Adrenaline depolarization in paravertebral sympathetic neurones of bullfrogs

Abstract

Responses to adrenaline (Ad) and their ionic mechanisms were analysed using intracellular recording and voltage-clamp methods in neurones of bullfrog sympathetic ganglia. Ad (5 μM–1 mM) applied directly to sympathetic neurones by pressure ejection through a micropipette produced three types of depolarizing responses (2–20 mV). Under voltage-clamp conditions, Ad (100 μM) produced fast, slow and mixed types of inward currents (Ad_Is) with amplitude of 2.9±1.3 nA. β-Adrenoceptors may be responsible for the generation of these Ad_Ds. The slow Ad_I which lasted for 1–5 min was associated with a decreased membrane conductance. The slow Ad_I decreased at hyperpolarized potential level and eventually nullified at −70 mV. No reversal of the slow Ad_I polarity was observed in the Ringer solution. Injection of Cs²⁺ into the ganglion cells produced a marked depression of the amplitude of the slow Ad_I. The slow Ad_I was blocked by bath-applied Ba²⁺ but not by TEA. Ad reduced the slow current relaxation, the M current, associated with voltage jumps in the membrane potential range −35 to −55 mV. The fast Ad response was associated with an increase in membrane conductance. When the membrane was depolarized, the fast Ad_I decreased and reversed its polarity at −36±8.3 mV. Removal of Cl ion from superfusing solution depressed the fast Ad_I, suggesting that activation of Cl⁻ conductances may be involved in the generation of the fast Ad_I. The mixed type of Ad response exhibited characteristics of both the fast and slow Ad responses. The results suggest that Ad increases the excitability of neurones in bullfrog sympathetic ganglia.