Shelby B. Dietz

Electrophysiological Characterization of V2a Interneurons and Their Locomotor-Related Activity in the Neonatal Mouse Spinal Cord

The V2a class of Chx10-expressing interneurons has been implicated in frequency-dependent control of left–right phase during locomotion in the mouse. We have used the Chx10::CFP mouse line to further investigate the properties and locomotion-related activity of V2a interneurons in the isolated neonatal spinal cord. V2a interneurons can be divided into three classes, based on their tonic, phasic, or delayed-onset responses to step depolarization. Electrical coupling is found only between neurons of same class and helps to synchronize neuronal activity within the class. Serotonin (5-HT) excites isolated tonic V2a interneurons by depolarizing the neurons and increasing their membrane input resistance, with no significant effects on action potential properties, a mechanism distinct from 5-HT excitation of commissural interneurons. During NMDA-/5-HT-induced locomotor-like activity, patch-clamp recordings and two-photon calcium imaging experiments show that approximately half of V2a interneurons fire rhythmically with ventral root-recorded motor activity; the rhythmic V2a interneurons fired during one half of the cycle, in phase with either the ipsilateral or the contralateral L2 ventral root bursts. The percentage of rhythmically firing V2a interneurons increases during higher-frequency fictive locomotion, and they become significantly more rhythmic in their firing during the locomotor cycle; this may help to explain the frequency-dependent shift in left–right coupling in Chx10::DTA mice, which lack these neurons. Our results together with data from the accompanying paper (Dougherty and Kiehn, 2009) reinforce earlier proposals that the V2a interneurons are components of the hindlimb central pattern generator, helping to organize left–right locomotor coordination in the neonatal mouse spinal cord.

Activity of Hb9 Interneurons during Fictive Locomotion in Mouse Spinal Cord

Hb9 interneurons (Hb9 INs) are putative components of the mouse spinal locomotor central pattern generator (CPG) and candidates for the rhythm-generating kernel. Studies in slices and hemisected spinal cords showed that Hb9 INs display TTX-resistant membrane potential oscillations, suggesting a role in rhythm generation. To further investigate the roles of Hb9 INs in the locomotor CPG, we used two-photon calcium imaging in the in vitro isolated whole neonatal mouse spinal cord preparation to record the activity of Hb9 INs, which were subsequently stained for unambiguous genetic identification. We elicited fictive locomotion by transmitter application or by electrically stimulating the caudal tip of the spinal cord. Although most Hb9 INs were rhythmically active during fictive locomotion, their activity was sparse and they failed to fire with each cycle of the episode. If Hb9 INs are the principal pacemakers of the CPG in the hemisegment in which they are located, they should direct the firing of motor neurons, with their activity preceding that of their ipsilateral segmental ventral roots. Instead, during each locomotor cycle, onset of Hb9 IN activity lagged behind the onset of the ipsilateral ventral root burst by a mean phase of 0.21 during electrical stimulation and 0.28 during transmitter application. Whole-cell recordings in intact and hemisected spinal cords confirmed the imaging results. Our data suggest that Hb9 INs participate in fictive locomotion, but the delayed onset of activity relative to ipsilateral motoneurons suggests that Hb9 INs are unlikely to be the sole intrasegmental rhythm-generating kernel of the CPG.

Contrasting short-term plasticity at two sides of the mitral–granule reciprocal synapse in the mammalian olfactory bulb

The mitral–granule reciprocal synapse shapes the response of the olfactory bulb to odour stimuli by mediating lateral and reciprocal inhibition. We investigated the short‐term plasticity of both the mitral‐to‐granule excitatory synapse and the granule‐to‐mitral inhibitory synapse in rat olfactory bulb slices, using whole‐cell patch clamp recordings. The granule‐to‐mitral inhibitory synapse invariably exhibited paired‐pulse depression at interstimulus intervals of less than a second, while the mitral‐to‐granule excitatory synapse showed heterogeneous responses, which on average yielded a moderate facilitation. Trains of stimuli led to a much greater depression at the granule‐to‐mitral synapse than at the mitral‐to‐granule synapse. Since mitral cells commonly respond to odours by burst firing with each inhalation cycle, we used bursts of stimuli to study recovery from depression. We found that recovery from depression induced by fast trains of stimuli was more rapid at the mitral‐to‐granule synapse than at the granule‐to‐mitral synapse. In addition, depression was enhanced by higher calcium concentrations, suggesting at least partial contribution of presynaptic mechanisms to short‐term depression. The observed short‐term plasticity could enable mitral cells to overcome autoinhibition and increase action potential propagation along lateral dendrites by burst firing.

