Antagonism by (1,2,5,6-tetrahydropyridine-4-yl) methylphosphinic acid of synaptic transmission in the neonatal rat spinal cord in vitro: an electrophysiological study

Shedding light on myopia by studying complete congenital stationary night blindness

Myopia is the most common eye disorder, caused by heterogeneous genetic and environmental factors. Rare progressive and stationary inherited retinal disorders are often associated with high myopia. Genes implicated in myopia encode proteins involved in a variety of biological processes including eye morphogenesis, extracellular matrix organization, visual perception, circadian rhythms, and retinal signaling. Differentially expressed genes (DEGs) identified in animal models mimicking myopia are helpful in suggesting candidate genes implicated in human myopia. Complete congenital stationary night blindness (cCSNB) in humans and animal models represents an ON-bipolar cell signal transmission defect and is also associated with high myopia. Thus, it represents also an interesting model to identify myopia-related genes, as well as disease mechanisms. While the origin of night blindness is molecularly well established, further research is needed to elucidate the mechanisms of myopia development in subjects with cCSNB. Using whole transcriptome analysis on three different mouse models of cCSNB (in Gpr179^-/-, Lrit3^-/- and Grm6^-/-), we identified novel actors of the retinal signaling cascade, which are also novel candidate genes for myopia. Meta-analysis of our transcriptomic data with published transcriptomic databases and genome-wide association studies from myopia cases led us to propose new biological/cellular processes/mechanisms potentially at the origin of myopia in cCSNB subjects. The results provide a foundation to guide the development of pharmacological myopia therapies.

Spinal Reflexes and Windup In Vitro: Effects of Analgesics and Anesthetics

The spinal cord is the first relay center for nociceptive information. Following peripheral injury, the spinal cord sensitizes. A sign of spinal sensitization is the hyper-reflexia which develops shortly after injury and can be detected in the isolated spinal cord as a "memory of pain." In this context, it is easy to understand that many analgesic compounds target spinally located sites of action to attain analgesia. In vitro isolated spinal cord preparations have been used for a number of years, and experience on the effects of compounds of diverse pharmacological families on spinal function has accumulated. Recently, we have proposed that the detailed study of spinal segmental reflexes in vitro may produce data relevant to the evaluation of the analgesic potential of novel compounds. In this review, we describe the main features of segmental reflexes obtained in vitro and discuss the effects of compounds of diverse chemical nature and pharmacological properties on such reflexes. Our aim was to compare the different profiles of action of the compounds on segmental reflexes in order to extract clues that may be helpful for pharmacological characterization of novel analgesics.

An Update on GABAρ Receptors

The present review discusses the functional and molecular diversity of GABAρ receptors. These receptors were originally described in the mammalian retina, and their functional role in the visual pathway has been recently elucidated; however new studies on their distribution in the brain and spinal cord have revealed that they are more spread than originally thought, and thus it will be important to determine their physiological contribution to the GABAergic transmission in other areas of the central nervous system. In addition, molecular modeling has revealed peculiar traits of these receptors that have impacted on the interpretations of the latest pharmacolgical and biophysical findings. Finally, sequencing of several vertebrate genomes has permitted a comparative analysis of the organization of the GABAρ genes.

Neurochemicals for the investigation of GABA(C) receptors

GABA(C) receptors are being investigated for their role in many aspects of nervous system function including memory, myopia, pain and sleep. There is evidence for functional GABA(C) receptors in many tissues such as retina, hippocampus, spinal cord, superior colliculus, pituitary and the gut. This review describes a variety of neurochemicals that have been shown to be useful in distinguishing GABA(C) receptors from other receptors for the major inhibitory neurotransmitter GABA. Some selective agonists (including (+)-CAMP and 5-methyl-IAA), competitive antagonists (such as TPMPA, (±)-cis-3-ACPBPA and aza-THIP), positive (allopregnanolone) and negative modulators (epipregnanolone, loreclezole) are described. Neurochemicals that may assist in distinguishing between homomeric ρ1 and ρ2 GABA(C) receptors (2-methyl-TACA and cyclothiazide) are also covered. Given their less widespread distribution, lower abundance and relative structural simplicity compared to GABA(A) and GABA(B) receptors, GABA(C) receptors are attractive drug targets.

