Pre- and postsynaptic modulation of spinal GABAergic neurotransmission by the neurosteroid, 5 beta-pregnan-3 alpha-ol-20-one

A behaviorally related developmental switch in nitrergic modulation of locomotor rhythmogenesis in larval Xenopus tadpoles

Locomotor control requires functional flexibility to support an animal's full behavioral repertoire. This flexibility is partly endowed by neuromodulators, allowing neural networks to generate a range of motor output configurations. In hatchling Xenopus tadpoles, before the onset of free-swimming behavior, the gaseous modulator nitric oxide (NO) inhibits locomotor output, shortening swim episodes and decreasing swim cycle frequency. While populations of nitrergic neurons are already present in the tadpole's brain stem at hatching, neurons positive for the NO-synthetic enzyme, NO synthase, subsequently appear in the spinal cord, suggesting additional as yet unidentified roles for NO during larval development. Here, we first describe the expression of locomotor behavior during the animal's change from an early sessile to a later free-swimming lifestyle and then compare the effects of NO throughout tadpole development. We identify a discrete switch in nitrergic modulation from net inhibition to overall excitation, coincident with the transition to free-swimming locomotion. Additionally, we show in isolated brain stem-spinal cord preparations of older larvae that NO's excitatory effects are manifested as an increase in the probability of spontaneous swim episode occurrence, as found previously for the neurotransmitter dopamine, but that these effects are mediated within the brain stem. Moreover, while the effects of NO and dopamine are similar, the two modulators act in parallel rather than NO operating serially by modulating dopaminergic signaling. Finally, NO's activation of neurons in the brain stem also leads to the release of NO in the spinal cord that subsequently contributes to NO's facilitation of swimming.

Neurosteroid influences on sensitivity to ethanol

This review will highlight a variety of mechanisms by which neurosteroids affect sensitivity to ethanol, including physiological states associated with activity of the hypothalamic-pituitary-adrenal (HPA) and hypothalamic-pituitary-gonadal (HPG) axes, and the effects of chronic exposure to ethanol, in addition to behavioral implications. To date, γ-aminobutyric acid (GABA(A)) receptor mechanisms are a major focus of the modulation of ethanol effects by neuroactive steroids. While NMDA receptor mechanisms are gaining prominence in the literature, these complex data would be best discussed separately. Accordingly, GABA(A) receptor mechanisms are emphasized in this review with brief mention of some NMDA receptor mechanisms to point out contrasting neuroactive steroid pharmacology. Overall, the data suggest that neurosteroids are virtually ubiquitous modulators of inhibitory neurotransmission. Neurosteroids appear to affect sensitivity to ethanol in specific brain regions and, consequently, specific behavioral tests, possibly related to the efficacy and potency of ethanol to potentiate the release of GABA and increase neurosteroid concentrations. Although direct interaction of ethanol and neuroactive steroids at common receptor binding sites has been suggested in some studies, this proposition is still controversial. It is currently difficult to assign a specific mechanism by which neuroactive steroids could modulate the effects of ethanol in particular behavioral tasks.

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 & K. T. Sillar (2003) SFN Abstracts 277.8].

Diminished neurosteroid sensitivity of synaptic inhibition and altered location of the alpha4 subunit of GABA(A) receptors in an animal model of epilepsy

In animal models of temporal lobe epilepsy (TLE), neurosteroid sensitivity of GABA(A) receptors on dentate granule cells (DGCs) is diminished; the molecular mechanism underlying this phenomenon remains unclear. The current study investigated a mechanism for loss of neurosteroid sensitivity of synaptic GABA(A) receptors in TLE. Synaptic currents recorded from DGCs of epileptic animals (epileptic DGCs) were less frequent, larger in amplitude, and less sensitive to allopregnanolone modulation than those recorded from DGCs of control animals (control DGCs). Synaptic currents recorded from epileptic DGCs were less sensitive to diazepam and had altered sensitivity to benzodiazepine inverse agonist RO 15-4513 (ethyl-8-azido-6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5alpha][1,4]benzodiazepine-3-carboxylate) and furosemide than those recorded from control DGCs. Properties of synaptic currents recorded from epileptic DGCs appeared similar to those of recombinant receptors containing the alpha4 subunit. Expression of the alpha4 subunit and its colocalization with the synaptic marker GAD65 was increased in epileptic DGCs. Location of the alpha4 subunit in relation to symmetric (inhibitory) synapses on soma and dendrites of control and epileptic DGCs was examined with postembedding immunogold electron microscopy. The alpha4 immunogold labeling was present more commonly within the synapse in epileptic DGCs compared with control DGCs, in which the subunit was extrasynaptic. These studies demonstrate that, in epileptic DGCs, the neurosteroid modulation of synaptic currents is diminished and alpha4 subunit-containing receptors are present at synapses and participate in synaptic transmission. These changes may facilitate seizures in epileptic animals.

