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M J Titmus

Publications and source records attributed to M J Titmus.

7 recordsLinked to original sources
PubMed1999

Effects of choline and other nicotinic agonists on the tectum of juvenile and adult Xenopus frogs: a patch-clamp study.

We have used anatomical methods and whole-cell patch-clamp recording to assess the distribution of nicotinic receptors in the tectum of Xenopus frogs and to measure effects of nicotinic ligands (carbachol, cytisine and nicotine) on glutamatergic spontaneous miniature excitatory postsynaptic currents. Our results confirm that retinotectal axons account for the majority of nicotinic receptors in the tectum and that nicotinic agonists exert presynaptic effects that increase the rate of transmitter release on to tectal cells. The nicotinic blockers mecamylamine and methyllycaconitine reduced responses to carbachol and cytisine. A small percentage of cells also showed postsynaptic responses. We have assessed whether there are developmental changes in the frequency of occurrence of spontaneous miniature excitatory postsynaptic currents. The first three months post-metamorphosis fall within the critical period for the dramatic plasticity displayed by binocular inputs during development in Xenopus. During this period, visual activity governs the formation of orderly maps relayed from the ipsilateral eye via the cholinergic projection from the nucleus isthmi to the tectum. In this study, we have found that critical-period tecta (two to 12 weeks postmetamorphosis) tend to have higher spontaneous activity than do older tecta (two to 69 weeks postmetamorphosis), and that nicotinic agonists increase that activity in both groups, with the result that the peak rates in response to nicotinic agonists are higher during the critical period than later. We also investigated the possible role of choline as an agonist of nicotinic receptors in the tectum. We have found that choline, as well as carbachol and cytisine, can cause a reversible increase in the rate of miniature excitatory postsynaptic currents. This result may help to explain how the isthmotectal projection, which accounts for the overwhelming majority of cholinergic input to the tectum, can exert effects on retinotectal terminals even though there are no morphologically identifiable synapses between the two populations. We have examined the morphology of cells filled with biocytin during the patch-clamp experiments, and we find that cells with dendrites in the stratum zonale, a layer with particularly dense input from the contralateral nucleus isthmi, have higher spontaneous activity than cells with dendrites that do not extend into that layer. Nicotinic agonists increased the activity recorded in both classes of cells. In addition, four pretectal cells were identified. Nicotinic agonists increased the rate of spontaneous activity recorded in that population. The results indicate that retinotectal transmission in the superior colliculus can be increased presynaptically by activity of the cholinergic projections of the nucleus isthmi. This modulation may be the basis for observations that blocking of cholinergic input disrupts the formation of topographic retinotectal projections. Moreover, the ability of choline to activate these receptors suggests that this metabolite of acetylcholine may permit paracrine activation of presynaptic receptors even though the tectum contains high acetylcholinesterase activity.

Aging↗
PubMed1996

Diffusion, not uptake, limits glycine concentration in the synaptic cleft.

1. The question of whether active uptake limits the duration of action of the inhibitory transmitter glycine has been addressed in vivo at inhibitory synapses on the goldfish Mauthner (M) cell. The kinetics of inhibitory postsynaptic potentials and inhibitory postsynaptic currents (IPSCs) evoked antidromically and by eighth-nerve stimulation were recorded in control and in conditions expected to block glycine uptake or slow its diffusion. Theoretical considerations, based on simulated quantal currents, predicted that if diffusion was slow, rapid uptake of glycine would be required and its block would prolong the synaptic responses. 2. Temperature coefficient values for IPSC decay time constants (tau S) are in the range of 2.0 for temperatures between 15 and 23 degrees C, suggesting that diffusion is not the rate-limiting step. 3. Li+, Ch+, or N-methyl-D-glucamine were substituted for 80% of the Na+ in the extracellular fluid to analyze the effects of blocking the Na(+)-dependent glycine uptake. These procedures enhanced the maximum inhibitory shunt produced by glycine iontophoresis, leading to the suggestion that uptake may buffer the concentration of the transmitter in the cleft. In contrast, the Na+ substitutes had no effect on the tau of the recurrent collateral IPSC, which involves synchronous activation of a pool of interneurons and has a monoexponential decay (tau approximately 10-11 ms). 4. The decay phase of the disynaptic IPSCs produced by stimulating the contralateral eighth nerve has fast and slow components, with a prolonged tail lasting up to 100 ms, particularly in the case of repetitive nerve stimulation. The tail is inhibitory, as revealed by its shunt of the antidromic action potential, and it is at least partially Cl- dependent. However, it can be accelerated by superfusion with the glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and DL-2-amino-5-phosphonopentanoic acid (APV). In the presence of these blockers, the IPSC decay remains biexponential (tau fast = 5.2 and 5.9 ms, tau slow = 94 and 130 ms for single and burst stimuli, respectively). Blocking uptake in this condition did not modify tau fast or tau slow. 5. We conclude that an active uptake mechanism does not shape glycinergic IPSCs, including the longer-lasting components that might include a contribution due to persistence of the transmitter. Rather, diffusion alone is sufficient to remove glycine at a rate faster than channel unbinding. 6. To test whether glycine might diffuse to adjacent excitatory synapses and enhance activation of N-methyl-D-aspartate receptors, CNQX and APV were applied locally, by pressure, to the M cell soma, but they had no effect on the prolonged decay of eighth-nerve-evoked responses. Thus the effects of the antagonists when added to the superfusate are exerted at the network level.

