Matters arising

0306-4522/92 $5.00 + 0.00 Pergamon Press Ltd IBRO

Neuroscience Vol. 50, No. 2, pp. 501 502, 1992

Printed in Great Britain

MATTERS ARISING

The C o m m e n t a r y entitled The Integrative Properties o f Spiny Distal Dendrites by S. W. Jaslove in Neuroscience Vol. 47, No. 3, pp. 495-519, 1992, contains many interesting points. This comment only aims at clarifying some aspects of the presence and properties of voltage-dependent potassium conductances in the spiny dendrites of Purkinje cells. So far, no evidence suggests that an M-like current regulates Purkinje cell dendritic excitability, as otherwise mentioned by Jaslove. 8 In Purkinje cells, a voltage-dependent, transient potassium current is present in the spiny dendrites, LS and activates well below threshold for both somatic Na + spikes and dendritic Ca 2+ spikes. The experiments show that the properties of the transient outward current resembles the A current, 3'~3 but it inactivates quite slowly, on a time-frame of seconds. The activation and inactivation properties of the transient A-like outward current are quite opposite an M-current, giving a very different temporal response. The conductance(s) underlying the transient outward current in Purkinje cell spiny dendrites is (are) not fully identified as 4-aminopyridine only partly blocks the effects of the transient outward current. 5 Several transient potassium conductances may therefore participate in the transient hyperpolarizing response. The amplitude of the dendritic Ca2+-response is strongly controlled by potassium conductances, grading the Ca2+-response from a local response to a

full spike, depending on the inactivation of the transient A-like current. This is also seen during excitatory synaptic stimulation. 2 This has also been shown with Fura-2 imaging. L2 The inhibitory input from stellate cells can inactivate the transient outward current, so a long-lasting effect follows the inhibitory input.l~ Furthermore, the fast afterhyperpolarization and reduction in Na ~-firing following a Ca2+-spike is reduced by an inhibitory postsynaptic potential which reduces the Ca 2~spike amplitude. IJ In this way the intrinsic conductances interact with both excitatory and inhibitory inputs. Tetraethylammonium tends to prolong the Ca 2+spike, 6-9'~° with little effect on the amplitude in comparison with 4-aminopyridine. This effect could be explained by the blocking action of tetraethylammonium on some Ca2+-activated potassium conductances. 4 This shows that several types of potassium conductances regulate Ca-'+-excitability in the dendrites of Purkinje cells. The experimental evidence shows that potassium conductances are indeed very important in the control of local dendritic excitability. Furthermore, the kinetic properties of the outward currents in Purkinje cells greatly expands the time-window for spatio-temporal integration to around 1 min. 67 J. MIDTGAARD

REFERENCES

1. Chan C., Hounsgaard J. and Nicholson C. (1988) Effects of electric fields on transmembrane potential and excitability of turtle cerebellar Purkinje cells in vitro. J. Physiol. 402, 751 771. 2. Chan C., Hounsgaard J. and Midtgaard J. (1989) Excitatory synaptic responses in turtle Purkinje cells in vitro. J. Physiol. 409, 143-156. 3. Connor J. A. and Stevens C. F. (1971) Voltage clamp studies of a transient outward membrane current in gastropod neural somata. J. Physiol. 213, 21 30. 4. Hille B. (1991) Ionic Channels" o f Excitable Membranes. Sinauer Associates. 5. Hounsgaard J. and Midtgaard J. (1988) Intrinsic determinants of firing pattern in turtle Purkinje cells in vitro. J. Physiol. 402, 731 749. 6. Hounsgaard J. and Midtgaard J. (1989) Synaptic control of excitability in turtle Purkinje cells in vitro. J. Physiol. 409, 157-170. 7. Hounsgaard J. and Midtgaard J. (1989) Dendrite processing in more ways than one. Trends Neurosci. 12, 313 315. 8. Jaslove S. W. (1992) The integrative properties of spiny distal dendrites. Neuroscience 47, 495-520. 9. Llinfis R. and Sugimori M. (1980) Electrophysiological properties of in vitro Purkinje cell somata in mammalian cerebeHar slices. J. Physiol. 305, 171-195. 10. Llinfis R. and Sugimori M. (1980) Electrophysiological properties of in vitro Purkinje cell dendrites in mammalian cerebellar slices. J. Physiol. 305, 197 213. 11. Midtgaard J. (1991) Interactions between postsynaptic membrane properties and ipsp's in turtle Purkinje cells in vitro. J. Physiol. 446, 157P. I2. Midtgaard J. Lasser-Ross N. and Ross W. N. (1991) Intrinsic determinants of dendritic Ca2+-influx in turtle Purkinje cell dendrites in vitro. Soc. Neurosci. Abstr. 628, 6. 13. Neher E. (1971) Two fast transient current components during voltage clamp on snail neurons. J. gen. Physiol. 58, 36 53. Continued

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Reply This is a valuable clarification of what is known about potassium currents in Purkinje cell dendrites. I did not mean to assert that Purkinje cells have M-currents. I believe we are in agreement that potassium channels probably play an important role in dendritic integration and secondarily modulated ones can underlie cell priming. As one originally trained in the Hodgkin-Huxley analysis of squid giant axon, I

am continually intrigued by how many variations of "the simple channel" have turned up, and how each plays a special role in shaping the cell potential of central neurons. I would like to take the opportunity to correct a sign inversion which appeared in my equation for flh on page 499. It should have read: flh = 4.5/{1 + e x p [ - ( V m + 35)/10]}. S. W. JASLOVE

(11 May 1992)