Role of excitatory amino acids in brainstem activation of spinal locomotor networks in larval lamprey

Brainstem control of locomotion and muscle tone with special reference to the role of the mesopontine tegmentum and medullary reticulospinal systems

The lateral part of the mesopontine tegmentum contains functionally important structures involved in the control of posture and gait. Specifically, the mesencephalic locomotor region, which may consist of the cuneiform nucleus and pedunculopontine tegmental nucleus (PPN), occupies the interest with respect to the pathophysiology of posture-gait disorders. The purpose of this article is to review the mechanisms involved in the control of postural muscle tone and locomotion by the mesopontine tegmentum and the pontomedullary reticulospinal system. To make interpretation and discussion more robust, the above issue is considered largely based on our findings in the experiments using decerebrate cat preparations in addition to the results in animal experimentations and clinical investigations in other laboratories. Our investigations revealed the presence of functional topographical organizations with respect to the regulation of postural muscle tone and locomotion in both the mesopontine tegmentum and the pontomedullary reticulospinal system. These organizations were modified by neurotransmitter systems, particularly the cholinergic PPN projection to the pontine reticular formation. Because efferents from the forebrain structures as well as the cerebellum converge to the mesencephalic and pontomedullary reticular formation, changes in these organizations may be involved in the appropriate regulation of posture-gait synergy depending on the behavioral context. On the other hand, abnormal signals from the higher motor centers may produce dysfunction of the mesencephalic-reticulospinal system. Here we highlight the significance of elucidating the mechanisms of the mesencephalic-reticulospinal control of posture and locomotion so that thorough understanding of the pathophysiological mechanisms of posture-gait disorders can be made.

Respiratory rhythms generated in the lamprey rhombencephalon

Brainstem networks generating the respiratory rhythm in lampreys are still not fully characterized. In this study, we described the patterns of respiratory activities and we identified the general location of underlying neural networks. In a semi-intact preparation including the brain and gills, rhythmic discharges were recorded bilaterally with surface electrodes placed over the vagal motoneurons. The main respiratory output driving rhythmic gill movements consisted of short bursts (40.9+/-15.6 ms) of discharge occurring at a frequency of 1.0+/-0.3 Hz. This fast pattern was interrupted by long bursts (506.3+/-174.6 ms) recurring with an average period of 37.4+/-24.9 s. After isolating the brainstem by cutting all cranial nerves, the frequency of the short respiratory bursts did not change significantly, but the slow pattern was less frequent. Local injections of a glutamate agonist (AMPA) and antagonists (6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or D,L-amino-5-phosphonopentanoic acid (AP5)) were made over different brainstem regions to influence respiratory output. The results were similar in the semi-intact and isolated-brainstem preparations. Unilateral injection of AP5 or CNQX over a rostral rhombencephalic region, lateral to the rostral pole of the trigeminal motor nucleus, decreased the frequency of the fast respiratory rhythm bilaterally or stopped it altogether. Injection of AMPA at the same site increased the rate of the fast respiratory rhythm and decreased the frequency of the slow pattern. The activity recorded in this area was synchronous with that recorded over the vagal motoneurons. After a complete transverse lesion of the brainstem caudal to the trigeminal motor nucleus, the fast rhythm was confined to the rostral area, while only the slow activity persisted in the vagal motoneurons. Our results support the hypothesis that normal breathing depends on the activity of neurons located in the rostral rhombencephalon in lampreys, whereas the caudal rhombencephalon generates the slow pattern.

