Malfunctions of potassium channels are increasingly implicated as causes of neurological disorders. However, the functional roles of the large-conductance voltage- and Ca2+-activated K+channel (BK ...channel), a unique calcium, and voltage-activated potassium channel type have remained elusive. Here we report that mice lacking BK channels ( BK-/-) show cerebellar dysfunction in the form of abnormal conditioned eye-blink reflex, abnormal locomotion and pronounced deficiency in motor coordination, which are likely consequences of cerebellar learning deficiency. At the cellular level, the BK-/-mice showed a dramatic reduction in spontaneous activity of the BK-/-cerebellar Purkinje neurons, which generate the sole output of the cerebellar cortex and, in addition, enhanced short-term depression at the only output synapses of the cerebellar cortex, in the deep cerebellar nuclei. The impairing cellular effects caused by the lack of postsynaptic BK channels were found to be due to depolarization-induced inactivation of the action potential mechanism. These results identify previously unknown roles of potassium channels in mammalian cerebellar function and motor control. In addition, they provide a previously undescribed animal model of cerebellar ataxia.
In the present report, we provide evidence that mesencephalic trigeminal (Mes-V) sensory neurons, a peculiar type of primary afferent cell with its cell body located within the CNS, may operate in ...different functional modes depending on the degree of their membrane polarization. Using intracellular recording techniques in the slice preparation of the adult rat brain stem, we demonstrate that when these neurons are depolarized, they exhibit sustained, high-frequency, amplitude-modulated membrane potential oscillations. Under these conditions, the cells discharge high-frequency trains of spikes. Oscillations occur at membrane potential levels more depolarized than -53 +/- 2.3 mV (mean +/- SD). The amplitude of these oscillations increases with increasing levels of membrane depolarization. The peak-to-peak amplitude of these waves is approximately 3 mV when the cells are depolarized to levels near threshold for repetitive firing. The frequency of oscillations is similar in different neurons (108.9 +/- 15.5 Hz) and was not modified, in any individual neuron, by changes in the membrane potential level. These oscillations are abolished by hyperpolarization and by TTX, whereas blockers of voltage-dependent K+ currents slow the frequency of oscillations but do not abolish the activity. These data indicate that the oscillations are generated by the activation of inward Na+ current/s and shaped by voltage-dependent K+ outward currents. The oscillatory activity is not modified by perfusion with low-calcium, high-magnesium, or cobalt-containing solutions nor is it modified in the presence of cadmium or Apamin. These results indicate that a calcium-dependent K+ current does not play a significant role in this activity. We postulate that the membrane oscillatory activity in Mes-V neurons is synchronized in adjoining electrotonically coupled cells and that this activity may be modulated in the behaving animal by synaptic influences.
In heterozygous Lurcher mice (Lc/+), the Purkinje cells (PCs) degenerate almost totally during postnatal development. On the other hand, their projection target, the deep cerebellar nuclei (DCN), ...shows few signs of degeneration and seems to play an important role in maintaining a residual cerebellar function in Lc/+. We asked whether the DCN in Lc/+ develop cellular adaptations allowing them to cope with the loss of GABAergic PC input. Using whole-cell patch-clamp recordings, we measured inhibitory postsynaptic currents from DCN of Lc/+ and wild-type mice (WT). In experiments on phenotypically striking Lc/+ studied well after the onset of the PC degeneration, we found enlarged average synaptic conductances (g(syn)) compared with WT. We next investigated postnatal mice before and after the onset of PC death. In younger animals </= postnatal day (p) 13, no difference was found in g(syn) between the two groups. At p14, g(syn) in Lc/+ showed an increase, while those in WT stayed on the level found in younger animals. A peak-scaled nonstationary fluctuation analysis suggests that an increase in the average number of channels open at peak is the basis for the change in g(syn). The changes in g(syn), suitable to increase the efficacy of GABAergic transmission, occur in close temporal relationship to PC death and, thus, may reflect a functional adaptation to the loss of the DCN's main GABAergic afferents.
