Determination of odor detection threshold in the Gottingen minipig

Abstract

The aim of the study was to examine the ability of Göttingen minipigs to acquire an olfaction-based operant conditioning task and to determine the detection threshold for ethyl acetate and ethanol. We used an automated olfactometer developed for rodents to train and test 14 pigs. Odor sampling and reliable responding were obtained after three to fifteen 160-trial sessions. Successful transfer of the task from ethyl acetate to ethanol was achieved in 1-4 sessions. Detection threshold for ethyl acetate varied between 10(-2)% and 10(-6)% v/v and for ethanol between 0.1% and 5 × 10(-6)% v/v. The results provide evidence that minipigs can successfully acquire 2-odorant discrimination using a food-rewarded instrumental conditioning paradigm for testing olfactory function. This olfactory discrimination paradigm provides reliable measures of olfactory sensitivity and thereby enables detection of changes in olfaction in a porcine model of Alzheimer's disease currently being developed.

Figure 1

Learning curves regarding olfactory discrimination for each of 14 individual minipigs. Mean percent correct responding (±standard error of the mean) on each 160-trial session. S+ was a 1% aqueous solution of ethyl acetate, and S- was water solvent. Gaps in lines denote a 1-month pause in testing during which the apparatus was optimized by moderate modifications. To master the task, individuals must obtain a minimum 75% correct responses in one 160-trial session. Success in reaching criterion is denoted by a blank circle. Females (ID F1-F8), males (ID M9-M14).

Figure 2

Learning curves regarding olfactory discrimination of a novel odorant for 9 minipigs. Mean percent correct responding on each 160-trial session. S+ was a 95% aqueous solution of ethanol, and S- was the water solvent. Horizontal reference line (y = 75) denotes criterion of success. To master the task, individuals should reach at least 75% correct responses in one 160-trial session.

Figure 3

Performance of minipigs in detecting descending concentrations of ethyl acetate. Each data point represents mean percentage correct choices from 5 to 8 blocks of 20 trials comprising a total of 100-160 decisions. In case of more than one replicate at one concentration, a line is drawn at the mean value (ID F5, F7, and M9). Horizontal reference line (y = 50) denotes chance level. To master the task, individuals should reach a mean of minimum 75% correct responses in one 100- to 160-trial session. Failure in reaching success criterion is denoted by a blank circle. ID M9 showed a peculiar performance as it was not readily possible to determine the olfactory detection threshold for ethyl acetate. However, when exchanging S+ with water in odor saturator bottle number one (▪), the pig still performed perfectly, whereas the performance reached a below chance level (35% correct response) when odor saturator bottle number 1 and 2 both containing water were interchanged (♦).

Figure 4

Determination of ethanol detection threshold for 4 minipigs. Each data point represents mean percentage correct choices from 5 to 8 blocks of 20 trials comprising a total of 100-160 decisions. In case of more than one replicate at one concentration, a line is drawn at the mean value. Horizontal reference line (y = 50) denotes chance level. To master the task, individuals should reach a mean of minimum 75% correct responses in one 100- to 160-trial session. Failure in reaching success criterion is denoted by a blank circle.