The effects of oxotremorine, epibatidine, atropine, mecamylamine and naloxone in the tail-flick, hot-plate, and formalin tests in the naked mole-rat (Heterocephalus glaber)

The naked mole-rat (Heterocephalus glaber) is a promising animal model for the study of pain mechanisms, therefore a thorough characterization of this species is essential. The aim of the present study was to establish the naked mole-rat as a model for studying the cholinergic receptor system in antinociception by investigating the involvement of muscarinic, nicotinic and opioid receptors in nociceptive tests in this species. The effects of systemic administration of the muscarinic receptor agonist oxotremorine and the nicotinic receptor agonist epibatidine were investigated in the tail-flick, the hot-plate, and the formalin tests. The effects of co-administration of the muscarinic receptor antagonist atropine, the nicotinic receptor antagonist mecamylamine, and the opioid receptor antagonist naloxone were also investigated. Oxotremorine and epibatidine induced a significant, dose-dependent antinociceptive effect in the tail-flick, hot-plate, and formalin tests, respectively. The effects of oxotremorine and epibatidine were blocked by atropine and mecamylamine, respectively. In all three nociceptive tests, naloxone in combination with oxotremorine or epibatidine enhanced the antinociceptive effects of the drugs. The present study demonstrated that stimulation of muscarinic and nicotinic receptors produces antinociceptive effects in the naked-mole rat. The reversal effect of atropine and mecamylamine suggests that this effect is mediated by cholinergic receptors. As naloxone increases the antinociceptive effects of cholinergic agonists, it is suggested that the cholinergic antinociception acts via a gateway facilitated by opioid receptor blockage; however, the precise interaction between these receptor systems needs further investigation.

LY2033298, a positive allosteric modulator at muscarinic M₄ receptors, enhances inhibition by oxotremorine of light-induced phase shifts in hamster circadian activity rhythms

RATIONALE

Entrainment of circadian rhythms to the light-dark cycle is essential for restorative sleep, and abnormal sleep timing is implicated in central nervous system (CNS) disorders like depression, schizophrenia, and Alzheimer's disease. Many transmitters, including acetylcholine, that exerts its actions via muscarinic receptors modulate the suprachiasmatic nucleus, the master pacemaker.

OBJECTIVES

Since positive allosteric modulators of muscarinic M(4) receptors are candidates for treatment of mood and cognitive deficits of CNS disorders, it is important to evaluate their circadian actions.

MATERIALS AND METHODS

The effects of intraperitoneally applied muscarinic agents on circadian wheel-running rhythms were measured employing hamsters, a model organism for studying activity rhythms.

RESULTS

Systemic administration of the muscarinic receptor agonist oxotremorine (0.01-0.04 mg/kg) inhibited light-induced phase delays and advances of hamster circadian wheel-running rhythms. The M₄ positive allosteric modulator, LY2033298 (10-40 mg/kg), had no effect on light-induced phase shifts when administered alone, yet significantly enhanced (at 20 mg/kg) the inhibitory influence of oxotremorine on light-induced phase delays. In addition, the muscarinic receptor antagonist, scopolamine, which was without effect on light-induced phase shifts when administered alone (0.001-0.1 mg/kg), antagonized (at 0.1 mg/kg) the inhibitory effect of oxotremorine and LY2033298 on light-induced phase delays.

CONCLUSIONS

These results are the first to demonstrate that systemically applied muscarinic receptor agonists modulate circadian activity rhythms, and they also reveal a specific role for M₄ receptors. It will be of importance to evaluate circadian actions of psychotropic drugs acting via M₄ receptors, since they may display beneficial properties under pathological conditions.

Muscarinic type 2 receptors in the lateral dorsal tegmental area modulate cocaine and food seeking behavior in rats

The cholinergic input from the lateral dorsal tegmental area (LDTg) modulates the dopamine cells of the ventral tegmental area (VTA) and plays an important role in cocaine taking. Specific pharmacological agents that block or stimulate muscarinic receptors in the LDTg change acetylcholine (ACh) levels in the VTA. Furthermore, manipulations of cholinergic input in the VTA can change cocaine taking. In the current study, the ACh output from the LDTg was attenuated by treatment with the selective muscarinic type 2 (M2) autoreceptor agonist oxotremorine.sesquifumarate (OxoSQ). We hypothesized that OxoSQ would reduce the motivation of rats to self-administer both natural and drug rewards. Animals were tested on progressive ratio (PR) schedules of reinforcement for food pellets and cocaine. On test days, animals on food and on cocaine schedules were bilaterally microinjected prior to the test. Rats received either LDTg OxoSQ infusions or LDTg artificial cerebrospinal fluid (aCSF) infusions in a within-subjects design. In addition, infusions were delivered into a dorsal brain area above the LDTg as an anatomical control region. OxoSQ microinjection in the LDTg, compared to aCSF, significantly reduced both the number of self-administered pellets and cocaine infusions during the initial half of the session; this reduction was dose-dependent. OxoSQ microinjections into the area just dorsal to the LDTg had no significant effect on self-administration of food pellets or cocaine. Animals were also tested in locomotor activity chambers for motor effects following the above microinjections. Locomotor activity was mildly increased by OxoSQ microinjection into the LDTg during the initial half of the session. Overall, these data suggest that LDTg cholinergic neurons play an important role in modifying the reinforcing value of natural and drug rewards. These effects cannot be attributed to significant alterations of locomotor behavior and are likely accomplished through LDTg muscarinic autoreceptors.

Injection of oxotremorine in nucleus accumbens shell reduces cocaine but not food self-administration in rats

Mesencephalic dopamine neurons form synapses with acetylcholine (ACh)-containing interneurons in the nucleus accumbens (NAcc). Although their involvement in drug reward has not been systematically investigated, these large aspiny interneurons may serve an important integrative function. We previously found that repeated activation of nicotinic cholinergic receptors enhanced cocaine intake in rats but the role of muscarinic receptors in drug reward is less clear. Here we examined the impact of local changes in muscarinic receptor activation within the NAcc on cocaine and food self-administration in rats trained on a progressive ratio (PR) schedule of reinforcement. Animals were given a minimum of 9 continuous days of drug access before testing in order to establish a stable breaking point (BP) for intravenous cocaine infusions (0.75 mg/kg/infusion). Rats in the food group acquired stable responding on the PR schedule within 7 days. On the test day, rats were bilaterally infused in the NAcc with the muscarinic receptor agonist oxotremorine methiodide (OXO: 0.1, 0.3 or 1 nmol/side), OXO plus the M(1) selective antagonist pirenzepine (PIRENZ; 0.3 nmol/side) or aCSF 15 min before cocaine or food access. OXO dose dependently reduced BP values for cocaine reinforcement (-17%, -44% [p<0.05] and -91% [p<0.0001] for 0.1, 0.3 and 1.0 nmol, respectively) and these reductions dissipated by the following session. Pretreatment with PIRENZ blocked the BP-reducing effect of 0.3 nmol OXO. Notably, OXO (0.1, 0.3 and 1.0 nmol/side) injection in the NAcc did not affect BP for food reward. The results suggest that muscarinic ACh receptors in the caudomedial NAcc may play a role in mediating the behavior reinforcing effects of cocaine.