GABAergic and glycinergic interneuron expression during spinal cord development: dynamic interplay between inhibition and excitation in the control of ventral network outputs

A key objective of neuroscience research is to understand the processes leading to mature neural circuitries in the central nervous system (CNS) that enable the control of different behaviours. During development, network-constitutive neurons undergo dramatic rearrangements, involving their intrinsic properties, such as the blend of ion channels governing their firing activity, and their synaptic interactions. The spinal cord is no exception to this rule; in fact, in the ventral horn the maturation of motor networks into functional circuits is a complex process where several mechanisms cooperate to achieve the development of motor control. Elucidating such a process is crucial in identifying neurons more vulnerable to degenerative or traumatic diseases or in developing new strategies aimed at rebuilding damaged tissue. The focus of this review is on recent advances in understanding the spatio-temporal expression of the glycinergic/GABAergic system and on the contribution of this system to early network function and to motor pattern transformation along with spinal maturation. During antenatal development, the operation of mammalian spinal networks strongly depends on the activity of glycinergic/GABAergic neurons, whose action is often excitatory until shortly before birth when locomotor networks acquire the ability to generate alternating motor commands between flexor and extensor motor neurons. At this late stage of prenatal development, GABA-mediated excitation is replaced by synaptic inhibition mediated by glycine and/or GABA. At this stage of spinal maturation, the large majority of GABAergic neurons are located in the dorsal horn. We propose that elucidating the role of inhibitory systems in development will improve our knowledge on the processes regulating spinal cord maturation.

Novel, potent, and selective GABAC antagonists inhibit myopia development and facilitate learning and memory

This study reports pharmacological and physiological effects of cis- and trans-(3-aminocyclopentanyl)butylphosphinic acid (cis- and trans-3-ACPBPA). These compounds are conformationally restricted analogs of the orally active GABA(B/C) receptor antagonist (3-aminopropyl)-n-butylphosphinic acid (CGP36742 or SGS742). cis-[IC(50)(rho1) = 5.06 microM and IC(50)(rho2) = 11.08 microM; n = 4] and trans-3-ACPMPA [IC(50)(rho1) = 72.58 microM and IC(50)(rho2) = 189.7 microM; n = 4] seem competitive at GABA(C) receptors expressed in Xenopus laevis oocytes, having no effect as agonists (1 mM) but exerting weak antagonist (1 mM) effects on human GABA(A) and GABA(B) receptors. cis-3-ACPBPA was more potent and selective than the trans-compound, being more than 100 times more potent at GABA(C) than GABA(A) or GABA(B) receptors. cis-3-ACPBPA was further evaluated on dissociated rat retinal bipolar cells and dose-dependently inhibited the native GABA(C) receptor (IC(50) = 47 +/- 4.5 microM; n = 6). When applied to the eye as intravitreal injections, cis- and trans-3-ACPBPA prevented experimental myopia development and inhibited the associated vitreous chamber elongation, in a dose-dependent manner in the chick model. Doses only 10 times greater than required to inhibit recombinant GABA(C) receptors caused the antimyopia effects. Using intraperitoneal administration, cis- (30 mg/kg) and trans-3-ACPBPA (100 mg/kg) enhanced learning and memory in male Wistar rats; compared with vehicle there was a significant reduction in time for rats to find the platform in the Morris water maze task (p < 0.05; n = 10). As the physiological effects of cis- and trans-3-ACPBPA are similar to those reported for CGP36742, the memory and refractive effects of CGP36742 may be due in part to its GABA(C) activity.

GABA(C) receptors in retina and brain

The expression of GABA(C) receptors has long been regarded as a specific property of bipolar cells in the inner retina where they control the information transfer from bipolar to retinal ganglion cells. A number of recent anatomical and physiological studies, however, have provided evidence that GABA(C) receptors are also expressed in many brain structures apart from the retina. The presence of GABA(C) receptors in many GABAergic neurons suggests that this receptor type may be involved in the regulation of local inhibition. This chapter focuses on the distribution of GABA(C) receptors and their possible function in various brain areas.

