Imaging synaptically mediated responses produced by brainstem inputs onto identified spinal neurons in the neonatal mouse

Descending inputs to spinal cord neurons in mammals have previously been characterized functionally using microelectrode recording of single neurons, a technique with high spatial and temporal resolution but low yield. Consequently our knowledge about the functional connections between the brain and the spinal cord has been accumulating at a very low pace. Here we describe a high throughput optical recording approach in an ex vivo brainstem–spinal cord preparation of the neonatal mouse that permits screening many spinal neurons simultaneously for synaptic inputs from descending axons.The fluorescent calcium indicator calcium green dextran amine was loaded retrogradely into specific spinal neuron populations, including motoneurons (MNs) of the medial and lateral motor columns and two populations of interneurons with descending axons (dINs) in the ventral funiculus. Focal electrical stimulation of brainstem neuron populations with descending axons generated synaptic responses revealed by transient increases in intracellular calcium concentration in all four populations of spinal neurons. The resultant fluorescence signals could be readily visualized in individual MNs directly through the ventral white matter. In the more deeply located dINs, responses could be readily visualized in individual neurons from the surface of an oblique cut through the spinal cord.The rapid optical investigation of functional connections between brainstem descending neurons and various populations of spinal neurons in the living mammalian preparation should help uncover some of the key features of supraspinal sensory and motor control and provide a valuable tool for examining the re-innervation of spinal neurons by descending axons after spinal cord regeneration.