The goal of this study was to determine whether PTP1b within the NTS contributes to the maintenance of resting baroreflex function. The key findings are that inhibition of PTP1b in the NTS results in: 1) transient decreases in resting AP and HR; 2) significant impairment of the BRS for bradycardia, independent of sustained changes in AP or HR; 3) no significant changes in the tachycardic BRS or responses to activation of cardiac vagal chemosensitive fibers. Furthermore, we provide evidence for PTP1b gene and protein expression within the dorsal medulla, an area integral for central cardiovascular and baroreflex regulation. Collectively, these results suggest a novel role for PTP1b in the modulation of the BRS at the level of the NTS and provide insight into molecular mechanisms participating in baroreflex regulation.
The most prominent finding of our study is the dramatic reduction in the BRS for bradycardia, to levels observed in hypertensive animals, (27
) in response to PTP1b inhibition. This supports a functional role for PTP1b in brain centers participating in activation of parasympathetic outflow pathways in response to baroreceptor activation. The BRS is typically assessed within 15 min after microinjections and was altered at this time point in the presence of the PTP1b inhibition. In the present study, changes in resting HR were still evident at 10 min after the injection, which could directly influence baroreflex function. Thus, we retested the BRS at 60 min when hemodynamic measures had returned to baseline levels and observed continued suppression of the baroreflex. This finding confirms that different neural mechanisms are involved in regulation of BRS versus resting hemodynamics.(15
) Although this PTP1b inhibitor is reported to be reversible, we did not examine the time course for recovery of the BRS. At the present time, we do not know the mechanisms underlying PTP1b modulation of baroreflex function. Potential mechanisms include alterations in the activity of kinases, neurotransmitter receptors (glutamate and GABA), voltage-gated channels or intracellular signaling pathways within the NTS.
The selective impairment of the BRS to increases but not decreases in pressure is consistent with the actions of other peptides in the NTS (9
) and mimics findings in human hypertension (29
). Although lower than the bradycardic BRS, the resting level of the tachycardic BRS is within the range of reported values for anesthetized rats.(9
) An inherent asymmetry in the HR responses to changes in pressure elicited by vasoactive drugs, with depressed responses to baroreflex deactivation produced by nitroprusside, has been previously observed.(31
) Providing evidence for modality-specific actions, the PTP1b inhibitor did not alter responses to activation of cardiac vagal chemosensitive afferent fibers which converge with baroreceptor inputs in the NTS (32
). We did not assess responses to different phenylbiguanide doses; however, the dose used is submaximal and has been previously used to show higher and lower responses in transgenic relative to Sprague-Dawley rats.(33
) Moreover, the attenuation of the BRS was seen across the dose range of responses for phenylephrine, suggesting the level of vagal activation is not a limiting factor in the effects of the phosphatase inhibition.
While there was no short-term effect, the PTP1b inhibitor reduced pressor responses to phenylephrine at 60 min after the injection. This is consistent with blunted vascular adrenergic responses to phenylephrine and downregulation of vascular α1-adrenergic receptor expression in PTP1b knockout mice.(7
) However, this is opposite to what is expected if the BRS is attenuated, where enhanced pressor effects in response to phenylephrine might occur. Long-term attenuation of the vagal component of BRS associated with the loss of the phosphatase would be expected to play a permissive role in the development of hypertension and favor sympathetic activation. In fact, such a pattern is reported in PTP1b knockout mice, in spite of the reduced responsiveness to phenylephrine observed in these animals. Consistent with the present study, peptides that produce systemic pressor effects and overall attenuation of BRS often elicit transient depressor and bradycardic responses in the NTS.(9
) While the mechanism of the depressor response to PTP1b inhibition is unknown, the acute decreases in pressure caused by angiotensin peptides in the NTS involve glutamate and substance P release.(34
There are potential limitations in the present study. First, it remains possible that the BRS suppression is mediated by non-specific actions of the PTP1b inhibitor. This seems unlikely given the inhibitor specificity for modification of baroreceptor afferent inputs and the bradycardic BRS, the consistency with long-term genetic deletion of the phosphatase (7
), and that the effect is opposite to that of inhibition of the PI3K pathway.(35
) Second, these studies were conducted in anesthetized animals; however, we employed an agent widely known for its preservation of autonomic function relative to other anesthetics.(36
) Under this anesthesia, we have shown that the level of resting BRS is similar between conscious and anesthetized animals and responses to other pharmacologic inhibitors are not confounded.(37
) Third, we examined acute, site-specific inhibition of PTP1b within the NTS. Further studies are needed to address whether chronic administration, either peripheral or central, results in baroreflex dysfunction and subsequent elevations in AP.