Analysis of the responses of frog motoneurons to epinephrine and norepinephrine

Epinephrine and norepinephrine were applied to the isolated superfused frog spinal cord hyperpolarized motoneurons. The hyperpolarization was related to both direct and indirect actions and the indirect effects were produced by activation of α 2-adrenoceptors. In about half of the spinal cords a slow depolarization caused by activation of β-receptors was seen and was largely attributable to direct actions of the catecholamines on motorneuron membranes. In a small number of preparations an early α 1-mediated depolarization was noted. The results suggest that catecholamines released from terminals in the frog ventral horn could exert a modulatory action on the motoneuron output from the spinal cord.