Biomechanical behavior of carbon fiber-reinforced polyetheretherketone as a dental implant material in implant-supported overdenture under mandibular trauma: A finite element analysis study

Context

Implant-supported overdentures are well-known and widely accepted treatment modality to increase retention which is a crucial factor for determining patient satisfaction. The placement of two implants in the anterior region can be selected as a first-line treatment in patients with the atrophic mandibular ridge.

Aims

The purpose of this research was to assess the biomechanical effects of carbon fiber-reinforced polyetheretherketone (CFR-PEEK) implant-supported overdenture in the event of 2,000 N forefront trauma to an atrophic edentulous mandible by using the finite element analysis method.

Materials and Methods

Three types of mandible models were simulated; the first one was an edentulous atrophic mandible model; in the second model, 3.5 × 11.5 mm CFR-PEEK implants; and in the third model, 4.3 × 11.5 mm CFR-PEEK implants were positioned in the region of the lateral incisor of the identical edentulous atrophic mandible.

Results

Maximum Von Misses stresses 979.261 MPa, 1,454.69 MPa, and 1,940.71 MPa and maximum principal stresses 1,112.74 MPa, 1,249.88 MPa, and 1,251.33 MPa have been detected at the condylar neck area and minimum principal stresses - 1,203.38 MPa, -1,503.21 MPa, and - 1,990.34 MPa have been recorded at the symphysis and corpus regions from M1 to M3, respectively. In addition, the M2 and M3 models showed low-stress distributions around the implant-bone interface, particularly where the implants were in contact with cancellous bone.

Conclusions

The results showed that the insertion of different diameters of CFR-PEEK implants led to low and homogenous stress distribution all around the implant-bone interface and stresses transferred directly to the condylar neck areas. Therefore, it was observed that CRF-PEEK implants did not change the basic behavior of the mandibula in response to frontal stresses.

Intra- versus intergroup variance in collective behavior

Animal collective motion arises from the intricate interactions between the natural variability among individuals, and the homogenizing effect of the group, working to generate synchronization and maintain coherence. Here, these interactions were studied using marching locust nymphs under controlled laboratory settings. A novel experimental approach compared single animals, small groups, and virtual groups composed of randomly shuffled real members. We found that the locust groups developed unique, group-specific behavioral characteristics, reflected in large intergroup and small intragroup variance (compared with the shuffled groups). Behavioral features that differed between single animals and groups, but not between group types, were classified as essential for swarm formation. Comparison with Markov chain models showed that individual tendencies and the interaction network among animals dictate the group characteristics. Deciphering the bidirectional interactions between individual and group properties is essential for understanding the swarm phenomenon and predicting large-scale swarm behaviors.

Differential control of temporal and spatial aspects of cockroach leg coordination

Ensembles of neuronal networks and sensory pathways participate in controlling the kinematic and dynamic parameters of animal movement necessary to achieve motor coordination. Determining the relative contribution of proprioceptive feedback is essential for understanding how animals sustain stable, coordinated locomotion in complex natural environments. Here, we focus on the role of chordotonal organs (COs), proprioceptors found in insect legs, in the spatial and temporal regulation of walking. We compare gait parameters of intact cockroaches (Periplaneta americana) and sensory-impaired ones, injected with pymetrozine, a chemical previously shown to abolish CO function in locusts. We verify that afferent CO activity in pymetrozine-treated cockroaches is inhibited, and analyze the effect of this sensory deprivation on inter-leg coordination. We find significant changes in tarsi placement and leg path trajectories after pymetrozine treatment. Leg touchdown accuracy, measured from relative tarsi positions of adjacent legs, is reduced in treated animals. Interestingly, despite poorer spatial coordination in both stance and swing, temporal properties of the gait remain largely the same as in the intact preparations, apart from changes in ipsilateral phase differences between front and middle legs. These findings provide insights into the role of COs in insect gait control and establish pymetrozine as a useful tool for further studies of insect locomotion.

