GABAergic effects on respiratory neuronal discharge during opossum development.

Changes in breathing pattern between immature and adult animals could be due in part to changing postsynaptic sensitivity to particular neurotransmitters by respiratory neurons and/or to the fate of these neurotransmitters after release. To probe for such effects, gamma-aminobutyric acid (GABA) and the GABAA antagonist, bicuculline, were pressure injected by micropipette in very small volumes (approximately 25 pl) near identified medullary respiratory neurons in Inactin-anesthetized adult and suckling opossums. At a concentration of 10 mM, GABA induced suppression of respiratory neurons firing in animals from about 3 wk of age (the youngest animals tested) onward, with the largest responses occurring in adults. For those age groups tested with 0.5 and 50 mM GABA, shorter and longer responses, respectively, were observed. Bicuculline increased the discharge of respiratory units at all ages tested, but responses normalized to initial firing rates did not systematically differ between sucklings down to 4 wk of age and adults. Bicuculline also influenced the onset and cessation of firing in both inspiratory and expiratory neurons. Discharge of respiratory neurons in immature suckling opossums is characterized by few spikes and low firing rates with each breath. However, recovery of neuronal firing from an exogenous load of GABA and release of neuronal firing after antagonism of GABAA receptors does not show a developmental pattern that would implicate GABA as the crucial mediator of these effects.