Larry M. Jordan

The role of serotonin in reflex modulation and locomotor rhythm production in the mammalian spinal cord

Over the past 40 years, much has been learned about the role of serotonin in spinal cord reflex modulation and locomotor pattern generation. This review presents an historical overview and current perspective of this literature. The primary focus is on the mammalian nervous system. However, where relevant, major insights provided by lower vertebrate models are presented. Recent studies suggest that serotonin-sensitive locomotor network components are distributed throughout the spinal cord and the supralumbar regions are of particular importance. In addition, different serotonin receptor subtypes appear to have different rostrocaudal distributions within the locomotor network. It is speculated that serotonin may influence pattern generation at the cellular level through modulation of plateau properties, an interplay with N-methyl-D-aspartate receptor actions, and afterhyperpolarization regulation. This review also summarizes the origin and maturation of bulbospinal serotonergic projections, serotonin receptor distribution in the spinal cord, the complex actions of serotonin on segmental neurons and reflex pathways, the potential role of serotonergic systems in promoting spinal cord maturation, and evidence suggesting serotonin may influence functional recovery after spinal cord injury.

Initiation of Locomotion in Mammals

Abstract: Several “locomotor regions” of the mammalian brain stem can be stimulated, either electrically or chemically, to induce locomotion. Active cells labeled with c‐fos within the mesencephalic locomotor region (MLR) have been found in the periaqueductal gray, the cuneiform nucleus, the pedunculopontine nucleus, and the locus coeruleus. Different subsets of these nuclei appear to be activated during locomotion produced in different behavioral contexts. The locomotor nuclei can be classified into areas associated with exploratory, appetitive, and defensive locomotion, in accordance with the proposal of Sinnamon (1993, Prog. Neurobiol. 41: 323‐344). The interpretation of lesion studies designed to reveal areas of the brain essential for locomotion must be based on knowledge of the nuclei which become active in the specific locomotor task being tested. An argument is put forward in favor of the continued use of the term “mesencephalic locomotor region.”

Dendritic L-type calcium currents in mouse spinal motoneurons: implications for bistability.

The intrinsic properties of mammalian spinal motoneurons provide them with the capability to produce high rates of sustained firing in response to transient inputs (bistability). Even though it has been suggested that a persistent dendritic calcium current is responsible for the depolarizing drive underlying this firing property, such a current has not been demonstrated in these cells. In this study, calcium currents are recorded from functionally mature mouse spinal motoneurons using somatic whole‐cell patch‐clamp techniques. Under these conditions a component of the current demonstrated kinetics consistent with a current originating at a site spatially segregated from the soma. In response to step commands this component was seen as a late‐onset, low amplitude persistent current whilst in response to depolarizing–repolarizing ramp commands a low voltage clockwise current hysteresis was recorded. Simulations using a neuromorphic motoneuron model could reproduce these currents only if a noninactivating calcium conductance was placed in the dendritic compartments. Pharmacological studies demonstrated that both the late‐onset and hysteretic currents demonstrated sensitivity to both dihydropyridines and the L‐channel activator FPL‐64176. Furthermore, the α1D subunits of L‐type calcium channels were immunohistochemically demonstrated on motoneuronal dendrites. It is concluded that there are dendritically located L‐type channels in mammalian motoneurons capable of mediating a persistent depolarizing drive to the soma and which probably mediate the bistable behaviour of these cells.

On the regulation of repetitive firing in lumbar motoneurones during fictive locomotion in the cat

