Depressor Motoneurons Research Articles

From Motor-Output to Connectivity: An In-Depth Study of in-vitro Rhythmic Patterns in the Cockroach Periplaneta americana.

The cockroach is an established model in the study of locomotion control. While previous work has offered important insights into the interplay among brain commands, thoracic central pattern generators, and the sensory feedback that shapes their motor output, there remains a need for a detailed description of the central pattern generators' motor output and their underlying connectivity scheme. To this end, we monitored pilocarpine-induced activity of levator and depressor motoneurons in two types of novel in-vitro cockroach preparations: isolated thoracic ganglia and a whole-chain preparation comprising the thoracic ganglia and the subesophageal ganglion. Our data analyses focused on the motoneuron firing patterns and the coordination among motoneuron types in the network. The burstiness and rhythmicity of the motoneurons were monitored, and phase relations, coherence, coupling strength, and frequency-dependent variability were analyzed. These parameters were all measured and compared among network units both within each preparation and among the preparations. Here, we report differences among the isolated ganglia, including asymmetries in phase and coupling strength, which indicate that they are wired to serve different functions. We also describe the intrinsic default gait and a frequency-dependent coordination. The depressor motoneurons showed mostly similar characteristics throughout the network regardless of interganglia connectivity; whereas the characteristics of the levator motoneurons activity were mostly ganglion-dependent, and influenced by the presence of interganglia connectivity. Asymmetries were also found between the anterior and posterior homolog parts of the thoracic network, as well as between ascending and descending connections. Our analyses further discover a frequency-dependent inversion of the interganglia coordination from alternations between ipsilateral homolog oscillators to simultaneous activity. We present a detailed scheme of the network couplings, formulate coupling rules, and review a previously suggested model of connectivity in light of our new findings. Our data support the notion that the inter-hemiganglia coordination derives from the levator networks and their coupling with local depressor interneurons. Our findings also support a dominant role of the metathoracic ganglion and its ascending output in governing the anterior ganglia motor output during locomotion in the behaving animal.

Intra- and intersegmental neural network architectures determining rhythmic motor activity in insect locomotion

Abstract The coordinated movement of the extremities of an animal during locomotion is achieved by the interaction between groups of neurons called central pattern generators (CPGs). In the absence of any sensory input this network creates a stable rhythmic motor activity that is essential for a successful coordination between limbs. Studying the structure and the interaction between different parts of the CPG network is therefore of particular interest. This work is motivated by recent experimental results reported by Mantziaris et al. (2017) [12] . By chemically activating both isolated and interconnected deafferented thoracic segments (ganglia) of the stick insect Mantziaris et al. (2017) [12] analyzed the interactions between contralateral networks that drive the levator-depressor muscle pairs, which are responsible for the upward-downward movement of the legs. The results of the experimental analysis showed that intrasegmental phase relations differ between isolated segments. In particular, in isolated segments where the control networks of the middle and hind legs reside, i.e. in the meso- and metathoracic ganglia, the phase relations between activities of the contralateral depressor motoneurons were in-phase and anti-phase, respectively. Moreover, the phase relations switched to in-phase and stabilized when the ganglia were interconnected. Using the phase reduction of an intersegmental network model of stick insect locomotion presented in our previous work (Yeldesbay et al. (2017) [22]), we built a reduced model of the intra- and intersegmental network controlling levator-depressor activity in the meso- and metathoracic ganglia. By examining the intra- and intersegmental phase differences in the model we identified the properties of the network couplings that replicate the results observed in the experiments. We applied the theoretical analysis to escape type CPGs and revealed a set of possible contra- and ipsilateral synaptic connections. Finally, we defined general features of the couplings between CPGs of any type that maintain the phase relations observed in the experiments.

Endogenous rhythm and pattern-generating circuit interactions in cockroach motor centres.

