Nasal Toxicity of Manganese Sulfate and Manganese Phosphate in Young Male Rats Following Subchronic (13-Week) Inhalation Exposure

Growing evidence suggests that nasal deposition and transport along the olfactory nerve represents a route by which inhaled manganese and certain other metals are delivered to the rodent brain. The toxicological significance of olfactory transport of manganese remains poorly defined. In rats, repeated intranasal instillation of manganese chloride results in injury to the olfactory epithelium and neurotoxicity as evidenced by increased glial fibrillary acidic protein (GFAP) concentrations in olfactory bulb astrocytes. The purpose of the present study was to further characterize the nasal toxicity of manganese sulfate (MnSO₄) and manganese phosphate (as hureaulite) in young adult male rats following subchronic (90-day) exposure to air, MnSO₄ (0.01, 0.1, and 0.5 mg Mn/m³), or hureaulite (0.1 mg Mn/m³). Nasal pathology, brain GFAP levels, and brain manganese concentrations were assessed immediately following the end of the 90-day exposure and 45 days thereafter. Elevated end-of-exposure olfactory bulb, striatum, and cerebellum manganese concentrations were observed following MnSO₄ exposure to ≥0.01, ≥0.1, and 0.5 mg Mn/m³, respectively. Exposure to MnSO₄ or hureaulite did not affect olfactory bulb, cerebellar, or striatal GFAP concentrations. Exposure to MnSO₄ (0.5 mg Mn/m³) was also associated with reversible inflammation within the nasal respiratory epithelium, while the olfactory epithelium was unaffected by manganese inhalation. These results confirm that high-dose manganese inhalation can result in nasal toxicity (irritation) and increased delivery of manganese to the brain; however, we could not confirm that manganese inhalation would result in altered brain GFAP concentrations.     

Drug Targeting to the Brain: Transfer of Picolinic Acid Along the Olfactory Pathways

Picolinic acid (PA) protects against quinolinic acid- and kainic acid-induced neurotoxicity in the brain. To study the uptake of PA to the brain, we administered [3 H]PA via a unilateral nasal instillation or iv injection to mice. Autoradiography demonstrated a rapid uptake of radioactivity in the olfactory nerve layer and in the ipsilateral olfactory bulb (OB) following nasal instillation of [3 H]PA. After 4 h, there was a high level of radioactivity in the central parts of the ipsilateral OB and olfactory peduncle. Moreover, iv injection of [3 H]PA demonstrated a selective uptake and retention of radioactivity in the OB. Gas chromatography-mass spectrometry (GC-MS) demonstrated the presence of PA and PA-glycine conjugate in the OB. In mice with reduced peripheral olfactory innervations there was a decreased uptake of [3 H]PA in the OB as compared to controls suggesting that an intact olfactory neuroepithelium is a prerequisite for an uptake of PA to the OB. There is an increased interest in brain targeting of drugs with limited ability to pass the blood-brain barrier. The present results demonstrate that PA fulfils structural requirements for a transfer along the olfactory pathways to the brain.     

Deposition of inhaled nanoparticles in the rat nasal passages: Dose to the olfactory region

In vivo experiments have shown that nanoparticles depositing in the rat olfactory region can translocate to the brain via the olfactory nerve. Quantitative predictions of the dose delivered by inhalation to the olfactory region are needed to clarify this route of exposure and to evaluate the dose-response effects of exposure to toxic nanoparticles. Previous in vivo and in vitro studies quantified the percentage of inhaled nanoparticles that deposit in the rat nasal passages, but olfactory dose was not determined. The dose to specific nasal epithelium types is expected to vary with inhalation rate and particle size. The purpose of this investigation, therefore, was to develop estimates of nanoparticle deposition in the nasal and, more specifically, olfactory regions of the rat. A three-dimensional, anatomically accurate, computational fluid dynamics (CFD) model of the rat nasal passages was employed to simulate inhaled airflow and to calculate nasal deposition efficiency. Particle sizes from 1 to 100?nm and airflow rates of 288, 432, and 576 ml/min (1, 1.5, and 2 times the estimated resting minute volume) were simulated. The simulations predicted that olfactory deposition is maximum at 6–9% of inhaled material for 3- to 4-nm particles. The spatial distribution of deposited particles was predicted to change significantly with particle size, with 3-nm particles depositing mostly in the anterior nose, while 30-nm particles were more uniformly distributed throughout the nasal passages.     

