Most mammals discern various chemical signals in the environment so that they may adapt their behaviour and response in order to improve their chances of survival. Although many higher order organisms utilize both olfaction and gustation for chemosensitivity, majority of animals rely on the former as the primary chemosensory modality (Reviewed from Lledo et al, 2005).
The mammalian olfactory system controls a plethora of functions such as homeostasis, emotions such as fear, anger, pleasure and anxiety, sexual and maternal functions as well as communal behaviours such as distinguishing between tribe or family and outsiders (Lledo et al, 2005).
Rodents have a separate organ known as the vomeronasal organ to detect chemical compounds such as pheromones and transmit information via the accessory olfactory bulb to communicate and encode social status or reproductive viability via integration of this organ’s activity precisely to sex and genetic makeup (Luo et al, 2003). The existence of this organ in humans, till date is widely debated and not well understood. Odorants are distinguished by approximately 1000 types of odorant receptors (ORs) which are expressed on the olfactory cilia of olfactory sensory neurons (OSNs). OSNs only express a single variant of OR and similar OSNs project their axons through the cribriform plate to the glomerulus, which is a spherical structure found in the olfactory bulb (OB). OSNs are part of the olfactory neuroepithelium alongside sustentacular cells (for structural support) and basal cells (stem cells by which new OSNs are generated). In the glomerulus, odor related information are transmitted to mitral/tufted (MT) cells which are second order neurons. These neurons are extensively regulated by intrabulbar circuits as well as centrifugal inputs. This allows for the establishment of a unique odor code. In addition to this, due to about 1000 different circuits from various ORs, the OB indeed has to contain an intricate circuitry to accommodate and adapt to the different needs of the rodent. Overview of OB circuitry Information from the OSNs are first discerned by the glomeruli of the OB. OSNs displaying a specific type of OR will project their axons to two glomeruli in the OB (medial and lateral side). OSNs can release neurotransmitter (glutamate) onto postsynaptic glutamatergic M/T cells, periglomerular (PG) as well as GABAergic and dopaminergic short axon (SA) cells (Figure 1A), both of which are located in the juxtaglomerular region. A single glomerulus may be innervated by the primary dendrites of 20-50 M/T cells, of which 5-20 may be mitral. M/T cells may also innervate surrounding GABAergic interneurons. A single PG cell usually only innervates a single glomerulus, and can be activated by OSNs and M/T cells, and in turn inhibit these neurons and neighbouring PG cells via GABAergic inhibition. The inhibitory synapses are reciprocal in nature, where both pre and post synapses are in close proximity, with GABA release being …show more content…
This now begs the question: Can plasticity in the rodent olfactory system be driven by odor-dependent structural and physiological changes?
Plasticity in the Rodent Olfactory System
The olfactory system is innervated by a surfeit of modulatory systems and a profound capability for information storage. This enables its circuitry to display both short and long term synaptic plasticity, experience-dependent neurogenesis and morphological changes in dendrites throughout development.
Short Term Plasticity
Short term plasticity has been noted to occur in the OB, which may be crucial for adaptation and sustainability towards repeated and extended durations as well as perhaps for calibration of sensory systems to cope with various situations such as erratic odours.
For example, when a short stimulus was presented to the OB, both spatial and temporal local patterns of OB circuitry were altered, suggesting a change in strength of synaptic transmission (Spors and Grinvald, 2002). The axon terminals of OR express D2 receptors (D2Rs) and neighbouring juxtaglomerular neurons express dopamine. When the D2Rs are activated in the OB, transmitter release from the OR axons are attenuated, thus decreasing afferent input gain (Reviewed from Wilson et al, …show more content…
Consistently, another investigation (Gutièrrez-Mecinas et al, 2005) which elucidated D2 anatomical distributions in the olfactory glomeruli found it agreeable that D2 receptors act pre-synaptically to regulate glutamate release from the olfactory onto the dendrites M/T cells and PG cells, regulate the glutamatergic synapses of these dendrites and control the strength of neurotransmission from GABAergic and dopaminergic PG cells onto M/T
The mammalian olfactory system controls a plethora of functions such as homeostasis, emotions such as fear, anger, pleasure and anxiety, sexual and maternal functions as well as communal behaviours such as distinguishing between tribe or family and outsiders (Lledo et al, 2005).
