Rostral ventrolateral medullary but not medullary lateral tegmental field neurons mediate sympatho-sympathetic reflexes in cats

Abstract

This study was designed to build on past work from this laboratory by testing the hypothesis that medullary lateral tegmental field (LTF) neurons play a critical role in mediating sympathoexcitatory responses to activation of sympathetic afferent fibers. We studied the effects of microinjection of N-methyl-d-aspartate (NMDA) or non-NMDA receptor antagonists or muscimol bilaterally into the LTF on the area under the curve of the computer-averaged sympathoexcitatory potential in the right inferior cardiac nerve elicited by short trains of stimuli applied to afferent fibers in the left inferior cardiac or left splanchnic nerve (CN, SN) of baroreceptor-denervated and vagotomized cats anesthetized with a mixture of diallylbarbiturate and urethane. In contrast to our hypothesis, sympathoexcitatory responses to stimulation of CN (n = 5-7) or SN (n = 4-7) afferent fibers were not significantly affected by these procedures. We then determined whether the rostral and caudal ventrolateral medulla (RVLM, CVLM) and nucleus tractus solitarius (NTS) were involved in mediating these reflexes. Blockade of non-NMDA, but not NMDA, receptors in the RVLM significantly reduced the area under the curve of the sympathoexcitatory responses to electrical stimulation of either CN (P = 0.0110; n = 6) or SN (P = 0.0131; n = 5) afferent fibers. Neither blockade of excitatory amino acid receptors nor chemical inactivation of CVLM or NTS significantly affected the responses. These data show that activation of non-NMDA receptors in the RVLM is a critical step in mediating the sympatho-sympathetic reflex.

Fig. 1.

Sites of microinjection of excitatory amino acid receptor antagonists or muscimol into the medulla. A: schematics of medullary cross sections with asterisks (*) marking target sites around which these injections were made into the lateral tegmental field (LTF), rostral and caudal ventrolateral medulla (RVLM, CVLM), and the nucleus tractus solitarius (NTS). Although only the left side of the medulla is shown, injections were placed symmetrically on the left and right sides. −1 to +5.5 refer to approximate distance (in mm) from the obex according to the stereotaxic atlas of Berman (12). B: representative histological sections showing the track made with the pipette used for injections into the NTS, CVLM, LTF, and RVLM of four cats. A, nucleus ambiguus; IO, inferior olive; Py, pyramid; 5Sp and 5St, spinal trigeminal nucleus and tract. Scale bar: 1 mm.

Fig. 2.

Evoked responses in the right inferior cardiac nerve produced by electrical stimulation of sympathetic afferent fibers. A: responses to stimulation of the cut central end of the left inferior cardiac nerve before (black trace) and 10 min after (solid gray trace) bilateral microinjection of 1,2,3,4-tetrahydro-6-nitro-2,3-dioxobenzo-[f]quinoxaline-7-sulfonamide (NBQX) into the medullary LTF and 5 min after cervical spinal cord transection (dashed gray trace). B: same as A, except stimuli were applied to the cut central end of the left splanchnic nerve. Traces here and in Figs. 4 and 5 show computer-averaged responses in the inferior cardiac nerve to 45 11-ms trains of three pulses (200 Hz; 1.0 ms) applied once every 2 s (applied at time 0 on the x-axis). Stimulus intensity was 1.0 mA in A and 0.3 mA in B. Vertical calibration, 35 μV in A and B. Area under the curve (AUC) of the evoked response is the area above the dotted horizontal black line.

Fig. 3.

Summary of effects of bilateral microinjection of drugs into the medulla on the area under the curve of the sympathoexcitatory responses in the right inferior cardiac nerve elicited by stimulation of afferents in the left inferior cardiac (CN) and left splanchnic (SN) nerves. A: changes in the area under the curve of the sympathoexcitatory responses (expressed as percent of control response) elicited by electrical stimulation of afferent fibers in CN and SN 5–10 min after bilateral microinjection of NBQX, D(−)-2-amino-5-phosphonopentanoic acid (d-AP5), or muscimol (Musc) into the LTF. B–D: same, except microinjections were made into RVLM, CVLM, and NTS, respectively. Data are expressed as means ± SE. *Statistically different from control. The number in the bars refers to the number of experiments.

Fig. 4.

Examples of sympatho-sympathetic reflexes recorded in the left inferior cardiac nerve before and after bilateral microinjection of drugs into the medulla and cervical spinal cord transection. A: response to stimulation of the left CN (0.5 mA) before (solid black trace) and 10 min after microinjection of d-AP5 into the LTF (gray trace) or the CVLM (dashed black trace). B: response to stimulation of the left SN (0.5 mA) before (black trace) and 10 min after microinjection of muscimol into the LTF (gray trace) and 5 min after cervical spinal cord transection (dashed gray trace). Vertical calibration, 50 μV (A) and 25 μV (B).

Fig. 5.

Effects of microinjection of drugs into the RVLM or cervical spinal cord transection on the sympatho-sympathetic reflex recorded in the right inferior cardiac nerve. A: response to left SN stimulation (0.6 mA) before (solid black trace), 10 min (gray trace), and 45 min after microinjection of NBQX into RVLM. B: response to left CN stimulation (0.6 mA) before (black trace) and after (solid gray trace) microinjection of d-AP5 into RVLM and 5 min after spinal cord transection (dashed gray trace). C: response to left SN stimulation (0.5 mA) before (black trace) and after (solid gray trace) microinjection of muscimol into RVLM and 5 min after spinal cord transection (dashed gray trace). Vertical calibration, 55 μV (A), 30 μV (B), and 30 μV (C).