Characterization of odor-evoked neural activity in the olfactory peduncle

Control of innate olfactory valence by segregated cortical amygdala circuits

Animals perform innate behaviors that are stereotyped responses to specific evolutionarily relevant stimuli in the absence of prior learning or experience. These behaviors can be reduced to an axis of valence, whereby specific odors evoke approach or avoidance. The cortical amygdala (plCoA) mediates innate attraction and aversion to odor. However, little is known about how this brain area gives rise to behaviors of opposing motivational valence. Here, we sought to define the circuit features of plCoA that give rise to innate olfactory behaviors of valence. We characterized the physiology, gene expression, and projections of this structure, identifying a divergent, topographic organization that selectively controls innate attraction and avoidance to odor. First, we examined odor-evoked responses in these areas and found sparse encoding of odor identity, but not valence. We next considered a topographic organization and found that optogenetic stimulation of the anterior and posterior domains of plCoA elicits attraction and avoidance, respectively, suggesting a functional axis for valence. Using single cell and spatial RNA sequencing, we identified the molecular cell types in plCoA, revealing an anteroposterior gradient in cell types, whereby anterior glutamatergic neurons preferentially express Slc17a6 and posterior neurons express Slc17a7 . Activation of these respective cell types recapitulates appetitive and aversive valence behaviors, and chemogenetic inhibition reveals partial necessity for valence responses to innate appetitive or aversive odors. Finally, we identified topographically organized circuits defined by projections, whereby anterior neurons preferentially project to medial amygdala, and posterior neurons preferentially project to nucleus accumbens, which are respectively sufficient and necessary for innate negative and positive olfactory valence. Together, these data advance our understanding of how the olfactory system generates stereotypic, hardwired attraction and avoidance, and supports a model whereby distinct, topographically distributed plCoA populations direct innate olfactory valence responses by signaling to divergent valence-specific targets, linking upstream olfactory identity to downstream valence behaviors, through a population code. This represents a novel circuit motif in which valence encoding is represented not by the firing properties of individual neurons, but by population level identity encoding that is routed through divergent targets to mediate distinct valence.

The olfactory striae: A historical perspective on the inconsistent anatomy of the bulbar projections

Central olfactory pathways (i.e., projection axons of the mitral and tufted cells), and especially olfactory striae, lack common terminology. This is due to their high degree of intra- and interindividual variability, which has been studied in detail over the past century by Beccari, Mutel, Klass, Erhart, and more recently, by Duque Parra et al. These variations led to some confusion about their number and anatomical arrangement. Recent advances in fiber tractography have enabled the precise in vivo visualization of human olfactory striae and the study of their projections. However, these studies require their algorithms to be set up according to the presumed anatomy of the analyzed fibers. A more precise definition of the olfactory striae is therefore needed, not only to allow a better analysis of the results but also to ensure the quality of the data obtained. By studying the various published works on the central olfactory pathways from the first systematic description by Soemmerring to the present, I have traced the different discussions on the olfactory tracts and summarized them here. This review adopts a systematic approach by addressing each stria individually and tracing the historical background of what was known about it in the past, compared to the current knowledge. The chronological and organized approach used provides a better understanding of the anatomy of these essential structures of the olfactory system.

Intracranial self-stimulation reverses impaired spatial learning and regulates serum microRNA levels in a streptozotocin-induced rat model of Alzheimer disease

BACKGROUND

The assessment of deep brain stimulation (DBS) as a therapeutic alternative for treating Alzheimer disease (AD) is ongoing. We aimed to determine the effects of intracranial self-stimulation at the medial forebrain bundle (MFB-ICSS) on spatial memory, neurodegeneration, and serum expression of microRNAs (miRNAs) in a rat model of sporadic AD created by injection of streptozotocin. We hypothesized that MFB-ICSS would reverse the behavioural effects of streptozotocin and modulate hippocampal neuronal density and serum levels of the miRNAs.

METHODS

We performed Morris water maze and light-dark transition tests. Levels of various proteins, specifically amyloid-β precurser protein (APP), phosphorylated tau protein (pTAU), and sirtuin 1 (SIRT1), and neurodegeneration were analyzed by Western blot and Nissl staining, respectively. Serum miRNA expression was measured by reverse transcription polymerase chain reaction.

RESULTS

Male rats that received streptozotocin had increased hippocampal levels of pTAU S202/T205, APP, and SIRT1 proteins; increased neurodegeneration in the CA1, dentate gyrus (DG), and dorsal tenia tecta; and worse performance in the Morris water maze task. No differences were observed in miRNAs, except for miR-181c and miR-let-7b. After MFB-ICSS, neuronal density in the CA1 and DG regions and levels of miR-181c in streptozotocin-treated and control rats were similar. Rats that received streptozotocin and underwent MFB-ICSS also showed lower levels of miR-let-7b and better spatial learning than rats that received streptozotocin without MFB-ICSS.

LIMITATIONS

The reversal by MFB-ICSS of deficits induced by streptozotocin was fairly modest.

CONCLUSION

Spatial memory performance, hippocampal neurodegeneration, and serum levels of miR-let-7b and miR-181c were affected by MFB-ICSS under AD-like conditions. Our results validate the MFB as a potential target for DBS and lend support to the use of specific miRNAs as promising biomarkers of the effectiveness of DBS in combatting AD-associated cognitive deficits.

The anterior olfactory nucleus revisited - an emerging role for neuropathological conditions?

Olfaction is an important sensory modality for many species and greatly influences animal and human behavior. Still, much about olfactory perception remains unknown. The anterior olfactory nucleus is one of the brain's central early olfactory processing areas. Located directly posterior to the olfactory bulb in the olfactory peduncle with extensive in- and output connections and unique cellular composition, it connects olfactory processing centers of the left and right hemispheres. Almost 20 years have passed since the last comprehensive review on the anterior olfactory nucleus has been published and significant advances regarding its anatomy, function, and pathophysiology have been made in the meantime. Here we briefly summarize previous knowledge on the anterior olfactory nucleus, give detailed insights into the progress that has been made in recent years, and map out its emerging importance in translational research of neurological diseases.