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.
Summary: Arachnoid granulation is a protrusion of the arachnoid membrane into the cranial sinus, and is thought to play an essential role in the cerebrospinal fluid (CSF) absorption. Because the cells covering the apex region of the arachnoid granulation have different morphological features compared to the ordinary endothelial cells lining of the cranial sinus lumen, it has been expected these covering endothelial cells perform some specific function in the CSF absorption mechanism. However, little is known about functional differences between the covering endothelium of the arachnoid granulation and the ordinary sinus endothelium. In the present study, the characteristics of the covering cells located at the apex of arachnoid granulations of human, monkey and dog brain were examined by histochemical and immunohistochemical methods. The endothelial cells lining the cranial sinus lumen generally expressed such proteins as von Willebrand factor (vWF), CD31 and glycoproteins containing GS-1 or LE-1 lectin reacting sugar residue which are endothelial cell markers. However, the endothelial cells specifically located at the apex of arachnoid granulations failed to show vWF immunoreactivity, whereas the other endothelial markers were positive in each species we examined. Double staining of vWF antibody with other markers has clearly demonstrated that the endothelial cells on the apex region of arachnoid granulations exhibit no expression of vWF whereas cells lining the lateral region of arachnoid granulations and the luminal surface of ordinary cranial sinuses showed co-localization of these markers. The structural and histochemical differences between endothelial cells located at the apex region of arachnoid granulations and those of the sinus wall may reflect functional differences.
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