Kinematics of soft-bodied, legged locomotion in Manduca sexta larvae

Caterpillar crawling is distinct from that of worms and molluscs; it consists of a series of steps in different body segments that can be compared to walking and running in animals with stiff skeletons. Using a three-dimensional kinematic analysis of horizontal crawling in Manduca sexta, the tobacco hornworm, we found that the phase of vertical displacement in the posterior segments substantially led changes in horizontal velocity and the segments appeared to pivot around the attached claspers. Both of the motions occur during vertebrate walking. In contrast, vertical displacement and horizontal velocity in the anterior proleg-bearing segments were in phase, as expected for running gaits coupled by elastic storage. We propose that this kinematic similarity to running results from the muscular compression and release of elastic tissues. As evidence in support of this proposal, the compression and extension of each segment were similar to harmonic oscillations in a spring, although changes in velocity were 70° out of phase with displacement, suggesting that the spring was damped. Measurements of segment length within, and across, intersegmental boundaries show that some of these movements were caused by folding of the body wall between segments. These findings demonstrate that caterpillar crawling is not simply the forward progression of a peristaltic wave but has kinetic components that vary between segments. Although these movements can be compared to legged locomotion in animals with stiff skeletons, the underlying mechanisms of caterpillar propulsion, and in particular the contribution of elastic tissues, remain to be discovered.</p

The contribution of the NMDA receptor glycine site to rhythm generation during fictive swimming in <em>Xenopus laevis</em> tadpoles

The presence of the N-methyl-D-aspartate (NMDA) receptor glycine-binding site and its role in locomotor activity have been examined using fictive swimming in stage 42 Xenopus laevis frog tadpoles as a simple model system. The specific NMDA/glycine site blocker L-689560 (0.1-20 mu M) impaired swimming rhythm generation and abolished NMDA-induced locomotor-like ventral root activity. D-serine (50 mu M), an agonist at the NMDA/glycine site, increased the duration of skin stimulus-induced fictive swimming episodes, and produced slow modulations of burst frequency and amplitude. These effects of D-serine were reversed by L-689560. In some animals, D-serine also induced an alternative intense, non-locomotory form of rhythmic motor output termed struggling. Glycine (100 mu M), another endogenous agonist at this site, triggered similar effects to D-serine, but only when applied in the presence of strychnine. Manipulations of endogenous glycine levels using sarcosine or ALX 5407 (inhibitors of the glycine re-uptake protein, GlyT1b), produced similar effects to glycine site agonists, including increased episode durations, and modulations in cycle period and burst amplitude. Sarcosine and ALX 5407 also induced struggling. In summary, these experiments support the hypothesis that NMDA receptors in the swimming network of Xenopus laevis tadpoles possess glycine-binding sites, not all of which are fully occupied under normal circumstances. Altering the strength of the NMDA receptor-mediated component of the synaptic drive for swimming by increasing or decreasing occupancy of this site potently influences the locomotor pattern.</p

Group I mGluRs increase locomotor network excitability in Xenopus tadpoles via presynaptic inhibition of glycinergic neurotransmission

The group I metabotropic glutamate receptor agonist (S)-3,5-dihyroxyphenylglycine (DHPG) increases the frequency of rhythmic swimming activity in Xenopus tadpoles. This study explores the possibility that group I receptor modulation occurs in part via depression of inhibitory synaptic transmission. Applications of the glycine receptor antagonist strychnine occluded the effects of DHPG, providing preliminary evidence that group I receptors affect motor network output by reducing glycinergic transmission. This evidence was supported further by intracellular and whole-cell patch-clamp recordings from presumed motorneurons. DHPG applications produced two prominent effects: (i) during swimming activity, glycinergic mid-cycle IPSPs were reduced in amplitude; and (ii) during quiescent periods, the frequency of spontaneous miniature IPSPs was also reduced. No change in membrane potential or input resistance following group I receptor activation was detected. The reduction in fast synaptic inhibition provides a plausible explanation for the increased excitability of the locomotor network, although other contributory mechanisms activated in parallel by group I receptors cannot be discounted. Aspects of this work have been published previously in abstract form [R. J. Chapman &amp; K. T. Sillar (2003) SFN Abstracts 277.8].</p

