Why is atropine a CNS stimulant, although it blocks the muscarinic receptors in the brain?

Question

I know that atropine is a muscarinic antagonist, so why does atropine have excitatory actions on the brain while it is blocking muscarinic receptors?

Answer 1

I answered your question in the video but I wanted to add something here. If you look at atropines action on the heart for example, It speeds up the rate even through its blocking muscarinic receptors on the heart. Again, thats because the activation of those receptors normally has an inhibitory action on the heart that slows down the heart rate.

Answer 2

If it blocks an inhibitory input, then you are removing that inhibition, and you get excitation. In many cases in the CNS, muscarinic receptors are coupled to an inhibitory G protein (Gi), and the downstream effect is for stimulation of muscarinic receptors to be inhibitory on cAMP, etc. Thus, blocking these receptors would have the opposite effect, i.e. stimulation. This is a biochemical stimulation, and not necessarily a physiological one. If you are looking for a neurostimulatory effect, then you can look at it as follows. If ACh stimulates muscarinic receptors on say GABA (inhibitory) neurons, then blocking this would remove the inhibition by GABA leading to stimulation of the target neuron.