Spatiotemporal Dynamics of Rhythmic Spinal Interneurons Measured With Two-Photon Calcium Imaging and Coherence Analysis

In rhythmic neural circuits, a neuron often fires action potentials with a constant phase to the rhythm, a timing relationship that can be functionally significant. To characterize these phase preferences in a large-scale, cell type-specific manner, we adapted multitaper coherence analysis for two-photon calcium imaging. Analysis of simulated data showed that coherence is a simple and robust measure of rhythmicity for calcium imaging data. When applied to the neonatal mouse hindlimb spinal locomotor network, the phase relationships between peak activity of >1,000 ventral spinal interneurons and motor output were characterized. Most interneurons showed rhythmic activity that was coherent and in phase with the ipsilateral motor output during fictive locomotion. The phase distributions of two genetically identified classes of interneurons were distinct from the ensemble population and from each other. There was no obvious spatial clustering of interneurons with similar phase preferences. Together, these results suggest that cell type, not neighboring neuron activity, is a better indicator of an interneurons response during fictive locomotion. The ability to measure the phase preferences of many neurons with cell type and spatial information should be widely applicable for studying other rhythmic neural circuits.

Adult spinal V2a interneurons show increased excitability and serotonin-dependent bistability

In mice, most studies of the organization of the spinal central pattern generator (CPG) for locomotion, and its component neuron classes, have been performed on neonatal [postnatal day (P)2-P4] animals. While the neonatal spinal cord can generate a basic locomotor pattern, it is often argued that the CPG network is in an immature form whose detailed properties mature with postnatal development. Here, we compare intrinsic properties and serotonergic modulation of the V2a class of excitatory spinal interneurons in behaviorally mature (older than P43) mice to those in neonatal mice. Using perforated patch recordings from genetically tagged V2a interneurons, we revealed an age-dependent increase in excitability. The input resistance increased, the rheobase values decreased, and the relation between injected current and firing frequency (F/I plot) showed higher excitability in the adult neurons, with almost all neurons firing tonically during a current step. The adult action potential (AP) properties became narrower and taller, and the AP threshold hyperpolarized. While in neonates the AP afterhyperpolarization was monophasic, most adult V2a interneurons showed a biphasic afterhyperpolarization. Serotonin increased excitability and depolarized most neonatal and adult V2a interneurons. However, in ∼30% of adult V2a interneurons, serotonin additionally elicited spontaneous intrinsic membrane potential bistability, resulting in alternations between hyperpolarized and depolarized states with a dramatically decreased membrane input resistance and facilitation of evoked plateau potentials. This was never seen in younger animals. Our findings indicate a significant postnatal development of the properties of locomotor-related V2a interneurons, which could alter their interpretation of synaptic inputs in the locomotor CPG.

Postnatal Development of Dendrodendritic Inhibition in the Mammalian Olfactory Bulb

The mitral–granule cell (MC–GC) reciprocal synapse is an important source of auto- and lateral-inhibition in the olfactory bulb (OB), and this local inhibition is critical for odor discrimination. We may gain insight into the role of MC autoinhibition in olfaction by correlating the functional development of the autoinhibition with the postnatal development of olfactory function. We have studied the functional development of the MC–GC reciprocal synapse using whole-cell patch-clamp recordings from MCs and GCs in acute OB slices from 3- to 30-day-old rats. The magnitude of dendrodendritic inhibition (DDI) measured by depolarizing a single MC and recording recurrent inhibition in the same cell increased up to the fifteenth day of life (P15), but dropped between P15 and P30. The initial increase and later decrease in DDI was echoed by a similar increase and decrease in the frequency of miniature inhibitory post-synaptic currents, suggesting an accompanying modulation in the number of synapses available to participate in DDI. The late decrease in DDI could also result, in part, from a decrease in GC excitability as well as an increase in relative contribution of N-methyl d-aspartate (NMDA) receptors to γ-amino butyric acid (GABA) release from GC synapses. Changes in release probability of GABAergic synapses are unlikely to account for the late reduction in DDI, although they might contribute to the early increase during development. Our results demonstrate that the functional MC–GC circuit evolves over development in a complex manner that may include both construction and elimination of synapses.

