Co-transmitting interneurons in the mouse olfactory bulb regulate olfactory detection and discrimination

Odor representation and coding by the mitral/tufted cells in the olfactory bulb

The olfactory bulb (OB) is the first relay station in the olfactory system and functions as a crucial hub. It can represent odor information precisely and accurately in an ever-changing environment. As the only output neurons in the OB, mitral/tufted cells encode information such as odor identity and concentration. Recently, the neural strategies and mechanisms underlying odor representation and encoding in the OB have been investigated extensively. Here we review the main progress on this topic. We first review the neurons and circuits involved in odor representation, including the different cell types in the OB and the neural circuits within and beyond the OB. We will then discuss how two different coding strategies-spatial coding and temporal coding-work in the rodent OB. Finally, we discuss potential future directions for this research topic. Overall, this review provides a comprehensive description of our current understanding of how odor information is represented and encoded by mitral/tufted cells in the OB.

Evolutionary innovations in the primate dopaminergic system

The human brain has evolved unique capabilities compared to other vertebrates. The mechanistic basis of these derived traits remains a fundamental question in biology due to its relevance to the origin of our cognitive abilities and behavioral repertoire, as well as to human-specific aspects of neuropsychiatric and neurodegenerative diseases. Comparisons of the human brain to those of nonhuman primates and other mammals have revealed that differences in the neuromodulatory systems, especially in the dopaminergic system, may govern some of these behavioral and cognitive alterations, including increased vulnerability to certain brain disorders. In this review, we highlight and discuss recent findings of human- and primate-specific alterations of the dopaminergic system, focusing on differences in anatomy, circuitry, and molecular properties.

Arcuate dopaminergic/GABAergic neurons project within the hypothalamus and to the median eminence

Cotransmission, meaning the release of multiple neurotransmitters from one synapse, allows for increased diversity of signaling in the brain. Dopamine (DA) and γ-aminobutyric acid (GABA) are known to coexpress in many regions such as the olfactory bulb and the ventral tegmental area. Tuberoinfundibular dopaminergic neurons (TIDA) in the arcuate nucleus of the hypothalamus (Arc) project to the median eminence (ME) and regulate prolactin release from the pituitary, and prior work suggests dopaminergic Arc neurons also cotransmit GABA. However, the extent of cotransmission, and the projection patterns of these neurons have not been fully revealed. Here, we used a genetic intersectional reporter expression approach to selectively label cells that express both tyrosine hydroxylase (TH) and vesicular GABA transporter (VGAT). Through this approach, we identified cells capable of both DA and GABA cotransmission in the Arc, periventricular (Pe), paraventricular (Pa), ventromedial, and the dorsolateral hypothalamic nuclei, in addition to a novel population in the caudate putamen. The highest density of labeled cells was in the Arc, 6.68% of DAPI-labeled cells at Bregma -2.06 mm, and in the Pe, 2.83% of DAPI-labeled cells at Bregma -1.94 mm. Next, we evaluated the projections of these DA/GABA cells by injecting an mCherry virus that fluoresces in DA/GABA cells. We observed a cotransmitting DA/GABA population, with projections within the Arc, and to the Pa and ME. These data suggest DA/GABA Arc neurons are involved in prolactin release as a subset of TIDA neurons. Further investigation will elucidate the interactions of dopamine and GABA in the hypothalamus.NEW & NOTEWORTHY Cotransmitting dopaminergic (DA) and γ-aminobutyric acid (GABA)ergic (DA/GABA) neurons contribute to the complexity of neural circuits. Using a new genetic technique, we characterized the locations, density, and projections of hypothalamic DA/GABA neurons. DA/GABA cells are mostly in the arcuate nucleus (Arc), from which they project locally within the arcuate, to the median eminence (ME), and to the paraventricular (Pa) nucleus. There is also a small and previously unreported group of DA/GABA cells in the caudate putamen.

Cholecystokinin-expressing superficial tufted cells modulate odour representation in the olfactory bulb and olfactory behaviours

In mammals, odour information within the olfactory bulb (OB) is processed by complex neural circuits before being ultimately represented in the action potential activity of mitral/tufted cells (M/Ts). Cholecystokinin-expressing (CCK+) superficial tufted cells (sTCs) are a subset of tufted cells that potentially contribute to olfactory processing in the OB by orchestrating M/T activity. However, the exact role of CCK+ sTCs in modulating odour processing and olfactory function in vivo is largely unknown. Here, we demonstrate that manipulating CCK+ sTCs can generate perception and induce place avoidance. Optogenetic activation/inactivation of CCK+ sTCs exerted strong but differing effects on spontaneous and odour-evoked M/T firing. Furthermore, inactivation of CCK+ sTCs disrupted M/T odour encoding and impaired olfactory detection and odour discrimination. These results establish the role of CCK+ sTCs in odour representation and olfactory behaviours. KEY POINTS: Mice could perceive the activity of CCK+ sTCs and show place avoidance to CCK+ sTC inactivation. Optical activation of CCK+ sTCs increased the percentage of cells with odour response but reduced the odour-evoked response in M/Ts in awake mice. Optical inactivation of CCK+ sTCs greatly decreased spontaneous firing and odour-evoked response in M/Ts. Inactivation of CCK+ sTCs impairs the odour decoding performance of M/Ts and disrupts odour detection and discrimination behaviours in mice. These results indicate that CCK+ sTCs participate in modulating the odour representation and maintaining normal olfactory-related behaviours.

Switching on generalized fear

Stress induces a neurotransmitter switch that leads to fear in harmless situations.