The role of gap junction proteins in the development of neural network functional topology

Gap junctions (GJs) provide a common form of intercellular communication in most animal cells and tissues, from Hydra to human, including electrical synaptic signalling. Cell coupling via GJs has an important role in development in general, and in neural network development in particular. However, quantitative studies monitoring GJ proteins throughout nervous system development are few. Direct investigations demonstrating a role for GJ proteins by way of experimental manipulation of their expression are also rare. In the current work we focused on the role of invertebrate GJ proteins (innexins) in the in vitro development of neural network functional topology, using two-dimensional neural culture preparations derived from the frontal ganglion of the desert locust, Schistocerca gregaria. Immunocytochemistry and quantitative real-time PCR revealed a dynamic expression pattern of the innexins during development of the cultured networks. Changes were observed both in the levels and in the localization of expression. Down-regulating the expression of innexins, by using double-strand RNA for the first time in locust neural cultures, induced clear changes in network morphology, as well as inhibition of synaptogenesis, thus suggesting a role for GJs during the development of the functional topology of neuronal networks.

Modeling of caterpillar crawl using novel tensegrity structures

Caterpillars are soft-bodied animals. They have a relatively simple nervous system, and yet are capable of exhibiting complex movement. This paper presents a 2D caterpillar simulation which mimics caterpillar locomotion using Assur tensegrity structures. Tensegrity structures are structures composed of a set of elements always under compression and a set of elements always under tension. Assur tensegrities are a novel sub-group of tensegrity structures. In the model, each caterpillar segment is represented by a 2D Assur tensegrity structure called a triad. The mechanical structure and the control scheme of the model are inspired by the biological caterpillar. The unique engineering properties of Assur tensegrity structures, together with the suggested control scheme, provide the model with a controllable degree of softness-each segment can be either soft or rigid. The model exhibits several characteristics which are analogous to those of the biological caterpillar. One such characteristic is that the internal pressure of the caterpillar is not a function of its size. During growth, body mass is increased 10 000-fold, while internal pressure remains constant. In the same way, the model is able to maintain near constant internal forces regardless of size. The research also suggests that caterpillars do not invest considerably more energy while crawling than while resting.

The locust foraging gene

Our knowledge of how genes act on the nervous system in response to the environment to generate behavioral plasticity is limited. A number of recent advancements in this area concern food-related behaviors and a specific gene family called foraging (for), which encodes a cGMP-dependent protein kinase (PKG). The desert locust (Schistocerca gregaria) is notorious for its destructive feeding and long-term migratory behavior. Locust phase polyphenism is an extreme example of environmentally induced behavioral plasticity. In response to changes in population density, locusts dramatically alter their behavior, from solitary and relatively sedentary behavior to active aggregation and swarming. Very little is known about the molecular and genetic basis of this striking behavioral phenomenon. Here we initiated studies into the locust for gene by identifying, cloning, and studying expression of the gene in the locust brain. We determined the phylogenetic relationships between the locust PKG and other known PKG proteins in insects. FOR expression was found to be confined to neurons of the anterior midline of the brain, the pars intercerebralis. Our results suggest that differences in PKG enzyme activity are correlated to well-established phase-related behavioral differences. These results lay the groundwork for functional studies of the locust for gene and its possible relations to locust phase polyphenism.

Memoirs of a locust: density-dependent behavioral change as a model for learning and memory

A locust outbreak is a stupendous natural phenomenon that remains in the memory of whoever has been lucky (or unlucky) enough to witness it. Recent years have provided novel and important insights into the neurobiology of locust swarming. However, the central nervous system processes that accompany and perhaps even lie at the basis of locust phase transformation are still far from being fully understood. Our current work deals with the memory of a locust outbreak from a new perspective: that of the individual locust. We take locust density-dependent phase transformation - a unique example of extreme behavioral plasticity, and place it within the context of the accepted scheme of learning and memory. We confirm that a short time period of exposure to a small crowd of locusts is sufficient to induce a significant behavioral change in a previously solitary locust. Our results suggest that part of the behavioral change is due to long-term habituation of evasive and escape responses. We further demonstrate that the memory of a crowding event lasts for at least 24h, and that this memory is sensitive to a protein synthesis blocker. These findings add much to our understanding of locust density-dependent phase polyphenism. Furthermore, they offer a novel and tractable model for the study of learning and memory-related processes in a very distinctive behavioral context.

