CALCIUM SPIKES AND CALCIUM PLATEAUS EVOKED BY DIFFERENTIAL POLARIZATION IN DENDRITES OF TURTLE MONONEURONES IN-VITRO

1. The ability of dendrites in turtle motoneurones to support calcium spikes and calcium plateaux was investigated using differential polari zation by applied electric fields. 2. Electric fields were generated b y passing current through transverse slices of the turtle spinal cord between two plate electrodes. The linear extracellular voltage gradien t generated by the field implied that the tissue was ohmic and homogen eous. 3. The transmembrane potential at the cell body of motoneurones was measured as the voltage difference between an intracellular and an extracellular microelectrode. 4. In normal medium an applied field in duced synaptic activity as well as intrinsic polarization of motoneuro nes. Synaptic activity was suppressed by tetrodotoxin (TTX, 1 mum). 5. In the presence of TTX and tetraethylammonium (TEA, 1-5 mm), applied fields evoked multicomponent Ca2+ spikes in both the soma-hyperpolariz ing and soma-depolarizing direction of the field. The different compon ents of Ca2+ spikes were discrete and additive. High amplitude compone nts had higher threshold and faster time course and were followed by l arger after-hyperpolarizations, than low amplitude components. The fre quency of field-evoked regenerative responses was relatively insensiti ve to somatic bias current. 6. TTX-resistant Ca2+-mediated plateau pot entials promoted by apamin were evoked by differential polarization in both the soma-depolarizing and somahyperpolarizing direction. 7. It i s concluded that Ca2+ channels responsible for Ca2+ spikes and Ca2+ pl ateaux are present in dendrites of spinal motoneurones of the turtle.