Cannabinoids and memory: animal studies

This review will consider studies concerning the effects of cannabinoid receptor agonists and antagonists on memory in laboratory animals. Two subtypes of cannabinoid receptors have been identified to date: the central CB1 subtype and the peripheral CB2 subtype. The receptor which specifically binds Delta9-tetrahydrocannabinol (Delta9-THC) and related compounds in rat and human brain has been discovered and cloned by a number of researchers. This cannabinoid receptor is localized with high concentrations in different brain areas, including hippocampus and amygdala, which play an important role in the modulation of memory. In recent years evidence has been obtained that cannabinoids influence memory processes. It has been shown, for example, that Delta9-THC impairs memory in rats, mice and monkeys tested in a variety of experimental conditions (radial maze, instrumental discrimination tasks, Morris water maze, etc.). In some of these researches the effect of Delta9-THC was antagonized by the CB1 receptor antagonist SR 141716A, showing the involvement of this subtype of cannabinoid receptor in its effect. Anandamide, arachidonylethanolamide, was recently discovered as the first endogenous ligand for the cannabinoid receptor. It has been reported to stimulate CB1 receptors and to mimic the pharmacological effects of cannabinoids. Experiments carried out by our group have shown that anandamide impairs memory consolidation in random bred mice (CD1), exerts genotype-dependent influences on memory in inbred strain of mice (C57 BL/6 and DBA/2), and that opioid and dopaminergic systems might be involved in its effects.     

Prenatal cocaine exposure does not alter working memory in adult rats

The present study was designed to assess working memory in adult rats exposed to intravenous cocaine in utero, as part of an examination of various cognitive and affective functions. The study included four groups: a saline control and three groups exposed to ascending doses of cocaine from gestational days 8 to 21 (0.5, 1.0, or 3.0 mg/kg). This exposure regimen (route of administration and dose) has been shown to accurately reproduce the pharmacokinetic profile and physiological effects of human recreational cocaine use. This report describes the results of a series of automated alternation tasks, in which the animals were rewarded for alternating their responses between two response ports on successive trials. In the final task, the delay between trials varied randomly between 0, 20, 40, and 80 s, thereby varying the retention interval. Although performance declined dramatically as the retention interval increased, the rate of this decline did not differ across treatment groups. These results suggest that prenatal cocaine exposure, at doses that model recreational use, does not produce lasting changes in explicit memory or working memory. However, subtle, sex-specific effects of prenatal cocaine exposure were seen on measures that indicate impairments in sustained attention and "readiness", as well as altered reactivity to task-related stressors such as waiting for long and unpredictable delays.     

Nitric oxide modulates apomorphine-induced recognition memory deficits in rats

Nitric oxide (NO) is an important intracellular messenger in the brain. The implication of NO in schizophrenia is well documented although it is not yet clear whether net over or underproduction of NO is typical of this disease. In line with this, either NO donors or NO synthase (NOS) inhibitors were found to abolish psychotomimetic effects, including cognition deficits, produced by N-methyl-d-aspartate (NMDA) receptor hypofunction. In addition, there is poor experimental evidence concerning the efficacy of NO to modulate memory deficits produced by dopamine (DA) dysfunction. The present study was designed to investigate the ability of NO modulators (NO donors and NOS inhibitors to reverse recognition memory impairments produced by the DA D₁/D₂ mixed receptor agonist apomorphine in rats. For these studies, the novel object recognition test (NORT) was used as the memory test. Apomorphine (0.05, 0.1, 0.5 and 1.0 mg/kg), dose-dependently, disrupted performance in this recognition memory procedure in rats. The NO donors molsidomine (2.0 and 4.0 mg/kg) and SNP (0.3 and 1.0 mg/kg), reversed the impairing effects of apomorphine (1.0 mg/kg) in the NORT. Administration of the NOS inhibitors L-NAME (1.0 and 3.0 mg/kg) or 7-NI (1.0 and 3.0 mg/kg) produced similar results. The present findings indicate a) that apomorphine dose-dependently impaired recognition memory and b) that a cognitive deficit produced by DA dysfunction is sensitive to NO.     