Lateral-line activity during undulatory body motions suggests a feedback link in closed-loop control of sea lamprey swimming

The lateral-line system is common to most aquatic organisms. It plays an important role in behaviours involving detection of other animals and obstacles. In gnathostome fishes, these behaviours appear to be dependent on an efferent inhibitory system that filters out stimuli caused by the animal’s own movement. Sea lampreys ( Petromyzon marinus L., 1758), the most basal extant vertebrate, possess a functional lateral-line system. Yet they completely lack the inhibitory efferent system. Thus, they may use the lateral line to sense their own swimming movements, helping to stabilize swimming. To test this hypothesis, we first investigated the kinematics of free-swimming lampreys. In an intact tethered preparation, we then generated undualatory body motions of comparable amplitude and frequency to swimming, while monitoring the evoked responses of the posterior lateral-line nerve. Last, we tested the effect of eliminating lateral-line inputs by cobalt treatment. In the tethered preparation, we recorded distinctive and consistent activity in the lateral-line nerve that was strongly dependent on characteristics of the motion. We found that distinct characteristics of the rhythmic movements are encoded in the temporal characteristics of the response. Swimming kinematics of cobalt-treated animals differed from controls, suggesting a complex, yet necessary role of the lateral-line system in closed-loop control of swimming.

Metamorphosis-related changes in the lateral line system of lampreys, Petromyzon marinus

Lamprey metamorphosis leads to considerable changes in morphology and behavior. We have recently reported that larval lampreys possess a functional lateral line system. Here we investigated metamorphic morphological changes in the lateral line system using light and electron microscopy. Functional modifications were studied by recording the trunk lateral line nerve activity of larvae and adults while stimulating neuromasts with approximately sinusoidal water motion. We found a general re-patterning of neuromasts on the head and trunk including an increase in numbers, redistribution within the pit lines, and shifts of the pit lines relative to external features. The trunk lateral line nerve response was qualitatively similar in adults and larvae. Both showed two neuronal populations responding to opposite directions of water flow. Magnitude of the response increased monotonically with stimulus amplitude. At low frequencies, the response lag relative to the stimulus maximum was approximately 220 degrees , and the gain depended approximately linearly on frequency, confirming that superficial neuromasts are velocity detectors. Changes in phase lag with increasing stimulus frequency were steeper in larvae, suggesting slower afferent conductance. The response gain with frequency was smaller for adults, suggesting a narrower frequency discrimination range and decreased sensitivity. These changes may be adaptations for the active lifestyle of adult lampreys.

Coemergence of regularity and complexity during neural network development

With the growing recognition that rhythmic and oscillatory patterns are widespread in the brain and play important roles in all aspects of the function of our nervous system, there has been a resurgence of interest in neuronal synchronized bursting activity. Here, we were interested in understanding the development of synchronized bursts as information-bearing neuronal activity patterns. For that, we have monitored the morphological organization and spontaneous activity of neuronal networks cultured on multielectrode-arrays during their self-executed evolvement from a mixture of dissociated cells into an active network. Complex collective network electrical activity evolved from sporadic firing patterns of the single neurons. On the system (network) level, the activity was marked by bursting events with interneuronal synchronization and nonarbitrary temporal ordering. We quantified these individual-to-collective activity transitions using newly-developed system level quantitative measures of time series regularity and complexity. We found that individual neuronal activity before synchronization was characterized by high regularity and low complexity. During neuronal wiring, there was a transient period of reorganization marked by low regularity, which then leads to coemergence of elevated regularity and functional (nonstochastic) complexity. We further investigated the morphology-activity interplay by modeling artificial neuronal networks with different topological organizations and connectivity schemes. The simulations support our experimental results by showing increased levels of complexity of neuronal activity patterns when neurons are wired up and organized in clusters (similar to mature real networks), as well as network-level activity regulation once collective activity forms.

