Differential modulation by tetraethylammonium of the processes underlying network bursting in the neonatal rat spinal cord in vitro

Contributions of h- and Na+/K+ Pump Currents to the Generation of Episodic and Continuous Rhythmic Activities

Developing spinal motor networks produce a diverse array of outputs, including episodic and continuous patterns of rhythmic activity. Variation in excitability state and neuromodulatory tone can facilitate transitions between episodic and continuous rhythms; however, the intrinsic mechanisms that govern these rhythms and their transitions are poorly understood. Here, we tested the capacity of a single central pattern generator (CPG) circuit with tunable properties to generate multiple outputs. To address this, we deployed a computational model composed of an inhibitory half-center oscillator (HCO). Following predictions of our computational model, we tested the contributions of key properties to the generation of an episodic rhythm produced by isolated spinal cords of the newborn mouse. The model recapitulates the diverse state-dependent rhythms evoked by dopamine. In the model, episodic bursting depended predominantly on the endogenous oscillatory properties of neurons, with Na⁺/K⁺ ATPase pump (I_Pump) and hyperpolarization-activated currents (I_h) playing key roles. Modulation of either I_Pump or I_h produced transitions between episodic and continuous rhythms and silence. As maximal activity of I_Pump decreased, the interepisode interval and period increased along with a reduction in episode duration. Decreasing maximal conductance of I_h decreased episode duration and increased interepisode interval. Pharmacological manipulations of I_h with ivabradine, and I_Pump with ouabain or monensin in isolated spinal cords produced findings consistent with the model. Our modeling and experimental results highlight key roles of I_h and I_Pump in producing episodic rhythms and provide insight into mechanisms that permit a single CPG to produce multiple patterns of rhythmicity.

The persistent sodium current generates pacemaker activities in the central pattern generator for locomotion and regulates the locomotor rhythm

Rhythm generation in neuronal networks relies on synaptic interactions and pacemaker properties. Little is known about the contribution of the latter mechanisms to the integrated network activity underlying locomotion in mammals. We tested the hypothesis that the persistent sodium current (I(NaP)) is critical in generating locomotion in neonatal rodents using both slice and isolated spinal cord preparations. After removing extracellular calcium, 75% of interneurons in the area of the central pattern generator (CPG) for locomotion exhibited bursting properties and I(NaP) was concomitantly upregulated. Putative CPG interneurons such as commissural and Hb9 interneurons also expressed I(NaP)-dependent (riluzole-sensitive) bursting properties. Most bursting cells exhibited a pacemaker-like behavior (i.e., burst frequency increased with depolarizing currents). Veratridine upregulated I(NaP), induced riluzole-sensitive bursting properties, and slowed down the locomotor rhythm. This study provides evidence that I(NaP) generates pacemaker activities in CPG interneurons and contributes to the regulation of the locomotor activity.

Kainate and metabolic perturbation mimicking spinal injury differentially contribute to early damage of locomotor networks in the in vitro neonatal rat spinal cord

Acute spinal cord injury evolves rapidly to produce secondary damage even to initially spared areas. The result is loss of locomotion, rarely reversible in man. It is, therefore, important to understand the early pathophysiological processes which affect spinal locomotor networks. Regardless of their etiology, spinal lesions are believed to include combinatorial effects of excitotoxicity and severe stroke-like metabolic perturbations. To clarify the relative contribution by excitotoxicity and toxic metabolites to dysfunction of locomotor networks, spinal reflexes and intrinsic network rhythmicity, we used, as a model, the in vitro thoraco-lumbar spinal cord of the neonatal rat treated (1 h) with either kainate or a pathological medium (containing free radicals and hypoxic/aglycemic conditions), or their combination. After washout, electrophysiological responses were monitored for 24 h and cell damage analyzed histologically. Kainate suppressed fictive locomotion irreversibly, while it reversibly blocked neuronal excitability and intrinsic bursting induced by synaptic inhibition block. This result was associated with significant neuronal loss around the central canal. Combining kainate with the pathological medium evoked extensive, irreversible damage to the spinal cord. The pathological medium alone slowed down fictive locomotion and intrinsic bursting: these oscillatory patterns remained throughout without regaining their control properties. This phenomenon was associated with polysynaptic reflex depression and preferential damage to glial cells, while neurons were comparatively spared. Our model suggests distinct roles of excitotoxicity and metabolic dysfunction in the acute damage of locomotor networks, indicating that different strategies might be necessary to treat the various early components of acute spinal cord lesion.

N-methyl-D-aspartate triggers neonatal rat hypoglossal motoneurons in vitro to express rhythmic bursting with unusual Mg2+ sensitivity

The brainstem nucleus hypoglossus innervates the tongue which must contract rhythmically during respiration, chewing and swallowing. Such rhythmic discharges are due to network bursting mediated by AMPA receptor-dependent glutamatergic transmission. The contribution by hypoglossal motoneurons themselves to rhythmicity remains, however, unclear as they might simply express cyclic patterns produced by premotoneurons or, in analogy to spinal motoneurons, might participate to bursting due to activation of their N-methyl-D-aspartate (NMDA) receptors. Using patch clamp recording from hypoglossal motoneurons in slice preparations of neonatal rat brainstem, we observed that NMDA directly depolarized motoneurons to generate various discharge patterns. Most motoneurons produced transient bursts which were consistently restored by repolarizing membrane potential to rest. Fewer motoneurons generated either sustained bursting or random firing. Rhythmic bursts were recorded from XII nerve rootlets even when single motoneuron bursting required hyperpolarization. NMDA evoked bursts were blocked by the Ca2+ antagonist Cd2+, the gap junction blocker carbenoxolone, or Mg2+ free solution, and partially inhibited by tetrodotoxin or nifedipine. Under voltage clamp, NMDA-induced bursting persisted at negative or positive potentials and was resistant to high extracellular Mg2+ in accordance with the observation of widespread motoneuron expression of NMDA 2D receptor subunits that confer poor Mg2+ sensitivity. It is proposed that NMDA depolarized motoneurons with the contribution of Mg2+ insensitive channels, and triggered bursting via cyclic activation/deactivation of voltage-dependent Na+, Ca2+ and K+ currents spread through gap junctions. The NMDA-evoked bursting pattern was similar to the rhythmic discharges previously recorded from the XII nerve during milk sucking by neonatal rats.