Chronic scopolamine treatment and brain cholinergic function

The Impact of Some Modulators of the Renin-Angiotensin System on the Scopolamine-Induced Memory Loss Mice Model

As some of the renin-angiotensin-aldosterone system (RAAS)-dependent mechanisms underlying the cognitive performance modulation could include oxidative balance alterations, in this study we aimed to describe some of the potential interactions between RAAS modulators (Losartan and Ramipril) and oxidative stress in a typical model of memory impairment. In this study, 48 white male Swiss mice were divided into six groups and received RAAS modulators (oral administration Ramipril 4 mg/kg, Losartan 20 mg/kg) and a muscarinic receptors inhibitor (intraperitoneal injection scopolamine, 0.5 mg/kg) for 8 consecutive days. Then, 24 h after the last administration, the animals were euthanized and whole blood and brain tissues were collected. Biological samples were then processed, and biochemical analysis was carried out to assess superoxide dismutase and glutathione activities and malondialdehyde concentrations. In the present experimental conditions, we showed that RAAS modulation via the angiotensin-converting enzyme inhibition (Ramipril) and via the angiotensin II receptor blockage (Losartan) chronic treatments could lead to oxidative stress modulation in a non-selective muscarinic receptors blocker (scopolamine) animal model. Our results showed that Losartan could exhibit a significant systemic antioxidant potential partly preventing the negative oxidative effects of scopolamine and a brain antioxidant potential, mainly by inhibiting the oxidative-stress-mediated cellular damage and apoptosis. Ramipril could also minimize the oxidative-mediated damage to the lipid components of brain tissue resulting from scopolamine administration. Both blood serum and brain changes in oxidative stress status were observed following 8-day treatments with Ramipril, Losartan, scopolamine, and combinations. While the serum oxidative stress modulation observed in this study could suggest the potential effect of RAAS modulation and scopolamine administration on the circulatory system, blood vessels endothelia, and arterial tension modulation, the observed brain tissues oxidative stress modulation could lead to important information on the complex interaction between renin-angiotensin and cholinergic systems.

Differential effects of post-training scopolamine on spatial and non-spatial learning tasks in mice

Muscarinic antagonist scopolamine has been extensively used to model amnesia in lab rodents, but most studies have focused on the effects of pre-training scopolamine administration. Here, we examined post-training scopolamine administration in C57BL/6JRj mice. Learning was assessed in three different procedures: odour discrimination in a digging paradigm, visual discrimination in a touchscreen-based setup, and spatial learning in the Morris water maze. Scopolamine administration affected performance in the odour discrimination task. More specifically, scopolamine decreased perseverance, which facilitated reversal learning. Similar results were obtained in the visual discrimination task, but scopolamine did not affect performance in the spatial learning task. It is unlikely that these results can be explained by non-memory-related cognitive effects (e.g., attention), non-cognitive behaviours (e.g., locomotor activity) or peripheral side-effects (e.g., mydriasis). They likely relate to the various neuropharmacological actions of scopolamine.

A one-week 5-choice serial reaction time task to measure impulsivity and attention in adult and adolescent mice

Many psychiatric disorders emerge during adolescence. The study of executive functions in animal models of these disorders critically requires short-duration tasks measuring these functions before the animal ages. Here, a novel 5-choice serial reaction time task (5-CSRTT) protocol is presented, to measure attention and impulsivity within one week, without scheduled food deprivation and with little animal handling. Mice were allowed 24-h/day task access from their home-cage, during which they could self-pace task progression and earn unlimited food rewards depending on task performance. Manipulation of task parameters in this self-paced 5-CSRTT protocol (SP-5C) affected attentional performance and impulsivity to a similar extent as previously observed in the 5-CSRTT. Task activity followed intrinsic circadian rhythm, distinctive for the SP-5C protocol, with task performance stable over the day. The sensitivity of the SP-5C protocol to detect strain differences between C57BL/6J, DBA/2 J, BXD16 and BXD62 mice was demonstrated as well as its suitability for testing adolescent mice. Acute administration of the muscarinic acetylcholine receptor antagonist scopolamine impaired attentional performance, providing initial pharmacological validation of the task. The SP-5C substantially shortens the assessment of impulsivity and attention, increases test efficiency and enables the assessment of adolescent mouse models of psychiatric disorders.

