Horea Szedlacsek, well what can I say - statistically significant, pharmaco-treatment-based results in the ever-dependable forced swim test AND the black-white box assay. I suppose that settles it. Perhaps some common sense parsimony-based thinking is in order.

• Maybe the attributed effect is simply due to an aversive reaction to the cholinergic agents, regardless of the given action.
• The local, neuroanatomical, application of drugs may provide little relevant information regarding systemic cholinergic effects expected in naturalistic neuromodulator modulation. Granted this is the gold standard compared to oral administration, but still limited, unfortunately, especially if you are interested in the drugs rather than the circuits.
• Presumably you are ultimately interested in how these agents will affect humans, not rodents. If so, then there is a simple solution. Note - I am being serious here. 1) Acquire both agents and note down your own differential affective reaction upon self-administration. Both agents are quite safe, I think, so I see issue making this a particularly unreasonable experiment. This is what the iconic and groundbreaking pharmacologists of the past did and their results made a much more significant impact than what has been seen in recent times.
• If this is not possible, find individuals who have taken these drugs for some clinical indication. Finding people who have been prescribed scopolamine, specifically, or benztropine, trihexyphenidyl, etc., should be easy. Physostigmine will be harder. Ask them about their subjective experience.
• My hypothesis is that scopolamine will be much less anxiogenic compared with physostigmine - cholinergic syndrome can be quite unpleasant.
• Of course, neither of these agents has any place in the modern psychiatric pharmacopoeia, so this would just be an intellectual exercise.
• While I am having a little fun with the above comment, so apologies for the tone, but again I am being serious. Is the goal to move knowledge forward or simply do yet another rodent experiment, for publication but never application.

I'm not an expert, and I had to look up the mechanism of action of both Scopolamine and Physostigmine. So we know that the former is a parasympatholytic, whereas the latter is a parasympathomimetic. However, that does not necessarily mean that they cause opposite effects in every instance.

1. So in the case of Scopolamine, we anticipate anxiogenic-like effects, since it is a parasympatholytic. An M1 antagonist in the CNS, Scopolamine blocks the M2 muscarinic receptors at the cardiac SA-node, and thus, increases heart rate (anxiogenic).

2. On the other hand, Physostigmine is a parasympathomimetic that works by inhibiting AChE, and thus, disinhibiting the usual degradation of ACh at the synaptic cleft. It is suggested that ACh induces anxiety through its action on M1 receptors.

Article Cholinergic signaling in the hippocampus regulates social st...

So when the action of AChE is inhibited, ACh can more freely bind M1 receptors and induce its anxiogenic-like effects. Despite the fact that Scopolamine blocks the M1 receptors in the CNS, it still can cause anxiety through the M2 receptors located in the cardiac tissue. Though, I'm not sure if this is the answer that you were looking for.

Miky Timothy, Milad Shirvaliloo thank you for your answers!

I am only interested in those damn circuits involved in the scopolamine action in rats, at the moment.

The broader picture is like this: we tested some drugs on rats treated with scopolamine in elevated plus maze.

The drugs were supposed to inhibit AMPAR internalization. Scopolamine induced anxiety, the drugs cancelled the effect. Not only it doesn't make much sense according to the literature ( Martinowich K, Schloesser RJ, Lu Y, et al. Roles of p75(NTR), long-term depression, and cholinergic transmission in anxiety and acute stress coping. Biol Psychiatry. 2012;71(1):75-83. doi:10.1016/j.biopsych.2011.08.014 ) but it also raises a more serious question of understanding the role of cholinergic system in anxiety. And when it comes to anxiety, the paper mentioned above seems to me to come in contradiction with the one mentioned by Milad Shirvaliloo ( Mineur YS, Obayemi A, Wigestrand MB, et al. Cholinergic signaling in the hippocampus regulates social stress resilience and anxiety- and depression-like behavior. Proc Natl Acad Sci U S A. 2013;110(9):3573-3578. doi:10.1073/pnas.1219731110 ).

So basically I am trying to find a mechanism involving AMPARs and scopolamine in the elevated-plus maze performance (time spent in the open arms).

Milad Shirvaliloo, I am looking for a mechanism, like the one you outlined. But as much as I understand it, it relies on the idea that scopolamine only inhibits M1. But scopolamine inhibits M1-5 equally ( Witkin JM, Overshiner C, Li X, et al. M1 and m2 muscarinic receptor subtypes regulate antidepressant-like effects of the rapidly acting antidepressant scopolamine. J Pharmacol Exp Ther. 2014;351(2):448-456. doi:10.1124/jpet.114.216804 ). But yes, I am looking for that type of explanation.

Thanks again!

You are comparing an ACHEi (indirect global agonist of M1-5), to a nonselective M antagonist ...

The key is to look at which M receptors scopolamine actually blocks. I suspect that it has a preference (i.e higher affinity) for certain M subtypes over others. If scopolamine is blocking cholinergic autoreceptors (M2 and M4, but double check), then this would disinhibit Ach neurons and this would increase Ach levels.

It is probably not as simple as above, but you can get a better idea of how scopolamine is exerting its effects