Alterations of reward mechanisms in bulbectomised rats

Depressive-like phenotype enhances relapse of nicotine seeking after forced abstinence in rats

OBJECTIVE

Comorbidity of depression and drug addiction is common, but effective treatment is missing. A rat model combining the olfactory bulbectomy (OBX) model and IV drug self-administration has provided evidence of differential reactivity of the OBX rats towards drugs of abuse. This study evaluates nicotine taking and seeking behaviour in this model.

METHODS

Adult male Wistar rats were used; in one group, the OBX was performed while the other group was sham-operated. After three weeks of nicotine self-administration (fixed ratio-1 schedule), rats underwent two weeks of forced abstinence followed by a drug-free relapse-like session. Two doses of nicotine were studied: 0.019 and 0.030 mg/kg per infusion. The locomotor test took place before the self-administration protocol and on the first day of abstinence.

RESULTS

OBX induced characteristic hyperactive locomotor phenotype. OBX rats self-administered more nicotine in the experiment using 0.019 mg/kg per infusion, but they reached lower drug intake in the study using 0.030 mg/kg per infusion. However, relapse of nicotine seeking after forced abstinence was significantly higher in the OBX groups in both cohorts.

CONCLUSION

These results are in line with previous studies showing OBX-induced dissimilarities in drug-seeking and drug-taking and represent complementary information to reports on other substances.

Effects of low-dose alcohol exposure in adolescence on subsequent alcohol drinking in adulthood in a rat model of depression

OBJECTIVE

Adolescence drinking and subsequent development of alcohol use disorder (AUD) is a worldwide health concern. In particular, mood dysregulation or early alcohol exposure can be the cause of heavy drinking in some individuals or a consequence of heavy drinking in others.

METHODS

This study investigated the effects of voluntary alcohol intake during adolescence, i.e. continuous 10% alcohol access between postnatal days (PND) 29 to 43 and olfactory bulbectomy (OBX) model of depression (performed on PND 59) on alcohol drinking in Wistar rats during adulthood (PND 80-120, intermittent 20% alcohol access). In addition, the effect of NBQX, an AMPA/kainate receptor antagonist (5 mg/kg, IP) on spontaneous alcohol consumption was examined.

RESULTS

Rats exposed to 10% alcohol during adolescence exhibited a lower 20% alcohol intake in the intermittent paradigm during adulthood, while the OBX-induced phenotype did not exert a significant effect on the drinking behaviour. NBQX exerted a transient reduction on alcohol intake in the OBX rats.

CONCLUSIONS

Our results indicate that exposure to alcohol during adolescence can affect alcohol drinking in adulthood and that further exploration of AMPA and/or kainate receptor antagonists in co-morbid alcoholism-depression is warranted.

Sex and Feeding Status Differently Affect Natural Reward Seeking Behavior in Olfactory Bulbectomized Rats

Substance abuse and depression are common psychiatric disorders with a high rate of comorbidity. Both conditions affect differently men and women and preclinical research has showed many sex differences in drug addiction and depression. The most common approach for modeling depression-addiction comorbidity is the combination of the intravenous drug self-administration and the olfactory bulbectomy (OBX) models in rats. Such a combination has revealed enhanced drug-taking and drug-seeking behaviors in OBX rats, but no study has investigated so far potential sex differences in operant responding and motivation for natural reinforcers in OBX rats. This study investigated for the first time operant self-administration of palatable food pellets in male and female OBX rats under different feeding status, i.e., ad libitum vs. restricted food, and schedules of reinforcement, i.e., a continuous ratio schedule fixed ratio 1 (FR1) vs. a complex (FR5_(x)) second order schedule of reinforcement. In the FR1 experiment, OBX rats of both sexes exhibited lower operant responding and intake of palatable food pellets than sham-operated controls, with food restriction leading to increased operant responding in both OBX and SHAM groups. Female rats showed higher responding than males but this effect was abolished by the OBX lesion. Similarly, in the (FR5_(x)) second order schedule of reinforcement both male and female OBX rats showed lower responding and food intake, with SHAM and OBX females showing higher operant responding than corresponding male groups. Overall, our findings showed that: (i) responding for food was lower in OBX than in SHAM rats under both FR1 and (FR5_(x)) schedules of reinforcement; (ii) sex and food restriction affect operant responding for palatable food; and (iii) the suppressing effect of OBX lesion on food intake was consistently present in both sexes and represents the most robust factor in the analysis. This may represent anhedonia which is associated with depressive-like phenotype and palatable food self-administration may serve as a robust behavioral index of anhedonia in the OBX model.

Olfactory bulbectomy increases reinstatement of methamphetamine seeking after a forced abstinence in rats

Drug addiction is commonly associated with depression and comorbid patients also suffer from higher cravings and increased relapse rate. To address this issue preclinically we combined the olfactory bulbectomy (OBX) model of depression and intravenous methamphetamine self-administration procedure in rats to assess differences in relapse-like behavior. Male Sprague-Dawley rats were divided randomly into two groups; in one group the bilateral olfactory bulbectomy (OBX) was performed while the other group was sham operated. After recovery, intracardiac catheter was implanted. Intravenous self-administration procedure was conducted in operant boxes using nose-poke operandi (Coulbourn Instruments, Inc., USA) under fixed ratio 1 schedule of reinforcement. Methamphetamine was available at dose 0.08 mg/kg/infusion. After stable methamphetamine intake was maintained, a period of forced abstinence was initiated and rats were kept in their home-cages for 14 days. Finally, one reinstatement session was conducted in operant boxes with no drug delivery. In the reinstatement session the mean of 138.4 active nose-pokes was performed by the OBX group, while the sham group displayed 41 responses, i.e. 140 % and 48 % of basal nose-poking during maintenance phase in OBX and sham operated group respectively. OBX group also showed significantly more passive nose-pokes indicating hyperactive behavioral traits in bulbectomized rats. However, the % of active operandum preference was equal in both groups. Olfactory bulbectomy model significantly increased reinstatement of methamphetamine seeking behavior. This paradigm can be used to evaluate potential drugs that are able to suppress the drug-seeking behavior.