A comparison of serotonin neuromodulation of mouse spinal V2a interneurons using perforated patch and whole cell recording techniques.

Whole cell recordings (WCRs) are frequently used to study neuronal properties, but may be problematic when studying neuromodulatory responses, due to dialysis of the cells cytoplasm. Perforated patch recordings (PPR) avoid cellular dialysis and might reveal additional modulatory effects that are lost during WCR. We have previously used WCR to characterize the responses of the V2a class of Chx10-expressing neurons to serotonin (5-HT) in the neonatal mouse spinal cord (Zhong et al., 2010). Here we directly compare multiple aspects of the responses to 5-HT using WCR and PPR in Chx10-eCFP neurons in spinal cord slices from 2 to 4 day old mice. Cellular properties recorded in PPR and WCR were similar, but high-quality PP recordings could be maintained for significantly longer. Both WCR and PPR cells could respond to 5-HT, and although neurons recorded by PPR showed a significantly greater response to 5-HT in some parameters, the absolute differences between PPR and WCR were small. We conclude that WCR is an acceptable recording method for short-term recordings of neuromodulatory effects, but the less invasive PPR is preferable for detailed analyses and is necessary for stable recordings lasting an hour or more.

An asymmetric model of the spinal locomotor central pattern generator: insights from afferent stimulations

We investigated the effects of dorsal root stimulation (flexor related dL2 and extensor related dL5) on fictive locomotion evoked pharmacologically (by 5-HT+NMDA+DA) in the isolated neonatal mouse spinal cord. In our experiments, electrical stimulation produced a wide variety of effects depending on stimulation frequency, intensity, and the drug concentrations used. At stimulation intensities near threshold we were able to produce phase advances and delays that resolved within a single step cycle. During these single-cycle alterations we observed different effects after stimulating dL2 and dL5. Stimulation of dL2 during ipsilateral extension typically produced an early onset of the next flexion and termination of current extension in the ipsilateral activity with or without rhythm disturbances on the contralateral side of the cord (subject to drug concentrations and stimulus intensity). These disturbances represented a true phase resetting characterized by a full flexor phase expression independent of ending the stimulation producing the disturbance. In contrast, stimulation of dL5 often produced a complex bilateral effect starting from a brief activation of ipsilateral extension (for the stimulus duration) with a corresponding reduction in the flexor activity followed by a reactivation of ipsilateral flexor activity (a full second burst that could represent a rebound evoked by end of stimulation) and then by an enhanced next extensor burst. Our results were consistent with the previously proposed two-level architecture of the spinal central pattern generator (CPG) consisting of a top-level rhythm generator (RG) and pattern formation (PF) circuits [1,3]. The above differences in the effects of flexor and extensor afferent stimulation are consistent with the previous suggestion that the spinal CPG has an asymmetric flexor-extensor organization, so that only the flexor half-center of the rhythm generator (RG) has intrinsic rhythmogenic capabilities [2]. To further evaluate the organization of the spinal CPG and its control by flexor and extensor afferents, we extended our previous model of the CPG in the neonatal rodent spinal cord [4] by incorporating bilateral flexor and extensor afferent pathways and used this model for simulating the above effects of afferent stimulations. The extended model contains left and right half-center RGs interacting via excitatory and inhibitory commissural interneurons (CIN). The flexor half-center of each RG can intrinsically generate rhythmic bursting, while the extensor half-centers do not have intrinsic rhythmic capabilities. Each RG half-center projects to the corresponding PF population controlling the activity of the corresponding motoneuron pools. Ipsilateral neural circuits include several populations of spinal interneurons, including experimentally identified CINs, two types of V2a interneurons, and motoneurons [4]. Network interactions have been organized to be consistent with the activity of these neurons observed during spontaneous experimental deletions [4]. The model reproduces the locomotor-like rhythm with bilaterally coordinated flexor and extensor activities and the neuronal firing patterns observed in the normal conditions and during resetting and non-resetting deletions [4], as well as the experimentally evoked effects of dorsal root stimulation described above. The model proposes mechanistic explanations for the asymmetric effect of afferent stimulation and provides novel insights into the organization of the locomotor central pattern generator.