Behavioural plasticity and the cholinergic system

Regional Distribution of Copper, Zinc and Iron in Brain of Wistar Rat Model for Non-Wilsonian Brain Copper Toxicosis

In previous studies, we have reported first in vivo evidence of copper deposition in the choroid plexus, cognitive impairments, astrocytes swelling (Alzheimer type II cells) and astrogliosis (increase in number of astrocytes), and degenerated neurons coupled with significant increase in the hippocampus copper and zinc content in copper-intoxicated Wistar rats. Nonetheless, hippocampus iron levels were not affected by chronic copper-intoxication. Notwithstanding information on distribution of copper, zinc and iron status in different regions of brain due to chronic copper exposure remains fragmentary. In continuation with our previous study, the aim of this study was to investigate the effects of intraperitoneally injected copper lactate (0.15 mg Cu/100 g body weight) daily for 90 days on copper, zinc and iron levels in different regions of the brain using atomic absorption spectrophotometry. Copper-intoxicated group showed significantly increased cortex, cerebellum and striatum copper content (76, 46.8 and 80.7 % increase, respectively) compared to control group. However, non-significant changes were observed for the zinc and iron content in cortex, cerebellum and striatum due to chronic copper exposure. In conclusion, the current study demonstrates that chronic copper toxicity causes differential copper buildup in cortex, cerebellum and striatum region of central nervous system of male Wistar rats; signifying the critical requirement to discretely evaluate the effect of copper neurotoxicity in different brain regions, and ensuing neuropathological and cognitive dysfunctions.

Ameliorative effects of vitamin C on short-term sensorimotor and cognitive changes induced by acute chlorpyrifos exposure in Wistar rats

Human and experimental animal studies have shown long- and short-term neurological sequelae following acute organophosphate (OP) exposure. Although the main molecular mechanism of OP neurotoxiicty involves acetylcholinesterase (AChE) inhibition, studies have also implicated the induction of oxidative stress. The present study was therefore aimed at evaluating the effect of acute chlorpyrifos (CPF) exposure on short-term sensorimotor and cognitive changes in Wistar rats, the role of brain lipoperoxidative changes and the effect of pretreatment with vitamin C. Twenty-eight rats divided into four groups of seven rats in each group served as subjects for this study. Rats in group I were given soya oil (2 ml/kg) while those in group II were dosed vitamin C (100 mg/kg). Group III were administered CPF only (42.5 mg/kg ∼50% of LD50), while group IV were pretreated with vitamin C (100 mg/kg) and then exposed to CPF (42.5 mg/kg), 30 min later. The regimens were administered once orally and the animals were examined for clinical signs, death and subjected to periodic neurobehavioral evaluation for motor strength, coordinated gait, neuromuscular coordination, learning and memory. At the end of 8 weeks of evaluation, the rats were sacrificed and the brain tissue evaluated for AChE activity and malonaldehyde (MDA) concentration, as an index of lipoperoxidative changes. The rats administered with CPF showed deficits in motor strength, coordinated gaits, neuromuscular coordination, learning and memory, slight decrease in AChE activity and an increase in brain MDA concentration. Pretreatment with vitamin C improved the neurobehavioral deficits and AChE activity, and caused a significant reduction in MDA concentration. In conclusion, the study has demonstrated that acute CPF exposure in Wistar rats caused short-term impairment in sensorimotor and cognitive functions partly due to brain lipoperoxidative changes, which were ameliorated by pretreatment with vitamin C.

Effects of single or repeated dermal exposure to methyl parathion on behavior and blood cholinesterase activity in rats

The effects of a single or repeated dermal administration of methyl parathion on motor function, learning and memory were investigated in adult female rats and correlated with blood cholinesterase activity. Exposure to a single dose of 50 mg/kg methyl parathion (75% of the dermal LD(50)) resulted in an 88% inhibition of blood cholinesterase activity and was associated with severe acute toxicity. Spontaneous locomotor activity and neuromuscular coordination were also depressed. Rats treated with a lower dose of methyl parathion, i.e. 6.25 or 12.5 mg/kg, displayed minimal signs of acute toxicity. Blood cholinesterase activity and motor function, however, were depressed initially but recovered fully within 1-3 weeks. There were no delayed effects of a single dose of methyl parathion on learning acquisition or memory as assessed by a step-down inhibitory avoidance learning task. Repeated treatment with 1 mg/kg/day methyl parathion resulted in a 50% inhibition of blood cholinesterase activity. A decrease in locomotor activity and impairment of memory were also observed after 28 days of repeated treatment. Thus, a single dermal exposure of rats to doses of methyl parathion which are lower than those that elicit acute toxicity can cause decrements in both cholinesterase activity and motor function which are reversible. In contrast, repeated low-dose dermal treatment results in a sustained inhibition of cholinesterase activity and impairment of both motor function and memory.

