A specific anti-aggressive effect of repeatedly administered lobeline

Regulation of aggressive behaviors by nicotinic acetylcholine receptors: Animal models, human genetics, and clinical studies

Neuropsychiatric disorders are frequently complicated by aggressive behaviors. For some individuals, existing behavioral and psychopharmacological treatments are ineffective or confer significant side effects, necessitating development of new ways to treat patients with severe aggression. Nicotinic acetylcholine receptors (nAChRs) are a large and diverse family of ligand-gated ion channels expressed throughout the brain that influence behaviors highly relevant for neuropsychiatric disorders, including attention, mood, and impulsivity. Nicotine and other drugs targeting nAChRs can reduce aggression in animal models of offensive, defensive, and predatory aggression, as well as in human laboratory studies. Human genetic studies have suggested a relationship between the CHRNA7 gene encoding the alpha-7 nAChR and aggressive behavior, although these effects are heterogeneous and strongly influenced by genetic background and environment. Here we review animal, human genetic, and clinical studies supporting a consistent role of nicotine and nAChR signaling in modulation of aggressive behaviors. We integrate findings from recent studies of aggression neuroscience, discuss the circuitry that may be involved in these effects of nAChRs, and identify multiple key questions that must be answered prior to safe and effective translation for human patients. This article is part of the special issue on 'Contemporary Advances in Nicotine Neuropharmacology'.

Modulation of aggressive behavior in mice by nicotinic receptor subtypes

Aggression is frequently comorbid with neuropsychiatric conditions and is a predictor of worse outcomes, yet current pharmacotherapies are insufficient and have debilitating side effects, precluding broad use. Multiple models of aggression across species suggest that the nicotinic acetylcholine receptor (nAChR) agonist nicotine has anti-aggressive (serenic) properties. Here we demonstrate dose-dependent serenic effects of acute nicotine administration in three distinct mouse strains: C57BL/6, BALB/c, and CD1. While acute nicotine administration (0.25mg/kg) modestly reduced solitary homecage locomotion, this could not account for nicotine's serenic effects since social encounters eliminated the hypolocomotor effect, and nicotine did not alter social interaction times. Pretreatment with the homomeric (α7 subunit) nAChR antagonist methyllycaconitine (5mg/kg), but not the heteromeric (β2 or β4 subunit-containing) nAChR antagonist dihydro-β-erythroidine (DHβE, 3mg/kg), blocked the serenic effects of nicotine. By contrast, pretreatment with DHβE blocked the effect of acute nicotine administration on locomotion, uncoupling nicotine's serenic and hypolocomotor effects. Finally, the α7 nAChR partial agonist GTS-21 reduced aggression in C57BL/6 mice. These results support the idea that acute nicotine administration has serenic effects and provide evidence for specificity of this effect distinct from effects on locomotion. Furthermore, pharmacological studies suggest that activation of α7 nAChRs underlies the serenic effects of nicotine. Further studies of nAChRs could enhance understanding of the neurobiology of aggression and may lead to the development of novel, more specific treatments for pathological aggression.

Effects of bupropion, alone or coadministered with nicotine, on social behavior in mice

Bupropion, administered alone or combined with nicotine, is presently used to treat nicotine dependence. Despite experimental evidence of the complex behavioral actions of this drug, there have been little data reported about its effects on social behavior. Our main aim was to investigate the effects of acute administration of bupropion, alone or plus nicotine, on social interaction in mice. OF1 group-housed male mice were confronted in a neutral cage with an anosmic opponent during a 10 minutes encounter. Time allocated to body care and digging was reduced by administration of bupropion (40 mg/kg) both when administered alone and with nicotine (1 and 0.5 mg/kg). The lowest dose of bupropion (10 mg/kg) also reduced digging when combined with 1 mg/kg of nicotine. Time spent on non-social exploration and exploration from a distance was significantly higher in mice treated with bupropion (40 mg/kg) alone or combined with nicotine (1 and 0.5 mg/kg). The lowest dose of bupropion (10 mg/kg) increased non-social exploration when combined with 0.5 mg/kg of nicotine and exploration from a distance when combined with 1 mg/kg of nicotine. Ethopharmacological assessment of the behavior of groups of mice treated with different combinations of the two drugs indicates that nicotine can potentiate some of the behavioral effects of low doses of bupropion. Results also indicate that bupropion, either alone or combined with nicotine, has no significant effects on social investigation, suggesting that this drug does not induce a clear anxiolytic profile in OF1 mice.

