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 ((MnHPO4)-Mn-54). 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 (MnHPO4)-Mn-54 aerosol (0.39 mg Mn-54/m(3); MMAD 1.68 mum, 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 (MnHPO4)-Mn-54 exposure and at 1, 2, 4, 8, and 21 d postexposure with gamma spectrometry and autoradiography. Rats with two patent nostrils had equivalent Mn-54 concentrations on both sides of the nose, olfactory bulb, and striatum, while asymmetrical Mn-54 delivery occurred in rats with one occluded nostril. High levels of Mn-54 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 (MnHPO4)-Mn-54 exposure, while brain and nose samples on the side ipsilateral to the nostril occlusion had negligible levels of Mn-54 activity. Our results demonstrate that the olfactory route contributes to Mn-54 delivery to the rat olfactory bulb and tubercle. However, this pathway does not significantly contribute to striatal Mn-54 concentrations following a single, short-term inhalation exposure to (MnHPO4)-Mn-54.