Exp Brain Res (1991) 84:426-433

Experimental BrainResearch 9 Springer-Verlag1991

Medial vestibular nucleus in the guinea-pig II. Ionic basis of the intrinsic membrane properties in brainstem slices M. Serafin 1, C. de Waele 2, A. Khateb 1, P.P. Vidal 2, and M. Miihlethaler 1 1 D~partement de Physiologie, CMU, 1 Rue Michel Servet, CH-1211 Gen~ve 4, Switzerland 2 Laboratoire de Physiologie Neurosensorielle, CNRS, 15 Rue de l'Ecole de M6decine, F-75270 Paris Cedex 06, France Received February 13, 1990 / Accepted September 6, 1990

Summary. In the preceding paper, medial vestibular nuclei neurones (MVNn) were shown to belong to two main classes, A MVNn and B MVNn, depending on their membrane properties in brainstem slices. In the following study we attempted to confirm this segregation by studying some of the ionic conductances that these cells are endowed with. Type A M V N n demonstrated small high threshold calcium spikes that could be potentiated by barium, a 4 - A P resistant A-like conductance and a calcium-dependant afterhyperpolarization. Type B MVNn, in contrast, had large high threshold calcium spikes and prolonged calcium-dependant plateau potentials. In addition, they had a calcium-dependant afterhyperpolarization as well as a subthreshold persistent sodium conductance. A subpopulation of B M V N n had also low threshold calcium spikes that gave them bursting properties. These data confirm the segregation of M V N neurones into two main classes and will be discussed with respect to the firing characteristics of vestibular neurones in vivo.

Key words: Calcium conductances - Sodium persistent

delayed slower A H P and a secondary range of firing in the first intervals of the f/I curves. In addition a subset of B M V N n displayed low threshold spike bursts, or subthreshold plateau potentials or a combination thereof. Type C MVNn on the other hand represent a nonhomogeneous cell population and were thus not studied further. In this paper we have attempted to study some of the membrane conductances o f M V N neurones in order to find out whether the segregation into two classes of cells can be further substantiated at that level. A complete description of ionic conductances, which is difficult to undertake with current-clamp methods in slices, was therefore beyond the scope of the present study. It will be seen that pharmacological studies do indeed confirm the segregation in two main classes and that these data might be interpreted in the light of the well known segregation of vestibular neurones into tonic (the A MVNn) and phasic (the B MVNn) cells (Shimazu and Precht 1965). These data have been presented previously in short communications (Serafin and Miihlethaler 1989; Serafin et al. 1989; Serafin et al. 1990).

conductance - Low threshold spike - Isolated whole brain - Guinea-pig

Material and methods

Introduction In the companion paper (Serafin etal. 1991a), in the absence of any pharmacological manipulation, we have shown that two main cellular types (79.4% of the total) of medial vestibular nuclei neurones (MVNn) are present: type A M V N neurones (A MVNn) and type B M V N neurones (B MVNn). In addition a small and non-homogeneous cell group, C MVNn (20.6%) was also identified. Type A M V N n (32.3%) had a broad action potential, a large single afterhyperpolarization (AHP), an A-type of rectification and a single range of firing. Type B MVNn (47.1%) had a thin action potential, an early fast and a Offprint requests to: M. Mfihlethaler (address see above)

The slice technique and electrophysiological method were discussed in the preceding paper (Serafin et al. 1991a). Drugs were applied in the perifusing saline at known concentrations. Tetrodotoxin (TTX, Calbiochem) was used at 10-6 M, tetraethylammonium (TEA-Br, Fluka) at 10-20 mM and 4-Aminopyridine (4-AP, Sigma) at 5-10 mM. For blocking calcium currents cobalt (2-3 raM) or cadmium at 0.5-1 mM were used as replacements for calcium. For this purpose a saline without KH2PO 4 and MgSO4 was used to avoid precipitation.

