SOMA POTENTIALS AND MODES OF ACTIVATION OF CRAYFISH MOTONEURONS

Molecular Logic of Synaptic Diversity Between Drosophila Tonic and Phasic Motoneurons

Although neuronal subtypes display unique synaptic organization and function, the underlying transcriptional differences that establish these features is poorly understood. To identify molecular pathways that contribute to synaptic diversity, single neuron PatchSeq RNA profiling was performed on Drosophila tonic and phasic glutamatergic motoneurons. Tonic motoneurons form weaker facilitating synapses onto single muscles, while phasic motoneurons form stronger depressing synapses onto multiple muscles. Super-resolution microscopy and in vivo imaging demonstrated synaptic active zones in phasic motoneurons are more compact and display enhanced Ca 2+ influx compared to their tonic counterparts. Genetic analysis identified unique synaptic properties that mapped onto gene expression differences for several cellular pathways, including distinct signaling ligands, post-translational modifications and intracellular Ca 2+ buffers. These findings provide insights into how unique transcriptomes drive functional and morphological differences between neuronal subtypes.

Synaptic Properties and Plasticity Mechanisms of Invertebrate Tonic and Phasic Neurons

Defining neuronal cell types and their associated biophysical and synaptic diversity has become an important goal in neuroscience as a mechanism to create comprehensive brain cell atlases in the post-genomic age. Beyond broad classification such as neurotransmitter expression, interneuron vs. pyramidal, sensory or motor, the field is still in the early stages of understanding closely related cell types. In both vertebrate and invertebrate nervous systems, one well-described distinction related to firing characteristics and synaptic release properties are tonic and phasic neuronal subtypes. In vertebrates, these classes were defined based on sustained firing responses during stimulation (tonic) vs. transient responses that rapidly adapt (phasic). In crustaceans, the distinction expanded to include synaptic release properties, with tonic motoneurons displaying sustained firing and weaker synapses that undergo short-term facilitation to maintain muscle contraction and posture. In contrast, phasic motoneurons with stronger synapses showed rapid depression and were recruited for short bursts during fast locomotion. Tonic and phasic motoneurons with similarities to those in crustaceans have been characterized in Drosophila, allowing the genetic toolkit associated with this model to be used for dissecting the unique properties and plasticity mechanisms for these neuronal subtypes. This review outlines general properties of invertebrate tonic and phasic motoneurons and highlights recent advances that characterize distinct synaptic and plasticity pathways associated with two closely related glutamatergic neuronal cell types that drive invertebrate locomotion.

The beginning of intracellular recording in spinal neurons: facts, reflections, and speculations

Intracellular (IC) recording of action potentials in neurons of the vertebrate central nervous system (CNS) was first reported by John Eccles and two colleagues, Walter Brock and John Coombs, in Dunedin, NZL in 1951/1952 and by Walter Woodbury and Harry Patton in Seattle, WA, USA in 1952. Both groups studied spinal cord neurons of the adult cat. In this review, we discuss the precedents to their notable achievement and reflect and speculate on some of the scientific and personal nuances of their work and its immediate and later impact. We then briefly discuss early achievements in IC recording in the study of CNS neurobiology in other laboratories around the world, and some of the methods that led to enhancement of CNS IC-recording techniques. Our modern understanding of CNS neurophysiology directly emanates from the pioneering endeavors of the five who wrote the seminal 1951/1952 articles.

THE MECHANISM OF DISCHARGE PATTERN FORMATION IN CRAYFISH INTERNEURONS

Excitatory and inhibitory processes which result in the generation of output impulses were analyzed in single crayfish interneurons by using intracellular recording and membrane polarizing techniques. Individual spikes which are initiated orthodromically in axon branches summate temporally and spatially to generate a main axon spike; temporally dispersed branch spikes often pace repetitive discharge of the main axon. Hyperpolarizing IPSP's sometimes suppress axonal discharge to most of these inputs, but in other cases may interact selectively with some of them. The IPSP's reverse their polarity at a hyperpolarized level of membrane potential; they sometimes exhibit two discrete time courses indicating two different input sources. Outward direct current at the main axon near branches causes repetitive discharges which may last, with optimal current intensities, for 1 to 15 seconds. The relation of discharge frequency to current intensity is linear for an early spike interval, but above 100 to 200 impulses/sec. it begins to show saturation. In one unit the current-frequency curve exhibited two linear portions, suggesting the presence of two spike-generating sites in the axon. Current threshold measurements, using test stimuli of different durations, showed that both accommodation and "early" or "residual" refractoriness contribute to the determination of discharge rate at different frequencies.

