Trends in Mortality From Novel Psychoactive Substances as "Legal Highs": Gender Differences in Manner of Death and Implications for Risk Differences for Women

Introduction

This study aimed to examine drug-related deaths in the UK in which novel psychoactive substances (NPS) are an implicated substance, and to focus on female deaths in comparison with male deaths. While male overdoses dominate epidemiological statistics, there is an increase in female drug-related deaths and a narrowing of the gap between gender mortality rates which is to date unexplained.

Method

This study analyzed data from the National Programme for Substance Abuse Deaths (NPSAD) database that records drug-related deaths in the UK from coronial records. A dataset was constructed using parameters to capture all drug-related cases during the period 2007-2017 when NPS were legal and highly available in the UK, in order to capture deaths recorded among both regular and occasional drug users, and to include all cases recorded during that period regardless of NPS status in order to make comparisons. The final dataset comprised 10,159 cases, with 456 NPS-related deaths. Data for NPS and non-NPS were compared, and comparisons were made between cohorts by gender. The dataset also includes coronial narrative notes which allowed a qualitative analysis of NPS female deaths to add contextual explanation.

Results

The proportion of male NPS deaths is significantly higher than that for female NPS deaths but does not reflect the generalized difference between male and female drug-related mortality of this period studied. Demographic and outcome data by gender difference were significant for all drug-related deaths, but not for NPS-only deaths, indicating a greater homogeneity among NPS deaths by gender. Older women using NPS were more likely to have methadone or diazepam as another drug implicated and have established histories of drug misuse.

Conclusion

Where NPS have been used, differences in drug death profiles are less likely to be accounted for by gender than other demographic or behavioral differences more typically found in opiate deaths. The social and health problems of older women may be key characteristics that differentiate female deaths from male deaths. These findings also support evidence of increasing uptake of NPS among older established drug users that adds further risk to polydrug use.

Seeing sadness: Comorbid effects of loneliness and depression on emotional face processing

BACKGROUND/OBJECTIVE

Loneliness and depression are highly comorbid, and both are associated with social processing deficits. However, there is a paucity of research aimed at differentiating emotional face-processing deficits that are comorbid to loneliness and depression versus those attributable to loneliness or depression only.

METHODS

502 participants were recruited and screened for loneliness (UCLA Loneliness Scale) and depression (Beck Depression Inventory). Of those, seventy-seven took part in a fully crossed 2 (loneliness; low/high) * 2 (depression; low/high) factorial between-subjects design study to assess individual and comorbid effects of loneliness and depression on a computerized morphed facial emotion processing task.

RESULTS

Comorbidity was confirmed by a significant positive correlation between loneliness and depression. On the emotion processing task, loneliness was associated with an increased accuracy for sad faces and decreased accuracy for fearful faces and depression with decreased accuracy in identifying happy faces. Comorbid loneliness and depression resulted in an increased misattribution of neutral faces as sad, an effect that was also seen in those who were either only lonely or only depressed.

CONCLUSION

This if the first study to tease out comorbid versus independent effects of loneliness and depression on social information processing. To the extent that emotional biases may act as risk factors for detrimental outcomes, our findings highlight the importance of treating both loneliness and depression.

Neuroticism related differences in working memory tasks

Two influential theories relating to personality traits, i.e. arousal-based theory (ABT) and attentional control theory (ACT), made predictions on how neuroticism may affect task performance. ABT suggested that high neurotics perform worse than low neurotics in all difficult tasks, whereas they perform similar in easy tasks. On the other hand, ACT suggested that high neurotics perform worse than low neurotics only if the task relies on central executive functions of working memory (WM), such as switching or inhibition. However, currently it is still unclear whether neuroticism affects all difficult tasks, as proposed by ABT, or whether it is specific to certain tasks, as proposed by ACT. To test this, we used the Cambridge Neuropsychological Tasks Automated Battery (CANTAB) as our test tool and we selected three working memory tasks which tested the effect of neuroticism on both the central executive system (CES) and the WM storage system (i.e. visuospatial sketchpad) in 21 low and 24 high neurotics. Results showed that high neurotics, as compared to low neurotics, exhibited lower performance only when the working memory task is specifically associated with switching and/or inhibition, but not in a task which is associated with the visuospatial sketchpad. We conclude that the results support the ACT rather than the ABT, because high levels of neuroticism impaired behavioural performance specifically in demanding tasks associated with switching and inhibition, but not in tasks associated with the visuospatial sketchpad.

Does perception of drug-related harm change with age? A cross-sectional online survey of young and older people

OBJECTIVES

To investigate how young and older people perceive the harms associated with legal and illegal drugs.

DESIGN

Cross-sectional study: adults aged 18-24 years versus 45+ completed an online survey ranking the perceived harms associated with 11 drugs on 16 drug-related harm criteria.

SETTING

Online survey.

PARTICIPANTS

184 participants aged 18-24 years (113 female: mean age 21: SD 1.3) and 91 participants aged 45+ (51 female: mean age 60: SD 8.5).

MAIN OUTCOME MEASURES

'Perception of drug-related harms': This was measured using a rating scale ranging from 1 (no risk of harm) to 4 (high risk of harm). Participants were also asked about sources which informed their perception on drug-related harms as well as their own personal self-reported drug experiences.

RESULTS

Of the illegal drugs, heroin, methamphetamine and cocaine were rated as the most harmful and cannabis was rated as the least harmful. Alcohol and tobacco were also rated as less harmful. The results showed that perceptions of drug-related harms were inconsistent with current knowledge from research on drugs. Furthermore, perceptions on drug harms were more conservative in the 45+ group for a number of illegal drugs and tobacco. However, the 45+ age group did not perceive alcohol as any more harmful than the younger group.