Unique Scopolamine Withdrawal Syndrome After Standard Transdermal Use

We report the case of a 62-year-old woman who developed a withdrawal syndrome after using a standard 1.5-mg transdermal scopolamine (TDS) patch behind the ear to prevent motion sickness during sailing. The patient, who had used TDS occasionally for years without significant adverse effects, more recently, having worn a patch continuously for 7 days, approximately 24 to 36 hours after removing the patch developed dizziness, nausea, sweating, fatigue, and drowsiness. All symptoms disappeared without therapy in about 2 days. Approximately 1 year after the first episode, though, a very similar, more severe disabling reaction developed on 2 occasions. Drowsiness and malaise were accompanied by severe asthenia, orthostatic sweating, inability to stand, and hypotension. All clinical tests (electrocardiogram; spirometry; blood cell count; plasma levels of cortisol, sodium, and potassium; and liver and kidney function tests) were negative, and symptoms disappeared slowly, after several days. Although we are certain that scopolamine was responsible for the symptoms, we are less clear as to the nature of the disorder. The effects being more severe after a more prolonged use of the TDS patch, the increase in severity each successive time, and the time lag between removing the patch and appearance of symptoms all indicated a withdrawal syndrome for which several mechanisms may be suggested.

Reverse translation of the rodent 5C-CPT reveals that the impaired attention of people with schizophrenia is similar to scopolamine-induced deficits in mice

Attentional dysfunction in schizophrenia (SZ) is a core deficit that contributes to multiple cognitive deficits and the resulting functional disability. However, developing procognitive therapeutics for neuropsychiatric disorders have been limited by a 'translational gap'--a lack of cognitive paradigms having cross-species translational validity and relevance. The present study was designed to perform an initial validation of the cross-species homology of the 5-choice Continuous Performance Test (5C-CPT) in healthy nonpsychiatric comparison subjects (NCS), SZ patients and mice under pharmacologic challenge. The 5C-CPT performance in SZ patients (n=20) was compared with age-matched NCS (n=23). The effects of the general muscarinic receptor antagonist scopolamine on mice (n=21) performing the 5C-CPT were also assessed. SZ subjects exhibited significantly impaired attention in the 5C-CPT, driven by reduced target detection over time and nonsignificantly increased impulsive responding. Similarly, scopolamine significantly impaired attention in mice, driven by reduced target detection and nonsignificantly increased impulsive responding. Scopolamine also negatively affected accuracy and speed of responding in mice, although these measures failed to differentiate SZ vs. NCS. Thus, mice treated with scopolamine exhibited similar impairments in vigilance as seen in SZ, although the differences between the behavioral profiles warrant further study. The availability of rodent and human versions of this paradigm provides an opportunity to: (1) investigate the neuroanatomic, neurochemical and genomic architecture of abnormalities in attention observed in clinical populations such as SZ; (2) develop and refine animal models of cognitive impairments; and (3) improve cross-species translational testing for the development of treatments for these impairments.

Anxiolytic effects of environmental enrichment attenuate sex-related anxiogenic effects of scopolamine in rats

In groups of four same-sexed animals, PVG/c hooded rats were housed for 4.5 months in standard or enriched cages containing several objects that could be explored and manipulated. On separate occasions, each rat then experienced two consecutive daily trials in an open field, a light-dark box or a Y maze with arm inserts that enabled an acquisition trial comprising one black and one white arm to be changed for a retention trial consisting of two black arms. Before their trials in the open field and light-dark box, and following each acquisition trial in the Y maze, the rats received an intraperitoneal injection of 2 mg/kg scopolamine or isotonic saline. In the open field, enrichment led to higher levels of ambulation, walking, rearing and occupancy of the center of the apparatus and shorter emergence latencies from the dark into the light compartment of the light-dark box accompanied by more entries of this compartment. Enrichment also increased entries of and time spent in the changed (or novel) Y-maze arm only for male rats treated with scopolamine. The drug decreased rearing and increased grooming in the open field as well as increasing emergence latencies and decreasing entries of and the time spent on the light compartment of the light-dark box. The main results were interpreted as enrichment having attenuated anxiogenic effects of the behavioral testing and the action of scopolamine for male (but not female) rats in their choices of the novel arm in the Y maze.