Long-term potentiation in the dentate gyrus in freely moving rats is reinforced by intraventricular application of norepinephrine, but not oxotremorine

Growing evidence suggests that processes of synaptic plasticity, such as long-term potentiation (LTP) occurring in one synaptic population, can be modulated by consolidating afferents from other brain structures. We have previously shown that an early-LTP lasting less than 4 h (E-LTP) in the dentate gyrus can be prolonged by stimulating the basolateral amygdala, the septum or the locus coeruleus within a specific time window. Pharmacological experiments have suggested that noradregeneric (NE) and/or cholinergic systems might be involved in these effects. We have therefore investigated whether the direct intraventricular application of agonists for NE- or muscarinic receptors is able to modulate synaptic plasticity. E-LTP was induced at the dentate gyrus of freely moving rats using a mild tetanization protocol that induces only an E-LTP. NE or oxotremorine (OXO) were applied icv 10 min after the tetanus. Results show that low doses of NE (1.5 and 5 nM) effectively prolong LTP. A higher dose (50 nM) was not effective. None of the OXO doses employed (5, 25, and 50 nM) showed similar effects. These results stress the importance of transmitter-specific modulatory influences on the time course of synaptic plasticity, in particular NE whose application mimics the reinforcing effect of directly stimulating limbic structures on LTP.

Facilitation of memory for extinction of drug-induced conditioned reward: role of amygdala and acetylcholine

These experiments examined the effects of posttrial peripheral and intra-amygdala injections of the cholinergic muscarinic receptor agonist oxotremorine on memory consolidation underlying extinction of amphetamine conditioned place preference (CPP) behavior. Male Long-Evans rats were initially trained and tested for an amphetamine (2 mg/kg) CPP. Rats were subsequently given limited extinction training, followed by immediate posttrial peripheral or intrabasolateral amygdala injections of oxotremorine. A second CPP test was then administered, and the amount of time spent in the previously amphetamine-paired and saline-paired apparatus compartments was recorded. Peripheral (0.07 or 0.01 mg/kg) or intra-amygdala (10 etag/0.5 microL) postextinction trial injections of oxotremorine facilitated CPP extinction. Oxotremorine injections that were delayed 2 h posttrial training did not enhance CPP extinction, indicating a time-dependent effect of the drug on memory consolidation processes. The findings indicate that memory consolidation for extinction of approach behavior to environmental stimuli previously paired with drug reward can be facilitated by posttrial peripheral or intrabasolateral amygdala administration of a cholinergic agonist.

Fruit extracts antagonize Aβ- or DA-induced deficits in Ca2+ flux in M1-transfected COS-7 cells

Evidence suggests that there is a selective sensitivity to oxidative stress (OSS) among muscarinic receptor (MAChR) subtypes with M1, M2 and M4 showing > OSS than M3 or M5 subtypes in transfected COS-7 cells. This may be important in determining the regional specificity in neuronal aging and Alzheimer disease (AD). We assessed the effectiveness of blueberry (BB) and other high antioxidant (HA) fruit extracts (boysenberry, BY; cranberry, CB; black currant, BC; strawberry, SB; dried plums, DP; and grape, GR) on the toxic effects of Abeta 25-35 (100 microM, 24 hrs) and DA (1 mM, 4 hrs) on calcium buffering (Recovery) following oxotremorine (750 microM) -induced depolarization in M1AChR-transfected COS-7 cells, and on cell viability following DA (4 hrs) exposure. The extracts showed differential levels of Recovery protection in comparisons to the non-supplemented controls that was dependent upon whether DA or Abeta was used as the pretreatment. Interestingly, assessments of DA-induced decrements in viability revealed that all of the extracts had some protective effects. These findings suggest that the putative toxic effects of Abeta or DA might be reduced by HA fruit extracts.

Characterization of muscarinic autoreceptors in the rabbit hippocampus and caudate nucleus

Oxotremorine-induced inhibition of electrically evoked release of 3H-acetylcholine from brain slices preincubated with 3H-choline was used to characterize muscarinic autoreceptors in rabbit hippocampus and caudate nucleus. From the shifts to the right of the concentration-response curves of oxotremorine in the presence of muscarinic receptor antagonists, the following pKB values [95% C.I.] were determined in the hippocampus: tripinamide: 8.7 [8.5, 8.8]; himbacine: 8.4 [8.3, 8.5]; AQ-RA 741: 8.3 [8.2, 8.5]; 4-DAMP: 8.2 [8.0, 8.3]; hexahydrosiladifenidol: 7.4 [7.2, 7.5]; AF-DX 116: 7.3 [7.1, 7.4]; pirenzepine: 6.8 [6.6, 7.0]; and PD102807: 6.3 [6.0, 6.5]. In the caudate nucleus: tripinamide: 9.1 [8.9, 9.2]; 4-DAMP: 8.3 [8.2, 8.5]; himbacine: 8.1 [8.0, 8.2]; AQ-RA 741: 8.1 [8.0, 8.3]; hexahydrosiladifenidol: 7.3 [7.2, 7.4]; AF-DX 116: 7.1 [7.0, 7.2]; pirenzepine: 6.7 [6.6, 6.8]; and PD102807: 6.5 [6.2, 6.8]. These pKB values fit best to literature values for M2 receptors, suggesting that the muscarinic autoreceptor of the rabbit hippocampus and caudate nucleus is the m2 gene product.

Muscarinic receptors of the neostriatum--their role in controlling operant behavior in dogs

Chronic experiments were performed on six dogs to study the effects of bilateral microinjections of muscarinic receptor agonists and blockers into the dorsal striatum on the performance of an operant defensive reflex consisting of maintenance of a specified posture and on the differentiation of stimuli. Microinjections of carbachol, a non-selective agonist of muscarinic receptors, were accompanied by increases in the tonic component of movements, inhibition of phasic movements, ordering of the postural rearrangement, and increases in the amplitudes of its components. Bilateral microinjections of the selective agonist oxotremorine into the neostriatum had significantly weaker effects on the amplitude of postural rearrangement, generally decreasing the amplitude. Although oxotremorine also increased the tonic component of the operant response, this effect was weaker than that seen with carbachol microinjections. In addition, oxotremorine, unlike carbachol increased the number of intersignal limb elevations. These data, along with data published in the literature, are used to suggest the hypothesis that the actions of oxotremorine are mediated not only via muscarinic M2 but also via M1 receptors in the neostriatum. Stronger changes in responses to differential stimuli were also obtained after microinjection of the non-selective agonist carbachol than after microinjection of oxotremorine, and the fact that changes in responses to differential stimuli were significantly greater than changes in those to defensive stimuli suggests that microinjections of muscarinic M1 and M4 receptor agonists into the striatum are also accompanied by improvements in attention to significant stimuli.

Intravenously administered oxotremorine and atropine, in doses known to affect pain threshold, affect the intraspinal release of acetylcholine in rats

Both systemically and intrathecally administered cholinergic agonists produce antinociception while cholinergic antagonists decrease pain threshold. The mechanism and the site of action of these substances are not known. In the present study it was hypothesized that systemically administered muscarinic agonists and antagonists modify nociceptive threshold by affecting intraspinal release of acetylcholine (ACh). Catheters were inserted into the femoral vein in rats maintained on isoflurane anaesthesia for administration of oxotremorine (10-300 microg/kg) and atropine (0.1, 10, 5000 microg/kg). Spinal microdialysis probes were placed intraspinally at approximately the C2-C5 spinal level for sampling of acetylcholine and dialysis delivery of atropine (0.1, 1, 10 nM). Additionally, the tail-flick behaviour was tested on conscious rats injected intraperitoneally with saline, atropine (10, 100 and 5000 microg/kg), or subcutaneously with oxotremorine (30, 100, 300 microg/kg). Subcutaneous administration of oxotremorine (30, 100, 300 microg/kg) significantly increased the tail-flick latency. These doses of oxotremorine dose-dependently increased the intraspinal release of acetylcholine. Intravenously administered atropine, in a dose that produced hyperalgesia (5000 microg/kg) in the tail-flick test, significantly decreased the intraspinal release of acetylcholine. Our results suggest an association between pain threshold and acetylcholine release in spinal cord. It is also suggested that an approximately 30% increase in basal ACh release produces antinociception and that a 30% decrease in basal release produces hyperalgesia.

