Biomedical subjects

Alan Roberts

Publications and source records attributed to Alan Roberts.

18 recordsLinked to original sources

PubMed2006

Persistent responses to brief stimuli: feedback excitation among brainstem neurons.

The ability of brief stimuli to trigger prolonged neuronal activity is a fundamental requirement in nervous systems, common to motor responses and short-term memory. Bistable membrane properties and network feedback excitation have both been proposed as suitable mechanisms to sustain such persistent responses. There is now good experimental evidence for membrane bistability. In contrast, the long-standing hypotheses based on positive feedback excitation have yet to be supported by direct evidence for mutual excitatory connections between appropriate neurons. In young frog tadpoles (Xenopus), we show that a small region of caudal hindbrain and rostral spinal cord is sufficient to generate prolonged swimming in response to a brief stimulus. We used paired whole-cell patch recordings to identify hindbrain neurons in this region that actively excite spinal neurons to drive sustained swimming. We show directly that some of these hindbrain neurons make reciprocal excitatory connections with each other. We use a population model of the hindbrain network to illustrate how feedback excitation can provide a robust mechanism to generate persistent responses. Our recordings provide direct evidence for feedback excitation among neurons within a network that drives a prolonged response. Its presence in a lower brain region early in development suggests that it is a basic feature of neuronal network design.

Animals↗

PubMed2006

Are black-white differences in females' body dissatisfaction decreasing? A meta-analytic review.

Proponents of the sociocultural model of eating disorders have suggested that ethnic differences in body dissatisfaction may be diminishing as the thin ideal of beauty becomes more widely disseminated among minority women. In a meta-analysis, the authors examined temporal trends in Black-White differences and also examined whether these differences generalize across various age groups and measures. Results confirmed more favorable body image evaluations among Black than White females, with the greatest differences at the age period of the early 20s. Although results confirmed that ethnic differences have diminished, this trend was limited to weight-focused measures. On more global body image measures, ethnic differences actually increased. These results suggest that the relationship between Black-White ethnicity and body image is more complex than previously suggested.

Adolescent↗

PubMed2005

Production of calcium-mobilizing metabolites by a novel member of the ADP-ribosyl cyclase family expressed in Schistosoma mansoni.

ADP-ribosyl cyclases are structurally conserved enzymes that are best known for catalyzing the production of the calcium-mobilizing metabolite, cyclic adenosine diphosphate ribose (cADPR), from nicotinamide adenine dinucleotide (NAD(+)). However, these enzymes also produce adenosine diphosphate ribose (ADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP(+)), both of which have been shown to modulate calcium mobilization in vitro. We have now characterized a new member of the cyclase family from Schistosoma mansoni, a member of the Platyhelminthes phylum. We show that the novel NAD(P)(+) catabolizing enzyme (NACE) expressed by schistosomes is structurally most closely related to the cyclases cloned from Aplysia but also shows significant homology with the mammalian cyclases, CD38 and CD157. NACE expression is developmentally regulated in schistosomes, and the GPI-anchored protein is localized to the outer tegument of the adult schistosome. Importantly, NACE, like all members of the cyclase family, is a multifunctional enzyme and catalyzes NAD(+) glycohydrolase and base-exchange reactions to produce ADPR and NAADP(+). However, despite being competent to generate a cyclic product from NGD(+), a nonphysiologic surrogate substrate, NACE is so far the only enzyme in the cyclase family that is unable to produce significant amounts of cADPR (<0.02% of reaction products) using NAD(+) as the substrate. This suggests that the other calcium-mobilizing metabolites produced by NACE may be more important for calcium signaling in schistosomes. Alternatively, the function of NACE may be to catabolize extracellular NAD(+) to prevent its use by host enzymes that utilize this source of NAD(+) to facilitate immune responses.

ADP-ribosyl Cyclase↗

PubMed2004

Glutamate and acetylcholine corelease at developing synapses.

