Intraseptal administration of muscimol produces dose-dependent memory impairments in the rat

Neurotransmitter receptors involved in post-training memory processing by the amygdala, medial septum, and hippocampus of the rat

Rats were trained and tested in habituation to a novel environment and step-down inhibitory avoidance. Immediately after training in each task the animals received intra-amygdala, intraseptal, or intrahippocampal micro-injections of agonists and antagonists of various neurotransmitter receptors. In the habitation task, intrahippocampal, but not intra-amygdala or intraseptal administration of the NMDA receptor antagonist aminophosphornopentanoic acid (AP5, 5.0 micrograms) or of the muscarinic receptor antagonist, scopolamine (2.0 micrograms) caused amnesia and the indirect antagonist of GABA-A receptors, picrotoxin (0.08 microgram), caused retrograde facilitation. Intrahippocampal administration of the respective agonists, glutamate, oxotremorine, and muscimol, had effects of their own opposite to those of the blockers, and norepinephrine (0.3 microgram) caused memory facilitation. In the avoidance task, results obtained with drug infusions given into the three structures were very similar: in all cases, AP5, scopolamine, and muscimol were amnestic, and glutamate, oxotremorine, norepinephrine, and picrotoxin caused memory facilitation. In addition, also in the three structures, picrotoxin counteracted the amnestic effect of AP5 and/or scopolamine and the beta-adrenoceptor blocker, timolol (0.3 microgram), while ineffective on its own, attenuated all the effects of picrotoxin. The results suggest that similar synaptic mechanisms in the amygdala, medial septum, and hippocampus are involved in memory consolidation: NMDA, muscarinic, and beta-noradrenergic receptors stimulate and GABA-A receptors inhibit this process, and beta-noradrenergic receptors modulate the GABAergic synapses. In the avoidance task these mechanisms operate in the three structures: in habituation only those in the hippocampus are operative. Possibly in each structure these mechanisms regulate, if not actually consolidate, a different aspect, component, or form of memory.

Galanin in the medial septal area impairs working memory

Galanin, a peptide of 29 amino acids, is co-localized with acetylcholine in a subpopulation of neurons of the medial septal area (MSA) that project to the hippocampus. Galanin reverses the actions of acetylcholine in several biochemical and behavioral procedures, and may be involved in memory processes. To test the possibility that galanin acts on the cell bodies of MSA neurons, two measures of septohippocampal function were assessed following intra-septal microinfusion of galanin or two synthetic fragments of galanin (1-16 and 21-29). The behavioral measure was choice accuracy in a working memory task in a T-maze. The electrophysiological measure was hippocampal theta activity recorded from the dentate hilus. The galanin fragment, 1-16, and the complete peptide, 1-29, decreased choice accuracy and decreased hippocampal theta activity in a dose-dependent fashion. Saline and the 21-29 fragment had no effect on choice accuracy and hippocampal theta. Sensorimotor performance was unaffected. These findings demonstrate that galanin impairs working memory when administered directly into the MSA and suggest that galanin inhibits MSA neural activity.

Chlordiazepoxide-induced working memory impairments: site specificity and reversal by flumazenil (RO15-1788)

The following studies examined the dose and time dependence, site specificity, and reversibility of chlordiazepoxide (CDP)-induced working memory impairments in adult male Sprague-Dawley rats. The rats were tested in a delayed non-match-to-sample radial-arm maze task in which a 1-h delay was imposed between the first four (predelay) and all subsequent (postdelay) arm choices. Intraperitoneal (ip) injection of 2.5 or 5.0 but not 1.25 mg/kg CDP immediately following the predelay session impaired performance in the task. CDP increased the number of errors and decreased the number of correct choices during the postdelay session. The observed working memory impairments also appeared to be site specific since injection of CDP into the medial septum, but not into the anterior amygdala nuclei, immediately following the predelay session also impaired working memory in a dose-related manner. Furthermore, there was a time window for CDP-induced working memory impairments since intraseptal injection of the drug immediately but not 15 min following the predelay session disrupted memory. This observation suggests that the performance deficits reflect disrupted working memory and not proactive effects on performance or the induction of state-dependent learning. In the final experiment, rats were injected ip with either saline or an amnestic dose of CDP (5.0 mg/kg) following the predelay session and then were immediately infused with 10 nmol flumazenil (RO15-1788), a benzodiazepine receptor antagonist or vehicle, into either the medial septum or anterior nuclei of the amygdala. Intraseptal injection of flumazenil prevented the working memory impairments produced by ip injection of CDP. In contrast, intra-amygdala injection of flumazenil did not attenuate, enhance, or modify the CDP-induced working memory impairment. These observations suggest that CDP disrupts working memory by interacting with benzodiazepine receptors in the medial septum.