The contribution of the NMDA receptor glycine site to rhythm generation during fictive swimming in Xenopus laevis 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 microm) impaired swimming rhythm generation and abolished NMDA-induced locomotor-like ventral root activity. D-serine (50 microm), 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 microm), 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.

Modulation of a spinal locomotor network by metabotropic glutamate receptors

We have explored the potential involvement of the three main classes of metabotropic glutamate receptor in the modulation of a spinal locomotor network using tadpoles of the anuran amphibian Xenopus laevis. Selective activation of group I receptors in Xenopus embryos and young larvae using the general group I agonist DHPG [(S)-3,5-dihyroxyphenylglycine] significantly increased the frequency of swimming and the number of spontaneously occurring swimming episodes, as monitored by extracellular recordings from ventral roots. Group I receptor activation was without significant effect on the duration or amplitude of motor bursts, the duration of swimming episodes, or the head-to-tail delay in the propagation of swimming activity. Activation of either group II or group III receptors, however, following bath applications of the specific agonists APDC [(2R,4R)-aminopyrrolidine-2,4-dicarboxylic acid] and L-AP4 (L-2-amino-4-phosphonobutanoate), respectively, produced a net inhibitory effect on many of the parameters of fictive swimming at both developmental stages, including a reduction in swimming frequency and episode duration, along with a significant reduction in motor burst amplitude and duration in larval animals only. Applications of selective antagonists provide evidence for activation of all three groups during swimming. The group II and III antagonists EGLU (1-ethyl-2-benzimidazolinone) and MAP4 [(S)-2-amino-2-methyl-4-phosphonobutanoate], respectively, increased, while group I antagonists, CPCCOEt and MPEP, decreased swim frequency. Our findings thus provide evidence for the presence and endogenous activation of three classes of metabotropic glutamate receptor which may function as an intrinsic modulatory control system during fictive swimming in Xenopus tadpoles.

Neurosteroid modulation of synaptic and extrasynaptic GABA(A) receptors

Certain naturally occurring pregnane steroids act in a nongenomic manner to potently and selectively enhance the interaction of the inhibitory neurotransmitter GABA with the GABA(A) receptor. Consequently such steroids exhibit anxiolytic, anticonvulsant, analgesic, sedative, hypnotic, and anesthetic properties. In both physiological and pathophysiological scenarios, the pregnane steroids may function as endocrine messengers (e.g., produced in the periphery and cross the blood-brain barrier) to influence behaviour. However, additionally "neurosteroids" can be synthesised in the brain and spinal cord to act in a paracrine or autocrine manner and thereby locally influence neuronal activity. Given the ubiquitous expression of the GABA(A) receptor throughout the mammalian central nervous system (CNS), physiological, pathophysiological, or drug-induced pertubations of neurosteroid levels may be expected to produce widespread changes in brain excitability. However, the neurosteroid/GABA(A) receptor interaction is brain region and indeed neuron specific. The molecular basis of this specificity will be reviewed here, including (1) the importance of the subunit composition of the GABA(A) receptor; (2) how protein phosphorylation may dynamically influence the sensitivity of GABA(A) receptors to neurosteroids; (3) the impact of local steroid metabolism; and (4) the emergence of extrasynaptic GABA(A) receptors as a neurosteroid target.

