In addition to AChE, OPs have been shown to interact or affect, other components of the cholinergic system. Several OPs were found to bind to nicotinic receptors, however, only at high concentrations (100 μM; [108]). OPs can also interact at lower concentration (nanomolar range) with muscarinic receptors, particularly the M2 subtype [109] and [110]; and references in 111], where they appear to act as agonists. In vivo, Enzymes involved in the metabolism of peptides have been reported to be affected by OPs. For example, acylpeptide hydrolase (APH) is inhibited by various oxons at concentrations lower than those required to inhibit AChE; inhibition was also observed after in vivo exposure to OPs [113]. APH is responsible for the removal of N-acetylated aminoacids from the N-terminal of short peptides, such as α-melanocyte-stimulating hormone or β-endorphin. Indirect evidence for inhibition of peptidases involved in the metabolism of enkephalins was reported in a comparative analysis of DFP- and physostigmine-induced antinociception in mice [114] and [115]. DFP, but not physostigmine, significantly increased met-enkephalin levels in the striatum. Both compounds caused antinociception, however, only that induced by DFP was antagonized by naloxone. These in vivo effects, however, were seen of doses that caused significant AChE inhibition.