Iain S. McGregor

Chronic cannabinoid exposure produces lasting memory impairment and increased anxiety in adolescent but not adult rats.

Although many studies have examined the acute behavioural effects of cannabinoids in rodents, few have examined the lasting effects of cannabinoids at different developmental ages. This study compared lasting effects of cannabinoid exposure occurring in adolescence to that occurring in early adulthood. Forty, 30-day old (adolescent) and 18, 56-day old (adult) female albino Wistar rats were injected with vehicle or incremental doses of the cannabinoid receptor agonist (-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydroxypropyl) cyclohexanol (CP 55,940) once per day for 21 consecutive days (150, 200 and 300 μg/kg i.p. for 3, 8 and 10 days, respectively). Following a 21-day drug-free period, working memory was assessed using an object recognition task. Locomotor activity was also measured in the object recognition apparatus via a ceiling-mounted passive infrared sensor. Three days later, anxiety was assessed using a social interaction test. In the object recognition task, significantly poorer working memory was observed in the adolescent but not adult CP 55,940-treated rats. Adolescent, but not adult CP 55,940-treated rats, also exhibited a significant decrease in social interaction with a novel conspecific. These results suggest that chronic exposure to a cannabinoid receptor agonist well after the immediate postnatal period, but before reaching sexual maturity, can lead to increased anxiety and a lasting impairment of working memory.

Adolescent rats find repeated Delta(9)-THC less aversive than adult rats but display greater residual cognitive deficits and changes in hippocampal protein expression following exposure.

The current study examined whether adolescent rats are more vulnerable than adult rats to the lasting adverse effects of cannabinoid exposure on brain and behavior. Male Wistar rats were repeatedly exposed to Δ-9-tetrahydrocannabinol (Δ9-THC, 5 mg/kg i.p.) in a place-conditioning paradigm during either the adolescent (post-natal day 28+) or adult (post-natal day 60+) developmental stages. Adult rats avoided a Δ9-THC-paired environment after either four or eight pairings and this avoidance persisted for at least 16 days following the final Δ9-THC injection. In contrast, adolescent rats showed no significant place aversion. Adult Δ9-THC-treated rats produced more vocalizations than adolescent rats when handled during the intoxicated state, also suggesting greater drug-induced aversion. After a 10–15 day washout, both adult and adolescent Δ9-THC pretreated rats showed decreased social interaction, while only Δ9-THC pretreated adolescent rats showed significantly impaired object recognition memory. Seventeen days following their last Δ9-THC injection, rats were euthanased and hippocampal tissue processed using two-dimensional gel electrophoresis proteomics. There was no evidence of residual Δ9-THC being present in blood at this time. Proteomic analysis uncovered 27 proteins, many involved in regulating oxidative stress/mitochondrial functioning and cytoarchitecture, which were differentially expressed in adolescent Δ9-THC pretreated rats relative to adolescent controls. In adults, only 10 hippocampal proteins were differentially expressed in Δ9-THC compared to vehicle-pretreated controls. Overall these findings suggest that adolescent rats find repeated Δ9-THC exposure less aversive than adults, but that cannabinoid exposure causes greater lasting memory deficits and hippocampal alterations in adolescent than adult rats.

The motivation for beer in rats: effects of ritanserin, naloxone and SR 141716.

Abstract Rats were given two weeks of home cage access to either “near-beer” (a beverage that tastes like beer but contains <0.5% ethanol v/v) or near-beer with added ethanol (4.5% v/v), which is simply referred to as “beer”. The two groups of rats (near-beer and beer) were then trained on a “lick-based progressive ratio paradigm” in operant chambers in which an ever increasing number of licks had to be emitted for each successive fixed unit of near-beer or beer delivered. Break points (the ratio at which responding ceased) for near-beer and beer were approximately equal under baseline conditions. Rats were then tested for the effects of the 5HT2A/2C receptor antagonist ritanserin (0.625, 2.5 or 10 mg/kg), the opioid receptor antagonist naloxone (0.625, 2.5 or 10 mg/kg) or the cannabinoid CB1 receptor antagonist SR 141716 (0.3, 1 or 3 mg/kg). All three drugs caused a dose-dependent reduction of break-points and locomotor activity in both the beer and near-beer groups. However, the effects of SR 141716 and naloxone, but not ritanserin, on break-points were significantly more pronounced on rats drinking beer compared to those drinking near-beer. There were no such differential effects of any of the drugs on locomotor activity across the two groups. These results suggest that both SR 141716 and naloxone differentially affect the motivation to consume alcoholic beverages and may thus have potential as drugs for the treatment of alcohol craving.

