Impulsivity and opioid drugs: differential effects of heroin, methadone and prescribed analgesic medication

BACKGROUND

Previous studies have provided inconsistent evidence that chronic exposure to opioid drugs, including heroin and methadone, may be associated with impairments in executive neuropsychological functioning, specifically cognitive impulsivity. Further, it remains unclear how such impairments may relate of the nature, level and extent of opioid exposure, the presence and severity of opioid dependence, and hazardous behaviours such as injecting.

METHOD

Participants with histories of illicit heroin use (n = 24), former heroin users stabilized on prescribed methadone (methadone maintenance treatment; MMT) (n = 29), licit opioid prescriptions for chronic pain without history of abuse or dependence (n = 28) and healthy controls (n = 28) were recruited and tested on a task battery that included measures of cognitive impulsivity (Cambridge Gambling Task, CGT), motor impulsivity (Affective Go/NoGo, AGN) and non-planning impulsivity (Stockings of Cambridge, SOC).

RESULTS

Illicit heroin users showed increased motor impulsivity and impaired strategic planning. Additionally, they placed higher bets earlier and risked more on the CGT. Stable MMT participants deliberated longer and placed higher bets earlier on the CGT, but did not risk more. Chronic opioid exposed pain participants did not differ from healthy controls on any measures on any tasks. The identified impairments did not appear to be associated specifically with histories of intravenous drug use, nor with estimates of total opioid exposure.

CONCLUSION

These data support the hypothesis that different aspects of neuropsychological measures of impulsivity appear to be associated with exposure to different opioids. This could reflect either a neurobehavioural consequence of opioid exposure, or may represent an underlying trait vulnerability to opioid dependence.

A high-fat-diet-induced cognitive deficit in rats that is not prevented by improving insulin sensitivity with metformin

AIMS/HYPOTHESIS

We previously demonstrated that animals fed a high-fat (HF) diet for 10 weeks developed insulin resistance and behavioural inflexibility. We hypothesised that intervention with metformin would diminish the HF-feeding-evoked cognitive deficit by improving insulin sensitivity.

METHODS

Rats were trained in an operant-based matching and non-matching to position task (MTP/NMTP). Animals received an HF (45% of kJ as lard; n = 24), standard chow (SC; n = 16), HF + metformin (144 mg/kg in diet; n = 20) or SC + metformin (144 mg/kg in diet; n = 16) diet for 10 weeks before retesting. Body weight and plasma glucose, insulin and leptin were measured. Protein lysates from various brain areas were analysed for alterations in intracellular signalling or production of synaptic proteins.

RESULTS

HF-fed animals developed insulin resistance and an impairment in switching task contingency from matching to non-matching paradigm. Metformin attenuated the insulin resistance and weight gain associated with HF feeding, but had no effect on performance in either MTP or NMTP tasks. No major alteration in proteins associated with insulin signalling or synaptic function was detected in response to HF diet in the hypothalamus, hippocampus, striatum or cortex.

CONCLUSIONS/INTERPRETATION

Metformin prevented the metabolic but not cognitive alterations associated with HF feeding. The HF diet protocol did not change basal insulin signalling in the brain, suggesting that the brain did not develop insulin resistance. These findings indicate that HF diet has deleterious effects on neuronal function over and above those related to insulin resistance and suggest that weight loss may not be sufficient to reverse some damaging effects of poor diet.

Influence of 5,7-dihydroxytryptamine lesions of the rat fornix-fimbria and cingulum bundles on spontaneous activity in an aversive maze

Exposure to aversive environmental stimuli stimulates the serotonergic neurones that project to the forebrain and inhibit spontaneous activity when studied in a simple maze. This study explored the putative role of the principal 5-hydroxytryptamine (5-HT) neurones that project to the hippocampus from the median raphe nucleus in this response to an aversive environment by lesioning the 5-HT fibres that project through the fornix/fimbria and cingulum bundles. The effects of the lesions were investigated in independent groups of animals tested in an enclosed four-arm maze and a more aversive elevated maze of the same dimensions composed entirely of four open arms. The rats were significantly less active in the open maze, the principal effect of maze design being observed during the first 5 min sub-trial of a 15 min trial. This response to the more aversive environment was totally abolished by the lesion. It is concluded that exposure to an explicitly aversive environment elicits a brief stimulation of the 5-HT neurones that project to the hippocampus from the median raphe nucleus and that this stimulation inhibits the initial burst of exploratory activity that is observed in animals placed in a less aversive novel environment.

Behavioural and neurochemical responses evoked by repeated exposure to an elevated open platform

Increased psychophysiological resistance to chronic stress has been related to increased 5-HT release in the dorsal hippocampus. This study investigated the changes in 5-HT release and turnover in the hippocampus evoked by acute and repeated exposure to an inescapable stressor, an elevated open platform, and compared them to the changes evoked in the frontal cortex. Repeated exposure to this stressor results in habituation of the plasma corticosterone response to the test, with full habituation being observed after 20 trials. Repeated exposure to the stressor for 5 or 10 occasions increased 5-HT turnover in the hippocampus. By contrast, 5-HT turnover in frontal cortex was increased by acute exposure to the stressor. Microdialysis studies showed that acute stress increased 5-HT overflow in prefrontal cortex but not dorsal hippocampus whereas repeated daily (10 days) exposure to the stressor increased basal extracellular 5-HT in the dorsal hippocampus, but not the prefrontal cortex. Prior exposure to the stressor on up to 10 occasions enhanced the plasma corticosterone response to a challenge in an elevated plus-maze performed 24h later whereas repeated, but not acute, exposure to the stressor, elicited anxiolytic-like behavioural responses in this test. It is concluded that acute exposure to this form of inescapable stress selectively stimulates the 5-HT projections to the frontal cortex; repeated stress elicits a sustained increase in 5-HT release and turnover in the hippocampus. The data are consistent with the hypothesis that increased 5-HT release in the hippocampus may be implicated in the mechanisms underlying habituation to inescapable stress.

Regulation of corticosteroid receptors in the rat brain: the role of serotonin and stress

It has been suggested that physiological resistance to repeated stress is associated with increased 5-hydroxytryptamine (5-HT) release in the dorsal hippocampus and that dysregulation of this neuroadaptation may be implicated in the psychopathology of depression. This study used 5,7-dihydroxytryptamine lesions to investigate the role of 5-HT projections to the hippocampus in physiological responses to repeated stress and putative changes in corticosteroid receptor immunoreactivity in the brain. Repeated exposure to elevated open platform stress (1 h/day) caused regionally selective changes in glucocorticoid and mineralocorticoid receptor immunoreactivity in the dorsal hippocampus that were not observed in ventral hippocampus, frontal cortex, hypothalamus or parietal cortex. Glucocorticoid receptor immunoreactivity in the dorsal hippocampus was decreased after 5 days but increased after 20 days of stress. Mineralocorticoid receptor immunoreactivity was increased after 5 or 10 days of stress. The increases in glucocorticoid and mineralocorticoid receptor immunoreactivity, evoked by repeated stress, were abolished by lesions of the principal 5-HT projections to the hippocampus. The lesions abolished the increased defecation observed in stressed animals, but had no effects on the plasma corticosterone response to the stressor or the habituation of this response observed following repeated stress. The experiments have revealed a dissociation in the regulation of corticosteroid receptor expression in the dorsal and ventral hippocampus by repeated stress and 5-HT. The data suggest that adaptation to inescapable stress is associated with regionally selective changes in corticosteroid receptor expression in dorsal hippocampus that are largely 5-HT-dependent, although these changes do not mediate habituation of the pituitary adrenocortical response to the stressor.