Behavioral sensitization to amphetamine induced by a single i.p. dose of oxotremorine in the rat

Earlier experiments have revealed that rats treated with a single dose of chlorphenvinphos (CVP), an irreversible acetylcholinesterase inhibitor, are hyposensitive to amphetamine (AMPH) given three weeks after CVP. Exposure to CVP results in an excess of acetylcholine with subsequent overactivation of the nicotinic as well as muscarinic cholinergic receptors. The purpose of the present experiment was to find out whether a selective activation of muscarinic receptors could induce behavioral hyposensitivity to AMPH. To attain this purpose, male rats were pretreated once with 0.00, 0.135, 0.27 or 0.55 mg/kg of oxotremorine, a muscarinic agonist, and challenged 15 days later with 1.0 mg/kg dose of AMPH. The pre- and postinjection open-field behavior of the rats was tested with the use of a computerized set of activity meters. The testing revealed that in oxotremorine pretreated animals the behavioral response to AMPH, i.e. increase in the ambulatory activity, was not diminished but, to the contrary, it was augmented. This effect was dose-dependent, being most pronounced in rats given the 0.55 mg/kg of oxotremorine. The possible cause of the difference between the effect of CVP and oxotremorine is discussed.

Central muscarinic mechanisms regulating voiding in rats

The influence of muscarinic receptor stimulation and blockade on the central regulation of micturition was evaluated in conscious female rats. Saline was infused into the bladder to induce repeated bladder contractions and voiding. Increasing doses of a muscarinic agonist, oxotremorine-M (OXO-M; 0.01 to 1 microg/rat) or antagonist, atropine (0.1 to 30 microg/rat) were administered. Intrathecal OXO-M (0.1 microg) increased bladder capacity (BC; 85 +/- 17%), but did not change maximal voiding pressure (MVP), pressure threshold (PT), postvoiding intravesical pressure, or voiding efficiency (VE). Intracerebroventricular OXO-M (0.1 microg) increased BC (97 +/- 6%), MVP (45 +/- 19%), PT (158 +/- 49%), and reduced VE (-17 +/- 5%). A larger dose of OXO-M (1 microg, either i.c.v. or i.t.) produced greater changes. These effects were not reproduced by i.v. injections of OXO-M. The effects of OXO-M were blocked by pretreatment with atropine in a dose (1 microg i.c.v. or i.t.), which alone had no effect on voiding parameters. A larger dose of atropine (10 microg) reduced MP (-31 +/- 7% i.c.v. and -34 +/- 6% i.t.) and VE (-21 +/- 3% i.c.v. and -25 +/- 5% i.t.) but increased BC (52 +/- 8% i.c.v.). These results indicate that activation of muscarinic receptors in the brain or spinal cord can suppress voluntary voiding, but also stimulates bladder activity during bladder filling. The muscarinic inhibitory mechanisms do not appear to be tonically active. The effects of atropine (i.c.v. and i.t.) indicate that muscarinic excitatory mechanisms are tonically active. These findings raise the possibility that voiding function is regulated by both inhibitory and excitatory cholinergic mechanisms in the central nervous system.

Changes in EEG power spectra and behavioral states in rats exposed to the acetylcholinesterase inhibitor chlorpyrifos and muscarinic agonist oxotremorine

Organophosphates (OPs) inhibit acetylcholinesterase (AChE) activity causing cholinergic stimulation in the central nervous system (CNS). Cholinergic systems are crucial in electroencephalogram (EEG) generation and regulation of behavior; however, little is known about how OP exposure affects the EEG and behavioral states. We recorded EEG, core temperature and motor activity before and after exposure to the OP pesticide chlorpyrifos (CHP) in adult female rats implanted with telemetric transmitters. The recording and reference electrodes were placed in the occipital and frontal bones, respectively. The animals received CHP, 25 mg/kg, p.o., or oxotremorine (OX), 0.2 mg/kg, s.c. CHP led to a significant increase in delta (0.1-3.5 Hz), slow theta (4-6.5 Hz), gamma 2 (35.5-50 Hz), reduction in fast theta (7-8.5 Hz), alpha/sigma (9-14 Hz), beta 1 (14.5-24 Hz), beta 2 (24.5-30 Hz) and gamma 1 (30.5-35 Hz) powers, slowing of peak frequencies in 1-9 Hz range, hypothermia and decrease in motor activity. The drop in 7-14 Hz was associated with cholinergic suppression of sleep spindles. Changes in behavioral state were characterized by dramatic diminution of sleep postures and exploring activity and prolongation of quiet waking. There was recovery in all bands in spite of continued inhibition of AChE activity [44,45] in rats exposed to CHP. OX-induced EEG and behavioral alterations were similar to CHP except there was no increase in delta and the onset and recovery were more rapid. We did not find a correlation between the EEG and core temperature alterations. Overall, changes in EEG (except in delta band) and behavior following CHP were attributable to muscarinic stimulation. Cortical arousal together with increased quiet waking and decreased sleep after CHP occurred independently from inhibition of motor activity and lowering of core temperature.

Activation of the mesocorticolimbic dopaminergic system by stimulation of muscarinic cholinergic receptors in the ventral tegmental area

OBJECTIVE

To investigate the function of muscarinic receptors in the ventral tegmental area in vivo, the release of endogenous monoamines was simultaneously measured in the somatodendritic (ventral tegmental area) and terminal (frontal cortex and nucleus accumbens) regions of the mesocorticolimbic dopaminergic system in rats, using dual probe microdialysis.

METHODS

Rats were implanted with dual microdialysis probes ipsilaterally into the ventral tegmental area (VTA) and nucleus accumbens (NAC) or frontal cortex (FC).

RESULTS

Intrategmental infusion of the muscarinic agonist oxotremorine M (OXO M, 0.1 and 1 mM) increased extracellular levels of dopamine and serotonin, but not noradrenaline, in the VTA to a maximum of 200% over baseline in both urethane-anaesthetized and unanaesthetized rats. In freely moving animals, this effect was accompanied by strong motor agitation. Both VTA dopamine and serotonin levels dropped to 60% or less of baseline when the perfusion medium was replaced by a calcium-free medium containing OXO M. In the NAC and FC, a similar increase in extracellular dopamine, but not serotonin and noradrenaline, was observed during OXO M infusion in the VTA. The removal of calcium during OXO M infusion in the VTA did not cause a decrease in NAC dopamine levels. Activation of serotonin and dopamine release by OXO M in the VTA and FC was dramatically reduced or prevented by the co-infusion of the muscarinic antagonist N-methylscopolamine (0.1 mM).

CONCLUSION

These data demonstrate that VTA dopamine cells possess functional muscarinic receptors whose activation stimulates the release of dopamine in the VTA, NAC and FC. These results also suggest that muscarinic receptors may modulate the synaptic release of serotonin in the VTA.