Results Potassium-dependant conductances in A and B M V N n

Both type A (Fig. 1A) and type B (Fig. 1C) M V N n displayed an anomalous rectification, seen as a sag i n t h e

427

A

I

B IO mV

Fig. 1A-D. Potassium-dependant 20 mV

0.5 nA J

50

50 ms

ms

C

D

20 mV

/

/

20 mV

0.5 nA -1

[ I 0 0 ms

Table 1. Summarized properties of 170 MVN neurones Type A neurones 9 Wide action potential 32.3% 9 Large single AHP 9 Single range firing 9 A-type rectification 9 Small high threshold calcium (HT-Ca 2+) spikes Type B neurones 9 Thin action potential 47.1% 9 Early fast and delayed slower AHP 9 Secondary range in the first intervals 9 Large HT-Ca z § spikes and Ca 2§ plateau potentials 9 And - 55.0% Na + plateau potentials (Na(P)) 16.5% Low threshold Ca 2+ spikes (LTS) - 16.5% LTS and Na (P) - 12.0% Absence of LTS and Na (P) Type C neurones 20.6%

voltage response of the m e m b r a n e to prolonged hyperpolarizing pulses. In both cases, this rectification was blockable by the external application of cesium at 1-3 m M (filled triangle in Fig. 1A, C). The presence of a single deep A H P in A M V N n and of a delayed slow A H P in B M V N n was suggestive for the presence in b o t h cell types of calcium-activated potassium conductances. Indeed in the presence of cadm i u m (1 m M ) or cobalt (3 m M ) both the single A H P of A M V N n (arrow in Fig. 1B) and the slow A H P of B M V N n (arrow in Fig. 1D) were reduced.

I0 ms

conductances in MVNn. A An A MVNn demonstrating an A-like outward rectification (double arrow) and an anomalous inward rectification (upper voltage trace). The anomalous rectifier was blocked by cesium at 2 mM (lower voltage trace, triangle). B In presence of cobalt (3 mM, upper voltage trace, arrow), the AHP (lower voltage trace, control medium) of an A MVNn, was strongly reduced. This induced a decrease in the interspike interval. C Anomalous rectification in a B MVNn (upper trace), was eliminated by cesium at 2 mM (lower trace, triangle). D Slow late AHP (lower trace) in a B MVNn was eliminated by cadmium at I mM (upper trace, arrow)

In contrast we were unable to block the transient A-like conductance of A M V N n (double arrow in Fig. 1A) with either cesium (1-3 raM) or T E A (20 raM). Even 4 A P , a more specific potassium blocker for A currents, was ineffective when tested at concentrations of up to 20 raM.

Calcium conductances in A M V N n Calcium conductances were difficult to reveal in all A M V N n (see Table 1). When these cells were perifused with T T X (10 -6 M) or with a combination (Fig. 2B or C) of T T X (10 -6 M), T E A (up to 20 m M ) and 4-AP (up to 10 mM), they only revealed very small TTX-resistant action potentials. These spikes were calcium-dependant as they could be eliminated by replacing calcium with cadmium at 1 m M (Fig. 2D). However A M V N n (Fig. 3A, B) were able to generate long lasting plateau potentials (Fig. 3C) when calcium was substituted with barium, in presence of T T X (10 -6 M), T E A (20 raM) and 4-AP (10 raM). These TTXresistant barium plateau potentials were eliminated (Fig. 3D) by the further addition of cadmium at 1 m M .

High threshold calcium spikes and plateau potentials in B MVNn In contrast to A M V N n , calcium conductances were easy to reveal in all B M V N n (see Table 1). In presence of T T X (10 -6 M), 4AP (10 m M ) and T E A (20 mM), all B M V N n developed high threshold TTX-resistant responses following short depolarizing or hyperpolarizing current pulses. When the stimuli were subthreshold,

428

B

A

TTX

,/

/

20 mV

I C

D

TTX, TEA, 4 - A P

1

[ 0.5 nA

TTX, TEA, 4-AP, Cd§247

20 mV

i

I

I

I 50 ms

Fig. 2A-D. Calcium spikes in an A MVNn. A Action potentials in

TTX-resistant responses were evoked. D All responsiveness was eliminated by the replacement of calcium by cadmium at 1 mM

control medium. B Lack of response in presence of TTX (10 -6 M). C In presence of added TEA (20 mM) and 4-AP (10 mM), small

A

B

j,

\

J

20 mV

t m

20 ms

C

Io.5 nA

1

20 ms

Ba 2.

