The Role of the Midbrain Tegmentum in the Regulation of Breathing Patterns in Carp (Cyprinus carpio) and Trout (Onchorhynchus mykiss).

In vertebrates, the basic respiratory rhythm is modified by both sensory feedback and input from higher centers to produce a broad range of breathing patterns. In carp (Cyprinus carpio L.), breathing is often episodic while in trout (Onchorhynchus mykiss) it is continuous and rhythmic except when water is hyperoxic. A previous study in carp revealed that stimulation of neurons at a site in the dorsal mesencephalic tegmentum (DMT) ventrolateral to the oculomotor nucleus, can terminate the apneas and initiate breathing episodes (Juch and Ballintijn, 1983). Such stimulation, however, did not eliminate the apneas or the breathing episodes. To determine whether this site also contains neurons involved in initiating periods of apnea that are intrinsic to episodic breathing, we attempted to lesion the DMT site in decerebrate, spinalectomized carp and trout using stereotaxic microinjections of 0.01mM kainic acid. Following decerebration and spinalectomy, 93% of carp and 33% of trout breathed in episodes in hyperoxic water. Kainic acid injections initially stimulated breathing in all fish. As the excitotoxic effects progressed, breathing frequency and breath amplitude returned to normal levels suggesting that sites within the areas we lesioned are not involved in establishing the overall level of respiratory drive. The ensuing lesions, however, eliminated the episodic breathing in hyperoxia in over 50% of the carp and 33% of the trout. This suggests that sites within the midbrain tegmentum are involved in establishing breathing patterns and that there is a non-uniform distribution of neurons within the midbrain tegmentum involved in producing apneas and clustering breaths into episodes.