Effect of L-prolyl-L-leucyl-glycinamide (MIF-I) on some neutrotransmitter-receptor interactions

Some neurotransmitter-receptor interactions have been studied in an attempt to determine how L-prolyl-L-leucyl-glycinamide (MIF-I) exerts its antiparkinson effect. MIF-I affected neither the contractile responses of isolated mouse vas deferens and guinea pig ileum to noradrenaline, acetylcholine, substance P and histamine, nor the inhibitory effects of dopamine and GABA on the rat vas deferens and guinea pig ileum. MIF-I, as well as L-leucine and Pro-Leu, antagonized the contractile response of the ileum to 5-hydroxytryptamine (5-HT). Behavioural tests were used to examine the action of MIF-I on CNS transmitter-receptor interactions. MIF-I did not modify the circling produced by either dopamine agonists in nigro-striatal lesioned rats of 5-HT agonists in rats with a lesion of the medial raphe nucleus. MIF-I affected neither 5-hydroxytryptophan-induced head twitches in mice, which is a measure of 5-HT receptor stimulation, nor striatally-evoked head turning in the rat, which is a model for brain GABA function. It is concluded that MIF-I, at the doses used, does not directly modify the function of any of the CNS transmitter examined. Other possibilities to explain its antiparkinson action are discussed.