Mini MCAT passage: Neurotransmitter removal from the synapse

Neurotransmitters are chemical messengers that transmit signals across a synapse from one neuron’s axon to another neuron’s dendrite. Neurotransmitters are released from the presynaptic neuron in vesicles that travel across the synaptic cleft where they are received by receptors on the postsynaptic neuron. Neurotransmitters can have either excitatory or inhibitory effects on the postsynaptic neuron by increasing or decreasing the probability that the postsynaptic neuron will create an action potential. Neurotransmitters are removed from the synaptic cleft by diffusion out of the cleft, removal by an enzyme, or via re-uptake pumps that actively pump the neurotransmitter back into the presynaptic axon.

A researcher is interested in mechanisms of neurotransmitter removal from the synaptic cleft following release from the axon terminal. He designs a series of experiments in mice to increase the rate of release of neurotransmitter to test effects on compensatory changes to removal processes. He also cultures neurons from the mice to test aspects of neurotransmitter release and removal in vitro.