Accessory Olfactory Bulb (AOB)

Accessory Olfactory Bulb (AOB)

Overview

<table class="infobox infobox-cell">
<tr>
<th class="infobox-header" colspan="2">Accessory Olfactory Bulb (AOB)</th>
</tr>
<tr>
<td class="label">Name</td>
<td><strong>Accessory Olfactory Bulb (AOB)</strong></td>
</tr>
<tr>
<td class="label">Type</td>
<td>Cell Type</td>
</tr>
</table>

The Accessory Olfactory Bulb (AOB) is a critical neural structure located in the ventral forebrain that processes pheromonal and social chemosensory information. Unlike the main olfactory bulb which detects volatile odors, the AOB is specialized for detecting non-volatile chemical signals, particularly pheromones and other social cues that mediate species-specific behaviors including mating, aggression, territorial marking, and parent-offspring recognition[@sakurai2004] [1](https://pubmed.ncbi.nlm.nih.gov/15535763/). The AOB receives input from the vomeronasal organ (VNO), also known as the Jacobson's organ, via the vomeronasal nerve, and projects to brain regions involved in social and reproductive behavior [2](https://pubmed.ncbi.nlm.nih.gov/10532334/).

Anatomical Organization

Location and Structure

The AOB is situated at the base of the forebrain, posterior and dorsal to the main olfactory bulb. In most mammals, it appears as a discrete, ovoid structure that is histologically distinct from the main olfactory bulb. The AOB contains several well-defined layers:

Accessory Olfactory Bulb (AOB)

Overview

<table class="infobox infobox-cell">
<tr>
<th class="infobox-header" colspan="2">Accessory Olfactory Bulb (AOB)</th>
</tr>
<tr>
<td class="label">Name</td>
<td><strong>Accessory Olfactory Bulb (AOB)</strong></td>
</tr>
<tr>
<td class="label">Type</td>
<td>Cell Type</td>
</tr>
</table>

The Accessory Olfactory Bulb (AOB) is a critical neural structure located in the ventral forebrain that processes pheromonal and social chemosensory information. Unlike the main olfactory bulb which detects volatile odors, the AOB is specialized for detecting non-volatile chemical signals, particularly pheromones and other social cues that mediate species-specific behaviors including mating, aggression, territorial marking, and parent-offspring recognition[@sakurai2004] [1](https://pubmed.ncbi.nlm.nih.gov/15535763/). The AOB receives input from the vomeronasal organ (VNO), also known as the Jacobson's organ, via the vomeronasal nerve, and projects to brain regions involved in social and reproductive behavior [2](https://pubmed.ncbi.nlm.nih.gov/10532334/).

Anatomical Organization

Location and Structure

The AOB is situated at the base of the forebrain, posterior and dorsal to the main olfactory bulb. In most mammals, it appears as a discrete, ovoid structure that is histologically distinct from the main olfactory bulb. The AOB contains several well-defined layers:

• Vomeronasal nerve layer: The most superficial layer containing incoming axons from the vomeronasal organ
• Glomerular layer: Where vomeronasal receptor neuron axons terminate in spherical structures called glomeruli
• Mitral cell layer: Contains the principal output neurons of the AOB
• Granule cell layer: Contains inhibitory interneurons that modulate mitral cell activity

Cellular Components

The AOB contains several distinct neuronal populations:

Vomeronasal Sensory Neurons (VSNs): Bipolar neurons in the vomeronasal epithelium that express specific vomeronasal receptors (VRs). These are divided into two main families:

• V1Rs (or VNr1): Found in the apical region, detect volatile pheromones
• V2Rs (or VNr2): Found in the basal region, detect non-volatile cues

Granule Cells: Small inhibitory interneurons that form reciprocal dendrodendritic synapses with mitral cells. They provide lateral inhibition and help shape the temporal pattern of AOB output.

Sustentacular Cells: Supporting glial-like cells that provide metabolic and structural support to the neuropil.