Olfactory transport: a direct route of delivery of inhaled manganese phosphate to the rat brain

Experiments examining the dosimetry of inhaled manganese generally focus on pulmonary deposition and subsequent delivery of manganese in arterial blood to the brain. Growing evidence suggests that nasal deposition and transport along olfactory neurons represents another route by which inhaled manganese is delivered to certain regions of the rat brain. The purpose of this study was to evaluate the olfactory uptake and direct brain delivery of inhaled manganese phosphate ((54)MnHPO(4)). Male, 8-wk-old, CD rats with either both nostrils patent or the right nostril occluded underwent a single, 90-min, nose-only exposure to a (54)MnHPO(4) aerosol (0.39 mg (54)Mn/m(3); MMAD 1.68 microm, sigma(g) 1.42). The left and right sides of the nose, olfactory pathway, striatum, cerebellum, and rest of the brain were evaluated immediately after the end of the (54)MnHPO(4) exposure and at 1, 2, 4, 8, and 21 d postexposure with gamma spectrometry and autoradiography. Rats with two patent nostrils had equivalent (54)Mn concentrations on both sides of the nose, olfactory bulb, and striatum, while asymmetrical (54)Mn delivery occurred in rats with one occluded nostril. High levels of (54)Mn activity were observed in the olfactory bulb and tubercle on the same side (i.e., ipsilateral) to the open nostril within 1-2 d following (54)MnHPO(4) exposure, while brain and nose samples on the side ipsilateral to the nostril occlusion had negligible levels of (54)Mn activity. Our results demonstrate that the olfactory route contributes to (54)Mn delivery to the rat olfactory bulb and tubercle. However, this pathway does not significantly contribute to striatal (54)Mn concentrations following a single, short-term inhalation exposure to (54)MnHPO(4).     

Visualization of In Vivo Olfactory Uptake and Transfer Using Fluorescein Dextran

Nasal administration of a 3 kDa fluorescein dextran (FD3) solution to rats resulted in transcellular absorption across the olfactory epithelium and transfer to the olfactory bulb within 15 min. After entering the lamina propria, FD3 was transferred in the connective tissue surrounding the olfactory nerve bundles to the olfactory bulb of the brain. More FD3 was absorbed across the olfactory epithelium than across the respiratory epithelium and to the nasal associated lymphoid tissue. Further, the amount of FD3 crossing the olfactory epithelium was region-dependent, with higher amounts absorbed in the turbinates than in the nasal septum. Plastic embedding and sectioning followed by fluorescence microscopy, enabled simultaneous visualization of FD3 in the mucosa and olfactory bulb, as well as the opportunity to store the tissue blocks for a prolonged period of time.     

Olfactory deposition of inhaled nanoparticles in humans

Abstract

Context: Inhaled nanoparticles can migrate to the brain via the olfactory bulb, as demonstrated in experiments in several animal species. This route of exposure may be the mechanism behind the correlation between air pollution and human neurodegenerative diseases, including Alzheimer’s disease and Parkinson’s disease.

Objectives: This article aims to (i) estimate the dose of inhaled nanoparticles that deposit in the human olfactory epithelium during nasal breathing at rest and (ii) compare the olfactory dose in humans with our earlier dose estimates for rats.

Materials and methods: An anatomically-accurate model of the human nasal cavity was developed based on computed tomography scans. The deposition of 1–100?nm particles in the whole nasal cavity and its olfactory region were estimated via computational fluid dynamics (CFD) simulations. Our CFD methods were validated by comparing our numerical predictions for whole-nose deposition with experimental data and previous CFD studies in the literature.

Results: In humans, olfactory dose of inhaled nanoparticles is highest for 1–2?nm particles with ～1% of inhaled particles depositing in the olfactory region. As particle size grows to 100?nm, olfactory deposition decreases to 0.01% of inhaled particles.

Discussion and conclusion: Our results suggest that the percentage of inhaled particles that deposit in the olfactory region is lower in humans than in rats. However, olfactory dose per unit surface area is estimated to be higher in humans in the 1--7?nm size range due to the larger inhalation rate in humans. These dose estimates are important for risk assessment and dose-response studies investigating the neurotoxicity of inhaled nanoparticles.     