Effects of beta-blockers and nicardipine on oxotremorine-induced tremor in common marmosets

Effects of beta-blockers (propranolol, arotinolol and nipradilol) and a Ca2+ channel blocker (nicardipine) on oxotremorine-induced tremor were studied in common marmosets. Generalized tremor was elicited by an intraperitoneal administration of 0.25 mg/kg oxotremorine. Intensity of the tremor was classified into 7 degrees, and it was evaluated every 10 min. The total intensity of oxotremorine-induced tremor for each drug was expressed as "points", which were the sum of tremor intensity scores evaluated every 10 min up to 190 min following the administration of oxotremorine. Beta-blockers significantly suppressed the tremor. On the other hand, the Ca2+ channel blocker exacerbated the tremor.

Intrathecal oxotremorine affects formalin-induced behavior and spinal nitric oxide synthase immunoreactivity in rats

The present research was undertaken to investigate, by behavioral and immunohistochemical methods, the effects of intrathecal (i.th.) injection of the muscarinic agonist oxotremorine on the response to the long-lasting nociceptive stimulus induced by injection of formalin into the rat hind paw. Formalin injection induced a biphasic, pain-induced behavioral response (paw jerks), as well as an increase in the number of nitric oxide (NO) synthase-labeled neurons in laminae I-III, IV, and X, but not in laminae V-VI. Oxotremorine (0.1-10 ng, i.th.) inhibited paw-jerk frequency in both phases of formalin-induced behavior. The immunohistochemical results showed that i.th.-injected oxotremorine differently affected the level of NO synthase in lumbar part of the spinal cord: no change or increase after the dose of 1 ng, and a significant reduction of nitric oxide synthase neurons after the higher dose (10 ng). These results evidenced a role of cholinergic system in the modulation of tonic pain and in nitric oxide synthase expression at the spinal cord level, which further suggests that these two systems could be involved in phenomena induced by long-lasting nociceptive stimulation.

Antinociception after both peripheral and intrathecal injection of oxotremorine is modulated by spinal nitric oxide

The present study investigated the role of spinal nitric oxide (NO) in the antinociception induced by intraperitoneal (i.p.) and intrathecal (i.th.) injection of oxotremorine. The experiments were carried out on male Wistar rats, which had cannulas chronically implanted in the lumbar enlargement of the spinal cord. Antinociceptive effects were evaluated using a tail-flick and a paw pressure test. To raise the spinal NO level, the rats received the NO donor, 3-morpholino-sydnonimine (SIN-1, 10 and 100 microg/5 microl); to lower the NO level, the inhibitor of NO synthase, N-nitro-L-arginine methyl ester (L-NAME, 50 and 400 microg/5 microl), was administered. Both those substances were injected i.th. Systemic injections of oxotremorine (0.02 and 0.1 mg/kg) produced a significant increase in the thermal nociceptive threshold, while the mechanical threshold was affected only by the higher dose (0.1 mg/kg) of the muscarinic agonist. I.th. injections of oxotremorine (0.1 ng, 1 ng, 1 microg/5 microl) produced significant antinociception in both those tests. I.th. administration of SIN-1 in doses which themselves did not affect the nociceptive threshold antagonized both the peripheral and central oxotremorine antinociception. I.th. administration of L-NAME (50 and 400 microg/5 microl) did not change the nociceptive threshold, but dose-dependently potentiated the effects of oxotremorine injected i.p. in both tests; however, the effect of i.th. administration of oxotremorine was potentiated only in the tail-flick test. Our results demonstrate that irrespective of the way of its injection, the antinociceptive effect of oxotremorine is modulated by activity of the spinal NO. Moreover, our results further support the hypothesis that NO present in the spinal cord exerts pronociceptive effects.

Oxotremorine suppresses thalamocortical oscillations via thalamic muscarinic acetylcholine receptors

We investigated whether the local intrathalamic infusion of a muscarinic acetylcholine receptor agonist (oxotremorine) at either the reticular nucleus of thalamus (NRT) or the ventroposteromedial nucleus of thalamus (VPM) suppresses thalamocortically generated neocortical high-voltage spindles (HVSs). In addition, we studied whether the intracerebroventricular (ICV) infusion of a selective muscarinic M2 acetylcholine receptor antagonist (methoctramine) could block the suppression of HVSs induced by either systemic (IP) administration of an anticholinesterase drug [tetrahydroaminoacridine (THA)] or ICV infusion of oxotremorine in rats. Intrathalamic administration of oxotremorine at 3 and 15 microg in the NRT, and at 15 microg in the VPM suppressed HVSs. ICV oxotremorine at 30 and 100 microg and IP THA at 3 mg/kg decreased HVSs. ICV methoctramine at 100 microg increased HVSs and completely blocked the decrease in HVSs produced by oxotremorine 100 microg and THA 3 mg/kg. The results suggest that activation of muscarinic M2 acetylcholine receptors in thalamic nuclei (NRT and VPM) can suppress thalamocortical oscillations and that ICV or systemically administered drugs that activate either directly (oxotremorine and methoctramine) or indirectly (THA) the muscarinic M2 acetylcholine receptors may modulate neocortical HVSs via the thalamus.

Pharmacological features of masculine sexual behavior in an animal model of depression

Neonatal treatment with clomipramine induces behavioral alterations during adulthood that resemble symptoms observed in human depression. Therefore, it has been proposed as an animal model of depression. Impairment of male sexual performance is one of the main effects of this treatment. Using this model of depression, we evaluated the effects of drugs that stimulate sexual performance by acting selectively on the adrenergic, serotonergic, or cholinergic system. Yohimbine, a selective antagonist of the alpha-2 receptors; 8-OH-DPAT, a selective agonist of the 5-HT1A receptors; and oxotremorine, a muscarinic agonist, were administered to male rats neonatally treated with clomipramine that showed sexual behavior impairments. Yohimbine and oxotremorine induced only a slight improvement of sexual deficiencies. 8-OH-DPAT not only restored sexual behavior to normal levels, but induced facilitation in most of the copulatory parameters. These results suggest that neonatal treatment with clomipramine induces sexual deficits acting mainly on the adrenergic and cholinergic systems, while the serotoninergic system seems to be preserved.

M1 muscarinic receptor stimulation decreases aspartate release in the rat neostriatum

This study investigates the effects of different muscarinic receptor agonists on extracellular glutamate and aspartate concentrations in the rat neostriatum. In vivo intracerebral perfusions were undertaken in the conscious rat using a concentric push-pull cannulae system. Amino acid concentrations in samples were determined by HPLC with fluorometric detection. The intrastriatal perfusion of arecoline, a M1-M2 muscarinic receptor agonist, produced a significant decrease in extracellular [ASP] (45% of decrease) but not in extracellular [GLU]. These effects were blocked by scopolamine, a M1-M2 muscarinic receptor antagonist. McN-A-343, a M1 muscarinic receptor agonist, but not the M2 muscarinic receptor agonist, oxotremorine, produced a significant decrease in extracellular [ASP] (40% of decrease) but not in extracellular [GLU]. The effects of McN-A-343 on extracellular [ASP] were blocked by pirenzepine, a M1 muscarinic receptor antagonist. These results suggest that the decrease in extracellular [ASP] could be mediated, at least in part, by M1 muscarinic receptor activation in the rat neostriatum.

