Neurotransmitters are the chemical substances that act to communicate across synaptic cleft. These neurotransmitters helps in transferring of information from one neuron to the other neuron.
A single neuron can synthesize and release different types of neurotransmitters, which can be identified by the principal neurotransmitter in their axon terminals.
Thus, the blanks can be filled with neurotransmitter, neurotransmitter.