Regional characteristics of the membrane response of an identified crayfish nonspiking interneuron to intracellularly injected current

Effects of active conductance distribution over dendrites on the synaptic integration in an identified nonspiking interneuron

The synaptic integration in individual central neuron is critically affected by how active conductances are distributed over dendrites. It has been well known that the dendrites of central neurons are richly endowed with voltage- and ligand-regulated ion conductances. Nonspiking interneurons (NSIs), almost exclusively characteristic to arthropod central nervous systems, do not generate action potentials and hence lack voltage-regulated sodium channels, yet having a variety of voltage-regulated potassium conductances on their dendritic membrane including the one similar to the delayed-rectifier type potassium conductance. It remains unknown, however, how the active conductances are distributed over dendrites and how the synaptic integration is affected by those conductances in NSIs and other invertebrate neurons where the cell body is not included in the signal pathway from input synapses to output sites. In the present study, we quantitatively investigated the functional significance of active conductance distribution pattern in the spatio-temporal spread of synaptic potentials over dendrites of an identified NSI in the crayfish central nervous system by computer simulation. We systematically changed the distribution pattern of active conductances in the neuron's multicompartment model and examined how the synaptic potential waveform was affected by each distribution pattern. It was revealed that specific patterns of nonuniform distribution of potassium conductances were consistent, while other patterns were not, with the waveform of compound synaptic potentials recorded physiologically in the major input-output pathway of the cell, suggesting that the possibility of nonuniform distribution of potassium conductances over the dendrite cannot be excluded as well as the possibility of uniform distribution. Local synaptic circuits involving input and output synapses on the same branch or on the same side were found to be potentially affected under the condition of nonuniform distribution while operation of the major input-output pathway from the soma side to the one on the opposite side remained the same under both conditions of uniform and nonuniform distribution of potassium conductances over the NSI dendrite.

Functional significance of passive and active dendritic properties in the synaptic integration by an identified nonspiking interneuron of crayfish

Nonspiking interneurons control their synaptic output directly by membrane potential changes caused by synaptic activities. Although these interneurons do not generate spikes, their dendritic membrane is endowed with a variety of voltage-dependent conductances whose functional significance in synaptic integration remains unknown. We quantitatively investigated how the passive and active dendritic properties affect the synaptic integration in an identified nonspiking interneuron of crayfish by computer simulation using its multicompartment model based on electrophysiological measurements and three-dimensional morphometry. At the resting potential level, the attenuation factor (V(s)/V(t)) of a unitary synaptic potential in the course of its spread from a dendritic terminal (V(s)) to other terminals (V(t)) ranged from 4.42 to 6.30 with no substantial difference between hyperpolarizing and depolarizing potentials. The compound synaptic responses to strong mechanosensory stimulation could be reproduced in calculation only as the result of spatial summation of attenuated potentials, not as any single large potential. The characteristic response could be reproduced by assuming that the active conductances were distributed only in the dendritic region where the synaptic summation was carried out. The active conductances in other parts of the cell affected neither the shape of the compound synaptic response nor the dendritic spread of synaptic potentials. These findings suggest that the active membrane conductances do not affect the spatial distribution of synaptic potentials over dendrites but function in sculpting the summed synaptic potential to enhance temporal resolution in the synaptic output of the nonspiking interneuron.

Quantitative analyses of anatomical and electrotonic structures of local spiking interneurons by three-dimensional morphometry in crayfish

We quantitatively investigated the three-dimensional structure of the dendrites of local spiking interneurons using a confocal laser scanning microscope in the terminal abdominal ganglion of crayfish. We also studied their passive membrane properties electrophysiologically using the single-electrode current clamp techniques to analyze their electrotonic structure. All of the local spiking interneurons examined in this study lacked distinctive axonal structure and had a monopolar cell body that was connected with a fine primary process to a thick main segment. Numerous fine secondary processes projected from the main segment in the ganglionic neuropile. The average anatomical length of a secondary process from the main segment to its terminal was 261.9 +/- 15.2 microm. The average input resistance and membrane time constant of local spiking interneurons, obtained from their voltage responses to intracellular injection of step current pulses in the main segment, were 15.2 +/- 1.6 MOmega and 13.9 +/- 1.9 msec, respectively. Calculation of the electrotonic length of dendritic processes based on morphological and physiological data obtained in this study revealed that the average electrotonic length of secondary processes in local spiking interneurons was significantly longer than in local nonspiking interneurons, although both types of local interneurons showed apparently similar anaxonic structure. The steady-state voltage attenuation factors for the secondary processes of local spiking interneurons were significantly greater than those of local nonspiking interneurons in both centrifugal and centripetal directions. The larger electrotonic structure of local spiking interneurons compared to that of nonspiking interneurons appears to be compensated for by their excitable dendritic membrane.

Information processing by nonspiking interneurons: passive and active properties of dendritic membrane determine synaptic integration

Nonspiking interneurons control activities of postsynaptic cells without generating action potentials in the central nervous system of many invertebrates. Physiological characteristics of their dendritic membrane have been analyzed in previous studies using single electrode current- and voltage-clamp techniques. We constructed a single compartment model of an identified nonspiking interneuron of crayfish. Experimental results allowed us to simulate how the passive and active properties of the dendritic membrane influence the integrative processing of synaptic inputs. The results showed that not only the peak amplitude but also the time course of synaptic potentials were dependent on the membrane potential level at which the synaptic activity was evoked. When the synaptic input came sequentially, each individual input was still discernible at depolarized levels at which the membrane time constant was short due to depolarization-dependent membrane conductances. In contrast, synaptic potentials merged with each other to develop a sustained potential at hyperpolarized levels where the membrane behaved passively. Thus, synaptic integration in a single nonspiking interneuron depends on the value of membrane potential at which it occurs. This probably reflects the temporal resolution required for specific types of information processing.

