Activity-dependent plasticity in desecending synaptic inputs to spinal motoneurons in an in vitro turtle brainstem-spinal cord preparation

Abstract

Long-term (>1 h) and short-term (sec-to-min) activity-dependent synaptic plasticity have been proposed to contribute to the patterning of rhythmic network activity. There is, however, very little experimental evidence to support this hypothesis. Previously, we demonstrated that electrically-evoked descending synaptic inputs to respiratory-related spinal motoneurons express long-term depression (LTD) or long-term potentiation (LTP) following spinal stimulation at different frequencies in an in vitro turtle brainstem-spinal cord preparation [1]. For example, evoked potentials in pectoralis (expiratory) nerves express LTD following 1 and 10 Hz spinal stimulation (900 pulses), while potentials in serratus (inspiratory) nerves express LTP following 100 Hz spinal stimulation (900 pulses). In this study, we hypothesized that activity-dependent synaptic plasticity is expressed in identified synaptic connections within the respiratory control system of adult turtles (Pseudemys), and that LTD is expressed in respiratory descending synaptic inputs to pectoralis motoneurons following 10 Hz spinal stimulation. Using in vitro turtle brainstem-spinal cord preparations (n = 6), the lateral funiculus at spinal segment C5 (rostral to pectoralis and serratus motoneurons) was electrically stimulated (10 Hz, 900 pulses, 400 μA) while the preparation spontaneously produced rhythmic respiratory motor output. One application of spinal stimulation immediately decreased respiratory burst amplitude by ~75% on both pectoralis and serratus (P 0.05). Closer examination showed that serratus burst amplitudes at 50 min post-stim were depressed by ~20% in 3/5 preparations, and potentiated by 100–150% in 2/5 preparations. Spinal stimulation did not change hypoglossal burst amplitude. In conclusion, 10 Hz spinal stimulation did not exclusively elicit LTD in spontaneous expiratory activity in pectoralis nerves, suggesting that LTD is expressed in nonrespiratory-related descending synaptic inputs to pectoralis motoneurons, or that spontaneous respiratory activity in pectoralis motoneurons overrides the expression of LTD. We hypothesize that the activity-dependent plasticity observed with spinal stimulation is due to spinal mechanisms because hypoglossal respiratory activity was unaltered.