D +Cd 2,

20 mV

20 mV I0.1 nA

r

I s o n l y s m a l l a f t e r d e p o l a r i z a t i o n s o c c u r r e d (Fig. 4 A , single arrow). I f the s t i m u l u s was increased, high t h r e s h o l d spikes ( H T S ) were t r i g g e r e d (Fig. 4 A , d o u b l e a r r o w s a n d Fig. 4B) o n t o p o f a high t h r e s h o l d p l a t e a u de-

---I i

10.5 nA

20 ms

Fig. 3A-D. Barium spikes in an A MVNn. A Control. B Under a slight DC hyperpolarization, characteristic sag (arrow) of an A MVNn. C In presence of TTX (10 -6 M), TEA (20 raM), 4-AP (10 raM) and barium (2.4 mM) large plateau potentials were evoked. D All responsiveness was eliminated by the replacement of barium by cadmium at 1 mM

p o l a r i z a t i o n ( H T P , single a r r o w in Fig. 4 B). D u e p o s sibly to a n e q u i l i b r i u m in b e t w e e n c a l c i u m c o n d u c tances a n d v a r i o u s subsisting p o t a s s i u m c o n d u c t a n ces, the oscillating T T X - r e s i s t a n t spikes p r o g r e s s i v e l y

429

A

//

B

TTX, TEA, 4-AP ,,,..

I 20 mV

/ -4

I nA 0.25

250 ms

C

D

TTX, TEA, 4-AP

E

TTX, TEA, 4-AP

+Cd 2.

/

2O mV

_l 500 ms

Fig. 4A-E. High threshold calcium spikes (HTS) and plateau potentials (HTP) in a B MVNn. A In presence of TTX ( 10 -6 M), TEA (20 raM) and 4-AP (10 raM), calcium HTS (double arrow) and HTP were elicited. B Fast sweed speed of the first part of the

,.A

B

j

500 ms 20 ms recording in A. C, D In another neurone, in the same conditions, only HTP were elicited, following either short depolarizations (C) or hyperpolarizations (D). E In presence of cadmium at 1 mM, the calcium HTP was eliminated (same cell as in C, D)

TTX

20 mV

I

1

._l

i--

"

i

50 ms

C

TTX

.....

I nA

50 ms

D

+Cd 2,-

Fig. 5A-D. Low threshold calcium

20 mV J

,oo

ms

I InA

q 50 ms

d e c r e a s e d in a m p l i t u d e ( a r r o w h e a d in Fig. 4A). A new p l a t e a u p o t e n t i a l was then r e a c h e d w h i c h c o u l d last for several seconds. I n c o n t r a s t t o H T P w h i c h o c c u r r e d in every B M V N n tested, o s c i l l a t i n g T T X - r e s i s t a n t spikes

spikes (LTS) in a B MVNn. A Low threshold response evoked by a depolarizing current pulse in control medium, under a 30 mV DC hyperpolarization. B, C Low threshold TTX-resistant spikes evoked by depolarizing (B) or hyperpolarizing current pulses (C). D Elimination of the TTX-resistant spikes by cadmium at 1 mM

were n o t a l w a y s evoked. I n d e e d , as d e m o n s t r a t e d in a n o t h e r n e u r o n e , a n H T P c o u l d be r a p i d l y r e a c h e d , a f t e r a n initial t r a n s i e n t spike, f o l l o w i n g either d e p o l a r i z i n g (Fig. 4C) o r h y p e r p o l a r i z i n g pulses (Fig. 4D). C a l c i u m