Muscarinic acetylcholine receptor inhibition in transgenic Alzheimer-like Tg2576 mice by scopolamine favours the amyloidogenic route of processing of amyloid precursor protein

The molecular mechanisms of the interrelationship between cholinergic neurotransmission, processing of amyloid precursor protein (APP) and beta-amyloid (Abeta) production in vivo are still less understood. To reveal any effect of cholinergic dysfunction on APP processing in vivo, 11-month-old transgenic Tg2576 mice with Abeta plaque pathology received intraperitoneal injections of scopolamine at a daily dosage of 2mg/kg body weight for 14 days in order to suppress cortical cholinergic transmission by chronic inhibition of muscarinic acetylcholine receptors. Scopolamine treatment of transgenic Tg2576 mice resulted in increased levels of fibrillar Abeta(1-40) and Abeta(1-42), while the soluble, SDS-extractable Abeta level remained unchanged as compared to vehicle-injected Tg2576 mice. alpha-Secretase activity determined in cortical tissue from scopolamine-treated Tg2576 mice was lower by about 30% as compared to that assayed in control mice, while beta-secretase activity and BACE1 protein expression appeared unaffected by scopolamine treatment. The amount of sAPPalpha, the product secreted by alpha-secretase-mediated APP cleavage, and the unprocessed APP were assayed in the soluble and membrane fraction, respectively, of cortical tissue preparations from treated and control mice by Western blotting. Using the anti antibody 6E10 which specifically labels human sAPPalpha and full length APP in transgenic Tg2576, an enhanced APP level was detected in the membrane fraction from treated mice as compared to controls, while in the soluble fraction scopolamine treatment did not affect the protein level of sAPPalpha. These data indicate an accumulation of APP in cortical membrane fraction in scopolamine-treated Tg2576 mice presumably due to the decreased level of alpha-secretase-mediated APP cleavage, and further suggest that chronic suppression of cortical muscarinic cholinergic transmission may alter the balance between alpha- and beta-secretory APP processing by favouring the amyloidogenic route.

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.

Time-related effects of stress on cholinergic sensitivity

The effect of the administration of the muscarinic cholinergic agonist oxotremorine on locomotor activity was investigated in DBA/2 mice subjected to chronic restraint stress of different durations (120 min daily for 10, 14 or 18 days). Oxotremorine induced a depressant effect on locomotion, which was reduced after 10 and 14 days of restraint, but not after a 18-day restraint stress. Acetylcholine (ACh) content was significantly reduced in prefrontal cortex after 10 and 14 days of stress but returned to control values after 18 days of restraint. No changes in ACh content were observed in nucleus accumbens and striatum. These results are discussed in terms of possible changes in muscarinic receptor sensitivity.

Involvement of muscarinic acetylcholine receptors in regulation of kitten visual cortex plasticity

Receptor autoradiographic methods specific for M3 and M1 muscarinic acetylcholine receptors were used to investigate the development and input-dependent laminar redistribution of these receptor populations during the critical period for kitten visual cortex plasticity. Analysis of the binding curves of [3H]4-diphenylacetyl-N-methyl-piperidine (4-DAMP) and [3H]pirenzepine (PZ) indicated that these two ligands bound heterogeneously to muscarinic acetylcholine receptors with different affinities. While [3H]4-DAMP showed a high affinity for M3 receptors and much lower affinities for M1 and M2 receptors, [3H]PZ displayed higher affinity for M1 receptors. By carefully choosing concentrations of labelled and unlabeled ligands, the patterns of laminar distribution for both receptor subtypes within visual cortex were obtained. Both receptors were most concentrated in cortical layer IV immediately after birth and during the most sensitive period of visual cortex plasticity. The binding density for both receptor subtypes thinned out progressively in this layer to concentrate in more superficial layers as plasticity waned with age. Moreover, interruption of visual or spontaneous input to visual cortex induced either by lesion or by tetrodotoxin infusion into lateral geniculate nucleus prevented the developmental redistribution of these receptors from layer IV to superficial layers, that is, the pattern of laminar distribution remained that of the age at which the lesion or tetrodotoxin infusion into the lateral geniculate nucleus was performed. The results indicate that the developmental expression of M3 and M1 muscarinic acetylcholine receptors in kitten visual cortex depends on cortical inputs.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased CSF HVA response to arecoline challenge in Alzheimer's disease