Colocalization of synaptic GABA(C)-receptors with GABA (A)-receptors and glycine-receptors in the rodent central nervous system

Fast inhibition in the nervous system is preferentially mediated by GABA- and glycine-receptors. Two types of ionotropic GABA-receptor, the GABA(A)-receptor and GABA(C)-receptor, have been identified; they have specific molecular compositions, different sensitivities to GABA, different kinetics, and distinct pharmacological profiles. We have studied, by immunocytochemistry, the synaptic localization of glycine-, GABA(A)-, and GABA(C)-receptors in rodent retina, spinal cord, midbrain, and brain-stem. Antibodies specific for the alpha1 subunit of the glycine-receptor, the gamma2 subunit of the GABA(A)-receptor, and the rho subunits of the GABA(C)-receptor have been applied. Using double-immunolabeling, we have determined whether these receptors are expressed at the same postsynaptic sites. In the retina, no such colocalization was observed. However, in the spinal cord, we found the colocalization of glycine-receptors with GABA(A)- or GABA(C)-receptors and the colocalization of GABA(A)- and GABA(C)-receptors in approximately 25% of the synapses. In the midbrain and brain-stem, GABA(A)- and GABA(C)-receptors were colocalized in 10%-15% of the postsynaptic sites. We discuss the possible expression of heteromeric (hybrid) receptors assembled from GABA(A)- and GABA(C)-receptor subunits. Our results suggest that GABA(A)- and GABA(C)-receptors are colocalized in a minority of synapses of the central nervous system.

(3-Aminocyclopentyl)methylphosphinic acids: Novel GABAC receptor antagonists

Our understanding of the role GABA(C) receptors play in the central nervous system is limited due to a lack of specific ligands. Here we describe the pharmacological effects of (+/-)-cis-3- and (+/-)-trans-3-(aminocyclopentyl)methylphosphinic acids ((+/-)-cis- and (+/-)-trans-3-ACPMPA) as novel ligands for the GABA(C) receptor showing little activity at GABA(A) or GABA(B) receptors. (+/-)-cis-3-ACPMPA has similar potency to (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA) at human recombinant rho1 (K(B)=1.0+/-0.2microM) and rat rho3 (K(B)=5.4+/-0.8microM) but is 15 times more potent than TPMPA on human recombinant rho2 (K(B)=1.0+/-0.3microM) GABA(C) receptors expressed in Xenopus oocytes. (+/-)-cis- and (+/-)-trans-3-ACPMPA are novel lead compounds for developing into more potent and selective GABA(C) receptor antagonists with increased lipophilicity for in vivo studies.

GABA receptor rho subunit expression in the developing rat brain

Ionotropic GABA(C) receptors are composed of rho1, rho2 and rho3 subunits. Although the distribution of rho subunit mRNAs in the adult brain has been studied, information on the developmental regulation of different rho subunits in the brain is scattered and incomplete. Here, GABA(C) receptor rho subunit expression was studied in the developing rat brain. In situ hybridization on postnatal brain slices showed rho2 mRNA expression from newborn in superficial gray layer (SGL) of superior colliculus (SuC), and from the first postnatal week in the hippocampal CA1 region and pretectal nucleus of the optic tract. rho2 mRNA was also expressed in the adult dorsal lateral geniculate nucleus. Quantitative RT-PCR revealed expression of all three rho subunits in the hippocampus and superior colliculus from the first postnatal day. In the hippocampus, rho2 mRNA expression clearly dominated over rho1 and rho3, whereas in the superior colliculus, rho1 mRNA expression levels were similar to rho2. In both areas, a clear up-modulation of rho2 and rho3 mRNA during the first postnatal week was detected. GABA(C) receptor protein expression was confirmed in adult hippocampus, superior colliculus and dorsal lateral geniculate nucleus by immunohistochemistry. Our results demonstrate for the first time the expression of all three rho subunit mRNAs in several regions of the developing and adult rat brain. Our quantitative data allows assessment of putative subunit combinations in the superior colliculus and hippocampus. From the selective distribution of rho subunits, it may be hypothesized that GABA(C) receptors are specifically involved in aspects of visual image motion processing in the rat brain.