2-Amino-5-phosphonovalerate↗
PubMed1990

Axotomy-induced alterations in the electrophysiological characteristics of neurons.

The electrophysiological alterations provoked by axotomy have now been studied for almost half a century, in a number of different cell types. Consequently, it is now possible to detail some common mechanisms underlying these changes and to sort out certain trends in the data. The major phenomena reviewed in this section and some possible future directions are summarized below. (1) It is now possible to advance a unified hypothesis for the effects of axotomy on the conduction velocity of myelinated fibers. The key is that axon diameter, which is directly correlated with conduction velocity, is regulated, at least in part, by neurofilament protein gene expression and transport into the axon. Thus, the largest myelinated axons, with the fastest conduction velocities, have the highest neurofilament contents, and in turn, experience greater or faster declines in neurofilament content, axon caliber, and conduction speed following nerve injury. This regulation of neurofilament gene expression also appears to be target- and/or accessory cell-dependent. In fact, Hoffman and colleagues (1988) have hypothesized that neuron interactions with specific targets (via as yet unknown target-induced signals) may either specify or permit specification of the level of neurofilament gene expression in neurons. Imposed on this primary size determinant is an influence of activity, which also underlies the differential atrophy and decrement in conduction velocity exhibited by motor and sensory fibers of comparable diameters in the same lesioned nerve. Unmyelinated axons, whose structures are not dominated by neurofilament content and metabolism, react very differently to axotomy. The structural and metabolic basis of their reaction is not known. (2) Passive membrane properties, in particular neuronal input resistance, remain relatively stable in the majority of neurons after axotomy. The major exceptions, vertebrate spinal motoneurons, lamprey dorsal interneurons, and mammalian vagal motoneurons, all show an increase in input resistance after axotomy. This change in input resistance appears to be correlated with structural or geometric simplification of dendritic trees and real or apparent changes in specific membrane resistance in one case and with a reduction in cell body size in the other two; however, changes in specific membrane resistance cannot be excluded even in the latter two cases. In the spinal motoneurons, input resistance changes may be more pronounced in those neurons with the most extensive or complex dendritic geometries (i.e. F-type motoneurons). More combined electrophysiological (ideally under voltage or patch clamp conditions) and morphological investigations of single neurons need be done to resolve these questions.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1986

Altered excitability of goldfish mauthner cell following axotomy. I. Characterization and correlations with somatic and axonal morphological reactions.

Axonal transection 7-10 mm distal to the cell body of the goldfish Mauthner (M) cell induced alterations in its excitability; namely, the antidromic spike recorded in the soma was converted from a single-component axon-hillock response to a larger amplitude, two-component impulse. The mean spike amplitude of the axotomized cells was approximately 50% greater (59.6 +/- 15.1 mV, n = 94) than that in controls (39.4 +/- 6.3 mV, n = 73). The onset of the induced increase in spike amplitude occurs at approximately 20 days postaxotomy, and the transition to a reactive spike is complete by approximately 30-35 days. Eighty-three percent of the M-cells axotomized for more than 30 days were physiologically reactive as judged by their large spike amplitudes and/or the presence of an additional spike component. Concomitant with the enhanced spike amplitudes, there was a depression of excitability in the initial segment-axon hillock region of the axotomized cells. This depression was suggested by a decrease in the initial segment (IS) spike height (from 39.4 +/- 6.3 mV, n = 73, in controls to 27.5 +/- 5.6 mV, n = 13, in axotomized cells), a decrease in its maximum rate of rise (from 153.6 +/- 24 V/s, n = 15, to 112.5 +/- 30 V/s, n = 29), and frequent failure of antidromic invasion into the initial segment and axon hillock. These changes in excitability could not be attributed to alterations in passive membrane properties, since the mean resting potential (77.8 +/- 5.2 mV, n = 37, control; 76.9 +/- 7.8 mV, n = 87, axotomized) and input resistance (170 +/- 21.3 K omega, n = 13, control; 176 +/- 26.6 K omega, n = 21, axotomized) were not altered significantly by axotomy. Threshold voltage was also unaffected (13.4 +/- 3.2 mV, n = 11, control; 11.9 +/- 2.5 mV, n = 11, axotomized). Sequential recordings of spike amplitudes from the axon hillock, soma, and lateral dendrite suggest that the generator of the axotomy-induced component is localized to the normally passive soma and proximal dendrite. In addition, the presumed soma-dendritic In addition, the presumed soma-dendritic component contributes very little if anything to the action potentials recorded in the axon. The onset and occurrence of alterations in excitability and cell body morphology (chromatolysis and nuclear associated changes) were compared in different M-cell populations and in the same identified M-cells. The comparisons suggested that these two events tend to occur in parallel.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1986