SummaryRepetitive firing of motoneurones was examined in decerebrate, unanaesthetised, paralysed cats in which fictive locomotion was induced by stimulation of the mesencephalic locomotor region. Repetitive firing produced by sustained intracellular current injection was compared with repetitive firing observed during fictive locomotion in 17 motoneurones. During similar interspike intervals, the afterhyperpolarisations (AHPs) during fictive locomotion were decreased in amplitude compared to the AHPs following action potentials produced by sustained depolarising current injections. Action potentials were evoked in 10 motoneurones by the injection of short duration pulses of depolarising current throughout the step cycles. When compared to the AHPs evoked at rest, the AHPs during fictive locomotion were reduced in amplitude at similar membrane potentials. The post-spike trajectories were also compared in different phases of the step cycle. The AHPs following these spikes were reduced in amplitude particularly in the depolarised phases of the step cycles. The frequency-current (f-I) relations of 7 motoneurones were examined in the presence and absence of fictive locomotion. Primary ranges of firing were observed in all cells in the absence of fictive locomotion. In most cells (6/7), however, there was no relation between the amount of current injected and the frequency of repetitive firing during fictive locomotion. In one cell, there was a large increase in the slope of the f-I relation. It is suggested that this increase in slope resulted from a reduction in the AHP conductance; furthermore, the usual elimination of the relation is consistent with the suggestions that the repetitive firing in motoneurones during fictive locomotion is not produced by somatic depolarisation alone, and that motoneurones do not behave as simple input-output devices during this behaviour. The correlation of firing level with increasing firing frequency which has previously been demonstrated during repetitive firing produced by afferent stimulation or by somatic current injection is not present during fictive locomotion. This lends further support to the suggestion that motoneurone repetitive firing during fictive locomotion is not produced or regulated by somatic depolarisation. It is suggested that although motoneurones possess the intrinsic ability to fire repetitively in response to somatic depolarisation, the nervous system need not rely on this ability in order to produce repetitive firing during motor acts. This capability to modify or bypass specific motoneuronal properties may lend the nervous system a high degree of control over its motor output.

Localization of a descending pathway in the spinal cord which is necessary for controlled treadmill locomotion

The spinal cord pathways which are important for controlled treadmill locomotion evoked by stimulation of the mesencephalic locomotor region (MLR) were investigated in cats subjected to subtotal spinal cord lesions at the C1-C2 level. Locomotion could be evoked following bilateral lesions of the dorsal columns, the dorsolateral funiculi, and the ventromedial funiculi, and after combined lesions of the dorsolateral and ventromedial funiculi, but not after bilateral lesions of the ventrolateral quadrant. Unilateral lesions of the ventrolateral quadrant abolished locomotion in the limbs is ipsilateral to the lesion. It is suggested that MLR stimulation may give rise to locomotion by activation of pontine and medullary reticulospinal pathways projecting through the ventrolateral quadrant.

Activity of interneurons within the L4 spinal segment of the cat during brainstem-evoked fictive locomotion

SummaryExtracellular recordings from interneurons located in the L4 spinal segment were made during fictive locomotion produced by electrical stimulation of the mesencephalic locomotor region (MLR) in the paralysed decerebrate cat. Only interneurons within the L4 segment which received group II input from quadriceps, sartorius or the pretibial flexor muscle afferents and which had axonal projections to motor nuclei in L7 were selected for analysis. During the fictive step cycle two thirds of these interneurons fired action potentials during the time of activity in the ipsilateral hindlimb flexor neurograms. These cells were also less responsive to peripheral input during the extension phase of the fictive locomotion cycle. The remaining one third of the interneurons examined were not rhythmically active during locomotion. The possible contributions of the midlumbar interneurons to motoneuron activity during locomotion are discussed.

Autoradiographic demonstration of the projections from the mesencephalic locomotor region

An autoradiographic tracing technique was used to examine the projections of the classically defined mesencephalic locomotor region (MRL). Injections of [3H]proline and [3H]leucine were made into sites in the caudal mesencephalon which can be stimulated to produce locomotion. The injection sites were confined to the cuneiform nucleus (stereotaxic coordinates P2.0, L4.0, H-1.0). Descending projections were primarily ipsilateral to the gigantocellular and magnocellular reticular formation of the pons and medulla, the dorsal tegmental reticular nucleus, and the nucleus raphe magnus. Some sparse contralateral projections were also observed within the magnocellular and gigantocellular reticular formation. Direct axonal connections with the spinal cord were not consistently observed. Ascending projections were observed to the subthalamic nucleus, caudal hypothalamic nuclei, the centrum medianum nucleus of the thalamus, the ventral tegmental area of Tsai, the superior colliculus, and the periaqueductal gray region. The ascending projections were also ipsilateral, with sparse contralateral labeling confined to areas which received ipsilateral projections. Projections to the contralateral cuneiform nucleus were also consistently observed. The results, when compared to those of another study, suggest that the classical MLR is anatomically distinct from the more medial sites in the mesencephalon which can also induce locomotion.