Ketamine blocks a taste recognition memory in fetal rats

Decisions about novelty/familiarity are critical in determining whether or not information should be attended to, and possibly encoded, for long-term storage. We have reported that fetal and neonatal rats exhibit an increase in orofacial movements (e.g., perseverative mouthing and mouth movements, and licks) upon tasting saccharin (SAC), if it was experienced previously. E19 rat fetuses can acquire this taste recognition memory and retain it for at least 5 days (P3). In the current study, we sought to evaluate the role of N-methyl-D-aspartate (NMDA) receptors in establishing a taste recognition memory. Pregnant Sprague-Dawley rats received ketamine (NMDA receptor antagonist) (doses: 0, 50, or 100 mg/kg, i.p.). One-half hour later, we performed a reversible spinal block on each pregnant dam, and E19 fetuses received an oral injection of 10 microl, 0.3% SAC or water (control) while in utero. The uterus was replaced and the pups were later born via a normal vaginal delivery. On P3, all pups experienced oral lavage of 10 microl, 0.3% SAC, and motor responses were recorded. As expected, non-drugged control neonates tasting familiar SAC exhibited significantly more perseverative mouth movements, as well as total mouth movements and licks, than did pups tasting novel SAC. However, this taste recognition memory response was not observed in rats exposed to ketamine in utero. The data suggest that early non-associative taste memories may be disrupted by NMDA receptor blockade.     

Desferoxamine reverses neonatal iron-induced recognition memory impairment in rats

We have previously demonstrated that rats given iron neonatally presented memory deficits. The aim of the present study was to evaluate the effect of desferoxamine, a metal chelating agent, on memory deficits in an iron overload model in rats. Male rats received vehicle or iron orally at postnatal days 12–14 and desferoxamine (30 or 300 mg/kg) in the adulthood. After desferoxamine treatment, they were trained in a novel-object recognition task. Iron-treated rats showed recognition memory impairments when compared to controls. Iron-treated rats that received desferoxamine 300 mg/kg, showed normal recognition memory, suggesting that desferoxamine can reverse recognition memory deficits associated with iron accumulation. Further research is required to examine whether the findings from animal models of iron overload have implications for humans.     

Cannabinoids modulate ultrasound-induced aversive responses in rats

Rationale Mechanisms and brain substrates mediating cannabinoid-induced modulation of behaviour towards aversive stimuli are poorly understood.Objectives To investigate the effects of systemic and intra-dorsal periaqueductal grey (PAG) administration of the cannabinoid receptor agonist HU210 on behaviour and plasma corticosterone levels in rats exposed to ultrasound and determine the contribution of CB₁ receptors.Methods In experiment 1, rats received vehicle or CB₁ receptor antagonist SR141716A (3 mg/kg, IP) 30 min prior to a second injection of vehicle or HU210 (5, 20 or 80 g/kg, IP). In experiment 2, rats received intra-dorsal PAG vehicle or SR141716A (30 g/rat) 10 min prior to intra-dorsal PAG vehicle or HU210 (5 g/rat). Following injections, rats were exposed to an aversive 20 kHz ultrasonic tone for 3 min. Behaviour, including hyperlocomotor activity and freezing, was monitored during and post-ultrasound. Plasma corticosterone levels 10 min post-ultrasound were measured.Results Ultrasound induced explosive running and freezing behaviour. Systemic administration of HU210 attenuated the expression of ultrasound-induced hyperlocomotor activity and increased freezing. The HU210-induced attenuation of hyperlocomotor activity was blocked by SR141716A. Intra-PAG administration of HU210 reduced the expression of ultrasound-induced hyperlocomotor activity, an effect not blocked by SR141716A. Systemic and intra-PAG administration of HU210 increased plasma corticosterone levels, an effect not blocked by SR141716A.Conclusions The cannabinoid receptor agonist HU210 modulates behaviour towards an aversive ultrasound stimulus in rats, an effect accompanied by increased HPA axis activity. These effects may be mediated, at least in part, by the dorsal PAG but cannot be explained solely by an action at CB₁ receptors.Both D.P.F. and M.D.J. contributed equally to this study.     

Nicotine improves memory in an object recognition task in rats.

Nicotine was investigated for its mnemonic effect in a two trials object recognition task. In the first trial, two copies of the same object were presented. In the second trial (24 h after), one of the familiar object and a new object were presented. The time spent exploring the new object by control rats was not significantly different from the exploration time of the familiar object, indicating that they did not remember the familiar object. Rats injected with nicotine before the first trial, after the first trial or before the second trial spent more time in exploring the new object than the familiar one at the second trial. These results suggest that, in normal rats, acute nicotine enhances acquisition, consolidation and restitution of the information in an object recognition task.     