Developmental differences in neuromodulation and synaptic properties in the lamprey spinal cord

Functional properties in the spinal cord change during development to adapt motor outputs to differing behavioral requirements. Here, we have examined whether there are also developmental differences in spinal cord plasticity by comparing the neuromodulatory effects of substance P in the larval lamprey spinal cord with its previously characterized effects in premigratory adults. The premigratory adult effects of substance P were all significantly reduced in larvae. As the adult effects of substance P depend on the N-methyl-d-aspartate (NMDA)-dependent potentiation of glutamatergic synaptic transmission, we examined if the developmental differences in neuromodulation were associated with differences in synaptic properties. We found that the amplitude, rise time, and half-width of excitatory postsynaptic potentials (EPSPs) from excitatory network interneurons were all significantly reduced in larvae compared with adults. These differences were associated with a reduction in the NMDA component of larval EPSPs, an effect that could have contributed to the reduced modulatory effects of substance P in larvae. In contrast to glutamatergic inputs, the amplitude, rise time, and half-width of inhibitory postsynaptic potentials (IPSPs) from ipsilateral inhibitory interneurons were all significantly increased in larvae compared with adults. Substance P also potentiated larval IPSP amplitudes, an effect not seen in adults. This increase in inhibition contributed to the reduced effects of substance P in larvae, as premigratory adult-like modulation could be evoked when inhibition was blocked with strychnine. These results suggest that opposite developmental changes in excitatory and inhibitory synaptic transmission and their modulation are associated with developmental differences in spinal cord neuromodulation.

Fast nongenomic effects of steroids on synaptic transmission and role of endogenous neurosteroids in spinal pain pathways

Steroids exert long-term modulatory effects on numerous physiological functions by acting at intracellular/nuclear receptors influencing gene transcription. Steroids and neurosteroids can also rapidly modulate membrane excitability and synaptic transmission by interacting with ion channels, that is, ionotropic neurotransmitter receptors or voltage-dependent Ca2+ or K+ channels. More recently, the cloning of a plasma membrane-located G protein-coupled receptor for progestins in various species has suggested that steroids/neurosteroids could also influence second-messenger pathways by directly interacting with specific membrane receptors. Here we review the experimental evidence implicating steroids/neurosteroids in the modulation of synaptic transmission and the evidence for a role of endogenously produced neurosteroids in such modulatory effects. We present some of our recent results concerning inhibitory synaptic transmission in lamina II of the spinal cord and show that endogenous 5alpha-reduced neurosteroids are produced locally in lamina II and modulate synaptic gamma-aminobutyric acid A(GABAA) receptor function during development, as well as during inflammatory pain. The production of 5alpha-reduced neurosteroids is controlled by the endogenous activation of the peripheral benzodiazepine receptor (PBR), which initiates the first step of neurosteroidogenesis by stimulating the translocation of cholesterol across the inner mitochondrial membrane. Tonic neurosteroidogenesis observed in immature animals was decreased during postnatal development, resulting in an acceleration of GABAA receptor-mediated miniature inhibitory postsynaptic current (mIPSC) kinetics observed in the adult. Stimulation of the PBR resulted in a prolongation of GABAergic mIPSCs at all ages and was observed during inflammatory pain. Neurosteroidogenesis might play an important role in the control of nociception at least at the spinal cord level.

Metamodulation of a spinal locomotor network by nitric oxide

Flexibility in the output of spinal networks can be accomplished by the actions of neuromodulators; however, little is known about how the process of neuromodulation itself may be modulated. Here we investigate the potential "meta"-modulatory hierarchy between nitric oxide (NO) and noradrenaline (NA) in Xenopus laevis tadpoles. NO and NA have similar effects on fictive swimming; both potentiate glycinergic inhibition to slow swimming frequency and GABAergic inhibition to reduce episode durations. In addition, both modulators have direct effects on the membrane properties of motor neurons. Here we report that antagonism of noradrenergic pathways with phentolamine dramatically influences the effect of the NO donor S-nitroso-N-acetylpenicillamine (SNAP) on swimming frequency, but not its effect on episode durations. In contrast, scavenging extracellular NO with 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO) does not influence any of the effects of NA on fictive swimming. These data place NO above NA in the metamodulatory hierarchy, strongly suggesting that NO works via a noradrenergic pathway to control glycine release but directly promotes GABA release. We confirmed this possibility using intracellular recordings from motor neurons. In support of a natural role for NO in the Xenopus locomotor network, PTIO not only antagonized all of the effects of SNAP on swimming but also, when applied on its own, modulated both swimming frequency and episode durations in addition to the underlying glycinergic and GABAergic pathways. Collectively, our results illustrate that NO and NA have parallel effects on motor neuron membrane properties and GABAergic inhibition, but that NO serially metamodulates glycinergic inhibition via NA.