Uptake of cobalt from the nasal mucosa into the brain via olfactory pathways in rats

In the olfactory epithelium the primary olfactory neurons are in contact with the environment in the nasal cavity and they are also connected to the olfactory bulbs of the brain. These neurons may therefore provide a pathway by which foreign materials may reach the brain. Inhalation of cobalt-containing dust or fumes occurs in several workplaces, which may result in high exposure of the nasal tissues. In the present study, we used autoradiography and gamma-spectrometry to examine the transport of cobalt in the olfactory system after intranasal administration of 57Co2+ in rats. The results showed an uptake of the metal in the olfactory mucosa and a transport to the olfactory bulbs of the brain. The metal accumulated in the olfactory nerve layer and the terminals of the primary olfactory neurons in the glomerular layer of the bulb. In addition, low levels of cobalt were seen to migrate into the interior of the bulbs and the anterior parts of the olfactory cortex, indicating that the metal is able to leave the terminals of the primary olfactory neurons. Occupational exposure to cobalt, which is a neurotoxic metal, occurs in several workplaces, e.g. the hard metal industry. Memory deficits have been observed among workers exposed to hard metal via inhalation, and it was considered that cobalt may be the neurotoxic component of the hard metal. We propose that inhaled hard metal (as a dust powder or in a mist form) is deposited in the nasal passages and that released cobalt, after uptake into the brain via the olfactory pathway, may cause neurotoxicity. We consider that the olfactory route of entry of cobalt into the brain may be important and should be taken into account when risk assessments are performed concerning occupational inhalation of this metal.     

Direct olfactory transport of inhaled manganese ((54)MnCl(2)) to the rat brain: toxicokinetic investigations in a unilateral nasal occlusion model

Inhalation exposure of humans to high concentrations of manganese (Mn) is associated with elevated Mn levels in the basal ganglia and an extrapyramidal movement disorder. In the rat, direct olfactory transport of Mn from the nose to the brain has been demonstrated following intranasal instillation of (54)MnCl(2). However, the contribution this route makes to brain Mn delivery following inhalation is unknown and was the subject of our study. Male 8-week old CD rats underwent a single 90-min nose-only exposure to a (54)MnCl(2) aerosol (0.54 mg Mn/m(3); MMAD 2.51 microm). The left and right sides of the nose and brain, including the olfactory pathway and striatum, were sampled at 0, 1, 2, 4, and 8 days postexposure. Control rats were exposed to (54)MnCl(2) with both nostrils patent to evaluate the symmetry of Mn delivery. Another group of rats had the right nostril plugged to prevent nasal deposition of (54)MnCl(2) on the occluded side. Gamma spectrometry (n = 6 rats/group/time point) and autoradiography (n = 1 rat/group/time point) were used to compare the levels of (54)Mn found on the left and right sides of the nose and brain to determine the contribution of olfactory uptake to brain (54)Mn levels. Brain and nose samples from the side with the occluded nostril had negligible levels of (54)Mn activity, validating the nasal occlusion procedure. High levels of (54)Mn were observed in the olfactory bulb and tract/tubercle on the side or sides with an open nostril within 1-2 days following inhalation exposure. These results demonstrated, for the first time, that the olfactory route contributes the majority (up to >90%) of the (54)Mn found in the olfactory pathway, but not in the striatum, of the rat brain up to 8 days following a single inhalation exposure. These findings suggest that the olfactory route may make a significant contribution to brain Mn levels following inhalation exposure in the rat.     

Inhaled iron, unlike manganese, is not transported to the rat brain via the olfactory pathway

Iron and manganese share structural, biochemical, and physiological similarities. The objective of this study was to determine whether iron, like manganese, is transported to the rat brain via the olfactory tract following inhalation exposure. Eight-week-old male CD rats were exposed to approximately 0.31 mg Fe per m(3) (mass median aerodynamic diameter = 2.99 microm; geometric standard deviation = 1.15) via inhalation for a target duration of 90 min. Following exposure, rats were euthanized immediately (0) or at 1, 2, 4, 8, or 21 days postexposure. In addition to nasal and regional brain tissues, blood, and viscera were also collected. 59Fe concentrations were determined by gamma spectrometry. Further, heads were collected and frozen, and autoradiograms were prepared to visualize the location of 59Fe from the nose to the brain. Finally, olfactory mucosa samples collected at 0, 2, 4, and 21 days postexposure were further analyzed using high-performance liquid chromatography (HPLC) plus gamma spectroscopy to determine the association between 59Fe and transferrin. Data obtained from gamma spectrometry revealed that most of the iron remained in the nasal regions of the olfactory system and that less than 4% of iron deposited on the olfactory mucosa was observed in the olfactory bulb. Autoradiograms confirmed the data obtained from gamma spectrometry. 59Fe activity was absent in the olfactory regions of the brain even 4 days postexposure. Further, HPLC-gamma spectroscopy analyses indicated that 59Fe in the olfactory mucosa was coeluted with transferrin. Hence iron, unlike manganese, is not readily transported to the brain via the olfactory tract.     