43O

A

CONTROL Cd 2+

I0 mV

0.5 nA

V~ 5 0 ms

B

C . 9

Cd2+ + TTX

Cd 2§ + T T X

~2~22

I0 mV

0.5 nA

Fq 5 0 ms

I0 ms

Fig. 6A-C. Sodium persistent conductances in a B MVNn. A Under a small hyperpolarizing DC current injection, and in presence of cadmium at 1 mM, the fast action potentials were riding on a plateau potential. B The plateau potential (*) and the fast spikes were both eliminated by TTX (10 -6 M, dot). C Lack of response in presence of cadmium at 1 m M and TTX (10 -6 M)

B

A

20 mV

1

1

0.5 nA

1

IO m s

I 0 0 ms

D

C

20 mV

2 0 0 ms

20 ms

Fig. 7A-D. Type B second-order vestibular neurone, identified in the isolated and perfused whole brain. A Orthodromic activation by stimulation of the VIII 'h nerve. B In presence of TTX (10 and TEA (20 raM) a plateau potential was elicited. C Irregular spontaneous activity in another neurone. D Double A H P of the neurone illustrated in C

6M)

431 HTS and HTP, were both eliminated (Fig. 4E, same cell than in C, D) by substituting calcium with cadmium at 1 raM.

Low threshold calcium spikes in B M V N n A subset of B MVNn (see Table 1) were shown in the preceding paper to be able to fire in bursts due to the presence of low threshold spikes (LTS). These LTS could be triggered if the neurones were hyperpolarized by a DC current. This removed the inactivation of a low threshold conductance, which could then be activated by depolarizing current pulses (Fig. 5A). When TTX (10 -6 M) was added to the solution, the low threshold response persisted, but the fast spikes were eliminated (Fig. 5 B). Rebound TTX-resistant spikes could also be evoked following hyperpolarizing current pulses (Fig. 5 C). These TTX-resistant responses were calciumdependant as they were eliminated by the replacement of calcium with 1 mM of cadmium (Fig. 5D).

Non-inactivating sodium plateau potentials in B MVNn We have shown in the preceding paper that in a large number of B MVNn (with or without LTS, see Table 1), subthreshold plateau potentials could be revealed after suppression of the spontaneous firing by a small DC hyperpolarization. These plateau potentials could then be easily elicited in response to short pulses of depolarizing current. They could still be triggered (Fig. 6A, B, star) when calcium was substituted with cadmium (1 raM). They were eliminated (Fig. 6B, dot) however by the addition of TTX at (10- 6 M) even if the depolarizing pulses were increased well above the control level (Fig. 6C). These subthreshold sodium plateau potentials, due presumably to the presence of a persistent Na + current were present altogether in 71.5% of B MVNn (Table 1, 55%+16.5%). Discussion

This paper confirms that in the MVN, as already suggested in the accompanying paper, different cell types can be distinguished. The segregation, which was based essentially on the shape of their action potentials and AHPs, was also present in their main ionic conductances in a very clear-cut manner. Altogether, these differences are summarized in Table 1 and will be discussed below from a pharmacological and functional point of view.

Membrane conductances Potassium conductances. Potassium conductances of MVN neurones were not investigated in detail in this study. From data obtained in other preparations one might assume that part of the AHP in both A and B MVNn is due to voltage-dependant potassium conduc-

tances as in many other studied vertebrate CNS neurones. Both cell types displayed in addition calciumdependant AHPs as also seen in other CNS neurones (Blatz and Magleby 1987; Halliwell 1990). In B MVNn, this conductance underlied the late slow AHP which must be triggered by the rather large calcium conductances present in these cells (see discussion below). In A MVNn, the calcium-dependant component appeared to represent only a fraction of the large AHP (Fig. 1 B), the rest being probably due to voltage-dependant conductances that remain to be investigated. One, presumably potassium, conductance was present only in A MVNn and used as a defining property. It could be triggered following either hyperpolarizing pulses from rest or depolarizing pulses, when the neurone was under a DC hyperpolarization. In both conditions it introduced a lag time in the firing of the cell. The current responsible for such a behavior has been originally described in invertebrates as the A current (Hagiwara et al. 1961) and is very much similar to what has now been described in many vertebrate CNS neurones (Rudy 1988; Llin~is 1988; Halliwell 1990). However in contrast to other studies, we have been unable to block it with 4-AP. It therefore remains to be seen, using voltageclamp techniques, what is the exact nature of this rectification.