The spinal GABAergic system is a strong modulator of burst frequency in the lamprey locomotor network

The spinal network coordinating locomotion is comprised of a core of glutamate and glycine interneurons. This network is modulated by several transmitter systems including spinal GABA interneurons. The purpose of this study is to explore the contribution of GABAergic neurons to the regulation of locomotor burst frequency in the lamprey model. Using gabazine, a competitive GABAA antagonist more specific than bicuculline, the goal was to provide a detailed analysis of the influence of an endogenous activation of GABAA receptors on fictive locomotion, as well as their possible interaction with GABAB and involvement of GABAC receptors. During N-methyl-D-aspartate (NMDA)-induced fictive locomotion (ventral root recordings in the isolated spinal cord), gabazine (0.1-100 microM) significantly increased the burst rate up to twofold, without changes in regularity or "burst quality." Gabazine had a proportionately greater effect at higher initial burst rates. Picrotoxin (1-7.5 microM), a less selective GABAA antagonist, also produced a pronounced increase in frequency, but at higher concentrations, the rhythm deteriorated, likely due to the unspecific effects on glycine receptors. The selective GABAB antagonist CGP55845 also increased the frequency, and this effect was markedly enhanced when combined with the GABAA antagonist gabazine. The GABAC antagonist (1,2,5,6-tetrahydropyridine-4-yl)methylphosphinic acid (TPMPA) had no effect on locomotor bursting. Thus the spinal GABA system does play a prominent role in burst frequency regulation in that it reduces the burst frequency by < or =50%, presumably due to presynaptic and soma-dendritic effects documented previously. It is not required for burst generation, but acts as a powerful modulator.

Function of gamma-aminobutyric acid receptor/channel rho 1 subunits in spinal cord

gamma-Aminobutyric acid (GABA) receptor/channel rho 1 subunits are important components in inhibitory pathways in the central nervous system. However, the precise locations and roles of these receptors in the central nervous system are unknown. We studied the expression localization of GABA receptor/channel rho 1 subunit in mouse spinal cord and dorsal root ganglia (DRG). The immunohistochemistry results indicated that GABA receptor/channel rho 1 subunits were expressed in mouse spinal cord superficial dorsal horn (lamina I and lamina II) and in DRG. To understand the functions of the GABA receptor/channel rho 1 subunit in these crucial sites of sensory transmission in vivo, we generated GABA receptor/channel rho 1 subunit mutant mice (rho 1-/-). GABA receptor/channel rho 1 subunit expression in the rho 1-/- mice was eliminated completely, whereas the gross neuroanatomical structures of the rho 1-/- mice spinal cord and DRG were unchanged. Electrophysiological recording showed that GABA-mediated spinal cord response was altered in the rho 1-/- mice. A decreased threshold for mechanical pain in the rho 1-/- mice compared with control mice was observed with the von Frey filament test. These findings indicate that the GABA receptor/channel rho 1 subunit plays an important role in modulating spinal cord pain transmission functions in vivo.

Phosphorylation of the recombinant rho1 GABA receptor

Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the mammalian brain. While a growing body of literature indicates that postsynaptic GABA receptors are regulated by phosphorylation, there is discrepancy as to the specific effects of phosphorylation on GABA receptor function. Here, we have identified phosphorylation sites on the human rho1 GABA receptor for six protein kinases widely expressed in the brain: protein kinase C (PKC); cAMP-dependent protein kinase (PKA); calmodulin-dependent kinase (CaMKII); casein kinase (CKII); mitogen-activated protein kinase (MAPK); and cGMP-dependent protein kinase (PKG). We demonstrate that in nearly all cases, the consensus sites and actual phosphorylation sites do not agree supporting the risk of relying on a sequence analysis to identify potential phosphorylation sites. In addition, of the six kinases examined, only CKII phosphorylated the human rho2 subunit. Site-directed mutagenesis of the phosphorylation sites, or activation/inhibition of select kinase pathways, did not alter the receptor sensitivity or maximal GABA-activated current of the rho1 GABA receptor expressed in Xenopus laevis oocytes suggesting phosphorylation of rho1 does not directly alter receptor properties. This study is a first and necessary step towards elucidating the regulation of rho1 GABA receptors by phosphorylation.