OLFACTORY TRANSPORT: A DIRECT ROUTE OF DELIVERY OF INHALED MANGANESE PHOSPHATE TO THE RAT BRAIN

Experiments examining the dosimetry of inhaled manganese generally focus on pulmonary deposition and subsequent delivery of manganese in arterial blood to the brain. Growing evidence suggests that nasal deposition and transport along olfactory neurons represents another route by which inhaled manganese is delivered to certain regions of the rat brain. The purpose of this study was to evaluate the olfactory uptake and direct brain delivery of inhaled manganese phosphate ( 54 MnHPO 4 ). Male, 8-wk-old, CD rats with either both nostrils patent or the right nostril occluded underwent a single, 90-min, nose-only exposure to a 54 MnHPO 4 aerosol (0.39 mg 54 Mn/m 3 ; MMAD 1.68 w m, σ g 1.42). The left and right sides of the nose, olfactory pathway, striatum, cerebellum, and rest of the brain were evaluated immediately after the end of the 54 MnHPO 4 exposure and at 1, 2, 4, 8, and 21 d postexposure with gamma spectrometry and autoradiography. Rats with two patent nostrils had equivalent 54 Mn concentrations on both sides of the nose, olfactory bulb, and striatum, while asymmetrical 54 Mn delivery occurred in rats with one occluded nostril. High levels of 54 Mn activity were observed in the olfactory bulb and tubercle on the same side (i.e., ipsilateral) to the open nostril within 1-2 d following 54 MnHPO 4 exposure, while brain and nose samples on the side ipsilateral to the nostril occlusion had negligible levels of 54 Mn activity. Our results demonstrate that the olfactory route contributes to 54 Mn delivery to the rat olfactory bulb and tubercle. However, this pathway does not significantly contribute to striatal 54 Mn concentrations following a single, short-term inhalation exposure to 54 MnHPO 4 .     

Demonstration of Nucleoside Transporter Activity in the Nose-to-Brain Distribution of [<Superscript>18</Superscript>F]Fluorothymidine Using PET Imaging

To evaluate the role of nucleoside transporters in the nose-to-brain uptake of [¹⁸F]fluorothymidine (FLT), an equilibrative nucleoside transporter (ENT1,2) and concentrative nucleoside transporter (CNT1–3) substrate, using PET to measure local tissue concentrations. Anesthetized Sprague-Dawley rats were administered FLT by intranasal (IN) instillation or tail-vein injection (IV). NBMPR (nitrobenzylmercaptopurine riboside), an ENT1 inhibitor, was administered either IN or intraperitoneally (IP). Dynamic PET imaging was performed for up to 40 min. A CT was obtained for anatomical co-registration and attenuation correction. Time-activity curves (TACs) were generated for the olfactory bulb (OB) and remaining brain, and the area-under-the-curve (AUC) for each TAC was calculated to determine the total tissue exposure of FLT. FLT concentrations were higher in the OB than in the rest of the brain following IN administration. IP administration of NBMPR resulted in increased OB and brain FLT exposure following both IN and IV administration, suggesting that NBMPR decreases the clearance rate of FLT from the brain. When FLT and NBMPR were co-administered IN, there was a decrease in the OB AUC while an increase in the brain AUC was observed. The decrease in OB exposure was likely the result of inhibition of ENT1 uptake activity in the nose-to-brain transport pathway. FLT distribution patterns show that nucleoside transporters, including ENT1, play a key role in the distribution of transporter substrates between the nasal cavity and the brain via the OB.     