Alpha-adrenoreceptor activation modulates swimming via glycinergic and GABAergic inhibitory pathways in Xenopus laevis tadpoles

This study focuses upon the network pathways underlying the adrenoreceptor-mediated modulation of fictive swimming in the immobilized Xenopus laevis tadpole. As shown recently, noradrenaline (NA) increases cycle periods while simultaneously reducing the rostrocaudal delay in head-to-tail firing and the duration of swimming episodes. Furthermore, both swimming frequency and duration are reduced by selective pharmacological activation of alpha1- and/or alpha2-adrenoreceptors, while alpha1-receptor activation also reduces rostrocaudal delays. We show that NA could still modulate aspects of swimming after blocking either glycine or GABA(A) receptors with strychnine and bicuculline, respectively. Furthermore, after prior application of NA, strychnine could counteract noradrenergic effects on cycle periods and rostrocaudal delays, while bicuculline could counteract effects on cycle periods, suggesting that these two fast inhibitory pathways are both involved in the NA modulation of swimming. In addition, blocking glycine receptors reduced the effects of alpha1-receptors on cycle periods and delays, while blocking GABA(A) receptors had no effect. Blocking either glycine or GABA(A) receptors, however, lessened the reduction in swimming frequency by alpha2-receptors. In addition, pre-application of bicuculline prevented a reduction in episode durations by NA, alpha1- and alpha2-receptors. Our findings suggest that the noradrenergic modulation of Xenopus swimming is mediated via alpha-adrenoreceptors interacting with both glycinergic and GABAergic inhibitory pathways. Both alpha1- and alpha2-receptor activation influences the GABAergic pathway controlling the duration of swimming episodes and is involved in the glycinergic modulation of the swimming rhythm and its longitudinal co-ordination, with alpha2-receptors additionally affecting swimming frequency through GABAergic pathways.

Synaptic drive to motoneurons during fictive swimming in the developing zebrafish

The development of swimming behavior and the correlated activity patterns recorded in motoneurons during fictive swimming in paralyzed zebrafish larvae were examined and compared. Larvae were studied from when they hatch (after 2 days) and are first capable of locomotion to when they are active swimmers capable of capturing prey (after 4 days). High-speed (500 Hz) video imaging was used to make a basic behavioral characterization of swimming. At hatching and up to day 3, the larvae swam infrequently and in an undirected fashion. They displayed sustained bursts of contractions ('burst swimming') at an average frequency of 60-70 Hz that lasted from several seconds to a minute in duration. By day 4 the swimming had matured to a more frequent and less erratic "beat-and-glide" mode, with slower (approximately 35 Hz) beats of contractions for approximately 200 ms alternating with glides that were twice as long, lasting from just a few cycles to several minutes overall. In whole cell current-clamp recordings, motoneurons displayed similar excitatory synaptic activity and firing patterns, corresponding to either fictive burst swimming (day 2-3) or beat-and-glide swimming (day 4). The resting potentials were similar at all stages (about -70 mV) and the motoneurons were depolarized (to about -40 mV) with generally non-overshooting action potentials during fictive swimming. The frequency of sustained inputs during fictive burst swimming and of repetitive inputs during fictive beat-and glide swimming corresponded to the behavioral contraction patterns. Fictive swimming activity patterns were eliminated by application of glutamate antagonists (kynurenic acid or 6-cyano-7-nitroquinoxalene-2,3-dione and DL-2-amino-5-phosphonovaleric acid) and were modified but maintained in the presence of the glycinergic antagonist strychnine. The corresponding synaptic currents underlying the synaptic drive to motoneurons during fictive swimming could be isolated under voltage clamp and consisted of cationic [glutamatergic postsynaptic currents (PSCs)] and anionic inputs (glycinergic PSCs). Either sustained or interrupted patterns of PSCs were observed during fictive burst or beat-and-glide swimming, respectively. During beat-and-glide swimming, a tonic inward current and rhythmic glutamatergic PSCs (approximately 35 Hz) were observed. In contrast, bursts of glycinergic PSCs occurred at a higher frequency, resulting in a more tonic pattern with little evidence for synchronized activity. We conclude that a rhythmic glutamatergic synaptic drive underlies swimming and that a tonic, shunting glycinergic input acts to more closely match the membrane time constant to the fast synaptic drive.

