Michael P. Nusbaum

A small-systems approach to motor pattern generation

How neuronal networks enable animals, humans included, to make coordinated movements is a continuing goal of neuroscience research. The stomatogastric nervous system of decapod crustaceans, which contains a set of distinct but interacting motor circuits, has contributed significantly to the general principles guiding our present understanding of how rhythmic motor circuits operate at the cellular level. This results from a detailed documentation of the circuit dynamics underlying motor pattern generation in this system as well as its modulation by individual transmitters and neurons.

The roles of co-transmission in neural network modulation

Neuromodulation provides considerable flexibility to the output of neural networks. In spite of the extensive literature documenting the presence of modulatory peptide co-transmitters in many neurons, considerably less is known about the specific roles of co-transmission in circuit function. This review describes some of the potential consequences of peptide co-transmission in functional circuits, using specific examples from recent work on the actions of identified peptidergic projection neurons acting on the multifunctional neural network within the crustacean stomatogastric ganglion. This system reveals that co-transmission provides projection neurons with a rich assortment of strategies for eliciting multiple outputs from a multifunctional network.

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.

Neuropeptidomic analysis of the brain and thoracic ganglion from the Jonah crab, Cancer borealis

Mass spectrometric methods were applied to determine the peptidome of the brain and thoracic ganglion of the Jonah crab (Cancer borealis). Fractions obtained by high performance liquid chromatography were characterized using MALDI-TOF MS and ESI-Q-TOF MS/MS. In total, 28 peptides were identified within the molecular mass range 750-3000Da. Comparison of the molecular masses obtained with MALDI-TOF MS with the calculated molecular masses of known crustacean peptides revealed the presence of at least nine allatostatins, three orcokinin precursor derived peptides, namely FDAFTTGFGHS, [Ala(13)]-orcokinin, and [Val(13)]-orcokinin, and two kinins, a tachykinin-related peptide and four FMRFamide-related peptides. Eight other peptides were de novo sequenced by collision induced dissociation on the Q-TOF system and yielded AYNRSFLRFamide, PELDHVFLRFamide or EPLDHVFLRFamide, APQRNFLRFamide, LNPFLRFamide, DVRTPALRLRFamide, and LRNLRFamide, which belong to the FMRFamide related peptide family, as well as NFDEIDRSGFA and NFDEIDRSSFGFV, which display high sequence similarity to peptide sequences within the orcokinin precursor of Orconectes limosus. Our paper is the first (neuro)peptidomic analysis of the crustacean nervous system.

Different Sensory Systems Share Projection Neurons But Elicit Distinct Motor Patterns

Considerable research has focused on issues pertaining to sensorimotor integration, but in most systems precise information remains unavailable regarding the specific pathways by which different sensory systems regulate any single central pattern-generating circuit. We address this issue by determining how two muscle stretch-sensitive neurons, the gastropyloric receptor neurons (GPRs), influence identified projection neurons that regulate the gastric mill circuit in the stomatogastric nervous system of the crab and then comparing these actions with those of the ventral cardiac neuron (VCN) mechanosensory system. Here, we show that the GPR neurons activate the gastric mill rhythm in the stomatogastric ganglion (STG) via their excitation of two identified projection neurons, modulatory commissural neuron 1 (MCN1) and commissural projection neuron 2 (CPN2), in the commissural ganglion. Support for this conclusion comes from the ability of the modulatory proctolin neuron (MPN), a projection neuron that suppresses the gastric mill rhythm via its inhibitory actions on MCN1 and CPN2, to inhibit the GPR-elicited gastric mill rhythm. Selective elimination of MCN1 and CPN2 access to the STG also prevents GPR activation of this rhythm. The VCN neurons also elicit the gastric mill rhythm by coactivating MCN1 and CPN2, but the GPR-elicited gastric mill rhythm is distinct. These distinct rhythms are likely to result partly from different MCN1 activity levels under these two conditions and partly from the presence of additional GPR actions in the STG. These results support the hypothesis that different sensory systems differentially regulate neuronal circuit activity despite their convergent actions on a single subpopulation of projection neurons.

