhibited (IC50 = 10 microM) the K(+)-evoked release of [3H]GABA from rat striatal slices and reduced at 0.5 and 1 microM, the population spike amplitude of the CA1 pyramidal cells induced by stimulation of the Schaffer's collateral commissural pathway in rat hippocampal slices. In mice, SR 46559A at a near lethal dose (200 mg/kg PO) did not induce the typical cholinergic syndrome nor did it modify at 30 mg/kg PO the oxotremorine-induced hypothermia. Like muscarinic agonists, SR 46559A (1 mg/kg PO) potentiated haloperidol-induced catalepsy in rats and inhibited (ED50 = 0.12 mg/kg PO) rotations induced in mice by intrastriatal injection of pirenzepine.(ABSTRACT TRUNCATED AT 250 WORDS)

Central activity of acetylcholinesterase oxime reactivators

The ability of various oximes to antagonize the sarin-induced hypothermia and reactivate phosphorylated acetylcholinesterase was used as an indicator of the central activity of oximes. HI-6, but neither toxogonin nor PAM Cl, antagonized sarin-induced hypothermia and reactivated brain acetylcholinesterase, in particular hypothalamic acetylcholinesterase. The sarin-induced hypothermia appears to be a muscarinic cholinergic action since atropine was also an effective antagonist of sarin-induced hypothermia. Neither HI-6 nor toxogonin antagonized oxotremorine-induced hypothermia, indicating that these oximes do not possess central cholinolytic activity. The results demonstrated that HI-6 penetrated the blood-brain barrier in a sufficient concentration to produce a biochemical and physiological action against sarin poisoning.

Flerobuterol: a potential antidepressant drug related to β‐adrenergic agonists. Experimental profile in mice

The potential antidepressant effect of flerobuterol (dl-(fluoro-2 phenyl)-1 t-butylamino-2 ethanol), a new drug related to beta-adrenoceptor agonists, was evaluated and compared with imipramine and salbutamol using classical psychopharmacological tests in mice. Like imipramine and salbutamol, flerobuterol (0.5-32 mg kg-1, ip) fully prevented apomorphine (16 mg kg-1, sc)- and partly reversed reserpine- and oxotremorine-induced hypothermia. At higher doses (16-32 mg kg-1), flerobuterol enhanced the toxic effects of yohimbine. Unlike imipramine, flerobuterol and salbutamol did not reduce immobility duration in the behavioural despair test. Salbutamol and flerobuterol decreased locomotor activity. Flerobuterol did not induce mydriasis, did not prevent oxotremorine-induced tremors or salivary and lacrimal gland secretion and did not reduce reserpine-induced palpebral ptosis. Propranolol (8 mg kg-1, ip) but not alpha-methyl-paratyrosine (75 mg kg-1, ip) prevented the flerobuterol-induced antagonism of apomorphine-induced hypothermia. Our results suggest that flerobuterol demonstrates potential antidepressant activity, which could be related to beta-adrenoceptor activation in mice.

Aminopyridazine muscarinic agonist, SR 95639A, is a functional M2 receptor antagonist in rat brain

AbstractThe minaprine analog SR 95639A (morpholinoethylamino‐3‐benzocylohepta‐(5,6‐c)pyridazine dihydrochloride), a putative muscarinic M1 receptor agonist, was evaluated for muscarinic activity in a number of biochemical and behavioral tests. Binding studies indicated that SR 95639A was 4‐fold selective for the M1 receptor. In contrast, both carbachol and the Merck “super agonist,” L 670,207, were 50‐fold selective for the M2 receptor, with Ki values of 47 μM and 100nM at the M1 receptor and 1 μM and 2nM at the M2 receptor, respectively. At concentrations up to 1 mM, SR 95639A had no effect on M1 receptor‐evoked phosphatidyl inositol (Pl) hydrolysis, while carbachol and L 670,207 had EC50 values of 116±10 μM and 0.60±0.12 μM respectively. SR 95639A (100 μM) did not inhibit the Pl response elicited by carbachol. At striatal M2 receptors linked to inhibition of forskolin‐mediated cyclic AMP production, SR 95639A at concentrations up to 100 μM had no agonist activity, while carbachol and L 670,207 had respective EC50 values of 540±31 nM and 2.9±0.5 nM. However, SR 95639A was an effective antagonist (K app = 3.50±0.45 μM; n = 5) of the carbachol response. In a rat radial arm maze paradigm, SR 95639A did not enhance cognitive performance and was unable to reverse deficits induced by scopolamine. The compound produced a dose‐dependent increase in mouse locomotor activity and did not attenuate the effects of scopolamine on habituation, but did potentiate oxotremorineinduced hypothermia.

