8and = 5) and rats treated with anti-DH-SAP (= 8). loss of 84% of bulbospinal adrenergic neurones does not alter the ability of RVLM to maintain SNA and arterial pressure at rest in anaesthetized rats, but this loss reduces the sympathoexcitatory and pressor responses evoked by RVLM stimulation. The data suggest sympathoexcitatory roles for both the CAB39L C1 cells and non-C1 cells of the RVLM and further suggest the C1 cells are critical for the full expression of sympathoexcitatory responses generated by the RVLM. The rostral ventrolateral medulla (RVLM) is critical for maintaining basal sympathetic vasomotor tone and is an essential component of many sympathetic reflexes (Reis 1989; Stornetta 1989; Guyenet, 1990; Verberne & Guyenet, 1992; Koshiya 1993; Dampney, 19941999). PIM447 (LGH447) The C1 adrenergic neurones coincide with a region of the RVLM that regulates sympathetic nerve activity (SNA) and arterial pressure (AP) (Ross 1981, 19841989), and these cells have been proposed to be important for the generation of sympathetic vasomotor tone and AP. The spinal axons of many C1 cells target sympathetic preganglionic neurones or their immediate vicinity (Ross 19841984; Milner 1988; Jansen 19951995; Schreihofer & Guyenet, 1997; Verberne 1999), a hallmark of the sympathetic vasomotor efferents that regulate blood pressure and cardiac output. However, the C1 cells are not the only efferent projection of the RVLM towards the thoracic spinal cord. This structure also contains other highly active, bulbospinal neurones that are inhibited by stimulation of arterial baroreceptors, but do not contain tyrosine hydroxylase (TH) or phenylethanolamine-1995; Schreihofer & Guyenet, 1997). The properties of these lightly myelinated, non-catecholaminergic cells (Lipski 1996) suggest that they could be a source of supraspinal glutamatergic drive to sympathetic preganglionic neurones (Deuchars 1995), which is essential for the generation of resting sympathetic tone and AP (Huangfu 1994). The relative contributions of the C1 cells and the non-catecholaminergic RVLM neurones to the generation of sympathetic vasomotor tone are not known. One way to address this question would be to identify deficits in the regulation of sympathetic tone that result from the selective destruction of one or the other cell type. Until recently, neither cell type could be targeted because C1 neurones are insensitive to the classic catecholaminergic neurotoxin 6-hydroxydopamine (6-OHDA; Jonsson 1976), and a specific marker to target the non-catecholaminergic neurones has not been identified. However, a newly introduced immunotoxin produced by conjugating the ribosomal toxin saporin to an anti-dopamine–hydroxylase antibody PIM447 (LGH447) (anti-DH-SAP) now provides the means to lesion C1 cells while sparing non-catecholaminergic neurones in the RVLM (Wrenn 1996). Membrane-bound DH is exteriorized during exocytosis and acts as a specific receptor for the internalization of anti-DH-SAP into noradrenergic and adrenergic neurones. Once inside, saporin blocks protein synthesis to PIM447 (LGH447) cause the death and eventual elimination of the cell (Stripe & Barbieri, 1986). Anti-DH-SAP has proved capable of selectively lesioning C1 neurones within the RVLM after administration into terminal fields or next to cell bodies (Wrenn 1996; Madden 1999; Schreihofer & Guyenet, 2000). We have previously shown that the depletion of bulbospinal C1 cells by intraspinal microinjection of anti-DH-SAP does not chronically alter AP or abolish sympathetic vasomotor tone in anaesthetized rats (Schreihofer & Guyenet, 2000). However, the sympathoexcitatory response to cyanide is virtually eliminated (Schreihofer & Guyenet, 2000). In the present study we examined whether the RVLM continues to maintain basal sympathetic tone after depletion of bulbospinal C1 cells with intraspinal microinjection of anti-DH-SAP. Because this treatment also depletes bulbospinal pontine noradrenergic neurones (Schreihofer & Guyenet, 2000), we also examined the effects of selective depletion of these.