Plethysmography as an alternative methodology to assess smell perception in guinea-pigs; a central and peripheral biomarker model for drug discovery research.

Catherine Sweatman, Isabelle Delescluse, Kathryn Hulland, Phil Stanley, John Adcock, Janet Nicholson, Ian Machin, Laura Corradini. Pfizer, Sandwich, United Kingdom.

The olfactory system is regulated by a variety of receptors localized in the central and peripheral nervous system. Opioid [1] and sodium channel (Nav) blockers [2] can modulate smell perception in man, as assessed by chemosensory performance testing, suggesting the smell test as a potential pharmacology biomarker for pain research. In addition, given the large number of targets localized in the olfactory bulb, the smell test could also represent a potential central biomarker for a variety of therapeutic areas. Here we aimed to develop a preclinical method for measuring smell perception (i.e. sniff behaviour) independent of locomotion, learning and reward. As rodents are obligate nasal breathers, sniffing behaviour has been reported to manifest in an increased respiratory rate (RR) [3]. Thus, RR was measured in male Dunkin-Hartley guinea-pigs (Harlan, 400-500g) using Buxco double-chamber plethysmography equipment. After 1 hour of acclimatisation, the change (Δ) from baseline RR was measured in response to the novel non-noxious odorant, n-butanol, delivered via a nebuliser for 12 sec. An initial range finding study showed guinea-pigs responded at 0.2% and 2% n-butanol v/v in saline but that the response was reduced by a previous exposure of the given chemical. Optimisation identified that a single exposure of 5µL of 8% n-butanol produced a maximal response (Δ 71.7±9.7 breaths per minute (bpm) vs Δ 4.2±2.1 bpm for controls; n = 8, p<0.01). We also confirmed that repeated exposure to the odorant one week apart resulted in a significantly reduced response (Δ 31.6±8.2 vs Δ 12.7±10.1 bpm; week1 vs week2, n=8, p<0.05), suggesting the model may only be successful with a single administration of a novel odorant. In subsequent experiments a group of guinea-pigs was treated with 30mg/kg, subcutaneously, of the non-selective Nav blocker, mexiletine. When tested two hours post dose the mexiletine-treated group showed a limited increase in RR in response to n-butanol compared to the vehicle-treated group (Δ 28.7±6.4 bpm vs Δ 50.3±5.4bpm; respectively, n=6, p<0.01). In summary, we have demonstrated that measuring RR using plethysmography could be a suitable preclinical model to assess drug-induced modulation of olfactory functions in non-anesthetised guinea-pigs, and that smell behaviour could represent a suitable pharmacological biomarker for Nav programs. Further work is required to assess the feasibility of this assay in other species and its suitability for other mechanisms being developed for potential pain therapy.

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