Hemisegmental localisation of rhythmic networks in the lumbosacral spinal cord of neonate mouse

In vitro isolated spinal cord preparations of newborn mice were used to examine the localisation of neuronal network(s) involved in the centrally-driven command of motor activities. Transections of reduced spinal cord preparations were performed under different extracellular bathing conditions, to obtain the smallest piece of cord capable of generating spinal motor rhythm. Under normal bathing medium, the whole lumbosacral cord from 0 to 2-day-old mice (P0-2 group) must be maintained to generate spontaneous motor bursts on lumbar ventral roots. In the P3-5 group, however, a three segment long section from the sacral part of the cord was still able to produce spontaneous episodes of rhythmic activity. Using a Mg2+-free medium to activate quiescent motor neuronal networks, transection procedures revealed that a double lumbar segment and a single segment (at both lumbar and sacral levels) of the cord continued to exhibit rhythmic locomotor-like discharges in P0-2 and P3-5 groups, respectively. In some experiments in which isolated reduced preparations did not generate any rhythmic activity in ventral roots, central inhibitory influences were blocked by addition of bicuculline (20-30 microM) or strychnine (20 microM) to the superperfusate. Under these conditions, a slow and synchronous rhythmic activity was typically recorded from lumbar and sacral outputs in both P0-2 and P3-5 groups. Finally, transection experiments showed that lumbar and sacral hemisegments of the cord retained the ability to generate a bicuculline- or strychnine-induced motor rhythm. These results suggest that (1) intersegmental connections appear to be stronger in P0-2 than in P3-5 group, since under both normal or Mg2+-free bathing medium, spinal rhythmic activity was more affected by transection procedures in preparations from the younger animals, and (2) neuronal networks producing rhythmic motor activities in mouse may be segmentally organised, each hemisegment being able to generate its own spinal motor rhythm.

Involvement of AMPA receptors in posterior locomotor activity in the rabbit: an in vivo study

Although AMPA receptors are known to be widely involved in excitatory synaptic neurotransmission at the spinal level, very little is known about their role in modulating motor activity in mammals. In curarized decerebrate or spinalized rabbit preparations, fictive locomotion was monitored on hindlimb nerves after either activation or blockade of AMPA receptors. In decerebrate preparations, the administration of the antagonist, NBQX (3.5 mg/kg i.p.) or the agonist, AMPA (0.5 mg/kg i.v.) produced, in both cases, a depression of locomotor activities induced by stimulation of cutaneous afferents (evoked locomotor activity). This potent effect was transient with AMPA (recovery after 20 min) and followed by the occurrence of spontaneous locomotor sequences, while no recovery was observed with NBQX treatment. In spinal preparations where a continuous 'spontaneous' locomotor activity resulted from the pharmacological activation of noradrenergic descending pathways (nialamide-DOPA pretreatment), the same drugs injected at higher doses (5 mg/kg NBQX i.p. and 1 mg/kg AMPA i.v.) only weakly affected the frequency of 'spontaneous' and evoked locomotor bursts while they exerted inhibitory and facilitatory effects on the burst amplitude respectively. The results suggest that AMPA receptors are involved at spinal level: 1) in direct mediation of cutaneous afferent excitatory effects on the posterior locomotor generators (pLG); 2) in indirect mediation of a supraspinal descending inhibition controlling, likely presynaptically, the cutaneous afferent activation; and 3) in transmission to motoneurons of the output signals from the pLG. Finally, tight spinal interactions between potent descending noradrenergic pathways and spinal AMPA neurotransmission were disclosed.

Role of sensory-evoked NMDA plateau potentials in the initiation of locomotion

Reticulospinal (RS) neurons constitute the main descending motor system of lampreys. This study reports on natural conditions whereby N-methyl-D-aspartate (NMDA)-mediated plateau potentials were elicited and associated with the onset of locomotion. Reticulospinal neurons responded in a linear fashion to mild skin stimulation. With stronger stimuli, large depolarizing plateaus with spiking activity were elicited and were accompanied by swimming movements. Calcium imaging revealed sustained intracellular calcium rise upon sensory stimulation. Blocking NMDA receptors on RS neurons prevented the plateau potentials as well as the associated rise in intracellular calcium. Thus, the activation of NMDA receptors mediates a switch from sensory-reception mode to a motor command mode in RS neurons.