Tapping the Brakes: Cellular and Synaptic Mechanisms that Regulate Thalamic Oscillations

Abstract

Thalamic oscillators contribute to both normal rhythms associated with sleep and anesthesia and abnormal, hypersynchronous oscillations that manifest behaviorally as absence seizures. In this review, we highlight new findings that refine thalamic contributions to cortical rhythms and suggest that thalamic oscillators may be subject to both local and global control. We describe endogenous thalamic mechanisms that limit network synchrony and discuss how these protective brakes might be restored to prevent absence seizures. Finally, we describe how intrinsic and circuit-level specializations among thalamocortical loops may determine their involvement in widespread oscillations and render subsets of thalamic nuclei especially vulnerable to pathological synchrony.

Keywords: absence epilepsy; oscillation; reticular nucleus; sparse networks; spindle; synchrony; thalamus.

Figure 1. Elements of the thalamic rhythm generator

Thalamocortical cells (TC, blue) make excitatory projections to related cortical areas, and emit axon collaterals that make excitatory synapses (e1) onto inhibitory neurons in reticular thalamus (RT, red). These connections are divergent (e1d) in that individual TC cells contact more than one RT cell and convergent (e1c) in that multiple TC cells contact each RT cell. RT cells, in turn, provide feedback inhibition back to TC cells (i1) that is convergent (i1c) and divergent (i1d). The intracellular records on the left show how i1 activation leads to generation of post-inhibitory rebound bursts (lower trace) that produce recurrent excitation of RT cells through synapse e1 (upper trace) that perpetuates the intrathalamic rhythm. In addition, RT cells contact each other through gap junctions (not shown) and chemical inhibitory synapses (i2); the latter will suppress thalamic rhythmicity. Excitatory feedback from the cortex projects to both RT (e2) and TC (e3), each with convergence and divergence. Each of these synapses is implicated in promoting, synchronizing, driving, and regulating thalamocortical oscillations. The individual compositions and functions of each major synapse are provided in Figure 3.

Figure 2. RT driven synaptic inhibition both drives spindle oscillations and desynchronizes them in vivo and in vitro.

A. Intracellular recordings from RT and TC (Th-Cx) cells during spontaneous spindle responses in felines. Upper two traces show sequences of 4 consecutive spindles, with the first, marked with asterisk, expanded in lower two traces. Note that RT cells fire on most cycles of the spindle sequence, while TC cells fire on many fewer cycles. Rhythmic IPSPs in TC cells lead to occasional rebound LTS responses, especially with the largest IPSPs, yet some large IPSPs that appear to be sufficient to do so, are instead followed by a subsequent IPSP that serves to veto the LTS. Some of these vetoed events are marked by open red arrows. B. Recordings from ferret LGN slices with spontaneous spindle like sequences evident in multi-unit extracellular recordings (upper trace) show similar periodic IPSPs (lower trace), some with rebound LTSs (Ca²⁺) that drive Na⁺ spikes, and other with clear vetoed events (red open arrows). C. Intracellular recordings from ventrobasal TC neurons in rat slices during evoked spindle-like oscillations. Multiple overlaid sequential responses are shown. Responses were evoked by extracellular stimulation of the internal capsule, which activates excitatory synapses (e2, e3) onto RT cells cause them to fire and produce inhibitory responses and rebound LTSs in the recorded TC cell. Note that sequential sweeps with the equivalent stimulus yielded LTSs with highly variable latencies. In many cases, a second IPSP arrives (open arrows) that delays or vetoes the LTS. D. Intracellular recordings from two (red and blue) nearby mouse TC neurons during 3 sequential optogenetic activations (green bar) of cholinergic inputs to RT. Note that sometime the blue TC cell leads the red cell (upper two traces), but that occasionally a second IPSP arrives (bottom trace) and vetoes (open red arrow) the early LTS leading to a response delayed by hundreds of ms. E. Reduced heterogeneity of TC IPSPs in the isolated thalamic network increases network synchrony. Autocorrelograms for TC cell multiunit spikes in rat VB-RT slices under various oscillatory conditions. The baseline autocorrelogram (black trace) shows very little structure and synchrony. Addition of the GABA_B antagonist CGP35348 (grey trace) increases network synchrony (increase in peak to valley ratio of autocorrelogram), and similar increases in synchrony are produced by the GABA_A antagonist picrotoxin (ptx). Thus, increasing IPSP homogeneity by abbreviating them to only contain GABA_A responses sped and synchronized the oscillation (inset), or by lengthening them to only contain slower GABA_B responses slowed and synchronized the oscillation. A. Modified from (Steriade and and Deschenes, 1988), B. Modified from (von Krosigk et al., 1993), C. modified from (Huguenard and Prince, 1994a), D. Modified from (Pita-Almenar et al., 2014), E. modified from (Jacobsen et al., 2001).

Figure 3. Synaptic elements in thalamic circuit that regulate synchrony and oscillations

A. RT →TC, synapse i1. This inhibitory synapse is responsible for the phasic inhibition that drives post-inhibitory rebound firing in TC cells. GABA is released to activate α1β2γ2 GABA_ARs within the synapse, and can spillover to activate extrasynaptic α4β2δ GABA_ARs and GABA_B receptors with slower kinetics. Spillover to extrasynaptic receptors is tightly regulated by GABA uptake via astrocytic GAT1 and GAT3. B. RT →RT, synapse i2. Chemical inhibitory signaling between RT cells is largely dependent on α3β3γ2 GABA_ARs, although a weak GABA_B component is also present. Astrocytes in RT appear to release an endogenous benzodiazepine site ligand derived from benzodiazepine binding inhibitor (DBI) producing a constitutive positive allosteric modulation of RT GABA_ARs, that suppresses synchrony in the network. C. RT →TC, synapse e1. This synapse provides excitatory feedback within the thalamic loop to reinforce spindle oscillations and SWD. Although gluA4 is the major AMPA receptor subunit in RT, it does not appear to contribute to excitation at this synapse, as it’s synaptic strength is unchanged by deletion of gria4, which encodes gluA4. D. CT →RT, synapse e2. This synapse provides feedback from the cortex that can reinforce global CTC oscillations. Inactivation of gria4 weakens this synapse, leading to underexcitation of RT, and failure to produce cortical feed-forward inhibition (synapse i1) with resultant hyperexcitation at the CT→TC synapse, e3 (not shown).