Background
Sympathetic preganglionic neurons (SPN) within the thoracic intermediolateral cell column (IML) affect an extensive range of end organs such as the heart, adrenal medulla and interscapular brown adipose tissue (iBAT). These SPN are intrinsically silent but receive substantial supraspinal input including fast neurotransmitters like GABA and slow peptides such as enkephalin.
Methods
To identify sympathetic premotor sources of GABA and enkephalin, immunohistochemical detection of cells retrogradely traced from the IML combined with in situ hybridisation to detect glutamate decarboxylase (GAD67) and preproenkephalin (PPE) mRNA was conducted.
To determine the autonomic functions of GABA and enkephalin in the spinal cord, intrathecal administration of the GABA-A antagonist bicuculline and mu-opioid antagonist naloxone was performed between T3-T6 in urethane-anaesthetised, vagotomised, paralysed rats.
Results
Of the five sympathetic premotor groups, the caudal raphe (including pallidus, obscurus and magnus) and RVMM each provided approximately 40% of total GAD67-expressing input to the spinal cord. The PVN possessed 38% of PPE-containing neurons projecting to the spinal cord.
Intrathecal bicuculline raised blood pressure, splanchnic sympathetic nerve activity and phrenic nerve activity in a burst-like manner, as previously published (Goodchild et al 2008), but also evoked an increase in iBAT temperature (2°C ± 0.13), end tidal CO2 (1.11% ± 0.11) and heart rate (60bpm ± 20).
Conclusions
These findings indicate that GABAergic and enkephalinergic inputs to the spinal cord primarily arise from different sympathetic premotor regions. These neurotransmitters act differentially in the spinal cord to affect multiple outflows including iBAT thermogenesis.
Disclosure
None