Phase-dependent changes in dorsal root potential during actual locomotion in rats

RORβ Spinal Interneurons Gate Sensory Transmission during Locomotion to Secure a Fluid Walking Gait

Animals depend on sensory feedback from mechanosensory afferents for the dynamic control of movement. This sensory feedback needs to be selectively modulated in a task- and context-dependent manner. Here, we show that inhibitory interneurons (INs) expressing the RORβ orphan nuclear receptor gate sensory feedback to the spinal motor system during walking and are required for the production of a fluid locomotor rhythm. Genetic manipulations that abrogate inhibitory RORβ IN function result in an ataxic gait characterized by exaggerated flexion movements and marked alterations to the step cycle. Inactivation of RORβ in inhibitory neurons leads to reduced presynaptic inhibition and changes to sensory-evoked reflexes, arguing that the RORβ inhibitory INs function to suppress the sensory transmission pathways that activate flexor motor reflexes and interfere with the ongoing locomotor program. VIDEO ABSTRACT.

Dynamic sensorimotor interactions in locomotion

Locomotion results from intricate dynamic interactions between a central program and feedback mechanisms. The central program relies fundamentally on a genetically determined spinal circuitry (central pattern generator) capable of generating the basic locomotor pattern and on various descending pathways that can trigger, stop, and steer locomotion. The feedback originates from muscles and skin afferents as well as from special senses (vision, audition, vestibular) and dynamically adapts the locomotor pattern to the requirements of the environment. The dynamic interactions are ensured by modulating transmission in locomotor pathways in a state- and phase-dependent manner. For instance, proprioceptive inputs from extensors can, during stance, adjust the timing and amplitude of muscle activities of the limbs to the speed of locomotion but be silenced during the opposite phase of the cycle. Similarly, skin afferents participate predominantly in the correction of limb and foot placement during stance on uneven terrain, but skin stimuli can evoke different types of responses depending on when they occur within the step cycle. Similarly, stimulation of descending pathways may affect the locomotor pattern in only certain phases of the step cycle. Section ii reviews dynamic sensorimotor interactions mainly through spinal pathways. Section iii describes how similar sensory inputs from the spinal or supraspinal levels can modify locomotion through descending pathways. The sensorimotor interactions occur obviously at several levels of the nervous system. Section iv summarizes presynaptic, interneuronal, and motoneuronal mechanisms that are common at these various levels. Together these mechanisms contribute to the continuous dynamic adjustment of sensorimotor interactions, ensuring that the central program and feedback mechanisms are congruous during locomotion.

GABA-Mediated Presynaptic Inhibition in Crayfish Primary Afferents by Non-A, Non-B GABA Receptors

GABAergic presynaptic inhibition has been investigated in primary afferents using an in vitro preparation of the crayfish, Procambarus clarkii. Presynaptic terminals of a leg proprioceptor, the coxo-basal (CB) chordotonal organ, were impaled in the neuorpil of the 5th thoracic ganglion. Pressure ejection of small volumes of the GABAA or GABAB receptor agonists, muscimol and 3-aminopropylphosphinic acid (3-APA), both induce depolarizing responses in the impaled CB sensory terminal. These depolarizations are not blocked by the specific GABAA and GABAB receptor antagonists, SR-95531 and phaclofen, but they are abolished by picrotoxin. Both muscimol- and 3-APA-induced depolarizations are carried by an increase in conductance to Cl-. The presynaptic increase in conductance to Cl- by GABA receptor activation leads to a depression of sensory synaptic transmission through a shunting of the incoming spikes. Monosynaptic EPSPs elicited in motor neurons by CB sensory nerve stimulation are depressed by muscimol and 3-APA. GABA-mediated presynaptic modulation occurs in crayfish primary afferents which can adjust the gain of reflexes. These results show that GABA-activated Cl- channels can induce a modulation of synaptic transmission, but also that the distinction between GABAA and GABAB receptors, as in vertebrates, is not applicable to the crustacean primary afferents.

Locomotor-related presynaptic modulation of primary afferents in the lamprey

Presynaptic modulation of sensory afferent transmission during rhythmic motor activity was investigated in the lamprey spinal cord in vitro. Intracellular recordings were performed from the somata and axons of the glutamatergic sensory neurons from the skin (dorsal cells) during locomotor activity induced by N-methyl-D-aspartate (NMDA). Dorsal cells were phasically depolarized during each ipsilateral ventral root burst. In some soma recordings no or only small amplitude depolarizations were seen, although intracellular recording of their axons revealed the existence of large depolarizations, suggesting that the input synapses are located on the axons. The amplitude of the depolarizations increased during intracellular injection of hyperpolarizing current. The amplitude of the depolarizations increased when the frequency of the locomotor rhythm was increased by elevating the NMDA concentration. The depolarizations were not blocked by specific GABA(A) (bicuculline) or GABA(B) (phaclofen and saclofen) antagonists. To investigate whether the phasic depolarization may influence the monosynaptic excitatory transmission to giant interneurons, the amplitude of the monosynaptic excitatory postsynaptic potential (EPSP) was compared between the onset of the ipsilateral locomotor burst and the burst mid-point. The compound monosynaptic EPSP evoked from dorsal column was significantly smaller during the peak depolarization than at burst onset. The reduction of the amplitude of the EPSPs was not associated with any change of the membrane potential or input resistance of the giant interneurons, suggesting that this effect is mediated by a presynaptic mechanism. Phase-dependent effects were also seen on burst and cycle duration following dorsal column stimulation. Thus, the locomotor-related depolarizations in dorsal cell axons may represent a mechanism for a phasic gain control of sensory transmission during fictive locomotion.

Phase-dependent modulation of primary afferent depolarization in single cutaneous primary afferents evoked by peripheral stimulation during fictive locomotion in the cat

Previous results from our laboratory have shown with intra-axonal recordings that hindfoot cutaneous primary afferents are subjected to rhythmic depolarizations during fictive locomotion (L-PAD) suggesting that cutaneous presynaptic mechanisms are activated by the central locomotor program. In this study, we examined the transmission in pathways responsible for primary afferent depolarizations (PAD) of cutaneous fibres during spontaneous fictive locomotion in decorticate cats and in spinal cats injected with nialamide and L-DOPA. PADs were evoked (E-PADs) by electrical stimulation of peripheral nerves and recorded intra-axonally with micropipettes in identified superficialis peroneal (SP; n = 7) and tibialis posterior (TP; n = 17) cutaneous primary afferents. Results showed that the amplitude of E-PADs, which were superimposed on the L-PAD, was deeply modulated throughout the locomotor cycle; decreasing to reach a minimum during the flexor phase and increasing to a maximum during the extensor phase. The results were not statistically different in fibres of the two nerves and in both types of preparation. The amplitude of E-PADs was always maximum during the extensor phase whether there was a large L-PAD or not during that phase. This suggests that the presynaptic mechanisms activated by central locomotor networks (L-PAD) and those activated by peripheral inputs (E-PAD) may in part be controlled differently. The results thus show that the transmission in PAD pathways activated by cutaneous inputs is phasically modulated by the central pattern generator for locomotion. This strongly suggests that the presynaptic inhibition in cutaneous fibres evoked by the movement-related feedback during real locomotion could be similarly modulated.