The influence of an extraneuronal compartment on the relaxation of the cat nictitating membrane in vivo.

1 Contractions of the cat nictitating membrane were elicited on stimulation of the internal carotid nerve, and the effects were studied of desipramine and two inhibitors of catechol-O-methyltransferase, U-0521 and pyrogallol, on the subsequent relaxation of the muscle. 2 The relaxation of the nictitating membrane occurred in at least two phases. The late phase of relaxation was prolonged after increase in the period of nerve stimulation and the duration of this phase was further prolonged after treatment with pyrogallol. 3 After inhibition of neuronal uptake of noradrenaline with desipramine both the early and late phases of relaxation were increased in duration, and subsequent administration of pyrogallol or U-0521 caused a further increase in the duration of the late phase of relaxation. 4 The results suggest that the late phase of relaxation of the nictitating membrane is influenced by efflux of noradrenaline from an extraneuronal pool.