ABSTRACTCockroaches are rapid and stable runners whose gaits emerge from the intricate, and not fully resolved, interplay between endogenous oscillatory pattern-generating networks and sensory feedback that shapes their rhythmic output. Here we studied the endogenous motor output of a brainless, deafferented preparation. We monitored the pilocarpine-induced rhythmic activity of levator and depressor motor neurons in the mesothoracic and metathoracic segments in order to reveal the oscillatory networks’ architecture and interactions. Data analyses included phase relations, latencies between and overlaps of rhythmic bursts, spike frequencies, and the dependence of these parameters on cycle frequency. We found that, overall, ipsilateral connections are stronger than contralateral ones. Our findings revealed asymmetries in connectivity among the different ganglia, in which meta-to-mesothoracic ascending coupling is stronger than meso-to-metathoracic descending coupling. Within-ganglion coupling between the metathoracic hemiganglia is stronger than that in the mesothoracic ganglion. We also report differences in the role and mode of operation of homologue network units (manifested by levator and depressor nerve activity). Many observed characteristics are similar to those exhibited by intact animals, suggesting a dominant role for feedforward control in cockroach locomotion. Based on these data we posit a connectivity scheme among components of the locomotion pattern generating system.

Task-dependent modification of leg motor neuron synaptic input underlying changes in walking direction and walking speed.

Animals modify their behavior constantly to perform adequately in their environment. In terrestrial locomotion many forms of adaptation exist. Two tasks are changes of walking direction and walking speed. We investigated these two changes in motor output in the stick insect Cuniculina impigra to see how they are brought about at the level of leg motor neurons. We used a semi-intact preparation in which we can record intracellularly from leg motor neurons during walking. In this single-leg preparation the middle leg of the animal steps in a vertical plane on a treadwheel. Stimulation of either abdomen or head reliably elicits fictive forward or backward motor activity, respectively, in the fixed and otherwise deafferented thorax-coxa joint. With a change of walking direction only thorax-coxa-joint motor neurons protractor and retractor changed their activity. The protractor switched from swing activity during forward to stance activity during backward walking, and the retractor from stance to swing. This phase switch was due to corresponding change of phasic synaptic inputs from inhibitory to excitatory and vice versa at specific phases of the step cycle. In addition to phasic synaptic input a tonic depolarization of the motor neurons was present. Analysis of changes in stepping velocity during stance showed only a significant correlation to flexor motor neuron activity, but not to that of retractor and depressor motor neurons during forward walking. These results show that different tasks in the stick insect walking system are generated by altering synaptic inputs to specific leg joint motor neurons only.

Control of motor activity, via motoneuron excitability and sensory-motor integration, by the steroid hormone 20-hydroxyecdysone in crayfish

We studied the effects of the molting hormone 20-hydroxyecdysone (20E) on leg sensory-motor networks of the red swamp crayfish, Procambarus clarkii. The hormone was injected in isolated crayfish and network activity was analyzed 3 days after injection using electrophysiology on an in vitro preparation of the leg locomotor network. This 20E treatment deeply reduced motor activity, by affecting both intrinsic motoneuron (MN) properties and sensory-motor integration. Indeed, we noticed a general decrease in motor nerve tonic activities, principally in depressor and promotor nerves. Moreover, intracellular recordings of depressor MNs confirmed a decrease of MN excitability due to a drop in input resistance. In parallel, sensory inputs originating from a proprioceptor, which codes joint movements controlled by these MNs, were also reduced. The shape of excitatory post-synaptic potentials (PSPs) triggered in MNs by sensory activity of this proprioceptor showed a reduction of polysynaptic components, whereas inhibitory PSPs were suppressed, demonstrating that 20E acted also on interneurons relaying sensory to motor inputs. Consequently, 20E injection modified the whole sensory-motor loop, as demonstrated by the alteration of the resistance reflex amplitude. These locomotor network changes induced by 20E were consistent with the decrease of locomotion observed in a behavioral test. In summary, 20E controls locomotion during crayfish premolt by acting on both MN excitability and sensory-motor integration. Among these cooperative effects, the drop of input resistance of MNs seems to be mostly responsible for the reduction of motor activity.