CONCLUSIONS

This survey demonstrates that the greatest misperception was in relation to alcohol-related harms which did not change with age. In order to minimise harms, this misperception needs to be addressed through education and policies that legislate drug use.

Low identification of alcohol use disorders in general practice in England

AIMS

The prevalence of alcohol use disorders (AUDs) in the United Kingdom is estimated at 25%, and primary care has been identified as the first line of treatment for this population. However, there is a paucity of evidence regarding the current rates of identification of AUDs in primary care. The aim of the present study was to compare the observed rates of AUDs in general practice with expected rates, which are based on general population prevalence rates of AUDs.

DESIGN, PARTICIPANTS AND MEASUREMENTS

Epidemiological data on individuals aged 16-64 years with an AUD was obtained from the General Practice Research Database. General population prevalence rates of AUDs were obtained from the Psychiatric Morbidity Survey. Chi(2) tests and identification ratios were used to analyse the data.

RESULTS

There was a significant relationship between type of AUD and identification (chi(2)=1466.89, P<0.001), and general practitioners were poorer at identifying harmful/hazardous drinkers when compared with dependent drinkers. No gender differences in the identification of hazardous/harmful drinking were found, but female dependent drinkers were significantly more likely to be identified than males (identification ratio 0.07; 95% confidence interval 0.06-0.07). The identification of AUDs was significantly lower for the 16-24-year age group compared with all other age groups.

CONCLUSION

Despite attempts at targeting hazardous/harmful drinkers for brief interventions in primary care, the present findings suggest that this group are still under-identified. Furthermore, this under-identification is even more apparent in men and in young people who have high general population prevalence rates for AUDs. In conclusion, increasing identification rates could be incorporated into brief intervention strategies in primary care.

Antidepressant-related deaths and antidepressant prescriptions in England and Wales, 1998-2000

BACKGROUND

Deaths from antidepressants continue to account for a substantial proportion of drug-related deaths.

AIMS

To investigate the relative toxicity of the major classes of antidepressant drugs, with the specific objective of assessing this in relation to the cause of death; and to analyse the deaths where there were multiple mentions of antidepressant drugs or other psychoactive drugs with antidepressants.

METHOD

Mortality data were collected from the National Programme of Substance Abuse Deaths, and antidepressant prescription data were collected.

RESULTS

Most deaths from antidepressant drugs were suicides (80%). Tricyclic antidepressants (TCAs) accounted for more drug mentions than did other antidepressant drugs (12 per million prescriptions). Selective serotonin reuptake inhibitors (SSRIs) were associated with a significantly lower risk of toxicity, but 93% of deaths from SSRIs occurred in combination with other drugs, especially TCAs (24.5%). In 'combination' deaths patients were significantly more likely to have had a history of drug misuse.

CONCLUSIONS

The efficacy and safety of augmentation therapy with TCAs in SSRI-resistant patients should be monitored carefully, and patients prescribed antidepressants should be screened for drug use/misuse.

Cause and manner of death in drug-related fatality: an analysis of drug-related deaths recorded by coroners in England and Wales in 2000

This study investigated causes and manner of drug-related fatalities recorded in 2000 in the United Kingdom, measuring the 'masked' manner of death in cases typically recorded as overdose. A retrospective cohort study was used of 1037 cases of accidental drug-related fatalities reported by coroners in England and Wales to the National Programme of Substance Abuse Deaths. Whilst 802 cases were identified as direct acute overdose, representing 77% of the total accidental deaths, 23% of 'overdose' fatalities were caused by asphyxiation (7%), drug-related medical conditions (7%), non-drug-related conditions (4%), traumatic accidents (3%) and infections (2%). Younger people show higher risk of overdose and asphyxiation; older people show higher risk from pre-existing medical conditions. This study not only confirmed the high risk of overdose associated with heroin and polydrug use, but it also identified other high fatality risk factors for heroin/morphine users such as contracting an acute infection leading to septicaemia or endocarditis, or contracting a chronic infection such as HIV, HBV or HCV. In contrast, stimulants particularly featured in traumatic accidents, with amphetamine use most associated with cardio-vascular fatality. These findings highlight the 'masked' manner of death in cases commonly recorded as overdose and demonstrate the need for a more-detailed and systematic method of recording drug-related deaths in order to inform drug education and harm reduction strategies.

Corticotropin releasing factor antagonist, alpha-helical CRF(9-41), reverses nicotine-induced conditioned, but not unconditioned, anxiety

RATIONALE

Unconditioned anxiogenic effects of nicotine have been observed in the social interaction (SI) test 5 min after injection of a low dose and both 5 min and 30 min after injection of a high dose. Conditioned anxiety has also been observed 24 h after testing in the SI with a high dose of nicotine.

OBJECTIVES

In order to determine whether these three anxiogenic effects shared a common mechanism, we investigated the role of corticotropin releasing factor (CRF). We therefore examined whether the CRF antagonist alpha-helical CRF(9-41) could block these three anxiogenic effects of nicotine.

METHODS

To test the unconditioned anxiogenic effects, pairs of male rats were tested in SI 5 min after s.c. vehicle or nicotine (0.1 mg/kg) or 30 min after s.c. vehicle or nicotine (0.45 mg/kg), and 30 min after i.c.v. artificial cerebrospinal fluid (aCSF) or alpha-helical CRF(9-41). To test conditioned anxiety, rats were exposed to the SI test on day 1, 5 min after vehicle or nicotine (0.1 mg/kg). On day 2, they were re-tested in SI 30 min after i.c.v. aCSF or alpha-helical CRF(9-41) (5 microg).

