Enteric reflexes and nitric oxide in the fish intestine

The aim of the present study was to elucidate the possible regulation of peristalsis in the intestine of the Atlantic cod Gadus morhua. For this purpose, the mid intestine was dissected out and placed in a partitioned bath. Balloon distension (0.1­0.4 ml) and intramural field stimulation (8 Hz, 10 V) were carried out and the responses of the circular muscle were recorded 1.5 cm orally and anally to the stimulus using force transducers. The preparations developed spontaneous contractions propagating in the anal direction with a frequency of about one contraction per 2 min. Distension of the muscle wall with a balloon did not evoke any recordable peristaltic reflexes. Intramural stimulation caused a contraction oral to the stimulation and a relaxation anal to the stimulation in most cases. Tetrodotoxin abolished the responses to electrical stimulation in both directions. Atropine reduced and methysergide abolished the oral contractions caused by electrical stimulation. Administration of the nitric oxide synthesis inhibitor l-NG-nitro-arginine methyl ester (l-NAME) abolished the anal relaxation caused by electrical stimulation and augmented the oral contractions. The results indicate the presence in teleost fish intestine of an ascending excitatory peristaltic reflex which involves a cholinergic­serotonergic pathway and a descending inhibitory reflex involving a nitrergic pathway. These observations suggest a high degree of conservation of peristaltic mechanisms during vertebrate evolution.