Selective antiaggressive effect of an alpha-2 adrenoceptor agonist naphthylmedetomidine in mice

Alpha-2 adrenoceptors (alpha(2)-ARs) are critically involved in regulating neurotransmitter release from sympathetic nerves and neurons and play an important role in the regulation of awareness, arousal and vigilance. In our recent study, dexmedetomidine, a full alpha(2)-AR agonist, produced antiaggressive effects in the social conflict test in mice at doses that were twice smaller than those producing sedation. The aim of this study was to ascertain antiaggressive effect of a novel drug naphthylmedetomidine, with a more selective alpha(2)-AR activity. Behavioral effects of naphthylmedetomidine (150-1200 microg/kg i.p.) were studied in the activity cage and in the social conflict tests in mice. Naphthylmedetomidine dose dependently decreased aggressive behavior during social conflict in aggressive mice with significant reduction already at the lowest doses tested (150 microg/kg), whereas locomotion and social investigation were significantly decreased only after four times bigger dose of naphthylmedetomidine (600 microg/kg) in aggressive mice. Naphthylmedetomidine had no effect on aggression in nonaggressive mice. Naphthylmedetomidine reduced locomotion in the activity cage significantly only at the highest doses tested (600 and 1200 microg/kg), and this effect was only partially reversed by administration of high doses of an alpha-2 antagonist atipamezole (3 and 10 mg/kg). In nonaggressive mice, the difference between the dose reducing dominant social behavior (social investigation) and locomotion (150 and 300 microg/kg, respectively) was smaller than in aggressive mice. In conclusion, naphthylmedetomidine showed a very strong and selective antiaggressive effect in aggressive mice, which was devoid of locomotion-inhibiting/sedative effect. This study suggests that naphthylmedetomidine may have clinical potential as antiaggressive drug.

Differential sensitivity to the effects of nicotine and bupropion in adolescent and adult male OF1 mice during social interaction tests

Few studies have compared the action of both nicotine (NIC) and bupropion (BUP), an antidepressant used to treat NIC dependence, on social and aggressive behavior at different ages. This study aims to determine whether these drugs produce differential effects in adolescent (postnatal day: 36-37) and adult (postnatal day: 65-66) mice that have been housed individually for 2 weeks in order to induce aggressive behavior. Mice received BUP (40, 20, or 10 mg/kg), NIC (1, 0.5, and 0.25 mg/kg as base), or vehicle earlier to a social interaction test. BUP (40 mg/kg) decreased social investigation and increased nonsocial exploration in both adolescent and adult mice. The same effects were also observed in adult mice administered with a lower dose of the same drug (20 mg/kg). In adolescents, NIC (1 mg/kg) decreased social investigation, but this effect did not reach statistical significance in adults. In conclusion, a differential sensitivity to the effects of NIC or BUP emerged in some of the behavioral categories when the two age groups were compared.

Effects of co-administration of bupropion and nicotinic agonists on the elevated plus-maze test in mice

There is evidence that the cholinergic nicotinic system is involved in the modulation of anxiety. Anxiolytic and anxiogenic effects of nicotine agonists have been reported in mice. Bupropion is an antidepressant drug which may alleviate some symptoms of nicotine withdrawal, although its effects on anxiety are not clear. It has been suggested that the interaction between bupropion and nicotinic mechanisms could be complex. The aim of the present study was to investigate acute effects of co-administration of bupropion and nicotinic agonists on the elevated plus-maze test in NMRI mice. Effects of nicotine, lobeline, and cytisine (0.35 and 0.175 mg/kg), administered alone or combined with bupropion (20 mg/kg) were tested in the plus-maze. Results indicated that nicotine (0.35 mg/kg) decreased number and percentage of entries and time spent in open arms, and increased percentage of protected stretched attend posture. Bupropion (20 mg/kg) plus lobeline (0.175 mg/kg) increased percentage of time spent in open arms, without altering total or closed arm entries. Our findings suggest that the highest dose of nicotine induces anxiogenic effects, which are counteracted when co-administered with bupropion. The combination of bupropion with a low dose of lobeline seems to have an anxiolytic profile in the conventional parameters of the plus-maze, although ethological measures do not support it clearly.

Dexmedetomidine selectively suppresses dominant behaviour in aggressive and sociable mice

Dexmedetomidine is a highly specific alpha2-adrenoreceptor agonist, which is now clinically used to induce sedation in patients in the intensive care units. Behavioural effects of dexmedetomidine have been little studied so far. The drug was reported to reduce behaviour such as locomotion or measures of anxiety or aggression in animals. The aim of the present study was to ascertain whether dexmedetomidine inhibits behaviour uniformly or with respect to particular stimuli or situations. Therefore, behavioural effects of dexmedetomidine were studied in the social conflict test in male mice (after three weeks of individual housing), which provides a wide spectrum of behavioural activities in two types of animals (aggressive and sociable mice) as well as in the activity cage. Dexmedetomidine (5-40 microg/kg i.p.) decreased locomotion in the activity cage and this effect was fully antagonized by atipamezole, a selective alpha2-adrenereceptor antagonist. However, dexmedetomidine did not reduce locomotion during social conflict. The only significant effects during social conflict were a selective and dose-dependent antiaggressive effect in aggressive mice and a selective reduction of social investigation ('sociability') in sociable mice. Thus, dexmedetomidine appears to inhibit predominantly dominant behaviour evoked by biologically important stimuli. The ability of dexmedetomidine to reduce aggression might be utilized for treatment of aggressive states. Sedation caused by dexmedetomidine can be easily disrupted and thus the drug may have an advantage over benzodiazepines or neuroleptics, which are used in this indication.