Brainstem control of activity and responsiveness in resting frog tadpoles: tonic inhibition

The hatchling Xenopus laevis tadpole was used to study the brain neurons controlling responsiveness. Tadpoles have reduced motor activity and responsiveness when they hang at rest, attached by cement gland mucus. Afferent input from cement gland mechanosensory neurons has both a phasic role in stopping swimming and a tonic role in reducing responsiveness while tadpoles hang attached. Both these roles depend on GABA(A)-mediated inhibition. We provide evidence supporting the hypothesis that long-term reduced responsiveness in attached tadpoles results from tonic activity in the reticulospinal GABAergic pathway mediating the stopping response. Two groups of putative stopping pathway interneurons were recorded in the caudal and rostral hindbrain of immobilised tadpoles. Both groups showed a sustained increase in activity during simulated attachment. This attached activity was irregular and unstructured. We consider whether low-level firing in cement gland afferents (at approximately 1 Hz) during simulated attachment is sufficient to explain the low-level firing (at approximately 0.5 Hz) in reticulospinal neurons. We then ask if a small population of these neurons (approximately 20) could produce sufficient inhibition of spinal neurons to reduce the whole tadpole's responsiveness. We conclude that for most of their 1st day of life GABAergic brainstem neurons could produce inhibition continuously while the tadpole is at rest.

Neurosteroid modulation of GABAA receptors

Certain metabolites of progesterone and deoxycorticosterone are established as potent and selective positive allosteric modulators of the gamma-aminobutyric acid type A (GABA(A)) receptor. Upon administration these steroids exhibit clear behavioural effects that include anxiolysis, sedation and analgesia, they are anticonvulsant and at high doses induce a state of general anaesthesia, a profile consistent with an action to enhance neuronal inhibition. Physiologically, peripherally synthesised pregnane steroids derived from endocrine glands such as the adrenals and ovaries function as hormones by crossing the blood brain barrier to influence neuronal signalling. However, the demonstration that certain neurons and glial cells within the central nervous system (CNS) can synthesize these steroids either de novo, or from peripherally derived progesterone, has led to the proposal that these steroids (neurosteroids) can additionally function in a paracrine manner, to locally influence GABAergic transmission. Steroid levels are known to change dynamically, for example in stress and during pregnancy. Given that GABA(A) receptors are ubiquitously expressed throughout the central nervous system, such changes in steroid levels would be predicted to cause a global enhancement of inhibitory neurotransmission throughout the brain, a scenario that would seem incompatible with a physiological role as a selective neuromodulator. Here, we will review emerging evidence that the GABA-modulatory actions of the pregnane steroids are highly selective, with their actions being brain region and indeed neuron dependent. Furthermore, the sensitivity of GABA(A) receptors is not static but can dynamically change. The molecular mechanisms underpinning this neuronal specificity will be discussed with particular emphasis being given to the role of GABA(A) receptor isoforms, protein phosphorylation and local steroid metabolism and synthesis.

Spinal inhibitory neurons that modulate cutaneous sensory pathways during locomotion in a simple vertebrate

During locomotion, reflex responses to sensory stimulation are usually modulated and may even be reversed. This is thought to be the result of phased inhibition, but the neurons responsible are usually not known. When the hatchling Xenopus tadpole swims, responses to cutaneous stimulation are modulated. This occurs because sensory pathway interneurons receive rhythmic glycinergic inhibition broadly in phase with the motor discharge on the same side of the trunk. We now describe a new whole-cell recording preparation of the Xenopus tadpole CNS. This has been used with neurobiotin injection to define the passive and firing properties of spinal ascending interneurons and their detailed anatomy. Paired recordings show that they make direct, glycinergic synapses onto spinal sensory pathway interneurons, and the site of contact can be seen anatomically. During swimming, ascending interneurons fire rhythmically. Analysis shows that their firing is more variable and not as reliable as other interneurons, but the temporal pattern of their impulse activity is suitable to produce the main peak of gating inhibition in sensory pathway interneurons. Ascending interneurons are not excited at short latency after skin stimulation but are strongly active after repetitive skin stimulation, which evokes vigorous and slower struggling movements. We conclude that ascending interneurons are a major class of modulatory neurons producing inhibitory gating of cutaneous sensory pathways during swimming and struggling.

