Lateral tegmental field neurons of cat medulla: a potential source of basal sympathetic nerve discharge

Abstract

A study was made of 170 neurons of the lateral tegmental field (LTF) of the cat medulla with spontaneous activity temporally related to the 2- to 6-Hz slow wave in inferior cardiac postganglionic sympathetic nerve discharge (as demonstrated with spike-triggered averaging). LTF neurons were excited by the iontophoresis of L-glutamate, and an inflection on the rising phase of their action potentials was observed. Thus, the site of extracellular unit recording presumably was in the region of the cell body. The lag between LTF unit spike occurrence and the peak of the 2- to 6-Hz slow wave in sympathetic nerve discharge (SND) was unchanged when blood pressure and, thus, baroreceptor nerve activity were lowered to a level at which the phase relationship between the slow wave and the cardiac cycle was disrupted. Thus, the discharges of LTF neurons apparently were more closely associated with those of elements of "efferent" brain stem networks controlling SND than with those of interneurons in the afferent limb of the baroreceptor reflex arc. LTF neurons with sympathetic nerve- and cardiac-related activity were classified into three types depending on their responses to elevated carotid sinus pressure (i.e., baroreceptor reflex activation). Of the 82 neurons tested, 33 were inhibited, 16 were excited, and 33 were unaffected by baroreceptor reflex activation. Using data collected in this and previous studies from our laboratory, we compared the firing times of neurons in the LTF, rostral ventrolateral medulla, and medullary raphe nuclei relative to the peak of the sympathetic nerve slow wave. LTF neurons that were inhibited by baroreceptor reflex activation are presumed to subserve a sympathoexcitatory function. These neurons fired significantly earlier during the sympathetic nerve slow wave than their counterparts in the rostral ventrolateral medulla and medullary raphe nuclei. LTF neurons classified as sympathoinhibitory (i.e., excited by baroreceptor reflex activation) fired significantly earlier than their counterparts in the medullary raphe nuclei. These data raise the possibility that LTF neurons are closer (at least in a temporal sense) to the region of origin of the 2- to 6-Hz component of SND than are ventrolateral medullary and raphe neurons. The firing times of sympathoexcitatory and sympathoinhibitory LTF neurons were not significantly different. These data are discussed relative to potential mechanisms involved in generating SND. Microstimulation of the second thoracic spinal segment was used to determine whether the axons of LTF neurons with sympathetic nerve-related activity projected to this level.(ABSTRACT TRUNCATED AT 400 WORDS)

Medullary lateral tegmental field: control of respiratory rate and vagal lung inflation afferent influences on sympathetic nerve discharge

“…In the models proposed by Koepchen et al (38) and Richter and Spyer (49), separate control of the cardiovascular and respiratory systems is manifested only at the level of bulbospinal and spinal neurons. In contrast to their models, our data support the view that selective control of SND is evident at the level of LTF neurons which are antecedent to RVLM-spinal sympathoexcitatory neurons (8,30).…”

“…Multiple injections were made so that the drug would spread through the portion of the LTF that contains neurons with activity correlated to SND, ranging from 1-4 mm rostral to the obex, 2-3 mm lateral to the midline, and from 2.25 to 4.75 mm below the dorsal surface (7-9, 12, 30). The most rostral injection site in the LTF is ϳ2 mm caudal, 1 mm medial, and ϳ1 mm dorsal to the region of the RVLM that contains neurons with activity correlated to SND (8,30). The most caudal injection site in the LTF is 1-1.5 mm medial and ϳ1 mm dorsal to the most rostral portion of the caudal ventrolateral medulla (CVLM) that contains neurons with activity correlated to SND (11).…”

“…However, the LTF also contains neurons with activity correlated to both SND and PNA in vagotomized cats (7). Because LTF neurons are likely antecedent to RVLM neurons in sympathoexcitatory pathways at least in the cat (8,30), it is possible that central respiratory and sympathetic circuits interact at a level antecedent to RVLM-spinal neurons. Thus a second objective of the current study was to test the hypothesis that LTF neurons are involved in central respiratory modulation of SND in vagotomized cats.…”

Medullary lateral tegmental field mediates the cardiovascular but not respiratory component of the Bezold-Jarisch reflex in the cat

“…The inferior cardiac postganglionic branch of the left stellate ganglia was exposed retropleurally by removing the head of the first rib (13), and the right phrenic nerve was isolated in the neck (29). The nerves were covered with silicone release agent (Dow Corning 7 compound).…”

“…The micropipette was positioned in tracks located ϳ2 and 3 mm rostral to the obex and 2.5-2.8 mm lateral to the midline; microinjections were made bilaterally at a depth of ϳ3.8 mm from the dorsal surface. Multiple injections were made so that the drug would spread through the portion of the LTF that contains neurons with activity correlated to SND (2,3,6,13). The most rostral injection site in the LTF is ϳ2 mm caudal, 1 mm medial, and ϳ1 mm dorsal to the region of the RVLM that contains neurons with activity correlated to SND (2, 13).…”