Presynaptic control of neurones in pattern-generating networks

Neural circuit regulation by identified modulatory projection neurons

Rhythmic behaviors (e.g., walking, breathing, and chewing) are produced by central pattern generator (CPG) circuits. These circuits are highly dynamic due to a multitude of input they receive from hormones, sensory neurons, and modulatory projection neurons. Such inputs not only turn CPG circuits on and off, but they adjust their synaptic and cellular properties to select behaviorally relevant outputs that last from seconds to hours. Similar to the contributions of fully identified connectomes to establishing general principles of circuit function and flexibility, identified modulatory neurons have enabled key insights into neural circuit modulation. For instance, while bath-applying neuromodulators continues to be an important approach to studying neural circuit modulation, this approach does not always mimic the neural circuit response to neuronal release of the same modulator. There is additional complexity in the actions of neuronally-released modulators due to: (1) the prevalence of co-transmitters, (2) local- and long-distance feedback regulating the timing of (co-)release, and (3) differential regulation of co-transmitter release. Identifying the physiological stimuli (e.g., identified sensory neurons) that activate modulatory projection neurons has demonstrated multiple “modulatory codes” for selecting particular circuit outputs. In some cases, population coding occurs, and in others circuit output is determined by the firing pattern and rate of the modulatory projection neurons. The ability to perform electrophysiological recordings and manipulations of small populations of identified neurons at multiple levels of rhythmic motor systems remains an important approach for determining the cellular and synaptic mechanisms underlying the rapid adaptability of rhythmic neural circuits.

Different microcircuit responses to comparable input from one versus both copies of an identified projection neuron

Neuronal inputs to microcircuits are often present as multiple copies of apparently equivalent neurons. Thus far, however, little is known regarding the relative influence on microcircuit output of activating all or only some copies of such an input. We examine this issue in the crab (Cancer borealis) stomatogastric ganglion, where the gastric mill (chewing) microcircuit is activated by modulatory commissural neuron 1 (MCN1), a bilaterally paired modulatory projection neuron. Both MCN1s contain the same co-transmitters, influence the same gastric mill microcircuit neurons, can drive the biphasic gastric mill rhythm, and are co-activated by all identified MCN1-activating pathways. Here, we determine whether the gastric mill microcircuit response is equivalent when stimulating one or both MCN1s under conditions where the pair are matched to collectively fire at the same overall rate and pattern as single MCN1 stimulation. The dual MCN1 stimulations elicited more consistently coordinated rhythms, and these rhythms exhibited longer phases and cycle periods. These different outcomes from single and dual MCN1 stimulation may have resulted from the relatively modest, and equivalent, firing rate of the gastric mill neuron LG (lateral gastric) during each matched set of stimulations. The LG neuron-mediated, ionotropic inhibition of the MCN1 axon terminals is the trigger for the transition from the retraction to protraction phase. This LG neuron influence on MCN1 was more effective during the dual stimulations, where each MCN1 firing rate was half that occurring during the matched single stimulations. Thus, equivalent individual- and co-activation of a class of modulatory projection neurons does not necessarily drive equivalent microcircuit output.

Repetition priming of motor activity mediated by a central pattern generator: the importance of extrinsic vs. intrinsic program initiators

Repetition priming is characterized by increased performance as a behavior is repeated. Although this phenomenon is ubiquitous, mediating mechanisms are poorly understood. We address this issue in a model system, the feeding network of Aplysia This network generates both ingestive and egestive motor programs. Previous data suggest a chemical coding model: ingestive and egestive inputs to the feeding central pattern generator (CPG) release different modulators, which act via different second messengers to prime motor activity in different ways. The ingestive input to the CPG (neuron CBI-2) releases the peptides feeding circuit activating peptide and cerebral peptide 2, which produce an ingestive pattern of activity. The egestive input to the CPG (the esophageal nerve) releases the peptide small cardioactive peptide. This model is based on research that focused on a single aspect of motor control (radula opening). Here we ask whether repetition priming is observed if activity is triggered with a neuron within the core CPG itself and demonstrate that it is not. Moreover, previous studies demonstrated that effects of modulatory neurotransmitters that induce repetition priming persist. This suggests that it should be possible to "prime" motor programs triggered from within the CPG by first stimulating extrinsic modulatory inputs. We demonstrate that programs triggered after ingestive input activation are ingestive and programs triggered after egestive input activation are egestive. We ask where this priming occurs and demonstrate modifications within the CPG itself. This arrangement is likely to have important consequences for "task" switching, i.e., the cessation of one type of motor activity and the initiation of another.