Allopregnanolone modulates spontaneous GABA release via presynaptic Cl- permeability in rat preoptic nerve terminals

The endogenous neurosteroid 3alpha-hydroxy-5alpha-pregnane-20-one (allopregnanolone) affects presynaptic nerve terminals and thereby increases the frequency of spontaneous GABA release. The present study aimed at clarifying the mechanisms underlying this presynaptic neurosteroid action, by recording the frequency of spontaneous GABA-mediated inhibitory postsynaptic currents (sIPSCs) in neurons from the medial preoptic nucleus (MPN) of rat. Acutely dissociated neurons with functional adhering nerve terminals were studied by perforated-patch recording under voltage-clamp conditions. It was shown that the sIPSC frequency increased with the external K(+) concentration ([K(+)](o)). Further, the effect of allopregnanolone on the sIPSC frequency was strongly dependent on [K(+)](o). In a [K(+)](o) of 5 mM, 2.0 microM allopregnanolone caused a clear increase in sIPSC frequency. However, the effect declined rapidly with increased [K(+)](o) and at high [K(+)](o) allopregnanolone reduced the sIPSC frequency. The effect of allopregnanolone was also strongly dependent on the external Cl(-) concentration ([Cl(-)](o)). In a reduced [Cl(-)](o) (40 mM, but with a standard [K(+)](o) of 5 mM), the effect on sIPSC frequency was larger than that in the standard [Cl(-)](o) of 146 mM. The dependence of the effect of allopregnanolone on [K(+)](o) and on estimated presynaptic membrane potential was also altered by the reduction in [Cl(-)](o). As in standard [Cl(-)](o), the effect in low [Cl(-)](o) declined when [K(+)](o) was raised, but reversed at a higher [K(+)](o). The GABA(A) receptor agonist muscimol also potentiated the sIPSC frequency. Altogether, the results suggest that allopregnanolone exerts its presynaptic effect by increasing the presynaptic Cl(-) permeability, most likely via GABA(A) receptors.

Fast inhibitory synapses: targets for neuromodulation and development of vertebrate motor behaviour

Locomotor networks must possess the inherent flexibility to adapt their output. In this review we discuss evidence from a simple vertebrate locomotor network that suggests fast inhibitory synapses are important targets for the forms of neuromodulation that afford this flexibility. Two important inhibitory transmitters, glycine and GABA, are present in the CNS of Xenopus tadpoles, where they each play distinct roles in the control of swimming. Glycine, but not GABA, contributes to the inhibitory mid-cycle component of each swim-cycle, the strength of which determines the frequency of swimming. Meanwhile, GABA release onto the swim network prematurely terminates swimming episodes. Hence, glycine controls how fast, whilst GABA controls how far the tadpole swims. Our work has focused on how the amines serotonin (5-HT) and noradrenaline (NA), and more recently the gas nitric oxide (NO), selectively target glycine and GABA release in the spinal cord to modulate swimming. In particular, we have identified three brainstem populations of nitrergic neurons, which suggests that nitric oxide may co-localise with 5-HT, NA and GABA. Here we review this work and suggest a hierarchy of brainstem modulatory systems, with NO acting as a metamodulator.

Nitric oxide selectively tunes inhibitory synapses to modulate vertebrate locomotion

We have explored the possible modulation by nitric oxide (NO) of inhibitory synaptic transmission mediated by either glycine or GABA during episodes of rhythmic fictive swimming in postembryonic Xenopus laevis tadpoles. Extracellular ventral-root recordings suggest a stage-dependent increase in the reliability and extent of the NO donor S-nitroso-n-acetylpenicillamine (SNAP; 0.1-1 mm) to inhibit swimming by reducing the frequency and shortening the duration of swim episodes. These effects of SNAP on the swimming rhythm at both developmental stages are corroborated by intracellular recordings from presumed motor neurons with sharp microelectrodes, which also suggest that NO inhibits swimming by facilitating both glycinergic and GABAergic inhibition. However, we found no evidence for NO modulation of the excitatory drive for swimming. In addition to presynaptic effects on inhibitory transmitter release, a pronounced postsynaptic membrane depolarization ( approximately 5-10 mV) and conductance decrease ( approximately 10-20%) are associated with bath application of SNAP. Hence, NO exerts inhibitory effects on swimming through multiple but selective actions on both the electrical properties of spinal neurons and on particular synaptic interconnections. The presynaptic and postsynaptic effects of NO act in concert to tune inhibitory synapses.