Dose- and time-dependent scopolamine-induced recovery of an inhibitory avoidance response after its extinction in rats

The present investigation was aimed at elucidating the dose and time dependency of scopolamine-induced recovery of inhibitory avoidance after its extinction. Two experiments were conducted: in the first, we analyzed the effects of four doses (1, 2, 4, and 8 mg/kg) of the musacrinic receptor antagonist scopolamine, on the expression of this conditioned response once it had been extinguished. Independent groups of rats were trained in a one-trial, step-through inhibitory avoidance task and submitted to daily retention (extinction) tests. After extinction had occurred, animals were injected intraperitoneally 10 min before retention testing, either with saline or scopolamine. Results show that scopolamine produced a dose-dependent recovery of the avoidance response. The second experiment was carried out in the same animals, which were now tested for retention of inhibitory avoidance at 1, 2, 3, 6, and 9 months after completion of the first experiment. All rats received counterbalanced injections of saline or scopolamine 10 min before testing at each time interval. Reliable recovery of the avoidance response was observed at the 1-month interval with a clear dose dependency while, after the second month, only the groups treated with the two higher doses continued responding. The results indicate that recovery of the extinguished response produced by muscarinic blockade follows dose- and time-dependent curves, and can be achieved long after a single training session. These data suggest that the inhibitory avoidance memory trace is retained in the brain after behavioural extinction of this response, thus supporting the view of extinction as new learning that affects the retrieval of the original memory, but does not modify its storage.

Effects of central muscarinic blockade on passive avoidance: anterograde amnesia, state dependency, or both?

It was recently reported that administration of relatively high intensities of footshock (overreinforcement) during training of passive avoidance protected animals against the amnesic effect of scopolamine, injected 5 min after training. This was interpreted in terms of a lesser involvement of acetylcholine in memory consolidation. An alternative explanation was that overreinforcement accelerated the consolidation process, which could have taken place before the injection of scopolamine. To test for this possibility, male Wistar rats were injected with 4, 8, or 12 mg/kg of scopolamine, 5 min before training with low or high levels of footshock and then tested for retention of the task. Scopolamine induced the expected memory deficit after the low-intensity footshock; after overreinforcement the higher doses of scopolamine induced state dependency, while no deficits were produced with the lower dose. It was concluded that: (a) acetylcholine is indeed involved in memory consolidation of passive avoidance; (b) scopolamine interacts with high footshock levels to produce state dependency; and (c) when relatively low doses of scopolamine are used in conditions of overreinforcement, protection against scopolamine-induced amnesia becomes evident.

Reversal of extinction by scopolamine

The aim of this experiment was to determine the effects of muscarinic blockade on extinction of passive avoidance conditioning. Rats were trained with a foot shock of 2.5, 3.0, or 6.0 mA and were tested for retention for 8 weeks (once weekly). Five minutes before the seventh test they were injected with 8 mg/kg scopolamine. The groups that had been trained with 2.5 and 3.0 mA showed extinction, which was reversed by the scopolamine; the overreinforced group (6.0 mA) did not show extinction and the scopolamine did not alter the conditioned response. The data support the hypothesis that extinction represents the learning of a new response sustained by a set of cholinergic neurons, different from that which mediated original passive avoidance learning.

Naltrexone pretreatment blocks microwave-induced changes in central cholinergic receptors

Repeated exposure of rats to pulsed, circularly polarized microwaves (2,450-MHz, 2-microseconds pulses at 500 pps, power density 1 mW/cm2, at an averaged, whole-body SAR of 0.6 W/kg) induced biphasic changes in the concentration of muscarinic cholinergic receptors in the central nervous system. An increase in receptor concentration occurred in the hippocampus of rats subjected to ten 45-min sessions of microwave exposure, whereas a decrease in concentration was observed in the frontal cortex and hippocampus of rats exposed to ten 20-min sessions. These findings, which confirm earlier work in the authors' laboratory, were extended to include pretreatment of rats with the narcotic antagonist naltrexone (1 mg/kg, IP) before each session of exposure. The drug treatment blocked the microwave-induced changes in cholinergic receptors in the brain. These data further support the authors' hypothesis that endogenous opioids play a role in the effects of microwaves on central cholinergic systems.