Revisiting the reticulum: feedforward and feedback contributions to motor program parameters in the crab cardiac ganglion microcircuit

The neurogenic heartbeat of crustaceans is controlled by the cardiac ganglion (CG), a central pattern generator (CPG) microcircuit composed of nine neurons. In most decapods, five "large" motor neurons (MNs) project from the CG to the myocardium, where their excitatory synaptic signals generate the rhythmic heartbeat. The processes of four "small" premotor neurons (PMNs) are confined to the CG, where they provide excitatory drive to the MNs via impulse-mediated chemical signals and electrotonic coupling. This study explored feedforward and feedback interactions between the PMNs and the MNs in the CG of the blue crab (Callinectes sapidus). Three methods were used to compare the activity of the MNs and the PMNs in the integrated CG to their autonomous firing patterns: 1) ligatures were tightened on the ganglion trunk that connects the PMNs and MNs; 2) TTX was applied focally to suppress selectively PMN or MN activity; and 3) sucrose pools were devised to block reversibly PMN or MN impulse conduction. With all treatments, the PMNs and MNs continued to produce autonomous rhythmic bursting following disengagement. Removal of PMN influence resulted in a significantly reduced MN duty cycle that was mainly attributable to a lower autonomous burst frequency. Conversely, after removal of MN feedback, the PMN duty cycle was increased, primarily due to a prolonged burst duration. Application of sucrose to block impulse conduction without eliminating PMN oscillations disclosed significant contributions of spike-mediated PMN-to-MN signals to the initiation and prolongation of the MN burst. Together, these observations support a view of the Callinectes CG composed of two classes of spontaneously bursting neurons with distinct endogenous rhythms. Compartmentalized feedforward and feedback signaling endow this microcircuit with syncytial properties such that the intrinsic attributes of the PMNs and MNs both contribute to shaping all parameters of the motor patterns transmitted to the myocardium.

Role of genital sensory information in the control of the functioning of the spinal generator for ejaculation

An intrinsic spinal rhythm mediates fictive ejaculation (FE). In this study, the effect of genital sensory stimulation on the functioning of the spinal generator of ejaculation was investigated. To this aim, the effect of (a) stimulation of internal and external genital structures; (b) repeated elicitation of FE and (c) genital stimulation during in progress expression of FE on the rhythmic genital motor pattern of ejaculation (GMPE) was analysed in sexually experienced, spinal male rats. Results showed that the spinal intrinsic ejaculatory rhythm can be modulated by genital inputs, and that repeated stimulation modifies this rhythm, progressively inhibiting its expression. Finally, in progress GMPEs could be reset by overlapping genital stimulation, supporting the notion of the spinal cord mediating the inhibition of FE following repeated genital inflow. Results reveal the nature of the modulatory role that genital afferent information exerts on the expression of FE.

Frequency-Dependent Regulation of Afferent Transmission in the Feeding Circuitry of Aplysia

During rhythmic behaviors, sensori-motor transmission is often regulated so that there are phasic changes in afferent input to follower neurons. We study this type of regulation in the feeding circuit of Aplysia. We characterize effects of the B4/5 interneurons on transmission from the mechanoafferent B21 to the radula closer motor neuron B8. In quiescent preparations, B4/5-induced postsynaptic potentials (PSPs) can block spike propagation in the lateral process of B21 and inhibit afferent transmission. B4/5 are, however, active during the retraction phase of motor programs, i.e., when mechanoafferent transmission to B8 presumably occurs. To determine whether mechanoafferent transmission is necessarily inhibited when B4/5 are active, we characterize the B4/5 firing frequency during retraction and show that, for the most part, it is low (below 15 Hz). There is, therefore, a low probability that spike propagation will be inhibited. The relative ineffectiveness of low frequency activity is not simply a consequence of insufficient PSP magnitude, because a single PSP can block spike propagation. Instead, it is related to the fact that PSPs have a short duration. When B4/5 fire at a low frequency, there is therefore a low probability that afferent transmission in the lateral process of B21 can be inhibited. In conclusion, we demonstrate that afferent transmission will not always be affected when a neuron that exerts inhibitory effects is active. Although a cell may be ineffective when it fires at a low frequency, ineffectiveness is not necessarily a consequence of spike frequency per se. Instead it may be due to spike timing.

Different proctolin neurons elicit distinct motor patterns from a multifunctional neuronal network

Distinct motor patterns are selected from a multifunctional neuronal network by activation of different modulatory projection neurons. Subsets of these projection neurons can contain the same neuromodulator(s), yet little is known about the relative influence of such neurons on network activity. We have addressed this issue in the stomatogastric nervous system of the crab Cancer borealis. Within this system, there is a neuronal network in the stomatogastric ganglion (STG) that produces many versions of the pyloric and gastric mill rhythms. These different rhythms result from activation of different projection neurons that innervate the STG from neighboring ganglia and modulate STG network activity. Three pairs of these projection neurons contain the neuropeptide proctolin. These include the previously identified modulatory proctolin neuron and modulatory commissural neuron 1 (MCN1) and the newly identified modulatory commissural neuron 7 (MCN7). We document here that each of these neurons contains a unique complement of cotransmitters and that each of these neurons elicits a distinct version of the pyloric motor pattern. Moreover, only one of them (MCN1) also elicits a gastric mill rhythm. The MCN7-elicited pyloric rhythm includes a pivotal switch by one STG network neuron from playing a minor to a major role in motor pattern generation. Therefore, modulatory neurons that share a peptide transmitter can elicit distinct motor patterns from a common target network.