Most neurons release a single fast-acting low-molecular-weight transmitter at synapses to activate and open postsynaptic ion channels. We challenge this principle with evidence for corelease of the two major excitatory transmitters, glutamate and acetylcholine (ACh), from single identified neurons in the developing frog tadpole spinal cord. Whole-cell patch electrodes were used to record from single spinal neurons. When action potentials and inhibition were blocked, spontaneous miniature excitatory postsynaptic currents (mEPSCs) were recorded. These were fully blocked only by joint application of glutamate [alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) and N-methyl-D-aspartate receptor (NMDAR)] and nicotinic ACh receptor (nAChR) antagonists. Fast nAChR and slow NMDAR mEPSCs were isolated pharmacologically. We then show that some mEPSCs have both the fast nAChR rise and slow NMDAR decay and conclude that some individual synaptic vesicles corelease glutamate and ACh. Whole-cell recordings from pairs of neurons were then used to identify the spinal interneurons coreleasing the two excitatory transmitters. One anatomical class of interneuron with descending axons was found to excite other spinal neurons by activating nAChR, AMPAR, and NMDAR simultaneously at its synapses. Although Jonas, Bischofberger, and Sandkuhler [Jonas, P., Bischofberger, J. & Sandkuhler, J. (1998) Science 281, 419-424] showed that the inhibitory transmitters GABA and glycine can be coreleased at spinal synapses, the Xenopus tadpole provides a case where the two main CNS excitatory transmitters are released from single vesicles, and where the presynaptic neuron coreleasing two transmitters has been identified.

Acetylcholine↗

PubMed2004

Primitive roles for inhibitory interneurons in developing frog spinal cord.

Understanding the neuronal networks in the mammal spinal cord is hampered by the diversity of neurons and their connections. The simpler networks in developing lower vertebrates may offer insights into basic organization. To investigate the function of spinal inhibitory interneurons in Xenopus tadpoles, paired whole-cell recordings were used. We show directly that one class of interneuron, with distinctive anatomy, produces glycinergic, negative feedback inhibition that can limit firing in motoneurons and interneurons of the central pattern generator during swimming. These same neurons also produce inhibitory gating of sensory pathways during swimming. This discovery raises the possibility that some classes of interneuron, with distinct functions later in development, may differentiate from an earlier class in which these functions are shared. Preliminary evidence suggests that these inhibitory interneurons express the transcription factor engrailed, supporting a probable homology with interneurons in developing zebrafish that also express engrailed and have very similar anatomy and functions.

Action Potentials↗

PubMed2004

Advantageous indirect interactions in systems of competition.

Ecological foodwebs display complex networks of species interactions. Here we discuss how two species, whether directly interacting or not, can crucially affect each other 'indirectly' through their mutual associations with intermediary species. A technique is presented for quantifying these "indirect effects", so that a simple measure emerges for the degree of overall harm or advantage that a particular species encounters from another in the context of a given foodweb. If the system is one of pure competition, the "direct" interaction between any two species is of course harmful to both. But when the method is applied to such systems it predicts that, if all interactions are allowed for including the indirect, in a large proportion of pair interactions at least one species will be deriving benefit from the other. Computer-generated samples confirm that this proportion is accurately predicted. The many pathways for indirect interaction can thus often reverse the direct connection, so that competing species achieve an interaction here called 'Advantageous in A Community Context' (ACC).

Animals↗

PubMed2004

Dorsal spinal interneurons forming a primitive, cutaneous sensory pathway.