Force encoding in stick insect legs delineates a reference frame for motor control

The regulation of forces is integral to motor control. However, it is unclear how information from sense organs that detect forces at individual muscles or joints is incorporated into a frame of reference for motor control. Campaniform sensilla are receptors that monitor forces by cuticular strains. We studied how loads and muscle forces are encoded by trochanteral campaniform sensilla in stick insects. Forces were applied to the middle leg to emulate loading and/or muscle contractions. Selective sensory ablations limited activities recorded in the main leg nerve to specific receptor groups. The trochanteral campaniform sensilla consist of four discrete groups. We found that the dorsal groups (Groups 3 and 4) encoded force increases and decreases in the plane of movement of the coxo-trochanteral joint. Group 3 receptors discharged to increases in dorsal loading and decreases in ventral load. Group 4 showed the reverse directional sensitivities. Vigorous, directional responses also occurred to contractions of the trochanteral depressor muscle and to forces applied at the muscle insertion. All sensory discharges encoded the amplitude and rate of loading or muscle force. Stimulation of the receptors produced reflex effects in the depressor motoneurons that could reverse in sign during active movements. These data, in conjunction with findings of previous studies, support a model in which the trochanteral receptors function as an array that can detect forces in all directions relative to the intrinsic plane of leg movement. The array could provide requisite information about forces and simplify the control and adaptation of posture and walking.

Neural Circuit Reconfiguration by Social Status

The social rank of an animal is distinguished by its behavior relative to others in its community. Although social-status-dependent differences in behavior must arise because of differences in neural function, status-dependent differences in the underlying neural circuitry have only begun to be described. We report that dominant and subordinate crayfish differ in their behavioral orienting response to an unexpected unilateral touch, and that these differences correlate with functional differences in local neural circuits that mediate the responses. The behavioral differences correlate with simultaneously recorded differences in leg depressor muscle EMGs and with differences in the responses of depressor motor neurons recorded in reduced, in vitro preparations from the same animals. The responses of local serotonergic interneurons to unilateral stimuli displayed the same status-dependent differences as the depressor motor neurons. These results indicate that the circuits and their intrinsic serotonergic modulatory components are configured differently according to social status, and that these differences do not depend on a continuous descending signal from higher centers.

The effect of the bioflavonoid quercetin on voltage-gated calcium channels in Periplaneta americana Df motoneuron

The effect of the natural bioflavonoid quercetin on voltage operated calcium channels was investigated in American cockroach (Periplaneta americana) fast depressor motoneuron (Df) using electrophysiological and pharmacological methods. For blockade of outward potassium currents, tetraethylammonium and cesium leakage from microelectrode tip was used. Under this condition and application of command potentials, current-potential relationship was obtained in the absence and presence of quercetin (10 µM). Meanwhile, frequency and amplitude of action potentials (calcium spikes) under current clamp were also determined. In the presence of quercetin, inward calcium current showed a significant increase (p<0.05-0.01) in the potential range of -40 to 0 mV as compared to vehicle. Meanwhile, the frequency of calcium spikes in the presence of quercetin significantly increased relative to vehicle (p<0.01) and quercetin did not significantly change their amplitude. On the other hand, it caused a significant reduction of afterhyperpolarization (AHP) (p<0.05). Quercetin also shifted the voltage dependence of the inactivation and activation curves to more negative potentials and caused a signicant increase in the slope of activation, which reflects a yield of more current for a given potential. Quercetin through augmentation of inward calcium currents (Ica, L), increasing frequency of calcium spikes, and reduction of AHP amplitude could increase excitability and firing of Df motoneurons and this may be of benefit in those brain diseases in which neuronal excitability is depressed. Key words: Quercetin, calcium channel, electrophysiology, Periplaneta americana.

