The impact of neuromodulators on active dendritic conductances was investigated by the use of intracellular recording techniques in spinal motoneurons in the adult cat. The well known lack of voltage control of dendritic regions during voltage clamp applied at the soma was used to estimate dendritic amplification of a steady monosynaptic input generated by muscle spindle Ia afferents. In preparations deeply anesthetized with pentobarbital, Ia current either decreased with depolarization or underwent a modest increase at membrane potentials above Ϫ40 mV. In unanesthetized decerebrate preparations (which have tonic activity in axons originating in the brainstem and releasing serotonin or norepinephrine), active dendritic currents caused strong amplification of Ia input. In the range of Ϫ50 to Ϫ40 mV, peak Ia current was over four times as large as that in the pentobarbital-anesthetized preparations. Exogenous administration of a noradrenergic agonist in addition to the tonic activity further enhanced amplification (sixfold increase). Amplification was not seen in preparations with spinal transections. Overall, the dendritic amplification with moderate or strong neuromodulatory drive was estimated to be large enough to allow the motoneurons innervating slow muscle fibers to be driven to their maximum force levels by remarkably small synaptic inputs. In these cells, the main role of synaptic input may be to control the activation of a highly excitable dendritic tree. The neuromodulatory control of synaptic amplification provides motor commands with the potential to adjust the level of amplification to suit the demands of different motor tasks. Key words: motoneuron; spinal cord; neuromodulation; electrophysiology; serotonin; norepinephrine; plateau potential; bistable; dendritic amplificationVoltage-sensitive conductances within the dendritic tree of the postsynaptic neuron play a major role in synaptic integration (Johnston et al., 1996;Yuste and Tank, 1996). Many types of voltage-sensitive conductances are under the control of neuromodulatory inputs acting via second messenger systems. Thus, the potential exists for the neuromodulatory inputs to control synaptic integration by altering the activation of voltage-sensitive currents in the dendrites of neurons.The spinal motoneuron is subject to potent neuromodulatory control by axons that originate in the brainstem and release either serotonin (5-HT) or norepinephrine (NE) (Hounsgaard et al., 1988;Takahashi and Berger, 1990;Wang and Dun, 1990;White et al., 1991;Binder et al., 1996). In the presence of these neuromodulators, motoneurons exhibit a persistent inward current (Hounsgaard and Kiehn, 1989;Svirskis and Hounsgaard, 1998;Lee and Heckman, 1999a). The composition of this current may vary in different types of motoneurons (Hounsgaard and Kiehn, 1989;Hsiao and Chandler, 1995;Zhang et al., 1995;Lee and Heckman, 1998b). In spinal motoneurons in the adult cat, the total persistent inward current (I PIC ) is exceedingly large Heckman, 1998a, 1999a). Activation of I...

No abstract

No abstract

Morphological and electrophysiological properties of neural cells are substantially influenced by their immediate extracellular surroundings, yet the features of this environment are difficult to mimic in vitro. Therefore, there is a tremendous need to develop a new generation of culture systems that more closely model the complexity of nervous tissue. To this end, we engineered novel electrophysiologically active 3D neural constructs composed of neurons and astrocytes within a bioactive extracellular matrix-based scaffold. Neurons within these constructs exhibited extensive 3D neurite outgrowth, expressed mature neuron-specific cytoskeletal proteins, and remained viable for several weeks. Moreover, neurons assumed complex 3D morphologies with rich neurite arborization and clear indications of network connectivity, including synaptic junctures. Furthermore, we modified whole-cell patch clamp techniques to permit electrophysiological probing of neurons deep within the 3D constructs, revealing that these neurons displayed both spontaneous and evoked electrophysiological action potentials and exhibited functional synapse formation and network properties. This is the first report of individual patch clamp recordings of neurons deep within 3D scaffolds. These tissue engineered cellular constructs provide an innovative platform for neurobiological and electrophysiological investigations, serving as an important step towards the development of more physiologically relevant neural tissue models.