Neural Correlates of Cat Odor-Induced Anxiety in Rats: Region-Specific Effects of the Benzodiazepine Midazolam

Cat odor elicits a profound defensive reaction in rats that is reduced by benzodiazepine drugs. The neural correlates of this phenomenon were investigated here using Fos immunohistochemistry. Rats received either midazolam (0.75 mg/kg, s.c.) or vehicle and were exposed to pieces of a collar that had been worn by a domestic cat or an unworn (dummy) collar. Cat odor caused midazolam-sensitive defensive behavioral responses, including avoidance of collar contact, inhibition of grooming, and prolonged rearing. Cat odor exposure induced Fos expression in the posterior accessory olfactory bulb (glomerular, mitral, and granule cell layers), with granule cell layer activation attenuated by midazolam. High basal Fos expression, and some cat odor-associated Fos expression, was evident in the main olfactory bulb (glomerular cell layer), and midazolam exerted a strong inhibitory effect in this region. Midazolam inhibited Fos expression in key limbic regions involved in pheromone transduction (medial amygdala and bed nucleus of the stria terminalis) and defensive behavior (prelimbic cortex, lateral septum, lateral and medial preoptic areas, and dorsal premammillary nucleus). However, midazolam failed to affect cat odor-related Fos expression in a range of key defense-related sites, including the ventromedial hypothalamic nucleus, paraventricular nucleus of the hypothalamus, periaqueductal gray, and cuneiform nucleus. These results indicate that midazolam exerts a region-specific effect on the neural substrates activated by predator odor, with effects in the lateral septum and dorsal premammillary nucleus likely to be of major importance. These findings also suggest the intriguing hypothesis that cat odor is processed by rats as a “pheromone-like” stimulus.

Modulation of anxiety-related behaviours following lesions of the prelimbic or infralimbic cortex in the rat.

A series of experiments examined behavioural and autonomic aspects of stress and anxiety in rats subjected to either: (1) electrolytic lesions of the infralimbic cortex subregion of the medial prefrontal cortex; (2) electrolytic lesions of the prelimbic cortex subregion of the medial prefrontal cortex; (3) sham lesions of infralimbic or prelimbic cortex (sham control); or (4) no lesions (control). In exploration-based models of anxiety, infralimbic- or prelimbic-lesioned rats spent less time in the centre of an open field and less time on the exposed arms of an elevated plus maze, indicating increased anxiety. Locomotor activity was normal in the lesioned rats when tested in a non-stressful enclosed environment. In a step-down passive avoidance task, infralimbic-lesioned rats stepped down more quickly than controls onto a grid floor where they had been shocked 24 h previously. Prelimbic-lesioned rats were no different to controls on this test, although they showed greater latencies to step down onto the grid floor during conditioning. In a final experiment, indirect calorimetry was used to show that both infralimbic- and prelimbic-lesioned rats have essentially normal alterations in oxygen consumption and energy substrate utilisation when exposed to brief footshock. Thus, the impaired passive avoidance in infralimbic-lesioned rats cannot be attributed to decreased nociception. It is concluded that both the prelimbic and infralimbic regions play a role in anxiety, and that this role may be subtly differentiated. In particular, the infralimbic cortex may have a specific role in mediating the inhibition of behaviours associated with aversive outcomes.

Breaking the loop: Oxytocin as a potential treatment for drug addiction

Drug use typically occurs within a social context, and social factors play an important role in the initiation, maintenance and recovery from addictions. There is now accumulating evidence of an interaction between the neural substrates of affiliative behavior and those of drug reward, with a role for brain oxytocin systems in modulating acute and long-term drug effects. Early research in this field indicated that exogenous oxytocin administration can prevent development of tolerance to ethanol and opiates, the induction of stereotyped, hyperactive behavior by stimulants, and the withdrawal symptoms associated with sudden abstinence from drugs and alcohol. Additionally, stimulation of endogenous oxytocin systems is a key neurochemical substrate underlying the prosocial and empathogenic effects of party drugs such as MDMA (Ecstasy) and GHB (Fantasy). Brain oxytocin systems exhibit profound neuroplasticity and undergo major neuroadaptations as a result of drug exposure. Many drugs, including cocaine, opiates, alcohol, cannabis, MDMA and GHB cause long-term changes in markers of oxytocin function and this may be linked to enduring deficits in social behavior that are commonly observed in laboratory animals repeatedly exposed to these drugs. Very recent preclinical studies have illustrated a remarkable ability of exogenously delivered oxytocin to inhibit stimulant and alcohol self-administration, to alter associated drug-induced changes in dopamine, glutamate and Fos expression in cortical and basal ganglia sites, and to prevent stress and priming-induced relapse to drug seeking. Oxytocin therefore has fascinating potential to reverse the corrosive effects of long-term drugs abuse on social behavior and to perhaps inoculate against future vulnerability to addictive disorders. The results of clinical studies examining intranasal oxytocin effects in humans with drug use disorders are eagerly awaited. This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior.

Repeated cannabinoid exposure during perinatal, adolescent or early adult ages produces similar longlasting deficits in object recognition and reduced social interaction in rats.