AM251, cannabinoids receptors ligand, improves recognition memory in rats

High density of cannabinoid receptors type 1 (CB1) in the brain suggests that endocannabinoid system plays an important role in the functioning of the central nervous system. Natural and synthetic cannabinoids are known to attenuate learning and memory processes. The adverse effects of cannabinoids are reversed by SR141716A, at first reported to be a selective CB1 receptor antagonist, later shown to possess also inverse agonist properties. The present study was performed in an attempt to determine the influence of different doses of AM251, a member of the same cannabinoid group as SR141716A, on recognition memory evaluated in an object recognition test. Because cannabinoids may alter motor function and affect anxiety, the influence of AM251 on psychomotor activity and anxiety was assessed in an "open-field" test and elevated plus maze, respectively. While the lowest dose of AM251 (1.0 mg/kg) significantly improved recognition memory, higher doses (2.5 mg/kg and 5.0 mg/kg) did not have an influence on it. Moreover, AM251 did not affect anxiety but in the highest dose significantly attenuated psychomotor activity in rats. The main finding of the present study indicates that AM251, at the dose of 1.0 mg/kg, improves recognition memory in rats without alteration of their psychomotor activity and anxiety. The pro-cognitive effect exerted by compounds belonging like AM251 to diarylpyrazole group may be beneficial in therapeutic use of these compounds, especially in patients with cognitive dysfunctions.     

Pilocarpine improves olfactory discrimination and social recognition memory deficits in 24 month-old rats

Muscarinic receptor agonists have been suggested as potential drugs to counteract age-related cognitive decline since critical changes in cholinergic system occur with aging. Recently, we demonstrated that single administration of the non-selective muscarinic receptor agonist pilocarpine prevents age-related spatial learning impairments in rats. In addition, increasing evidence suggests that areas in the central nervous system processing olfactory information are affected at the early stages of age-related diseases, such as Alzheimer's disease, and that specific olfactory testing may represent an important tool in the diagnosis of these diseases. In the present study, olfactory discrimination and short-term social memory of 3 and 24 month-old rats were assessed with the olfactory discrimination and social recognition memory tasks, respectively. The actions of the repeated treatment with pilocarpine (30 mg/kg, i.p.; once per day for 21 days) in relation to age-related effects on olfactory and cognitive functions were also studied. The 24 month-old rats exhibited significantly impaired performance in both models, demonstrating deficits in their odour discrimination and in their ability to recognize a juvenile rat after a short period of time. The treatment with pilocarpine improved in a specific manner these age-related deficits in 24 month-old rats without altering their motor performance. The present results extend the notion of the participation of muscarinic receptors in control of olfactory functions and reinforce the potential of muscarinic receptor agonists for the treatment of age-related cognitive decline.     

Pregnenolone sulfate infused in lateral septum of male rats impairs novel object recognition memory

In the present paper we show for the first time that pregnenolone sulfate (Preg-S) impairs rats’ memory for novel object recognition when injected in lateral septum (1.2 μΜ). The effect of Preg-S is clearly related to the moment the reagent is administered: if administered shortly after the training phase, or prior to the test phase of the experiment, there is no amnesic effect. It is only amnesic when administered 30 min before training. Accordingly, Preg-S does not appear to affect the storage of new memories or their retrieval but rather the acquisition itself. Based on the described afferences and efferences of lateral septum, we suggest a possible stimulatory effect of Preg-S regarding glutamate receptors and/or an inhibitory effect of GABA receptors located in local interneurons or recurrent axon collaterals, both of which have been reported to exist in the aforementioned nucleus.     

Cannabinoids

Since the discovery of an endogenous cannabinoid system, research into the pharmacology and therapeutic potential of cannabinoids has steadily increased. Two subtypes of G-protein coupled cannabinoid receptors, CB(1) and CB(1), have been cloned and several putative endogenous ligands (endocannabinoids) have been detected during the past 15 years. The main endocannabinoids are arachidonoyl ethanolamide (anandamide) and 2-arachidonoyl glycerol (2-AG), derivatives of arachidonic acid, that are produced "on demand" by cleavage of membrane lipid precursors. Besides phytocannabinoids of the cannabis plant, modulators of the cannabinoid system comprise synthetic agonists and antagonists at the CB receptors and inhibitors of endocannabinoid degradation. Cannabinoid receptors are distributed in the central nervous system and many peripheral tissues, including immune system, reproductive and gastrointestinal tracts, sympathetic ganglia, endocrine glands, arteries, lung and heart. There is evidence for some non-receptor dependent mechanisms of cannabinoids and for endocannabinoid effects mediated by vanilloid receptors. Properties of CB receptor agonists that are of therapeutic interest include analgesia, muscle relaxation, immunosuppression, anti-inflammation, antiallergic effects, improvement of mood, stimulation of appetite, antiemesis, lowering of intraocular pressure, bronchodilation, neuroprotection and antineoplastic effects. The current main focus of clinical research is their efficacy in chronic pain and neurological disorders. CB receptor antagonists are under investigation for medical use in obesity and nicotine addiction. Additional potential was proposed for the treatment of alcohol and heroine dependency, schizophrenia, conditions with lowered blood pressure, Parkinson's disease and memory impairment in Alzheimer's disease.     

