Descending projections of hypothalamic neurons with sympathetic nerve-related activity.

1. Spike-triggered averaging was used to identify 104 hypothalamic (HYP) neurons whose spontaneous or L-glutamate-induced action potentials were synchronized to inferior cardiac postganglionic-sympathetic nerve discharge (SND) in 39 pentobarbital sodium-anesthetized cats. Neurons were located primarily in the lateral hypothalamus but also in the posterior, dorsal, ventromedial, and anterior hypothalamus, as well as in the paraventricular region. Most neurons tested (41/60) were classified as sympathoexcitatory (SE) because their firing rate decreased during baroreceptor reflex activation. Because the firing rate of 15 neurons increased during the pressor response produced by aortic obstruction, they were classified as sympathoinhibitory (SI). The firing rate of the other four neurons tested was unaffected by baroreceptor reflex activation. 2. Microstimulation of the medullary lateral tegmental field (LTF; stereotaxic plane P10.5-P12, 2.3-3 mm lateral to the midline) antidromically activated 11 of 58 HYP neurons with sympathetic nerve-related activity, including seven SE neurons and one SI neuron. Antidromic mapping was used to trace the axonal trajectories of HYP neurons that were activated by LTF microstimulation. The results of these experiments suggested that the axons of eight of these neurons branched or terminated in the LTF. The data obtained from another series of experiments were consistent with the view that these HYP neurons excited LTF-SE neurons. LTF-SE neurons were synaptically activated by electrical stimulation of the posterior or lateral hypothalamus. This stimulus also increased SND. The modal onset latency (36 +/- 7.2 ms, mean +/- SE) of synaptic activation of LTF-SE neurons was similar to the onset latency (38 +/- 6.8 ms) of antidromic activation of HYP neurons by LTF microstimulation. These data support the view that LTF-SE neurons are involved in mediating HYP influences on SND. 3. Rostral ventrolateral medullary (RVLM)-SE neurons, including those whose axons projected to the thoracic intermediolateral nucleus (IML), also appear to be involved in mediating HYP-stimulus-induced increases in SND. HYP stimulation synaptically activated these neurons with a modal onset latency of 36 +/- 9.6 ms. Microstimulation of the region containing RVLM-SE neurons antidromically activated 16 of 60 HYP neurons with sympathetic nerve-related activity. The nine neurons tested were classified as SE. antidromic mapping revealed that RVLM microstimulation activated the main axon rather than an axonal branch or terminal of 9 of 12 of these HYP neurons. 4. Microstimulation of the mesencephalic periaqueductal gray (PAG) at stereotaxic planes A2-A3.5 antidromically activated 30 of 61 HYP neurons with sympathetic nerve-related activity, including 13 SE neurons and three SI neurons.(ABSTRACT TRUNCATED AT 400 WORDS)