Rodents have a separate organ known as the vomeronasal organ to detect chemical compounds such as pheromones and transmit information via the accessory olfactory bulb to communicate and encode social status or reproductive viability via integration of this organ’s activity precisely to sex and genetic makeup (Luo et al, 2003). The existence of this organ in humans, till date is widely debated and not well understood. Odorants are distinguished by approximately 1000 types of odorant receptors (ORs) which are expressed on the olfactory cilia of olfactory sensory neurons (OSNs). OSNs only express a single variant of OR and similar OSNs project their axons through the cribriform plate to the glomerulus, which is a spherical structure found in the olfactory bulb (OB). OSNs are part of the olfactory neuroepithelium alongside sustentacular cells (for structural support) and basal cells (stem cells by which new OSNs are generated). In the glomerulus, odor related information are transmitted to mitral/tufted (MT) cells which are second order neurons. These neurons are extensively regulated by intrabulbar circuits as well as centrifugal inputs. This allows for the establishment of a unique odor code. In addition to this, due to about 1000 different circuits from various ORs, the OB indeed has to contain an intricate circuitry to accommodate and adapt to the different needs of the rodent. Overview of OB circuitry Information from the OSNs are first discerned by the glomeruli of the OB. OSNs displaying a specific type of OR will project their axons to two glomeruli in the OB (medial and lateral side). OSNs can release neurotransmitter (glutamate) onto postsynaptic glutamatergic M/T cells, periglomerular (PG) as well as GABAergic and dopaminergic short axon (SA) cells (Figure 1A), both of which are located in the juxtaglomerular region. A single glomerulus may be innervated by the primary dendrites of 20-50 M/T cells, of which 5-20 may be mitral. M/T cells may also innervate surrounding GABAergic interneurons. A single PG cell usually only innervates a single glomerulus, and can be activated by OSNs and M/T cells, and in turn inhibit these neurons and neighbouring PG cells via GABAergic inhibition. The inhibitory synapses are reciprocal in nature, where both pre and post synapses are in close proximity, with GABA release being …show more content…
This now begs the question: Can plasticity in the rodent olfactory system be driven by odor-dependent structural and physiological changes?
Plasticity in the Rodent Olfactory System
The olfactory system is innervated by a surfeit of modulatory systems and a profound capability for information storage. This enables its circuitry to display both short and long term synaptic plasticity, experience-dependent neurogenesis and morphological changes in dendrites throughout development.
Short Term Plasticity
Short term plasticity has been noted to occur in the OB, which may be crucial for adaptation and sustainability towards repeated and extended durations as well as perhaps for calibration of sensory systems to cope with various situations such as erratic odours.
For example, when a short stimulus was presented to the OB, both spatial and temporal local patterns of OB circuitry were altered, suggesting a change in strength of synaptic transmission (Spors and Grinvald, 2002). The axon terminals of OR express D2 receptors (D2Rs) and neighbouring juxtaglomerular neurons express dopamine. When the D2Rs are activated in the OB, transmitter release from the OR axons are attenuated, thus decreasing afferent input gain (Reviewed from Wilson et al, …show more content…
Consistently, another investigation (Gutièrrez-Mecinas et al, 2005) which elucidated D2 anatomical distributions in the olfactory glomeruli found it agreeable that D2 receptors act pre-synaptically to regulate glutamate release from the olfactory onto the dendrites M/T cells and PG cells, regulate the glutamatergic synapses of these dendrites and control the strength of neurotransmission from GABAergic and dopaminergic PG cells onto M/T