There is mounting evidence that chronic cannabis use might result in lasting neurobehavioural changes, although it remains unclear whether vulnerability diminishes with age. The current study compared the effects of cannabinoid exposure at three developmental periods on subsequent measures of memory and anxiety. Male rats aged 4 days (perinatal), 30 days (adolescent) and 56 days (young adult) were injected with vehicle or incremental doses of the cannabinoid receptor agonist CP 55940, daily for 21 consecutive days (0.15, 0.20 or 0.30mg/kg for 7 days per dose, respectively). Following a 28-day drug-free period, working memory was assessed in an object recognition task. One week later, social anxiety was assessed in a social interaction test. Two days later, generalized anxiety was assessed in an emergence test. Results revealed that CP 55940 impaired working memory and social interaction similarly at all three ages. CP 55940 had no effects in five of six emergence test measures, but a modest but significant reduction in anxiety was noted in one measure following adolescent exposure. We conclude that chronic cannabinoid exposure leads to long-term memory impairments and increased anxiety, irrespective of the age at which drug exposure occurrs.

A role for oxytocin and 5-HT1A receptors in the prosocial effects of 3,4 methylenedioxymethamphetamine ("ecstasy")

The drug 3,4 methylenedioxymethamphetamine (MDMA; ecstasy) has a widely documented ability to increase feelings of love and closeness toward others. The present study investigated whether oxytocin, a neuropeptide involved in affiliative behavior, may play a role in this effect. A moderate (5 mg/kg, i.p.) dose of MDMA increased social interaction in male Wistar rats, primarily by increasing the amount of time rats spent lying adjacent to each other. MDMA (5 mg/kg) activated oxytocin-containing neurons in the supraoptic and paraventricular nuclei of the hypothalamus, as shown by Fos immunohistochemistry. MDMA (5 mg/kg i.p.) also increased plasma oxytocin levels and this effect was prevented by pre-treatment with the 5-HT(1A) antagonist N-[2-[4-(2-methyoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide maleate salt (WAY 100,635; 1 mg/kg i.p.). The oxytocin receptor antagonist tocinoic acid (20 microg, i.c.v.) had no effect on social behavior when given alone but significantly attenuated the facilitation of social interaction produced by MDMA (5 mg/kg). The 5-HT(1A) agonist 8-hydroxy-2-(di-n-propylamino)-tetraline) (8-OH-DPAT, 0.25 mg/kg, i.p.) increased social behavior in a similar way to MDMA and this effect was also significantly attenuated by tocinoic acid. Taken together, these results suggest that oxytocin release, stimulated by MDMA through 5-HT(1A) receptors, may play a key role in the prosocial effects of MDMA and underlie some of the reinforcing effects of the drug.

Aversive effects of the synthetic cannabinoid CP 55,940 in rats

A series of experiments investigated the behavioral and hedonic effects of the synthetic cannabinoid CP 55,940 in male Wistar rats. CP 55,940 had a biphasic effect on locomotor activity, with a 10 micrograms/kg dose causing locomotor stimulation and a 100 micrograms/kg dose causing profound hypoactivity. CP 55,940 (100 micrograms/kg) also caused a marked hypothermia for at least 3 h following administration, while lower doses (2.5 and 10 micrograms/kg) had no effect. CP 55,940 (100 micrograms/kg) had anorexic and hyperdipsic effects for up to 24 h following administration and caused significant reductions in body weight. CP 55,940 (100 micrograms/kg) also caused significant avoidance to a flavoured fluid (saccharin) with which it was paired. In the conditioned place preference paradigm both the 10 micrograms/kg and 100 micrograms/kg doses of CP 55,940 produced significant place avoidance. It is concluded that CP 55,940 is aversive to rats. The possible mechanisms underlying this aversion are discussed.

From ultrasocial to antisocial: a role for oxytocin in the acute reinforcing effects and long‐term adverse consequences of drug use?

Addictive drugs can profoundly affect social behaviour both acutely and in the long‐term. Effects range from the artificial sociability imbued by various intoxicating agents to the depressed and socially withdrawn state frequently observed in chronic drug users. Understanding such effects is of great potential significance in addiction neurobiology. In this review we focus on the ‘social neuropeptide’ oxytocin and its possible role in acute and long‐term effects of commonly used drugs. Oxytocin regulates social affiliation and social recognition in many species and modulates anxiety, mood and aggression. Recent evidence suggests that popular party drugs such as MDMA and gamma‐hydroxybutyrate (GHB) may preferentially activate brain oxytocin systems to produce their characteristic prosocial and prosexual effects. Oxytocin interacts with the mesolimbic dopamine system to facilitate sexual and social behaviour, and this oxytocin‐dopamine interaction may also influence the acquisition and expression of drug‐seeking behaviour. An increasing body of evidence from animal models suggests that even brief exposure to drugs such as MDMA, cannabinoids, methamphetamine and phencyclidine can cause long lasting deficits in social behaviour. We discuss preliminary evidence that these adverse effects may reflect long‐term neuroadaptations in brain oxytocin systems. Laboratory studies and preliminary clinical studies also indicate that raising brain oxytocin levels may ameliorate acute drug withdrawal symptoms. It is concluded that oxytocin may play an important, yet largely unexplored, role in drug addiction. Greater understanding of this role may ultimately lead to novel therapeutics for addiction that can improve mood and facilitate the recovery of persons with drug use disorders.