Anomalous rectifyier. During long-lasting hyperpolarizing current steps, there was a sag in the membrane potential in both cell types, which is similar in time course to what has been described in many other CNS neurones (Halliwell 1990). At the return of the hyperpolarizing pulse, there was a slow rebound in type B cells. In type A cells, the presence of the A-like conductance probably masked in most cases the occurrence of such rebounds. As shown elsewhere this rectification was sensitive to small doses of cesium. In other preparations it has been shown to be due to a mixed sodium and potassium conductance (Rudy 1988; Llin/ts 1988; Halliwell 1990). High and low threshold calcium conductances. Two types of membrane conductances to calcium were uncovered in MVNn. The first type is the low threshold calcium conductance (LTS), which was present only in a subpopulation of B MVNn (33 %). This conductance, first uncovered in olivary cells (Llinfis and Yarom 1981), was later shown to underlie the burst firing of a number of CNS neurones (Llinfis 1988). The rationale for its presence in MVNn has been previously discussed (Serafin et al. 1989; Serafin et al. 1990). The second type is a high threshold calcium conductance, again similar in that respect to many other mammalian CNS neurones (Llinfis 1988). In Purkinje cells and inferior olivary neurones (Llin/ts and Sugimori 1980b; Llimls and Yarom 1981), in which their location was studied, they were demonstrated to be at the dendritic level. In B MVNn, these conductances are easier to trigger than in A MVNn and they give rise to large high threshold spikes (HTS) and plateau potentials (HTP). The HTP might represent an equilibrium between calcium conductances and various unbloeked voltage-de-

432 pendant and/or calcium-dependant potassium conductances (Llinfis and Sugimori 1980a; Llinfis and Sugimori 1980b). It is not known in MVNn whether the calcium conductances of the HTS and the HTP are one and the same or different. In motoneurones, on pharmacological grounds alone, they were shown to differ (Hounsgaard and Mintz 1988). The HTP corresponded to an L-channel and the spikes to an N-channel (Nowycky et al. 1985). This problem is however best dealt with using voltageclamp methods, and was thus not investigated further in the present study. In A MVNn, calcium conductances were also present, but difficult to evoke in the absence of barium. This might be due to differences in between type A and B MVNn calcium conductances. However it might be simpler to assume that large, partially or completely unblocked potassium conductances involved in the AHP (Fig. 2C), impeded the development of the HTP. Alternatively the site of origin of calcium conductances in A MVNn might be located distally in the dendritic tree. Sodium persistent conductance. B MVNn were endowed with a sodium persistent conductance, already described in several CNS neurones (Llin/ts 1988). It continuously drove the neurone towards the threshold for fast sodium spikes and was thus the major determinant of the spontaneous activity of B MVNn. In addition, depending on the level of the membrane potential, this conductance might, in the low threshold range, contribute to shape the bursts in B MVNn, in conjunction with the LTS. Coexistence of a sodium persistent conductance and LTS was already described in a number of other nuclei (Jahnsen and Llinfis 1984; Llinfis and Mfihlethaler 1988). Functional aspects In vivo, two main classes of second-order vestibular neurones have been described (Precht and Shimazu 1965; Shimazu and Precht 1965), the tonic and kinetic neurones. Kinetic neurones have a more irregular resting discharge, and a higher sensitivity to acceleration than the tonic neurones. In vitro, our results demonstrate that the majority of the medial vestibular nuclei neurones can also be classified using two categories, the A MVNn and the B MVNn. Is there a correspondence between these two classifications ? A MVNn have a deep, long-lasting AHP. This AHP will remove the inactivation of an A-like conductance which can be subsequently activated during the repolarizing phase of the AHP. This will slow down the firing of these neurones and favor the regularity of their resting discharge. Moreover, the large size of the AHP might render these neurones less susceptible to excitatory drives. In vivo, such a neurone should therefore have a regular resting discharge. It should have also a long recovery period due to the large amplitude of the AHP. This is reminiscent of the characteristics of the tonic vestibular neurones (Precht and Shimazu 1965; Shimazu and Precht 1965). In contrast, B MVNn have a non-inactivating sodium conductance which constantly maintain them near

