Clonidine reverses the amnesia induced by dopamine beta hydroxylase inhibition

Neuromodulatory signaling in hippocampus-dependent memory retrieval

Considerable advances have been made toward understanding the molecular signaling events that underlie memory acquisition and consolidation. In contrast, less is known about memory retrieval, despite its necessity for utilizing learned information. This review focuses on neuromodulatory and intracellular signaling events that underlie memory retrieval mediated by the hippocampus, for which the most information is currently available. Among neuromodulators, adrenergic signaling is required for the retrieval of various types of hippocampus-dependent memory. Although they contribute to acquisition and/or consolidation, cholinergic and dopaminergic signaling are generally not required for retrieval. Interestingly, while not required for retrieval, serotonergic and opioid signaling may actually constrain memory retrieval. Roles for histamine and non-opioid neuropeptides are currently unclear but possible. A critical effector of adrenergic signaling in retrieval is reduction of the slow afterhyperpolarization mediated by β1 receptors, cyclic AMP, protein kinase A, Epac, and possibly ERK. In contrast, stress and glucocorticoids impair retrieval by decreasing cyclic AMP, mediated in part by the activation of β2 -adrenergic receptors. Clinically, alterations in neuromodulatory signaling and in memory retrieval occur in Alzheimer's disease, Down syndrome, depression, and post-traumatic stress disorder, and recent evidence has begun to link changes in neuromodulatory signaling with effects on memory retrieval.

Hypericum perforatum as a nootropic drug: enhancement of retrieval memory of a passive avoidance conditioning paradigm in mice

Depression, among other non-cognitive symptoms, is common in patients with dementia. The effect of Hypericum perforatum (St. John's Wort) extract, with well-documented antidepressant activity, was tested on memory retrieval 24 h after training on a one-trial passive avoidance task in mice. Acute administration of Hypericum extract (4.0, 8.0, 12.0, and 25.0 mg/kg i.p.) before retrieval testing increased the step-down latency during the test session. The same doses of Hypericum extract, on the other hand, failed to reverse scopolamine-induced amnesia of a two-trial passive avoidance task. The involvement of serotonergic, adrenergic, and dopaminergic mechanisms in the facilitatory effect of Hypericum extract on retrieval memory was investigated. Pretreatment of the animals with serotonergic 5-HT1A receptor antagonist (-)-pindolol (0.3, 1.0, and 3.0 mg/kg), serotonergic 5-HT2A receptor blocker spiperone (0.01, 0.03, and 0.1 mg/kg), alpha adrenoceptor antagonist phentolamine (1, 5, and 10 mg/kg), beta receptor antagonist propranolol (5, 7.5, and 10 mg/kg), dopaminergic D1 receptor antagonist SCH 23390 (0.01, 0.05, and 0.1 mg/kg), and dopaminergic D2 receptor antagonist sulpiride (5, 7.5, and 10 mg/kg) revealed the involvement of adrenergic and serotonergic 5-HT1A receptors in the facilitatory effect of Hypericum extract on retrieval memory. It is concluded that Hypericum extract may be a better alternative for treatment of depression commonly associated with dementia than other antidepressants known to have anticholinergic side effects causing delirium, sedation and even exacerbating already existing impaired cognition. In dementias of old age, Hypericum perforatum would, therefore, serve as one medication targeting both depression and amnesia with lower potential side effects.

