Selective GABAergic control of higher-order thalamic relays.

GABAergic signaling is central to the function of the thalamus and has been traditionally attributed primarily to the nucleus reticularis thalami (nRT). Here we present a GABAergic pathway, distinct from the nRT, that exerts a powerful inhibitory effect selectively in higher-order thalamic relays of the rat. Axons originating in the anterior pretectal nucleus (APT) innervated the proximal dendrites of relay cells via large GABAergic terminals with multiple release sites. Stimulation of the APT in an in vitro slice preparation revealed a GABA(A) receptor-mediated, monosynaptic IPSC in relay cells. Activation of presumed single APT fibers induced rebound burst firing in relay cells. Different APT neurons recorded in vivo displayed fast bursting, tonic, or rhythmic firing. Our data suggest that selective extrareticular GABAergic control of relay cell activity will result in effective, state-dependent gating of thalamocortical information transfer in higher-order but not in first-order relays.

Regulation of recombinant and native hyperpolarization-activated cation channels.

Ionic currents generated by hyperpolarization-activated cation-nonselective (HCN) channels have been principally known as pacemaker h-currents (Ih), because they allow cardiac and neuronal cells to be rhythmically active over precise intervals of time. Presently, these currents are implicated in numerous additional cellular functions, including neuronal integration, synaptic transmission, and sensory reception. These roles are accomplished by virtue of the regulation of Ih by both voltage and ligands. The article summarizes recent developments on the properties and allosteric interactions of these two regulatory pathways in cloned and native channels. Additionally, it discusses how the expression and properties of native channels may be controlled via regulation of the transcription of the HCN channel gene family and the assembly of channel subunits. Recently, several cardiac and neurological diseases were found to be intimately associated with a dysregulation of HCN gene transcription, suggesting that HCN-mediated currents may be involved in the pathophysiology of excitable systems. As a starting point, we briefly review the general characteristics of Ih and the regulatory mechanisms identified in heterologously expressed HCN channels.

Functional stabilization of weakened thalamic pacemaker channel regulation in rat absence epilepsy.

Aberrant function of pacemaker currents (Ih), carried by hyperpolarization-activated cation non-selective (HCN) channels, affects neuronal excitability and accompanies epilepsy, but its distinct roles in epileptogenesis and chronic epilepsy are unclear. We probed Ih function and subunit composition during both pre- and chronically epileptic stages in thalamocortical (TC) neurones of the Genetic Absence Epilepsy Rat from Strasbourg (GAERS). Voltage gating of Ih was unaltered in mature somatosensory TC cells, both in vivo and in vitro. However, the enhancement of Ih by phasic, near-physiological, cAMP pulses was diminished by approximately 40% and the half-maximal cAMP concentration increased by approximately 5-fold. This decreased responsiveness of Ih to its major cellular modulator preceded epilepsy onset in GAERS, persisted throughout the chronic state, and was accompanied by an enhanced expression of the cAMP-insensitive HCN1 channel mRNA (> 50%), without changes in the mRNA levels of HCN2 and HCN4. To assess for alterations in TC cell excitability, we monitored the slow up-regulation of Ih that is induced by Ca2+-triggered cAMP synthesis and important for terminating in vitro synchronized oscillations. Remarkably, repetitive rebound Ca2+ spikes evoked normal slow Ih up-regulation in mature GAERS neurones; that sufficed to attenuate spontaneous rhythmic burst discharges. These adaptive mechanisms occurred upstream of cAMP turnover and involved enhanced intracellular Ca2+ accumulation upon repetitive low-threshold Ca2+ discharges. Therefore, HCN channels appear to play a dual role in epilepsy. Weakened cAMP binding to HCN channels precedes, and likely promotes, epileptogenesis in GAERS, whereas compensatory mechanisms stabilizing Ih function contribute to the termination of spike-and-wave discharges in chronic epilepsy.

Pacemaker channels in mouse thalamocortical neurones are regulated by distinct pathways of cAMP synthesis.

A crucial aspect of pacemaker current (Ih) function is the regulation by cyclic nucleotides. To assess the endogenous mechanisms controlling cAMP levels in the vicinity of pacemaker channels, Ih regulation by G-protein-coupled neurotransmitter receptors was studied in mouse thalamocortical neurones. Activation of beta-adrenergic receptors with (-)-isoproterenol (Iso) led to a small steady enhancement of Ih amplitude, whereas activation of GABAB receptors with (+/-)-Baclofen (Bac) reduced Ih, consistent with an up- and down-regulation of basal cAMP levels, respectively. In contrast, a transient (taudecay, approximately 200 s), supralinear up-regulation of Ih was observed upon coapplication of Iso and Bac that was larger than that observed with Iso alone. This up-regulation appeared to involve a cAMP synthesis pathway distinct from that recruited by Iso, as it was associated with a reversible acceleration in Ih activation kinetics and an occlusion of modulation by photolytically released cAMP, yet showed an 11 mV as opposed to a 6 mV positive shift in the activation curve and an at least seven-fold increase in duration. GABA, in the presence of the GABAA antagonist picrotoxin, mimicked, whereas N-ethylmaleimide, an inhibitor of Gi-proteins, blocked the up-regulation, supporting a requirement for GABAB receptor activation in the potentiation. Activation of synaptic GABAB responses via stimulation of inhibitory afferents from the nucleus reticularis potentiated Iso-induced increments in Ih, suggesting that synaptically located receptors couple positively to cAMP synthesis induced by beta-adrenergic receptors. These findings indicate that distinct pathways of cAMP synthesis target the pacemaker current and the recruitment of these may be controlled by GABAergic activity within thalamic networks.