Modulation of pacemaker channel function in a model of thalamocortical hyperexcitability by demyelination and cytokines.

A consensus is yet to be reached regarding the exact prevalence of epileptic seizures or epilepsy in multiple sclerosis (MS). In addition, the underlying pathophysiological basis of the reciprocal interaction among neuroinflammation, demyelination, and epilepsy remains unclear. Therefore, a better understanding of cellular and network mechanisms linking these pathologies is needed. Cuprizone-induced general demyelination in rodents is a valuable model for studying MS pathologies. Here, we studied the relationship among epileptic activity, loss of myelin, and pro-inflammatory cytokines by inducing acute, generalized demyelination in a genetic mouse model of human absence epilepsy, C3H/HeJ mice. Both cellular and network mechanisms were studied using in vivo and in vitro electrophysiological techniques. We found that acute, generalized demyelination in C3H/HeJ mice resulted in a lower number of spike-wave discharges, increased cortical theta oscillations, and reduction of slow rhythmic intrathalamic burst activity. In addition, generalized demyelination resulted in a significant reduction in the amplitude of the hyperpolarization-activated inward current (Ih) in thalamic relay cells, which was accompanied by lower surface expression of hyperpolarization-activated, cyclic nucleotide-gated channels, and the phosphorylated form of TRIP8b (pS237-TRIP8b). We suggest that demyelination-related changes in thalamic Ih may be one of the factors defining the prevalence of seizures in MS.

The Hyperpolarization-Activated HCN4 Channel is Important for Proper Maintenance of Oscillatory Activity in the Thalamocortical System.

Hyperpolarization-activated cation channels are involved, among other functions, in learning and memory, control of synaptic transmission and epileptogenesis. The importance of the HCN1 and HCN2 isoforms for brain function has been demonstrated, while the role of HCN4, the third major neuronal HCN subunit, is not known. Here we show that HCN4 is essential for oscillatory activity in the thalamocortical (TC) network. HCN4 is selectively expressed in various thalamic nuclei, excluding the thalamic reticular nucleus. HCN4-deficient TC neurons revealed a massive reduction of Ih and strongly reduced intrinsic burst firing, whereas the current was normal in cortical pyramidal neurons. In addition, evoked bursting in a thalamic slice preparation was strongly reduced in the mutant mice probes. HCN4-deficiency also significantly slowed down thalamic and cortical oscillations during active wakefulness. Taken together, these results establish that thalamic HCN4 channels are essential for the production of rhythmic intrathalamic oscillations and determine regular TC oscillatory activity during alert states.

Impairment of frequency-specific responses associated with altered electrical activity patterns in auditory thalamus following focal and general demyelination.

Multiple sclerosis is characterized by intermingled episodes of de- and remyelination and the occurrence of white- and grey-matter damage. To mimic the randomly distributed pathophysiological brain lesions observed in MS, we assessed the impact of focal white and grey matter demyelination on thalamic function by directing targeted lysolecithin-induced lesions to the capsula interna (CI), the auditory cortex (A1), or the ventral medial geniculate nucleus (vMGN) in mice. Pathophysiological consequences were compared with those of cuprizone treatment at different stages of demyelination and remyelination. Combining single unit recordings and auditory stimulation in freely behaving mice revealed changes in auditory response profile and electrical activity pattern in the thalamus, depending on the region of the initial insult and the state of remyelination. Cuprizone-induced general demyelination significantly diminished vMGN neuronal activity and frequency-specific responses. Targeted lysolecithin-induced lesions directed either to A1 or to vMGN revealed a permanent impairment of frequency-specific responses, an increase in latency of auditory responses and a reduction in occurrence of burst firing in vMGN neurons. These findings indicate that demyelination of grey matter areas in the thalamocortical system permanently affects vMGN frequency specificity and the prevalence of bursting in the auditory thalamus.

Modulation of thalamocortical oscillations by TRIP8b, an auxiliary subunit for HCN channels.

Hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channels have important functions in controlling neuronal excitability and generating rhythmic oscillatory activity. The role of tetratricopeptide repeat-containing Rab8b-interacting protein (TRIP8b) in regulation of hyperpolarization-activated inward current, I h, in the thalamocortical system and its functional relevance for the physiological thalamocortical oscillations were investigated. A significant decrease in I h current density, in both thalamocortical relay (TC) and cortical pyramidal neurons was found in TRIP8b-deficient mice (TRIP8b-/-). In addition basal cAMP levels in the brain were found to be decreased while the availability of the fast transient A-type K+ current, I A, in TC neurons was increased. These changes were associated with alterations in intrinsic properties and firing patterns of TC neurons, as well as intrathalamic and thalamocortical network oscillations, revealing a significant increase in slow oscillations in the delta frequency range (0.5-4 Hz) during episodes of active-wakefulness. In addition, absence of TRIP8b suppresses the normal desynchronization response of the EEG during the switch from slow-wave sleep to wakefulness. It is concluded that TRIP8b is necessary for the modulation of physiological thalamocortical oscillations due to its direct effect on HCN channel expression in thalamus and cortex and that mechanisms related to reduced cAMP signaling may contribute to the present findings.