Expression and dendritic mRNA localization of GABAC receptor rho1 and rho2 subunits in developing rat brain and spinal cord

The cellular distribution of GABAC receptor rho1 and rho2 subunits in the rat central nervous system remains controversial. We investigated how these subunits were distributed in cerebellum, hippocampus and spinal cord at postnatal day 1, 7 or in adult life. We found that in the adult cerebellum rho1 and rho2 mRNAs were expressed in Purkinje cells and basket-like cells only. In the hippocampus both subunits were expressed throughout the CA1 pyramidal layer, dentate gyrus and scattered interneurons with maximum staining intensity at P7. In the adult hippocampus in situ staining was predominantly found on interneurons. GABAC antibody labelling in P7 and adult hippocampus was largely overlapping with the in situ staining. Western blot analysis showed GABAC receptor in retina, ovary and testis. In the spinal cord the rho2 signal was consistently stronger than rho1 with overlapping expression patterns. At P1, the most intensely labelled cells were the motoneurons while on P7 and adult sections, interneurons and motoneurons were likewise labelled. On spinal neurons both rho1 and rho2 mRNAs showed somatodendritic localization, extending out for >100 microm with punctate appearance especially in adult cells. A similar spinal distribution pattern was provided with polyclonal antibody labelling, suggesting close correspondence between mRNA and protein compartmentalization. Electrophysiological experiments indicated that P1 spinal motoneurons did possess functional GABAC receptors even though GABAC receptors played little role in evoked synaptic transmission. Our results suggest a pattern of rho1 and rho2 subunit distribution more widespread than hitherto suspected with strong developmental regulation of subunit occurrence.

Elimination of the rho1 subunit abolishes GABA(C) receptor expression and alters visual processing in the mouse retina

Inhibition is crucial for normal function in the nervous system. In the CNS, inhibition is mediated primarily by the amino acid GABA via activation of two ionotropic GABA receptors, GABA(A) and GABA(C). GABA(A) receptor composition and function have been well characterized, whereas much less is known about native GABA(C) receptors. Differences in molecular composition, anatomical distributions, and physiological properties strongly suggest that GABA(A) receptors and GABA(C) receptors have distinct functional roles in the CNS. To determine the functional role of GABA(C) receptors, we eliminated their expression in mice using a knock-out strategy. Although native rodent GABA(C) receptors are composed of rho1 and rho2 subunits, we show that after rho1 subunit expression was selectively eliminated there was no GABA(C) receptor expression. We assessed GABA(C) receptor function in the retina because GABA(C) receptors are highly expressed on the axon terminals of rod bipolar cells and because this site modulates the visual signal to amacrine and ganglion cells. In GABA(C)rho1 null mice, GABA-evoked responses, normally mediated by GABA(C) receptors, were eliminated, and signaling from rod bipolar cells to third order cells was altered. These data demonstrate that elimination of the GABA(C)rho1 subunit, via gene targeting, results in the absence of GABA(C) receptors in the retina and selective alterations in normal visual processing.

Inhibition of spinal or hypoglossal motoneurons of the newborn rat by glycine or GABA

The function of GABA or glycine during early postnatal development remains controversial as their action is reported as either excitatory or inhibitory. The present study addressed the question of the functional role of GABA or glycine on rat motoneurons shortly after birth. For this purpose, using in vitro preparations from immature rats (postnatal age, P0-P4 days), we recorded from lumbar spinal motoneurons and hypoglossal motoneurons. All data were obtained under current clamp conditions (recording with potassium methylsulphate containing electrodes) from cells at about -70 mV resting potential. On spinal motoneurons we used the glycinergic and GABAergic recurrent postsysnaptic potential (PSP) mediated by Renshaw cells to assess its impact on excitatory synaptic inputs from dorsal afferent fibres. Despite its depolarizing nature, the recurrent PSP consistently inhibited synaptic excitation of lumbar motoneurons. On hypoglossal motoneurons, exogenously applied GABA or glycine produced depolarization with decreased input resistance. This response was always associated with inhibition of cell firing induced by intracellular current pulses. Even when the membrane potential was repolarized to resting level in the presence of GABA or glycine, hypoglossal motoneurons failed to generate spikes. Conversely, similar depolarization produced by glutamate consistently facilitated spike firing. GABAergic and glycinergic synaptic potentials evoked by focal stimulation of the reticular formation inhibited firing and/or increased firing latency in the majority of hypoglossal motoneurons. These results indicate that, immediately after birth, rat motoneurons were inhibited by synaptically released or exogenously applied GABA or glycine.