The function of intersegmental connections in determining temporal characteristics of the spinal cord rhythmic output

Recent renewed interest in the study of rhythmic behaviors and pattern-generating circuits has been inspired by the currently well-established role of oscillating neuronal networks in all aspects of the function of our nervous system: from sensory integration to central processing, and of course motor control. An integrative rather than reductionist approach in the study of pattern-generating circuits is in accordance with current developments. The lamprey spinal cord, a relatively simple and much-studied preparation, is a useful model for such a study. It is an example of a chain of coupled oscillatory units that is characterized by its ability to demonstrate robust coordinated rhythmic output when isolated in vitro. The preparation allows maximum control over the chemical (neuromodulators and hormones) as well as neuronal environment (sensory and descending inputs) of the single oscillatory unit: the pattern-generating circuit. The current study made use of recently developed tools for nonlinear analysis of time-series, specifically neurophysiological signals. These tools allow us to reveal and characterize biological-functional complexity and information capacity of the neuronal output recorded from the lamprey model network. We focused on the importance of different types of inputs to an oscillatory network and their effect on the network's functional output. We show that the basic circuit, when isolated from short- and long-range neuronal inputs, demonstrates its full potential of information capacity: maximal variation quantities and elevated functional complexity. Morphological and functional constraints result in the network exhibiting only a limited range of the above. This constitutes an important substrate for plasticity in neuronal network function.

Larval lampreys possess a functional lateral line system

Morphology of larval lampreys' neuromasts was found to be very similar to that of adults. Activity in the lateral line nerve, elicited by a vibrating ball, indicated a functional lateralis system. Analysis revealed at least two populations of afferents, responding to opposite directions of water flow, with adapting responses. The response magnitude increased monotonically with stimulus amplitude. Larval lampreys' neuromasts were less sensitive than those of teleosts. At low frequencies the response showed a phase lead of 200-220 degrees with respect to the maximum of the ball displacement and a gain that was approximately linearly proportional to frequency.

Neuromodulation of the locust frontal ganglion during the moult: a novel role for insect ecdysis peptides

In insects, continuous growth requires the periodic replacement of the exoskeleton during the moult. A moulting insect displays a stereotypical set of behaviours that culminate in the shedding of the old cuticle at ecdysis. Moulting is an intricate process requiring tightly regulated physiological changes and behaviours to allow integration of environmental cues and to ensure the proper timing and sequence of its components. This is under complex hormonal regulation, and is an important point of interaction between endocrine and neural control. Here, we focus on the locust frontal ganglion (FG), an important player in moulting behaviour, as a previously unexplored target for ecdysis peptides. We show that application of 10(-7) mol l(-1) ecdysis-triggering hormone (ETH) or 10(-7) mol l(-1) and 10(-6) mol l(-1) Pre-ecdysis-triggering hormone (PETH) to an isolated FG preparation caused an increase in bursting frequency in the FG, whereas application of 10(-6) mol l(-1) eclosion hormone (EH) caused an instantaneous, though temporary, total inhibition of all FG rhythmic activity. Crustacean cardioactive peptide (CCAP), an important peptide believed to turn on ecdysis behaviour, caused a dose-dependent increase of FG burst frequency. Our results imply a novel role for this peptide in generating air-swallowing behaviour during the early stages of ecdysis. Furthermore, we show that the modulatory effects of CCAP on the FG motor circuits are dependent on behavioural state and physiological context. Thus, we report that pre-treatment with ETH caused CCAP-induced effects similar to those induced by CCAP alone during pre-ecdysis. Thus, the action of CCAP seems to depend on pre-exposure to ETH, which is thought to be released before CCAP in vivo.