Dominance of local sensory signals over inter-segmental effects in a motor system: experiments

Legged locomotion requires that information local to one leg, and inter-segmental signals coming from the other legs are processed appropriately to establish a coordinated walking pattern.However, very little is known about the relative importance of local and inter-segmental signals when they converge upon the central pattern generators (CPGs) of different leg joints.We investigated this question on the CPG of the middle leg coxa–trochanter (CTr)-joint of the stick insect which is responsible for lifting and lowering the leg.We used a semi-intact preparation with an intact front leg stepping on a treadmill, and simultaneously stimulated load sensors of the middle leg.We found that middle leg load signals induce bursts in the middle leg depressor motoneurons(MNs). The same local load signals could also elicit rhythmic activity in the CPG of the middle leg CTr-joint when the stimulation of middle leg load sensors coincided with front leg stepping. However, the influence of front leg stepping was generally weak such that front leg stepping alone was only rarely accompanied by switching between middle leg levator and depressor MN activity. We therefore conclude that the impact of the local sensory signals on the levator–depressor motor system is stronger than the inter-segmental influence through front leg stepping.

Encoding of force increases and decreases by tibial campaniform sensilla in the stick insect, Carausius morosus

Detection of force increases and decreases is important in motor control. Experiments were performed to characterize the structure and responses of tibial campaniform sensilla, receptors that encode forces through cuticular strains, in the middle leg of the stick insect (Carausius morosus). The sensilla consist of distinct subgroups. Group 6A sensilla are located 0.3mm distal to the femoro-tibial joint and have oval shaped cuticular caps. Group 6B receptors are 1mm distal to the joint and have round caps. All sensilla show directional, phasico-tonic responses to forces applied to the tibia in the plane of joint movement. Group 6B sensilla respond to force increases in the direction of joint extension while Group 6A receptors discharge when those forces decrease. Forces applied in the direction of joint flexion produce the reverse pattern of sensory discharge. All receptors accurately encode the rate of change of force increments and decrements. Contractions of tibial muscles also produce selective, directional sensory discharges. The subgroups differ in their reflex effects: Group 6B receptors excite and Group 6A sensilla inhibit tibial extensor and trochanteral depressor motoneurons. The tibial campaniform sensilla can, therefore, encode force increases or decreases and aid in adapting motor outputs to changes in load.

Social Interactions Determine Postural Network Sensitivity to 5-HT

The excitability of the leg postural circuit and its response to serotonin (5-HT) were studied in vitro in thoracic nervous system preparations of dominant and subordinate male crayfishes. We demonstrate that the level of spontaneous tonic activity of depressor and levator motoneurons (MNs) (which control downward and upward movements of the leg, respectively) and the amplitude of their resistance reflex are larger in dominants than in subordinates. Moreover, we show that serotonergic neuromodulation of the postural circuit also depends on social status. Depressor and levator MN tonic firing rates and resistance reflex amplitudes were significantly modified in the presence of 10 mum 5-HT in dominants but not in subordinates. Using intracellular recording from depressor MNs, we show that their input resistance was not significantly different in dominants and subordinates in control conditions. However, 5-HT produced a marked depolarization in dominants and a significantly weaker depolarization in subordinates. Moreover, in the presence of 5-HT, the amplitude of the resistance reflex and the input resistance of MNs increased in dominants and decreased in subordinates. The peak amplitude and the decay phase of unitary EPSPs triggered by sensory spikes were significantly increased by 5-HT in dominants but not in subordinates. These observations suggest that neural networks are more reactive in dominants than in subordinates, and this divergence is even reinforced by 5-HT modulation.

Intersegmental coordination of cockroach locomotion: adaptive control of centrally coupled pattern generator circuits