Postsynaptic alpha-2 receptor stimulation improves memory in aged monkeys: indirect effects of yohimbine versus direct effects of clonidine

Very low doses (0.00001 mg/kg) of the alpha-2 adrenergic antagonist, yohimbine, improved working memory performance in a subset of aged monkeys. Improvement appeared to result from increased norepinephrine (NE) release onto postsynaptic alpha-2 adrenoceptors, as the response was blocked by the "postsynaptic" alpha-2 antagonist, SKF104078. Cognitive-enhancing effects of low dose yohimbine treatment may depend on aged animals retaining an intact, endogenous NE system. In contrast to yohimbine, the alpha-2 agonist, clonidine, has improved working memory in all aged animals examined. In the present study, clonidine's beneficial effects were also blocked by the postsynaptic antagonists SKF104078 and SKF104856, suggesting that clonidine acts by directly stimulating postsynaptic alpha-2 adrenoceptors. Beneficial doses of clonidine (0.01 mg/kg) and yohimbine (0.00001 mg/kg) were combined to see if they would produce additive effects on memory enhancement. This strategy was successful in young monkeys with intact NE systems but was not effective in the aged monkeys. These findings demonstrate that drugs that indirectly stimulate postsynaptic alpha-2 receptors by increasing NE release are not as reliable in aged monkeys as directly acting agonists that can replace NE at postsynaptic alpha-2 receptors.

Reserpine impairs spatial working memory performance in monkeys: reversal by the alpha 2-adrenergic agonist clonidine

Repeated daily treatment with the catecholamine-depleting agent, reserpine, dramatically reduced performance on the delayed response task, a test of spatial working memory that depends upon the integrity of the prefrontal cortex. Delayed response performance fell from an average of 27.2/30 trials correct before reserpine treatment to an average of 20.4/30 trials correct after repeated reserpine administration. Injection of the alpha 2-adrenergic agonist, clonidine (0.0001-0.05 mg/kg), to chronic reserpine-treated monkeys significantly restored performance on the delayed response task; performance after an optimal dose averaged 27.8/30 trials correct. Clonidine's beneficial effects on delayed response performance were longlasting; monkeys remained improved for more than 24 h after a single clonidine injection. The finding that clonidine is efficacious in reserpinized animals supports the hypothesis that alpha 2-adrenergic agonists improve cognitive function through actions at postsynaptic, alpha 2-adrenergic receptors on non-adrenergic cells. In contrast to the delayed response task, reserpine had little effect on performance of a visual discrimination task, a reference memory task which does not depend on the prefrontal cortex. These results emphasize the importance of postsynaptic, alpha 2-adrenergic mechanisms in the regulation of working memory.

Behavioral and receptor binding analysis of the alpha 2-adrenergic agonist, 5-bromo-6 [2-imidazoline-2-yl amino] quinoxaline (UK-14304): evidence for cognitive enhancement at an alpha 2-adrenoceptor subtype

The ability of the alpha 2-agonists clonidine, B-HT920 (6-allyl-2-amino-5,6,7,8-tetrohydro-4H-thiazolo-[4,5-d]-azepine) and guanfacine to improve memory in aged monkeys has been related to their affinity to bind at a proposed rauwolscine-insensitive (Ri) subtype of alpha 2-adrenergic receptor, while their hypotensive and sedating effects have been related to affinity at a rauwolscine-sensitive site (Rs) (Arnsten et al., 1988). The present study examined the alpha 2-agonist UK-14304 (5-bromo-6 [2-imidazoline-2-yl amino] quinoxaline) for its binding characteristics in tissue from the brain of the rat and for its behavioral effects in aged monkeys. The drug UK-14304 was found to have slightly higher affinity for the Ri than the Rs site (Ki values of 138 and 245 nM, respectively), but was not as selective as the alpha 2-agonist guanfacine (Ki values of 23 and 340 nM, respectively). Consistent with this binding profile, very small doses of UK-14304 (0.00017-0.17 micrograms/kg) produced a reliable but modest improvement in memory in the aged monkeys (average improvement of 16.7% +/- 2.6% following an optimal dose). No hypotensive or sedating side effects were observed at these small doses. However, hypotension and sedation emerged rapidly when the dose was raised above 1.7 micrograms/kg and at the largest doses tested (50.0-100.0 micrograms/kg), hypotension was severe (systolic pressure below 70 mm Hg) and the animals were too sedated to complete cognitive testing. The separation between doses that improved memory and those that produced hypotension and sedation was not as great for UK-14304 as it was for guanfacine, consistent with the greater selectivity of guanfacine for the Ri site. These results offer a fourth example whereby the ability of an alpha 2-agonist to improve cognitive function, without side effects, could be related to the relative affinities for the Ri and Rs sites.