Cellular, synaptic, network, and modulatory mechanisms involved in rhythm generation

The membrane properties and the synaptic interactions of individual neurons, as well as the interactions between neuronal networks, all contribute to the formation of the complex patterns of activity that underlie rhythmic motor patterns and slow-wave sleep rhythms. These properties and interactions are potential points of modulation for further refining network output. Recent work illustrates the range of these properties and interactions and suggests how they may be modulated.

Identification of rat brainstem multisynaptic connections to the oral motor nuclei using pseudorabies virus. III. Lingual muscle motor systems

The present experiments complete our investigations of higher order afferent control of the orofacial muscles by examining the premotor systems controlling the lingual musculature. Pseudorabies virus (PRV) was injected into the extrinsic (protruders: genioglossus and geniohyoid; retractors: hyoglossus and styloglossus) and intrinsic tongue muscles in bilaterally sympathectomized rats. Injection volumes ranged from 1 to 12 microl with average titers of 4 x 10(8) pfu/ml and maximum survival times of 90 h. Consistent labeling patterns and distributions occurred across each of the individual muscles and between extrinsic and intrinsic muscle groups, as well as in comparison to the results from the previous masticatory and facial muscle experiments. Virus injections produced a predictable myotopic labeling pattern in the hypoglossal nucleus (Mo 12). Transneuronally labeled neurons occurred in regions known to project directly to Mo 12 motoneurons including the nucleus subcoeruleus, trigeminal sensory areas, parvicellular reticular formation, and the dorsal medullary reticular fields. Maximum survival times revealed more distant connections from medial and lateral reticular zones including the periaqueductal gray, dorsal raphe, laterodorsal and pedunculopontine tegmental areas, and substantia nigra in the midbrain, the gigantocellular region, pontine nucleus caudalis and ventralis, and lateral paragigantocellular region in the pons, and the nucleus of the solitary tract, paratrigeminal region, and paramedian field in the medulla. Thus, injections of PRV into the orofacial muscles revealed a complex, but remarkably uniform network of multisynaptic connections in the brainstem that control and coordinate the activity of the masticatory, facial, and lingual muscles.

Neural modulation: following your own rhythm

Recent studies of an invertebrate neural circuit show how presynaptic inhibition can play a key role in the generation of oscillatory activity, and can allow the directly affected axon terminal to engage in rhythmic activity independently of the rest of the neuron.

Intrinsic neuromodulation: altering neuronal circuits from within

There are two sources of neuromodulation for neuronal circuits: extrinsic inputs and intrinsic components of the circuits themselves. Extrinsic neuromodulation is known to be pervasive in nervous systems, but intrinsic neuromodulation is less recognized, despite the fact that it has now been demonstrated in sensory and neuromuscular circuits and in central pattern generators. By its nature, intrinsic neuromodulation produces local changes in neuronal computation, whereas extrinsic neuromodulation can cause global changes, often affecting many circuits simultaneously. Studies in a number of systems are defining the different properties of these two forms of neuromodulation.

Oscillation in motor pattern-generating networks

Oscillation in motor pattern generators is driven either by pacemaker neurons with inherent bursting properties or through network interactions. In a few examples in invertebrates and lower vertebrates, the mechanisms by which reciprocal inhibition combines with inherent membrane properties to produce network oscillation are beginning to emerge. A recently developed theoretical framework provides a context for understanding and comparing these findings.

A switch between two modes of synaptic transmission mediated by presynaptic inhibition

Presynaptic inhibition reduces chemical synaptic transmission in the central nervous system between pairs of neurons, but its role(s) in shaping the multisynaptic interactions underlying neural network activity are not well studied. We therefore used the crustacean stomatogastric nervous system to study how presynaptic inhibition of the identified projection neuron, modulatory commissural neuron 1 (MCN1), influences the MCN1 synaptic effects on the gastric mill neural network. Tonic MCN1 discharge excites gastric mill network neurons and activates the gastric mill rhythm. One network neuron, the lateral gastric (LG) neuron, presynaptically inhibits MCN1 and is electrically coupled to its terminals. We show here that this presynaptic inhibition selectively reduces or eliminates transmitter-mediated excitation from MCN1 without reducing its electrically mediated excitatory effects, thereby switching the network neurons excited by MCN1. By switching the type of synaptic output from MCN1 and, hence, the activated network neurons, this presynaptic inhibition is pivotal to motor pattern generation.