Animals’ ability to demonstrate both stereotyped and adaptive locomotor behavior is largely dependent on the interplay between centrally generated motor patterns and the sensory inputs that shape them. We utilized a combined experimental and theoretical approach to investigate the relative importance of CPG interconnections vs. intersegmental afferents in the cockroach: an animal that is renowned for rapid and stable locomotion. We simultaneously recorded coxal levator and depressor motor neurons (MN) in the thoracic ganglia of Periplaneta americana, while sensory feedback was completely blocked or allowed only from one intact stepping leg. In the absence of sensory feedback, we observed a coordination pattern with consistent phase relationship that shares similarities with a double-tripod gait, suggesting central, feedforward control. This intersegmental coordination pattern was then reinforced in the presence of sensory feedback from a single stepping leg. Specifically, we report on transient stabilization of phase differences between activity recorded in the middle and hind thoracic MN following individual front-leg steps, suggesting a role for afferent phasic information in the coordination of motor circuits at the different hemiganglia. Data were further analyzed using stochastic models of coupled oscillators and maximum likelihood techniques to estimate underlying physiological parameters, such as uncoupled endogenous frequencies of hemisegmental oscillators and coupling strengths and directions. We found that descending ipsilateral coupling is stronger than ascending coupling, while left–right coupling in both the meso- and meta-thoracic ganglia appear to be symmetrical. We discuss these results in comparison with recent findings in stick insects that share similar neural and body architectures, and argue that the two species may exemplify opposite extremes of a fast–slow locomotion continuum, mediated through different intersegment coordination strategies.

Differential effects of octopamine and tyramine on the central pattern generator for Manduca flight

The biogenic amine, octopamine, modulates a variety of aspects of insect motor behavior, including direct action on the flight central pattern generator. A number of recent studies demonstrate that tyramine, the biological precursor of octopamine, also affects invertebrate locomotor behaviors, including insect flight. However, it is not clear whether the central pattern generating networks are directly affected by both amines, octopamine and tyramine. In this study, we tested whether tyramine affected the central pattern generator for flight in the moth, Manduca sexta. Fictive flight was induced in an isolated ventral nerve cord preparation by bath application of the octopamine agonist, chlordimeform, to test potential effects of tyramine on the flight central pattern generator by pharmacological manipulations. The results demonstrate that octopamine but not tyramine is sufficient to induce fictive flight in the isolated ventral nerve cord. During chlordimeform induced fictive flight, bath application of tyramine selectively increases synaptic drive to depressor motoneurons, increases the number of depressor spikes during each cycle and decreases the depressor phase. Conversely, blocking tyramine receptors selectively reduces depressor motoneuron activity, but does not affect cycle by cycle elevator motoneuron spiking. Therefore, octopamine and tyramine exert distinct effects on the flight central pattern generating network.

Recruitment in a heterogeneous population of motor neurons that innervates the depressor muscle of the crayfish walking leg muscle

According to the size principle the fine control of muscle tension depends on the orderly recruitment of motor neurons from a heterogeneous pool. We took advantage of the small number of excitatory motor neurons (about 12) that innervate the depressor muscle of the crayfish walking leg to determine if the size principle applies to this muscle. We found that in accordance with the size principle, when stimulated by proprioceptive input, neurons with small extracellular spikes were recruited before neurons with medium or large spikes. Because only a small fraction of the motor neurons responded strongly enough to sensory input to be recruited in this way, we extended our analysis to all neurons by characterizing properties that have classically been associated with recruitment order such as speed of axonal conduction and extracellular spike amplitude. Through a combination of physiological and anatomical criteria we were able to identify seven classes of excitatory depressor motor neurons. The majority of these classes responded to proprioceptive input with a resistance reflex, while a few responded with an assistance reflex, and yet others did not respond. Our results are in general agreement with the size principle. However, we found qualitative differences between neuronal classes in terms of synaptic input and neuronal structure that would in theory be unnecessary, according to a strict interpretation of the size principle. We speculate that the qualitative heterogeneity observed may be due to the fact that the depressor is a complex muscle, consisting of two muscle bundles that share a single insertion but have multiple origins.