Cannabinoids

The British Pharmacological Society conference commenced with the GC Clark symposium on cannabinoids (CBs), an area that was often represented during the general pharmacology sessions of the conference. Endocannabinoids were identified as playing important physiological roles and the CB receptors appear to offer novel therapeutic targets in a range of clinical conditions. This report concentrates on the CB aspects of the meeting.     

The cannabinoid agonist WIN 55,212-2 reduces sensorimotor gating and recognition memory in rats

Cannabinoids can disrupt short-term memory in humans and animals and induce learning deficits and other cognitive impairments. In the present study we examined the role of a full cannabinoid agonist in short-term memory, sensorimotor gating, and the acquisition and expression of an operant learning paradigm in rats. We tested the effects of the synthetic cannabinoid WIN 55,212-2 (0.6 and 1.2 mg/kg) on short-term memory in social and object recognition tests, on prepulse inhibition (PPI) of startle, as well as on lever pressing for palatable food. Injections of 0.6 and 1.2 mg/kg WIN 55,212-2 impaired recognition memory and PPI in a dose-dependent manner, but had no effect on lever-pressing acquisition or expression, or on food preference. The PPI deficit was reversed by the administration of 0.1 mg/kg haloperidol. These data suggest that the synthetic cannabinoid WIN 55,212-2 does not lead to a general impairment of learning in an appetitive instrumental task, but significantly affects short-term memory and sensorimotor integration. The impairment in recognition and PPI might be due to deficits in attention-based short-term information processing.     

Cannabinoids: neurochemical aspects after oral chronic administration to rats

Male and female Fischer rats were treated orally with Δ⁹-THC doses between 50 and 500 mg/kg or with crude marihuana extract between 50 and 1500 mg/kg, for 28 or 91 consecutive days. At necropsy, brains were weighed and kept frozen until 10% homogenates in 0.32 m sucrose could be made and analyzed. Homogenate samples were assayed for total protein, RNA, lipids, and acetylcholinesterase, succinic dehydrogenase and monoamine oxidase activities. Significant decreases were obtained for protein, RNA and acetylcholinesterase activity at 28 days and monoamine oxidase at 91 days. No changes in total lipids, glycolipids or cholesterol concentrations were observed. The neurochemical alterations coincided with behavioral symptoms of hyperactivity and convulsive activity. Both neurotoxicity and neurochemical changes were partially reversed after the longer interval of treatment.     

The atypical antipsychotic risperidone reverses the recognition memory deficits induced by post-weaning social isolation in rats

Rationale

Rearing rats in isolation from weaning is an established preclinical neurodevelopmental model which induces behavioural deficits with apparent translational relevance to some core symptoms of schizophrenia.

Objective

This study evaluated the ability of the atypical antipsychotic risperidone to reverse behavioural deficits induced by post-weaning social isolation of rat pups and to further characterise the predictive validity of this model.

Method

Forty-five male Lister hooded rats were housed in groups of 3–4 (n?=?16) or singly (n?=?29) for 4 weeks immediately after weaning on postnatal day (PND) 22–24. On PND 51, novel cage-induced locomotor activity (LMA) was assessed to subdivide rats into groups balanced for behavioural response. On PNDs 58, 59, 65 and 72, rats received either vehicle (1 ml/kg; i.p.) or risperidone (0.2 or 0.5 mg/kg; i.p.) 30 min prior to testing in LMA, novel object discrimination (NOD), prepulse inhibition (PPI) of acoustic startle and conditioned emotional response (CER) learning paradigms, respectively.

Results

Isolation rearing had no effect on PPI, but produced LMA hyperactivity and impaired NOD and CER compared to group-housed controls. Risperidone caused a dose-dependent reduction in LMA, irrespective of rearing condition, but selectively reversed the NOD deficit in isolation-reared rats. Risperidone did not reverse the isolation rearing-induced CER deficit.

Conclusions

Similar to its clinical profile, risperidone only partially reverses the schizophrenic symptomology; since it reversed some, but not all, of the learning and memory deficits induced by post-weaning isolation, the isolation rearing model may be useful to predict antipsychotic activity of novel therapeutic agents.     