RESULTS

alpha-Helical CRF(9-41) did not block the unconditioned anxiogenic effect of either dose of nicotine. Nicotine (0.1 mg/kg, 5 min) elicited a conditioned anxiogenic response that was significantly reversed by alpha-helical CRF(9-41). The CRF antagonist alone had no effect.

CONCLUSIONS

CRF is an important mediator of the conditioned anxiety to nicotine, but may not play a role in mediating the acute anxiogenic effects.

Anxiety conditioned to nicotine in the elevated plus-maze is time dependent

Conditioning to the anxiogenic effects of nicotine has previously been demonstrated in the social interaction test and there was no generalization of conditioning between the social interaction and elevated plus-maze tests. Because the two tests generate distinct states of anxiety, the conditioning could have occurred to the cues associated with the test environment and/or to those associated with the type of anxiety generated by the test. The elevated plus-maze permits separation of these two factors, because quite distinct states of anxiety are generated on trials 1 and 2, whereas the apparatus cues remain the same. Rats that had been tested on day 1 in the plus-maze, 5 min after nicotine (0.45 mg/kg), showed a conditioned anxiogenic response when tested undrugged on day 2. This was shown by significantly lower percentages of open-arm entries and percentage of time spent on the open arms, compared with control groups. Thus, conditioning to apparatus cues is sufficient to mediate a conditioned anxiogenic effect. The importance of the timing of the nicotine-associated cues was demonstrated by the failure to obtain conditioned anxiogenic effects when rats were exposed to the plus-maze on day 1, 30 min after nicotine (0.45 or 0.1 mg/kg).

Conditioned anxiety to nicotine

RATIONALE

Despite its reinforcing properties nicotine has also been reported to produce anxiety in humans and anxiogenic effects in animal tests of anxiety.

OBJECTIVE

The aims of this study were three-fold: (a) to investigate whether anxiety can be conditioned to cues associated with an acute anxiogenic dose of nicotine, (b) to investigate whether the conditioned anxiety is specific to a particular test of anxiety, and (c) to investigate whether nicotine pre-exposure influences the development of a conditioned anxiogenic effect.

METHODS

An anxiogenic dose of nicotine was administered to rats either before or after experience with the social interaction (SI) test. The retention of a conditioned anxiogenic response was examined when the rats were re-tested undrugged in the SI test 24 h later. To test whether conditioned anxiety was test specific, rats that had been tested in the elevated plus-maze with an anxiogenic dose of nicotine were retested undrugged in the SI test 24 h later, and vice versa. We then examined the effects of 4 days or 4 weeks pre-exposure to nicotine on the development of a conditioned anxiogenic response in the SI test.

RESULTS

Rats injected with nicotine (0.45 mg/kg s.c.) 5 min before the social interaction test spent significantly less time in SI, indicating an unconditioned anxiogenic effect than did vehicle-injected controls or rats injected with nicotine after the test. After 24 h when all groups were tested undrugged only those previously tested in SI after nicotine injection showed a significant conditioned anxiogenic effect. This conditioned anxiety was test specific. Rats injected with nicotine before the SI test did not show an anxiogenic response when tested 24 h later undrugged in the plus-maze, and vice versa. Furthermore, although 4 days exposure to nicotine (0.45 mg/kg s.c.) did not prevent the development of a conditioned anxiogenic response, 4 weeks self-administration of nicotine (total dose, 0.45 mg/kg i.v) in an operant chamber did not affect the acute anxiogenic response to nicotine in the SI test, but it did prevent the development of conditioned anxiety.

CONCLUSIONS

The present findings suggest that anxiety can be conditioned following exposure to an anxiogenic dose of nicotine, and that this anxiety is specific to the contextual cues associated with the SI test.

Nicotinic--serotonergic interactions in brain and behaviour

This review focuses on nicotinic--serotonergic interactions in the central nervous system (CNS). Nicotine increases 5-hydroxytryptamine (5-HT) release in the cortex, striatum, hippocampus, dorsal raphé nucleus (DRN), hypothalamus, and spinal cord. As yet, there is little firm evidence for nicotinic receptors on serotonergic terminals and thus nicotine's effects on 5-HT may not necessarily be directly mediated, but there is strong evidence that the 5-HT tone plays a permissive role in nicotine's effects. The effects in the cortex, hippocampus, and DRN involve stimulation of 5-HT(1A) receptors, and in the striatum, 5-HT(3) receptors. The 5-HT(1A) receptors in the DRN play a role in mediating the anxiolytic effects of nicotine and the 5-HT(1A) receptors in the dorsal hippocampus and lateral septum mediate its anxiogenic effects. The increased startle and anxiety during nicotine withdrawal is mediated by 5-HT(1A) and 5-HT(3) receptors. The locomotor stimulant effect of acute nicotine is mediated by 5-HT(1A) receptors and 5-HT(2) receptors may play a role in the expression of a sensitised response after chronic nicotine treatment. Unfortunately, the role of 5-HT(1A) receptors in mediating nicotine seeking has not yet been investigated and would seem an important area for future research. There is also evidence for nicotinic--serotonergic interactions in the acquisition of the water maze, passive avoidance, and impulsivity in the five-choice serial reaction task.

Antagonism of the anxiolytic effect of nicotine in the dorsal raphe nucleus by dihydro-beta-erythroidine

Nicotine has been reported to reduce anxiety in humans and in a number of animal tests. In the social interaction test of anxiety, administration of low doses of nicotine into the dorsal raphé nucleus (DRN) increases the time spent in social interaction without producing accompanying changes in locomotor activity, suggesting that nicotine acts specifically to reduce anxiety in this brain region. The present study examined the ability of the high-affinity competitive nicotinic receptor antagonist dihydro-beta-erythroidine hydrobromide (DH beta E) to antagonise the anxiolytic effect of nicotine following intra-DRN infusion using the social interaction test. The increase in social interaction observed after administration of nicotine (5 ng) into the DRN was completely reversed by coadministration of 100 ng DH beta E. DH beta E (100 ng), when administered alone into the DRN, did not modify the time spent in social interaction. However, it did significantly increase locomotor activity, and this effect was not antagonised by coadministration of nicotine (5 ng) into the DRN. Because of the pharmacological profile of DH beta E, our results suggest that the anxiolytic effect of nicotine in the DRN is mediated by the alpha 4 beta 2 nicotinic receptor subtype.

