Altered behavioral responses mediated by serotonin receptors in the genetically dystonic (dt) rat

The genetically dystonic (dt) rat is an animal model of dystonia that displays sustained abnormal movements that include: torticollis, clasping of the hindlimbs, rigidity of the limbs, and contortions of the trunk. Since serotonin (5-HT) has been shown to be involved in some animal models of movement disorders, the functional responsiveness of the 5-HT system in dt rats and phenotypical normal littermates was examined by administering 5-HT agonists selective for different receptor subtypes and observing behavioral responses associated with the activation of specific 5-HT receptor subtypes. The dt rats were 6-fold more sensitive to the ability of the 5-HT_1A agonist 8-OH-2-(di-n-propylamino)tetralin (8-OH-DPAT) to produce the 5-HT behavioral syndrome. The dt rats demonstrated a diminished head-shaking response following administration of the 5-HT₂ agonist 1-(2,5-dimethoxy-4-bromophenyl)-2-aminopropane (DOB). However, the dt rats also displayed significantly fewer head shakes following mechanical stimulation of the aural pinnae. The inability of the dt rat to demonstrate head-shaking behavior following stimulation of 5-HT₂ receptors is probably due to the dt rat's difficulty in producing the motor responses involved in this behavioral response and do not reflect alterations in 5-HT₂ receptor sensitivity. These results suggest the the 5-HT system, particularly 5-HT_1A receptors, may have an integral role in the abnormal movements displayed by the genetically dystonic rat and movement disorders in general.