Functional organization of crayfish abdominal ganglia: I. The flexor systems

For insect ganglia, Altman (Advances in Physiological Science, Vol. 23. Neurobiology of Invertebrates. New York: Pergamon Press, pp. 537-555, '81) proposed that individual neuropils control different motor activities. A corollary of this hypothesis is that motor neurons involved in many behavioral functions should branch in more neuropils than those active in fewer behaviors. In crayfish, the abdominal fast-flexor muscles are active only during the generation of the powerstroke for tailflips, whereas the slow-flexor muscles are involved in the maintenance of body posture. The slow flexors are thus active in many of the crayfish's behavioral activities. To test the generality of Altman's idea, we filled groups of crayfish fast-flexor and slow-flexor motor neurons with cobalt chloride and described their shapes with respect to the ganglionic structures through which they pass. Individual fast flexors were also filled intracellularly with HRP. Ganglia containing well-filled neurons were osmicated, embedded in plastic, and sectioned. Unstained sections were examined by light microscopy and pertinent sections were photographed. We found that the paths of the larger neurites were invariant, that the dendritic domains of fast and slow motor neurons occupied distinctive sets of neuropils, and that dendrites of slow motor neurons branched in more ganglionic structures than did those of fast motor neurons. These results are consistent with Altman's hypothesis.

Arsenazo III transients and calcium current in a normally non-spiking neuronal soma of crayfish

Arsenazo III was used to investigate Ca2+ transients in the normally non-excitable soma of the motor giant neurones of the crayfish Procambarus clarkii. Two kinds of regenerative potentials could be obtained depending on membrane potential conditioning: a fast spike after a pre-hyperpolarization to -90 mV and a slow action potential after a pre-depolarization to -50 mV. Only the second of these was accompanied by an Arsenazo III transient. In voltage-clamped, somata injected, with tetraethylammonium chloride, an absorbance change could be obtained by pulsing the membrane potential above -44 mV. The relationship between absorbance change and potential peaked between 0 and +10 mV then fell off to zero at ca. +150 mV. Changes in light absorbance studied using double-pulse protocols suggested that the inactivation of Ca2+ entry was predominantly mediated by the intracellular free Ca2+ concentration. External application of 1 mM-CdCl2 abolished both the absorbance changes and the (Ca2+) inward current. The voltage dependence of this current was similar to that of the absorbance change. For positive membrane potential the current-voltage relationship showed a voltage-dependent conductance property, the origin of which is discussed.

Potentiation of a transient outward current by Na+ influx in crayfish neurones

In voltage-clamped "motor-giant" neurones of the crayfish Orconectes limosus a depolarizing voltage step elicits a transient inward current carried by Na+ which is followed by an early and a delayed outward current. The early outward current is reduced if the Na+ current is suppressed by tetrodotoxin or the removal of external Na+. It is also abolished if the K+ channel blocking agents tetraethylammonium and 3,4-diaminopyridine are applied to the neurone. The outward current was not depressed if Li+ was substituted for Na+ in the external solution or if the Na-K pump was inhibited by ouabain or the removal of external K+. Ionophoretic injections of EGTA did not depress the early outward current. Short ionophoretic injections of Na+ into the neurone increased the outward current elicited by a depolarization but did not affect the leakage current. It is suggested that the influx of Na+ leads to a transient increment of the Na+ concentration near K+ channels and that internal Na+ ions exert an activating or modulating effect on K+ channels.

Anemonia sulcata toxins modify activation and inactivation of Na+ currents in a crayfish neurone

The effects of three toxins (ATX I, II, III) isolated from the sea anemone Anemonia sulcata were studied in the soma membrane of a crustacean neurone under voltage-clamp conditions. All three toxins affected the action potentials and the Na+ currents in a similar manner. The lowest concentrations tested (10 nM, 20 nM and 50 nM for ATX I, II and III, respectively) had pronounced selective effects on the Na+ current. No effect on K+ or Ca2+ currents was observed with concentrations up to 5 microM. In the presence of ATX the Na+ inactivation was incomplete even with pulses of 700 ms length or strong depolarizing prepulses. Besides the effects on the inactivation process ATX affected also the activation of the Na+ current. In cells treated with ATX the negative resistance branch of the peak Na+ current voltage relation was shifted by -5 mV to -20 mV. The time to peak was increased for small depolarizations (up to -30 mV) and the rate of rise (delta I/delta t) was enlarged by ATX. A slow activating current component was also observed after depolarizing prepulses or if the Na+ current was outward. The decay of the Na+ tail currents was considerably prolonged after the application of ATX if the membrane was repolarized to potentials more positive than about -60 mV. Repetitive stimulation led to a shortening of the action potential in ATX II treated neurones. A simultaneous and parallel decrement of the peak and plateau current was observed with depolarizing voltage steps.

