Cardiovascular function is altered by picomole injections of glutamate into rat medulla

Abstract

Local neural circuitry in the nucleus tractus solitarius (NTS) involved in cardiovascular control was studied by injecting nanoliter volumes of excitatory amino acids into the structure. Experiments were performed on urethane-anesthetized, artificially ventilated rats. Multibarrel micropipettes were used for pressure ejection of drugs or a dye for marking ejection sites. Ejected volumes, ranging from 200 pl to 25 nl, were directly monitored for every injection. Injections of as little as 200 fmol of L-glutamate in 200 pl into the medial and lateral NTS region rostral to the obex elicited marked, site-specific decreases in arterial pressure and heart rate. The majority of these responses were eliminated by blockade of parasympathetic and sympathetic neural outflow. At sites caudal to obex, in the commissural region of the NTS, L-glutamate injections produced marked elevations in heart rate and arterial pressure which were sympathetically mediated. Responses to L- glutamate were attenuated by concurrent injection of glutamic acid diethyl ester and DL-2-amino-4-phosphonobutyrate, or lidocaine. These results indicate a heterogeneity in the spatial organization of brain- stem circuitry underlying cardiovascular control that has not been previously described.

Stimulation of NTS A1 adenosine receptors evokes counteracting effects on hindlimb vasculature

“…In our previous study, predominantly pressor or depressor responses to microinjections of CPA were evoked from similar sites of the caudal subpostremal NTS, indicating that the type of the response was not related to anatomically specific groups of NTS neurons, for example, "depressor" (rostral and subpostremal) versus "pressor" (caudal) portion of the NTS, which respond reciprocally to microinjections of small doses of glutamate (13). In the present study, the depressor responses to activation of NTS A 1 receptors seem to be more frequent in the rostral than caudal portion of the subpostremal NTS (Fig.…”

Respiratory motoneuronal activity is altered by injections of picomoles of glutamate into cat brain stem

“…The effects of even smaller injections or the injection of drugs with subtle effects are likely to be discernible with more sophisticated analysis. Finally, brain functions other than respiratory and cardiovascular Nelson et al, 1985) control may also have intrinsic properties that can be revealed by the nanoinjection technique. The use of this technique, in conjunction with appropriate drugs and physiological controls, could be an important tool in understanding local information transfer in brain and its consequences in terms of behavior.…”

Inhibition of neurons in commissural nucleus of solitary tract reduces sympathetic nerve activity in SHR

“…Stimulation of arterial chemoreceptors evokes pressor responses via a sympathoexcitatory pathway from the commissural NTS (commNTS) that excites vasoconstrictor sympathetic premotor neurons in the RVLM either directly (21) or through brain stem pathways that involve neurons in the A5 region (13). Although their relationship to the chemoreceptor reflex is not known, the presence of sympathoexcitatory neurons in commNTS has been suggested by: 1) the increase in arterial pressure (AP) elicited by microinjection of glutamate into commNTS (8,26,31) and 2) the ability of inhibition of commNTS neurons to block both afferent stimulation-evoked increases in AP (24) and those evoked by inhibition of neurons in the CVLM (30).Although electrolytic lesions in commNTS abolish the pressor response evoked by stimulation of the arterial chemoreceptor reflex with potassium cyanide in both normotensive rats and spontaneously hypertensive rats (SHR) (6, 37), it was only in SHR that such lesions reduced the level of basal AP. These data raise the possibility that a tonically elevated level of discharge of commNTS neurons in SHR contributes to the maintenance of hypertension in this model.…”

“…Stimulation of arterial chemoreceptors evokes pressor responses via a sympathoexcitatory pathway from the commissural NTS (commNTS) that excites vasoconstrictor sympathetic premotor neurons in the RVLM either directly (21) or through brain stem pathways that involve neurons in the A5 region (13). Although their relationship to the chemoreceptor reflex is not known, the presence of sympathoexcitatory neurons in commNTS has been suggested by: 1) the increase in arterial pressure (AP) elicited by microinjection of glutamate into commNTS (8,26,31) and 2) the ability of inhibition of commNTS neurons to block both afferent stimulation-evoked increases in AP (24) and those evoked by inhibition of neurons in the CVLM (30).…”