Non‐targeted Effects of a Targeted Toxin

Abstract

Saporin (SAP) alone or conjugated either with stabilized substance P (SSP‐SAP) or with an antibody to dopamine β‐hydroxylase (anti‐DBH‐SAP) leads to attenuation of arterial baroreflexes when injected bilaterally into the nucleus tractus solitarii (NTS) of rats. Effects on cardiovascular reflex control could be due to an impact of the toxin on cells that express not only the target protein but also other receptors. To test that hypothesis we assessed effects of glutamate, N‐methyl‐D‐aspartate (NMDA), or α‐amino‐3‐hydroxy‐5‐methylisoxazole‐4‐proprionic acid (AMPA) injected into the NTS at the same site as a prior injection of SAP, anti‐DBH‐SAP, SSP‐SAP, 6‐hydroxydopamine (6‐OHDA) or phosphate buffered saline (PBS). 6‐OHDA was included because our previous studies showed that its injection into the NTS led to loss of noradrenergic neurons while not affecting other neuronal types. Also, 6‐OHDA did not lead to loss of astrocytes as seen after injection of anti‐DBH‐SAP, SSP‐SAP, or SAP into NTS. In control rats that received PBS, glutamate agonists produced dose‐related hypotensive, bradycardic effects, which were significantly reduced in animals treated with SAP containing toxins. Prior injection of 6‐OHDA did not affect bradycardic or hypotensive responses elicited by glutamate receptor agonists in the NTS. This study provides a cautionary note in interpreting physiological responses resulting from use of saporin conjugates in that, while selectively damaging neurons with a specific target, the toxins will also disrupt other neuronal functions medicated by the same neuron but through different mechanisms. Further, the study supports the concept that astrocytes in NTS play a role in mediating central cardiovascular control. Support: VA Merit Review and NIH R01 HL59593