Altered excitability of goldfish Mauthner cell following axotomy. II. Localization and ionic basis.

The ionic basis and spatial localizations of spike generation were examined in normal and axotomized goldfish Mauthner (M-) cells using intra- and extracellular recordings and pharmacological manipulation of ionic conductances, including localized iontophoretic drug applications. Tetrodotoxin (TTX) abolished both the initial segment (IS) spike in normal cells and the larger, two-component action potential in axotomized cells, whereas calcium (Ca2+) blockers did not. Thus, sodium (Na+) appears to be the major inward current carrier in both cases. A shoulder or plateau following the fast-rising Na+-dependent action potential was unmasked in both normal and axotomized M-cells by intracellular injections of tetraethylammonium (TEA), either alone or in conjunction with 4-aminopyridine (4-AP) or cesium (Cs+). This plateau potential was abolished by superfusing with saline containing the Ca2 antagonists, Co2+, Mn2+, or Cd2+. However, barium (Ba2+), which normally substitutes for Ca2+ and also blocks K+ conductances, did not produce a plateau spike, and no action potentials could be evoked in the presence of TTX. Simultaneous extra- and intracellular recordings from the soma and lateral dendrite revealed that both the full-sized axotomized spike and its individual labile components were always maximal at the soma. These data support the earlier suggestion that the axotomy-induced electrogenicity is primarily localized to that region. Iontophoretic application of TTX inside the axon cap, a distinctive neuropil surrounding the initial segment and the axon hillock and circumscribed by a glial border, and at various positions along the lateral dendrite confirmed the Na+-dependency of the action potentials recorded in normal and axotomized cells and further demonstrated that the soma generates the additional spike component in the latter. The results suggest that axotomy causes a persistent change in voltage-gated Na+ channel distribution in the M-cell, with Na+ channels appearing or becoming more numerous in the soma while becoming less concentrated in the initial segment-axon hillock. Possible related shifts in other voltage-dependent conductances are also discussed. Finally, these are the first detailed studies of the ionic basis of axotomy-induced electrogenicity in a vertebrate neuron, central or peripheral, and the similarity to the results obtained with invertebrate neurons suggests common mechanisms underlying the axon reaction.

Action Potentials↗
PubMed1981

Ultrastructure of identified fast excitatory, slow excitatory and inhibitory neuromuscular junctions in the locust.

The specialized jumping muscle of the locust, the metathoracic extensor tibiae (ETi), is innervated by four physiologically different motoneurons, including FETi, a phasic excitor, SETi, a tonic excitor, and CI, a tonic common inhibitor. FETi neuromuscular junctions were examined in three phasic ETi bundles innervated by FETi. FETi terminals were characterized by patchy contacts on to granular sarcoplasm. The ETi accessory extensor, innervated by both SETi and CI, contains two morphologically different types of axon ending. When this muscle was soaked in horseradish peroxidase, stimulation of SETi led to selective uptake in vesicles in terminals similar to those of FETi axons but containing smaller vesicles, while stimulation by CI caused increased uptake into terminals with more extensive contact directly on to fibrillar sarcoplasm. As has been observed in excitatory and inhibitory synapses in some crustacean and vertebrate nervous systems, the synaptic vesicles in the locust excitatory endings are round and electron-lucent while those in the inhibitory endings are more irregular in shape. The tonic neuromuscular junctions, SETi and CI, are more densely packed with vesicles, larger in cross-sectional area and appear to be of more complex shape than the smaller, vesicle-sparse, phasic FETi terminals. Following long duration stimulation at 10 Hz, the tonic neuromuscular junctions showed little morphological change. FETi endings, which fatigue within minutes at the same stimulation frequency, showed a 20% decrease in synaptic vesicle density and an increase in irregularly shaped membrane inclusions.

Animals↗