Translocation of Inhaled Ultrafine Particles to the Brain

Ultrafine particles (UFP, particles <100 nm) are ubiquitous in ambient urban and indoor air from multiple sources and may contribute to adverse respiratory and cardiovascular effects of particulate matter (PM). Depending on their particle size, inhaled UFP are efficiently deposited in nasal, tracheobronchial, and alveolar regions due to diffusion. Our previous rat studies have shown that UFP can translocate to interstitial sites in the respiratory tract as well as to extrapulmonary organs such as liver within 4 to 24 h postexposure. There were also indications that the olfactory bulb of the brain was targeted. Our objective in this follow-up study, therefore, was to determine whether translocation of inhaled ultrafine solid particles to regions of the brain takes place, hypothesizing that UFP depositing on the olfactory mucosa of the nasal region will translocate along the olfactory nerve into the olfactory bulb. This should result in significant increases in that region on the days following the exposure as opposed to other areas of the central nervous system (CNS). We generated ultrafine elemental ¹³C particles (CMD = 36 nm; GSD = 1.66) from [¹³C] graphite rods by electric spark discharge in an argon atmosphere at a concentration of 160 μg/m³. Rats were exposed for 6 h, and lungs, cerebrum, cerebellum and olfactory bulbs were removed 1, 3, 5, and 7 days after exposure. ¹³C concentrations were determined by isotope ratio mass spectroscopy and compared to background ¹³C levels of sham-exposed controls (day 0). The background corrected pulmonary ¹³C added as ultrafine ¹³C particles on day 1 postexposure was 1.34 μg/lung. Lung ¹³C concentration decreased from 1.39 μg/g (day 1) to 0.59 μg/g by 7 days postexposure. There was a significant and persistent increase in added ¹³C in the olfactory bulb of 0.35 μg/g on day 1, which increased to 0.43 μg/g by day 7. Day 1 ¹³C concentrations of cerebrum and cerebellum were also significantly increased but the increase was inconsistent, significant only on one additional day of the postexposure period, possibly reflecting translocation across the blood–brain barrier in certain brain regions. The increases in olfactory bulbs are consistent with earlier studies in nonhuman primates and rodents that demonstrated that intranasally instilled solid UFP translocate along axons of the olfactory nerve into the CNS. We conclude from our study that the CNS can be targeted by airborne solid ultrafine particles and that the most likely mechanism is from deposits on the olfactory mucosa of the nasopharyngeal region of the respiratory tract and subsequent translocation via the olfactory nerve. Depending on particle size, >50% of inhaled UFP can be depositing in the nasopharyngeal region during nasal breathing. Preliminary estimates from the present results show that ～20% of the UFP deposited on the olfactory mucosa of the rat can be translocated to the olfactory bulb. Such neuronal translocation constitutes an additional not generally recognized clearance pathway for inhaled solid UFP, whose significance for humans, however, still needs to be established. It could provide a portal of entry into the CNS for solid UFP, circumventing the tight blood–brain barrier. Whether this translocation of inhaled UFP can cause CNS effects needs to be determined in future studies.     

Distribution of nimodipine in brain following intranasal administration in rats

AIM: To determine whether nasally applied nimodipine (NM) could improve its systemic bioavailability and be transported directly from the nasal cavity to the brain. METHODS: NM was administered nasally, intravenously (iv), and orally to male Sprague-Dawley rats. At different times post dose, blood, cerebrospinal fluid (CSF), and brain tissue samples were collected, and the concentrations of NM in the samples were analyzed byHPLC. RESULTS: Oral systemic bioavailability of NM in…     

Methyl methacrylate toxicity in rat nasal epithelium: investigation of the time course of lesion development and recovery from short term vapour inhalation

An investigation of the time course of development and recovery of the nasal lesion induced in rats by inhalation of methyl methacrylate (MMA) was conducted. Groups of 45 female F344 rats (five animals per time point) were exposed whole body for 6 hours per day to 0 (control), 110 or 400 ppm MMA for 1, 2, 5, 10 or 28 consecutive days. Additional animals were retained for a period of 4, 13, 24 or 36 weeks following exposure to assess reversibility of any nasal tissue effects. After inhalation of MMA there was damage to the olfactory epithelium at 110 and 400 ppm. This was apparent following the first day of exposure, but recovery/regeneration was evident during the subsequent days of the exposure phase of the study. The most severely affected section of the nasal passages was that which included the ethmoturbinates. Focal adhesions between the septum and turbinates and between the turbinates themselves were seen in some animals exposed to 400 ppm MMA at time points after 5 days of exposure. There were no lesions in the squamous, transitional or respiratory epithelia and none in control rats. Lesions that developed in rats exposed to 110 ppm MMA subsequently repaired during the exposure period. At 400 ppm, the majority of the olfactory epithelium had returned to normal within 13 weeks of the end of the exposure phase, but minimal respiratory metaplasia remained evident and there were some focal adhesions between the septum and turbinates and between the turbinates themselves.     