Chronic stress and memory: Implication of the central cholinergic system

Restraint stress for ten days (two times two hours daily) induces a hypersensitivity of the central cholinergic system, reflected by antagonism to amnesia induced by scopolamine at 0.1 mg/kg in a passive avoidance test and by hypersensitivity to the hypothermic effect of oxotremorine at 1 mg/kg. A restraint stress for 30 days, on the other hand, diminishes animal retention in the passive avoidance test and causes a hyposensitivity to oxotremorine-induced hypothermia, reflecting a hypoactivity of the central cholinergic system. An acute 24-hour stress causes no change. The relationship between chronic stress and associated memory deficits is discussed.

Are antidepressant-like effects of triiodothyronine (T3) in rodents related to an hyperthyroid state?

1.1. The physiological and behavioral effects of T3 and corresponding plasma T3 levels were studied in mice.2.2. On the tests performed (antagonism of apomorphine- and oxotremorine-induced hypothermia, potentiation of yohimbine toxicity, L-5-HTP-induced head twitches and the learned-helplessness paradigm), T3 was active after subchronic treatment (1 injection per day for 3 days, ending 24 hours before testing).3.3. In these tests T3 exhibited the same profile as antidepressant drugs in rodents.4.4. The similar activity of beta-agonists in these tests and the ability of T3 to potentiate the effect of clenbuterol agree with the hypothesis that T3 can induce beta-adrenergic hypersensitivity.5.5. Under the present experimental conditions these effects were obtained with doses of T3 which did not induce hyper-triiodothyroninemia. Thus, the lowest doses significantly affecting apomorphine- and oxotremorine-induced hypothermia were respectively .008 and .032 mg/kg/day.6.6. Doses as low as .032 mg/kg/day were active in the yohimbine test.7.7. L-5-HTP-induced head twitches were potentiated by a dose of .25 mg/kg/day and in the learned-helpless paradigm, the lowest effective dose was .06 mg/kg/day.8.8. Plasma T3 values obtained in the same conditions were not significantly different from control at doses less than .5 mg/kg/day, and increased dramatically with higher doses, suggesting an accumulation of the hormone in plasma.9.9. The doses inducing an hyper-triiodothyroninemia coincided with physiological signs of hyperthyroidism in the animals (i.e. loss of weight and slight hyperthermia). Thus, the active dose range of T3 was below the lowerst dose required to produce a significant hyperthyroid state.10.10. This results suggest that a clinical benefit could be obtained with low doses of T3 that do not significantly induce an hyperthyroidism.

Potentiation of oxotremorine-induced hypothermia by alaproclate in PCA lesioned and non-lesioned rats

Stimulation of muscarinic cholinergic receptors with the highly potent and selective receptor agonist oxotremorine produced hypothermia in rats. Alaproclate, a purported selective serotonergic reuptake inhibitor, potentiated this response. Destruction of central presynaptic serotonergic terminals with the potent cytotoxin p-chloroamphetamine (PCA) failed to attenuate the hypothermic response to oxotremorine in alaproclate-pretreated animals. These results could be taken to suggest that alaproclate may act, at least in part, via a non-serotonergic mechanism to potentiate the oxotremorine-induced hypothermic response.

Psychopharmacological profile of 1-(m-(trifluoromethyl) phenyl) piperazine (TFMPP)

The effect of TFMPP, an agonist of the 5-HT 1b receptors, was studied in mice on several psychopharmacological parameters. In contrast to imipramine-like drugs, TFMPP neither antagonized reserpine-induced hypothermia nor increased yohimbine-induced toxicity. Similarly to imipramine-like drugs, TFMPP antagonized oxotremorine-induced hypothermia and was active in the behavioural despair test. In addition, TFMPP normalized a social behavioural deficit induced by isolation. The effects of TFMPP on oxotremorine-induced hypothermia in the behavioural despair test and in the isolation-induced social behavioural deficit are all antagonized by d-1 propranolol. It is concluded that TFMPP seems to possess psychotropic activity resembling only in part that of imipramine-like drugs and that these actions may be mediated through 5-HT 1b receptors.

Pharmacological adaptations and muscarinic receptor plasticity in hypothalamus of senescent rats treated chronically with cholinergic drugs.