threshold, a steeper slope in the f/I curve and a minority of them have an LTS which endowes them with bursting capability. These properties, together with a smaller AHP than in A MVNn, should make them more susceptible to excitatory drives. One might assume that in vivo these characteristics will confer to the B MVNn an irregular firing pattern, a higher excitability, and a lower threshold, than the A MVNn. These characteristics are reminiscent of the properties of the kinetic vestibular neurones (Precht and Shimazu 1965; Shimazu and Precht 1965). One limitation of the present study is however the lack of identification of the different cell types in the medial vestibular nucleus. Indeed the MVNn should be defined by their excitatory or inhibitory nature, their mono or polysynaptic connections with the primary vestibular afferents and their axonal projections. Hence this work is currently being completed by a preliminary study in the in vitro whole brain (M/ihlethaler and Serafin 1990; Miihlethaler et al. 1990), which allows such an identification. An example of a second-order, monosynaptically activated vestibular neurone, having the properties of a B MVNn, is illustrated in Fig. 7. Another point deserves discussion: we are arguing that the B MVNn probably behave, in vivo, like the kinetic neurones. However, in brainstem slices, they only display a regular discharge. Obviously in vivo it is the random synaptic noise of the irregular primary afferents which generates the fluctuation of the interspike interval of the irregular MVNn. Therefore in slices, without this drive, the putative irregular (type B) vestibular neurones cannot be differentiated on the basis of their spontaneous activity. Preliminary results however in the isolated and perfused brain, in which spontaneous synaptic activity is preserved, show that B MVNn have indeed as expected an irregular firing rate (Fig. 7C). Finally it should be mentioned that, in a parallel study performed in slices and in an isolated whole brain, we found the same segregation in 2 cell classes in a nucleus functionally related to the vestibular system, the Gigantocellular reticular nucleus of the medulla (Serafin et al. 1987; Serafin et al. 1991 b). Conclusions Our data point to a role of intrinsic membrane properties in the functional segregation of vestibular neurones into phasically and tonically firing cells. At that stage however this is still speculative and alternative explanations might be considered. Type B cells, for example, are endowed with a set of calcium and sodium conductances eliciting plateau potentials. This should make them very susceptible to neuromodulation by various substances such as norepinephrine, serotonine, histamine, acetylcholine or others, which are known (among their diverse actions) to block potassium conductances (Nicoll 1988). Such transmitters, which all were recently shown to excite vestibular neurones in vitro (our own unpublished results), could eventually in vivo turn a type B phasic cell into a tonically firing one. It is noteworthy that this is exactly what has been proposed to occur in spinal moto-

433 neurones. In the presence o f s e r o t o n i n , w h i c h b l o c k s a p o t a s s i u m c o n d u c t a n c e , these cells can be s w i t c h e d i n t o a t o n i c firing m o d e d u e to the presence o f l o n g - l a s t i n g calcium plateau potentials (Hounsgaard and Kiehn 1989). T h e r e f o r e , in a d d i t i o n to the role o f intrinsic a n d s y n a p t i c m e c h a n i s m s , the differences in b e t w e e n t o n i c a l l y a n d p h a s i c a l l y firing cells c o u l d also d e p e n d o n the a m o u n t a n d t y p e o f n e u r o m o d u l a t o r y c o n t r o l exerted at a n y given time.