In mammals, sensory projection pathways are provided by just three classes of spinal interneuron that develop from the roof-plate. We asked whether similar sensory projection interneurons are present primitively in a developing lower vertebrate where function can be more readily studied. Using an immobilized Xenopus tadpole spinal cord preparation, we define the properties and connections of spinal sensory projection interneurons using whole cell patch recordings from single neurons or pairs, identified by dye filling. Dorsolateral interneurons lie in the tadpole equivalent of the spinal dorsal horn, and have dorsally located dendrites and an ipsilateral ascending axon that projects into the midbrain. These neurons receive direct, mainly AMPA receptor (AMPAR)-mediated, excitation from skin touch sensory neurons. They in turn produce AMPAR and N-methyl-d-aspartate receptor (NMDAR)-mediated excitation of spinal locomotor pattern generator neurons rostrally on the same side of the cord. During swimming, they receive glycinergic modulatory inhibition from ascending interneurons in the spinal locomotor central pattern generator. We conclude that spinal dorsolateral interneurons are a primitive class of excitatory sensory projection neurons activated by ipsilateral cutaneous afferents and carrying excitation ipsilaterally and rostrally as far as the midbrain to initiate or accelerate swimming.

Action Potentials↗

PubMed2004

A direct comparison of whole cell patch and sharp electrodes by simultaneous recording from single spinal neurons in frog tadpoles.

High-impedance, sharp intracellular electrodes were compared with whole cell patch electrodes by recording from single spinal neurons in immobilized frog tadpoles. A range of neuron properties were examined using sharp or patch test electrodes while making simultaneous recordings with a second control patch electrode. Overall, test patch electrodes did not significantly alter the activity recorded by the control electrode, and recordings from the two electrodes were essentially identical. In contrast, sharp electrode recordings differed from initial control patch recordings. In some cases, differences were due to real changes in neuron properties: the resting membrane potential became less negative and the neuron input resistance (R(i)) fell; this fall was larger for neurons with a higher R(i). In other cases, the control patch electrode revealed that differences were due to the recording properties of the sharp electrode: tip potentials were larger and more variable; resting potentials appeared to be more negative; and spike amplitude was attenuated. However, sharp electrode penetration did not, in most cases, significantly alter the pattern of neuron firing in response to injected current or the normal pattern of activity following sensory stimulation or during fictive swimming. We conclude that sharp electrodes introduce a significant leak to the membrane of tadpole spinal neurons compared with patch electrodes but that this does not change the fundamental firing characteristics or activity of the neurons.

Action Potentials↗

PubMed2004

Mechanisms and significance of reduced activity and responsiveness in resting frog tadpoles.

Hatchling Xenopus laevis tadpoles spend most of their time attached to objects or the water surface by mucus secreted by a gland on the head. While attached, swimming activity and responsiveness to swim-initiating stimuli are reduced over long periods of time. We have investigated the mechanisms and significance of this apparent long-term inhibition. In behavioural experiments we show, firstly, that innervation of the cement gland and GABA(A)-mediated inhibition are necessary for attachment to reduce responsiveness, and secondly, that denervation of the cement gland increases tadpole activity and increases their predation by damselfly nymphs (Zygoptera). To investigate the neuronal pathway from the cement gland to GABA(A) inhibition, we have devised an immobilized, inverted tadpole preparation where a weight attached to the mucus simulates the force as it hangs. Simulated attachment reduces responsiveness and spontaneous fictive swimming activity. We have recorded the activity and responses of trigeminal neurons innervating the cement gland. They are spontaneously active and simulating attachment results in a sustained increase in this activity. We propose that hanging from a mucus strand increases firing in cement gland afferents. This leads to tonic GABA inhibition that reduces tadpole activity and responses, and leads to fewer attacks by predators.

Animals↗

PubMed2003

The spinal interneurons and properties of glutamatergic synapses in a primitive vertebrate cutaneous flexion reflex.

