Reinforcing and neural activating effects of norharmane, a non-nicotine tobacco constituent, alone and in combination with nicotine.

Tobacco use is the leading cause of preventable death. Although the health risks are well known, cessation rates remain low. Whereas behavioral and neuroanatomical studies on tobacco addiction conventionally use nicotine, there is evidence that other constituents, such as monoamine oxidase inhibitors, may be important factors for modeling smoking. The aims of the present study were therefore to determine whether norharmane, a tobacco constituent and monoamine oxidase inhibitor, is self-administered alone and/or in combination with nicotine, and to evaluate the neural mechanisms underlying acquisition of self-administration of the two drugs. Sprague-Dawley rats were catheterized and allowed to intravenously self-administer either saline, nicotine (7.5 µg/kg/inj), norharmane (0.25 or 2.5 µg/kg/inj), alone or combined together (7.5+2.5 µg/kg/inj) for five days at fixed ratio (FR)1, two days each at FR2 and FR5, and one day at progressive ratio. Animals acquired self-administration of norharmane alone (2.5 µg/kg/inj), and the reinforcing effects of nicotine and norharmane were additive. For neuroanatomical analyses, rats self-administered the same treatments for six days at FR1, then brains were collected and processed by in situ hybridization for cfos mRNA expression. Treatment-specific profiles of regional cfos expression and correlations between cfos mRNA levels and behavioral responding were observed. Thus, not only was norharmane behaviorally reinforcing but, when combined with nicotine, resulted in patterns of neural activation distinct from that of norharmane or nicotine alone. This suggests that non-nicotine constituents can have central activating effects independent of nicotine, further substantiating the need for their inclusion in preclinical investigations of tobacco dependence.

The monoamine oxidase (MAO) inhibitor tranylcypromine enhances nicotine self-administration in rats through a mechanism independent of MAO inhibition.

Our current study aims to evaluate the mechanisms of tranylcypromine (TCP)-mediated enhancement of nicotine self-administration. We replicated our previous findings which demonstrate that 1 h pretreatment with TCP (3 mg/kg, i.p.) enhances nicotine self-administration (7.5 µg/kg/inj, i.v.) when compared with vehicle-treated rodents. We tested whether TCP-mediated enhancement of nicotine self-administration was due to MAO inhibition or off-target effects by (i) extending the TCP pretreatment time from 1 to 20 h, and (ii) evaluating the role of the individual TCP stereoisomers in nicotine self-administration studies. While 20 h and (-)TCP pretreatment induced significant inhibition of MAO (60-90%), animals found nicotine only weakly reinforcing. Furthermore, while both (+) and (±)TCP treatment induced nearly 100% MAO inhibition, (+)TCP pretreated animals took longer to acquire nicotine self-administration compared to (±)TCP pretreated animals. Stable nicotine self-administration in (+)TCP pretreated animals was influenced by nicotinic receptor activation but not nicotine-paired cues. The opposite was found in (±)TCP pretreated animals. Treatment with (-) or (±)TCP increased dopamine and serotonin overflow, while the (+) and (±)TCP treatment enhanced monoamine overflow subsequent to nicotine. Together, our data suggests TCP enhancement of nicotine self-administration are mediated through mechanisms independent of MAO inhibition, including nicotine-paired cues and monoamine uptake inhibition.