Muscarinic acetylcholine receptors on rat thymocytes: their possible involvement in DNA fragmentation

Several studies have shown that the nervous (and hormonal) system controls immune functions. In the present study, we examined the presence of muscarinic acetylcholine receptors and the effect of carbachol on DNA fragmentation in adult rat thymocytes. Rat thymocytes possessed high affinity binding sites for the muscarinic antagonist [3H]3-quinuclidinyl benzilate (QNB). The average number of binding sites per cells was 3000, and the equilibrium dissociation constant of [3H]QNB on intact cell was approximately 80 nM. The binding was inhibited by an M1- and M3-selective antagonist, 4-diphenylacetoxy-N-methylpiperidine methiodine (4-DAMP). Hydrocortisone (100 mg/kg, s.c.) treatment of rats for 2 days prior to sacrifice increased the average number of [3H]QNB binding sites on thymocytes by 82 +/- 33%. The gel electrophoresis of DNA extracted from carbachol-treated thymocytes revealed a ladder pattern typical of intranucleosomal fragmentation. The addition of oxotremorine-M also induced DNA fragmentation and the effects of muscarinic agonists were inhibited by the addition of atropine or 4-DAMP. The results suggest the existence of muscarinic receptors and the possible involvement in apoptosis in thymocytes.

Oxotremorine infusions into the medial septal area of middle-aged rats affect spatial reference memory and ChAT activity

Age-related spatial memory deficits are correlated with septohippocampal cholinergic system degeneration. The present study examined the effect of intraseptal infusions of the cholinergic agonist, oxotremorine, on spatial reference memory in middle-aged rats using place discrimination in the water maze, and on cholinergic activity using choline acetyltransferase (ChAT) activity. Oxotremorine mildly improved the rate of place discrimination acquisition of middle-aged rats during initial sessions only, but did not affect asymptotic levels of performance achieved. Of the brain regions assayed, ChAT activity increased with age in the temporal cortex and dorsal CA2/3 region of the hippocampus. Oxotremorine significantly decreased ChAT activity in the dorsal hippocampus. In contrast to our previous results in aged rats indicating a more robust effect of oxotremorine on spatial working memory, the present results suggest a modest effect of intraseptal oxotremorine on the acquisition of a spatial reference memory task.

Modulation of rhythmical slow activity, long-term potentiation and memory by muscarinic receptor agonists

We investigated the cholinergic modulation of hippocampal rhythmical slow activity (or theta activity), long-term potentiation and a behavioral memory task. The intravenous administration of the muscarinic receptor agonists, AF102B ((+/-)-cis-2-methyl-spiro(1,3-oxathiolane-5,3') quinuclidine hydrochloride hemihidrate) and oxotremorine, induced rhythmical slow activity at doses of 1.0 mg/kg and 0.01 mg/kg, respectively. Long-term potentiation of population spike amplitude in the hippocampal CA1, which was induced by tetanic stimulation to the Schaffer collateral/commissural fiber, was increased by AF102B (1.0 mg/kg i.v.) and oxotremorine (0.01 mg/kg i.v.). Oral administration of AF102B and oxotremorine improved scopolamine-induced memory deficits in a passive avoidance task in mice at doses of 1.0 mg/kg and 0.2 mg/kg, respectively. The correspondence of the effective doses of muscarinic receptor agonists in these three experiments suggested the cholinergic correlation of rhythmical slow activity, long-term potentiation and memory.

Method for screening drug and chemical effects in laboratory rats using computerized quantitative electroencephalography

A minimally-invasive method of quantitative electroencephalography (qEEG) that requires no anesthetics and parallels techniques of naturalistic stimulation was developed and validated for regulatory testing of drugs and chemicals in rats. Male and female Fischer 344 rats were utilized in a randomized-block design to measure qEEG target parameters associated with a range of cholinesterase inhibition. For this study, physostigmine was administered ip at doses of 0.05, 0.2 or 1.0 mg/kg, resulting in average cholinesterase inhibition in plasma (28, 38 and 70%), erythrocytes (19, 24 and 36%), and brain (2, 10 and 31%) which correlated well with increased total power and amplitude changes. Additional treatment-related effects consisted of decreased relative alpha and beta, increased relative delta, and a left-shift in the spectral-edge frequency. In a second study, male and female Sprague-Dawley rats were utilized in a treatment-by-subjects design to determine qEEG target parameter changes due to the M2 autoreceptor agonist oxotremorine. Repeated incremental doses (0.05, 0.1, 0.2 mg/kg; ip) of oxotremorine resulted in increased beta contribution, a right-shift in the spectral-edge frequency and decreased alpha contribution. These qEEG results with physostigmine and oxotremorine correlate well with receptor-specific and general muscarinic effects, making it a reliable contribution to analysis of agonist and antagonist effects of cholinergic compounds.

Method for screening drug and chemical effects in laboratory rats using computerized quantitative electroencephalography

A minimally-invasive method of quantitative electroencephalography (qEEG) that requires no anesthetics and parallels techniques of naturalistic stimulation was developed and validated for regulatory testing of drugs and chemicals in rats. Male and female Fischer 344 rats were utilized in a randomized-block design to measure qEEG target parameters associated with a range of cholinesterase inhibition. For this study, physostigmine was administered ip at doses of 0.05, 0.2 or 1.0 mg/kg, resulting in average cholinesterase inhibition in plasma (28, 38 and 70%), erythrocytes (19, 24 and 36%), and brain (2, 10 and 31%) which correlated well with increased total power and amplitude changes. Additional treatment-related effects consisted of decreased relative alpha and beta, increased relative delta, and a left-shift in the spectral-edge frequency. In a second study, male and female Sprague-Dawley rats were utilized in a treatment-by-subjects design to determine qEEG target parameter changes due to the M2 autoreceptor agonist oxotremorine. Repeated incremental doses (0.05, 0.1, 0.2 mg/kg; ip) of oxotremorine resulted in increased beta contribution, a right-shift in the spectral-edge frequency and decreased alpha contribution. These qEEG results with physostigmine and oxotremorine correlate well with receptor-specific and general muscarinic effects, making it a reliable contribution to analysis of agonist and antagonist effects of cholinergic compounds.

Interaction of chlorphenvinphos with cholinergic receptors in the rabbit hypothalamus

The purpose of this study was to find out whether chlorphenvinphos (CVP), an organophosphorous pesticide, interacts with the muscarinic cholinergic receptors in CNS. To attain this goal, the effects of intrahypothalamic injections of oxotremorine (Ox), a muscarinic agonist, and physostigmine (Phys), a carbamate anticholinesterase, were compared with those produced by intrahypothalamic injections of CVP in the rabbit. It was found that the infusion of Ox (20 micrograms) as well as Phys (200 micrograms) into the anterior hypothalamus leads to an increase in the 4-7 Hz theta rhythm in the hippocampus and to the appearance of behavioral symptoms suggestive of a threat response. In the case of Ox, the effects could be prevented by injections of 20 micrograms scopolamine, a muscarinic antagonist. Pretreatment of the hypothalamus with 100 micrograms hemicholinium (HC-3) did not prevent the effects of Phys injected 2 h later. (HC-3 prevents the resynthesis of acetylcholine by blocking choline reuptake. This leads to a gradual depletion of ACh stores and to an inhibition of the cholinergic transmission). It suggests that Phys activates directly postsynaptic muscarinic receptors. Intrahypothalamic injections of CVP in doses of up to 1360 micrograms produced no overt changes in behavior nor in the hippocampal EEG of the rabbit and did not prevent the effect of subsequent injections of Ox. This suggests that CVP is neither an agonist nor antagonist of the muscarinic receptors in the rabbit hypothalamus.

