PHARMACOLOGICALLY EVOKED FICTIVE MOTOR PATTERNS IN THE ACUTELY SPINALIZED MARMOSET MONKEY (CALLITHRIX-JACCHUS)

The existence of a spinal network capable of generating rhythmic alter nating activity resembling locomotion still has not been firmly establ ished in primates, including man, although evidence for one is accumul ating. The present study investigated whether it is possible to activa te such a network by administration of a variety of pharmacological ag ents to acutely spinalized marmoset monkeys (Callithrix jacchus) in th e absence of phasic afferent input to the spinal cord. Fourteen marmos et monkeys were decerebrated, spinalized, and paralyzed. The nerves su pplying both hindlimbs were cut and recorded from. In 5 monkeys the ef fect of electrical stimulation of the brainstem was investigated befor e spinalization. In 3 of these monkeys, rhythmic activity alternating between extensors and flexor nerves was seen. In the 2 other monkeys o nly synchronized activity was elicited. In acutely spinalized monkeys, administration of L-3,4-dihydroxyphenylalanine (L-dopa; 3-4 h after t reatment with nialamide) failed to evoke any rhythmic alternating acti vity. In contrast, administration of clonidine elicited alternating ac tivity in all of 8 monkeys tested. In 4 of these monkeys, the activity was restricted to alternation between ipsilateral and contralateral f lexor nerves, whereas alternating activity between ipsilateral flexors and extensors was also seen in the other 4 monkeys, Administration of excitatory amino acids (NMDA or NMA) also elicited rhythmic alternati ng activity in 7 of 10 spinalized monkeys. In 4, rhythmic alternating activity was seen between extensors and flexors on one limb as well as between ipsilateral and contralateral flexors. In 3 monkeys NMDA/NMA produced alternation between extensors and flexors of one limb without alternation between the ipsilateral and contralateral sides. Administ ration of noradrenaline failed to elicit any rhythmic activity, but ra ther completely depressed already existing activity. Administration of serotonin (5-HT) was ineffective in facilitating alternating activity in 6 of 8 monkeys and was facilitatory to rhythmic activity in the ot her 2. We suggest that these data provide further evidence of a networ k capable of eliciting rhythmic alternating activity resembling locomo tion in the primate spinal cord, The network, however, seems to be mor e difficult to activate pharmacologically in those conditions than in other mammals. This may especially be the case in higher primates, inc luding man.