6.5 - D - Acetylcholine
Acetylcholine is a neurotransmitter used in many synapses, most notably those between neurons and muscle fibers. After acetylcholine is secreted into the synapse, it diffuse towards receptors on the post-synaptic cell.
Acetylcholine only remains bound to the receptor for a short amount of time due to acetylcholinesterase which is present in the synapse. This enzyme rapidly digests acetylcholine into choline and acetate. The choline molecules are then reabsorbed into the pre-synaptic neuron and used to synthesize new acetycholine neurotransmitters. This is done by combining the choline with an acetyl group.
The transmission of signals across a synapse can be slowed or blocked through a number of mechanisms, most of which prevent the neurotransmitter from binding to its receptor. Neonicotinoids are an example of this as molecules in the class are able to bind to acetylcholine receptors on post-synaptic cells in insects.
Since acetylcholinesterase cannot digest neonictonoids, when they bind to receptors, they permanently block acetylcholine from binding. Neonicotinoids specifically target receptors in the central nervous system of insects, which has made it a effective form of pest control.
Neonicotinoid-based pesticides are currently widely used crops as they are not highly toxic to humans. However, there has recently been concern that the pesticide is negatively affecting honeybees and other beneficial insect species.