The Effect of Propionic Acid on the Habituation to Social and Non-social Odour Cues in Adult Male Rats

The enteric bacterial metabolite, propionic acid (PPA), elicits physiological and behavioural changes in rodents reminiscent of autism spectrum disorder (ASD). This includes abnormal sensory processing and social behaviour. ASD may contribute to social deficits through impaired habituation; therefore, the present study examined the effects of intraperitoneal PPA on the habituation to social and non-social odours. Adult male rats were injected daily with PPA or the vehicle control, and for 3 days, habituated to a conspecific odour or vanilla extract for 10 minutes. On day 4, rats were exposed to a novel conspecific odour or almond extract for 10 minutes to observe dishabituation. Behaviours were measured in the open-field and analyzed via an automated system and by the manual scoring of video-recordings. Results from both scoring methods strongly correlated with one another. PPA treatment significantly increased repetitive behaviours and hypoactivity. Drug and odour had no significant effects on odour habituation, although PPA non-social rats displayed reduced habituation for entries into the odour quadrant and sniffing. Group differences were insignificant. No dishabituation to the odour was observed in all groups. However, an insignificant, subtle reduction in dishabituation to the open-field was seen in PPA groups for total distance travelled. PPA may influence odour discrimination in rodents and contribute to sensory habituation deficits in ASD. Differences in body temperature and weight post-injection were measured to monitor a potential sickness response from the bacterial by-product. Results for PPA rats did not differ from controls, suggesting that PPA exerts its effects through other mechanisms

Social factors and the neurobiology of pathogen avoidance

Although the evolutionary causes and consequences of pathogen avoidance have been gaining increasing interest, there has been less attention paid to the proximate neurobiological mechanisms. Animals gauge the infection status of conspecifics and the threat they represent on the basis of various sensory and social cues. Here, we consider the neurobiology of pathogen detection and avoidance from a cognitive, motivational and affective state (disgust) perspective, focusing on the mechanisms associated with activating and directing parasite/pathogen avoidance. Drawing upon studies with laboratory rodents, we briefly discuss aspects of (i) olfactory-mediated recognition and avoidance of infected conspecifics; (ii) relationships between pathogen avoidance and various social factors (e.g. social vigilance, social distancing (approach/avoidance), social salience and social reward); (iii) the roles of various brain regions (in particular the amygdala and insular cortex) and neuromodulators (neurotransmitters, neuropeptides, steroidal hormones and immune components) in the regulation of pathogen avoidance. We propose that understanding the proximate neurobiological mechanisms can provide insights into the ecological and evolutionary consequences of the non-consumptive effects of pathogens and how, when and why females and males engage in pathogen avoidance