Acknowledgements. This work was supported by grants from the Swiss NSF (nos. 3.288.0.85 and 3.560.0.86), the Sandoz Foundation, I~ and the French Minist6re des Affaires Etrang6res. We thank Ms. D. Machard for her excellent technical assistance. References Blatz AL, Magleby KL (1987) Calcium activated potassium channels. Trends Neurosci 10:463-467 Hagiwara S, Kusano K, Saito N (1961) Membrane changes of Onchodium nerve cell in potassium-rich media. J Physiol (Lond) 155 : 470-489 Halliwell JV (1990) K + channels in the central nervous system. In: Cook NS (ed) Potassium channels, structure, classification, function and therapeutic potential. John Wiley & Sons, New York, pp 348-381 Hounsgaard J, Kiehn O (1989) Serotonin-induced bistability of turtle motoneurones caused by a nifedipine-sensitive calcium plateau potential. J Physiol (Lond) 414:265-282 Hounsgaard J, Mintz I (1988) Calcium conductances and firing properties of spinal motoneurones in the turtle. J Physiol (Lond) 398:591-603 Jahnsen H, Llin/ts R (1984) Ionic basis for the electro-responsiveness and oscillatory properties of guinea-pig thalamic neurones in vitro. J Physiol (Lond) 349:227-247 Llinfis R, Miihlethaler M (1988) Electrophysiology of guinea-pig cerebellar nuclear cells in the in vitro brainstem-cerebellar preparation. J Physiol (Lond) 404:241-258 Llinfis R (1988) The intrinsic electrophysiological properties of mammalian neurons : insights into central nervous system function. Science 242:1654-1664 Llinfis R, Sugimori M (1980a) Electrophysiological properties of in vitro Purkinje cell somata in mammalian cerebellar slices. J Physiol (Lond) 305:171-195

Llinits R, Sugimori M (1980b) Electrophysiological properties of in vitro Purkinje cell dendrites in mammalian cerebellar slices. J Physiol (Lond) 305:197-213 Llinfis R, Yarom Y (1981) Electrophysiology of mammalian inferior olivary neurones in vitro: different types of voltage-dependent ionic conductances. J Physiol (Lond) 315:54%567 Mtihlethaler M, Serafin M (1990) Thalamic spindles in an isolated and perfused preparation in vitro. Brain Res 524:17 21 Mi.ihlethaler M, Cand P, Henauer R, Serafin M (1990) Design of a microprocessor controlled thermoregulated chamber for perfusing in vitro mammalian brain preparation. J Neurosci Meth 31:215-223 NicoI1 RA (1988) The coupling of neurotransmitters receptors to ion channels in the brain. Science 241 : 545-551 Nowycky MC, Fox AP, Tsien RW (1985) Three types of neuronal calcium channels with different calcium agonist sensitivity. Nature 316: 440-443 Precht W, Shimazu H (1965) Functional connections of tonic and kinetic vestibular neurons with primary vestibular afferents. J Neurophysiol 28:1014-1028 Rudy B (1988) Diversity and ubiquity of K channels. Neuroscience 25:729 749 Serafin M, Vidal PP, Miihlethaler M (1987) Electrophysiological and pharmacological properties of reticular neurons in nucleus gigantocellularis : a study in brainstem slices and in an isolated whole brain of guinea-pig. Soc Neurosci Abstr 13:853 Serafin M, Miihlethaler M (1989) Electrophysiological study of vestibular nuclei neurons in vitro. Experientia 45:A13 Serafin M, Vidal PP, de Waele C, Khateb A, M/ihlethaler M (1989) Burst firing of medial vestibular nuclei neurones in vitro. Soc Neurosci Abstr 15: 452 Serafin M, Khateb A, de Waele C, Vidal PP, Mfihlethaler (1990) Low threshold calcium spikes in medial vestibular nuclei neurones in vitro: a role in the generation of the vestibular nystagmus quick phase in vivo? Exp Brain Res 82:187-190 Serafin M, de Waele C, Khateb A, Vidal PP, Mtihlethaler M (1991 a) Medial vestibular nucleus in the guinea-pig: I. Intrinsic membrane properties in brainstem slices. Exp Brain Res 84:417-425 Serafin M, Khateb A, de Waele C, Vidal PP, Miihlethaler M (1991 b) Electrophysiology of vestibular and gigantocellular nuclei neurones in vitro. In: Berthoz A, Vidal PP, Graf W (eds) Head-neck motor control. Wiley, New York (in press) Shimazu H, Precht W (1965) Tonic and kinetic responses of cat's vestibular neurons to horizontal angular acceleration. J Neurophysiol 28 : 991-1013