The brain of the African wild dog. II. The olfactory system.

Employing a range of neuroanatomical stains, we detail the organization of the main and accessory olfactory systems of the African wild dog. The organization of both these systems follows that typically observed in mammals, but variations of interest were noted. Within the main olfactory bulb, the size of the glomeruli, at approximately 350 µm in diameter, are on the larger end of the range observed across mammals. In addition, we estimate that approximately 3,500 glomeruli are present in each main olfactory bulb. This larger main olfactory bulb glomerular size and number of glomeruli indicates that enhanced peripheral processing of a broad range of odorants is occurring in the main olfactory bulb of the African wild dog. Within the accessory olfactory bulb, the glomeruli did not appear distinct, rather forming a homogenous syncytia-like arrangement as seen in the domestic dog. In addition, the laminar organization of the deeper layers of the accessory olfactory bulb was indistinct, perhaps as a consequence of the altered architecture of the glomeruli. This arrangement of glomeruli indicates that rather than parcellating the processing of semiochemicals peripherally, these odorants may be processed in a more nuanced and combinatorial manner in the periphery, allowing for more rapid and precise behavioral responses as required in the highly social group structure observed in the African wild dog. While having a similar organization to that of other mammals, the olfactory system of the African wild dog has certain features that appear to correlate to their environmental niche.

Morphology and physiology of olfactory neurons in the lateral protocerebrum of the silkmoth Bombyx mori.

Insect olfaction is a suitable model to investigate sensory processing in the brain. Olfactory information is first processed in the antennal lobe and is then conveyed to two second-order centres-the mushroom body calyx and the lateral protocerebrum. Projection neurons processing sex pheromones and plant odours supply the delta area of the inferior lateral protocerebrum (∆ILPC) and lateral horn (LH), respectively. Here, we investigated the neurons arising from these regions in the brain of the silkmoth, Bombyx mori, using mass staining and intracellular recording with a sharp glass microelectrode. The output neurons from the ∆ILPC projected to the superior medial protocerebrum, whereas those from the LH projected to the superior lateral protocerebrum. The dendritic innervations of output neurons from the ∆ILPC formed a subdivision in the ∆ILPC. We discuss pathways for odour processing in higher order centres.

Anatomical basis and physiological role of cerebrospinal fluid transport through the murine cribriform plate.

Cerebrospinal fluid (CSF) flows through the brain, transporting chemical signals and removing waste. CSF production in the brain is balanced by a constant outflow of CSF, the anatomical basis of which is poorly understood. Here, we characterized the anatomy and physiological function of the CSF outflow pathway along the olfactory sensory nerves through the cribriform plate, and into the nasal epithelia. Chemical ablation of olfactory sensory nerves greatly reduced outflow of CSF through the cribriform plate. The reduction in CSF outflow did not cause an increase in intracranial pressure (ICP), consistent with an alteration in the pattern of CSF drainage or production. Our results suggest that damage to olfactory sensory neurons (such as from air pollution) could contribute to altered CSF turnover and flow, providing a potential mechanism for neurological diseases.

Cranial Pair I: The Olfactory Nerve.

The olfactory nerve constitutes the first cranial pair. Compared with other cranial nerves, it depicts some atypical features. First, the olfactory nerve does not form a unique bundle. The olfactory axons join other axons and form several small bundles or fascicles: the fila olfactoria. These fascicles leave the nasal cavity, pass through the lamina cribrosa of the ethmoid bone and enter the brain. The whole of these fascicles is what is known as the olfactory nerve. Second, the olfactory sensory neurons, whose axons integrate the olfactory nerve, connect the nasal cavity and the brain without any relay. Third, the olfactory nerve is composed by unmyelinated axons. Fourth, the olfactory nerve contains neither Schwann cells nor oligodendrocytes wrapping its axons. But it contains olfactory ensheathing glia, which is a type of glia unique to this nerve. Fifth, the olfactory axons participate in the circuitry of certain spherical structures of neuropil that are unique in the brain: the olfactory glomeruli. Sixth, the axons of the olfactory nerve are continuously replaced and their connections in the central nervous system are remodeled continuously. Therefore, the olfactory nerve is subject to lifelong plasticity. Finally seventh, the olfactory nerve can be a gateway for the direct entrance of viruses, neurotoxins and other xenobiotics to the brain. In the same way, it can be used as a portal of entry to the brain for therapeutic substances, bypassing the blood-brain barrier. In this article, we analyze some features of the anatomy and physiology of the first cranial pair. Anat Rec, 302:405-427, 2019. © 2018 Wiley Periodicals, Inc.