Hemicholinium-3 impairs spatial learning and the deficit is reversed by cholinomimetics

The effects of hemicholinium-3 (HC-3) on spatial discrimination learning were studied. Rats were equipped with indwelling cannulae in the right lateral ventricle and, following recovery, were trained on a two platform spatial discrimination task in a water maze. In this task a visible escape platform remains in a fixed position in the pool during a single training session, whilst the location of an identical "float" (which affords no escape) is randomly varied. For each session the location of the fixed escape platform was changed and the rats were retrained to criterion following pretreatment either with artificial cerebrospinal fluid (CSF) or HC-3 (2.5, 5.0 micrograms/rat/ICV) 1 h before training. Each rat received every treatment according to a latin square design. The results showed that spatial learning was dose dependently impaired by HC-3, choice accuracy being reduced to chance levels by the higher dose. There was no evidence of motoric difficulty, as choice latencies were not significantly increased. Experiments were then conducted to test for reversal of the deficit using a range of psychotropic drugs. Rats were treated with CSF or HC-3 (5 micrograms/rat ICV) 60 min prior to testing and test drugs were injected 15 min before testing. Some doses of physostigmine (46-460 micrograms/kg/SC) and tetrahydroaminoacridine (THA) (2.2-10 mg/kg/SC) reversed the spatial learning deficit. The muscarinic agonists arecoline (0.046-1 mg/kg/SC), aceclidine (1-10 mg/kg/SC), oxotremorine (30-100 micrograms/kg/SC) and RS-86 (0.46, 1.0 microgram/kg/SC) were also effective. Pilocarpine (0.22-2.2 mg/kg/SC) showed marginal activity and isoarecoline (4.6-10 mg/kg/SC) was inactive. Nicotine (0.32, 1, 3.2 mg/kg/SC) and piracetam (10, 30, 100 mg/kg IP) were also inactive. The alpha 2 agonist, clonidine (46, 100 micrograms/kg SC) and the antagonist idazoxan (32, 100 micrograms/kg SC) were also inactive. Learning deficits were not reversed by haloperidol (20, 60 micrograms/kg), amphetamine (0.1, 0.46 mg/kg), the selective 5-HT1A agonist 8-OH-DPAT (30, 100 micrograms/kg) or by the benzodiazapine antagonist ZK-93426 (1, 3.2, 10 mg/kg). The results show that forebrain Ach depletion by HC-3 impairs spatial discrimination learning and these deficits are reversed by cholinesterase inhibitors and some muscarinic receptor agonists. Some degree of pharmacological selectivity is indicated by the failure of a range of other drugs to reverse the impairments.

Attenuation of forgetting by pharmacological stimulation of aminergic neurotransmitter systems

Mice were trained in one-way active avoidance to a criterion of 3/4 avoidances and tested under extinction conditions one week later when substantial forgetting had occurred. Thirty min prior to testing animals were injected with either saline or different doses of drugs which activate the noradrenergic (phenylephrine, salbutamol, clonidine) dopaminergic (L-dopa(Sinemet) transdihydrolisuride, apomorphine) and serotonergic (fluoxetine, 5-methoxy DMT) neurotransmitter systems. Results showed that all agents alleviated forgetting in a dose dependent fashion. Untrained mice treated with the most effective dose of representative drugs from each class did not exhibit avoidance behavior at testing indicating that the improved performance of trained animals was probably not the result of increased activity or other non-memorial effects of the drugs. It was concluded that pharmacological agents which stimulate monoamine systems may improve memory retrieval by activating a non-specific neural system which controls arousal, attention and motor readiness.

