RVLM sympathetic premotor neurons are not synaptically linked to each other

Sympathetic premotor neurons residing within the rostral ventrolateral medulla (RVLM) control the activity of sympathetic nerves and blood pressure. In vivo, the activities of sympathetic nerves and spinally projecting cells in the RVLM display rhythmic bursting that is independent of baroreceptor input. We have previously proposed that such synchronisation could result from functional coupling between spinally projecting neurons. In this study we directly test that hypothesis by examining the prevalence of synaptic connections between pairs of RVLM sympathetic premotor neurons, identified by retrograde transport of tracer microinjected into the spinal cord. We simultaneously recorded membrane voltage and currents in pairs (34 pairs from x neurons) of bulbospinal neurons recorded in whole cell mode in acute brainstem slices prepared from P8 - P20 rats. We injected depolarizing currents to evoke trains of action potentials in the ‘presynaptic’ neuron and used ‘presynaptic’ action potentials to trigger averages of holding current recorded in the ‘post-synaptic’ neuron. No evidence of monosynaptic connections was observed in any case, despite long and stable recordings from neurons in close proximity to one another. We conclude that direct synaptic connections between bulbospinal neurons are sparse or non-existent and that direct connections between these cells toes not play a significant functional role.1 page(s