Coupling of sympathetic and somatic motor outflows from the spinal cord in a perfused preparation of adult mouse in vitro

1. The relationship between sympathetic and somatic motor outflows from thoraco-lumbar spinal cord was investigated in a novel arterially perfused trunk-hindquarters preparation of adult mouse. 2. Ongoing activity was present in both somatic motor (obturator, sciatic or femoral nerves) and sympathetic outflows (either renal nerve or abdominal sympathetic chain). Sympathetic activity was rhythmic with bursts frequencies of 0.6-2.2 Hz. No obvious rhythmic activity was found in the somatic motor outflow. There were periods during which sympathetic and somatic motor activity were correlated. 3. Addition of NMDA (20-80 mu M) to the perfusate elicited repetitive burst discharges in the somatic motor outflow which were sometimes rhythmic. The frequency of these burst discharges/rhythmic activity varied between preparations but in all cases increased with increasing NMDA concentration. 4. NMDA induced burst discharges in the sympathetic outflow. This bursting activity was of the same frequency as the somatic motor outflow and the two were coupled as revealed by correlation analysis. Periods of coupling persisted for up to 3 min. 5. Administration of hexamethonium (300 mu M), to block sympathetic ganglionic transmission, had no effect on the somatic motor activity but severely attenuated sympathetic nerve discharge. 6. The thoraco-sacral cord therefore has the neuronal machinery necessary for generating and coupling activity in somatic motor and sympathetic outflows. Our findings indicate a dynamic control over the degree of coupling. We discuss that the synchronization of these neural outflows reflects either coupling between two independent mechanisms or the presence of a common synaptic driver impinging on both somatic motor and sympathetic neurones.