One additional complication of dopamine (which also applies to the other neuromodulatory transmitters that we discuss later in the book, such as noradrenaline) is that there are different types of dopamine receptors that are present on neurons. Some of these (known as D1-type receptors) have the effect of increasing the excitability of the neurons where they are present (turning up the volume), while others (D2-type receptors) have the effect of reducing the excitability of those neurons (turning down the volume).