Alpha-adrenoreceptor activation modulates swimming via glycinergic and GABAergic inhibitory pathways in Xenopus laevis tadpoles

This study focuses upon the network pathways underlying the adrenoreceptor-mediated modulation of fictive swimming in the immobilized Xenopus laevis tadpole. As shown recently, noradrenaline (NA) increases cycle periods while simultaneously reducing the rostrocaudal delay in head-to-tail firing and the duration of swimming episodes. Furthermore, both swimming frequency and duration are reduced by selective pharmacological activation of alpha1- and/or alpha2-adrenoreceptors, while alpha1-receptor activation also reduces rostrocaudal delays. We show that NA could still modulate aspects of swimming after blocking either glycine or GABA(A) receptors with strychnine and bicuculline, respectively. Furthermore, after prior application of NA, strychnine could counteract noradrenergic effects on cycle periods and rostrocaudal delays, while bicuculline could counteract effects on cycle periods, suggesting that these two fast inhibitory pathways are both involved in the NA modulation of swimming. In addition, blocking glycine receptors reduced the effects of alpha1-receptors on cycle periods and delays, while blocking GABA(A) receptors had no effect. Blocking either glycine or GABA(A) receptors, however, lessened the reduction in swimming frequency by alpha2-receptors. In addition, pre-application of bicuculline prevented a reduction in episode durations by NA, alpha1- and alpha2-receptors. Our findings suggest that the noradrenergic modulation of Xenopus swimming is mediated via alpha-adrenoreceptors interacting with both glycinergic and GABAergic inhibitory pathways. Both alpha1- and alpha2-receptor activation influences the GABAergic pathway controlling the duration of swimming episodes and is involved in the glycinergic modulation of the swimming rhythm and its longitudinal co-ordination, with alpha2-receptors additionally affecting swimming frequency through GABAergic pathways.

Adrenoreceptor-mediated modulation of the spinal locomotor pattern during swimming in Xenopus laevis tadpoles

This study focused on the contribution of different adrenoreceptor subtypes to the modulation of fictive swimming activity in a relatively simple, yet intact, lower vertebrate system, the immobilized Xenopus laevis tadpole and explored their possible role in mediating the noradrenergic modulation of spinal motor networks. In Xenopus embryos, near the time of hatching, activation of alpha(1) adrenoreceptors increased the duration of episodes of fictive swimming, whilst in larvae, 24 h after hatching, they were decreased. Activation of alpha(2) adrenoreceptors, however, markedly reduced episode duration at both developmental stages. Cycle periods in both stages were increased by the activation of alpha(1) and/or alpha(2) receptor subclasses, whereas beta adrenoreceptors were not apparently involved in the modulation of cycle periods or the duration of swim episodes. However, both beta and alpha(1) receptor activation decreased the intersegmental delay in the head-to-tail propagation of swimming activity, while alpha(2) receptors did not influence these rostro-caudal delays. Activation of neither alpha, nor beta, receptor subclasses had any consistent effect on the duration of ventral motor bursts. Our findings suggest that noradrenergic modulation of the swim-pattern generator in Xenopus tadpoles is mediated through the activation of alpha and beta adrenoreceptors. In addition, activation of particular receptor subclasses might enable the selective modulation of either the segmental rhythm generating networks, the intersegmental coordination of those networks or control at both levels simultaneously.

Neurosteroid modulation of GABA IPSCs is phosphorylation dependent

The neurosteroid 3alpha-hydroxy-5alpha-pregnan-20-one (allopregnanolone) facilitates GABA(A) receptor-mediated ionic currents via allosteric modulation of the GABA(A) receptor. Accordingly, allopregnanolone caused an increase in the slow decay time constant of spontaneous GABA-mediated IPSCs in magnocellular neurons recorded in hypothalamic slices. The allopregnanolone effect on IPSCs was inhibited by a G-protein antagonist as well as by blocking protein kinase C and, to a lesser extent, cAMP-dependent protein kinase activities. G-protein and protein kinase C activation in the absence of the neurosteroid had no effect on spontaneous IPSCs but enhanced the effect of subsequent allopregnanolone application. These findings together suggest that the neurosteroid modulation of GABA-mediated IPSCs requires G-protein and protein kinase activation, although not via a separate G-protein-coupled steroid receptor.