Molecular Markers

The AOB expresses a distinctive set of molecular markers that distinguish it from the main olfactory bulb:

• Vomeronasal receptor type 1 (V1R) family: G-protein-coupled receptors specialized for pheromone detection
• Vomeronasal receptor type 2 (V2R) family: Class C GPCRs that detect MHC peptides and other non-volatile signals
• Gαo: The predominant G-protein α-subunit in vomeronasal signaling
• TRPC2: Transient receptor potential channel essential for vomeronasal transduction
• Atypical protein kinase C (aPKC): Involved in AOB development and plasticity

Connectivity

Afferent Inputs

The primary input to the AOB comes from vomeronasal sensory neurons in the VNO. These neurons project via the vomeronasal nerve through the cribriform plate to terminate in glomeruli within the AOB. Each glomerulus receives input from neurons expressing the same vomeronasal receptor, creating a chemotopic map[@hawkins2018] [4](https://pubmed.ncbi.nlm.nih.gov/29897642/).

Efferent Outputs

AOB mitral and tufted cells project to several brain regions:

• Medial amygdala: The primary target, involved in social behavior regulation
• Bed nucleus of the stria terminalis (BNST): Involved in stress and anxiety responses
• Hypothalamic nuclei: Including the medial preoptic area and ventromedial hypothalamus, controlling reproductive and parental behaviors
• Piriform cortex: Provides secondary olfactory processing

Functional Role in Social Behavior

The AOB is essential for processing pheromonal signals that drive species-typical behaviors:

Reproductive Behavior: AOB-mediated signals regulate mate choice, mating behavior, and pregnancy block (the Bruce effect). Female mice require AOB function to recognize pheromonal cues from appropriate mates[@keverne1999] [5](https://pubmed.ncbi.nlm.nih.gov/10532334/).

Aggression: Territorial and intermale aggression is mediated by AOB-processed pheromonal signals. Androgen-dependent urinary signals activate AOB circuits that modulate aggressive behavior.

Parental Behavior: Pheromonal cues from pups activate AOB circuits in mothers, promoting nursing and pup-directed behaviors. The AOB shows increased activity during lactation.

Social Recognition: The AOB is critical for individual recognition based on chemosensory cues. Both mice and rats require AOB function to distinguish familiar from novel conspecifics.

Olfactory Dysfunction in Neurodegeneration

While the AOB is not a primary focus of neurodegeneration research, olfactory dysfunction is a well-established early biomarker for several neurodegenerative diseases, and the olfactory system including the AOB shows pathological changes in these conditions.

Alzheimer's Disease

Olfactory dysfunction is recognized as one of the earliest preclinical signs of Alzheimer's disease (AD), often preceding cognitive impairment by several years[@suzuki2018] [6](https://pubmed.ncbi.nlm.nih.gov/29505958/). The olfactory system, including both main and accessory olfactory bulbs, shows:

• Amyloid deposition: Aβ plaques have been identified in the olfactory bulb and piriform cortex
• Tau pathology: Neurofibrillary tangles accumulate in olfactory structures
• Neuroinflammation: Activated microglia in olfactory regions
• Olfactory bulb atrophy: Reduced volume observed in AD patients via MRI

Parkinson's Disease

Hyposmia is a well-documented early sign of Parkinson's disease (PD), often preceding motor symptoms by years [8](https://pubmed.ncbi.nlm.nih.gov/22720756/). Pathological changes in the olfactory system include:

• α-Synuclein deposition: Lewy bodies found in olfactory bulb neurons, including mitral and granule cells
• Olfactory bulb volume reduction: MRI studies show decreased bulb volume in PD patients [9](https://pubmed.ncbi.nlm.nih.gov/29633768/)
• Olfactory neuron loss: Post-mortem studies reveal reduced neuronal numbers

Other Neurodegenerative Conditions

Olfactory dysfunction is observed in several other neurodegenerative diseases:

• Dementia with Lewy bodies: Often more severe than in AD
• Multiple system atrophy: Early olfactory impairment
• Frontotemporal dementia: Variable olfactory involvement
• Huntington's disease: Olfactory deficits present

Adult Neurogenesis in the Olfactory Bulb

The mammalian olfactory system, including the AOB, exhibits continuous neurogenesis throughout adulthood. Neural stem cells in the subventricular zone (SVZ) generate new neurons that migrate via the rostral migratory stream to the olfactory bulb [11](https://pubmed.ncbi.nlm.nih.gov/28554234/). In the AOB, new granule cells are continuously added and integrated into existing circuits.

