Stimulants behavioral effects

The characteristic behavioral effects of acute and chronic psychomotor stimulant diugs are locomotor activation, stereotypy, and conditioned reward and stimulus-reward learning. The most important brain regions involved in these effects are summarized in Table 3. 

The various stimulants have no obvious chemical relationships and do not share primary neurochemical effects, despite their similar behavioral effects. Cocaines chemical strucmre does not resemble that of caffeine, nicotine, or amphetamine. Cocaine binds to the dopamine reuptake transporter in the central nervous system, effectively inhibiting dopamine reuptake. It has similar effects on the transporters that mediate norepinephrine and serotonin reuptake. As discussed later in this chapter in the section on neurochemical actions mediating stimulant reward, dopamine is very important in the reward system of the brain the increase of dopamine associated with use of cocaine probably accounts for the high dependence potential of the drug. 

Stimulants induce both tolerance and sensitization to their behavioral effects. Tolerance develops to the anorectic and euphoric effects of stimulants (Schuster 1981) however, chronic intermittent use of low doses of stimulants delays the development of tolerance. With the doses commonly used in clinical practice, patients treated for narcolepsy or for depressive or apathetic states find that the stimulant properties usually persist without development of tolerance however, the persistence of antidepressant effects remains a matter of controversy. Sensitization has been linked to the development of amphetamine-induced psychosis (Yui et al. 1999). Sensitization to the induction of psychosis is suggested because psychosis is induced by progressively lower doses and shorter periods of consumption of amphetamine following repeated use over time (Sato 1986). Sensitization for amphetamine-induced psychosis may persist despite long periods of abstinence. 

D Mello, G.D. Comparison of some behavioral effects of and electrical brain stimulation of the mesolimbic dopamine system in rats. Psychopharmacology (Berlin) 75 184-192, 1981. 

J.F. Temporal and sequential patterns of agonistic behavior. Effects of alcohol, anxiolytics and psychomotor stimulants. Psychopharmacology 97 149-151, 1989. 

The behavioral effects of amphetamine, methamphetamine, MDMA, MDA, p-chloroamphetamine, and fenfluramine are not identical. Except for the last drug, all can cause some degree of behavioral stimulation, but exact behavioral effects differ markedly. More complete definition of their behavioral differences is a prerequisite to a better understanding of the mechanism(s) of these drugs. 

Mating animals that display a required trait and selecting offspring that also display the trait reveals genetic differences in behavioral responses such as cognition. Rats have been selectively bred for high or low emotionality on the basis of defecation rates and these two strains have been found to differ in their sensitivity to the stimulant/depressant effects of nicotine [49]. 

Alpha-methylation of DMT reduced its behavioral activity in animals, while alpha-methylation of N-methyltryptamine (27) resulted in an agent with stimulant properties (137,228). Alpha-methyltryptamine (a-MeT structure 77), however, is hallucinogenic in man at doses of about 30 mg. Thus it is two to three times more active than DMT (for review see refs. 24, 81, and 196). 5-Methoxy-a-methyltryptamine (5-OMe-a-MeT 78) was also determined to be twice as active in man as its dialkyl counterpart, 5-OMeDMT. In human trials, 5-OMe-a-MeT produced behavioral effects at about 3 mg (204). A comparison of the activities of the individual isomers of 78 in man has not been reported. However, Glennon and co-workers (76,83,90) found that the (+)-isomers of both a-MeT and 5-OMe-a-MeT are more active than their racemates in tests of discriminative control of behavior in rats. Although (+)-5-OMe-a-MeT was four times more active than its enantiomer, (-)-a-MeT did not produce effects similar to either racemic a-MeT or 5-OMeDMT. 

Electrical stimulation in the vicinity of the raphe nuclei has been reported to produce depressant effects on MSRs (32,142). These depressant effects could be attributed to activation of postsynaptic inhibitory receptors analogous to those characterized in the forebrain, except for the fact that the depression was blocked by 5-HT antagonists. The effectiveness of the antagonists suggests that the receptor is more akin to the excitatory 5-HT receptor characterized on motoneurons (134,184). The depressant behavioral effects could result from... 

The behavioral effects of nicotine have been defined as both stimulant and depressant, effects that are influenced by the present mental status and expectations of the smoker. Smokers may feel alert and relaxed. Nicotine produces myriad effects on the central nervous system (CNS), almost all of which appear to be mediated through nicotinic receptors. Additionally, nicotine influences multiple neuronal systems. One of its most prominent effects is stimulated release of dopamine, particularly in the nucleus accumbens, which is a major component of the reward system. Nicotine also stimulates the release of endogenous opioids and glucocorticoids. 

---