Neurophysiological studies of flight-related density-dependent phase characteristics in locusts

Locusts show an extreme example of density-dependent phase polymorphism, demonstrating within the species differences in morphology as well as biology, dependent on the population density. Behavior is the primary density-dependent change which facilitates the appearance of various morphological and physiological phase characteristics. We have studied density dependent differences in flight related sensory and central neural elements in the desert locust Schistocerca gregaria. Wind generated high frequency spiking activity in the tritocerebral commissure giant (TCG, an identified interneuron that relay inputs from head hair receptors to thoracic motor centers) that was much less intense in solitary locusts, compared to gregarious ones. In addition the solitary locusts' TCG demonstrated much stronger adaptation of its response. In cases when flight was initiated high frequency TCG activity was independent of the locust phase. The tritocerebral commissure dwarf (TCD) is a GABAergic flight related interneuron that is sensitive to ambient illumination intensity. An increase in the TCD spontaneous activity under dark vs. light conditions was significantly higher in gregarious locusts then in solitary ones, implying a flight-related inhibitory mechanism that is far more active in gregarious locusts under dark conditions. Thus, density-dependent phase differences in interneuron activity pattern and properties well reflect and may be at least partially responsible to behavioral flight-related characteristics.

The locust frontal ganglion: a multi-tasked central pattern generator

The locust frontal ganglion (FG) constitutes a major source of innervation to the foregut dilator muscles and thus plays a key role in control of foregut movements. This paper reviews our recent studies on the generation and characteristics of FG motor outputs in two distinct and fundamental locust behaviors: feeding and molting. In an in vitro preparation, isolated from all descending and sensory inputs, the FG was spontaneously active and generated rhythmic multi-unit bursts of action potentials, which could be recorded from all efferent nerves. Thus the FG motor pattern is generated by a central pattern generator within the ganglion. Intracellular recordings suggest that only a small fraction (10-20%) of the FG 100 neurons demonstrate rhythmic activity. The FG motor output in vivo was relatively complex, and strongly dependent on the locust's physiological and behavioral state. Rhythmic activity of the foregut was found to depend on the amount of food present in the crop; animals with full crop demonstrated higher FG burst frequency than those with empty crop. At the molt, the FG generates a distinct motor pattern that could be related to air-swallowing behavior.

Neuromodulation for behavior in the locust frontal ganglion

Neuromodulators orchestrate complex behavioral routines by their multiple and combined effects on the nervous system. In the desert locust, Schistocerca gregaria, frontal ganglion neurons innervate foregut dilator muscles and play a key role in the control of foregut motor patterns. To further investigate the role of the frontal ganglion in locust behavior, we currently focus on the frontal ganglion central pattern generator as a target for neuromodulation. Application of octopamine, a well-studied insect neuromodulator, generated reversible disruption of frontal ganglion rhythmic activity. The threshold for the modulatory effects of octopamine was 10(-6) mol l(-1), and 10(-4) mol l(-1) always abolished the ongoing rhythm. In contrast to this straightforward modulation, allatostatin, previously reported to be a myoinhibitor of insect gut muscles, showed complex, tri-modal, dose-dependent effects on frontal ganglion rhythmic pattern. Using a novel cross-correlation analysis technique, we show that different allatostatin concentrations have very different effects not only on cycle period but also on temporal characteristics of the rhythmic bursts of action potentials. Allatostatin also altered the frontal ganglion rhythm in vivo. The analysis technique we introduce may be instrumental in the study of not fully characterized neural circuits and their modulation. The physiological significance of our results and the role of the modulators in locust behavior are discussed.

