Th17 lymphocytes drive vascular and neuronal deficits in a mouse model of postinfectious autoimmune encephalitis.

Antibodies against neuronal receptors and synaptic proteins are associated with a group of ill-defined central nervous system (CNS) autoimmune diseases termed autoimmune encephalitides (AE), which are characterized by abrupt onset of seizures and/or movement and psychiatric symptoms. Basal ganglia encephalitis (BGE), representing a subset of AE syndromes, is triggered in children by repeated group A Streptococcus (GAS) infections that lead to neuropsychiatric symptoms. We have previously shown that multiple GAS infections of mice induce migration of Th17 lymphocytes from the nose into the brain, causing blood-brain barrier (BBB) breakdown, extravasation of autoantibodies into the CNS, and loss of excitatory synapses within the olfactory bulb (OB). Whether these pathologies induce functional olfactory deficits, and the mechanistic role of Th17 lymphocytes, is unknown. Here, we demonstrate that, whereas loss of excitatory synapses in the OB is transient after multiple GAS infections, functional deficits in odor processing persist. Moreover, mice lacking Th17 lymphocytes have reduced BBB leakage, microglial activation, and antibody infiltration into the CNS, and have their olfactory function partially restored. Th17 lymphocytes are therefore critical for selective CNS entry of autoantibodies, microglial activation, and neural circuit impairment during postinfectious BGE.

Place cell firing cannot support navigation without intact septal circuits.

Though it has been known for over half a century that interference with the normal activity of septohippocampal neurons can abolish hippocampal theta rhythmicity, a definitive answer to the question of its function has remained elusive. To clarify the role of septal circuits and theta in location-specific activity of place cells and spatial behavior, three drugs were delivered to the medial septum of rats: Tetracaine, a local anesthetic; muscimol, a GABA-A agonist; and gabazine, a GABA-A antagonist. All three drugs disrupted normal oscillatory activity in the hippocampus. However, tetracaine and muscimol both reduced spatial firing and interfered with the rat's ability to navigate to a hidden goal. After gabazine, location-specific firing was preserved in the absence of theta, but rats were unable to accurately locate the hidden goal. These results indicate that theta is unnecessary for location-specific firing of hippocampal cells, and that place cell activity cannot support accurate navigation when septal circuits are disrupted.

Electrophysiological Recordings from Identified Cell Types in the Olfactory Cortex of Awake Mice.

Neural circuits consist of a myriad of distinct cell types, each with specific intrinsic properties and patterns of synaptic connectivity, which transform neural input and convey this information to downstream targets. Understanding how different features of an odor stimulus are encoded and relayed to their appropriate targets will require selective identification and manipulation of these different elements of the circuit. Here, we describe methods to obtain dense, extracellular electrophysiological recordings of odor-evoked activity in olfactory (piriform) cortex of awake, head-fixed mice, and optogenetic tools and procedures to identify genetically defined cell types within this circuit.

Recurrent circuitry is required to stabilize piriform cortex odor representations across brain states.

Pattern completion, or the ability to retrieve stable neural activity patterns from noisy or partial cues, is a fundamental feature of memory. Theoretical studies indicate that recurrently connected auto-associative or discrete attractor networks can perform this process. Although pattern completion and attractor dynamics have been observed in various recurrent neural circuits, the role recurrent circuitry plays in implementing these processes remains unclear. In recordings from head-fixed mice, we found that odor responses in olfactory bulb degrade under ketamine/xylazine anesthesia while responses immediately downstream, in piriform cortex, remain robust. Recurrent connections are required to stabilize cortical odor representations across states. Moreover, piriform odor representations exhibit attractor dynamics, both within and across trials, and these are also abolished when recurrent circuitry is eliminated. Here, we present converging evidence that recurrently-connected piriform populations stabilize sensory representations in response to degraded inputs, consistent with an auto-associative function for piriform cortex supported by recurrent circuitry.

Group I metabotropic glutamate receptors control metaplasticity of spinal cord learning through a protein kinase C-dependent mechanism.

Neurons within the spinal cord can support several forms of plasticity, including response-outcome (instrumental) learning. After a complete spinal transection, experimental subjects are capable of learning to hold the hindlimb in a flexed position (response) if shock (outcome) is delivered to the tibialis anterior muscle when the limb is extended. This response-contingent shock produces a robust learning that is mediated by ionotropic glutamate receptors (iGluRs). Exposure to nociceptive stimuli that are independent of limb position (e.g., uncontrollable shock; peripheral inflammation) produces a long-term (>24 h) inhibition of spinal learning. This inhibition of plasticity in spinal learning is itself a form of plasticity that requires iGluR activation and protein synthesis. Plasticity of plasticity (metaplasticity) in the CNS has been linked to group I metabotropic glutamate receptors (subtypes mGluR1 and mGluR5) and activation of protein kinase C (PKC). The present study explores the role of mGluRs and PKC in the metaplastic inhibition of spinal cord learning using a combination of behavioral, pharmacological, and biochemical techniques. Activation of group I mGluRs was found to be both necessary and sufficient for metaplastic inhibition of spinal learning. PKC was activated by stimuli that inhibit spinal learning, and inhibiting PKC activity restored the capacity for spinal learning. Finally, a PKC inhibitor blocked the metaplastic inhibition of spinal learning produced by a group I mGluR agonist. The data strongly suggest that group I mGluRs control metaplasticity of spinal learning through a PKC-dependent mechanism, providing a potential therapeutic target for promoting use-dependent plasticity after spinal cord injury.