Unlike the monosynaptic "stretch" reflex, the exact neuronal pathway for a simple cutaneous reflex has not yet been defined in any vertebrate. In young frog tadpoles, we made whole-cell recordings from pairs of spinal neurons. We found direct, excitatory, glutamatergic synapses from touch-sensitive skin-sensory neurons to sensory pathway interneurons, and then from these sensory interneurons to motoneurons and premotor interneurons on the other side of the body. We conclude that the minimal pathway for this primitive reflex, in which stroking the skin on one side leads to flexion on the other side, is disynaptic. This detailed circuit information has allowed us to ask whether the properties of glutamatergic synapses during the first day of CNS development are tuned to their function in the tadpole's responses. Stroking the skin excites a few sensory neurons. These activate primarily AMPA receptors producing short, strong excitation that activates many sensory pathway interneurons but only allows temporal summation of closely synchronous inputs. In contrast, the excitation produced in contralateral neurons by the sensory pathway interneurons is weak and primarily mediated by NMDA receptors. As a result, considerable summation is required for this excitation to lead to postsynaptic neuron firing and a contralateral flexion. We conclude that from their early functioning, synapses from sensory neurons are strong and those from sensory pathway interneurons are weak. The distribution of glutamate receptors at synapses in this developing circuit is tuned so that synapses have properties suited to their roles in the whole animal's reflex responses.

Afferent Pathways↗

PubMed2002

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.

Afferent Pathways↗

PubMed2002

Sensory activation and role of inhibitory reticulospinal neurons that stop swimming in hatchling frog tadpoles.

Activity in neuronal networks underlying locomotion and other rhythmic actions can start and stop in response to specific sensory stimuli. In vertebrate locomotion, some reticulospinal neurons such as Mauthner neurons can initiate activity, but the neurons controlling stopping are not defined. We have studied swimming in tadpoles of the frog, Xenopus, which is started by touching the skin and stops when the head contacts a solid surface. Using an immobilized tadpole preparation, the same stimuli control fictive swimming. When head contact is imitated by pressure to the head skin sensory neurons in the trigeminal ganglion are active, spinal neurons receive GABAergic inhibition, and swimming stops. Here we record intracellularly from neurons in the hindbrain that are excited by pressure or electrical stimulation to the head skin. By intracellular filling with neurobiotin, we identify these anatomically as mid-hindbrain reticulospinal neurons (MHRs). These have contralateral descending projections to the spinal cord and GABA-like immunoreactivity. They are rhythmically inhibited during fictive swimming. Individual MHRs reliably stopped ongoing swimming when brief firing was induced by intracellular current injection. The ability of individual MHRs to stop swimming was blocked by the GABA(A) antagonist bicuculline. Our evidence indicates that MHRs receive direct excitation from trigeminal sensory neurons and in turn release GABA to directly inhibit spinal neurons and turn off the swimming central pattern generator.

Action Potentials↗

PubMed2002

Effect of different protocols of caffeine intake on metabolism and endurance performance.

Competitive athletes completed two studies of 2-h steady-state (SS) cycling at 70% peak O(2) uptake followed by 7 kJ/kg time trial (TT) with carbohydrate (CHO) intake before (2 g/kg) and during (6% CHO drink) exercise. In Study A, 12 subjects received either 6 mg/kg caffeine 1 h preexercise (Precaf), 6 x 1 mg/kg caffeine every 20 min throughout SS (Durcaf), 2 x 5 ml/kg Coca-Cola between 100 and 120 min SS and during TT (Coke), or placebo. Improvements in TT were as follows: Precaf, 3.4% (0.2-6.5%, 95% confidence interval); Durcaf, 3.1% (-0.1-6.5%); and Coke, 3.1% (-0.2-6.2%). In Study B, eight subjects received 3 x 5 ml/kg of different cola drinks during the last 40 min of SS and TT: decaffeinated, 6% CHO (control); caffeinated, 6% CHO; decaffeinated, 11% CHO; and caffeinated, 11% CHO (Coke). Coke enhanced TT by 3.3% (0.8-5.9%), with all trials showing 2.2% TT enhancement (0.5-3.8%; P < 0.05) due to caffeine. Overall, 1) 6 mg/kg caffeine enhanced TT performance independent of timing of intake and 2) replacing sports drink with Coca-Cola during the latter stages of exercise was equally effective in enhancing endurance performance, primarily due to low intake of caffeine (approximately 1.5 mg/kg).

