Cortical interneuron-mediated inhibition delays the onset of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis is a fatal disease resulting from motor neuron degeneration in the cortex and spinal cord. Cortical hyperexcitability is a hallmark feature of amyotrophic lateral sclerosis and is accompanied by decreased intracortical inhibition. Using electrophysiological patch-clamp recordings, we revealed parvalbumin interneurons to be hypoactive in the late pre-symptomatic SOD1*G93A mouse model of amyotrophic lateral sclerosis. We discovered that using adeno-associated virus-mediated delivery of chemogenetic technology targeted to increase the activity of the interneurons within layer 5 of the primary motor cortex, we were able to rescue intracortical inhibition and reduce pyramidal neuron hyperexcitability. Increasing the activity of interneurons in the layer 5 of the primary motor cortex was effective in delaying the onset of amyotrophic lateral sclerosis-associated motor deficits, slowing symptom progression, preserving neuronal populations, and increasing the lifespan of SOD1*G93A mice. Taken together, this study provides novel insights into the pathogenesis and treatment of amyotrophic lateral sclerosis.

Olfactory memory representations are stored in the anterior olfactory nucleus

The anterior olfactory nucleus (AON) is the initial recipient of odour information from the olfactory bulb, and the target of dense innervation conveying spatiotemporal cues from the hippocampus. We hypothesized that the AON detects the coincidence of these inputs, generating patterns of activity reflective of episodic odour engrams. Using activity-dependent tagging combined with neural manipulation techniques, we reveal that contextually-relevant odour engrams are stored within the AON and that their activity is necessary and sufficient for the behavioural expression of odour memory. Our findings offer a new model for studying the mechanisms underlying memory representations.

Odours are powerful stimuli used by most organisms to guide behaviour. Here, the authors identify populations of neurons within the anterior olfactory nucleus (AON) which are necessary and sufficient for the behavioural expression of odour memory.

Behavioral Evaluation of Odor Memory in Mice

Behavioural tests based on the spontaneous recognition paradigm have been used extensively for examining the memory capacity of rodents. By exploiting their innate preference to investigate novel stimuli, inferences can be drawn about the perceived familiarity of encountered objects. Olfaction is the dominant sense used by mice to navigate their environment, yet these tests are often conducted using visual objects. By employing odors, one can reduce the high level of variability commonly observed between subjects. In this paper, we describe a protocol for assessing context-dependent odor memory by probing spatial and temporal associations separately or in conjunction with each other. We also detail a context-independent novel odor recognition protocol. These tests offer a simple and effective method for measuring odor memory in rodents using cheap and easily obtained materials.

Hippocampal projections to the anterior olfactory nucleus differentially convey spatiotemporal information during episodic odour memory

The hippocampus is essential for representing spatiotemporal context and establishing its association with the sensory details of daily life to form episodic memories. The olfactory cortex in particular shares exclusive anatomical connections with the hippocampus as a result of their common evolutionary history. Here we selectively inhibit hippocampal projections to the anterior olfactory nucleus (AON) during behavioural tests of contextually cued odour recall. We find that spatial odour memory and temporal odour memory are independently impaired following inhibition of distinct, topographically organized hippocampal-AON pathways. Our results not only reveal a longstanding unknown function for the AON but offer new mechanistic insights regarding the representation of odours in episodic memory.

Topographic Organization of Hippocampal Inputs to the Anterior Olfactory Nucleus

Top-down processes conveying contextual information play a major role in shaping odor representations within the olfactory system, yet the underlying mechanisms are poorly understood. The hippocampus (HPC) is a major source of olfactory top-down modulation, providing direct excitatory inputs to the anterior olfactory nucleus (AON). However, HPC-AON projections remain uncharacterized. In an effort to understand how hippocampal inputs are distributed within the AON, we systematically outlined their organization using anterograde and retrograde tracing methods. We found that AON-projecting hippocampal pyramidal neurons are located mostly in the ventral two-thirds of the HPC and are organized topographically such that cells with a ventral to intermediate hippocampal point of origin terminate, respectively, at the medial to lateral AON. Our neuroanatomical findings suggest a potential role for the HPC in the early processing and contextualization of odors which merits further investigation.

Top-down modulation of olfactory-guided behaviours by the anterior olfactory nucleus pars medialis and ventral hippocampus

Olfactory processing is thought to be actively modulated by the top-down input from cortical regions, but the behavioural function of these signals remains unclear. Here we find that cortical feedback from the anterior olfactory nucleus pars medialis (mAON) bidirectionally modulates olfactory sensitivity and olfaction-dependent behaviours. To identify a limbic input that tunes this mAON switch, we further demonstrate that optogenetic stimulation of ventral hippocampal inputs to the mAON is sufficient to alter olfaction-dependent behaviours.