Anxiolytic actions of the substance P (NK1) receptor antagonist L-760735 and the 5-HT1A agonist 8-OH-DPAT in the social interaction test in gerbils

The gerbil social interaction test has previously detected anxiolytic effects of nicotine and diazepam. In the present study, the high affinity substance P (NK(1)) receptor antagonist L-760735 (3 mg/kg) significantly increased the time spent in social interaction, whereas its low affinity analogue L-781773 (3 mg/kg) was without effect. Diazepam (0.1 mg/kg) and the 5-HT(1A) receptor agonist 8-OH-DPAT (0.003 and 0.01 mg/kg) also increased social interaction, whereas an acute dose of the selective serotonin re-uptake inhibitor fluoxetine (10 mg/kg) decreased the time spent in social interaction. Diazepam (0.1 mg/kg) significantly increased locomotor activity, but this effect was independent of the increase in social interaction. The other drugs tested were without effect on locomotor activity. The present findings suggest that the gerbil social interaction may well provide a useful assay for detecting both anxiolytic and anxiogenic compounds, and suggests that the high affinity NK(1) receptor antagonist L-760735 may prove to be useful as an anxiolytic therapy.

In adolescence, female rats are more sensitive to the anxiolytic effect of nicotine than are male rats

Anxiety may play an important role in the onset of smoking, particularly in young girls. This study examined whether there were sex differences in the effects of nicotine on anxiety in adolescent rats and whether social isolation modified these effects. Male and female adolescent rats were housed in groups of the same sex or in social isolation for seven days prior to testing in the social interaction test of anxiety. Nicotine increased social interaction in both males and females, and because there was no concomitant change in locomotor activity, this indicated anxiolytic effects. However, there was a 5-fold sex difference in the lowest dose required to enhance social interaction, with an anxiolytic effect in females at 0.05 mg/kg, but in males only at 0.25mg/kg. Furthermore, in males the anxiolytic effect was seen only in socially isolated animals, whereas in the females it was present in both housing conditions. The depressant effect of nicotine on locomotor activity also depended on both the sex of the animal and on their housing conditions, with greater effects in singly housed animals and in males. This sex difference in sensitivity to nicotine's anxiolytic effects suggests there may be sex differences in the factors initiating and maintaining teenage smoking.

Tolerance to midazolam's anxiolytic effects after short-term nicotine treatment

In the social interaction test of anxiety, microinjections of midazolam (2-8 microg) into the dorsal hippocampus or dorsal raphé nucleus significantly increased the time spent in active social interaction, without changing locomotor activity, thus indicating specific anxiolytic effects. However, tolerance developed to these effects in rats that had been pre-treated for 6 days with (-)-nicotine (0.1 mg/kg/day; subcutaneous). Thus, cross-tolerance to the anxiolytic effects of midazolam develops rapidly following a short period of treatment with a low dose of nicotine, which contrasts with the more slowly developing tolerance (about 3 weeks) that develops after benzodiazepine treatment. Following 6 days of nicotine treatment there was a significant reduction in [(3)H]flunitrazepam binding at 2 and 10 nM in the hippocampus, but no change in the midbrain. The decrease in benzodiazepine binding could explain tolerance to the effects of midazolam when administered to the dorsal hippocampus, but other mechanisms, such as indirect effects on the serotonergic (5-HT) system, might be involved in tolerance to the effects of dorsal raphé nucleus administration.

Different treatment regimens and the development of tolerance to nicotine's anxiogenic effects

The effects of different treatment regimens were investigated on the development of tolerance to the anxiogenic effect of nicotine (0.45 mg/kg) in the social interaction test of anxiety. Rats received nicotine (0.45 mg/kg/day) by intravenous injections (5 days/week), subcutaneous injections (5 or 7 days/week) or continuous infusion by osmotic minipump. In all groups, 4 days of nicotine treatment resulted in significant decreases in social interaction compared with the vehicle control groups, without changes in locomotor activity, indicating a specific anxiogenic effect. These significant anxiogenic effects persisted even after 4 weeks of treatment although they were less marked, indicating development of partial tolerance. No significant changes in the time spent in social interaction were found when rats were tested undrugged 24 and 72 h after the termination of nicotine treatment. There was no evidence that the treatment regimen affected the rate of development of tolerance, despite very different peak plasma nicotine concentrations.

