How does the neuron know when to release the neurotransmitters?

All neurotransmitter receptors should be thought of as having two functions: First, to detect a particular neurotransmitter, and second, to do something when they detect it. The receptor determines what the neurotransmitter's effect is. So it's not always right to call a neurotransmitter inhibitory or excitatory. Glutamate, for example, is among the most common neurotransmitters, and it's almost always excitatory... Except when it binds to a particular type of glutamate receptor, which is inhibitory. Done dopamine receptors are excitatory, some are inhibitory, and not all receptors have effects that fit neatly into those two categories. Sometimes a receptor will have an effect on something completely different... When the NMDA subtype of glutamate receptor is activated, for example, it can cause the postsynaptic cell to change what receptors it puts at that synapse (a cell can have different receptors at different synapses!). Your welcome!