Development of a spinal locomotor rheostat

Locomotion in immature animals is often inflexible, but gradually acquires versatility to enable animals to maneuver efficiently through their environment. Locomotor activity in adults is produced by complex spinal cord networks that develop from simpler precursors. How does complexity and plasticity emerge during development to bestow flexibility upon motor behavior? And how does this complexity map onto the peripheral innervation fields of motorneurons during development? We show in postembryonic Xenopus laevis frog tadpoles that swim motorneurons initially form a homogenous pool discharging single action potential per swim cycle and innervating most of the dorsoventral extent of the swimming muscles. However, during early larval life, in the prelude to a free-swimming existence, the innervation fields of motorneurons become restricted to a more limited sector of each muscle block, with individual motorneurons reaching predominantly ventral, medial, or dorsal regions. Larval motorneurons then can also discharge multiple action potentials in each cycle of swimming and differentiate in terms of their firing reliability during swimming into relatively high-, medium-, or low-probability members. Many motorneurons fall silent during swimming but can be recruited with increasing locomotor frequency and intensity. Each region of the myotome is served by motorneurons spanning the full range of firing probabilities. This unfolding developmental plan, which occurs in the absence of movement, probably equips the organism with the neuronal substrate to bend, pitch, roll, and accelerate during swimming in ways that will be important for survival during the period of free-swimming larval life that ensues

New proctolin analogues and their myotropic effects on heart of yellow mealworm Tenebrio molitor L. and foregut of locust - Schistocerca gregaria L.

We have extended our work on structure/activity relationship of neuropeptide proctolin (H-Arg-Tyr-Leu-Pro-Thr-OH) by evaluating the effects of the following proctolin analogues: H-X1-Tyr-Leu-Pro-Thr-OH, where X1 - D-Arg (1), N-Me-Arg (2), Can (3), D, Tyr2, D-Leu3, D-Thr5]-proctolin (12). In analogues 1-9, the N-terminal Arg-residue was replaced by basic amino acid derivatives with peptides containing amino acid residue was replaced by basic amino acid derivatives with peptides containing amino acid residues with an isosteric system on the back side chain relative to Arg (compounds 3, 5 and 6) or homo-Arg (compound 7). Analogues 1-12 were evaluated for myotropic action on in vitro heart preparation of Tenebrio molitor, whereas peptides 2, 5 and 7-12 were tested for contractile action on isolated foregut of Schistocerca gregaria. Peptides 2 and 3 retained full cardiotropic activity in Tenebrio molitor while peptides 5 and 7 preserved 40% and 15%, respectively, locust-gut contracting activity of proctolin. Peptides 11 and 12 showed antagonistic activity in Schistocerca gregaria foregut.</p

New proctolin analogues:Synthesis and biological investigation in insects

We have extended our work on structure/activity relationship studies of the neuropeptiden proctolin (H-Arg-Tyr-Leu-Pro-Thr-OH) by evaluating the effects of the following proctolin analogues: H-X1-Tyr-Leu-Pro-Thr-OH, where X1 = D-Arg (I), N-Me-Arg (II), Can (III), Orn(di-Me) (IV), Orn(iPr) (V), Lys(N, N-di-Me) (VI), Lys(iPr) (VII), Lys(Nic) (VIII) and D-Lys(Nic) (IX). In analogues I-IX, the N-terminal Arg residue was replaced by basic amino acid derivatives with peptides containing amino acid residues with an isosteric system on the back side chain relative to Arg (compounds III, V and VI) or homo-Arg (compound VII). Analogues I-IX were evaluated for myotropic activity on the in vitro heart preparation of Tenebrio molitor, whereas peptides II, V, and VII-IX were tested for contractile activity on the isolated foregut of locust Schistocerca gregaria. Peptide II and III showed full cardiotropic activity in T. molitor while peptides V and VII showed 40% and 15%, respectively, locust-gut contracting activity of proctolin.</p