Plasticity of non-giant flexion circuitry in chronically cut abdominal nerve cords of the crayfish, Procambarus clarkii

We have investigated the pattern of neuronal activity involved in the gradual return of sensory-evoked abdominal flexions in crayfish with chronically transected nerve cords. Recordings were made from eight types of identified neurone that mediate phasic abdominal movements, in a preparation consisting of the isolated abdominal nerve cord and tailfan. Responses of the cells to pinches and dorsiflexions of the tailfan were compared in two groups of animals: animals whose cords had been cut at the thoracic-abdominal junction 4-17 weeks earlier (chronic preparations), and animals whose cords had been cut at the same site either just before the experiment or up to 6 days earlier (acute preparations). Sensory stimuli produced bursts of spikes in 73% of the fast flexor motoneurones impaled in chronic preparations, but never fired these neurones in acute preparations. However, fast flexor motoneurones in both preparations were fired with approximately equal frequency by single impulses in the giant axons, suggesting that the firing thresholds of these motoneurones had not changed. Sensory stimuli also caused spiking in the extensor inhibitor and the flexor inhibitor in chronic preparations; in contrast, responses in the fast extensor motoneurones were always subthreshold and occasionally hyperpolarizing. None of these cells was fired by similar stimuli in acute preparations. Neurones restricted to the giant axon pathways (lateral, medial, segmental and motor giants) were silent during sensory-evoked flexor discharges in chronically transected cords. Flexor discharges were accompanied by intense activity in non-giant axons recorded from the dorsal cord. Two identified, non-giant interneurones with axons in the dorsal cord were substantially depolarized but never fired by sensory input in chronic preparations. Sensory-evoked firing in the fast flexor motoneurones was not abolished by removal of the posterior stump of the nerve cord at the transection site. About 20% of chronic preparations generated cyclic motor output in response to unpatterned sensory stimulation. The pattern of motor activity that develops in chronically transected cords resembles that seen in normal crayfish during non-giant tailflips. Because cord transection permanently isolates the abdomen from rostral neural centres normally required for the generation of such tailflips, the return of co-ordinated motor output in chronically cut cords may result from the sensory activation of non-giant circuitry within the abdominal nervous system.

Retinal illumination produces synaptic inhibition of a neurosecretory organ in the crayfish, Pacifastacus leniusculus (Dana)

We have identified a cluster of neurosecretory cells in the crayfish eyestalk that possess dendrites in the second optic neuropil (Medulla) and project axons to the first optic neuropil (Lamina). Illumination of the ipsilateral retina produces a synaptic inhibition of these cells that is mimicked by iontophoresis of gamma-aminobutyric acid within the medullary neuropil. The neurosecretory nature of the cells, the efferent projection of their axons, and the strong inhibition of their spiking activity upon retinal illumination suggest that they may be involved in the feedback control of dark adaptation and/or circadian changes in visual sensitivity.

The ionic mechanism of intracellular pH regulation in crayfish neurones

1. Intracellular pH (pHi) regulation in crayfish neurones was studied using pH-, Na+-, and Cl- sensitive micro-electrodes. Neuronal pH regulation has previously been studied only in molluscs. 2. The average resting pHi of crayfish neurones was 7.12 +/- 0.09, which is 1 pH unit more alkaline than that predicted were H+ ions distributed in equilibrium with the membrane potential. 3. When the cytoplasm was acidified (by NH4Cl loading, CO2 application, or HCl injection), pHi recovered towards its resting value. 4. Removal of Na+ from the external solution inhibited pHi recovery from an acid load by more than 90%. pHi recovery resumed immediately when external Na+ was reintroduced. 5. The resting intracellular Na+ concentration ([Na+]i) of crayfish neurones was 15-25 mM. During pHi recovery from an acid load, [Na+]i increased by 10-50 mM. 6. Reducing the external HCO3(-) concentration from 5 mM to 0 mM slowed pHi recovery by an average of about 45%. This slowing was appreciable even in cells in which Na+ removal almost totally blocked pHi recovery. 7. The resting intracellular Cl- concentration ([Cl-]i) was 30-40 mM, indicating that these cells actively accumulate Cl-. During pHi recovery from an acid load, [Cl-]i decreased by 3-5 mM. 8. In the presence of the anion exchange inhibitor SITS (4-acetamide-4'-isothiocyanostilbene-2,2'-disulphonic acid), pHi recovery was slowed to the rate which was normally seen in HCO3(-)-free Ringer solution. SITS abolished the dependence of pHi recovery on the external HCO3(-) concentration. 9. It is concluded that pHi regulation in crayfish neurones involves two separate mechanisms: a Na+-dependent, HCO3(-)-independent acid extrusion process, and a Cl---HCO3(-) exchange which is probably also Na+-dependent.

Dendritic bottlenecks of crustacean motoneurons

Cobalt-labelled fast flexor motoneurons of the crayfish (Procambarus) were studied by electron microscopy after treatment with diaminobenzidine. The neurons were traced into the abdominal ganglion to locations at which they made contacts with the lateral giant fibres of the nerve cord. Fine secondary dendritic branches extended from the primary dendrites of the fast flexor motoneurons to the lateral giant fibre. These fine branches had bottlenecks at various places along their lenghts and also at their junctions with primary dendrites. Chemical synapses occurred at the bottlenecks and at other locations on the fine branches. It is postulated that chemical synapses at dendritic bottlenecks could act to modify the effectiveness of the excitatory drive provided by the lateral giant fibres to the fast flexor motoneurons, most likely by 'gating' electrical signals conveyed by the fine dendrites.