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 1. 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. [1]
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: [2]
The AOB exhibits significant species variation in size and complexity, with rodents having particularly well-developed AOB structures compared to primates 3. [3]
The AOB contains several distinct neuronal populations: [4]
Vomeronasal Sensory Neurons (VSNs): Bipolar neurons in the vomeronasal epithelium that express specific vomeronasal receptors (VRs). These are divided into two main families: [5]
Mitral/Tufted Cells: The principal output neurons of the AOB. These cells receive synaptic input from VSNs in the glomerular layer and project to higher brain centers. AOB mitral cells show characteristic physiological properties including slower conduction velocities and broader action potentials compared to main olfactory bulb mitral cells. [6]
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. [7]
Sustentacular Cells: Supporting glial-like cells that provide metabolic and structural support to the neuropil. [8]
The AOB expresses a distinctive set of molecular markers that distinguish it from the main olfactory bulb: [9]
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 4. [10]
AOB mitral and tufted cells project to several brain regions: [11]
The AOB is essential for processing pheromonal signals that drive species-typical behaviors: [12]
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 5. [13]
Aggression: Territorial and intermale aggression is mediated by AOB-processed pheromonal signals. Androgen-dependent urinary signals activate AOB circuits that modulate aggressive behavior. [14]
Parental Behavior: Pheromonal cues from pups activate AOB circuits in mothers, promoting nursing and pup-directed behaviors. The AOB shows increased activity during lactation. [15]
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. [16]
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.
Olfactory dysfunction is recognized as one of the earliest preclinical signs of Alzheimer's disease (AD), often preceding cognitive impairment by several years 6. The olfactory system, including both main and accessory olfactory bulbs, shows:
The AOB may be particularly vulnerable due to its unique neuronal properties and connectivity with limbic structures 7.
Hyposmia is a well-documented early sign of Parkinson's disease (PD), often preceding motor symptoms by years 8. Pathological changes in the olfactory system include:
The pattern of olfactory involvement in PD supports the "nose-to-brain" hypothesis of α-synuclein propagation, where pathology may originate in the olfactory epithelium and spread via the olfactory nerve to the brain 10.
Olfactory dysfunction is observed in several other neurodegenerative diseases:
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. In the AOB, new granule cells are continuously added and integrated into existing circuits.
This adult neurogenesis has several implications:
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. These cells have attracted interest for their therapeutic potential:
Olfactory assessment has emerged as a valuable clinical tool:
Standard tests include the University of Pennsylvania Smell Identification Test (UPSIT), Sniffin' Sticks, and Brief Smell Identification Test (B-SIT) 13.
Several strategies are being explored to preserve or restore olfactory function:
Aging is associated with progressive olfactory decline, affecting both detection and discrimination. Age-related changes in the AOB include:
These changes may compound pathological alterations in neurodegeneration, making older individuals more vulnerable to olfactory dysfunction 14.
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.
Research on the AOB in neurodegeneration continues to evolve:
Olfactory information processing in the accessory olfactory bulb. 2004. ↩︎
The vomeronasal organ. 1999. ↩︎
Targeting odorant receptors. 1996. ↩︎
Olfactory dysfunction in neurodegenerative diseases. 2019. ↩︎
Olfaction in Parkinson disease. 2012. ↩︎
Olfactory bulb involvement in neurodegenerative diseases. 2015. ↩︎
Olfactory bulb volume in PD. 2018. ↩︎
Vomeronasal organ aging. 2021. ↩︎