Antagonism of GABAergic transmission within the septum disrupts working/episodic memory in the rat

Male Sprague-Dawley rats, trained to perform a standard or delayed-non-match-to-sample radial arm maze task, were implanted with a single cannula aimed at the medial septal nucleus. A within-subjects design was utilized to examine the effects of intraseptal administration of the GABAergic antagonist bicuculline on performance of these tasks. Bicuculline (0-0.5 microgram/0.5 microliter) infusion produced dose-dependent impairments when administered prior to performance of a standard radial arm maze task. Post-training infusion of bicuculline (0.-0.25 microgram/0.5 microliter) also induced dose-dependent impairments in the delayed version (4 h) of the task. Further testing indicated that post-training administration of a low dose of bicuculline (0.05 microgram) in the delayed version of the task induced a deficit at a 4-h, but not a 1-h, retention interval. The latter indicates that the impairment varied as a function of bicuculline dose and increasing task difficulty (longer retention intervals). Previous observations indicated that post-training administration of the GABAergic agonist muscimol and the antagonist bicuculline could induce deficits in the performance of the delayed task. The present findings demonstrate that intraseptal bicuculline treatment can disrupt ongoing radial maze performance, as well as the maintenance and/or retrieval of memories necessary for performance of the delayed version of the task. These findings suggest that either activation or blockade of intraseptal GABA receptors is sufficient to disrupt working/episodic memory processes. The role of septum and septohippocampal pathway in working/episodic memory is discussed.

Assessment of a cholinergic contribution to chlordiazepoxide-induced deficits of place learning in the Morris water maze

This investigation sought to characterize the interaction between benzodiazepine and cholinergic systems in place learning in the Morris water maze. In the first experiment, rats were treated with scopolamine (1 mg/kg) alone or concomitantly with one of two doses of flumazenil (15 and 30 mg/kg) or with chlordiazepoxide (5 mg/kg) alone or concomitantly with flumazenil (15 mg/kg). Chlordiazepoxide and scopolamine severely impaired place learning but not cue learning. The low dose of flumazenil completely reversed the impairment produced by chlordiazepoxide and both high and low doses of flumazenil attenuated the place learning deficit produced by scopolamine. Neither dose of flumazenil affected place learning when administered alone. In the second experiment, rats were administered chlordiazepoxide (5 mg/kg) or scopolamine (1 mg/kg) alone or concomitantly with one of four doses of physostigmine (0.05, 0.10, 0.25, and 0.5 mg/kg). Once again, both chlordiazepoxide and scopolamine impaired place but not cue learning. Physostigmine reversed the impairment produced by scopolamine in a dose-dependent manner but failed at every dose to attenuate the impairment produced by chlordiazepoxide. The higher doses of physostigmine impaired place learning when administered alone. None of the drug treatments impaired cue learning. Together, these results suggest that the scopolamine-induced impairment of place learning is due to an increase in benzodiazepine/GABA activity, and contradict the notion that benzodiazepines impair memory by cholinergic mechanisms.

Delayed-non-match-to-sample performance in the radial arm maze: effects of dopaminergic and gabaergic agents

Central dopaminergic transmission has been implicated in memory processes. The present experiments examined the effects of several direct acting dopaminergic agents on performance of a delayed-non-match-to-sample radial arm maze task. Preadministration of apomorphine (D1-D2 agonist; 0.25, 0.5, and 1.0 mg/kg), quinpirole (D2 agonist; 0.1 mg/kg), or SKF38393 (D1 agonist; 3 mg/kg) increased the latency of choices but did not affect any index of accuracy with a 1 h retention interval. Post-training administration of quinpirole (0.1, 0.2, 1.0, and 2.0 mg/kg), SKF38393 (0.3, 3.0, and 6.0 mg/kg), sulpiride (D2 antagonist; 3, 10, and 30 mg/kg), or SCH23390 (D1 antagonist; 0.01, 0.1, and 1.0 mg/kg) also did not affect accuracy, although quinpirole produced a dose-dependent increase in the latency of choices, assessed 10 h post-treatment. For comparison, pretraining and post-training administration of the benzodiazepine chlordiazepoxide (1, 3, 5 mg/kg) was also tested and produced dose-dependent impairments in mnemonic performance at either a 1 or 4 h retention interval. The effects of chlordiazepoxide are consistent with evidence indicating that GABAergic agents can influence memory processes. In contrast, the present findings indicate that (peripheral administration of dopaminergic agents IS) not sufficient to alter the mnemonic processes required for accurate performance of this DNMTS-RAM task.