Social isolation modifies nicotine's effects in animal tests of anxiety

1. These experiments determined whether the housing conditions of rats influenced the effects of nicotine in two animal tests of anxiety, social interaction and elevated plus-maze tests. 2. In animals housed singly for 7 days, (-)nicotine (0.025 mg kg(-1) s.c.) was ineffective, but 0.05, 0.1 and 0.25 mg kg(-1) (s.c.) significantly increased the time spent in social interaction, without changing locomotor activity, thus indicating anxiolytic actions. (-)Nicotine (0.45 mg kg(-1) s.c.) significantly reduced social interaction, indicating an anxiogenic effect. 3. However, in group-housed animals, (-)nicotine (0.025 mg kg(-1) s.c.) had a significant anxiolytic effect in the social interaction test, but 0.01, 0.05, 0.1, 0.25 and 0.45 mg kg(-1) were ineffective. (-)Nicotine (1 mg kg(-1)) reduced motor activity and social interaction in the group-housed animals. 4. In the elevated plus-maze, the time-course and the dose-response curve to nicotine were investigated. In both singly- and group-housed rats, (-) nicotine (0.1 - 0.45 mg kg(-1) s.c.) decreased the per cent entries into, and per cent time spent on, the open arms, indicating anxiogenic effects. 5. The housing condition influenced the time course, with significant effects at 5 and 30 min after injection in group-housed rats, and significant effects at 30 and 60 min in singly-housed rats. 6. In the social interaction test there was no difference in the scores of the first and last rats removed from group cages, whereas the order of removal from the cages did affect the scores in the elevated plus-maze. 7. These results provide further evidence that the two animal tests model distinct states of anxiety, and show how social isolation powerfully modifies both anxiolytic and anxiogenic effects of nicotine.

The dorsal raphé nucleus is a crucial structure mediating nicotine's anxiolytic effects and the development of tolerance and withdrawal responses

RATIONALE

Smokers frequently report that they obtain anxiety-reducing (anxiolytic) effects from smoking, and this may be one factor which contributes to nicotine dependence.

OBJECTIVE

The aim of this study was to investigate the role of the dorsal raphé nucleus (DRN) in mediating the acute anxiolytic effect of nicotine, the development of tolerance to this effect and the anxiogenic response observed on withdrawal from chronic nicotine.

METHODS

The social interaction test of anxiety was used to investigate the effects of a range of doses of (-)-nicotine (2.5-4000 ng) following DRN infusion, and whether co-administration of the specific 5-HT1A receptor antagonist WAY 100635 could antagonise the anxiolytic action of nicotine. We then examined the effects of intra-DRN nicotine (2.5-7 ng) following six daily injections of subcutaneous (s.c.) (-)-nicotine (0.1 mg/kg). Finally, we examined whether s.c. or intra-DRN (-)-nicotine could antagonise the anxiogenic response seen 72 h after the termination of 7 days of nicotine treatment.

RESULTS

Acute nicotine administration into the DRN produced dose-related effects: low doses (2.5-10 ng) induced an anxiolytic effect, intermediate doses were behaviourally silent (100-1000 ng), and an anxiogenic effect was seen following administration of a high dose (4 micrograms). The anxiolytic effect of (-)-nicotine (5 ng) was reversed by co-administration of a behaviourally inactive dose of WAY 100635 (200 ng). Following 6 days of treatment with s.c. 0.1 mg/kg per day (-)-nicotine, tolerance developed to its anxiolytic action in the DRN. Rats withdrawn for 72 h following this chronic treatment showed an anxiogenic response which was reversed by (-)-nicotine injected s.c. (0.1 mg/kg) or into the DRN (5 ng).

CONCLUSIONS

The present findings therefore suggest that the DRN plays an important role in mediating the acute effects of nicotine on anxiety, as measured in the social interaction test, and that the anxiolytic effect is mediated by activation of somatodendritic 5-HT1A autoreceptors. The DRN is also concerned with mediating the development of tolerance to nicotine's anxiolytic effects and because there is an anxiogenic response 72 h after withdrawal from chronic nicotine, this suggests that an oppositional, compensatory mechanism is mediating the tolerance.

Development of tolerance to nicotine's anxiogenic effect in the social interaction test

The purpose of the present experiment was to explore the role of the dorsal hippocampus in mediating the development of tolerance to the anxiogenic effect of nicotine in the social interaction test of anxiety, and to determine whether tolerance develops to the effects of nicotine on [3H]-5-HT release in this area. Nicotine (1 microg) administered bilaterally into the dorsal hippocampus significantly reduced the time spent in social interaction in vehicle pre-treated rats, indicating an anxiogenic effect, but tolerance to this effect was seen in the rats pre-treated for 6 days with s.c. nicotine (0.1 mg/kg/day). In rats that had been pre-treated with vehicle for 6 days, nicotine (50-200 microM), significantly stimulated [3H]-5-HT release from dorsal hippocampal slices. This stimulation was significantly reduced in rats pre-treated with nicotine (0.1 mg/kg/day) for 6 days, indicating the development of tolerance to the effects of nicotine on 5-HT release. This suggests that tolerance to the anxiogenic effect of nicotine administered into the dorsal hippocampus could be mediated by a reduction in the nicotine enhancement of 5-HT release in this area.

Tolerance to nicotine's effects in the elevated plus-maze and increased anxiety during withdrawal

In the elevated plus-maze test of anxiety, nicotine (0.1 mg/kg sc; 30 min after injection) had a significant anxiogenic effect, shown by specific decreases in the percentage of time spent on the open arms and in the percentage of open-arm entries. Tolerance developed to this anxiogenic effect after 7 days of nicotine treatment (0.1 mg/kg/day). Five minutes after an acute injection, nicotine (0.1 mg/kg) was ineffective, but after 7 days of treatment a significant anxiolytic effect, shown by specific increases in the percentage of time spent on the open arms and in the percentage of open-arm entries, emerged. After 14 days of nicotine treatment, tolerance developed to this anxiolytic effect. There was a complete dissociation between the effects of nicotine on the measures of anxiety, and on the locomotor activity as measured by closed-arm entries. No changes in closed-arm entries were found after acute administration of nicotine, but rats tested 30 min after their 7th injection made significantly fewer, and those tested 5 min after their 14th injection made significantly more, entries than their respective controls. Rats that were tested after 24 h withdrawal from six daily nicotine injections showed a significant anxiogenic effect. A low dose of nicotine (5 ng) injected into the dorsal hippocampus was without effect in vehicle pretreated rats, but it was able to reverse the anxiogenic effect found after 24 h of withdrawal from 6 days of nicotine treatment.

