Opposite effects of PCA and chlorimipramine on ICSS and on its facilitation by amphetamine

Reward and the serotonergic system

Anhedonia, as a failure to experience rewarding stimuli, is a key characteristic of many psychiatric disorders including depression and schizophrenia. Investigations on the neurobiological correlates of reward and hedonia/anhedonia have been a growing subject of research demonstrating several neuromodulators to mediate different aspects of reward processing. Whereas the majority of research on reward mainly focused on the dopamine and opioid systems, a serotonergic mechanism has been neglected. However, recent promising results strengthen the pivotal role of serotonin in reward processing. Evidence includes electrophysical and pharmacological as well as genetic and imaging studies. Primate research using single-unit recording of neurons within the dorsal raphe nucleus argues for a serotonergic mediation of reward value, whereas studies using intracranial self-stimulation point to an important contribution of serotonin in modulating motivational aspects of rewarding brain stimulation. Pharmacological studies using agonists and antagonists of serotonergic receptor subtypes and approaches investigating an increase or decrease of the extracellular level of serotonin offer strong evidence for a serotonergic mediation, ranging from aversion to pleasure. This review provides an argument for serotonin as a fundamental mediator of emotional, motivational and cognitive aspects of reward representation, which makes it possibly as important as dopamine for reward processing.

Involvement of serotonin in nicotine dependence: processes relevant to positive and negative regulation of drug intake

The neurobiological substrate of nicotine dependence has been the subject of extensive preclinical and clinical research. Many experimental reports have implicated the brain serotonin (5-HT) systems in processes relevant to nicotine dependence, but the specific role of this neurotransmitter system largely remains to be elucidated. This review will focus on the role of 5-HT in the acute and chronic effects of nicotine. In particular, the evidence for a role of 5-HT neurotransmission in brain processes thought to be involved in positive and negative control of nicotine use will be examined, and potential clinical implications discussed.

Nicotine as an Addictive Substance: A Critical Examination of the Basic Concepts and Empirical Evidence

The present review is a critical analysis of the concepts behind and the empirical data supporting the view that tobacco use represents an addiction to nicotine. It deals with general aspects of the notion of addiction, while concentrating on specific problems associated with incorporating nicotine into current frameworks. The notion of addiction suffers from unprecedented definitional difficulties. The definitions offered by various authorities are very different, even contradictory. Definitions that reasonably include nicotine are so broad and vague that they allow many trivial things, such as salt, sugar, and watching television, to be considered addictive. Definitions that exclude the trivia also exclude nicotine. The addiction hypothesis, in general, is strongly shaped by views that certain drugs bring about a molecular level subversion of rationality. The main human evidence for this is verbal reports of smokers who say that they can't quit. On the other hand, the existence of many millions of successful quitters suggests that most people can quit. Some smokers don't quit, but whether they can't is another matter. The addiction hypothesis would be greatly strengthened by the demonstration that any drug of abuse produces special changes in the brain. It has yet to be shown that any drug produces changes in the brain different from those produced by many innocuous substances and events. The effects of nicotine on the brain are similar to those of sugar, salt, exercise, and other harmless substances and events. Apart from numerous conceptual and definitional inadequacies with the addiction concept in general, the notion that nicotine is addictive lacks reasonable empirical support. Nicotine does not have the properties of reference drugs of abuse. There are so many findings that conflict so starkly with the view that nicotine is addictive that it increasingly appears that adhering to the nicotine addiction thesis is only defensible on extra-scientific grounds.