Multiple monoaminergic modulation of posturo-locomotor network activity in the newborn rat spinal cord

Abstract

Studies devoted to understanding locomotor control have mainly addressed the functioning of the neural circuits controlling leg movements and relatively little is known of the operation of networks that activate trunk muscles in coordination with limb movements. The aim of the present work was (1) to identify the exogenous neurotransmitter cocktail that most strongly activates postural thoracic circuitry; (2) to investigate how the biogenic amines serotonin (5-HT), dopamine (DA), and noradrenaline (NA) modulate the coordination between limb and axial motor networks. Experiments were carried out on in vitro isolated spinal cord preparations from newborn rats. We recorded from ventral roots to monitor hindlimb locomotor and axial postural network activity. Each combination of the three amines with excitatory amino acids (EAAs) elicited coordinated rhythmic motor activity at all segmental levels with specific characteristics. The variability in cycle period was similar with 5-HT and DA while it was significantly higher with NA. DA elicited motor bursts of smaller amplitude in thoracic segments compared to 5-HT and NA, while both DA and NA elicited motor bursts of higher amplitude than 5-HT in the lumbar and sacral segments. The amines modulated the phase relationships of bursts in various segments with respect to the reference lumbar segment. At the thoracic level there was a phase lag between all recorded segments in the presence of 5-HT, while DA and NA elicited synchronous bursting. At the sacral level, 5-HT and DA induced an intersegmental phase shift while relationships became phase-locked with NA. Various combinations of EAAs with two or even all three amines elicited rhythmic motor output that was more variable than with one amine alone. Our results provide new data on the coordinating processes between spinal cord networks, demonstrating that each amine has a characteristic "signature" regarding its specific effect on intersegmental phase relationships.

Keywords: dopamine; locomotion; neuromodulation; noradrenaline; posture; serotonin; spinal cord.

FIGURE 1

Comparison of lumbar locomotor network activation in vitro by 5-HT, or DA or NA. (A) Each amine was added to the bathing saline containing the NMDA excitatory amino acid receptor agonist NMA (15 μM), while extracellular recordings were made from the second left and right lumbar ventral root (lL2, rL2) and the fifth left lumbar ventral root (lL5) of the isolated spinal cord. (B) Plots of mean cycle period value obtained for different amine concentrations. A dose-dependent effect is only observable for DA. (C) Plots of the coefficient of variation (COV) of cycle period for each amine (NMA 15 μM, 5-HT 15 μM, DA 50 μM, NA 50 μM), indicating that NA leads to a more unstable motor activity. (D) Plots of mean relative amplitude changes normalized to the amplitude observed in the NMA/5-HT condition (green bar). DA and NA elicit larger burst amplitude in the lumbar ventral roots. In this and in subsequent figures, the number on each bar histogram indicates the number of experiments. ^∗p < 0.05.

FIGURE 2

Comparison of axial locomotor network activation in vitro by 5-HT, or DA or NA. (A) Each amine was added to 15 μM NMA-containing saline during recordings from the sixth and eleventh left thoracic ventral roots (lT6, lT11), the second left lumbar ventral root (lL2), and the first and third left sacral ventral roots (lS1, lS3). (B) Plots indicating the percentage of experiments in which rhythmic motor activity was observed at a given segmental level. In these experiments, lumbar locomotor activity was always expressed. (C) Plots of mean relative amplitude changes normalized to the amplitude observed in the NMA/5-HT condition (green bar). Note the higher noradrenergic sensitivity of the more distal segments. ^∗p < 0.05.

FIGURE 3

Control of phase relationships by 5-HT, DA, or NA. (A) Averaged and replicated cycle sequences during locomotor-like activity induced by a combination of NMA (15 μM) and one of the three amines (5-HT, 15 μM; DA, 50 μM; NA, 50 μM). Bursts were normalized for the amplitude. Note phase differences relative to reference L2 bursts shift occurring in the thoracic and sacral segments. (B) Circular plots showing the mean phase values for burst activity in the six ventral roots recorded in A relative to the L2 ventral root in the presence of each three amines. Each point is the phase value measured from one experiment. The vector direction represents the grand mean phase value and its length is a function of the coupling strength. Data were pooled from all concentrations tested. i; ipsilateral; c, contralateral. ^∗p < 0.05.

FIGURE 4

Control of activity phase relationships by various combinations of NMA and 5-HT/DA/NA. (A) Simultaneous recordings of seven ventral roots in the presence of all four drugs. (B) Plots of mean cycle periods under various drug combinations. (C) Plots of mean relative amplitude changes normalized to the amplitude observed in the NMA/5-HT condition (green bar). (D1) Averaged and replicated cycles during locomotor-like activity induced by a combination of NMA (15 μM) and the three amines in various combinations. (D2) Circular plots calculated from the recordings in D1 showing the mean phase values for the six ventral roots relative to L2 in the presence of NMA and the different amine cocktails. All data presented were obtained at the following drug concentrations: NMA, 15 μM; 5-HT, 15 μM; DA, 50 μM; NA, 50 μM. ^∗p < 0.05.