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To investigate the role of P/Q type Ca(2+) channels in determining the firing pattern of Purkinje cells (PCs) we compared the somatically evoked discharge of action potentials (APs) in PCs from 3 to 4 week old cerebellar slice cultures obtained with ataxic mice lacking alpha(1A)-subunit (alpha(-/-)) and with normal mice (non-ataxic alpha(+/-) or alpha(+/+)) using the whole-cell configuration of the patch-clamp recording method. Whereas evoked responses of PCs in normal mice were mainly fast APs, those of PCs from ataxic mice were mainly low-threshold Ca(2+) spikes (LTS). Furthermore, a sustained plateau potential due to the activation of cadmium sensitive Ca(2+) conductances was not observed in PCs from ataxic mice by blocking K(+) channels. These results confirm that P/Q Ca(2+) channels elicit Ca(2+)-dependent plateau potentials and control the propagation of the dendritic LTS to the soma., (Copyright 2002 Elsevier Science Ireland Ltd.)

Mature rat cerebellar oligodendrocytes were isolated in culture using a serum free medium and identified using anti-galactocerebroside (GalC) and OL-1 antibodies. The morphology of such oligodendrocytes changes with time in culture from a multipolar shape at about 4 days in vitro (DIV) to a monopolar shape at about 8 DIV, a transition that has been previously described only in situ. Voltage-gated ionic currents were characterized at both oligodendrocyte developmental stages using the whole cell configuration of the patch-clamp technique. No differences between these two stages were detected, both types expressed a large K+ inward rectifying current similar to that described for oligodendrocytes from other vertebrate neuronal structures. This current could play an important role in the control of oligodendrocyte resting membrane potential. Our culture system provides a valuable model, close to the situation encountered in situ, not only to study the process of oligodendrocyte maturation, but also to identify the possible interactions between oligodendrocytes and cerebellar neurons such as granular and Purkinje cells during development.

Whole-cell recordings of cranial sensory neurons were obtained with CsCl- and tetraethylammonium (TEA)-loaded cells as used for ICa studies. When replacing calcium (Ca) with potassium (K) in the external medium, a transient inward current developed at low threshold, if the holding potential was below -100 mV. Its activation started above -60 mV and reached its maximum at -20 mV. Its reversal potential followed the predicted value of the K equilibrium potential (EK) characteristic of a K current. Pharmacologically, it resembles the classical outward A current since it is insensitive to TEA up to 20 mM and blocked by 4-aminopyridine (4-AP) and 3,4-AP in the millimolar range. This inward A current rises rapidly in less than 5 ms and decays monoexponentially with a time constant of about 60 ms at all potentials studied. Its steady-state inactivation occurred between -100 and -60 mV with a half-inactivation at -86 mV. All these characteristics hold for a voltage-dependent A current devoid of any Ca-dependent component. The activation and inactivation of this A inward current are compared to those of ICa,t, the other low threshold current encountered on those cells. The contribution of the two currents in the control of discharge frequency is discussed.

Cerebellar Purkinje cells (PC) can be maintained in culture for one to two weeks in L15, a rich medium known to allow expression of a normal excitability in peripheral neurones. When examined using whole cell recordings, PC proved to be inexcitable in these conditions, and this inexcitability could be related to the presence of large outward K currents. Depolarizing steps of -100 mV revealed a voltage-dependent biphasic K current with a large early transient phase followed by a small plateau phase. The early transient phase could be selectively eliminated by holding the cell at -40 mV or by extracellularly applying 5 mM 4-aminopyridine (4-AP), whereas the plateau was abolished by 15 mM tetraethylammonium (TEA). Hereafter, these currents will be identified as the IA and the delayed current respectively, IA being the predominant current. IA activated between -25 and +65 mV with a midpoint at +3 mV; inactivation occurred between -70 and -20 mV with a midpoint at -57 mV. Current decay followed an exponential time course with a time constant of about 30 ms between -20 and +10 mV. In the cell-attached recording configuration, depolarization elicited openings of two types of K channels, one inactivating and one non-inactivating. The non-inactivating K channel probably corresponded to the delayed K current and had a conductance of 22 pS in a physiological K gradient.

Conditions were devised to isolate in cranial sensory neurones transfer of Na ions: K and Ca were omitted from the extracellular medium, and simultaneously cells were intracellularly loaded with 120 mM caesium and 20 mM TEA at [Ca]i = 10(-8) M. A tetrodotoxin (TTX)-resistant current was shown to be elicited by step depolarization from -25 MV upwards. This current successively activates and inactivates at increasing rates on further depolarization and at 0 mV (where peak amplitude is reached) its time course is of 20-50 ms. Absence of TTX-sensitivity (up to 15 microM), slow time course and an activation curve shifted by 15 mV towards the depolarized potentials differentiate this current from the more classical fast Na current which can be elicited on the same cells. Inactivation was provoked by a prepulse of varying amplitude and duration: with a prepulse command to -20 mV, inactivation was of 50% within a delay of 300 ms and almost 100% in about 1 min. After complete inactivation by command to 0 mV for 300 ms, recovery by holding the potential at -80 mV was of 50% in 205 ms, and of 100% after 1-4 s. It is concluded that a charge transfer of Na accounts for most of the hump which prolongs the action potential of these sensory neurones, and thus it can be proposed that spike duration as modulated by neurotransmitters may also involve Na in addition to Ca.