Ultrastructural evidence for prominent postsynaptic localization of alpha2C-adrenergic receptors in catecholaminergic dendrites in the rat nucleus locus coeruleus.

Alpha-2-adrenergic receptor (alpha2-AR) agonists potently inhibit the activity of noradrenergic neurons of the locus coeruleus (LC), an effect that may be mediated by the A- and/ or C-subtypes of alpha2-AR (alpha2A- and alpha2C-AR). To gain insight into the functional significance of these alpha2-AR subtypes in the LC, we have examined their ultrastructural localization by using subtype-specific antibodies. We recently demonstrated that alpha2A-ARs are localized prominently in axon terminals and catecholaminergic dendrites in the LC. In the present study, we sought to identify the subcellular substrates underlying alpha2C-AR actions in the LC by analyzing the ultrastructural distribution of alpha2C-AR immunoreactivity (alpha2C-AR-IR) in sections that were dually labeled for the catecholamine-synthesizing enzyme tyrosine hydroxylase (TH). Alpha-2C-AR-IR was predominantly localized in dendrites, most of which also contained immunolabeling for TH. Within such dendrites, alpha2C-AR-IR was associated with the plasma membrane and occasionally Golgi cisternae and tubulovesicles. The vast majority of dendrites containing alpha2C-AR-IR received asymmetric (excitatory) contacts from unlabeled axon terminals that often contained dense core vesicles. Alpha-2C-AR-IR was observed in some unmyelinated axons and astrocytic processes that were apposed to TH-immunoreactive dendrites but was rarely associated with axon terminals. These results provide the first ultrastructural evidence that alpha2C-ARs (1) are localized postsynaptically in catecholaminergic neurons of the LC and (2) may be strategically situated to modulate the activation of LC neurons by excitatory inputs.