The distribution of NADPH-diaphorase-labelled interneurons and the role of nitric oxide in the swimming system of Xenopus laevis larvae

The possible involvement of the free radical gas nitric oxide (NO) in the modulation of spinal rhythm-generating networks has been studied using Xenopus laevis larvae. Using NADPH-diaphorase histochemistry, three putative populations of nitric oxide synthase (NOS)-containing cells were identified in the brainstem. The position and morphology of the largest and most caudal population suggested that a proportion of these neurons is reticulospinal. The possible contribution of nitrergic neurons to the control of swimming activity was examined by manipulating exogenous and endogenous NO concentrations in vivo with an NO donor (SNAP, 100–500 micromol l(−)(1)) and NOS inhibitors (l-NAME and l-NNA, 0.5-5 mmol l(−)(1)), respectively. In the presence of SNAP, swim episode duration decreased and cycle period increased, whereas the NOS inhibitors had the opposite effects. We conclude from these data that the endogenous release of NO from brainstem neurons extrinsic to the spinal cord of Xenopus laevis larvae exerts a continuous modulatory influence on swimming activity, functioning like a ‘brake’. Although the exact level at which NO impinges upon the swimming rhythm generator has yet to be determined, the predominantly inhibitory effect of NO suggests that the underlying mechanisms of NO action could involve modulation of synaptic transmission and/or direct effects on neuronal membrane properties.

Effects of a naturally occurring neurosteroid on GABAA IPSCs during development in rat hippocampal or cerebellar slices

1. The effects of the naturally occurring neurosteroid tetrahydrodeoxycorticosterone (THDOC) on GABAA receptor-mediated miniature, spontaneous and evoked IPSCs was tested using patch-clamp techniques in slices of hippocampus and cerebellum from rats at two developmental stages ( approximately 10 and approximately 20 days postnatal). The cells studied were hippocampal granule cells and cerebellar Purkinje and granule cells. 2. Most miniature GABAergic currents (mIPSCs) decayed with two exponentials and neurosteroids caused a approximately 4-fold increase in the decay time constant of the second exponential at the highest concentration used (2 microM). Similar effects were seen at high concentrations of THDOC (1-2 microM) in all cell groups tested. No effects were seen on amplitude or rise time of mIPSCs. 3. The effects of THDOC (1 microM) were shown to be stereoselective and rapidly reversible, indicating that the neurosteroid binds to the GABAA receptor, rather than acting genomically. 4. At concentrations of THDOC likely to occur physiologically (50-100 nM), the decay time of IPSCs was also enhanced (25-50 %) in all cerebellar cell groups tested. In contrast, at 100 nM THDOC, seven of 11 hippocampal granule cells were sensitive from the 10 day group but the 20 day hippocampal granule cells showed no significant enhancement in the presence of these lower concentrations of THDOC. 5. The differences in sensitivity of hippocampal and cerebellar cells to THDOC are compared to data reported in the literature on regional development of expression of different receptor subunits in the brain and it is suggested that the progressive relative insensitivity of the 20 day hippocampal cells may depend on increasing expression of the delta subunit of the GABAA receptor and possibly an increase in the alpha4 subunit.

Development and role of GABA(A) receptor-mediated synaptic potentials during swimming in postembryonic Xenopus laevis tadpoles