Evidence for a possible interaction between noradrenergic and serotonergic neurotransmission in the retrieval of a previously learned aversive habit in mice

The present series of experiments examined the effects of pretreating pirenperone-injected mice with a variety of non-serotonergic receptor antagonists on retrieval of a one-trial inhibitory (passive) avoidance task. Water-deprived mice were trained to avoid drinking from a water spout located in the avoidance chamber by pairing foot-shock with licks from the water spout. Retention was measured as the suppression of drinking (latency to drink) 48 h later. Pre-test administration of pirenperone (1.0 mg/kg) significantly enhanced retrieval (increased latencies). The suppression of drinking could not be attributed to the non-specific effects of pirenperone on behavior in general, as the performance of non-contigently shocked mice injected with the same dose of pirenperone did not exhibit a similar elevation in latencies. Of the seven pretreatment drugs examined, only phenoxybenzamine (1.0 mg/kg) completely blocked the pirenperone-induced response. Bicuculline (1.0 mg/kg) partially attenuated the enhanced performance resulting from pre-test pirenperone administration. The results suggest that the pirenperone-induced response may be partly due to activation of noradrenergic (alpha) neurotransmission.

Learning impairments after 6-OHDA treatment: a comparison with the effects of thiamine deficiency

Cortical norepinephrine, dopamine and 3,4-dihydroxyphenylacetic acid were reduced by injection of 6-hydroxydopamine (6-OHDA) jointly into the cisterna magna and the dorsal noradrenergic bundle. On subsequent behavioral testing, deficits were observed for spatial delayed alternation learning, but not for active or passive avoidance. Treatment with clonidine resulted in a significant improvement in spatial delayed alteration for experimental as compared to control animals. Injections of 6-OHDA into the cisterna magna alone had no significant effect on brain chemistry or behavioral measures. These results are similar to previous observations following a bout of thiamine deficiency, in which cortical catecholamines were depleted in animals that had exhibited deficits for spatial delayed alternation learning. We argue that the cortical catecholamine deficits observed in post-thiamine-deficient animals are sufficient to account for the delayed alternation deficits observed in this animal model of Korsakoff's psychosis.

Thiamine deficiency depletes cortical norepinephrine and impairs learning processes in the rat

Several lines of evidence indicate that thiamine deficiency causes the Wernicke-Korsakoff syndrome, a human memory disorder. The present study examined behavioral deficits in rats after recovery from a bout of thiamine deficiency. Following behavioral testing, the brains were dissected into regions and assayed biochemically for levels of dopamine, norepinephrine, serotonin and the primary metabolites of these monoamines. Based on previous findings in this laboratory, we predicted that thiamine deficiency not only produces behavioral deficits but loss in catecholamines as well. Impairments were observed for a spatial delayed alternation task that had been learned prior to experimental treatment. In addition, experimental animals were impaired in their ability to acquire two novel tasks, active and passive shock avoidance, after recovery from the acute effects of thiamine deficiency. Comparable deficits were not observed for a number of reflex responses that were measured to assess the general neurological state of the animals. Biochemical analyses revealed that the concentration of norepinephrine was reduced significantly in cortex-hippocampus and olfactory bulb but not in other regions, while dopamine and serotonin levels were not altered in any brain region examined. These data demonstrate that a bout of thiamine deficiency can produce persistent deficits in brain norepinephrine and concomitant decrements in behavioral measures of learning and memory. These results are consistent with our hypothesis and evidence that noradrenergic deficits contribute to the amnesic symptoms of Korsakoff's psychosis.

Intracerebroventricular apomorphine alleviates spontaneous forgetting and increases cortical noradrenaline

Rats were trained in a 6 unit spatial discrimination maze for food reinforcement; they were then implanted with intracerebroventricular (i.c.v.) cannulae and tested 25 days after training. Control animals displayed significant forgetting; 10 micrograms apomorphine alleviated this forgetting, while a higher dose did not. A hole board activity study revealed that i.c.v. Apomorphine does not produce the shaped dose-response activity curve found with systemic injections. Neurochemical analysis of forebrain structures after injections showed that the dose which facilitated memory retrieval also increased cortical and hippocampal noradrenaline, while the larger dose did not.