Endonasal anatomy of the olfactory neural network: Surgical implications.

OBJECTIVES/HYPOTHESIS: Define the anatomic distribution of the olfactory filaments within specific mucosal regions of the nasal cavity. STUDY DESIGN: Cadaveric study. METHODS: Seventeen cadaveric specimens (34 sides) were dissected to study the anatomical distribution and density of olfactory fila within different regions of the nasal cavity. Olfactory fila were dissected retrogradely to their point of entry into the anterior cranial fossa through the cribriform plate. Anatomic relationships among various components of the olfactory system and their corresponding arterial supply were determined subjectively. RESULTS: The highest density of olfactory fila was found at the mucosa of the ethmoid roof and superior turbinates. Olfactory fila were found at regions not previously considered to be part of the olfactory system: lateral wall of the nose, ethmoidal bullae, and between the os sphenoidale and arc of the posterior choana. Furthermore, at the septum, 20% of the olfactory fila crossed contralaterally before exiting the nose. The anterior ethmoidal arteries were the primary blood supply to the olfactory epithelium. CONCLUSIONS: This study suggests that olfactory filaments extend beyond previously established boundaries. These findings may have clinical implications regarding oncologic resections and could serve as the foundation for the development of techniques that better preserve olfactory function. LEVEL OF EVIDENCE: NA Laryngoscope, 2473-2477, 2018.

Anatomy of the olfactory nerve: A comprehensive review with cadaveric dissection.

Because of their anatomical situation, the olfactory nerves constitute a natural obstacle to exploring the anterior cranial fossa, making them vulnerable to traumatic, tumor, or iatrogenic lesions. Consequently, accurate knowledge of their microsurgical anatomy is of particular importance for surgeons to ensure the correct execution of surgical procedures with minimal sequelae, the least functional deterioration, and better therapeutic results. This review describes the functional and microsurgical anatomy of the olfactory nerves, illustrated with pictures of simulations based on cadaveric dissections and original illustrations of the central connections of the olfactory pathway. Clin. Anat, 2017. © 2017 Wiley Periodicals, Inc.

Anatomic relationship of the first olfactory neuron and trochlea: cadaveric study with surgical implications.

BACKGROUND: Iatrogenic injury to the trochlea is a potential complication of the Draf III or endoscopic modified Lothrop procedure; however, the location of the trochlea from within the sinus cavity has yet to be clearly characterized. We performed the first cadaveric study assessing the position of the trochlea in relation to the first olfactory neuron, a commonly identified landmark during the Draf III procedure. METHODS: Thirteen external dissections of the trochlea were performed on 7 disarticulated cadaveric heads via an extended Lynch type incision. An endoscopic Draf III procedure was then performed on all cadavers. A burr hole was then created at the trochlear fovea. A straight vector between the trochlear attachment and the ipsilateral first olfactory neuron was then created. The vector was then viewed from within the sinuses using a 30-degree endoscope. The endoscope was maneuvered to create an upright midline view centered on the olfactory neurons. An image was subsequently captured and analyzed using ImageJ software to calculate a vector angle. The linear distance between the 2 structures was also recorded. RESULTS: Relative to the first olfactory neuron, the trochlea was found to be 19.52 ± 4.68 mm away and at an angle of 39.42 ± 8.54 degrees in the anterolateral, superior direction along the frontal bone. CONCLUSION: We report the first anatomical dissection characterizing the position of the trochlea relative to the ipsilateral first olfactory neuron. Intraoperative recognition of this relationship during Draf III procedures can prevent potential injury to the trochlea.

Number of olfactory receptor neurons in the Chinese soft-shelled turtle.

The olfactory organ of turtle consists of the upper chamber epithelium (UCE) and the lower chamber epithelium (LCE), detecting air-borne odorants and water-borne odorants, respectively. In this study, we investigated the number of olfactory receptor neurons (ORNs) in the UCE and LCE of soft-shelled turtle in order to find their possible differences among terrestrial, semi-aquatic and highly-aquatic turtles. The number of ORNs in the soft-shelled turtle was higher in the LCE than in the UCE, suggesting its close relationship to the environment the turtle lives. In addition, relative abundance of the ORNs in the LCE to the UCE varied in accordance with the size of individuals, although its functional significance remains elusive.