Nasal toxicity of manganese sulfate and manganese phosphate in young male rats following subchronic (13-week) inhalation exposure

Growing evidence suggests that nasal deposition and transport along the olfactory nerve represents a route by which inhaled manganese and certain other metals are delivered to the rodent brain. The toxicological significance of olfactory transport of manganese remains poorly defined. In rats, repeated intranasal instillation of manganese chloride results in injury to the olfactory epithelium and neurotoxicity as evidenced by increased glial fibrillary acidic protein (GFAP) concentrations in olfactory bulb astrocytes. The purpose of the present study was to further characterize the nasal toxicity of manganese sulfate (MnSO(4)) and manganese phosphate (as hureaulite) in young adult male rats following subchronic (90-day) exposure to air, MnSO(4) (0.01, 0.1, and 0.5 mg Mn/m(3)), or hureaulite (0.1 mg Mn/m(3)). Nasal pathology, brain GFAP levels, and brain manganese concentrations were assessed immediately following the end of the 90-day exposure and 45 days thereafter. Elevated end-of-exposure olfactory bulb, striatum, and cerebellum manganese concentrations were observed following MnSO(4) exposure to > or = 0.01, > or = 0.1, and 0.5 mg Mn/m(3), respectively. Exposure to MnSO(4) or hureaulite did not affect olfactory bulb, cerebellar, or striatal GFAP concentrations. Exposure to MnSO(4) (0.5 mg Mn/m(3)) was also associated with reversible inflammation within the nasal respiratory epithelium, while the olfactory epithelium was unaffected by manganese inhalation. These results confirm that high-dose manganese inhalation can result in nasal toxicity (irritation) and increased delivery of manganese to the brain; however, we could not confirm that manganese inhalation would result in altered brain GFAP concentrations.     

Transfer of Some Carboxylic Acids in the Olfactory System Following Intranasal Administration

Abstract

The uptake of [¹⁴C]benzoic acid, 4-chloro[¹⁴C]benzoic acid, [³H]phthalic acid and [¹⁴C]salicylic acid in the nasal passages and brain was determined following a unilateral intranasal instillation in mice. An uptake of radioactivity from the nasal mucosa to the ipsilateral olfactory bulb was observed up to 4 h after administration following intranasal instillation of these carboxylic acids whereas the level was low in the contralateral olfactory bulb. Autoradiography of mice given [¹⁴C]benzoic acid and [¹⁴C]salicylic acid by intranasal instillation showed a preferential localization of radioactivity in the axonal and glomerular layer of the olfactory bulb 1 h after the administration. Four hours after administration the radioactivity was present as a gradient from the axonal layer towards the center of the olfactory bulb. Pretreatment of mice with a compound known to damage the olfactory neuroepithelium resulted in a decreased uptake of [¹⁴C]benzoic acid in the olfactory bulb. Thin layer chromatography of supernatants from the ipsilateral olfactory bulbs of mice given [¹⁴C]benzoic acid by nasal instillation indicated that the radioactivity in the bulbs represented unchanged compound. These results suggest that there is a transfer of some aromatic carboxylic acids in the olfactory pathways.     

Drug targeting to the brain: transfer of picolinic acid along the olfactory pathways

Picolinic acid (PA) protects against quinolinic acid- and kainic acid-induced neurotoxicity in the brain. To study the uptake of PA to the brain, we administered [3H]PA via a unilateral nasal instillation or iv injection to mice. Autoradiography demonstrated a rapid uptake of radioactivity in the olfactory nerve layer and in the ipsilateral olfactory bulb (OB) following nasal instillation of [3H]PA. After 4 h, there was a high level of radioactivity in the central parts of the ipsilateral OB and olfactory peduncle. Moreover, iv injection of [3H]PA demonstrated a selective uptake and retention of radioactivity in the OB. Gas chromatography-mass spectrometry (GC-MS) demonstrated the presence of PA and PA-glycine conjugate in the OB. In mice with reduced peripheral olfactory innervations there was a decreased uptake of [3H]PA in the OB as compared to controls suggesting that an intact olfactory neuroepithelium is a prerequisite for an uptake of PA to the OB. There is an increased interest in brain targeting of drugs with limited ability to pass the blood-brain barrier. The present results demonstrate that PA fulfils structural requirements for a transfer along the olfactory pathways to the brain.     