The effects of the muscarinic agonist, arecoline, on the concentration of homovanillic acid (HVA) in the cerebrospinal fluid of patients with Alzheimer's disease (AD) and controls were examined. Patients and controls received intravenous infusions of arecoline and a lumbar puncture was performed four hours after the infusion began. Arecoline induced a significant increase in the concentration of HVA in cerebrospinal fluid of Alzheimer's disease patients (p < .01) but not in controls. The differential HVA response to a muscarinic agonist in Alzheimer's disease is suggestive of an alteration in muscarinic receptor response. This finding may have potential implications for the pathophysiology and treatment of Alzheimer's disease.

Chronic treatment with scopolamine and physostigmine changes nerve growth factor (NGF) receptor density and NGF content in rat brain

Nerve growth factor (NGF) and NGF receptors were measured in cortex and hippocampus of rats treated with drugs affecting cholinergic neurotransmission. High (Kd = 0.045 nM) and low (Kd = 21 nM) affinity 125I-NGF binding sites were present in both cortical and hippocampal membranes with hippocampus containing higher numbers of both sites than cortex. Chronic treatment of rats with the muscarinic receptor antagonist scopolamine (5 mg/kg, twice daily) decreased the density of high- and low-affinity sites by 50-90% in cortical and hippocampal membranes. These changes were seen after 7 days, but not 3 days, of scopolamine treatment. Chronic infusion of physostigmine (1 mg/kg/day) using minipumps increased the number of high- and low-affinity sites in cortex 3- and 6-fold, respectively. The changes in receptor-binding parameters induced by physostigmine were transient as they were evident after 3 days of treatment, but returned to control levels after 7 days. NGF content in cortex and hippocampus was reduced by about 50% following 7, but not 3, days of chronic physostigmine infusion. In contrast, scopolamine treatment failed to change NGF levels in the cholinergic neuronal target regions but it decreased NGF content in the septal area. The content of NGF mRNA in the cortex measured by Northern blot analysis failed to change following either scopolamine or physostigmine treatment. The results suggest that the levels of NGF and NGF receptors in the target regions of cholinergic neurons are regulated by the extent of cholinergic neurotransmitter activity.

Difference in scopolamine sensitivity during ageing in rats: dichotomy between behavioural repertoire and learning performances

The effects of different doses of scopolamine in adult and aged rats were compared in two behavioural experiments. The first experiment involved visual reversal learning, while the second experiment investigated individual and social behavioural patterns. Results showed that the same dose of scopolamine was more effective in impairing the reverse discrimination in aged than in adult rats. In contrast, low doses which were effective in modifying behavioural patterns in adults, were inactive in aged rats. This difference in scopolamine sensitivity (supersensitivity versus hyposensitivity), observed between the two types of behaviour, is discussed in relation to age-related functional receptor adaptation, selective changes in muscarinic receptors in the different brain areas and dopaminergic- cholinergic interactions.

Muscarinic antagonist enhances one-trial food-aversion learning in the mollusc Pleurobranchaea

One hour before training, in two replicate studies on the sea slug Pleurobranchaea californica, all animals (N = 114) received body-cavity injections of scopolamine, oxotremorine, or the equivalent volume of the saline/seawater vehicle that was used to inject the drugs. The low drug doses (2 mumol/kg) were near the threshold for generating observable neurophysiological responses, but did not affect feeding thresholds arising to a stimulus derived from beer (Sbr) and to one derived from squid (Ssq). Before training, the animals did not discriminate between Sbr and Ssq, as indicated by similar thresholds to both stimuli. During training, experimental animals in each injection group received Sbr alone for 10 sec and then paired with electric shocks for 50 sec; control animals received shocks 1 hr after Sbr. Postconditioning tests began 12 hr after training and were repeated once daily thereafter. After training, all experimental groups exhibited the expected aversive behavior to Sbr, as indicated by 100- to 1000-fold increases in feeding thresholds, and retained low thresholds to Ssq, but the scopolamine animals were better able to discriminate between Sbr and Ssq than either of the other experimental groups. The aversive responses to Sbr increased over a 3-day period, but there appeared to be no difference between injection groups on such a long-duration "consolidation" phase. Of the control groups, only the scopolamine animals exhibited low feeding thresholds to both Sbr and Ssq. The other control groups exhibited similar behavior as the experimental animals, indicating that associative factors relating to the 1-hr separation between Sbr and shocks may have produced the behavior in these control animals. Thus, by comparison to the other injections, scopolamine 1) increased the ability of the experimental animals to make the discrimination between Sbr and Ssq, and 2) prevented learning to avoid Sbr in the control animals. An accompanying paper provides a detailed characterization of muscarinic receptor pharmacology in Pleurobranchaea.