Reversal of hemorrhagic shock in rats by oxotremorine: the role of muscarinic and nicotinic receptors, and AV3V region

In an experimental model of hemorrhagic shock resulting in the death of almost all rats within 20-30 min, centrally active cholinomimetic drugs are reported to induce a prompt, sustained and dose-dependent improvement in blood pressure and survival rate claimed to be due to nicotinic, but not muscarinic actions. In the present study, cholinergic receptor agonist, oxotremorine (50 micrograms/kg, i.v.) increased mean arterial pressure (from 22 +/- 1 to 123 +/- 3 mm Hg) and 60 min-survival rate (from 0% to 92%) in rats bled to hypovolemic shock. Atropine (2 mg/kg, i.v.) pretreatment inhibited the pressor effect of oxotremorine significantly, but did not modify its effect on survival rate. On the other hand, pretreatment with mecamylamine (50 micrograms, i.c.v.) almost abolished the reduction in mortality rate, but inhibited the pressor effect of oxotremorine, partially. These results indicate that oxotremorine-induced pressor response and decrease in mortality in rats with severe hemorrhagic shock are primarily mediated via central muscarinic and nicotinic receptors, respectively. AV3V region was previously reported to be involved in pressor and natriuretic effects of i.c.v. carbachol in normotensive rats. In the present study, the electrolytic lesions of AV3V region significantly inhibited oxotremorine-induced increases in both blood pressure and survival rate in rats subjected to hemorrhagic shock. These findings indicate that AV3V region plays a major role in cholinergic cardiovascular control in hypotensive animals as well as normotensives.

Acetylcholine release in the hippocampus: effects of cholinergic and GABAergic compounds in the medial septal area

The medial septal area (MSA) contains cholinergic and GABAergic neurons that send projections to the hippocampus. These neurons have both cholinergic and GABAergic receptors. This study was designed to determine the effects of intraseptal infusions of cholinergic and GABAergic drugs, which alter mnemonic processes, on hippocampal acetylcholine (ACh) release. Hippocampal ACh release was assessed using in vivo microdialysis and HPLC-EC. Oxotremorine and scopolamine produced a dose-dependent decrease in hippocampal ACh release. Muscimol decreased hippocampal ACh release at both high and low doses, although not in a dose-dependent manner. The effects of scopolamine and muscimol are consistent with a role of ACh in mnemonic processing.

Activation of the medial septal area attenuates LTP of the lateral perforant path and enhances heterosynaptic LTD of the medial perforant path in aged rats

Age-related memory impairments may be due to dysfunction of the septohippocampal system. The medial septal area (MSA) provides the major cholinergic projection to the hippocampus and is critical for memory. Knowledge of the neurobiological mechanisms by which the cholinergic system can attenuate age-related memory loss can facilitate the development of effective cognitive enhancers. At present, one of the best neurobiological models of memory formation is long-term potentiation/long-term depression (LTP/LTD). In previous studies, intraseptal infusion of the muscarinic agonist oxotremorine, which excites MSA neurons, improved memory in aged rats. The present study examined LTP and LTD in aged Fisher 344 rats following intraseptal infusion of oxotremorine. LTP and LTD were assessed using the slope of the EPSP recorded from the hilar region of the dentate gyrus. Induction of LTP was blocked in the lateral perforant path, but not in the medial perforant path, following intraseptal infusions of oxotremorine. The generation and amplitude of heterosynaptic LTD was enhanced in the medial perforant path, but not in the lateral perforant path. The results provide evidence that pharmacological activation of the MSA can modulate LTP and LTD in the hippocampus of aged rats. The implications of these results with respect to memory and synaptic plasticity in the hippocampus are discussed.

Effect on vasopressin release of microinjection of cholinergic agonists into the rat supraoptic nucleus

It is likely that central cholinergic pathways to the paraventricular and supraoptic nuclei participate in the control of vasopressin release. We have shown previously that this is due, in part, to activation of muscarinic, but not nicotinic, receptors in the paraventricular nucleus. There is, however, reason to believe that this cholinergic effect in the supraoptic nucleus may be the result of activation of nicotinic receptors. To test this possibility, we have studied in conscious unrestrained rats the effect of microinjection of muscarinic and nicotinic agonists into the supraoptic nucleus on vasopressin release, mean arterial blood pressure, and heart rate. Under ether anesthesia, a stainless steel guide cannula was placed in the supraoptic nucleus 5-7 days before the experiment, and femoral, arterial, and venous catheters were implanted 1 day before the experiment. Microinjection of nicotine into the supraoptic nucleus at doses of 1 and 10 micrograms resulted in transient increases in the plasma vasopressin concentration that were 7-fold and 11-fold greater, respectively, than control values at 3 min. There were also small transient increases in mean arterial blood pressure, but heart rate was unchanged. The microinjection of 2 and 20 ng of oxotremorine, a muscarinic agonist, into the supraoptic nucleus had no effect on the plasma vasopressin concentration, mean arterial blood pressure, or heart rate. These doses of oxotremorine were previously shown to have potent stimulatory effects on vasopressin release when microinjected into the paraventricular nucleus. These findings suggest that the central cholinergic stimulation of vasopressin release is due, in part, to activation of muscarinic receptors in the paraventricular nucleus and nicotinic receptors in the supraoptic nucleus.

The pretectal cholinergic system is involved through two opposite ways in frog monocular OKN asymmetry

Frog monocular horizontal optokinetic nystagmus (OKN) has been studied by coil recordings, before and after unilateral microinjection of cholinergic drugs into the pretectum. The recorded eye was either contralateral or ipsilateral to the injected structure. Before injection, monocular OKN displayed a directional asymmetry, reacting only to stimulations in the temporonasal (T-N) direction. The intrapretectal administration of a cholinergic muscarinic agonist (oxotremorine), as well as that of a nicotinic antagonist (D-tubocurarine), abolished the monocular OKN asymmetry, inducing the appearance of the naso-temporal (N-T) component; the difference between the slow phase velocity gain of both components was no longer significant. These data suggest that acetylcholine (ACh), at the level of the pretectum, acts in opposite ways through muscarinic and nicotinic binding sites; monocular OKN asymmetry could result, at least partially, from a facilitating nicotinic effect and an inhibitory muscarinic effect. Possible interactions with other transmitter systems are discussed.

Inhibition of rubral neurons by noxious and non-noxious pressure

The role of the red nucleus (RN) in nociception was investigated in this study. Extracellular recordings from spontaneously active RN neurons were conducted in the rat while noxious pressure was delivered to the hindpaws or tail. Cells in the RN were predominantly inhibited by the stimuli. The units were most responsive when noxious pressure was applied to the contralateral hindpaw. Furthermore, more cells in the magnocellular division of the RN responded to the stimuli than cells in the parvocellular division. Delivery of a graded pressure stimulus to the contralateral hindpaw revealed 4 cell types in the RN: non-responsive cells; cells only responsive during the early, non-noxious portion of the stimulus; cells only responsive during the later, noxious portion of the stimulus; and cells that showed an initial response during the non-noxious part of the stimulus and a second, later response during the noxious portion of the stimulus. To further examine the putative role of the RN in nociception, oxotremorine, gamma-aminobutyric acid (GABA), serotonin, glutamate, and morphine were unilaterally microinjected into the RN and the responses of the animals in the tail flick test were assessed. Only morphine produced a significant antinociception in the animals following intrarubral microinjection. However, it is unclear whether this alteration was mediated through the RN because an antinociception of equal magnitude could be elicited from the reticular formation surrounding the RN and lesions of the RN did not alter the antinociception produced by systemic administration of morphine. Although other explanations cannot be ruled out, it appears that the RN may be involved in coordinating the motor response to pain rather than modulating sensory transmission.

