Co-localization of two-color rAAV2-retro confirms the dispersion characteristics of efferent projections of mitral cells in mouse accessory olfactory bulb

AOBmi (Accessory olfactory bulb, mitral layer) MBA:204 projects to BST (Bed nuclei of the stria terminalis) MBA:351 · Mouse

Retrograde rAAV2-retro injected into the bed nucleus of the stria terminalis labelled neurons in the mitral cell layer of the accessory olfactory bulb (on average about 26 labelled neurons per AOB slice, the fewest of the four targets, n = 10 mice). In dual-tracer experiments, about 41% of BST-labelled MiA neurons were co-labelled from PMCo and about 71% were co-labelled from MeA.

Fig. 1A, 1B, 1I; Figs. 2, 5

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MEA (Medial amygdalar nucleus) MBA:403 projects to COApm (Cortical amygdalar area, posterior part, medial zone) MBA:663 · Mouse

The retrograde tracer injected into PMCo also labelled cells at the MeA injection site, indicating MeA neurons project to PMCo (reported as mutual connectivity).

Fig. 4B, 4C

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COApm (Cortical amygdalar area, posterior part, medial zone) MBA:663 projects to BST (Bed nuclei of the stria terminalis) MBA:351 · Mouse

Fluorescence of the color injected into BST was found at the PMCo injection site, indicating that PMCo neurons project to BST (reported as mutual connectivity; images not shown).

Results, 'AOB projects to BST and PMCo simultaneously' (Fig. 2)

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COAa (Cortical amygdalar area, anterior part) MBA:639 projects to COApm (Cortical amygdalar area, posterior part, medial zone) MBA:663 · Mouse

With retrograde virus in ACo and a different-color retrograde virus in PMCo, both colors were seen at each injection site, indicating that ACo neurons were retrogradely labelled from PMCo and thus project there.

Fig. 3B, 3C

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AOBmi (Accessory olfactory bulb, mitral layer) MBA:204 projects to MEA (Medial amygdalar nucleus) MBA:403 · Mouse

Retrograde rAAV2-retro injected into the medial amygdaloid nucleus retrogradely labelled many neurons in the mitral cell layer of the accessory olfactory bulb (about 111 cells per slice, among the highest of the four targets, n = 16 mice). Portions of MeA-labelled MiA neurons were also labelled from PMCo (~35%), BST (~14%) and ACo (~16%).

Fig. 1E, 1F, 1I; Figs. 4, 5, 6

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COAa (Cortical amygdalar area, anterior part) MBA:639 projects to MEA (Medial amygdalar nucleus) MBA:403 · Mouse

Red fluorescence from the MeA-injected retrograde virus was observed in the ACo (image not shown), indicating ACo neurons project to MeA.

Results, 'AOB projects to ACo and MeA simultaneously' (Fig. 6)

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BST (Bed nuclei of the stria terminalis) MBA:351 projects to COApm (Cortical amygdalar area, posterior part, medial zone) MBA:663 · Mouse

After injecting one retrograde virus in BST and the other in PMCo, the tracer from the PMCo injection appeared in BST neurons and vice versa, which the authors interpreted as reciprocal connections between the two regions; here BST cells were labelled from the PMCo injection.

Results, 'AOB projects to BST and PMCo simultaneously' (Fig. 2)

Made by an AI model reading the paper (claude-opus-5, extract@0.3.0); a second AI model (claude-opus-5) agrees.

COApm (Cortical amygdalar area, posterior part, medial zone) MBA:663 projects to COAa (Cortical amygdalar area, anterior part) MBA:639 · Mouse

The tracer injected in ACo also labelled cells at the PMCo injection site, indicating PMCo neurons project to ACo; the authors describe mutual connectivity between ACo and PMCo.

Fig. 3B, 3C

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AOBmi (Accessory olfactory bulb, mitral layer) MBA:204 projects to COAa (Cortical amygdalar area, anterior part) MBA:639 · Mouse

Retrograde rAAV2-retro injected into the anterior cortical amygdalar area labelled mitral-layer neurons of the accessory olfactory bulb (about 48 labelled cells per slice, n = 6 mice). In dual-injection experiments about 64% of ACo-labelled MiA neurons were also labelled from PMCo and about 39% from MeA.

Fig. 1C, 1D, 1I; Figs. 3, 6

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MEA (Medial amygdalar nucleus) MBA:403 projects to BST (Bed nuclei of the stria terminalis) MBA:351 · Mouse

The tracer injected into BST also labelled cells at the MeA injection site, indicating MeA neurons project to BST (described as mutual connectivity between BST and MeA).

Fig. 5B, 5C

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AOBmi (Accessory olfactory bulb, mitral layer) MBA:204 projects to COApm (Cortical amygdalar area, posterior part, medial zone) MBA:663 · Mouse

Retrograde rAAV2-retro injected into the posteromedial cortical amygdaloid area labelled mitral-layer neurons of the accessory olfactory bulb (about 117 cells per slice, the most of the four targets, n = 11 mice). Fractions of PMCo-labelled MiA neurons were co-labelled from BST (~6%), ACo (~20%) and MeA (~96%), indicating extensive collateralization.

Fig. 1G, 1H, 1I; Figs. 2, 3, 4

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COApm (Cortical amygdalar area, posterior part, medial zone) MBA:663 projects to MEA (Medial amygdalar nucleus) MBA:403 · Mouse

With separate retrograde viruses in MeA and PMCo, both fluorophores appeared at both injection sites; PMCo neurons were labelled by the MeA-injected tracer, indicating a PMCo-to-MeA projection.

Fig. 4B, 4C

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BST (Bed nuclei of the stria terminalis) MBA:351 projects to MEA (Medial amygdalar nucleus) MBA:403 · Mouse

After retrograde virus injections in BST and MeA with different fluorophores, both colors appeared at both sites; BST neurons labelled by the MeA-injected tracer indicate a BST-to-MeA projection.

Fig. 5B, 5C

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MEA (Medial amygdalar nucleus) MBA:403 projects to COAa (Cortical amygdalar area, anterior part) MBA:639 · Mouse

The EYFP retrograde virus injected into ACo produced green-labelled cells in the MeA, which the authors interpret as a projection from MeA to ACo.

Fig. 6C

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