Genetic and pharmacological models of cholinergic supersensitivity and affective disorders

Increased muscarinic sensitivity has been associated with altered hormonal states (hypothyroidism and hyperadrenocorticism), chronic administration of muscarinic antagonists or antidepressants with muscarinic actions, selective breeding for anticholinesterase sensitivity, and certain inbred strains of rats and mice. Thus, both genetic and environmental factors may influence muscarinic receptor sensitivity. The reasonably detailed studies on the selectively-bred rats have revealed that the Flinders Sensitive Line (FSL) rats weigh less, are less active, are more sensitive to muscarinic agonists and to stressors, and have higher concentrations of hippocampal and striatal muscarinic receptors than 'normal', or the selectively-bred, Flinders Resistant Line (FRL) rats. Thus, there are a number of parallels between FSL rats and depressed humans. The FSL rats may be the first animal model of depression to mimic the actual trait of depression, and not just the state.

Genetic influences on tolerance development with chronic oxotremorine infusion

Mice of four inbred strains (BALB, C57BL, DBA and C3H) were administered either saline or oxotremorine, a muscarinic agonist, at a dose of 0.5 mg/kg/hr by constant infusion through cannulas implanted in the right jugular veins. Chronic treatment resulted in the development of tolerance to the effects of oxotremorine both on rotarod performance and on body temperature. For drug-treated BALB mice, the dose-response curves for both measures were parallel to those for saline-treated mice, while for DBA and C3H mice the slopes of the dose-response curves were significantly less for treated mice than they were for controls. The equi-effective doses for the drug-treated animals were at least 8-fold greater than those for saline-treated mice. Drug treatment resulted in a significant decrease in the total number of muscarinic receptors in cortex as measured by the binding of [3H]quinuclidinyl benzilate (QNB) without effect on the KD for this ligand. Similarly, drug treatment did not affect the affinity of carbamylcholine as an inhibitor of QNB binding, but did significantly decrease the levels of both the high- and low-affinity agonist binding sites in cortex. The number of M1 muscarinic receptors measured by high affinity [3H]pirenzepine (PZ) binding was also significantly decreased in cortex without effect on the KD. The experiments were extended to five other brain regions. Full saturation curves were not constructed, however. Oxotremorine treatment significantly reduced QNB binding in every brain region. While the binding to agonist affinity states measured by carbamylcholine inhibition of QNB binding and M1 receptor levels measured by high affinity PZ binding tended to decrease with oxotremorine treatment not all changes were statistically significant. The changes in muscarinic receptor subtype levels induced by oxotremorine infusion did not differ among the strains. The results demonstrate that chronic treatment with a muscarinic agonist results in substantial tolerance to the effects of the drug in all four mouse strains. Although some differences in tolerance development exist, these differences are not readily explained by differences in the number or affinity states of brain muscarinic receptors.

Cholinergic agonists suppress play fighting in juvenile rats

Previous research has suggested that acetylcholine might activate play fighting in juvenile rats through its actions on central muscarinic receptors. To test this hypothesis we evaluated the effects on play fighting by the muscarinic agonists pilocarpine and arecoline given alone or in combination with the muscarinic antagonists scopolamine or methylscopolamine. Scopolamine, but not methylscopolamine which penetrates the brain poorly, suppressed play as indexed by frequency of pinning. Pilocarpine and arecoline also suppressed pinning at higher doses. Concurrent treatment with various agonist-antagonist dose combinations produced additive rather than counteractive effects. These data do not support the supposition that central muscarinic circuits are involved in the activation of play fighting.