Frequency Regulation of a Slow Rhythm by a Fast Periodic Input

Many nervous systems contain rhythmically active subnetworks that interact despite oscillating at widely different frequencies. The stomatogastric nervous system of the crab Cancer borealis produces a rapid pyloric rhythm and a considerably slower gastric mill rhythm. We construct and analyze a conductance-based compartmental model to explore the activation of the gastric mill rhythm by the modulatory commissural neuron 1 (MCN1). This model demonstrates that the period of the MCN1-activated gastric mill rhythm, which was thought to be determined entirely by the interaction of neurons in the gastric mill network, can be strongly influenced by inhibitory synaptic input from the pacemaker neuron of the fast pyloric rhythm, the anterior burster (AB) neuron. Surprisingly, the change of the gastric mill period produced by the pyloric input to the gastric mill system can be many times larger than the period of the pyloric rhythm itself. This model illustrates several mechanisms by which a fast oscillatory neuron may control the frequency of a much slower oscillatory network. These findings suggest that it is possible to modify the slow rhythm either by direct modulation or indirectly by modulating the faster rhythm.

Mechanosensory Activation of a Motor Circuit by Coactivation of Two Projection Neurons

Individual neuronal circuits can generate multiple activity patterns because of the influence of different projection neurons. However, in most systems it has been difficult to identify and assess the relative contribution of all upstream neurons responsible for the activation of any single activity pattern by a behaviorally relevant stimulus. To elucidate this issue, we used the stomatogastric nervous system (STNS) of the crab. The STNS includes the gastric mill (chewing) motor circuit in the stomatogastric ganglion (STG) and no more than 20 projection neurons that innervate the STG. We previously identified at least some (four) of the projection neurons that are activated directly by the ventral cardiac neuron (VCN) system, a population of mechanosensory neurons that activates the gastric mill circuit. Here we show that two of these projection neurons, the previously identified modulatory commissural neuron 1 (MCN1) and commissural projection neuron 2 (CPN2), are necessary and likely sufficient for the initiation/maintenance of the VCN-elicited gastric mill rhythm. Selective inactivation of either MCN1 or CPN2 still enabled a VCN-elicited gastric mill rhythm. However, because MCN1 and CPN2 have different actions on gastric mill neurons, these manipulations resulted in rhythms distinct from each other and from that occurring in the intact system. After removal of both MCN1 and CPN2, VCN stimulation failed to activate the gastric mill rhythm. Selective conjoint stimulation of MCN1 and CPN2, approximating their VCN-elicited activity patterns and firing frequencies, elicited a VCN-like gastric mill rhythm. Thus the VCN mechanosensory system elicits the gastric mill rhythm via its activation of a subset of the relevant projection neurons.

Proprioceptor Regulation of Motor Circuit Activity by Presynaptic Inhibition of a Modulatory Projection Neuron

Phasically active sensory systems commonly influence rhythmic motor activity via synaptic actions on the relevant circuit and/or motor neurons. Using the crab stomatogastric nervous system (STNS), we identified a distinct synaptic action by which an identified proprioceptor, the gastropyloric muscle stretch receptor (GPR) neuron, regulates the gastric mill (chewing) motor rhythm. Previous work showed that rhythmically stimulating GPR in a gastric mill-like pattern, in the isolated STNS, elicits the gastric mill rhythm via its activation of two identified projection neurons, modulatory commissural neuron 1 (MCN1) and commissural projection neuron 2, in the commissural ganglia. Here, we determine how activation of GPR with a behaviorally appropriate pattern (active during each gastric mill retractor phase) influences an ongoing gastric mill rhythm via actions in the stomato gastric ganglion, where the gastric mill circuit is located. Stimulating GPR during each retractor phase selectively prolongs that phase and thereby slows the ongoing rhythm. This selective action on the retractor phase results from two distinct GPR actions. First, GPR presynaptically inhibits the axon terminals of MCN1, reducing MCN1 excitation of all gastric mill neurons. Second, GPR directly excites the retractor phase neurons. Because MCN1 transmitter release occurs during each retractor phase, these parallel GPR actions selectively reduce the buildup of excitatory drive to the protractor phase neurons, delaying each protractor burst. Thus, rhythmic proprioceptor feedback to a motor circuit can result from a global reduction in excitatory drive to that circuit, via presynaptic inhibition, coupled with a phase-specific excitatory input that prolongs the excited phase by delaying the onset of the subsequent phase.

