I am reading about dopamine, and I have realized that dopaminergic neurons can fire phasically or tonically. What is the difference between these two modes of firing?
Tonic firing typically occurs without presynaptic input and can be viewed at as background activity. Tonic activity is often characterized by a steady action potential firing at a constant frequency. Note that not all neurons may have tonic activity at rest. It may serve as keeping a steady background level of a certain neurotransmitter or it can serve as a mechanism where both an inhibition or increase in presynaptic input can be transmitted. When a neuron is silent at rest, only an increase in presynaptic activity can be transmitted postsynaptically.
In contrast, phasic firing occurs after a neuron is activated due to presynaptic activity and it incurs activity on top of any background activity a neuron may have. It is typically restricted to one, a few, or a short burst of action potentials, whereafter the activity quickly returns to the resting state.
Tsai et al., Science (2017); 324: 1080-4