Electrophysiological and theoretical analysis of depolarization-dependent outward currents in the dendritic membrane of an identified nonspiking interneuron in crayfish

Depolarization-dependent outward currents were analyzed using the single-electrode voltage clamp technique in the dendritic membrane of an identified nonspiking interneuron (LDS interneuron) in situ in the terminal abdominal ganglion of crayfish. When the membrane was depolarized by more than 20 mV from the resting potential (65.0 +/- 5.7 mV), a transient outward current was observed to be followed by a sustained outward current. Pharmacological experiments revealed that these outward currents were composed of 3 distinct components. A sustained component (I(s)) was activated slowly (half rise time > 5 msec) and blocked by 20 mM TEA. A transient component (I(t1)) that was activated and inactivated very rapidly (peak time < 2.5 msec, half decay time < 1.2 msec) was also blocked by 20 mM TEA. Another transient component (I(t2)) was blocked by 100 microM 4AP, activated rapidly (peak time < 10.0 msec) and inactivated slowly (half decay time > 131.8 msec). Two-step pulse experiments have revealed that both sustained and transient components are not inactivated at the resting potential: the half-maximal inactivation was attained at -21.0 mV in I(t1), and -38.0 mV in I(t2). I(s) showed no noticeable inactivation. When the membrane was initially held at the resting potential level and clamped to varying potential levels, the half-maximal activation was attained at -36.0 mV in I(s), -31.0 mV in I(t1) and -40.0 mV in I(t2). The activation and inactivation time constants were both voltage dependent. A mathematical model of the LDS interneuron was constructed based on the present electrophysiological records to simulate the dynamic interaction of outward currents during membrane depolarization. The results suggest that those membrane conductances found in this study underlie the outward rectification of the interneuron membrane as well as depolarization-dependent shaping of the excitatory synaptic potential observed in current-clamp experiments.

Quantitative analyses of anatomical and electrotonic structures of crayfish nonspiking interneurons by three-dimensional morphometry

The three-dimensional structure of premotor nonspiking interneurons in the terminal abdominal ganglion of crayfish have been studied quantitatively by using a confocal laser-scanning microscope. Their passive membrane properties have also been studied electrophysiologically to analyze their electrotonic structure. In either one of the two major morphological types, anterolateral (AL) and posterolateral (PL), that are characterized by different locations of cell bodies in the ganglion, the monopolar cell body is connected with a fine primary process to a thick main segment projecting numerous fine secondary processes. These two types of cells share a common dendritic field in the neuropil, showing similar anatomical characteristics of dendrites. Electrotonic analyses based on the present anatomical and physiological measurements have revealed that the steady-state voltage-attenuation factors for the secondary processes were not statistically different between the AL- and PL-type cells. Comparison between the premotor nonspiking interneurons and an identified sensory nonspiking interneuron, which was studied previously, has revealed that voltage attenuation over secondary processes in both the centripetal and the centrifugal directions was significantly greater in the sensory than in the premotor interneurons, although the anatomical length of each secondary process from its terminal to the main segment was not different between them. Differences in the electrotonic structure between sensory and premotor nonspiking interneurons indicate their different modes of synaptic integration in the control of postsynaptic nerve cells.

Passive properties of swimmeret motor neurons

Four different functional types of motor neurons innervate each swimmeret: return-stroke excitors (RSEs), power-stroke excitors (PSEs), return-stroke inhibitors (RSIs), and power-stroke inhibitors (PSIs). We studied the structures and passive electrical properties of these neurons, and tested the hypothesis that different types of motor neurons would have different passive properties that influenced generation of the swimmeret motor pattern. Cell bodies of neurons innervating one swimmeret were clustered in two anatomic groups in the same ganglion. The shapes of motor neurons in both groups were similar, despite the differences in locations of their cell bodies and in their functions. Diameters of their axons in the swimmeret nerve ranged from <2 to approximately 35 microm. Resting membrane potentials, input resistances, and membrane time constants were recorded with microelectrodes in the processes of swimmeret motor neurons in isolated abdominal nerve cord preparations. Membrane potentials had a median of -59 mV, with 25th and 75th percentiles of -66.0 and -53 mV. The median input resistance was 6.4 M omega, with 25th and 75th percentiles of 3.4 and 13.7 M omega. Membrane time constants had a median of 9.3 ms, with 25th and 75th percentiles of 5.7 and 15.0 ms. Excitatory and inhibitory motor neurons had similar passive properties. RSE motor neurons were typically more depolarized than the other types, but the passive properties of RSE, PSE, RSI, and PSI neurons were not significantly different. Membrane time constants measured from cell bodies were briefer than those measured from neuropil processes, but membrane potentials and input resistances were not significantly different. The relative sizes of different motor neurons were measured from the sizes of their impulses recorded extracellularly from the swimmeret nerve. Smaller motor neurons had lower membrane potentials and were more likely to be active in the motor pattern than were large motor neurons. Motor neurons of different sizes had similar input resistances and membrane time constants. Motor neurons that were either oscillating or oscillating and firing in phase with the swimmeret motor pattern had lower average membrane potentials and longer time constants than those that were not oscillating. When the state of the swimmeret system changed from quiescence to continuous production of the motor pattern, the resting potentials, input resistances, and membrane time constants of individual swimmeret motor neurons changed only slightly. On average, both input resistance and membrane time constant increased. These similarities are considered in light of the functional task each motor neuron performs, and a hypothesis is developed that links the brief time constants of these neurons and graded synaptic transmission by premotor interneurons to control of the swimmeret muscles and the performance of the swimmeret system.