A comparison of the effects of scopolamine and diazepam on acquisition and retention of inhibitory avoidance in mice

Administration of either the muscarinic antagonist scopolamine or the benzodiazepine diazepam prior to training produced a dose-dependent impairment in the retention of one-trial inhibitory avoidance training in mice. To investigate the nature of this drug effect, the effects of scopolamine and diazepam were subsequently assessed on both acquisition and retention of inhibitory avoidance using a multiple-trial, training-to-criterion procedure. The training was conducted using either continuous trials in which the mouse was free to shuttle back and forth between shock and safe compartments or discrete trials in which the mouse was moved from the shock compartment of the safe compartment at the start of each trial. In either case, training continued until the mouse refrained from crossing into the shock compartment for a specified length of time on a single trial. Scopolamine (1.0 mg/kg) administered before training significantly increased the number of trials required to attain criterion, but did not affect retention when these mice were tested 2, 16, or 28 days later. In contrast, diazepam (1.0 mg/kg) did not significantly alter the number of trials necessary to reach criterion, but impaired retention of the inhibitory response in mice trained using discrete trials. The differences in the amnestic effects of scopolamine and diazepam revealed by this detailed analysis suggest that diazepam does not impair inhibitory avoidance performance through an effect on cholinergic function.

Repeated exposure to diisopropylfluorophosphate (DFP) produces increased sensitivity to cholinergic antagonists in discrimination retention and reversal

This experiment examined the effects of repeated exposure to diisopropylfluorophosphate (DFP), an organophosphate anticholinesterase, on the retention and reversal of a visual discrimination and on the number of muscarinic receptors in the brain. Rats were trained in a serial reversal procedure. After achieving stable performance, the rats were divided into two groups. One group received repeated injections of DFP, the other group received injections. To determine whether DFP-treated rats would be more sensitive than normal rats to stresses on the cholinergic system, each rat was injected with saline or one of three doses of scopolamine, a muscarinic receptor blocker, prior to testing on every 6th day. DFP alone caused no impairment in performance. Scopolamine produced a greater impairment in DFP-treated rats than in control rats. Similar results were obtained in a second behavioral task, match-to-sample in a water maze, using the same DFP treatment protocol and only one dose of scopolamine. The number of muscarinic receptors and acetylcholinesterase activity levels were reduced on the 2nd and 15th day after the end of DFP treatment. These results demonstrate that although repeated exposure to organophosphate anticholinesterases may not alter discrimination behavior directly, it may compromise the central nervous system so that it cannot react normally when challenged.

Effects of different doses of galanthamine, a long-acting acetylcholinesterase inhibitor, on memory in mice

The effects of galanthamine, a long-acting acetylcholinesterase inhibitor, on passive avoidance and a modified Morris swim task were studied in mice. Lesions of the nucleus basalis magnocellularis (nBM) produced significant decreases in cortical choline acetyltransferase (ChAT) activity and profound deficits on the 24-h retention of a passive avoidance response and the reversal phase of the swim task. Galanthamine, administered 4 h before testing, improved performance of the two tasks in a dose-dependent fashion. In both tasks, galanthamine produced a U-shaped dose-response curve: the optimal dose was 3.0 mg/kg, IP on passive avoidance and 2.0 mg/kg on the swim task. The improvements in performance were not due to differences in motor activity or sensitivity to electric footshock. Behavioral tolerance did not occur from repeated doses of galanthamine; in fact, prior doses of galanthamine appeared to have a priming effect on later performance. In contrast to the effects in nBM-lesioned mice, galanthamine impaired performance of control mice on both tasks. Several characteristics of galanthamine suggest that it may be effective in treating the central cholinergic deficits in Alzheimer's disease: 1) its ability to attenuate cognitive deficits in nBM-lesioned mice, 2) its relatively long half-life, and 3) its lack of tolerance effects in mice during 2 weeks of repeated dosing.

Repeated noise exposure affects muscarinic cholinergic receptors in the rat brain

We examined the effect of repeated exposure to 100-dB white noise (10 daily 45-min sessions) on muscarinic cholinergic receptors in different regions of the rat brain. Twenty-four hours after the last exposure session, increase in concentration (Bmax) of [3H]quinuclidinyl benzilate ([3H]-QNB) binding sites was observed in the hippocampus, but no significant change was seen in the striatum, frontal cortex, and hypothalamus. No significant effect of noise on receptor binding affinity (Kd) was found. Pretreating the rats with naltrexone (1 mg/kg, IP) before exposure blocked the noise-induced increase in cholinergic receptors in the hippocampus.