Recurrent cortical circuits implement concentration-invariant odor coding.

Animals rely on olfaction to find food, attract mates, and avoid predators. To support these behaviors, they must be able to identify odors across different odorant concentrations. The neural circuit operations that implement this concentration invariance remain unclear. We found that despite concentration-dependence in the olfactory bulb (OB), representations of odor identity were preserved downstream, in the piriform cortex (PCx). The OB cells responding earliest after inhalation drove robust responses in sparse subsets of PCx neurons. Recurrent collateral connections broadcast their activation across the PCx, recruiting global feedback inhibition that rapidly truncated and suppressed cortical activity for the remainder of the sniff, discounting the impact of slower, concentration-dependent OB inputs. Eliminating recurrent collateral output amplified PCx odor responses rendered the cortex steeply concentration-dependent and abolished concentration-invariant identity decoding.

A transformation from temporal to ensemble coding in a model of piriform cortex.

Different coding strategies are used to represent odor information at various stages of the mammalian olfactory system. A temporal latency code represents odor identity in olfactory bulb (OB), but this temporal information is discarded in piriform cortex (PCx) where odor identity is instead encoded through ensemble membership. We developed a spiking PCx network model to understand how this transformation is implemented. In the model, the impact of OB inputs activated earliest after inhalation is amplified within PCx by diffuse recurrent collateral excitation, which then recruits strong, sustained feedback inhibition that suppresses the impact of later-responding glomeruli. We model increasing odor concentrations by decreasing glomerulus onset latencies while preserving their activation sequences. This produces a multiplexed cortical odor code in which activated ensembles are robust to concentration changes while concentration information is encoded through population synchrony. Our model demonstrates how PCx circuitry can implement multiplexed ensemble-identity/temporal-concentration odor coding.

Complementary codes for odor identity and intensity in olfactory cortex.

The ability to represent both stimulus identity and intensity is fundamental for perception. Using large-scale population recordings in awake mice, we find distinct coding strategies facilitate non-interfering representations of odor identity and intensity in piriform cortex. Simply knowing which neurons were activated is sufficient to accurately represent odor identity, with no additional information about identity provided by spike time or spike count. Decoding analyses indicate that cortical odor representations are not sparse. Odorant concentration had no systematic effect on spike counts, indicating that rate cannot encode intensity. Instead, odor intensity can be encoded by temporal features of the population response. We found a subpopulation of rapid, largely concentration-invariant responses was followed by another population of responses whose latencies systematically decreased at higher concentrations. Cortical inhibition transforms olfactory bulb output to sharpen these dynamics. Our data therefore reveal complementary coding strategies that can selectively represent distinct features of a stimulus.

Uncontrollable stimulation undermines recovery after spinal cord injury.

Prior studies have shown that neurons within the spinal cord are sensitive to response-outcome relations, a form of instrumental learning. Spinally transected rats that receive shock to one hind leg learn to maintain the leg in a flexed position that minimizes net shock exposure (controllable shock). Prior exposure to uncontrollable stimulation (intermittent shock) inhibits this spinally mediated learning. Here it is shown that uncontrollable stimulation undermines the recovery of function after a spinal contusion injury. Rats received a moderate injury (12.5 mm drop) and recovery was monitored for 6 weeks. In Experiment 1, rats received varying amounts of intermittent tailshock 1-2 days after injury. Just 6 min of intermittent shock impaired locomotor recovery. In Experiment 2, rats were shocked 1, 4, or 14 days after injury. Delaying the application of shock exposure reduced its negative effect on recovery. In Experiment 3, rats received controllable or uncontrollable shock 24 and 48 h after injury. Only uncontrollable shock disrupted recovery of locomotor function. Uncontrollably shocked rats also exhibited higher vocalization thresholds to aversive stimuli (heat and shock) applied below the injury. Across the three experiments, exposure to uncontrollable shock, (1) delayed the recovery of bladder function; (2) led to greater mortality and spasticity; and (3) increased tissue loss (white and gray matter) in the region of the injury. The results indicate that uncontrollable stimulation impairs recovery after spinal cord injury and suggest that reducing sources of uncontrolled afferent input (e.g., from peripheral tissue injury) could benefit patient recovery.