Las estrías olfatorias del ser humano y propuesta de inclusión en la terminologia anatomica de estría olfatoria intermedia / Olfactory stria in the human being and proposal for inclusion in terminologia anatomica of olfactory intermediate stria

Las estrías olfatorias del ser humano son dos divisiones del tracto olfatorio, tradicionalmente identificadas como medial y lateral, reconocibles a nivel del trígono olfatorio y asociadas con la corteza piriforme. El objetivo del presente trabajo fue estudiar tomando como referentes treinta y seis nervios olfatorios de cerebros frescos, la presencia de más de dos estrías olfatorias y verificar si hay estría olfatoria intermedia. Se concluye que las estrías olfatorias son variables en número de dos a tres y que se debe incluirse en la Terminologia Anatomica el término estría intermedia.

The human olfactory stria are two divisions of the olfactory tract, traditionally identified as medial and lateral olfactory stria, recognizable at the level of the olfactory trigone, and associated with the piriform cortex. The aim of this work was to study taking as reference thirty six fresh brain's olfactory nerves, the presence of more than two olfactory stria and check for intermediate olfactory stria. It is concluded that the olfactory stria are variable in number from two to three and this should be included in the Terminologia Anatomica as the term stria intermedia.

First olfactory fiber as an anatomical landmark for frontal sinus surgery.

OBJECTIVES/HYPOTHESIS: Access to the frontal sinuses is technically challenging owing to their anterosuperior location, diverse anatomy, close proximity to critical structures and the need to work in a relatively narrow space with angled-lens endoscopes and instruments. This study attempts to study the relationship of the first olfactory fiber with the frontal sinus posterior wall, assessing its fidelity as a surgical landmark during frontal sinus surgery. STUDY DESIGN: Anatomic study. METHODS: Fifteen cadaveric specimens were studied. Measurements were obtained bilaterally using the data from individual CT scans. Median A-P was defined as the anteroposterior (A-P) diameter measured just lateral to the intersinus septum, paramedian A-P was measured 5 mm lateral to the septum, and maximum A-P was defined as the maximum anteroposterior diameter on axial images. A surgical navigation device was used to calculate the distance between the first olfactory fiber and the posterior table of the frontal sinus. RESULTS: The mean distance between the first olfactory fiber and the posterior wall of the frontal sinus was (4.03 ± 2.7) mm on the right side and (4.2 ± 2.9) mm on the left. This distance strongly correlated with the maximum A-P diameter of the sinus. CONCLUSIONS: In a cadaveric model, the first olfactory fiber was found to be an average of 4.0 mm posterior to the frontal sinus. The significant variability of this distance should be considered when using the first olfactory fiber to establish the posterior boundary of a frontal sinusotomy. Drilling no further posterior than 7 mm rostral to the first olfactory fiber would be safe in 91% of patients. LEVEL OF EVIDENCE: NA Laryngoscope, 126:1039-1045, 2016.

Primer caso descrito de parálisis aislada, completa y fluctuante del III nervio craneal como forma de inicio de un mieloma múltiple / First case described of isolated, complete and fluctuating cranial nerve III palsy heralding multiple myeloma

Introducción. El mieloma múltiple es la neoplasia de células plasmáticas más frecuente. Al ser incurable, el tratamiento persigue obtener el mayor tiempo de supervivencia libre de clínica. Constituye una causa extremadamente rara de afectación de los nervios craneales y es producido habitualmente por un plasmocitoma intracraneal. Presentamos un caso de mieloma múltiple, que asociaba un plasmocitoma intracraneal y que comenzó clínicamente con parálisis aislada, completa y fluctuante del III nervio craneal. Caso clínico. Mujer de 63 años que acudió a urgencias por presentar un cuadro clínico oscilante, consistente en diplopía binocular horizontal y, posteriormente, cefalea. La exploración neurooftalmológica reveló una parálisis completa del III nervio craneal derecho. Se solicitó una tomografía axial computarizada craneal urgente, que reveló múltiples lesiones osteolíticas diploicas, asociando una de ellas componente de partes blandas en la hendidura esfenoidal derecha. La paciente fue ingresada, y se le diagnosticó posteriormente un mieloma múltiple IgA-k. Tras recibir inducción quimioterápica y ser sometida a un trasplante autólogo de progenitores hematopoyéticos, alcanzó la remisión completa. Conclusiones. El mieloma múltiple es un trastorno raro de los nervios craneales, una causa muy infrecuente de parálisis aislada y completa del III nervio craneal y menos aún fluctuante, y no se ha encontrado ningún caso publicado con este inicio clínico. Tener en cuenta las posibles manifestaciones neurooftalmológicas del mieloma múltiple puede contribuir a un diagnóstico precoz y a una incidencia positiva sobre el curso de esta enfermedad (AU)

