IONIC MECHANISMS MEDIATING 5-HYDROXYTRYPTAMINE-EVOKED AND NORADRENALINE-EVOKED DEPOLARIZATION OF ADULT-RAT FACIAL MOTONEURONS

1. The actions of 5-hydroxytryptamine (5-HT) and noradrenaline (NA) on the membrane properties of facial motoneurones in slices from the adult rat brainstem in vitro were examined using intracellular recording techniques. 2. In voltage clamp recording, hyperpolarizing voltage steps (> 20 mV), from holding potentials at or close to the resting potential, induced a slowly activating, voltage-dependent inward current possessing properties similar to the hyperpolarization-activated current (I(h)) seen in other cell types. From tail current analysis two groups of facial motoneurones can be distinguished in terms of the activation range for I(h), one with a half-maximal activation at -81 mV and the other at -94 mV but with similar shapes. 3. 5-HT (120/126) and NA (21/21) depolarized facial motoneurones. The reversal potentials (E(m)) obtained from peak voltage amplitude I-V plots in varying extracellular potassium concentrations suggested mechanisms involving a decrease in K+ conductance. 4. Under voltage clamp, close to the resting potential, both 5-HT (39/41) and NA (13/13) evoked inward currents. 5. I-V plots and plots of 5-HT-sensitive current at different membrane potentials, obtained from currents evoked by voltage steps and measured before the development of I(h) (instantaneous current), indicated that the 5-HT-evoked inward current was predominately associated with a decrease in conductance but with a range of reversal potentials for 5-HT (E5-HT) from close to, to much more negative than the reversal potential for a potassium conductance (E(K)). In some cases no change or increases in instantaneous conductance were observed. 6. Steady-state I-V relationships and plots of 5-HT-sensitive current, measured after development of I(h), indicated a 5-HT-associated conductance increase with a time and voltage dependence close to that of I(h), which could be abolished by extracellular caesium (2-5 mM). 7. The NA-evoked inward current was always associated with a decrease in conductance. Instantaneous and steady-state I-V relationships as well as plots of NA-sensitive current indicated a reversal potential at E(K). 8. The activation curve for I(h) was shifted to more positive potentials in the presence of 5-HT. The time constant for activation of I(h) showed a similar shift. 9. 5-Carboxamidotryptamine (5-CT), a 5-HT receptor agonist, was selective for the enhancement of Ih and only evoked an inward current when the holding potential was within the activation range of I(h). 10. It is concluded that 5-HT depolarizes facial motoneurones through a combination of mechanisms. One involves a decrease in a K+ conductance active at all membrane potentials, the other an enhancement of a hyperpolarization-activated current, I(h). NA-evoked depolarization appears to involve only K+ channel closure.