An M3-like muscarinic autoreceptor regulates the in vivo release of acetylcholine in rat striatum

Selective muscarinic antagonists were used in an attempt to characterize the muscarinic autoreceptor modulating the release of acetylcholine in the striatum of the rat. In vivo microdialysis was applied to infuse atropine, 4-DAMP (4-diphenylacetoxy-N-methylpiperidine), pirenzepine or AF-DX 116 (11-[[2-[(diethylamino)methyl]-1-piperidinyl]acetyl]-5, 11-dihydro[2,3-b][1,4]benzodiazepine-6-one), leading to a dose-dependent increase in the overflow of acetylcholine, the order of potency being: atropine greater than 4-DAMP greater than pirenzepine greater than AF-DX 116. We conclude from these data that the muscarinic receptor modulating release in the striatum is of the M3 type.

Differential effects of M1- and M2-muscarinic drugs on striatal dopamine release and metabolism in freely moving rats

A dialysis loop cannula was implanted into rat striatum under anesthetized condition, and the area was perfused with Ringer's solution under freely moving condition after 3 days for surgical recovery. Dopamine (DA) and 3,4-dihydroxyphenylacetic acid recovered in the dialysate were measured by high-performance liquid chromatography with electrochemical detection. The effects of M1- and M2-muscarinic receptor agents, which were perfused continuously into the striatum through the dialysis membrane, were investigated. Continuous perfusion of AF102B, an M1-selective agonist, and oxotremorine, a non-selective agonist, resulted in a dose-dependent increase in the striatal DA release. Pirenzepine (10(-5) and 10(-7) M), an M1-selective antagonist, decreased the release of DA, and the stimulatory effect of AF102B (10(-5) M) was completely inhibited by 10(-5) and 10(-7) M pirenzepine, while the stimulatory effect of oxotremorine (10(-4) M) was only partly inhibited by 10(-5) M pirenzepine. AF-DX116 (10(-5) M), an M2-selective antagonist, increased the DA release, and showed an additive effect on the DA release evoked by AF102B (10(-5) M), whereas it produced no significant effect on oxotremorine (10(-5) M)-evoked DA release. These results suggest that in vivo DA release in the rat striatum is modulated by different subtypes of muscarinic receptors; i.e., the stimulatory effect is mainly mediated by M1-sites and inhibitory effect is mainly mediated by M2-sites. The changes in the DA release induced by the various drugs were prevented by pretreatment with tetrodotoxin (TTX). Since action potential-dependent DA release (exocytosis) is blocked by the pretreatment with TTX, those drugs affect DA release by means of action potential-dependent processes.

[Analysis of sleep-wakefulness structure in parkinsonian syndrome in rats induced by 1-methyl-4-phenyl-1,2,2,6-tetrahydropyridine and oxotremorine]

The structure of the sleep-wakefulness cycle on the white rats with experimental parkinsonian syndrome was investigated, which was induced by administration of MPTP and oxotremorine. The EEG was recorded during 6 hours. The recordings were made 1.5 hours after the administration of the drugs. It was established that MPTP and oxotremorine broke the structure of the sleep-wakefulness cycle almost with complete reduction of paradoxal sleep and the decrease of SWS. The number of sleep spindles was reduced and changed the characteristics of the hippocampal theta rhythms. MPTP induces weakness of the phasic compound and strengthens the tonic compound.

Effect of vasopressin antagonist on antidiuresis by oxotremorine microinjected into the hypothalamic supraoptic and paraventricular nuclei in a water-loaded and ethanol-anesthetized rat

Microinjection of the muscarinic agonist oxotremorine into the hypothalamic supraoptic (SON) and paraventricular (PVN) nuclei which contain cell bodies of vasopressinergic neurons induced potent antidiuretic effects in water-loaded and ethanol-anesthetized rats. The effects included both decreases in urine outflow and increases in urine osmotic pressure. However, no significant changes in various visceral functions other than antidiuresis such as mean blood pressure, heart rate, respiration rate and rectal temperature were observed when oxotremorine was microinjected into the SON. Only a slight change in mean blood pressure (approx. 10 mmHg decrease) was observed by the microinjection into the PVN. Intravenous preinjection of a vasopressin (AVP) V1 V2 antagonist that has one of the most potent V2 (antidiuretic)-antagonist activities, d(CH2)5-D-Tyr(Et)VAVP, inhibited nearly completely the antidiuretic effects induced by the microinjection of oxotremorine. The results demonstrated that oxotremorine stimulated muscarinic receptors in the hypothalamic SON and PVN, released AVP and induced an antidiuretic effect through AVP-receptors in the kidney.

Two-drug combinations of memory enhancers: effect of dose ratio upon potency and therapeutic window, in mice

Two-drug combinations have been reported to enhance retention more effectively than when either drug was administered alone at the same dose. Some combinations of cholinergic drugs enhance retention even though the total drug dosage is reduced by as much as 97% compared to the dose needed to improve retention when the same drugs are administered singly. The choice of dose ratio is usually arbitrary or based on empirical results. The present study systematically varied the ratio of two drugs in a combination and at the same time varied the dosage of each drug. The drug combinations were administered to mice immediately after training on T-maze footshock avoidance task. Retention was tested one week later. Three two-drug combinations were selected for presentation because they differed considerably as to (a) the lowest effective total dose that improved memory-retention, (b) the optimal ratio that improved retention and (c) the width of the therapeutic window. The effect of a drug combination on retention was found to be dependent on the particular drugs in the combination, the ratio and the dose administered.

Effects of diazepam on muscarinic acetylcholine receptor binding in vivo and on oxotremorine-induced tremor and hypothermia in mice

Diazepam has previously been shown to affect the acetylcholine synthesizing system in mouse brain. This paper reports studies on the effect of diazepam on muscarinic receptor density and on pharmacological effects of oxotremorine. The receptor density was studied using a new technique that allows such studies to be performed in vivo under physiological conditions. The method is based on the fact that L-hyoscyamine, the active antipode of atropine, binds specifically to muscarinic receptors in the brain, and can be measured with high sensitivity by gas chromatography--mass spectrometry. Diazepam was found to modify the binding properties of muscarinic receptors in CNS, thereby decreasing the functional receptor pool. It also prevented tremor induced by the muscarinic agonist oxotremorine. Diazepam could however not prevent the hypothermia induced, but rather accentuated this effect of oxotremorine. It is concluded that diazepam, directly or indirectly, influences the effect of cholinergic stimulators by modulating the size of the muscarinic receptor pool.

Antidiuretic effects of oxotremorine microinjected into the hypothalamic supraoptic and paraventricular nuclei in a water-loaded and ethanol-anesthetized rat

The effect of oxotremorine, a muscarinic agonist, on urine outflow compared with the effects of other cholinergic agonists and inhibitory effects of cholinergic antagonists upon the cholinergic actions were studied by microinjecting drugs stereotaxically, unilaterally into the supraoptic (SON) or paraventricular nuclei (PVN) in the hypothalamus of the rat which was loaded with water and anesthetized with ethanol. Oxotremorine decreased the urine outflow in dose- and time-dependent manners when microinjected into these nuclei. The median effective doses (ED50) were approx. 0.3 and 0.2 nmol in SON an PVN, respectively, being much less than ED50 values for nicotine. The time course of the antidiuretic effects was relatively slow, with the minimal urine outflow at approx. 30 min and the duration of one or longer hours. The antidiuretic effects of oxotremorine in these nuclei as well as the effects of acetylcholine and nicotine were completely blocked by pretreatment with atropine. The pretreatment with hexamethonium inhibited partially the effects of nicotine, but was unable to inhibit the effects of oxotremorine and acetylcholine. The data suggest that the antidiuretic effects of cholinergic agonists in SON and PVN are predominantly mediated through a muscarinic type of acetylcholine receptor. A possible mechanism for the antidiuretic effects is discussed.