Atropine effects on delayed discrimination performance of rats

The effects of atropine sulfate (ATS) and atropine methyl nitrate (ATM) on the conditional discrimination behavior of rats were investigated in eight-hour experimental sessions. Responding of rats was reinforced on either a lighted or a darkened lever depending on whether lights over both levers had been on during the preceding sample portion of the trial. Zero-delay and four-second-delay trials were randomly interspersed. Quality of performance was analyzed using the A' sensitivity measure of signal detection theory. Both drugs reduced both sensitivity and the percentage of trials on which responding occurred (percent response) below saline treatment levels. The two drugs did not reliably differ from each other in their effects on sensitivity during the zero-delay condition, but reliable differences between the two drugs emerged during the four-second-delay condition at doses above 0.8 mg/kg. Percent response recovered more rapidly for animals treated with ATS than responding occurred (percent response) below saline treatment levels. The two drugs did not reliably differ from each other in their effects on sensitivity during the zero-delay condition, but reliable differences between the two drugs emerged during the four-second-delay condition at doses above 0.8 mg/kg.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of an irreversible muscarinic agonist (BM123) on avoidance and spontaneous alternation performance

The present study sought to assess whether the compound N-[4-(2-chloro-ethylmethylamine)-2-butynyl]-2-pyrrolidone (BM123), a potent muscarinic agonist that binds irreversibly to the muscarinic receptor (mAChR), has long-lasting functional effects which may be related to a reduction in functional mAChRs. Passive (inhibitory) avoidance performance, one-way active avoidance learning, and spontaneous alternation behavior were studied in rats. The results confirmed the acute muscarinic stimulating effects of BM123, including tremor, salivation, chromodacryorrhea and hypothermia. In addition, when measured 3-4 days after administration, rats treated with BM123 had disrupted spontaneous alternation performance and tended to have impaired performance for the inhibitory avoidance task with facilitated acquisition of active avoidance. This spectrum of effects is consistent with previous reports showing a 20-40% reduction in mAChRs at these times after BM123. The reversible muscarinic agonist, oxotremorine, was without significant effect. In a further experiment, it was found that pretreatment with methyl atropine did not prevent the disruption of spontaneous alternation behavior by BM123, whereas pretreatment with atropine did. Thus, these long-lasting behavioral effects of BM123 are related to its alkylation of and subsequent reduction in central mAChRs.

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.

Long-term enhancement of evoked potentials in cat somatosensory cortex produced by co-activation of the basal forebrain and cutaneous receptors

Averaged evoked potentials from primary somatosensory cortex (SEPs) were recorded before and after pairing the peripheral stimuli with electrical activation of the basal forebrain (BF) in anesthetized cats. Four pulses at 400 Hz were delivered to the BF 120 ms before each cutaneous stimulus and 10 to 660 such pairings were found to produce an enlargement of the SEP in 10 of 11 animals. The average increase in amplitude of the initial peak of the SEP was 69%. The SEP remained enhanced in five of six animals that were tested an hour or more after the pairing, and in one case the SEP was tested 4.5 h after pairing without diminution. The effective BF sites were located in the substantia innominata and at the rostral pole of the globus pallidus, regions known to contain many cholinergic cell bodies. Enhancement occurred consistently only if stimulation of the BF site elicited a positive wave in the cortex at a latency of 11 to 18 ms. Repeated BF stimulation without cutaneous input did not produce a change in subsequent SEPs. The long-term changes described here may be involved in experimentally- and naturally-induced cortical reorganization.

Castration and tolerance induces changes in the levels of the activity of acetylcholinesterase in the isolated vas deferens of the rat

Changes in the activity of acetylcholinesterase (AChE) of the isolated vas deferens from normal, castrated, morphine and ethanol-tolerant rats were studied. Three days after the termination of treatment with morphine and on the last day of treatment with ethanol, a significant inhibition of the activity of AChE was detected. This reduction in the enzymatic activity persisted in morphine-tolerant rats for 15 days, but not for 30 days, at which time the levels of AChE were determined to be normal. However, in ethanol-tolerant rats, there were no significant changes found at days 15 or 30. The activity of AChE was decreased significantly in castrated rats, but this effect was reversed by treatment with testosterone. During withdrawal from morphine or ethanol, the levels of AChE were significantly increased. The results indicate that morphine and ethanol may be inducing changes in the feedback mechanism which regulates the levels of AChE at post-synaptic sites, and these changes could play an important role in the development of tolerance to morphine and to ethanol.

