Alpha2 adrenergic receptors and the central control of breathing in the cane toad, Bufo marinus

The effect of adrenergic agents on the central control of breathing in the cane toad, Bufo marinus, was tested by applying adrenergic agents to the ventral medullary surface of decerebrate adult toads. Toads were unidirectionally ventilated while recording lung, buccal, and artery blood pressures (BPI), as well as heart rate (HR). Following a control period, filter paper pledgets soaked in the appropriate solution (epinephrine - 0.023, 0.05, 0.10, and 0.23 mM; norepinephrine - 0.002, 0.016, 0.032, and 0.16 mM; clonidine - 0.00375, 0.0375, and 0.375 mM; or yohimbine - 0.43) mM) were placed bilaterally on the ventral medullary surface. Epinephrine significantly increased the number of breaths (26%), lung amplitude (9%), and episode duration (21%), but had no effect on BP or HR. The alpha2-agonist. clonidine, significantly increased respiratory activity at moderate doses (0.0375 mM) and decreased activity at high doses (0.375 mM), however, it failed to elicit significant changes in BP or HR. Pretreatment with the alpha2-adrenoceptor antagonist, yohimbine (0.43 mM), blocked the clonidine induced changes in respiratory activity. Yohimbine had no effect on cardiorespiratory parameters. Norepinephrine had no effect on either cardiovascular or respiratory variables. Thus, it appears that an alpha2 adrenergic mechanism is involved in the central control of respiration in this lower vertebrate.