Zimeldine: a review of its effects on ethanol consumption

This review evaluates the literature and describes an extensive series of experiments which examined the effects of zimeldine , its metabolite norzimeldine and other serotonin and norepinephrine reuptake inhibitors on voluntary ethanol consumption in rats. The results of these experiments indicate that drugs which specifically inhibit serotonin reuptake are capable of decreasing voluntary ethanol consumption. The behavioral mechanism through which these drugs exert their effects seems to be extinction of the primary reinforcing properties of alcohol. These effects seem to be partially attenuated both by drugs which modulate the norepinephrine system as well as by the serotonin postsynaptic receptor blocker methergoline. The data presented in this review are discussed in terms of the involvement of the serotonin and norepinephrine systems in the mechanism of action of these drugs. In addition, several alternative hypotheses concerning the nature of the phenomenon are offered. Finally, the implications of these data for the possible development of a treatment procedure for problem drinkers is discussed.

Facilitation of memory retrieval by centrally administered catecholamine stimulating agents

Amnesia for inhibitory avoidance learning induced in mice by a post-training injection of the protein synthesis inhibitor anisomycin was alleviated in a dose-dependent fashion by introcerebroventricular injections of D-amphetamine (20 micrograms), dopamine (10, 25 and 50 micrograms) and the dopamine agonist lisuride (0.5 and 1.0 micrograms), given 5 min before the retention test. Animals which received avoidance training in a different apparatus did not show increased test latencies following drug treatment thus eliminating non-specific behavioral suppression as an explanation for the findings. Neither norepinephrine nor the norepinephrine agonist clonidine was able to reverse the amnesia. These findings indicate that central dopamine systems may play a role in the retrieval of inhibitory avoidance learning.

Effects of cycloheximide, a protein synthesis inhibitor, on mouse brain catecholamine biochemistry

Cycloheximide (CXM), a protein synthesis inhibitor, has been shown to result in a marked inhibition of central catecholamine (CA) synthetic mechanisms at doses that cause amnesia in animals. Unlike other inhibitors of CA synthesis no significant depletion of whole brain NE or DA concentrations was observed 0.75, 1, 2, 3, 4, 6, 17, or 24 hours after administration of CXM (120 mg/kg) to C57BL/6J mice. In order to investigate the underlying basis of maintenance of CA levels in face of CA synthesis inhibition, the effects of CXM on in vitro release of 3H-NE was studied in mouse hypothalamic slices. CXM, in a dose related manner, significantly inhibited the potassium stimulated release of NE from hypothalamic slices. Anisomycin, another protein synthesis inhibitor, similarly inhibited NE release. These studies further document the effects of protein synthesis inhibitors on CA mechanisms and suggest that disruption of CA biochemistry may play a role in the amnesia observed after administration of protein synthesis inhibitors.

Amnesia due to beta-antagonists in a passive avoidance task in the chick

The beta-adrenergic antagonists sotalol, nadolol and timolol (which act at both beta 1- and beta 2-receptors) induce amnesia in the domestic chick when given systemically after a one trial passive avoidance task. State dependent learning and effects on performance at test (e.g. interference with recall) almost certainly do not cause the observed amnesia. All three effective beta-antagonists induce amnesia only when injected within a limited period after training, suggesting effects on memory formation. Sotalol differs markedly from nadolol and timolol in: (a) showing a sharp and markedly earlier loss of effectiveness as administration is moved to progressively later times after training (25-30 min rather than 40-50 min), (b) producing when given after training a delayed and gradual loss of retention rather an immediate and rapid loss. These differences seem not to reflect a lesser effectiveness of sotalol, nor a greater delay in the onset of its action, but instead qualitative differences in effects on memory formation.