Possible interaction between central cholinergic muscarinic and opioid peptidergic systems during memory consolidation in mice

Naloxone (0.01-1.00 mg/kg, ip) facilitated retention of a one-trial inhibitory avoidance task, when administered to male Rockland mice immediately after training, as indicated by performance on a retention test 48 hr later. The dose-response curve was an inverted U in this range of dose. In these conditions naloxone did not lengthen latencies to step-through during the retest of unshocked mice. Higher doses of naloxone (3.00 and 10.00 mg/kg, ip) tended to increase latencies to step-through of both shocked and unshocked mice. These facts rule out an aversive effect of naloxone for low and moderate doses but not for high doses. The influence of naloxone (0.10 mg/kg, ip) on retention was time dependent, which suggests that naloxone facilitated memory consolidation processes. The effects of naloxone were prevented by morphine in both an amnesic and a nonamnesic dose (1.0 and 0.5 mg/kg, ip, respectively). Therefore, naloxone probably facilitated retention as a function of its opiate antagonist properties. The memory facilitation induced by naloxone (0.10 mg/kg, ip) was antagonized by atropine (0.5 mg/kg, ip) but not by methylatropine (0.5 mg/kg, ip), mecamilamine (5 mg/kg, ip), or hexametonium (5 mg/kg, ip). Further, there was a mutual potentiation for both naloxone (0.01 mg/kg, ip) and the muscarinic agonist oxotremorine (6.25 and 12.5 micrograms/kg, ip) administered simultaneously, in doses which had no effect on their own. Moreover, an amnesic dose of atropine (10.00 mg/kg, ip) prevented the enhancement of retention induced by naloxone, while an amnesic dose of morphine (1.00 mg/kg, ip) did not modify the facilitatory effect of oxotremorine (50 micrograms/kg, ip) on retention. An inhibitory modulatory role for endogenous opioid systems on the activity of central cholinergic muscarinic systems during memory consolidation is suggested.

Psychopharmacological profile of diphenylhydantoin in mice

The pharmacological profile of diphenylhydantoin has some common features with that of classical antidepressants: it decreases motor activity in mice it shortens the period of immobility of mice in the "behavioral despair" test it antagonizes palpebral ptosis induced by reserpine in mice.

Quantitation of tolerance development after chronic oxotremorine treatment

A new procedure was developed to quantitate the tolerance which develops as mice are chronically infused with the muscarinic agonist, oxotremorine. Cumulative dose-response curves were constructed for the effects of oxotremorine on body temperature and rotarod performance by administering sequential injections to individual animals. These dose-response curves compare favorably to those constructed by injecting individual animals with one of several doses. The sequential injection technique was used to assess the magnitude of tolerance development to oxotremorine. A linear relationship between oxotremorine infusion rate (dose) and magnitude of change of the ED50 value for impairment of rotarod performance was observed, with animals receiving an infusion rate of 1.0 mg/kg/hr showing a 24-fold increase in ED50. Dose-response curves for tolerant animals were parallel to those constructed for naive animals. The oxotremorine dose required to decrease body temperature to 35 degrees C (ED35 degrees) was 80-fold greater than control in the group treated with 1.0 mg/kg/hr. The dose-response curves for tolerant animals were not parallel to those seen in naive animals. Time courses of recovery from a challenge dose of oxotremorine suggest little change in metabolism occurred during chronic infusion. Chronic oxotremorine infusion resulted in a decrease in the total number of QNB binding sites. Both high- and low-affinity sites were reduced in number. Since no change in K1 for the muscarinic agonist, carbamylcholine, was observed, it seems unlikely that a change occurs in the affinity of the muscarinic receptor for agonists. Significant change in receptor number was detected only in animals that received higher doses of oxotremorine. Chronic oxotremorine treatment had no effect on choline uptake by synaptosomes prepared from any of five brain regions.

[Effect of kynurenines on the actions of acetylcholine, oxotremorine, and nicotine]

Kynurenine, 3-hydroxyanthranilic, anthranilic and nicotinic acids at concentrations 10(-4) and 10(-5) M potentiated contractions of the isolated rat rectum produced by acetylcholine. Contractile response of the phrenico-diaphragmatic preparation of young rats to acetylcholine was diminished by kynurenine, 3-hydroxykynurenine and by quinolinic acid. In rats of both sexes, 3-hydroxyanthranilic, anthranilic, picolinic and nicotinic acids in doses of 10 and 20 mg/kg shortened the latency of oxotremorine tremor. Quinolinic and nicotinic acids reduced the latency of nicotinic tremor and potentiated hypothermia.

Brain catecholamines modifications. The effects on memory facilitation induced by oxotremorine in mice

the immediate posttrial injection of oxotremorine (0.250 mu Mol/kg, IP) can facilitate the retention of a passive-avoidance response in mice. After the administration of alfa-methyl-p-tyrosine methylester (alpha-MPT) by intracerebroventricular injection at doses that had no effect on retention (100 microgram, 10 microliter, 60 min before trial), the immediate posttrial injection of oxotremorine did not enhance retention. The employed dose of alpha-MPT reduced brain levels of norepinephrine by about 40% and those of dopamine by about 25%. Pretreatment with nialamide (30 mg/kg, 20h IP), which prevents the catecholamine depletion induced by alpha-MPT, counteracted the effects depletion induced by alpha-MPT, counteracted the effects of alpha-MPT on the actions of oxotremorine on retention. These results suggest a participation of brain catecholamines on the actions of oxotremorine on retention and a possible interaction of cholinergic neurons with catecholaminergic system in memory processes.

Influence of post-training intrahippocampally applied oxotremorine on the consolidation of a brightness discrimination

The posttraining intrahippocampal injection of oxotremorine revealed an improvement of the retention performance in a brightness discrimination task. The oxotremorine effect seemed to be dependent on distinct variables of training and was restricted to rats exhibiting a good acquisition performance. Scopolamine impaired the retention performance of animals with few training errors. The role of hippocampal cholinergic synapses for consolidation was discussed.

Application of principles of steady-state kinetics to the estimation of brain acetylcholine turnover rate: effects of oxotremorine and physostigmine

We have measured the turnover rate of acetylcholine (ACh) in the brains of mice injected with doses of oxotremorine and physostigmine that cause a prolonged increase of ACh concentration in brain. The method used to measure turnover rate of ACh is an application of principles of steady-state kinetics to the change with time of brain choline (Ch) and ACh specific radioactivities after an intravenous pulse injection of phosphorylcholine. We have found that when the concentration of brain ACh and Ch is increased to a new steady state as a result of oxotremorine and physostigmine injections the turnover rate of brain ACh decreases from 0.34 mumol/g/hr (in saline-treated mice) to 0.12 and 0.061 mumol/g/hr, respectively. The possibility that an increase of brain Ch or ACh concentrations plays a role in the control of brain ACh turnover rate is discussed.

Drugs affecting the cholinergic system in the intact mammalian eye. I. Evaluation of the miotic activity of acetylcholine-like drugs in the mouse eye

The effect of concentration and pH on the miotic activity of five acetylcholine-like drugs were studied in the intact mouse eye. The data suggests that it is mainly the nonionized form of the drug which contributes to its miotic activity. Acetylcholine-like drugs, degradable by cholinesterases, exert their miotic activity at higher concentrations than do those which are resistant to enzymatic hydrolysis. More active acetylcholine-like drugs appear to be less sensitive to concentration changes.