Enduring effects of muscarinic receptor activation on adult hippocampal neurogenesis, microRNA expression and behaviour.

The cholinergic system is one of the most important neurotransmitter systems in the brain with key roles in autonomic control and the regulation of cognitive and emotional responses. However, the precise mechanism by which the cholinergic system influences behaviour is unclear. Adult hippocampal neurogenesis (AHN) is a potential mediator in this context based on evidence, which has identified this process as putative mechanism of antidepressant action. More recently, post-transcriptional regulation by microRNAs is another candidate mechanism based on its involvement in the regulation of AHN and neurotransmission. Taking into account this background, we evaluated the behavioural effects of a non-convulsant dose of pilocarpine - a non-selective muscarinic receptor (mAChR) agonist - in adult Wistar rats. Furthermore, we quantified the expression of different microRNAs implicated in the regulation of AHN. Our results suggests that pilocarpine treatment increases AHN in the granular cell layer but also induced ectopic neurogenesis. Pilocarpine treatment reduced immobility time in forced swimming test but did not affect fear conditioning response, sucrose preference or novelty supressed feeding behaviour. In addition, treatment with pilocarpine down-regulated the expression of 6 microRNAs implicated in the regulation of neurotrophin signalling and axon guidance pathways. Therefore, we suggest that the low-dose stimulation of the cholinergic system is sufficient to alter AHN of rats through post-transcriptional mechanisms, which might contribute to long-lasting behavioural effects.