Systematic variation in effects of serotonin and norepinephrine on repetitive firing properties of ventral horn neurons.

Spinal interneurons are essential integrators of descending and peripheral input that receive profuse monoaminergic influence from brainstem nuclei. In this study, the effects of the monoamines serotonin and norepinephrine on the intrinsic properties of ventral horn interneurons were investigated in a slice preparation of the lumbar cord of 7-19 day old rats. Three cell groups with distinct firing patterns in response to steps of injected current were observed and classified as repetitive-firing, initial-burst or single-spiking. Input conductance tended to be largest in single-spiking cells whereas repetitive-firing cells showed the greatest tendency for spontaneous firing and had the fastest rate of rise for the action potential. Rhythmic firing behaviors were defined by the frequency-current relation evoked by linearly increasing current ramps. The monoaminergic modulation of firing patterns and frequency-current relations was primarily studied in repetitive-firing cells. The frequency-current threshold current was decreased in cells with high pre-drug values and increased in cells with low pre-drug values. Therefore, monoamine administration decreased the input-output heterogeneity of the repetitive-firing cells by compressing the range of frequency-current threshold currents. This action of monoamines may have a key role in the suppression of sensory-evoked reflexes and the production of coordinated movement.

Spinal interneurons that receive input from muscle afferents are differentially modulated by dorsolateral descending systems.

The possibility that descending systems have differential actions on the spinal interneurons that receive input from muscle afferents was investigated. Prolonged, physiological inputs were generated by stretch of the triceps surae muscles. The resulting firing patterns of 25 lumbosacral interneurons were recorded before and during a reversible cold block of the dorsolateral white matter at the thoracic level in nonparalyzed, decerebrate preparations. The strength of group I muscle afferent input was assessed from the response to sinusoidal tendon vibration, which activated muscle spindle Ia afferents directly and tendon organ Ib afferents via the resulting reflex force. The stretch-evoked responses of interneurons with strong responses to vibration were markedly suppressed by dorsal cold block, whereas the stretch-evoked responses of interneurons with weak vibration input were enhanced. The cells most strongly activated by vibration received their primary input from Ia afferents and all of these cells were inhibited by the cold block. These results suggest that a disruption of the descending system, such as occurs in spinal cord injury, will lead to a suppression of the interneuronal pathways with group Ia input while enhancing excitability within interneuronal pathways transmitting actions from higher threshold afferents. One possible consequence of this suppression would be a decreased activity among the Ia inhibitory interneurons that mediate reciprocal inhibition, resulting in abnormal reciprocal relations between antagonists and promoting anomalous muscle cocontraction.