Diazepam and nicotine increase social interaction in gerbils: a test for anxiolytic action

The effects of two drugs with anxiolytic actions, diazepam (0.1, 0.3 and 1 mg/kg) and nicotine (0.1 and 0.5 mg/kg) were examined on the time spent in social interaction by pairs of male gerbils. In a test arena lit by high light, diazepam (0.1 mg/kg) increased social interaction, without changing locomotor activity. Diazepam (0.3 and 1 mg/kg) produced a dose-related increase in locomotor activity, which reached significance at the higher dose. Nicotine produced a dose-related increase in social interaction, which reached significance at 0.5 mg/kg, but was without effect on locomotor activity. The specific increases in social interaction observed with diazepam and nicotine are similar to those seen in the well-validated social interaction test of anxiety in rats and suggest that social interaction in gerbils may also be used to screen for anxiolytic action of novel compounds.

Hippocampal and septal injections of nicotine and 8-OH-DPAT distinguish among different animal tests of anxiety

1. Different animal tests model different anxiety disorders. Thus, the social interaction test is a model of generalised anxiety disorder, plus-maze Trial 1 models elements of panic disorder and Trial 2 in the elevated plus-maze is a model of specific phobia. 2. Studies of the neuroanatomical and neurochemical pathways controlling behaviour in these different tests provides information on the neurobiological mechanisms modulating anxiety disorders. 3. In the social interaction test, nicotine and 8-OH-DPAT had anxiogenic effects when injected into the dorsal hippocampus or the lateral septum. 4. These ligands were without effect on Trial 1 in the plus-maze when injected into the dorsal hippocampus, but had anxiogenic effects when injected into the lateral septum. 5. On Trial 2 in the elevated plus-maze, nicotine had an anxiolytic effect, but 8-OH-DPAT had an anixiogenic effect when injected into the dorsal hippocampus.

The role of 5-HT1A receptors in mediating the anxiogenic effects of nicotine following lateral septal administration

The purpose of the present study was to determine the role of the 5-HT1A receptors in the lateral septum in the mediation of the anxiogenic effects of nicotine in the social interaction and elevated plus maze tests of anxiety in the rat. Bilateral infusion of (-)-nicotine (4 and 8 microg) and of the 5-HT1A receptor agonist 8-OH-DPAT (200 and 500 ng) into the lateral septum decreased the time spent in social interaction, indicating anxiogenic effects. The anxiogenic effect of 8-OH-DPAT (500 ng) was completely reversed by coadministration of a behaviourally inactive dose of the 5-HT1A receptor antagonist, WAY 100635 (200 ng). The anxiogenic effect of the lower dose of (-)-nicotine (4 microg) was completely reversed by WAY 100635 (200 ng), but the reversal was only partial following administration of 8 microg nicotine. In a second test of anxiety, the elevated plus maze, lateral septal administration of 8-OH-DPAT (500 ng) and nicotine (4 microg) induced anxiogenic effects. In this test, the anxiogenic effect of nicotine (4 microg) was completely reversed by coadministration of WAY 100635 (200 ng). The effects of 8-OH-DPAT demonstrate that stimulation of 5-HT1A receptors in the lateral septum has anxiogenic effects in two animal tests and that the anxiogenic effects of nicotine are mediated at least in part by these 5-HT1A receptors.

The role of the dorsal hippocampal serotonergic and cholinergic systems in the modulation of anxiety

A review of the literature suggests that the dorsal hippocampal serotonergic system, and, in particular, the postsynaptic 5-HT(1A) receptor, mediates an anxiogenic response, whereas endogenous dorsal hippocampal cholinergic tone mediates an anxiolytic response. Accordingly, it has been shown that direct dorsal hippocampal administration of the 5-HT(1A) receptor agonist, 8-OH-DPAT, the nicotinic receptor antagonist, mecamylamine, and the M(1) muscarinic receptor antagonist, pirenzepine, all have anxiogenic effects in rats tested in the social interaction test. It is therefore surprising that nicotine also has an anxiogenic effect in this test following dorsal hippocampal administration. However, the anxiogenic effects of mecamylamine and nicotine in the dorsal hippocampus are blocked by coadministration of the 5-HT(1A) receptor antagonist, WAY 100635, suggesting that both of these compounds act by enhancing hippocampal serotonergic transmission, thereby stimulating postsynaptic 5-HT(1A) receptors. This conclusion is supported by the observation that both nicotine and mecamylamine stimulate basal [3H]-5-HT release from dorsal hippocampal slices. A possible mechanism by which nicotinic receptor ligands modulate hippocampal 5-HT release is discussed, and it is proposed that the dorsal hippocampal serotonergic and cholinergic systems are tightly coupled and function antagonistically in the modulation of anxiety, as measured in the social interaction test. These systems are relatively unimportant in controlling behaviour on trial 1 in the plus-maze. On trial 2 in the elevated plus-maze, a model of specific phobia, the endogenous cholinergic system, nicotine, and the M(1) receptor agonist, McN-A-343, all mediate an anxiolytic effect, whereas stimulation of 5-HT(1A) receptors mediates an anxiogenic effect. It is proposed that the hippocampus may predominantly control the avoidance components of phobic anxiety, with other regions, such as the dorsomedial hypothalamus, controlling the escape components.