Adult↗

PubMed1992

Phase-dependent Modulation of a Cutaneous Sensory Pathway by Glycinergic Inhibition from the Locomotor Rhythm Generator in Xenopus Embryos.

In immobilized Xenopus laevis embryos two classes of sensory interneuron are excited by mechanosensory Rohon - Beard neurons and rhythmically inhibited during fictive swimming. Dorsolateral commissural (DLC) interneurons are inhibited in time with rhythmic motor discharge on the same side as their soma, while unidentified dorsolateral (DLX) interneurons are inhibited in the opposite phase of the swimming rhythm. The inhibition is abolished by bath application of strychnine sulphate at 1 - 10 microM, but not by the gamma-aminobutyric acid antagonists bicuculline (20 - 40 microM) or curare (70 - 100 microM). The inhibitory postsynaptic potentials (IPSPs) involve an increase in chloride conductance since they are reversed in sign to become depolarizing following intracellular injection of chloride ions. The conductance increase during inhibition was able to block impulses evoked by intracellular current in a phase-dependent manner, suggesting that postsynaptic inhibition is sufficient to account for the gating of afferent input to the spinal cord during swimming. An interneuron receives IPSPs that are predominantly in one phase of the rhythm, but most interneurons are also inhibited sporadically in the opposite phase. The amplitude and time course of the IPSPs closely follow the frequency of the swimming rhythm, with maximal inhibition occurring near the starts of episodes, when swimming frequency is at its highest. Towards the end of an episode, when swimming frequency declines, the level of inhibition is low, the membrane potential of the interneurons returns to rest between cycles, and IPSPs often fail to occur. Inhibition suppresses sensory excitation in a phase-dependent manner (cf. Sillar and Roberts, Nature, 331, 262 - 265, 1988). Sensory interneurons fire a single impulse in response to a brief sensory stimulus, but they will usually fire multiple impulses when depolarized with sufficient intracellular current. In some sensory interneurons a short-latency IPSP follows the impulse evoked by skin stimulation that could curtail impulse activity. However, when the inhibition is blocked by strychnine, sensory interneurons still fire a single short-latency impulse, favouring the conclusion that brief, synchronized afferent excitation elicits a single impulse in neurons that are capable of firing multiply. Since the inhibition of DLC interneurons occurs in phase with activity on the same side it probably originates from spiking in ipsilateral glycinergic commissural interneurons which have ipsilateral as well as contralateral projections. The inhibition of DLX interneurons in the opposite phase probably originates from the contralateral projections of commissural interneurons.

Journal Article↗

PubMed1990

Characterization and Function of Spinal Excitatory Interneurons with Commissural Projections in Xenopus laevis embryos.

The multipolar somata of dorsolateral commissural (dlc) interneurons (Roberts and Clarke, 1982) lie in a superficial dorsolateral position in the spinal cord of Xenopus laevis embryos. By applying horseradish peroxidase to one-half of the 100 microm diameter spinal cord, these neurons have been backfilled. Their dendritic branching pattern, commissural axonal projection and distribution near the time of hatching is described. Using Lucifer yellow-filled microelectrodes a population of sensory interneurons with dlc morphology has been identified. They have multipolar somata in a dorsolateral superficial position, obliquely projecting dendrites and a ventral commissural axon. They receive presumed monosynaptic excitation in response to electrical stimulation of sensory neurites in the skin on the same side as the soma. During fictive swimming activity in curarized embryos the dlc interneurons are rhythmically inhibited in time with ventral root discharge on the same side. Dlc interneurons can fire multiple impulses and can turn on fictive swimming when stimulated by intracellular current injection. Skin stimulation is followed by excitatory postsynaptic potentials (EPSPs) in contralateral ventral rhythmic neurons. These EPSPs are reduced by the application of NMDA receptor antagonist. We conclude that dlc interneurons are excited by primary skin afferent Rohon - Beard neurons, carry sensory information across the spinal cord to excite neurons on the opposite side by release of an excitatory amino acid transmitter and participate in reflexes and in the initiation of swimming.