The role of Renshaw cells in locomotion: antagonism of their excitation from motor axon collaterals with intravenous mecamylamine

SummaryThe contribution of Renshaw cell (RC) activity to the production of fictive locomotion in the mesencephalic preparation was examined using the nicotinic antagonist mecamylamine (MEC). After the i.v. administration of 3 doses of MEC (1.0 mg/kg) the following observations were made: 1) ventral root (VR) evoked discharge of RCs was decreased by up to 87.7%, 2) recurrent inhibitory postsynaptic potentials recorded in alpha motoneurons were greatly reduced or abolished, and 3) the rhythmic firing of RCs during the fictive step cycle was abolished in 83% of the cells examined. Locomotor drive potentials (LDPs) in motoneurons persisted during the fictive step cycle after MEC administration. Bursts of motoneuron firing during each fictive step cycle were characterized by increased frequency and number of spikes after MEC, although the burst duration was unaltered for similar step cycle lengths. A greater number and frequency of spikes per burst was also observed in Ia inhibitory interneurons (IaINs), which remained rhythmically active after MEC administration. It is concluded that Renshaw cells are not an integral part of the spinal central pattern generator for locomotion, nor do they control the timing of the motoneuron or IaIN bursts of firing during fictive locomotion. The data are consistent with a role for RCs in limiting the firing rates of motoneurons and IaINs during each burst.

State‐dependent hyperpolarization of voltage threshold enhances motoneurone excitability during fictive locomotion in the cat

1 Experiments were conducted on decerebrate adult cats to examine the effect of brainstem‐evoked fictive locomotion on the threshold voltage (Vth) at which action potentials were initiated in hindlimb motoneurones. Measurements of the voltage threshold of the first spike evoked by intracellular injection of depolarizing ramp currents or square pulses were compared during control and fictive locomotor conditions. The sample of motoneurones included flexor and extensor motoneurones, and motoneurones with low and high rheobase currents. 2 In all 38 motoneurones examined, action potentials were initiated at more hyperpolarized membrane potentials during fictive locomotion than in control conditions (mean hyperpolarization ‐8.0 ± 5.5 mV; range ‐1.8 to ‐26.6 mV). Hyperpolarization of Vth occurred immediately at the onset of fictive locomotion and recovered in seconds (typically < 60 s) following the termination of locomotor activity. 3 The Vth of spikes occurring spontaneously without intracellular current injection was also reduced during locomotion. 4 Superimposition of rhythmic depolarizing current pulses on current ramps in the absence of locomotion did not lower Vth to the extent seen during fictive locomotion. We suggest that Vth hyperpolarization results from an as yet undetermined neuromodulatory process operating during locomotion and is not simply the result of the oscillations in membrane potential occurring during locomotion.The hyperpolarization of Vth for action potential initiation during locomotion is a state‐dependent increase in motoneurone excitability. This Vth hyperpolarization may be a fundamental process in the generation of motoneurone activity during locomotion and perhaps other motor tasks.

Reversible cooling of the brainstem reveals areas required for mesencephalic locomotor region evoked treadmill locomotion

SummaryThe evidence suggests that the mesencephalic locomotor region (MLR) may not be a unitary region since anatomical and functional variations in the descending projections are clearly indicated. Reversible cooling of midline reticular structures can effectively block locomotion evoked by stimulation of lateral MLR (L3.5–4) sites while not significantly affecting the locomotion evoked from more medial MLR (L2–2.5) sites. In contrast, locomotion evoked by stimulation of the medial MLR sites is blocked by cooling of the ipsilateral lateral brainstem region which corresponds to the pontomedullary strip (PLS). Ipsilateral PLS cooling was not effective for blocking lateral MLR evoked locomotion, and contralateral PLS cooling was not effective for blocking either medial or lateral MLR evoked stepping. The evidence indicates that the lateral MLR relays through medial reticular nuclei while the medial MLR sites relay largely through the lateral brainstem structures often referred to as the PLS.