Prospective strategies for cholinergic interventions in Alzheimer's disease

The cholinergic hypothesis of memory dysfunction has guided most of the recent proposals for treating the primary symptoms of AD. The efficacy of these treatments has been severely limited. This review examines two major lines of evidence which suggest that the cholinergic hypothesis may have to be expanded and revised. The cholinergic hypothesis focuses on pre-synaptic defects. It assumes cholinoceptive neurons would function normally with adequate stimulation. Evidence is not sufficient to support this assumption. In addition, dissociations have been demonstrated between muscarinic receptor number and functional response of cholinoceptive neurons. Various measures are proposed to investigate the functional integrity of muscarinic receptors in AD patients. AD often has been characterized as a disorder produced by generalized cholinergic hypoactivity. Evidence for cortisol hypersecretion, abnormal dexamethasone suppression, and the occurrence of depressive symptoms, motoric dysfunction and sleep abnormalities in AD patients is more consistent with regional cholinergic hyperactivity than generalized hypoactivity. Resolution of these discrepancies could shed new light on the pathophysiology and treatment strategies for AD. Cholinoceptive neurons could be hypersensitive, subsensitive or have unaltered responsivity. These options would have very different treatment implications. New developments in outcome assessment which are capable of discriminating varieties of differential response to treatment can spur treatment development and improve quality of care for patients with complex disorders such as AD.

Reduced muscarinic receptor plasticity in frontal cortex of aged rats after chronic administration of cholinergic drugs

Age-related differences in muscarinic receptor plasticity were observed in young, adult and senescent Fischer 344 rats (3, 9 and 27 months old, respectively) following the chronic, intracerebroventricular (ivt) administration of a cholinergic agonist, oxotremorine, or antagonist, methylatropine. After three weeks treatment of young rats with ivt oxotremorine, the maximum number (Bmax) of 3H-QNB binding sites in frontal cortex, determined by saturation experiments, was reduced by 27%, with no apparent change in the affinity (Kd) of 3H-QNB for the muscarinic receptor. Conversely, chronic ivt methylatropine administered to 3 month old animals caused a 29% increase in Bmax with no significant change in Kd. Adult animals showed a somewhat lesser degree of muscarinic receptor plasticity (16% down-regulation after oxotremorine, 22% up-regulation after methylatropine). However, 3H-QNB binding parameters in frontal cortex from senescent rats were not significantly altered following identical treatments with oxotremorine or methylatropine. Thus, muscarinic receptor adaptation to chronic, cholinergic drug administration was impaired in aged animals. This reduced receptor plasticity with aging could have important implications for the long-term drug treatment of elderly patients and for the therapeutic efficacy of cholinergic drugs in age-related neurological disorders, such as Alzheimer's disease.

Effect of chronic lithium on cholinergically mediated responses and [3H]QNB binding in rat brain

Lithium (Li) has been previously reported to increase acetylcholine turnover and release in rat brain and to potentiate the neurotoxicity of cholinergic agents. We studied the effect of chronic Li administration, alone and in combination with the muscarinic antagonist, scopolamine, on two cholinergically-mediated responses and on muscarinic cholinergic receptor (MCR) binding in rat brain. Administered separately, Li and scopolamine enhanced the cataleptic and hypothermic responses to pilocarpine; combined administration resulted in an additive effect on both these measures. [3H]Quinuclidinyl benzilate ([3H]QNB) binding was increased by Li in the corpus striatum but not in the cortex, hippocampus and hypothalamus. Scopolamine increased [3H]QNB binding in the striatum, cortex and hippocampus; Li and scopolamine effects on striatal MCR were not additive. Contrary to a previous report, antagonist-induced MCR supersensitivity was not prevented by concurrent Li administration in any of the brain areas studied. The additive effect of Li and scopolamine on pilocarpine-induced catalepsy and a trend in this direction for pilocarpine-induced hypothermia suggest that the actions of the two agents to enhance cholinergically mediated responses may be achieved by different mechanisms. Supersensitive responses following scopolamine may be attributed to antagonist-induced up-regulation of postsynaptic muscarinic receptors as demonstrated in the binding studies. The effects of Li to enhance cholinergically-mediated catalepsy and hypothermia are interpreted as extending previous reports that Li stimulates brain cholinergic function by a presynaptic increase in acetylcholine turnover and release.