This adult neurogenesis has several implications:

• Plasticity: Allows for continuous refinement of olfactory circuits
• Regeneration: Potential for recovery after injury
• Aging: Neurogenesis declines with age, potentially contributing to olfactory decline
• Disease: Impaired neurogenesis may contribute to olfactory dysfunction in neurodegeneration

Olfactory Ensheathing Cells

The AOB and olfactory system contain specialized glial cells called olfactory ensheathing cells (OECs) that envelop olfactory axons and guide them from the periphery to the brain [12](https://pubmed.ncbi.nlm.nih.gov/32851853/). These cells have attracted interest for their therapeutic potential:

• Cell therapy: OECs have been explored for spinal cord injury repair
• Regeneration support: They promote axonal regeneration
• Neuroprotection: Potential to support degenerating neurons

Clinical Implications

Olfactory Testing in Neurodegeneration

Olfactory assessment has emerged as a valuable clinical tool:

• Early diagnosis: Olfactory tests can identify individuals at risk
• Disease staging: Olfactory function correlates with disease progression
• Differential diagnosis: Olfactory patterns differ between diseases

Therapeutic Approaches

Several strategies are being explored to preserve or restore olfactory function:

• Neuroprotective agents: Targeting oxidative stress and neuroinflammation
• Cell-based therapies: Transplantation of olfactory ensheathing cells or stem cells
• Olfactory training: Structured exposure to odors to enhance function
• Growth factor therapy: BDNF and other factors to support olfactory neurons

Aging and the Olfactory System

Aging is associated with progressive olfactory decline, affecting both detection and discrimination. Age-related changes in the AOB include:

• Neuronal loss: Reduced mitral and granule cell numbers
• Synaptic changes: Decreased synaptic density
• Neurogenesis decline: Reduced production of new neurons
• Glial alterations: Changes in supporting cell function

Research Models

Study of the AOB and olfactory dysfunction in neurodegeneration employs multiple approaches:

Animal Models: Rodent models allow detailed study of AOB anatomy and physiology. Transgenic models expressing human AD or PD proteins show olfactory pathology.

In vitro systems: Organotypic slice cultures and primary neuron cultures enable mechanistic studies.

Human studies: MRI volumetry, post-mortem studies, and olfactory testing provide clinical evidence.

iPSC models: Induced pluripotent stem cells from patients allow generation of olfactory cell types for study.

Future Directions

Research on the AOB in neurodegeneration continues to evolve:

• Biomarker development: Refining olfactory tests for early detection
• Mechanistic studies: Understanding how neurodegeneration affects olfactory circuits
• Therapeutic development: Targeting olfactory pathways for intervention
• Preventive strategies: Identifying ways to preserve olfactory function

See Also

• [Olfactory System](/brain-regions/olfactory-system)
• [Vomeronasal Organ](/cell-types/vomeronasal-organ)
• [Alzheimer's Disease](/diseases/alzheimers-disease)
• [Parkinson's Disease](/diseases/parkinsons-disease)
• [Olfactory Dysfunction](/mechanisms/olfactory-dysfunction-neurodegeneration)
• [Olfactory Ensheathing Cells](/cell-types/olfactory-ensheathing-cells)

Brain Atlas Resources

• [Allen Human Brain Atlas](https://human.brain-map.org/) — gene expression data
• [BrainSpan Atlas](https://brainspan.org/) — developmental transcriptome
• [Allen Mouse Brain Atlas](https://mouse.brain-map.org/) — mouse brain gene expression

References

External Links

• [NIH - Olfactory System](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2749925/)
• [PubMed: Olfactory bulb](https://pubmed.ncbi.nlm.nih.gov/?term=accessory+olfactory+bulb)
• [PubMed: Olfactory dysfunction neurodegenerative](https://pubmed.ncbi.nlm.nih.gov/?term=olfactory+dysfunction+neurodegenerative+disease)