Neurobiological mechanisms by which nicotine mediates different types of anxiety

The effects of nicotine administration into the dorsal hippocampus and lateral septum provide further evidence that different neurochemical and neuroanatomical substrates control behaviour in different animal tests. Thus, in the social interaction test (a model of generalised anxiety disorder), bilateral administration of nicotine (1-4 microg) into both regions has anxiogenic effects in test conditions that generate moderate anxiety. The anxiogenic effects are mediated by a nicotine-evoked increase in 5-hydroxytryptamine (5-HT) release and are reversed by co-administration of the 5-HT(1A) receptor antagonist, N-(2-(6-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridyl)-cyclohex -ane carboxamide trichloride (WAY 100,635). On trial 1 in the elevated plus-maze (which models the escape components of panic disorder), nicotine is without effect when administered to the dorsal hippocampus, but has anxiogenic effects after lateral septal administration. On trial 2 in the elevated plus-maze (a model of specific phobia), nicotine (1 microg) has anxiolytic effects when administered to the dorsal hippocampus, but is ineffective (4 and 8 microg) in the lateral septum.

Anxiogenic effects of nicotine in the dorsal hippocampus are mediated by 5-HT1A and not by muscarinic M1 receptors

After direct administration into the dorsal hippocampus nicotine decreased the time spent in social interaction, without changing locomotor activity, indicating an anxiogenic effect. The possibility that post-synaptic M1 muscarinic receptors mediated this effect was examined by determining whether dorsal hippocampal administration of a specific M1 receptor agonist (McN-A-343) had anxiogenic effects, and whether the anxiogenic effect of nicotine could be reversed by co-administration of the M1 receptor antagonist, pirenzepine. McN-A-343 (0.3, 1.6, 3.2, 15.8 nmol) was without effect on social interaction, and pirenzepine (0.7 and 2.4 nmol) injection into the dorsal hippocampus failed to reverse the decrease in social interaction caused by nicotine (6.3 nmol) injection into this area. However, the decrease in social interaction after nicotine (50 nmol) was completely reversed by the specific 5-HT1A receptor antagonist, WAY 100635 (0.4 nmol) after co-administration of both drugs into the dorsal hippocampus. Thus, the anxiogenic effect of nicotine in this brain region seems to be mediated by 5-HT1A, but not M1, receptors. In contrast to the effect of nicotine in naive animals, those retested after a second injection of 50 nmol did not show a significant anxiogenic effect. The theoretical implications of this are discussed and from a practical point of view this suggests caution in the retesting of animals after central injections.

Time-course of changes in the social interaction test of anxiety following acute and chronic administration of nicotine

The purpose of these experiments was to explore the hypothesis that the effects of nicotine on anxiety depend on the time since administration and the duration of treatment. In the social interaction test of anxiety, acute nicotine administration (0.1 mg/kg, subcutaneously) decreased social interaction when rats were tested 5 min after injection, but increased it when they were tested 30 min after injection. Social interaction was also decreased 1 h post-injection, but levels returned to baseline between 3 and 30 h. As these changes were independent of any changes in locomotor activity, nicotine seemed to be having both anxiogenic and anxiolytic effects at different times after injection. An anxiolytic effect was also observed 30 min after the second nicotine injection, and the anxiogenic effect observed 5 min after injection remained after 4 days of nicotine administration. However, after 7 days of nicotine treatment, tolerance was observed to both these effects. When rats were tested 72 h after the last of 7 or 14 days of nicotine treatment, an anxiogenic withdrawal response was observed. Thus, an oppositional mechanism may underlie tolerance to the anxiolytic effects, whereas there is as yet no evidence for this type of mechanism mediating tolerance to the anxiogenic effects.

"Depression" increases "craving" for sweet rewards in animal and human models of depression and craving

This study consisted of two experiments, one in rats and one in human volunteers, that used the identical progressive ratio (PR) operant procedure. In both experiments, responding was reinforced under a progressively increasing work requirement, and different groups of subjects received reinforcers that varied in sweetness. In experiment 1, rats were subjected to chronic mild stress, a well-validated animal model of depression. Performance under the PR schedule increased in subjects reinforced with conventional precision pellets (which contain 10% sucrose) or very sweet pellets, but not in subjects reinforced with sugar-free pellets. In experiment 2, volunteers were subjected to a depressive musical mood induction. Performance under the PR schedule increased in subjects reinforced with chocolate buttons, but not in subjects reinforced with with buttons made from the relatively unpalatable chocolate substitute carob. In experiment 2, depressive mood induction also increased chocolate craving, as measured by a novel questionnaire, and there were significant correlations between chocolate craving and chocolate-reinforced PR performance. These results suggest that performance under the PR schedule provides a measure of craving rather than reward, and that craving for sweet rewards is increased by depressive mood induction in both animal and human models. Implications for the interpretation of pharmacological studies using the PR procedure are also discussed.

Real time measurement of stimulated dopamine release in the conscious rat using fast cyclic voltammetry: dopamine release is not observed during intracranial self stimulation

Fast cyclic voltammetry (FCV) was used to measure real time release of electrically stimulated endogenous dopamine in the nucleus accumbens (NAc) of conscious freely moving rats for up to 17 days. The method of electrode construction, implantation, electrical stimulation and recording of changes of extracellular dopamine concentration in the conscious rat are described. Rats trained on a continuous reinforcement schedule to perform intracranial self stimulation (ICSS) were implanted with electrodes for FCV. During ICSS, no faradaic signal was observed at an electrode implanted in the NAc. Decreasing the intensity of the stimulating current abolished ICSS, increasing the stimulating current disrupted ICSS. Operator delivered electrical stimulations using currents greater than those needed for ICSS yielded dopamine signals. It is concluded that during ICSS, sufficient dopamine does not reach the extracellular fluid space to yield a faradaic signal detectable by FCV.

