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Orexin-immunoreactive inputs to rat sympathetic preganglionic neurons.
I J Llewellyn-Smith1, C L Martin, J N Marcus
1Cardiovascular Neuroscience Group, Cardiovascular Medicine and Centre for Neuroscience, Flinders University, Bedford Park, SA 5042, Australia. ida.llewellyn-smith@flinders.edu.au
Neuroscience Letters
|October 30, 2003
Summary
Orexin neurons influence blood pressure. This study found orexin-immunoreactive axons primarily target upper spinal cord neurons, not those controlling cardiovascular functions, suggesting indirect blood pressure regulation.
Area of Science:
- Neuroscience
- Cardiovascular Physiology
- Autonomic Nervous System Research
Background:
- Orexin is known to elevate blood pressure.
- Orexin-containing nerve fibers extensively project to the spinal cord.
Purpose of the Study:
- To investigate the direct anatomical connections between orexin-immunoreactive (IR) axons and sympathetic preganglionic neurons (SPN).
- To determine if orexin directly modulates SPN involved in cardiovascular regulation.
Main Methods:
- Immunohistochemistry was used to visualize orexin-IR axons.
- SPN were identified using choline acetyltransferase (ChAT) or retrograde tracing with cholera toxin B from the superior cervical ganglion (SCG).
- Ultrastructural analysis identified synaptic contacts.
Main Results:
- Orexin-IR axons were found in close proximity to SPN in the upper thoracic segments (T1-T2) but rarely in lower thoracic and lumbar segments (T4-L2).
- Orexin fibers also contacted ChAT-IR neurons in the intercalated nucleus and central autonomic area.
- Ultrastructural evidence confirmed orexin-IR synapses on SPN projecting to the SCG.
Conclusions:
- The distribution of orexin innervation suggests it does not directly act on SPN controlling cardiovascular functions, which are located in the mid- and lower thoracic cord.
- Orexin likely influences blood pressure through supraspinal pathways rather than direct spinal sympathetic preganglionic neuron activation.