All neurotransmitters are agonists – chemicals that bind to receptor neurons and activate them to respond. They act like keys in post-synaptic receptor neuron ‘locks.’ They fit into their own receptor neuron and bind to it to produce a voltage change called an action potential. When they do this they are having an excitatory effect in the synapse. An example of a neurotransmitter that does this is acetylcholine, which binds with receptors, especially in the hippocampal area, to improve encoding in memory. Kihara and Shimohama (2004; 2018) – found that a decrease in acetylcholine receptors has a leading role in the development of Alzheimer’s disease
Some drugs are also agonists, and bind to or mimic the effect of the neurotransmitter, provoking the same response in the receptor neuron. Alcohol does this for dopamine – activating dopamine receptors in the nucleus accumbens area of the brain.
However, although they are all agonists, not all neurotransmitters have an excitatory effect in the synapse. Sometimes they make it less likely for the receptor neuron to fire an action potential. GABA and serotonin both do this, decreasing the receptor neuron activity and having an inhibitory effect in the synapse.
Several neurotransmitters can have either excitatory or inhibitory effects, depending where in the brain they are acting and with which receptor neurons they are interacting. This Youtube video is very informative regarding neurons and neurotransmitters https://tinyurl.com/qo8yqxp