We have investigated the contribution of GABA(A) receptor activation to swimming in Xenopus tadpoles during the first day of postembryonic development. Around the time of hatching stage (37/8), bicuculline (10-50 microM) causes a decrease in swim episode duration and cycle period, suggesting that GABA(A) receptor activation influences embryonic swimming. Twenty-four hours later, at stage 42, GABA(A) receptor activation plays a more pronounced role in modulating larval swimming activity. Bicuculline causes short, intense swim episodes with increased burst durations and decreased cycle periods and rostrocaudal delays. Conversely, the allosteric agonist, 5beta-pregnan-3alpha-ol-20-one (1-10 microM) or the uptake inhibitor, nipecotic acid (200 microM) cause slow swimming with reduced burst durations and increased cycle periods. These effects appear to be mainly the result of GABA release from the spinal terminals of midhindbrain reticulospinal neurons but may also involve spinal GABAergic neurons. Intracellular recordings were made using KCl electrodes to reverse the sign and enhance the amplitude of chloride-dependent inhibitory postsynaptic potentials (IPSPs). Recordings from larval motoneurons in the presence of strychnine (1-5 microM), to block glycinergic IPSPs, provided no evidence for any GABAergic component to midcycle inhibition. GABA potentials were observed during episodes, but they were not phase-locked to the swimming rhythm. Bicuculline (10-50 microM) abolished these sporadic potentials and caused an apparent decrease in the level of tonic depolarization during swimming activity and an increase in spike height. Finally, in most larval preparations, GABA potentials were observed at the termination of swimming. In combination with the other evidence, our data suggest that midhindbrain reticulospinal neurons become involved in an intrinsic pathway that can prematurely terminate swim episodes. Thus during the first day of larval development, endogenous activation of GABA(A) receptors plays an increasingly important role in modulating locomotion, and GABAergic neurons become involved in an intrinsic descending pathway for terminating swim episodes.

Neurosteroid modulation of synaptic and GABA-evoked currents in neurons from the rat medial preoptic nucleus

The effects of the neurosteroid 3alpha-hydroxy-5alpha-pregnane-20-one (allopregnanolone) on synaptic and GABA-evoked currents in acutely dissociated neurons from the medial preoptic nucleus of rat were investigated by perforated-patch recordings under voltage-clamp conditions. The effect of 2.0 microM allopregnanolone on GABA-evoked currents depended strongly on the GABA concentration: the currents evoked by 100 microM GABA were markedly depressed and the desensitization was faster, but the decay after GABA application was prolonged. In contrast, the currents evoked by 1.0 microM GABA were markedly potentiated, the activation was faster, a prominent desensitization was induced, and the decay after GABA application was prolonged. In the absence of externally applied GABA, 2.0 microM allopregnanolone induced a slow current that could be attributed to Cl-. Allopregnanolone did not significantly affect the amplitude of spontaneous tetrodotoxin-insensitive (miniature) synaptic currents (mIPSCs) originating from synaptic terminals releasing GABA onto the dissociated neurons. However, the mIPSC decay phase was dramatically prolonged, with half-maximal effect at approximately 50 nM allopregnanolone. A qualitatively similar effect of allopregnanolone was seen when KCl was used to evoke synchronous GABA release. The frequency of mIPSCs was also affected, on average increased 3.5-fold, by 2.0 microM allopregnanolone, suggesting a presynaptic steroid action.

Development and aminergic neuromodulation of a spinal locomotor network controlling swimming in Xenopus larvae

In this article we review our research on the development and intrinsic neuromodulation of a spinal network controlling locomotion in a simple vertebrate. Swimming in hatchling Xenopus embryos is generated by a restricted network of well-characterized spinal neurons. This network produces a stereotyped motor pattern which, like real swimming, involves rhythmic activity that alternates across the body and progresses rostrocaudally with a brief delay between muscle segments. The stereotypy results from motoneurons discharging a single impulse in each cycle; because all motoneurons appear to behave similarly there is little scope for altering the output to the myotomes from one cycle to the next. Just one day later, however, Xenopus larvae generate a more complex and flexible motor pattern in which motoneurons can discharge a variable number of impulses which contribute to ventral root bursts in each cycle. This maturation of swimming is due, in part, to the influence of serotonin released from brain-stem raphespinal interneurons whose axonal projections innervate the cord early in larval life. Larval swimming is differentially modulated by both serotonin and by noradrenaline: serotonin leads to relatively fast, intense swimming whereas noradrenaline favors slower, weaker activity. Thus, these two biogenic amines select opposite extremes from the spectrum of possible output patterns that the swimming network can produce. Our studies on the cellular and synaptic effects of the amines indicate that they can control the strength of reciprocal glycinergic inhibition in the spinal cord. Serotonin and noradrenaline act presynaptically on the terminals of glycinergic commissural interneurons to weaken and strengthen, respectively, crossed glycinergic inhibition during swimming. As a result, serotonin reduces and noradrenaline increases interburst intervals. The membrane properties of spinal neurons are also affected by the amines. In particular, serotonin can induce intrinsic oscillatory membrane properties in the presence of NMDA. These depolarizations are slow compared to the cycle periods during swimming and so may contribute to enhancement of swimming over several consecutive cycles of activity.