Introduction. Multiple myeloma is the most common plasma-cell malignancy. To be incurable, treatment aims to obtain the longest non-clinical survival time. Cranial nerve palsy in multiple myeloma is extremely rare and is usually due to an intracranial plasmacytoma. We present a multiple myeloma case, with an intracranial plasmacytoma, which debuted clinically with isolated, complete and fluctuating cranial nerve III palsy. Case report. A 63-year-old woman presented an oscillating clinical picture, consisting of horizontal binocular diplopia and later, headache. The neuro-ophthalmologic examination revealed a complete cranial nerve III palsy of the right eye. An urgent cranial CT-scan was requested. It showed multiple diploic osteolytic lesions, associating soft-parts component in the right superior orbital fissure. The patient was admitted, being diagnosed subsequently of IgA-k multiple myeloma. After receiving induction-chemotherapy and undergoing autologous stem cell transplantation, she achieved full remission. Conclusions. Multiple myeloma is a rare cranial nerves disorder, very uncommon cause of cranial nerve III full isolated paralysis and even less fluctuating, not having found any case published with this clinical onset. Awareness of possible multiple myeloma neuro-ophthalmic manifestations may bring about an early diagnosis and a positive impact on the disease course (AU)

Comparative neuroanatomy of the antennal lobes of 2 homopteran species.

We compared the morphology of the primary olfactory center, the antennal lobe (AL), in 2 homopteran insects, Hyalesthes obsoletus Signoret (Homoptera: Cixiidae) and Scaphoideus titanus Ball (Homoptera: Cicadomorpha). The comparison between the ALs of the 2 species is particularly interesting considering that, although both use volatile cues to locate their host plants, their feeding behavior differs considerably: specifically, H. obsoletus is a highly polyphagous species, whereas S. titanus is strictly monophagous (on grapevine). Our investigation of the AL structure using immunocytochemical staining and antennal backfills did not reveal any sexual dimorphism in either the size of the ALs or in the size of individual glomeruli for either species. Instead, the AL of H. obsoletus displayed numerous and well-delineated glomeruli (about 130 in both sexes) arranged in a multilayered structure, whereas the smaller AL of S. titanus contained fewer than 15 glomerular-like structures. This difference is likely to reflect the comparatively reduced olfactory abilities in S. titanus, probably as a consequence of the reduced number of volatiles coming from the single host plant. Instead, in H. obsoletus, the ability to distinguish among several host plants may require a more complex olfactory neuronal network.

Computer-controlled stimulation for functional magnetic resonance imaging studies of the neonatal olfactory system.

AIM: Olfactory sensation is highly functional early in human neonatal life, with studies suggesting that odours can influence behaviour and infant-mother bonding. Due to its good spatial properties, blood oxygen level-dependent (BOLD) contrast functional magnetic resonance imaging (fMRI) has the potential to rapidly advance our understanding of the neural activity which underlies the development of olfactory perception in this key period. We aimed to design an 'olfactometer' specifically for use with neonatal subjects for fMRI studies of odour perception. METHODS: We describe a fully automated and programmable, fMRI compatible system capable of presenting odorant liquids. To prevent contamination of the system and minimize between-subject infective risk, the majority of the olfactometer is constructed from single-use, readily available clinical equipment. The system was used to present the odour of infant formula milk in a validation group of seven neonatal subjects at term equivalent postmenstrual age (median age 40 weeks). RESULTS: A safe, reliable and reproducible pattern of stimulation was delivered leading to well-localized positive BOLD functional responses in the piriform cortex, amygdala, thalamus, insular cortex and cerebellum. CONCLUSIONS: The described system is therefore suitable for detailed studies of the ontology of olfactory sensation and perception during early human brain development.

The role of Caspar Bartholin the Elder in the evolution of the terminology of the cranial nerves.

In the 17th century, the Bartholin family contributed greatly to the advancement of anatomical and medical science. Caspar Bartholin the Elder (1585-1629) introduced the terms nervus olfactorius and nervus vagus in 1611 in the Institutiones Anatomicae. Thomas Bartholin (1616-1680) extended their use to figures added to later revised editions of Institutiones Anatomicae.

Nuclear imaging with radiolabeled odor molecules in patients with olfactory disorder.

