Ircland
5-Hydroxytryptamine (5HT) has been implicated in numerous physiological functions that vary from cardiovascular and respiratory activity to various mood states and neuroendocrine and neuroimmune processes. However, the role of 5HT in the control of motor function has only recently been explored. Thus serotonergic neurons are known to directly innervate both primary and secondary motor areas and there is evidence that 5HT plays an important role in the functioning of the larger skeletal muscles. Knowledge of the anatomical distribution of the serotonergic system in the mammalian brain, and its numerous physiological functions, assists in a better understanding of the adverse effect of the SSRIs on the functioning of the motor system. The Occurrence dystonia, akathesia, and the exacerbation of Parkinson's disease by the SSRIs has frequently been ascribed to an inhibition of dopamine release from the nigro striatal pathway arising from the inhibitory serotonergic input from the raphk. However, there is increasing evidence that at least some SSRIs can cause extrapyramidal movement disorders (such as torticollis and dystonia) by acting on the sigma 2 receptors located in the red nucleus. Thus while 5HT may be involved in movement disorders associated with use of the SSRIs, some of the adverse effects of these drugs could be due to direct activation of sigma 2 receptors in the rubro-cerebellar circuit.