GABAA receptor modulation of memory: the role of endogenous benzodiazepines

GABAA receptors are known to downregulate memory consolidation processes: picrotoxin and bicuculline enhance memory, and benzodiazepines and muscimol depress it. The discovery of naturally occurring benzodiazepines in the brain prompted a recent investigation of whether these compounds could act as physiological regulators of the GABAA receptors involved in memory modulation. Different forms of learning cause a rapid reduction of benzodiazepine-like immunoreactivity in septum, amygdala and hippocampus; microinjection of the benzodiazepine antagonist flumazenil into these regions, at the time that consolidation is taking place, enhances memory. Ivan Izquierdo and Jorge Medina suggest that these and other findings indicate that benzodiazepines released in the septum, amygdala and hippocampus do indeed physiologically downregulate memory storage processes; moreover, benzodiazepine release could be modulated by the anxiety and/or stress associated with each type of learning.

Amygdala and dorsal hippocampus lesions block the effects of GABAergic drugs on memory storage

These experiments examined the effects of posttraining systemic administration of the GABAergic agonist muscimol and the GABAergic antagonist bicuculline on retention in mice with bilateral lesions of the amygdala, dorsal hippocampus or caudate nucleus. Unoperated male CD1 mice and mice with either sham lesions or electrolytically induced lesions of these 3 brain regions were trained in a one-trial inhibitory avoidance task and, immediately after training, received i.p. injections of either muscimol, (1.0, 2.0 or 3.0 mg/kg), bicuculline, (0.25, 0.5 or 1.0 mg/kg), or control solutions. Retention was tested 24 h after training. Lesions of the 3 brain regions produced comparable impairment of retention. In the unoperated controls and sham controls muscimol and bicuculline produced dose-dependent impairment and enhancement, respectively, of retention. The drug effects on retention were blocked by lesions of the amygdala and hippocampus, but were not blocked by lesions of the caudate nucleus. These findings are consistent with other recent evidence suggesting that the amygdala and hippocampus are involved in mediating posttraining neuromodulatory influences on memory storage.

Irreversible effects in rats of toluene (inhalation) exposure for six months

The irreversible CNS effects of six months' exposure to toluene (0, 500, and 1500 p.p.m.) in rats was studied applying a multi-disciplinary approach. After an exposure-free period, neurobehavioural, morphometric, pathological, and biochemical examinations were performed. No neurobehavioural or gross pathological changes were found. Morphometric measurements did not show loss of neurones. At 500 p.p.m. the mean nuclear volume and mean perikaryonal volume and the variation of the values of these parameters was increased in the exposed groups compared to the controls. Noradrenaline (NA), dopamine (DA), and 5-hydroxytryptamine (5-HT) levels were significantly changed in various brain regions. It is concluded that this investigation failed to reveal overt toluene-induced CNS-neurotoxicity, however, certain irreversible effects were found which further add to the accumulating evidence of the chronic CNS-neurotoxicity of toluene.

Intraseptal administration of bicuculline produces working memory impairments in the rat

Male Sprague-Dawley rats, trained to perform a delayed-non-match-to-sample eight-arm radial maze task, were implanted with a single cannula aimed at the medial septal nucleus. A within-subjects design was utilized to examine the effects of intraseptal administration of bicuculline (0.5 micrograms) on performance of this task with 1- and 4-h delay intervals imposed between choices four and five. Administration of bicuculline immediately following the first four choices produced an impairment in maze performance at both a 1- and a 4-h delay interval. This treatment also produced an increase in latency per choice. Bicuculline-induced impairments were not observed when administered 2 h following the predelay session (2 h prior to testing). These data support previous observations that pharmacological manipulation of GABAergic activity within the septum modifies working memory processes.

The role of interactions between the cholinergic system and other neuromodulatory systems in learning and memory

Extensive evidence indicates that disruption of cholinergic function is characteristic of aging and Alzheimer's disease (AD), and experimental manipulation of the cholinergic system in laboratory animals suggests age-related cholinergic dysfunction may play an important role in cognitive deterioration associated with aging and AD. Recent research, however, suggests that cholinergic dysfunction does not provide a complete account of age-related cognitive deficits and that age-related changes in cholinergic function typically occur within the context of changes in several other neuromodulatory systems. Evidence reviewed in this paper suggests that interactions between the cholinergic system and several of these neurotransmitters and neuromodulators--including norepinephrine, dopamine, serotonin, GABA, opioid peptides, galanin, substance P, and angiotensin II--may be important in learning and memory. Thus, it is important to consider not only the independent contributions of age-related changes in neuromodulatory systems to cognitive decline, but also the contribution of interactions between these systems to the learning and memory deficits associated with aging and AD.