Smell loss originates from peripheral disorders, like intranasal obstruction and olfactory cell injury, as well as central pathway diseases. Information derived from electrophysiological and psychophysical tests are useful for identifying loss of smell, but not for discriminating between central and peripheral deficits. This is because conventional imaging modalities are unable to deliver information about functional olfactory performance. Although functional imaging is able to show abnormal changes in central olfactory pathways, it seems that it is only possible to observe such abnormalities in olfactory cell dysfunction. We hypothesize that the scanning of peripheral olfactory systems by radiolabeled odor molecules may specifically reveal olfactory dysfunction and may be useful for differentiating peripheral from central olfactory disorders.

Strength testing of the human olfactory nerve at the frontal skull base.

Olfactory dysfunction may influence the quality of life tremendously. This study investigated the strength of the human olfactory nerve at the frontal skull base using cadavers. A total of 180 olfactory nerves were examined in 90 human cadaveric heads. The cut edges of the olfactory nerves were pulled until they were pulled out from the skull base. In the first set of 30 cases, each right olfactory nerve was pulled 0° laterally and 0° upward, and each left olfactory nerve was pulled 0° laterally and 15° upward. In the second set of 30 cases, each right olfactory nerve was pulled 0° laterally and 15° upward, and each left olfactory nerve was pulled 15° laterally and 15° upward. In the third set of 30 cases, each right olfactory nerve was pulled 15° laterally and 15° upward, and each left olfactory nerve was pulled 30° laterally and 15° upward. The strength of the olfactory nerve was measured when pulled in each direction. There was no significant difference in the strength of the olfactory nerves when pulling them in the postero-upward direction between 0° and 15° upward. The strengths of the olfactory nerves when pulling them in the postero-lateral direction 0° and 15° laterally were 3.14±1.87 and 4.05±1.70 g (mean ± standard deviation [SD]), respectively; the difference was almost significant. The olfactory nerve could be pulled more laterally than posteriorly because the retraction force is absorbed by the lateral wall of the olfactory fossa.

Microsurgical anatomy for intraoperative preservation of the olfactory bulb and tract.

Damage to the olfactory bulb and tract is a frequently described complication of brain surgery in the frontal region, and it seems to be influenced by the surgical approaches. Eighty cerebral hemispheres and 5 formalin-fixed cadavers filled with colored latex were used. Parameters were directly measured, and after olfactory bulb and tract were mobilized with careful dissections, retraction of the frontal lobe was noted. The anterior border of the olfactory bulb is 22.21 (SD, 5.45) mm posterior to the frontomarginal sulcus, and arachnoidal dissection should be performed parallel to olfactory structures using sharp instruments to allow early visualization. Overall mobilization of the olfactory bulb and tract as 29.3 (SD, 6.4) mm in length is possible without disrupting the structures and enables a greater degree of the frontal-lobe elevation window up to 13.1 (SD, 3.2) mm. Using the morphometric data and anatomic knowledge may prevent unwanted anosmia complication during surgical approaches.

Alteraciones olfativas como diagnóstico precoz de la poliposis nasosinusal incipiente / Smell disorders as early diagnosis in the early stage of Sinonasal Polyposis

Introducción: La creciente demanda de atención sobre la patología del olfato, junto con la persistente presencia de la poliposis nasosinusal, ha abierto la necesidad de tratar dichas patologías a niveles muy tempranos. Objetivos: Demostrar primero que los estadios incipientes de la poliposis nasosinusal son detectables por olfatometría antes que por imagen radiológica; segundo, tal detección se halla vinculada a la ausencia de ocupación del espacio «extrameatal» (donde se halla la hendidura olfativa). Métodos: Estudio basado en los datos obtenidos a partir de un grupo (n=121) de pacientes con poliposis nasosinusal de grado 0 o grado 1, libres de alergias, tríada de ASA y asma, sometidos a estudio fibroendoscópico, olfatometría (par craneal I y V) y TAC nasosinusal, comparándose el resultado con un grupo control (n=120). Resultados: Se confirmaron valores significativos (p<0,05) de afectación de los nervios olfatorio y trigémino, en los pacientes con grado 0 y grado 1 de poliposis. Conclusiones: La presencia de alteraciones olfativas con ausencia de ocupación del espacio «extrameatal» (poliposis grado 1 o grado 2), acompañado de TAC nasosinusal con inicio de ocupación etmoidal, debería ser interpretado como inicio de poliposis nasosinusal (AU)