Identification of the neurotransmitters involved in modulation of transmitter release from the central terminals of the locust wing hinge stretch receptor

The flight motor system of the locust represents a model preparation for the investigation of neuromodulation. At the wing hinges are stretch receptors important in generating and controlling the flight motor pattern. The forewing stretch receptor (fSR) makes direct cholinergic synapses with depressor motor neurons (MN) controlling that wing, including the first basalar MN (BA1). The fSR/BA1 synapse is modulated by muscarinic cholinergic receptors located on gamma-aminobutyric acid (GABA)-ergic interneurons (Judge and Leitch [1999a] J. Comp. Neurol. 407:103-114; Judge and Leitch [1999b] J. Neurobiol. 40:420-431). However, electrophysiology has shown that fSR/BA is also modulated by biogenic amines (Leitch et al. [2003] J. Comp. Neurol. 462:55-70). We have used electron microscopic immunocytochemistry (ICC) to identify the neurotransmitters in neurons presynaptic to the fSR and to determine the relative proportion of these different classes of modulatory inputs. Approximately 55% of all inputs to the fSR are glutamate-IR, indicating that glutamatergic neurons may also play an important role in presynaptically modulating the fSR terminals. Anti-GABA ICC confirmed that over 40% of inputs to the fSR are GABA-IR (Judge and Leitch [1999a] J. Comp. Neurol. 407:103-114). Labelling sections with an antioctopamine antibody revealed neurons containing distinctive large, electron-dense granules, which could reliably be used to identify them. Aminergic neurons that modulate the synapse may have very few morphologically recognizable synaptic outputs. Although putative octopaminergic processes were found in close contact to horseradish peroxidase-filled fSR profiles, no morphologically recognizable synaptic inputs to the fSR were evident. Collectively, these data suggest that most inputs to the fSR are from either glutamatergic or GABAergic neurons.

Neural Control and Coordination of Jumping in Froghopper Insects

The thrust for jumping in froghopper insects is produced by a rapid, synchronous depression of both hind legs generated by huge, multipartite trochanteral depressor muscles in the thorax and smaller levator muscles in the coxae. A three-phase motor pattern activates these muscles in jumping. First, a levation phase lasts a few hundred milliseconds, in which a burst of spikes in the trochanteral levator motor neurons moves the hind legs into their fully cocked position and thus engages a mechanical lock between a coxa and a femur. Second, a cocked phase lasts a few seconds, in which a trochanteral depressor motor neuron spikes continuously at a frequency gradually rising to 50 Hz, although the hind legs remain stationary. Levator motor spikes are sporadic. Third, the jump movement lasts <1 ms, in which the spikes in the depressors stop abruptly and the legs rapidly depress. This pattern may vary in the speed of the initial levation and in the duration of the cocked phase. Recordings from the depressor muscles on both sides showed remarkable synchrony of their motor spikes. In one 4.9-long cocked phase all 174 spikes were synchronous and in another 27 s period of continuous spiking all but one of 1,176 spikes were synchronous. When a single hind leg moves rapidly, these depressor spikes are nevertheless independent of those of the other leg. These features of the motor pattern and the coupling between motor neurons to the two hind legs ensure powerful movements to propel rapid jumping.

Kinematics and motor activity during tethered walking and turning in the cockroach, Blaberus discoidalis

When insects turn from walking straight, their legs have to follow different motor patterns. In order to examine such pattern change precisely, we stimulated single antenna of an insect, thereby initiating its turning behavior, tethered over a lightly oiled glass plate. The resulting behavior included asymmetrical movements of prothoracic and mesothoracic legs. The mesothoracic leg on the inside of the turn (in the apparent direction of turning) extended the coxa-trochanter and femur-tibia joints during swing rather than during stance as in walking, while the outside mesothoracic leg kept a slow walking pattern. Electromyograms in mesothoracic legs revealed consistent changes in the motor neuron activity controlling extension of the coxa-trochanter and femur-tibia joints. In tethered walking, depressor trochanter activity consistently preceded slow extensor tibia activity. This pattern was reversed in the inside mesothoracic leg during turning. Also for turning, extensor and depressor motor neurons of the inside legs were activated in swing phase instead of stance. Turning was also examined in free ranging animals. Although more variable, some trials resembled the pattern generated by tethered animals. The distinct inter-joint and inter-leg coordination between tethered turning and walking, therefore, provides a good model to further study the neural control of changing locomotion patterns.