Actions of a histaminergic/peptidergic projection neuron on rhythmic motor patterns in the stomatogastric nervous system of the crab Cancer borealis

Histamine is a neurotransmitter with actions throughout the nervous system of vertebrates and invertebrates. Nevertheless, the actions of only a few identified histamine‐containing neurons have been characterized. Here, we present the actions of a histaminergic projection neuron on the rhythmically active pyloric and gastric mill circuits within the stomatogastric ganglion (STG) of the crab Cancer borealis. An antiserum generated against histamine labeled profiles throughout the C. borealis stomatogastric nervous system. Labeling occurred in several somata and neuropil within the paired commissural ganglia as well as in neuropil within the STG and at the junction of the superior oesophageal and stomatogastric nerves. The source of all histamine‐like immunolabeling in the STG neuropil was one pair of neuronal somata, the previously identified inferior ventricular (IV) neurons, located in the supraoesophageal ganglion. These neurons also exhibited FLRFamide‐like immunoreactivity. Activation of the IV neurons in the crab inhibited some pyloric and gastric mill neurons and, with inputs from the commissural ganglia eliminated, terminated both rhythms. Focal application of histamine had comparable effects. The actions of both applied histamine and IV neuron stimulation were blocked, reversibly, by the histamine type‐2 receptor antagonist cimetidine. With the commissural ganglia connected to the STG, IV neuron stimulation elicited a longer‐latency activation of commissural projection neurons which in turn modified the pyloric rhythm and activated the gastric mill rhythm. These results support the hypothesis that the histaminergic/peptidergic IV neurons are projection neurons with direct and indirect actions on the STG circuits of the crab C. borealis. J. Comp. Neurol. 469:153–169, 2004.

Divergent co‐transmitter actions underlie motor pattern activation by a modulatory projection neuron

Co‐transmission is a common means of neuronal communication, but its consequences for neuronal signaling within a defined neuronal circuit remain unknown in most systems. We are addressing this issue in the crab stomatogastric nervous system by characterizing how the identified modulatory commissural neuron (MCN)1 uses its co‐transmitters to activate the gastric mill (chewing) rhythm in the stomatogastric ganglion (STG). MCN1 contains γ‐aminobutyric acid (GABA) plus the peptides proctolin and Cancer borealis tachykinin‐related peptide Ia (CabTRP Ia), which it co‐releases during the retractor phase of the gastric mill rhythm to influence both retractor and protractor neurons. By focally applying each MCN1 co‐transmitter and pharmacologically manipulating each co‐transmitter action during MCN1 stimulation, we found that MCN1 has divergent co‐transmitter actions on the gastric mill central pattern generator (CPG), which includes the neurons lateral gastric (LG) and interneuron 1 (Int1), plus the STG terminals of MCN1 (MCN1STG). MCN1 used only CabTRP Ia to influence LG, while it used only GABA to influence Int1 and the contralateral MCN1STG. These MCN1 actions caused a slow excitation of LG, a fast excitation of Int1 and a fast inhibition of MCN1STG. MCN1‐released proctolin had no direct influence on the gastric mill CPG, although it likely indirectly regulates this CPG via its influence on the pyloric rhythm. MCN1 appeared to have no ionotropic actions on the gastric mill follower motor neurons, but it did use proctolin and/or CabTRP Ia to excite them. Thus, a modulatory projection neuron can elicit rhythmic motor activity by using distinct co‐transmitters, with different time courses of action, to simultaneously influence different CPG neurons.