Effects on breathing of medullary bicuculline microinjections in immature opossums

The role of medullary gamma-aminobutyric acid A (GABAA)-receptor activation in the expression of breathing was studied in suckling opossums (Didelphis virginiana) from 4 to 8 wk of age. Animals were anesthetized with a thiobarbiturate, and the ventral medulla was exposed so that drugs could be microinjected into the medulla with a two-barrel glass micropipette. The GABAA antagonist bicuculline at approximately 1-3 pmol total dose was injected in volumes < 1 nl; injections of saline with pontamine sky blue dye were used as controls and to mark location. Breathing pattern was assessed using diaphragm electromyography (EMG). Effective sites for bicuculline microinjection were obtained in the ventrolateral medulla, with both lateral reticular and inferior olive nuclei present as rostrocaudal markers. Responses among 18 tested animals included increased duration of breaths (Ttot) and duration of inspiration (Ti). Peak amplitude of the diaphragm EMG and peak amplitude of the diaphragm/Ti were not consistently affected. Transient apnea, lasting at least 8 s, occurred in five of the animals. Control solutions did not elicit respiratory responses. These results suggest that neural circuitry in the medulla, using the neurotransmitter GABA, can have an early role in the modulation of breathing pattern.