Neural correlates to flight-related density-dependent phase characteristics in locusts

Locust phase polymorphism is an extreme example of behavioral plasticity; in response to changes in population density, locusts dramatically alter their behavior. These changes in behavior facilitate the appearance of various morphological and physiological phase characteristics. One of the principal behavioral changes is the more intense flight behavior and improved flight performance of gregarious locusts compared to solitary ones. Surprisingly, the neurophysiological basis of the behavioral phase characteristics has received little attention. Here we present density-dependent differences in flight-related sensory and central neural elements in the desert locust. Using techniques already established for gregarious locusts, we compared the response of locusts of both phases to controlled wind stimuli. Gregarious locusts demonstrated a lower threshold for wind-induced flight initiation. Wind-induced spiking activity in the locust tritocerebral commissure giants (TCG, a pair of identified interneurons that relay input from head hair receptors to thoracic motor centers) was found to be weaker in solitary locusts compared to gregarious ones. The solitary locusts' TCG also demonstrated much stronger spike frequency adaptation in response to wind stimuli. Although the number of forehead wind sensitive hairs was found to be larger in solitary locusts, the stimuli conveyed to their flight motor centers were weaker. The tritocerebral commissure dwarf (TCD) is an inhibitory flight-related interneuron in the locust that responds to light stimuli. An increase in TCD spontaneous activity in dark conditions was significantly stronger in gregarious locusts than in solitary ones. Thus, phase-dependent differences in the activity of flight-related interneurons reflect behavioral phase characteristics.

Molecular underpinnings of motor pattern generation: differential targeting of shal and shaker in the pyloric motor system

The patterned activity generated by the pyloric circuit in the stomatogastric ganglion of the spiny lobster, Panulirus interruptus, results not only from the synaptic connectivity between the 14 component neurons but also from differences in the intrinsic properties of the neurons. Presumably, differences in the complement and distribution of expressed ion channels endow these neurons with many of their distinct attributes. Each pyloric cell type possesses a unique, modulatable transient potassium current, or A-current (I(A)), that is instrumental in determining the output of the network. Two genes encode A-channels in this system, shaker and shal. We examined the hypothesis that cell-specific differences in shaker and shal channel distribution contribute to diversity among pyloric neurons. We found a stereotypic distribution of channels in the cells, such that each channel type could contribute to different aspects of the firing properties of a cell. Shal is predominantly found in the somatodendritic compartment in which it influences oscillatory behavior and spike frequency. Shaker channels are exclusively localized to the membranes of the distal axonal compartments and most likely affect distal spike propagation. Neither channel is detectably inserted into the preaxonal or proximal portions of the axonal membrane. Both channel types are targeted to synaptic contacts at the neuromuscular junction. We conclude that the differential targeting of shaker and shal to different compartments is conserved among all the pyloric neurons and that the channels most likely subserve different functions in the neuron.

Monoamine control of the pacemaker kernel and cycle frequency in the lobster pyloric network

The monoamines dopamine (DA), serotonin (5HT), and octopamine (Oct) can each sculpt a unique motor pattern from the pyloric network in the stomatogastric ganglion (STG) of the spiny lobster Panulirus interruptus. In this paper we investigate the contribution of individual network components in determining the specific amine-induced cycle frequency. We used photoinactivation of identified neurons and pharmacological blockade of synapses to isolate the anterior burster (AB) and pyloric dilator (PD) neurons. Bath application of DA, 5HT, or Oct enhanced cycle frequency in an isolated AB neuron, with DA generating the most rapid oscillations and Oct the slowest. When an AB-PD or AB-2xPD subnetworks were tested, DA often reduced the ongoing cycle frequency, whereas 5HT and Oct both evoked similar accelerations in cycle frequency. However, in the intact pyloric network, both DA and Oct either reduced or did not alter the cycle frequency, whereas 5HT continued to enhance the cycle frequency as before. Our results show that the major target of 5HT in altering the pyloric cycle frequency is the AB neuron, whereas DA's effects on the AB-2xPD subnetwork are critical in understanding its modulation of the cycle frequency. Octopamine's effects on cycle frequency require an understanding of its modulation of the feedback inhibition to the AB-PD group from the lateral pyloric neuron, which constrains the pacemaker group to oscillate more slowly than it would alone. We have thus demonstrated that the relative importance of the different network components in determining the final cycle frequency is not fixed but can vary under different modulatory conditions.

