C. elegans is a model organism and was the first eukaryotic organism to have its genome sequenced. The free-living nematode is often used in laboratories investigating nervous system development with all 302 of its neurons and their effectors categorized. Interesting control mechanisms have been investigated with reproduction, particularly with egg-laying in the nematode. There are 16 muscles, 2 types of neurons, and multiple receptors involved in the process of laying eggs in C. elegans, and there are mutations in all of those structures for the study of the process. One particular mutation that prevents the laying of eggs by the worm is rescued by the neurotransmitter, serotonin. That rescue suggests that the mutation is most likely in which of the following? A. one of the muscles needed for egg-laying
B. a receptor for serotonin on cells needed for egg-laying
C. a pre-synaptic neuron involved in egg-laying
D. a post-synaptic neuron involved in egg-laying

C. a pre-synaptic neuron involved in egg-laying

Explanation:

Serotonin (5-HT) is a neurotransmitter, i.e., a molecule that binds to specific receptors and thus transmits signals from one pre-synaptic neuron to another target cell. In humans, serotonin is involved in many physiological functions including sleep, mood, emotions, appetite, digestion, etc. Moreover, Caenorhabditis elegans is a model organism widely used in neuroscience research in order to understand neuromodulation at the level of individual neurons. In C. elegans, serotonin modifies behavior by regulating neuronal excitability and synaptic efficiency. Moreover, also in C. elegans, it has been shown that serotonin induces egg-laying and also provides a feedback regulatory mechanism by inhibiting the activity of the pre-synaptic neurons that release it. In consequence, in this case, it is expected that serotonin rescues the loss of function of serotonin receptors required for egg-laying by providing a regulatory feedback loop that contributes to serotonin adaptation.

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