Differential expression of catecholamine biosynthetic enzymes in the rat ventrolateral medulla

Adrenergic (CI) neurons located in the rostral ventrolateral medulla are co nsidered a key component in the control of arterial blood pressure. Classic ally, C1 cells have been identified by their immunoreactivity for the catec holamine biosynthetic enzymes tyrosine hydroxylase (TH) and/or phenylethano lamine N-methyltransferase (PNMT). However, no studies have simultaneously demonstrated the expression of aromatic L-amino acid decarboxylase (AADC) a nd dopamine P-hydroxylase (DBH) in these neurons. We examined the expressio n and colocalization of all four enzymes in the rat ventrolateral medulla u sing immunohistochemistry and reverse transcription-polymerase chain reacti on (RT-PCR) analysis. Retrograde tracer injected into thoracic spinal segme nts T2-T4 was used to identify bulbospinal neurons. Using fluorescence and confocal microscopy, most cells of the C1 group were shown to be double or triple labeled with TH, DBH, and PNMT, whereas only 65-78% were immunoreact ive for AADC. Cells that lacked detectable immunoreactivity for AADC were l ocated in the rostral C1 region, and approximately 50% were spinally projec ting. Some cells in this area lacked DBH immunoreactivity (6.5-8.3%) but we re positive for TH and/or PNMT. Small numbers of cells were immunoreactive for only one of the four enzymes. Numerous fibres that were immunoreactive for DBH but not for TH or PNMT were noted in the rostral C1 region. Single- cell RT-PCR analysis conducted on spinally projecting C1 neurons indicated that only 76.5% of cells that contained mRNA for TH, DBH, and PNMT containe d detectable message for AADC. These experiments suggest that a proportion of C1 cells may not express all of the enzymes necessary for adrenaline syn thesis. (C) 2001 Wiley-Liss, Inc.