To study the effect of muscarinic vs nicotinic cholinergic inhibition, and basal forebrain optogenetic cholinergic activation in a rodent switch timing assay.

Cholinergic deficit is a major feature in Alzheimer’s disease and Parkinson’s disease dementia. Patients with Alzheimer’s or Parkinson’s disease can have impairments in interval timing-the ability to control movements in time. Interval timing requires attention to time and working memory for temporal rules. Our recent work showed that scopolamine, a muscarinic receptor antagonist, impairs interval timing through disrupting stimulus-processing rather than temporal processing in the medial prefrontal cortex. However, the roles of muscarinic vs. nicotinic cholinergic circuitry in timing is unclear.  

Experiment 1: Mice (C57BL/6J, n=17, 10 males, 7 females) were well-trained on a switch timing task prior to intraperitoneal injections of: normal saline, scopolamine(2mg/kg), mecamylamine(a nicotinic receptor antagonist; 3mg/kg), physostigmine(a cholinesterase inhibitor; 0.35mg/kg) or nicotine(2mg/kg). The switch timing behavioral assay started 30 min after drug administration. All sessions were randomized and blinded. Experiment 2: A separate cohort of ChAT-Cre transgenic mice (n=4, 3 males, 1 female) were trained on the switch timing task before bilateral basal forebrain stereotactic AAV-DIO-ChR2 injection and optic canulae implantation (AP -2.35 @20^o, ML +/-1.8, DV AAV injection -4.6/-5.6, DV canulae -4.6). Laser (473 nm/20 Hz) stimulation was randomly assigned on/off across trials, and switch timing behavioral assay was started 30 min after scopolamine or normal saline administration.

Scopolamine decreased response times (p=0.009, Cohen’s D=0.7), while physostigmine increased response times (p=0.03, Cohen’s D=0.7). Nicotine and mecamylamine had no effect. Scopolamine induced timing deficits were partially reversed by optogenetic activation of basal forebrain cholinergic neurons.

Muscarinic, not nicotinic, cholinergic circuitry is critical for timing behavior in a switch interval timing task. Basal forebrain cholinergic activation improves interval timing. These data have relevance for cholinergic dysfunction in human diseases such as Alzheimer’s and Parkinson’s disease.