Neuronal bursting induced by NK3 receptor activation in the neonatal rat spinal cord in vitro

Intracellular recording from lumbar motoneurons and extracellular recording from ventral roots of the neonatal rat isolated spinal cord were used to study the mechanisms responsible for the excitation mediated by NK3 tachykinin receptors. The selective NK3 agonists senktide or [MePhe7]neurokinin B induced a slow depolarization with superimposed oscillations (mean period +/- SD was 2.8 +/- 0.8 s) that, in the majority of cases, showed left-right alternation at segmental level and were synchronous between L2 and L5 of the same side. During agonist wash out (5-20 min) a delayed form of hyperexcitability emerged consisting of bursts lasting 8 +/- 2 s (average interburst interval 55 +/- 21 s) with superimposed oscillations usually with homosegmental alternation and heterosegmental synchronicity. Such bursting was accompanied by depression of GABAergic dorsal root potentials evoked by dorsal root stimulation and of the recurrent inhibitory postsynaptic potential recorded from motoneurons. Despite bursting, motoneuron membrane potential returned to baseline while input resistance was increased. Bursts were a network-dependent phenomenon triggered by previous NK3 receptor activation because bursting was suppressed by glutamate receptor antagonists and was insensitive to motoneuron membrane potential or subsequent application of an NK3 receptor antagonist. NK3 receptors operated synergistically with N-methyl-D-aspartate (NMDA) and 5-hydroxytryptamine (5-HT) to trigger fully alternating locomotor-like rhythms while NK3 receptor antagonism disrupted the same rhythm. In summary, in the neonatal rat spinal cord NK3 receptors could trigger rhythmic activity predominantly with alternation at segmental level but with synchronous coupling between ipsilateral motor pools. NK3 receptor activation could also facilitate fictive locomotor patterns induced by NMDA and 5-HT.

Structure and function of GABA(C) receptors: a comparison of native versus recombinant receptors

In less than a decade our knowledge of the GABA(C) receptor, a new type of Cl(-)-permeable ionotropic GABA receptor, has greatly increased based on studies of both native and recombinant receptors. Careful comparison of properties of native and recombinant receptors has provided compelling evidence that GABA receptor rho-subunits are the major molecular components of GABA(C) receptors. Three distinct rho-subunits from various species have been cloned and the pattern of their expression in the retina, as well as in various brain regions, has been established. The pharmacological profile of GABA(C) receptors has been refined and more specific drugs have been developed. Molecular determinants that underlie functional properties of the receptors have been assigned to specific amino acid residues in rho-subunits. This information has helped determine the subunit composition of native receptors, as well as the molecular basis underlying subtle variations among GABA(C) receptors in different species. Finally, GABA(C) receptors play a unique functional role in retinal signal processing via three mechanisms: (1) slow activation; (2) segregation from other inhibitory receptors; and (3) contribution to multi-neuronal pathways.

Interaction between disinhibited bursting and fictive locomotor patterns in the rat isolated spinal cord

Using a transverse barrier that allowed discrete application of neurochemicals to certain lumbar regions of the rat isolated spinal cord, we studied the intersegmental organization of rhythmic patterns recorded extracellularly from ventral roots and intracellularly from single motoneurons. Fictive locomotor patterns were elicited by serotonin (5-HT) and/or N-methyl-D-aspartate (NMDA) or high K(+) solution applied to the rostral or caudal lumbar region of the cord. Neither 4-aminopyridine nor Mg(2+)-free solution shared this property. Coapplication of strychnine and bicuculline (blockers of fast synaptic inhibition) to the caudal part elicited slow bursting in the whole cord. These bursts could trigger episodes of fictive locomotion patterns in the rostral roots. When the rostral region was exposed to 5-HT and/or NMDA (during continuous application of strychnine and bicuculline caudally) a standard locomotor-like pattern was generated during each interburst interval and was surprisingly expressed with its typical pattern alternation even in the caudal area despite the local presence of strychnine and bicuculline. Midsagittal splitting of the caudal region did not change this alternating pattern, indicating that it was driven by rostral regions above the surgical cut. Discrete changes in the concentrations of NMDA rostrally modulated the burst amplitude recorded in the same region after caudal application of strychnine and bicuculline. The period of fictive locomotor patterns changed bimodally depending on the temporal relation with disinhibited bursts, indicating a tight interaction between these two rhythmic activities. These results are interpreted on the basis of a model that assumes a modular arrangement for the locomotor central pattern generator, made up by a series of unit burst generators with serial and crossed connections.