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

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)