Malfunctions of potassium channels are increasingly implicated as causes of neurological disorders. However, the functional roles of the large-conductance voltage- and Ca
2+
-activated K
+
channel ...(BK channel), a unique calcium, and voltage-activated potassium channel type have remained elusive. Here we report that mice lacking BK channels (BK
-/-
) show cerebellar dysfunction in the form of abnormal conditioned eye-blink reflex, abnormal locomotion and pronounced deficiency in motor coordination, which are likely consequences of cerebellar learning deficiency. At the cellular level, the BK
-/-
mice showed a dramatic reduction in spontaneous activity of the BK
-/-
cerebellar Purkinje neurons, which generate the sole output of the cerebellar cortex and, in addition, enhanced short-term depression at the only output synapses of the cerebellar cortex, in the deep cerebellar nuclei. The impairing cellular effects caused by the lack of postsynaptic BK channels were found to be due to depolarization-induced inactivation of the action potential mechanism. These results identify previously unknown roles of potassium channels in mammalian cerebellar function and motor control. In addition, they provide a previously undescribed animal model of cerebellar ataxia.
Malfunctions of potassium channels are increasingly implicated as causes of neurological disorders. However, the functional roles of the large-conductance voltage- and Ca super(2+)-activated K ...super(+) channel (BK channel), a unique calcium, and voltage-activated potassium channel type have remained elusive. Here we report that mice lacking BK channels (BK super(-/-)) show cerebellar dysfunction in the form of abnormal conditioned eye-blink reflex, abnormal locomotion and pronounced deficiency in motor coordination, which are likely consequences of cerebellar learning deficiency. At the cellular level, the BK super(-/-) mice showed a dramatic reduction in spontaneous activity of the BK super(-/-) cerebellar Purkinje neurons, which generate the sole output of the cerebellar cortex and, in addition, enhanced short-term depression at the only output synapses of the cerebellar cortex, in the deep cerebellar nuclei. The impairing cellular effects caused by the lack of postsynaptic BK channels were found to be due to depolarization-induced inactivation of the action potential mechanism. These results identify previously unknown roles of potassium channels in mammalian cerebellar function and motor control. In addition, they provide a previously undescribed animal model of cerebellar ataxia.
The interaction between cortical input frequency and intrinsic thalamic neuron (TN) properties were investigated using intracellular recordings from mice TNs in thalamocortical (TC) slices. ...Excitatory postsynaptic potentials (EPSPs) of corticothalamic (CT) origin were recorded at TN membrane potentials (
V
m) held, by current clamp means, between −59 and −55
mV to avoid low-threshold calcium currents (
I
T) activation. EPSPs elicited in ventrobasal neurons (
n=25) by stimulation in the internal capsule showed constant latency, relatively fast rise time (2.9±0.56
ms) and short duration (26.6±9.11
ms). EPSPs evoked by threshold stimulation (
n=10) showed similar characteristics (mean rise time, 2.74±0.42
ms; mean duration, 30±8.00
ms). The time course of CT synaptic facilitation was determined using pairs of stimuli. Paired-pulse facilitation (PPF) of CT EPSPs peaked at 25–30
ms stimulus intervals and decayed exponentially with an average time constant of 130
ms (
n=50). Application of the NMDA receptor blocker APV (25
μM,
n=4) did not modify PPF for any interstimulus interval studied but suppressed frequency facilitation evoked by trains of CT stimuli. We compared the number of spikes per stimulus (
F
s) evoked in TNs by repetitive CT stimulation over a range of frequencies at different
V
m. At hyperpolarized
V
m (below −65
mV) and frequencies of stimulation ≥10
Hz,
F
s decreased along the train while at depolarized
V
m (above −59
mV)
F
s increased along the train. Decremental patterns resulted from the activation of
I
T while facilitatory patterns emerged from superposition of synaptic and intrinsic mechanisms. At hyperpolarized
V
m, steady-state
F
s was maximal for frequencies ≤2
Hz, intermediate for frequencies between 2 and 10
Hz and zero at ≥10
Hz. At depolarized
V
m, steady-state
F
s increased with increasing frequencies (from 1 to 40
Hz).
We conclude that the CT–TN junctions are tuned to establish stable thalamocortical resonant dynamics.
Brainstem and spinal cord motoneurons that innervate somatic musculature serving antigravity functions are postsynaptically inhibited during active sleep. However, it has been reported that ...hypoglossal motoneurons (which do not innervate antigravity muscles) are not postsynaptically inhibited during active sleep, but are disfacilitated. In the present report we describe changes, during active sleep, in the excitability and membrane potential of digastric and synergistic motoneurons of the trigeminal motor pool; these neurons do not perform antigravity functions. The experiments described in the present report were performed in chronic cats that were prepared for intracellular recording. The motoneurons hyperpolarized an average of 11 mV (S.D. +/- 1.29, n = 8, P < 0.005) during active sleep compared to quiet sleep. Hyperpolarization was accompanied by a reduction in the excitability of the somadendritic regions of the neurons, as indicated by an increase in the delay of propagation of antidromic spikes from the initial segment to the somadendritic portion of the cell. High gain membrane potential recordings from these motoneurons revealed the occurrence of a remarkably large number of hyperpolarizing potentials during active sleep. When K-chloride-filled microelectrodes were utilized and chloride ions were injected intracellularly, the polarity of these potentials was reversed. During phasic episodes of active sleep, there was a clear increase in hyperpolarizing potential activity, a blockade of somadendritic spikes and phasic reductions in the amplitude of the initial segment spikes. Hyperpolarizing potentials occurred in conjunction with ponto-geniculo-occipital waves.
