Mifepristone treatment affects the response to repeated amphetamine injections, but does not attenuate the expression of sensitization

Mifepristone decreases nicotine intake in dependent and non-dependent adult rats

BACKGROUND

Addiction to tobacco and nicotine products has adverse health effects and afflicts more than a billion people worldwide. Therefore, there is an urgent need for new treatments to reduce tobacco and nicotine use. Glucocorticoid receptor blockade shows promise as a novel treatment for drug abuse and stress-related disorders.

AIM

These studies aim to investigate whether glucocorticoid receptor blockade with mifepristone diminishes the reinforcing properties of nicotine in rats with intermittent or daily long access to nicotine.

METHODS

The rats self-administered 0.06 mg/kg/inf of nicotine for 6 h per day, with either intermittent or daily access for 4 weeks before treatment with mifepristone. Daily nicotine self-administration models regular smoking, while intermittent nicotine self-administration models occasional smoking. To determine whether the rats were dependent, they were treated with the nicotinic acetylcholine receptor antagonist mecamylamine, and somatic signs were recorded.

RESULTS

The rats with intermittent access to nicotine had a higher level of nicotine intake per session than those with daily access but only the rats with daily access to nicotine showed signs of physical dependence. Furthermore, mecamylamine increased nicotine intake during the first hour of access in rats with daily access but not in those with intermittent access. Mifepristone decreased total nicotine intake in rats with intermittent and daily access to nicotine. Moreover, mifepristone decreased the distance traveled and rearing in the open field test and operant responding for food pellets.

CONCLUSION

These findings indicate that mifepristone decreases nicotine intake but this effect may be partially attributed to the sedative effects of mifepristone.

Resetting the Stress System with a Mifepristone Challenge

Psychotic depression is characterized by elevated circulating cortisol, and high daily doses of the glucocorticoid/progesterone antagonist mifepristone for 1 week are required for significant improvement. Using a rodent model, we find that such high doses of mifepristone are needed because the antagonist is rapidly degraded and poorly penetrates the blood–brain barrier, but seems to facilitate the entry of cortisol. We also report that in male C57BL/6J mice, after a 7-day treatment with a high dose of mifepristone, basal blood corticosterone levels were similar to that of vehicle controls. This is surprising because after the first mifepristone challenge, corticosterone remained elevated for about 16 h, and then decreased towards vehicle control levels at 24 h. At that time, stress-induced corticosterone levels of the 1xMIF were sevenfold higher than the 7xMIF group, the latter response being twofold lower than controls. The 1xMIF mice showed behavioral hyperactivity during exploration of the circular hole board, while the 7xMIF mice rather engaged in serial search patterns. To explain this rapid reset of corticosterone secretion upon recurrent mifepristone administration, we suggest the following: (i) A rebound glucocorticoid feedback after cessation of mifepristone treatment. (ii) Glucocorticoid agonism in transrepression and recruitment of cell-specific coregulator cocktails. (iii) A more prominent role of brain MR function in control of stress circuit activity. An overview table of neuroendocrine MIF effects is provided. The data are of interest for understanding the mechanistic underpinning of stress system reset as treatment strategy for stress-related diseases.

Effect of the glucocorticoid receptor antagonist RU486 on MK-801 induced behavioural sensitisation

Stress is known to modulate sensitisation to repeated psychostimulant exposure. However, there is no direct evidence linking glucocorticoids and sensitisation achieved by repeated administration of the NMDA receptor antagonist MK-801. We tested the hypothesis that co-administration of RU486, a glucocorticoid receptor (GR) antagonist, prior to repeated daily MK-801 injections would block the expression of locomotor sensitisation due to its dual effects on corticosterone and dopamine. We employed a repeated MK-801 administration locomotor sensitisation paradigm in male Sprague Dawley rats. RU486 or a dimethyl sulfoxide (DMSO) vehicle was co-administered with MK-801 or saline during the induction phase. Subsequent to withdrawal, rats were challenged with MK-801 alone to test for the expression of sensitisation. In a separate cohort of rats, plasma corticosterone levels were quantified from blood samples taken on the 1st, 4th and 7th day of induction and at expression. One day after challenge, nucleus accumbens tissue levels of dopamine and its metabolites DOPAC and HVA were measured. During the induction phase, RU486 progressively enhanced locomotor sensitisation to MK-801. RU486 and MK-801 both showed stimulatory effects on corticosterone levels and this was further augmented when given in combination. Contrary to our hypothesis, RU486 did not block the expression of locomotor sensitisation to MK-801 and actually increased levels of dopamine, DOPAC and HVA in nucleus accumbens tissue. Our results showed that RU486 has augmentative rather than inhibitory effects on MK-801-induced sensitisation. This study indicates a divergent role for glucocorticoids in sensitisation to MK-801 compared to sensitisation with other psychostimulants.

Mechanisms of Action and Persistent Neuroplasticity by Drugs of Abuse

Adaptation of the nervous system to different chemical and physiologic conditions is important for the homeostasis of brain processes and for learning and remembering appropriate responses to challenges. Although processes such as tolerance and dependence to various drugs of abuse have been known for a long time, it was recently discovered that even a single pharmacologically relevant dose of various drugs of abuse induces neuroplasticity in selected neuronal populations, such as the dopamine neurons of the ventral tegmental area, which persist long after the drug has been excreted. Prolonged (self-) administration of drugs induces gene expression, neurochemical, neurophysiological, and structural changes in many brain cell populations. These region-specific changes correlate with addiction, drug intake, and conditioned drugs effects, such as cue- or stress-induced reinstatement of drug seeking. In rodents, adolescent drug exposure often causes significantly more behavioral changes later in adulthood than a corresponding exposure in adults. Clinically the most impairing and devastating effects on the brain are produced by alcohol during fetal development. In adult recreational drug users or in medicated patients, it has been difficult to find persistent functional or behavioral changes, suggesting that heavy exposure to drugs of abuse is needed for neurotoxicity and for persistent emotional and cognitive alterations. This review describes recent advances in this important area of research, which harbors the aim of translating this knowledge to better treatments for addictions and related neuropsychiatric illnesses.