GABAergic control of basal forebrain cholinergic neurons and memory

The involvement of the GABAergic innervation of basal forebrain neurons in the rats' conditional visual discrimination performance was examined. Performance in such a task is based on the subjects's ability to retrieve information about response rules, and previous experiments have demonstrated that basal forebrain lesions interfere with this ability. Following the acquisition of the task, chronic guide cannulae were stereotaxically implanted into the substantia innominata of both hemispheres, and the animals were retrained. Administration of the GABAA-agonist muscimol into the substantia innominata (0, 25, 50 ng/0.5 microliters/hemisphere) dose-dependently decreased the number of correct responses, increased the number of errors of omission, increased response latency, but did not affect side bias. Systemic co-administration of the cholinesterase inhibitor physostigmine (0, 0.1, 0.2 mg/kg; i.p.) exclusively interacted with the effects of muscimol on correct responding. Specifically, physostigmine dose-dependently intensified and attenuated the muscimol-induced reduction in correct responding. Although it cannot be excluded that alternative neuronal mechanisms were involved in the mediation of the effects of muscimol and their interaction with physostigmine, these findings support previous evidence indicating that the activity of basal forebrain cholinergic neurons is controlled by a GABAergic input, and that this neuronal link is involved in mnemonic processing.

Cholinergic-dopaminergic interactions in cognitive performance

Both acetylcholinergic (ACh) and dopaminergic (DA) systems have been found to be crucial for the maintenance of accurate cognitive performance. In a series of studies examining those aspects of cognitive function revealed by the radial-arm maze, we have found that these two neurotransmitter systems interact in a complex fashion. Choice accuracy deficits in the radial-arm maze can be induced by blockade of either muscarinic- or nicotinic-ACh receptors. The choice accuracy deficit induced by blockade of muscarinic receptors with scopolamine can be reversed by the DA receptor blocker, haloperidol. The specific DA D1 blocker SCH 23390 also has this effect, whereas the specific D2 blocker raclopride does not, implying that it is D1 blockade that is critical for reversing the scopolamine effect. On the other hand, the choice accuracy deficit induced by nicotinic blockade with mecamylamine is potentiated by haloperidol. This effect is also seen with the D2 antagonist raclopride, but not with the D1 antagonist SCH 23390, implying that it is the D2 receptor which is important for the potentiation of the mecamylamine effect. The relevance of the D2 receptor for nicotinic actions on cognitive function is emphasized by the finding that the selective D2 agonist LY 171555 reverses the choice accuracy deficit caused by mecamylamine. Nicotinic and muscarinic blockade are synergistic in the deficit they produce. Antagonist doses subthreshold when given alone produce a pronounced impairment when given together. This latter deficit can be reversed by the D2 agonist LY 171555. These studies have outlined the complex nature of ACh-DA interactions with regard to cognitive function. Possible neural circuits for these interactions are discussed. The effectiveness of these selective DA treatments in reversing cognitive deficits due to ACh underactivation suggests a novel approach to treating cognitive dysfunction in syndromes such as Alzheimer's disease.

Muscimol injections in the medial septum impair spatial learning

These experiments examined the role of GABAergic systems in modulating septohippocampal cholinergic influences on learning. Microinjections of the GABA(A) agonist muscimol (0.5, 1.0 or 5.0 nmol) or physiological saline were administered (0.5 microliters) into the medial septum of rats via chronically implanted cannulae just prior to daily training in the Morris water maze spatial learning task. The animals received 3 training trials on each of 4 days. The escape latencies of rats trained with a submerged escape platform at a fixed location were significantly shorter than those trained with a randomly located platform. Rate of learning of the fixed location was significantly impaired in rats given pretraining muscimol injections in the medial septum at doses (1.0 and 5.0 nmol) that significantly reduced hippocampal high-affinity choline uptake (HACU). Analyses of responses on a probe trial with no pretraining injections and no platform revealed that, in comparison with controls, animals that had received muscimol prior to each training session were likely to swim in the region where the platform had been located. The finding that muscimol-injected rats were subsequently able to learn the task when trained without muscimol injections indicates that the acquisition impairment was not due to a lasting effect of the drug injections. Our results are consistent with the view that the septal GABAergic modulation of the septohippocampal cholinergic pathway is involved in regulating the acquisition of spatial information.