We have proposed that neurotrophins, in addition to their trophic actions, act as neuromodulators in the adult central nervous system. As a first step to test this hypothesis, we examined in the ...adult rat slice preparation whether nerve growth factor and neurotrophin-3 are capable of altering the excitability of neurons of the mesencencephalic trigeminal nucleus. In contrast to vehicle pressure microapplication, which did not evoke changes in the electrophysiological properties of these neurons, neurotrophin application produced a significant increase in amplitude of the membrane potential oscillatory activity that is observed in these cells and a significant decrease in their threshold current. The latency of these effects ranged from 2 to 80
s and the duration ranged from 2 to 11
min. Neurotrophin-3 induced a decrease in input resistance and resting membrane potential in 58% of the cells; nerve growth factor induced a decrease in input resistance and resting membrane potential in 35% of the neurons. The spike configuration and action potential afterhyperpolarization potential remained unchanged following neurotrophin application. Tetrodotoxin blocked the membrane potential oscillatory activity of trigeminal mesencephalic neurons. Neurotrophin-induced effects were not blocked by the tyrosine kinase inhibitor K-252a, whereas IgG-192, an antibody directed to the neurotrophin low-affinity receptor, enhanced excitability, as did neurotrophins.
These results demonstrate that neurotrophins are capable of producing a rapid increase in the excitability of trigeminal mesencephalic neurons and suggest that their effects may be mediated by low-affinity neurotrophin receptors.
We have proposed that neurotrophins, in addition to their trophic actions, act as neuromodulators in the adult central nervous system. As a first step to test this hypothesis, we examined in the ...adult rat slice preparation whether nerve growth factor and neurotrophin-3 are capable of altering the excitability of neurons of the mesencencephalic trigeminal nucleus. In contrast to vehicle pressure microapplication, which did not evoke changes in the electrophysiological properties of these neurons, neurotrophin application produced a significant increase in amplitude of the membrane potential oscillatory activity that is observed in these cells and a significant decrease in their threshold current. The latency of these effects ranged from 2 to 80 s and the duration ranged from 2 to 11 min. Neurotrophin-3 induced a decrease in input resistance and resting membrane potential in 58% of the cells; nerve growth factor induced a decrease in input resistance and resting membrane potential in 35% of the neurons. The spike configuration and action potential afterhyperpolarization potential remained unchanged following neurotrophin application. Tetrodotoxin blocked the membrane potential oscillatory activity of trigeminal mesencephalic neurons. Neurotrophin-induced effects were not blocked by the tyrosine kinase inhibitor K-252a, whereas IgG-192, an antibody directed to the neurotrophin low-affinity receptor, enhanced excitability, as did neurotrophins. These results demonstrate that neurotrophins are capable of producing a rapid increase in the excitability of trigeminal mesencephalic neurons and suggest that their effects may be mediated by low-affinity neurotrophin receptors.
Trigeminal motoneurons of the guinea pig brain stem slice preparation were studied using intracellular recording techniques. The voltage response to a 100-ms constant-current pulse was studied and a ...population of cells was found which did not exhibit sag or overshoot of their voltage response to a pulse of hyperpolarizing current of < 1 nA but did exhibit both phenomena when a current pulse of > 1 nA was used. The sag and overshoot observed with large-current pulses were reduced or blocked when 4 mM CsCl was added to the bathing solution. This observation supports the hypothesis that these phenomena were due to the voltage- and time-dependent activation of the Q-current. The method of peeling exponentials was then used to correct raw voltage data from cells in which the Q-current was present. The mean membrane time constant was within 1% and the mean input resistance was within 2% of the means for these parameters when measured in these same cells under conditions in which the Q-current was absent. We conclude from these experiments that the method of peeling exponentials is valid for obtaining estimates of the membrane time constant and input resistance from cells that exhibit sag and overshoot due to voltage- and time-dependent changes in the magnitude of the Q-current.
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