Distributed effects of dopamine modulation in the crustacean pyloric network

It is now clear that neuromodulators can reconfigure a single motor network to allow the generation of a family of related movements. Using dopamine modulation of the 14-neuron pyloric network from the crustacean stomatogastric ganglion as an example, we describe two major mechanisms by which network output is modulated. First, the baseline electrophysiological properties of the network neurons can be altered. Dopamine can affect the activity of each neuron independently. For example, DA modulates IA in nearly every neuron in the pyloric network, but in opposite directions in different cells. Furthermore, DA usually modulates combinations of ionic currents. In some cases, currents with opposing actions on cell excitability are simultaneously affected, and the net response reflects the sum of these opposing effects. Second, neuromodulators can alter the strength of synaptic interactions within the network, quantitatively "rewiring" the network. Every synapse in the network is affected by DA, with some increased and others decreased in strength. DA acts both pre- and postsynaptically to affect transmission: these actions are frequently opposing in sign, and the net response arises as the sum of these opposing actions. Finally, spike-evoked and graded transmission at the same synapse can be oppositely affected by DA. These results emphasize the distributed nature of modulation in motor networks.

Interaction of dopamine and cardiac sac modulatory inputs on the pyloric network in the lobster stomatogastric ganglion

Dopamine and cardiac sac network activity each have strong, and different, modulatory actions on the pyloric rhythm in the stomatogastric ganglion of the spiny lobster. When combined, the two modulatory inputs have a complex effect. Dopamine and cardiac sac activity cancel one another's effects to restore normal pyloric activity to 4 of the 6 classes of pyloric neurons. In the remaining two pyloric neurons, dopamine's strong modulatory effects are completely overruled during cardiac sac network related activity. Possible cellular mechanisms underlying these interactions are discussed.

Dopamine modulates graded and spike-evoked synaptic inhibition independently at single synapses in pyloric network of lobster

Bath application of dopamine (DA) modifies the rhythmic motor pattern generated by the pyloric network in the stomatogastric ganglion of the spiny lobster, Panulirus interruptus. Synaptic transmission between network members is an important target of DA action. All pyloric neurons employ both graded transmitter release and action-potential-mediated synaptic inhibition. DA was previously shown to alter the graded synaptic strength of every pyloric synapse. In this study, we compared DA's effects on action-potential-mediated and graded synaptic inhibition at output synapses of the lateral pyloric (LP) neuron. At each synapse the postsynaptic cell tested was isolated from other descending and pyloric synaptic inputs. DA caused a reduction in the size of the LP spike-evoked inhibitory postsynaptic potentials (IPSPs) in the pyloric dilator (PD) neuron. The change in IPSP size was significantly and linearly correlated with DA-induced reduction in the input resistance of the postsynaptic PD neuron. In contrast, graded inhibition, tested in the same preparations after superfusing the stomatogastric ganglion (STG) with tetrodotoxin (TTX), was consistently enhanced by DA. DA shifted the amplitude of spike-evoked IPSPs in the same direction as the alteration of the postsynaptic cell input resistance at two additional synapses tested: DA weakened the LP spike-mediated inhibition of the ventricular dilator (VD) and reduced the VD input resistance, while strengthening the LP --> pyloric constrictor (PY) synapse and increasing PY input resistance. As previously reported, graded inhibition was enhanced at these two LP output synapses. We conclude that DA can differentially modulate the spike-evoked and graded components of synapses between members of a central pattern generator network. At the synapses we studied, actions on the presynaptic cell predominate in the modulation of graded transmission, whereas effects on postsynaptic cells predominate in the regulation of spike-evoked IPSPs.