Immunocytochemical Localization of .BETA.2-Adrenergic Receptors in the Rat Spinal Cord and Their Spatial Relationships to Tyrosine Hydroxylase-immunoreactive Terminals

Abstract

The distribution of beta2-adrenergic receptors (beta2-ARs) in the rat spinal cord was investigated by immunocytochemistry using an antibody against beta2-ARs. The relationship between beta2-ARs and catecholaminergic terminals containing the catecholamine synthesizing enzyme, tyrosine hydroxylase (TH), in the dorsal horn was also examined by light and electron microscopy. beta2-AR-immunoreactivity (beta2-AR-IR) showing the appearance of fibers and puncta was particularly dense in laminae I and II of the dorsal horn. Moderate immunoreactivity was observed in the intermediolateral cell column, in the ventral horn and around the central canal. Additional immunoreactivity was detected in ependyma lining the central canal. Capsaicin treatment reduced, but did not eliminate, the immunostaining in the dorsal horn. Double immunofluorescence histochemistry for beta2-AR and TH showed no colocalization of the two antigens. By electron microscopy, beta2-AR-IR was found in dendrites as well as unmyelinated axons and axon terminals which contained many small clear vesicles and several large granular vesicles. Such terminals usually formed asymmetric synapses on labeled or unlabeled dendrites. TH-labeled terminals were often near both axonal and dendritic profiles containing beta2-AR-IR and sometimes made synaptic contacts with beta2-AR-labeled dendrites. However, beta2-AR-IR was found on the extrasynaptic portion of the plasma membrane. No synaptic contact was made between TH-labeled terminals and beta2-AR-labeled varicosities. These results demonstrated that beta2-ARs are localized on both nociceptive primary afferents and on dendrites in the rat dorsal horn and provide the ultrastructural evidence that beta2-ARs on both axonal and dendritic profiles are activated by catecholamines released from catecholaminergic terminals via volume transmission.