Introduction: The increasing demand for attention to olfactory disorders, along with the persistent presence of sinonasal polyposis, has opened the need to treat these pathologies in very early stages. Objectives: To demonstrate that incipient stages in sinonasal polyposis are detectable by olfactometry before radiological images, and that this detection is linked with a non-blocked area around the nasal meatus (where the olfactory cleft is located). Methods: This study is based on data obtained from a sinonasal polyposis (degree 0 or 1) patient group (n=121) without allergies or asthma backgrounds. The patients underwent both fibroscopic and olfactometry explorations (first and fifth cranial nerve) and computed axial tomography (CT) assessment. The results were compared with the control group (n=120). Results: Significant values (p<0.05) of affectation were found in decreasing olfactory levels (olfactory and trigeminal nerves) in patients with degree 0 or 1 polyposis. Conclusion: Olfactory disorders linked to a non-blocked area around the nasal meatus (degree 1 or 2 polyposis), together with sinonasal CT scans showing beginnings of ethmoidal inflammation, should be interpreted as incipient sinonasal polyposis (AU)

Endoscopic endonasal anatomy and approaches to the anterior skull base: a neurosurgeon's viewpoint.

OBJECTIVES: The objective of this study was to review the endoscopic anatomy of the anterior skull base, defining the pitfalls of endoscopic endonasal approaches to this region. Recently, these approaches are gaining popularity among neurosurgeons, and the details of the endoscopic anatomy and approaches are highlighted from the neurosurgeons' point of view, correlated with demonstrative cases. MATERIALS AND METHODS: Twelve fresh adult cadavers were studied (n = 12). We used Karl Storz 0 and 30 degrees, 4 mm, 18- and 30-cm rod lens rigid endoscope in our dissections. After preparation of the cadaveric specimens, we approached the anterior skull base by the extended endoscopic endonasal approach. RESULTS: After resection of the superior portion of the nasal septum, bilateral middle and superior turbinates, and bilateral anterior and posterior ethmoidal cells, we could obtain full exposure of the anterior skull base. The distance between optic canal and the posterior ethmoidal artery ranged from 8 to 16 mm (mean, 11.08 mm), and the distance between posterior ethmoidal artery and the anterior ethmoidal artery ranged from 10 to 17 mm (mean, 13 mm). After resecting the anterior skull base bony structure and the dura between the 2 medial orbital walls, we could visualize the olfactory nerves, interhemispheric sulcus, and gyri recti. With dissecting the interhemispheric sulcus, we could expose the first (A1) and second (A2) segments of the anterior cerebral artery, anterior communicating artery, and Heubner arteries. CONCLUSIONS: This study showed that extended endoscopic endonasal approaches are sufficient in providing wide exposure of the bony structures, and the extradural and intradural components of the anterior skull base and the neighboring structures providing more controlled manipulation of pathologic lesions. These approaches need specific skill and learning curve to achieve more minimally invasive interventions and less postoperative complications.

PHR1 is a vesicle-bound protein abundantly expressed in mature olfactory neurons.

OBJECTIVES/HYPOTHESIS: To characterize the role of Phr1, a gene highly expressed in primary sensory neurons where it encodes an integral membrane protein with an N-terminal pleckstrin homology domain and a C-terminal transmembrane domain, in the olfactory system. METHODS: We studied the immunelocalization of the PHR1 protein in mouse olfactory epithelium both at steady state and during regeneration following methyl bromide (MeBr) exposure using scanning confocal microscopy. Additionally, we examined the electrophysiologic role of Phr1 in olfaction and short-term olfactory adaptation. RESULTS: We found that PHR1 is abundantly and specifically expressed in olfactory neurons. It is widely distributed in punctate, vesiculated organelles throughout the cell bodies, axons, and glomeruli of primary olfactory neurons but is specifically excluded from the olfactory cilia. In the regenerating olfactory epithelium, PHR1 expression appears at 14 days following MeBr ablation coinciding with the onset of olfactory neuron maturity. Despite the abundant and specific expression throughout the olfactory neurons, mice lacking Phr1 did not exhibit differences in the distribution of the components of olfactory signal transduction system, the rate of olfactory regeneration following MeBr exposure, olfactory function, or short-term adaptation to odors. CONCLUSIONS: Phr1 is widely and abundantly expressed throughout mature olfactory neurons and other primary sensory neurons, but its absence does not appear to affect olfactory morphology, regeneration, sensory function, or adaptation. The exact function of Phr1 remains to be discovered.