NORADRENERGIC INPUT TO NOCICEPTIVE MODULATORY NEURONS IN THE RAT ROSTRAL VENTROMEDIAL MEDULLA

Within the rostral ventromedial medulla (RVM), there are two classes o f putative pain modulation neurons: ON cells and OFF cells, which resp ectively burst or pause prior to withdrawal reflexes elicited by noxio us stimulation. Alpha-adrenergic agonists injected into the RVM produc e changes in the latency of spinal nocifensive reflexes and, when iont ophoretically applied, alter the firing of RVM ON but not OFF cells. T o provide further information about the contribution of norepinephrine to RVM neuron function, we analyzed the distribution of tyrosine hydr oxylase immunoreactive (TH-ir) appositions upon RVM ON and OFF cells. In the lightly anesthetized rat, seven ON and five OFF cells were iden tified by changes in their discharge rate in relation to nociceptive w ithdrawal reflexes and were labeled by intracellular injection of neur obiotin. Sections containing labeled cells were visualized by using av idin conjugated to a Texas Red fluorophore. Tissue with labeled cells was subsequently processed for TH-ir by using a Bodipy fluorophore con jugated secondary antibody. The distribution of the Bodipy-labeled fib ers and terminals upon the Texas Red-labeled neurons was mapped using a confocal laser-scanning microscope. All the labeled neurons exhibite d close TH-ir appositions. Appositions were of two types: swellings an d fibers. Although the numbers and density of appositions varied among the cells, there were no consistent differences that correlated with physiological properties. Thus the overall density of appositions for ON cells (29.0 +/- 22.2 x 10(4) mu m(2)) did not differ significantly from that for OFF cells (25.4 +/- 22.2 x 10(4) mu m(2)). Tyrosine hydr oxylase immunoreactive (TH-ir) appositions upon ON and OFF cells varie d with their location along the dorso-ventral axis with more ventral n eurons having a greater density of TH-ir swelling-type appositions. In a separate study, TH-ir and dopamine-beta-hydroxylase-like immunoreac tivity (DBH-ir) were mapped in the same sections by using confocal mic roscopy. Nearly 97% of the TH-ir profiles co-localized with DBH-ir. Th ese observations provide evidence that both ON and OFF cells in the RV M are targeted by noradrenergic inputs. J. Comp. Neurol. 377:381-391,1 997. (C) 1997 Wiley-Liss, Inc.