Influence of atropine and N-methyl atropine pretreatments on behavioral and physiological effects of the irreversible muscarinic agonist, BM123

The irreversible muscarinic agonist, BM123 (63 mu moles kg-1, IV), was shown to produce central and peripheral physiological signs characteristic of cholinergic agonists. It also induced hypothermia, elevated nociceptive thresholds, reduced locomotor activity and disrupted spontaneous alternation performance in rats. The centrally acting muscarinic antagonist, atropine (50 mu mole kg-1) prevented or reduced all the above effects of BM123 when given SC 40 min prior to the BM123 injection. In contrast, the peripherally acting muscarinic antagonist, N-methyl atropine, prevented only the peripheral effects and the elevated nociceptive thresholds. Habituation of activity during a 20 min session was observed in all groups despite different levels of general activity. These findings are consistent with a model in which atropine and N-methyl atropine compete with BM123 for reversible association with the muscarinic receptor. In the case of BM123 administered alone, the association results, first, in agonist effects and proceeds to form an irreversible complex. Our present results show that by competing with BM123 for mAChR sites during the initial, reversible state of the interaction, atropine blocks the cholinomimetic effects of the agonist during both this state and its otherwise subsequent irreversible state.

Neurochemical and behavioral effects of N-ethyl-acetylcholine aziridinium chloride in mice

N-ethyl-choline aziridinium (ECA) and N-ethyl-acetylcholine aziridinium (EAA) were shown to be inhibitors of high affinity choline uptake in vitro (IC50 = 0.4 microM and 1.5 microM, respectively), and intraventricular administration showed that EAA was more selective in its inhibition of hippocampal choline uptake in vivo. EAA significantly reduced the activity of choline acetyltransferase in the hippocampus 3 to 28 days following intraventricular infusion, but not in the striatum or parahippocampal cortex. Neither muscarinic receptor binding nor glutamic acid decarboxylase activity were affected in any of the three brain regions. EAA (12 or 16 nanomoles, intraventricular) significantly impaired memory performance of mice in a radial arm maze when tested two weeks after treatment. A subgroup analysis implicated long-term reference memory as the mechanism disrupted.

Scopolamine disrupts visual reversal without affecting the first discrimination

The effect of scopolamine (0.5 mg/kg) was determined in a brightness discrimination test (Y maze) motivated by electrical shocks (escape avoidance). Male adult Sprague Dawley rats were used. Results show that scopolamine impairs significantly the visual reversal without affecting the first brightness discrimination. The qualitative analysis reveals that the anticholinergic drug-induced deficit involves both perseveration, i.e. failure to suppress inappropriate response, and a tendency to adopt a position habit. A parallelism with hippocampal and frontal lobe damage symptoms is discussed and an interpretation in terms of disinhibition and incapacity to solve a more difficult problem is proposed.

Effects of intrahippocampal injections of the cholinergic neurotoxin AF64A on open-field activity and avoidance learning in the rat

The effects of direct intrahippocampal administration of the cholinergic neurotoxin, AF64A, were investigated in male rats. Bilateral injections of AF64A (5 nmole/2 microliters) produced a significant decrease in choline acetyltransferase (CAT) activity in the dorsal hippocampus (25%) and overlying frontoparietal cortex (30%) but no changes in the striatum. Rats lesioned with AF64A exhibited increased levels of open-field activity, which was most marked at 1 week after the lesion; however, the rates of intrasession habituation were similar in lesioned and control rats. Lesioned rats also displayed deficits in acquisition and retention of a passive avoidance task and less dramatic deficits in acquisition of two-way shuttle box avoidance. These findings indicate that lesioning of cholinergic terminals in the hippocampus and/or cerebral cortex with AF64A leads to long-term deficits in learning and memory as well as increases in open-field activity.

Selective breeding for increased cholinergic function: development of a new animal model of depression

Two lines of rats that were selectively bred to vary in their sensitivity to the anticholinesterase DFP exhibited different degrees of behavioral depression after injection of the muscarinic agonist arecoline (2 mg/kg). The line of rats with increased behavioral depression after arecoline (the Flinders sensitive or S-line) also exhibited a greater reduction of activity in an open field chamber following exposure to foot shock and greater immobility in a forced swim test than the line of rats with reduced behavioral depression after arecoline (the Flinders resistant or R-line). In addition, the Flinders S-line exhibited a better memory on an inhibitory avoidance task. These differences were not related to differences in shock sensitivity between the lines. Thus, the Flinders S-line of rats reacts to both mild stressors and a cholinergic agonist with greater behavioral depression and may, therefore, be a useful new animal model of human depressive disorders, one that focuses on cholinergic supersensitivity.