Quercetin does not alter lipopolysaccharide-induced fever in rats

Fever is considered an important component of the acute phase response of the body in defence against invading organisms such as bacteria. Quercetin, an important representative of the flavonoid class, has been extensively studied as an anti-inflammatory agent. In the present study, we investigated the effect of quercetin, administered orally (5, 25 and 50 mg kg(-1)) or intraperitoneally (50 mg kg(-1)), on the febrile response induced by either intraperitoneally (50 mug kg(-1)) or intravenously (5 mug kg(-1)) injected lipopolysaccharide (LPS from Escherichia coli) in rats. In contrast with the well known anti-inflammatory activity of quercetin, the results demonstrate that quercetin, at the doses used, did not alter the fever induced by LPS, regardless of the route of administration.     

Cannabinoids in clinical practice

Cannabis has a potential for clinical use often obscured by unreliable and purely anecdotal reports. The most important natural cannabinoid is the psychoactive tetrahydrocannabinol (delta9-THC); others include cannabidiol (CBD) and cannabigerol (CBG). Not all the observed effects can be ascribed to THC, and the other constituents may also modulate its action; for example CBD reduces anxiety induced by THC. A standardised extract of the herb may be therefore be more beneficial in practice and clinical trial protocols have been drawn up to assess this. The mechanism of action is still not fully understood, although cannabinoid receptors have been cloned and natural ligands identified. Cannabis is frequently used by patients with multiple sclerosis (MS) for muscle spasm and pain, and in an experimental model of MS low doses of cannabinoids alleviated tremor. Most of the controlled studies have been carried out with THC rather than cannabis herb and so do not mimic the usual clincal situation. Small clinical studies have confirmed the usefulness of THC as an analgesic; CBD and CBG also have analgesic and antiinflammatory effects, indicating that there is scope for developing drugs which do not have the psychoactive properties of THC. Patients taking the synthetic derivative nabilone for neurogenic pain actually preferred cannabis herb and reported that it relieved not only pain but the associated depression and anxiety. Cannabinoids are effective in chemotherapy-induced emesis and nabilone has been licensed for this use for several years. Currently, the synthetic cannabinoid HU211 is undergoing trials as a protective agent after brain trauma. Anecdotal reports of cannabis use include case studies in migraine and Tourette's syndrome, and as a treatment for asthma and glaucoma. Apart from the smoking aspect, the safety profile of cannabis is fairly good. However, adverse reactions include panic or anxiety attacks, which are worse in the elderly and in women, and less likely in children. Although psychosis has been cited as a consequence of cannabis use, an examination of psychiatric hospital admissions found no evidence of this, however, it may exacerbate existing symptoms. The relatively slow elimination from the body of the cannabinoids has safety implications for cognitive tasks, especially driving and operating machinery; although driving impairment with cannabis is only moderate, there is a significant interaction with alcohol. Natural materials are highly variable and multiple components need to be standardised to ensure reproducible effects. Pure natural and synthetic compounds do not have these disadvantages but may not have the overall therapeutic effect of the herb.     

Cannabinoids in postmortem toxicology

Cannabinoids are often excluded from postmortem toxicology screens due to their ubiquitous nature, interpretative difficulties and unanswered questions regarding their postmortem redistribution. In this study, we review 30 postmortem cases where a drug screen gave a positive cannabinoids result and a confirmation identified Δ?-tetrahydrocannabinol (THC), 11-hydroxy-Δ?-tetrahydrocannabinol (11-OH-THC), and/or 11-nor-9-carboxy-Δ?-tetrahydrocannabinol (THC-COOH) in peripheral (BL-P) or cardiac/central blood (BL-C) and/or urine (UR). Had cannabinoids not been included in these toxicologic evaluations, incomplete or erroneous inferences would have been drawn in a substantial number of cases regarding cause/manner of death. THC was detected in 28 BL-C and in all 30 BL-P. THC and THC-COOH were confirmed present in 2 and 23 UR, respectively. 11-OH-THC was detected in 4 BL-C, 6 BL-P, and 0 UR. The mean THC concentrations in BL-C and BL-P were 8.0 and 15.8 ng/mL, respectively. The mean THC-COOH concentrations in BL-C and BL-P were 55.2 and 60.6 ng/mL, respectively. The mean 11-OH-THC concentrations in BL-C and BL-P were 17.0 and 12.5 ng/mL, respectively. Postmortem interval (PMI) for each case was determined and evaluated in relation to BL-C/BL-P concentration ratios with THC-COOH exhibiting a possible trend. This study is the first of its kind and demonstrates the usefulness of cannabinoid analyses as part of death investigations. Furthermore, it provides distribution data that will improve the ability of toxicologists and pathologists to evaluate cannabinoid concentrations in human postmortem specimens.