Changes in sleep architecture following chronic mild stress

Chronic exposure to mild unpredictable stress causes subsensitivity to rewards (anhedonia). These effects are reversible by chronic treatment with antidepressant drugs, and have been proposed as an animal model of depression. In the present study, sleep architecture, particularly the rapid eye movement (REM) component, was mapped in rats following exposure to chronic mild stress. The study used a unique large scale automated sleep system to record and analyze the sleep signals from 32 rats simultaneously. The effects of stress on sleep were maximal following 21 days of stress, at which time the stressed animals demonstrated decreases in active waking and deep sleep, and disruptions of REM sleep. The changes in REM sleep included increases in the duration of and transitions into REM sleep over the sleep part of the sleep-wake cycle, and most importantly, a reduced latency to the onset of the first REM period. These sleep abnormalities, and in particular the decrease in REM latency, are consistent with those reported in endogenous depression. The results provide further support for the validity of the chronic mild stress paradigm as an animal model to study the mechanisms underlying endogenous depression.

Effects of reinforcer sweetness and the D2/D3 antagonist raclopride on progressive ratio operant performance

Previous studies have reported that DA receptor antagonists suppress most behaviours; however, a paradoxical increase in performance may be seen in tests of operant or consummatory behaviours maintained by very sweet rewards, which lie on the descending limb of the inverted U-shaped concentration-performance function (i.e., under conditions where performance decreases as sweetness increases). Despite the low performance levels associated with very sweet reinforcers, preference studies indicate that they are nonetheless more rewarding. In the present study, the hypothesis that reinforcer efficacy is monotonically related to reinforcer sweetness was tested using a geometric progressive ratio reinforcement schedule, in which increasing numbers of responses were required to earn successive reinforcers (1, 2, 4, 8,.); the amount of work the animal emits in order to obtain an increasingly infrequent reinforcer is assumed to provide a measure of the magnitude of its rewarding effect. Three groups of rats were trained on this schedule, using as reinforcers food pellets containing 1%, 10% and 95% sucrose, respectively. Under conditions of continuous sucrose-pellet reinforcement, the highest response rates were maintained by the 10% sucrose pellets. However, under the progressive ratio schedule, performance was monotonically related to sucrose concentration. The dopamine D2/D3 receptor antagonist raclopride dose-dependently suppressed progressive ratio performance in all three groups.

Stereospecific reversal of stress-induced anhedonia by mianserin and its (+)-enantiomer

Chronic sequential exposure to a variety of mild unpredictable stressors has previously been found to depress the consumption of a dilute (1%) sucrose solution and to inhibit food-induced place preference conditioning. In the present study, using a simplified version of the mild stress procedure, the decreased sucrose intake was reversed by chronic (4 weeks) treatment with the atypical antidepressant mianserin. The racemic compound (+/-)-mianserin (5 mg/kg per day) and one of its enantiomers, (+)-mianserin (2.5 mg/kg) were effective in this model; a lower dose of (+/-)-mianserin (2.5 mg/kg), and the other enantiomer. (-)-mianserin (2.5 mg/kg), were ineffective. Vehicle-treated stressed animals were also subsensitive to food reward in the place conditioning procedure: normal place preference conditioning was reinstated by chronic treatment with (+/-)-mianserin (5 mg/kg) or (+)-mianserin, but not by the lower dose of (+/-)-mianserin (2.5 mg/kg) or by (-)-mianserin. Raclopride (100 micrograms/kg) reinstated the decrease in sucrose intake in stressed animals successfully treated with (+/-)- or (+)-mianserin. The results suggest that (+)-mianserin is the active enantiomer in reversing chronic mild stress-induced anhedonia, and further support the hypothesis of a dopaminergic mechanism of antidepressant action in this paradigm.

Reversal of stress-induced anhedonia by the dopamine receptor agonist, pramipexole

Chronic exposure to mild unpredictable stress has previously been found to depress the consumption of a palatable (1%) sucrose solution, and to attenuate food-induced place preference conditioning. In this study the effects of pramipexole (SND-919), a dopamine D2 agonist, were studied during 7-9 weeks of chronic treatment. Pramipexole (1.0 mg/kg per day) reversed the suppression of sucrose intake in stressed animals, increasing sucrose intakes above the levels seen in untreated nonstressed controls. Pramipexole also increased sucrose intake in nonstressed animals; these effects were accompanied by increases in water intake and tended to correlate with weight loss. Drug-treated stressed animals also lost weight, but in this case water intake was unaffected. A second group of animals received a higher dose of pramipexole (2.0 mg/kg per day). The effects of the two doses were very similar. After three weeks of treatment, these animals were switched to a lower dose of pramipexole (0.1 mg/kg per day). Increases in sucrose intake were maintained over three weeks of treatment at the lower dose, with significant recovery of body weight. Two further groups received the same doses of pramipexole (1.0 mg/kg for 6 weeks or 2.0 mg/kg for 3 weeks followed by 0.1 mg/kg thereafter), but received intermittent (twice-weekly) drug treatment. Intermittent pramipexole treatments also tended to increase sucrose intakes, but the results were less consistent from week to week. Following 6-8 weeks of pramipexole treatment, food-induced place preference conditioning was studied in all animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Environmental influences on behavioural sensitization to the dopamine agonist quinpirole

A runway was used to measure locomotor responses to quinpirole (200µg/kg), in rats. The locomotor stimulant effect of quinpirole increased progressively over successive trials at 3-day intervals. Animals administered quinpirole in the home cage were also sensitized, but to a lesser degree than animals tested in the runway following quinpirole injections. Exposure to an open field, following quinpirole injections, sensitized responsiveness in the runway to an extent comparable to that seen following runway exposure. Animals exposed to a movable running wheel, following quinpirole injections, were more sensitized to the effect of quinpirole in the runway than animals exposed to a locked running wheel. The results suggest that the extent of sensitization to quinpirole is determined by the behaviour elicited by the drug, rather than the environment in which it is administered. An operant conditioning model is proposed to account for these effects.