Faculty Bibliography

1. Voltage clamp studies were performed in squid giant synapse after blockage of the voltage-dependent sodium and potassium conductances. 2. Presynaptic depolarization under these conditions demonstrates the presence of voltage-dependent calcium conductance change for the duration of the voltage step, and a tail current at the break of the pulse. 2. This calcium current triggers a postsynaptic response which can be measured directly at the postsynaptic fiber. 4. These voltage clamp experiments have allowed the development of a mathematical model that describes the kinetics of the calcium current and the relationship between calcium current and transmitter release.

1. The electrical activity of Purkinje cells was studied in guinea-pig cerebellar slices in vitro. Intracellular recordings from Purkinje cell somata were obtained under direct vision, and antidromic, synaptic and direct electroresponsiveness was demonstrated. Synaptic potentials produced by the activation of the climbing fibre afferent could be reversed by direct membrane depolarization. 2. Input resistance of impaled neurones ranged from 10 to 19 M omega and demonstrated non-linearities in both hyperpolarizing and depolarizing directions. 3. Direct activation of a Purkinje cell indicated that repetitive firing of fast somatic spikes (s.s.) occurs, after a threshold, with a minimum spike frequency of about 30 spikes/sec, resembling the '2-class' response of crab nerve (Hodgkin, 1948). 4. As the amplitude of the stimulus was increased, a second form of electroresponsiveness characterized by depolarizing spike bursts (d.s.b.) was observed and was often accomppanied by momentary inactivation of the s.s. potentials. Upon application of tetrodotoxin (TTX) or removal of Na+ ions from the superfusion fluid, the s.s. potentials were abolished while the burst responses remained intact. However, Ca conductance blockers such as Co, Cd, Mn and D600, or the replacement of Ca by Mg, completely abolish d.s.b.s. 5. If Ca conductance was blocked, or Ca removed from the superfusion fluid without blockage of Na conductance, two types of Na-dependent electroresponsiveness were seen: (a) the s.s. potentials and (b) slow rising all-or-none responses which reached plateau at approximately -15 mV and could last for several seconds. These all-or-none Na-dependent plateau depolarizations outlasted the stimulus and were accompanied by a large increase in membrane conductance. Within certain limits the rate of rise and amplitude of the plateau were independent of stimulus strength. The latency, however, was shortened as stimulus amplitude was increased. These potentials were blocked by TTX or by Na-free solutions. 6. Substitution of extracellular Ca by Ba or intracellular injection of tetraethylammonium generated prolonged action potentials lasting for several seconds and showing a plateau more ositive than those obtained in norrmal circumstances by either non-inactivating Na or Ca currents. 7. Spontaneous firing of the Purkinje cell was characterized by burst-like activity consisting of both s.s. and d.s.b. responses. Addition of TTX to the bath left the basic spontaneous activity and its frequency unaltered, indicating tha Ca spiking and Ca-dependent K conductance changes are the main events underlying this oscillatory behaviour. 8

1. Intradendritic recordings from Purkinje cells in vitro indicate that white matter stimulation produces large synaptic responses by the activation of the climbing fibre afferent, but antidromic potentials do not actively invade the dendritic tree. 2. Climbing fibre responses may be reversed in a manner similar to that observed at the somatic level. However, the reversal does not show the biphasicity often seen at somatic level. 3. Input resistance of these dendrites was found to range from 15 to 30 M omega. The non-linear properties seen at the somatic level for depolarizing currents are also encountered here. However, there seems to be less anomalous rectification. 4. Detailed analysis of repetitive firing of Purkinje cells elicited by outward DC current shows that, as in the case of the antidromic invasion, the fast somatic potentials (s.s.) do not invade the dendrite actively. However, the dendritic spike bursts (d.s.b.s) interposed between the s.s. potentials are most prominent at dendritic level. 5. Two types of voltage-dependent Ca responses were observed. At low stimulus level a plateau-like depolarization is accompanied by a prominent conductance change; further depolarization produces large dendritic action potentials. These two classes of response are TTX-resistant but are blocked by Cd, Co, Mn or D600, or by the removal of extracellular Ca. 6. Following blockage of the Ca conductance, plateau potentials produced by a non-inactivating Na conductance are observed mainly near the soma indicating that this voltage-dependent conductance is probably associated with the somatic membrane. 7. Spontaneous firing in Purkinje cell dendrites is very similar to that observed at the soma. However, the amplitude of these bursts is larger at dendritic level. It is further concluded that these TTX-insensitive spikes are generated at multiple sites along the dendritic tree. 8. Six ionic conductances seem to be involved in Purkinje cell electroresponsiveness: (a) an inactivating and (b) a non-inactivating Na conductance at or near the soma, (c) a spike- and (d) a plateau-generating Ca conductance, and (e) voltage-dependent and (f) Ca-dependent K currents. 9. The possible role of these conductances in Purkinje cell integration is discussed