Nicotine and opioid peptides

From disordered eating to addiction: the "food drug" in bulimia nervosa

The high prevalence of substance abuse in individuals with bulimia nervosa (BN) and the pervasive symptom substitution in many types of drug addiction suggest that a number of substances--including food--can impair an individual's self-control, even in the presence of negative consequences. Nonetheless, the neurobiological similarities between BN and drug addiction are not clearly established. This review explores how the specific eating patterns seen in BN (binge eating and purging, with intermittent dietary restriction) are particularly addictive and differentiate BN from other eating disorders and obesity. A number of peripheral and central biological aberrations seen in BN may result in altered reward sensitivity in these individuals, particularly through effects on the dopaminergic system. Neurobiological findings support the notion that BN is an addictive disorder, which has treatment implications for therapy and pharmacological manipulations.

Neurobiology of aversive states

Hoebel and colleagues are often known as students of reward and how it is coded in the CNS. This article, however, attempts to focus on the significant advances by Hoebel and others in dissecting out behavioral components of distinct aversive states and in understanding the neurobiology of aversion and the link between aversive states and addictive behaviors. Reward and aversion are not necessarily dichotomous and may reflect an affective continuum contingent upon environmental conditions. Descriptive and mechanistic studies pioneered by Bart Hoebel have demonstrated that the shift in the reward-aversion spectrum may be, in part, a result of changes in central dopamine/acetylcholine ratio, particularly in the nucleus accumbens. The path to aversion appears to include a specific neurochemical signature: reduced dopamine release and increased acetylcholine release in "reward centers" of the brain. Opioid receptors may have a neuromodulatory role on both of these neurotransmitters.

Neurobiology of addiction. An integrative review

Evidence that psychoactive substance use disorders, bulimia nervosa, pathological gambling, and sexual addiction share an underlying biopsychological process is summarized. Definitions are offered for addiction and addictive process, the latter being the proposed designation for the underlying biopsychological process that addictive disorders are hypothesized to share. The addictive process is introduced as an interaction of impairments in three functional systems: motivation-reward, affect regulation, and behavioral inhibition. An integrative review of the literature that addresses the neurobiology of addiction is then presented, organized according to the three functional systems that constitute the addictive process. The review is directed toward identifying candidate neurochemical substrates for the impairments in motivation-reward, affect regulation, and behavioral inhibition that could contribute to an addictive process.

A sensitization–homeostasis model of nicotine craving, withdrawal, and tolerance: Integrating the clinical and basic science literature

Recent reports suggest that nicotine withdrawal symptoms are common among adolescents after a few weeks of intermittent tobacco use. No current model of nicotine dependence had predicted the rapid development of symptoms of dependence and withdrawal before the development of tolerance. We present a model that integrates neuroscience with clinical observations regarding how nicotine dependence develops, progresses, and resolves in humans. The central tenet of this sensitization-homeostasis model is that nicotine's dependence liability derives from its ability to stimulate neural pathways responsible for the suppression of craving. As a result of sensitization, the craving suppression produced by nicotine is magnified to superphysiological levels. The overinhibition of neurons responsible for craving initiates compensatory homeostatic measures that stimulate the craving pathways and result in craving when nicotine is absent. Separate homeostatic mechanisms are responsible for craving, withdrawal, and tolerance. The sensitization-homeostasis model is unique in its attribution of dependence to craving suppression, its attention to the temporal relationships among clinical features of nicotine dependence, and its extensive integration of clinical observations and basic science. It provides a framework for theory-based research.

Cytisine-induced behavioral activation: delineation of neuroanatomical locus of action

Systemic injections of nicotine increase locomotor activity. The present study was designed to determine whether there is a circumscribed region in the ventral tegmentum that mediates the locomotor-activating effects of nicotine. The mapping technique was used to delineate this region: bilateral injections of cytisine (1 nmol/0.5 microliters per side) were made into sites in and around the ventral tegmentum and the amount of locomotion associated with each site was quantified. The distribution of injection sites spanned between 0.8 and 4.6 mm posterior to bregma. The amount of locomotion varied with relation to the area into which cytisine was injected and a region was identified within which injections of cytisine preferentially increased locomotion. This region appears to have reasonably well-defined anterior and posterior boundaries. Since a portion of the delineated area overlaps the dopamine-containing cell-body region, the possibility remains that the activation of the mesolimbic dopamine system may contribute to the behavioral activation associated with ventral tegmental injections of cytisine. This same system may contribute to the locomotor-activating effects associated with systemic injections of nicotine.

Cholinergic regulation of rat preproenkephalin RNA in the adrenal medulla

Expression of the rat preproenkephalin (ppENK) gene involves transsynaptic cholinergic mechanisms. We evaluated the effects of cholinergic agonist treatments in vivo on the expression of adrenomedullary ppENK RNA. Cholinergic treatment with nicotinic + muscarinic receptor agonists resulted in a synergistic 100-fold rise in steady-state ppENK messenger RNA levels, but only a 30- to 35-fold rise in initiation of steady-state ppENK RNA transcripts. The levels of initiated ppENK steady-state RNA peaked at two days, whereas mature (1.45 kb) ppENK mRNA levels continued to rise, peaking at four days. This suggested that other transcriptional (attenuation or alternative splicing) or post-transcriptional (RNA stabilization) regulatory mechanisms must be operative. As multiple ppENK RNA start sites exist, we examined how usage of multiple sites was altered by cholinergic treatments. The predominant start site changed from E2 in the basal state, to E4 after primary cholinergic stimulation, to E3 after re-treatment. This represents novel example of differential usage of multiple RNA initiation start sites in vivo. Differences in initiated and mature transcripts are consistent with at least four mechanisms involved in control of cholinergic-induced ppENK RNA expression: (i) simply new initiation of RNA transcripts, (ii) differential usage of the multiple RNA start sites, (iii) stabilization of mRNA transcripts, and (iv) attenuation and/or alternative RNA splicing of RNA transcripts.