STRYCHNINE ELIMINATES RECIPROCATION AND AUGMENTATION OF RESPIRATORY BURSTS OF THE IN-VITRO FROG BRAIN-STEM

We have recorded rhythmic bursts of efferent action potentials from ne rves of respiratory muscles in the frog (Rana pipiens), using a modifi ed in vitro preparation, in which the brainstem lies in situ in the ve ntral half of the skull. The burst in the sternohyoid branch of the hy poglossal nerve (Hsh) was augmenting, and alternated with a relatively brief augmenting burst in the main branch of the hypoglossal nerve (H m). The laryngeal branch of the vagus nerve (Xl) displayed a biphasic burst, beginning before peak activity of Hsh and spanning the Hm burst . The spatio-temporal patterns of these bursts closely resemble those recorded from the same nerves in intact and in decerebrate frogs, indi cating that the bursting rhythm of this in situ preparation constitute s fictive breathing. The nature of neurotransmission responsible for b urst reciprocity and augmentation was investigated by applying the gly cine receptor blocker, strychnine. Low levels of strychnine (1 and 5 M ) increased the frequency of fictive breathing without changing the sh ape or timing of Hsh, Hm and Xl bursts; at higher doses (10 and 20 M) the bursts in all nerves abruptly changed shape and timing to become s ynchronous and decrementing. The strychnine-induced changes were assoc iated with the appearance of a prominent peak (10-20 Hz) on the spectr al analysis of the nerve discharge, possibly indicating a fundamental change in neurogenesis of the respiratory pattern. We conclude that th e burst augmentation and reciprocation discharge characteristics of fi ctive breathing in the frog require strychnine-sensitive inhibitory ne tworks. (C) 1997 Elsevier Science Ireland Ltd.