Receptor plasticity is an important compensatory process by which the central nervous system adapts to pathological insult or long-term exposure to drugs. Senescent animals may show an age-related impairment of muscarinic receptor up- or down-regulation after chronic exposure to cholinergic drugs. The purpose of this study was to assess biochemical and pharmacological endpoints of muscarinic receptor plasticity in young, adult and senescent animals. Male, Fischer 344 rats (ages 3, 9, and 27 months) were administered methylatropine or oxotremorine intracerebroventricularly (IVT) for 3 weeks and tested for their functional response to a muscarinic agonist. The density of hypothalamic, muscarinic receptors was also estimated from analysis of 3H-QNB binding isotherms. In young rats, parallel changes in muscarinic receptors and response were noted, but chronic administration of cholinergic drugs to senescent animals had no effect. Thus, 3H-QNB binding in hypothalamus of young and adult rats was increased (31% and 17%) after chronic IVT methylatropine and decreased (20% and 15%) after IVT oxotremorine. Also, young rats treated with IVT methylatropine were supersensitive to the hypothermic effects of a muscarinic agonist (oxotremorine), while young and adult animals administered chronic IVT oxotremorine exhibited marked tolerance. In contrast, identically treated senescent rats showed no changes in 3H-QNB binding or oxotremorine-induced hypothermia. These results demonstrate the impaired ability of senescent rats to up- or down-regulate brain muscarinic receptors and to exhibit functional adaptations seen in young animals treated chronically with cholinergic drugs.

Antidepressant-like effects of triiodothyroacetic acid in mice.

Triiodothyroacetic acid (TA3) is a natural thyroid analogue which possesses some of the properties of triiodothyronine (T3). In particular, it has an inhibitory effect on the thyreotropic axis, but its peripheral effects are reduced. Given the activity of T3 on psychopharmacological models of depression in rodents, we investigated the effects of TA3 on some pharmacological and behavioral tests in mice. TA3 antagonized apomorphine- and oxotremorine-induced hypothermia, potentiated yohimbine-induced toxicity and L-5-hydroxy-tryptophan-induced head twitches, but did not affect forced swimming-induced immobility or reserpine-induced hypothermia. Thus, TA3 was effective on the same psychopharmacological tests which have previously been used to demonstrate the antidepressant-like effects of T3. Moreover, TA3 under the same experimental conditions as T3 has little effect on the metabolic clearance rate parameters, and might, therefore, be preferable to T3 for clinical use in depression.

Amitriptyline Produces Dose-Dependent Supersensitivity of a Central Cholinergic Mechanism

The discontinuation of tricyclic antidepressants (TCAs) can produce symptoms suggesting cholinergic overdrive. The authors previously proposed that these withdrawal states are the consequence of TCA affected cholinergic system supersensitivity. Evidence that chronic treatment with amitriptyline produces dose-dependent supersensitization of a central cholinergic mechanism is now presented. Core temperature is subject to a hypothalamic muscarinic mechanism. The thermic response of adult male rats to oxotremorine was telemetrically measured after 7 days of treatment with saline or amitriptyline 3, 10, and 20 mg/kg given intraperitoneally twice daily. Treatment with the TCA produced dose-dependent enhancement of oxotremorine-induced hypothermia. The data support the hypothesis that at least some TCA withdrawal phenomena involves supersensitization of muscarinic systems.

Differences in behavioral responses to oxotremorine and physostigmine in New Zealand black (NZB/B1NJ) and C57BL/6 mice

The NZB/B1NJ (NZB) mice are an autoimmune-prone strain, known to develop brain-reactive antibodies in serum at much earlier chronological ages than normal mice. Measurement of locomotor activity in 8–10 month old C57BL/6 (C57) mice following the administration of either oxotremorine or physostigmine, revealed a biphasic response consisting of inhibition at small doses, but increased motor activity at large doses. In contrast, age-matched NZB mice exhibited little inhibition at the smaller doses, but had much greater increases in activity after the larger doses. Similarly, when compared to C57 mice, NZB mice were less sensitive to oxotremorine-induced salivation, diarrhea and visible tremors. Moreover, oxotremorine-induced hypothermia occurred at smaller doses in C57 mice than in NZB mice and was of a greater magnitude. Thus, at an age when NZB mice possess high levels of brain-reactive antibodies, and exhibit impairment in tests of learning/memory, these mice also show diminished responses in several tests of cholinomimetic-induced behavior and physiological alterations.

Psychopharmacological profile of salsolinol

1. SAL is a tetraisoquinolein (T.I.Q.), resulting from the condensation of acetaldehyde and dopamine. 2. SAL, injected intraperitoneally, is active in tests commonly used to screen potential antidepressants. 3. This effect is especially studied by using antagonism of apomorphine, reserpine, oxotremorine-induced hypothermia. 4. The noradrenergic system seems to be involved in the mechanism of action.