α adrenergic receptors include 3 highly homologous subtypes: α, α, and α. these receptors have a critical role in regulating neurotransmitter release from sympathetic nerves and from adrenergic neurons in the central nervous system. studies in mice revealed that both the α and α subtypes were required for normal presynaptic control of transmitter release from sympathetic nerves in the heart and from central noradrenergic neurons; the α subtype inhibited transmitter release at high stimulation frequencies, whereas the α subtype modulated neurotransmission at lower levels of nerve activity