Effects of neck and circumoesophageal connective lesions on posture and locomotion in the cockroach

Few studies in arthropods have documented to what extent local control centers in the thorax can support locomotion in absence of inputs from head ganglia. Posture, walking, and leg motor activity was examined in cockroaches with lesions of neck or circumoesophageal connectives. Early in recovery, cockroaches with neck lesions had hyper-extended postures and did not walk. After recovery, posture was less hyper-extended and animals initiated slow leg movements for multiple cycles. Neck lesioned individuals showed an increase in walking after injection of either octopamine or pilocarpine. The phase of leg movement between segments was reduced in neck lesioned cockroaches from that seen in intact animals, while phases in the same segment remained constant. Neither octopamine nor pilocarpine initiated changes in coordination between segments in neck lesioned individuals. Animals with lesions of the circumoesophageal connectives had postures similar to intact individuals but walked in a tripod gait for extended periods of time. Changes in activity of slow tibial extensor and coxal depressor motor neurons and concomitant changes in leg joint angles were present after the lesions. This suggests that thoracic circuits are sufficient to produce leg movements but coordinated walking with normal motor patterns requires descending input from head ganglia.

Remodeling of an identified motoneuron during metamorphosis: Hormonal influences on the growth of dendrites and axon terminals

During metamorphosis of the tobacco hawkmoth Manduca sexta, the femoral depressor motoneuron (FeDe MN) undergoes remodeling of its dendrites and motor terminals. Previous studies have established that remodeling of MNs during metamorphosis is mediated by the same hormones that control metamorphosis: the ecdysteroids and juvenile hormone (JH). During the pupal stage, the ecdysteroids promote adult-specific growth of MNs in the absence of JH, but JH or its synthetic mimics can interfere with ecdysteroid-mediated growth if applied during early sensitive periods. Hence, the application of a JH mimic (JHM) either systemically or locally to a target muscle has been used to distinguish those aspects of motor-terminal remodeling that are controlled by ecdysteroid action on the CNS from those that are influenced by ecdysteroid action on the peripheral targets. Here, we have extended the analysis of central and peripheral hormonal influences on MN remodeling by injecting JHM locally into the CNS thus altering the hormonal environment of the FeDe MN soma without altering the hormonal environment of its target muscle. Our results demonstrate that adult dendritic growth and motor-terminal growth can be experimentally uncoupled, suggesting that each is regulated independently. JHM application to the CNS perturbed dendritic growth, but had no measurable impact on motor-terminal growth. Peripheral actions of ecdysteroids, therefore, appear sufficient to promote the development of adult-specific motor terminals but not the development of an adult-specific dendritic arbor.

Serotonin enhances the resistance reflex of the locomotor network of the crayfish through multiple modulatory effects that act cooperatively.

Serotonin (5HT) is an endogenous amine that modifies posture in crustacea. Here, we examined the mechanisms of action of 5HT on the resistance reflex in crayfish legs. This reflex, which counteracts movements imposed on a limb, is based on a negative feedback system formed by proprioceptors that sense joint angle movements and activate opposing motoneurons. We performed intracellular recordings from depressor motoneurons while repetitively stretching and releasing a leg joint proprioceptor in a resting in vitro preparation (i.e., a preparation that lacks spontaneous rhythmic activity). 5HT increased the amplitude of the depolarization during the release phase of the proprioceptor (corresponding to an upward movement of the leg) and the discharge frequency of the motoneurons. The 5HT-induced increase in the resistance reflex is caused, to a large extent, by polysynaptic pathways because it was very attenuated in the presence of high divalent cation solution. In addition to this activation of the polysynaptic pathways, 5HT also has postsynaptic effects that enhance the resistance reflex. 5HT causes a tonic depolarization, as well as an increase in the time constant and input resistance of motoneurons. We developed a simple mathematical model to describe the integrative properties of the motoneurons. The conclusion of this study is that the input frequency and the decay time constant of the EPSPs interact in such a way that small simultaneous changes in these parameters can cause a large effect on summation. Therefore, the conjunction of presynaptic and postsynaptic changes produces a strong cooperative effect on the resistance reflex response.