Journal Article↗

PubMed1989

The Influence of Magnesium Ions on the NMDA Mediated Responses of Ventral Rhythmic Neurons in the Spinal Cord of Xenopus Embryos.

Xenopus embryos immobilized in tubocurarine respond to natural skin stimulation with fictive swimming. This can also occur in saline without Mg2+ and is blocked by NMDA antagonists. Ventral spinal cord neurons which are rhythmically active during swimming are depolarized by bath applied N-methyl-D-aspartate (NMDA) (in 1 microM tetrodotoxin (TTX) to block indirect effects). By using current clamp techniques this depolarization is shown to be partially blocked by 0.5 and 1 mM Mg2+ in a voltage-dependent manner similar to that described in cultured neurons. Mg2+ partially and reversibly reduces the slow NMDA-mediated component of excitatory post-synaptic potentials (EPSPs) in ventral neurons. However, in 1 mM Mg2+ fictive swimming can still be evoked by natural stimulation. The frequency of swimming is slightly lower than in nominally 0 mM Mg2+, but the pattern of ventral root activity and synaptic drive to ventral neurons seems little affected. Fictive swimming can also be induced by applying NMDA to spinal preparations. In 0 mM Mg2+, such rhythmic activity is unstable and transient over a narrow NMDA concentration range. In 0.5 mM Mg2+, continuous rhythmic activity is induced over a wide range of NMDA concentrations. Lower spinal preparations need higher NMDA concentrations to induce activity. We conclude that the neurons rhythmically active in swimming have NMDA receptor channels which show a voltage dependent block in the presence of Mg2+. However, while Mg2+ exerts a powerful stabilizing influence on rhythmic activity induced in spinal embryos by exogenous NMDA, its influence on 'naturally' evoked fictive swimming is less clear. The fictive swimming machinery in the brain and spinal cord can produce stable swimming with or without Mg2+ induced voltage dependency of the NMDA channels.

Journal Article↗

PubMedDate not supplied

Modelling inter-segmental coordination of neuronal oscillators: synaptic mechanisms for uni-directional coupling during swimming in Xenopus tadpoles.

Locomotion requires longitudinal co-ordination. We have examined uni-directional synaptic coupling processes between two classes of neuronal network oscillators: autonomously active "intrinsic" oscillators, and "potential" oscillators that lack sufficient excitatory drive for autonomous activity. We model such oscillator networks in the bilaterally-symmetrical, Xenopus tadpole spinal cord circuits that co-ordinate swimming. "Glutamate" coupling EPSPs can entrain a second oscillator of lower frequency provided their strength is sufficient. Fast (AMPA) EPSPs advance spiking on each cycle, while slow (NMDA) EPSPs increase frequency over many cycles. EPSPs can also enable rhythmicity in "potential" oscillators and entrain them. IPSPs operate primarily on a cycle-by-cycle basis. They can advance or delay spiking to entrain a second "intrinsic" oscillator with higher, equal or lower frequency. Bilaterally symmetrical coupling connections operate twice per cycle: once in each half-cycle, on each side of the receiving oscillator. Excitatory and inhibitory coupling allow entrainment in complimentary areas of parameter space.

Action Potentials↗

PubMedDate not supplied

Communication skills and undergraduate psychiatry: a description of an innovative approach to prepare Australian medical students for their clinical psychiatry attachment.

OBJECTIVE: To address the increasing diversity in the linguistic and ethnic backgrounds of medical students at the University of Melbourne, a program was developed to prepare students for the particular challenges in cultural literacy and idiomatic English language proficiency, especially when exploring sensitive areas of patients' personal history. The authors outline the University of Melbourne program and present two case studies. RESULTS: Post hoc confidential survey of students' appraisals of the program is presented. CONCLUSION: The survey found a high level of acceptance and approval among most students.

Adult↗