Olfactory toxicity of methyl iodide in the rat

 The monohalomethanes (methyl iodide, methyl bromide and methyl chloride) are widely used industrial methylating agents with pronounced acute and chronic toxicity in both experimental animals and man. Recently inhalation exposure of rats to methyl bromide has been shown to result in severe olfactory toxicity. This study examined the effects on the rat nasal cavity of inhalation of methyl iodide (100 ppm for 0.5 – 6 h), and demonstrated that methyl iodide is a more potent olfactory toxin than methyl bromide. Within the nasal cavity the olfactory epithelium was the principle target tissue, and it was only at high doses (600 ppm.h) that limited damage to transitional epithelium occurred. The squamous and respiratory epithelia were consistently unaffected. Within olfactory epithelium the sustentacular cells were the primary cellular target and damage to sensory cells appeared to be a secondary event. Methyl iodide induced olfactory damage was reversible, and 2 weeks after exposure almost complete repair had taken place. Received: 7 March 1995 / Accepted: 3 May 1995     

Transfer of morphine along the olfactory pathway to the central nervous system after nasal administration to rodents

The aim of this study was to investigate whether morphine can be transferred along the olfactory pathway to the CNS, thereby circumventing the blood–brain barrier, after nasal administration to rodents. Radiolabelled and unlabelled morphine were administered via the right nostril to mice and rats. Olfactory bulbs, brain tissue and blood samples were collected. Morphine-derived radioactivity was measured using liquid scintillation (LS) and the concentrations of morphine and its metabolite morphine-3-glucuronide (M3G) were also assessed with high-performance liquid chromatography. The location of morphine-derived radioactivity in the rat brain was visualised by autoradiography. Overall, the levels of morphine in the right olfactory bulbs (ROBs) significantly exceeded those in the left olfactory bulbs (LOBs) and brain tissue samples 15, 60 and 240 min after right-sided nasal administration. Fifteen minutes after intravenous administration, there were no significant differences between olfactory bulbs and the other brain areas. Five minutes after nasal administration, autoradiography revealed radioactivity surrounding the ROB and reaching one of the ventricles in the brain. After 60 min, radioactivity had reached the peripheral parts of the ROB. All the techniques used in this study demonstrate that morphine was transferred along the olfactory pathway to the CNS after nasal administration to rodents.     

Brain uptake of dihydroergotamine after intravenous and nasal administration in the rat

This study was conducted to determine the uptake of dihydroergotamine (DHE) into the brain after intravenous and intranasal administration in rats. Eight anesthetized rats received either an intravenous (i.v.) or two successive intranasal (i.n.) doses of tritium labeled dihydroergotamine (³H-DHE) with ¹⁴C-inulin as a non-BBB (blood–brain barrier) permeable marker. Radioactivity concentrations in plasma were determined at designated times within 30 min postdose, and in blood and seven brain regions (olfactory bulb, frontal cortex, parietal cortex, occipital cortex, cerebellum, mid-brain areas, and brain stem) at 30 min. The plasma-to-brain permeability*area product (PeA) following an i.v. dose was calculated based on the 30-min brain tissue concentration and the area under the plasma concentration–time curve (AUC_{0–30 min, i.v.}) assuming unidirectional transport from plasma to brain. Direct transport from nasal cavity to brain was assessed based on the amount of radioactivity in brain determined experimentally and predicted based on plasma AUC_{0–30 min, i.n.} and PeA obtained from i.v. data. Following an i.v. dose, DHE distributed into the brain with a brain-to-plasma concentration ratio of ∼5% at 30 min postdose. The PeA value of DHE ranged from 8.6×10⁻⁴ to 37.5×10⁻⁴ mL min⁻¹ g⁻¹ in different brain regions. Following i.n. doses the experimentally determined concentration in olfactory bulb was approximately 51 times, and in other regions three to seven times, greater than predicted values based only on PeA and plasma AUC, suggesting a direct transport pathway from the nasal cavity to the brain. As a result, the brain tissue concentrations at 30 min were similar to (0.31–1.04 times) those following an i.v. dose except for the olfactory bulb, in which the concentration was approximately four times greater than that following an i.v. dose. In conclusion, ³H-DHE penetrated the BBB following intravenous administration. Following i.n. doses, ³H-DHE was able to enter the brain directly from the nasal cavity, with the olfactory bulb being a part of the direct passage from nasal cavity to brain. Copyright © 1998 John Wiley & Sons, Ltd.