Effects of a metabotropic glutamate agonist, trans-ACPD, on cortical epileptiform activity

The effect of the metabotropic glutamate receptor agonist trans-1-amino-1,3,cyclopentanedicarboxylic acid (trans-ACPD) on epileptiform activity induced in rat neocortical slices by exposure to Mg(2+)-free medium was examined. Trans-ACPD dose dependently (10-200 microM) decreased the frequency of spontaneous epileptiform events whilst increasing both the duration of afterpotentials and the number of afterbursts associated with single events. This effect on afterpotentials and afterbursting was particularly pronounced in 14-17 day-old rats and was blocked by the sigma ligand ditolyguanidine (DTG) 10 microM. The putative metabotropic glutamate receptor antagonist L-AP3 did not antagonise the actions of trans-ACPD. The results suggest a role for metabotropic glutamate receptors in epilepsy, possibly in the transition from interictal to ictal activity.

D-CPP-ene (SDZ EAA 494), a potent and competitive N-methyl-D-aspartate (NMDA) antagonist: effect on spontaneous activity and NMDA-induced depolarizations in the rat neocortical slice preparation, compared with other CPP derivatives and MK-801

D- and L-enantiomers of the competitive NMDA antagonists CPP and CPP-ene, as well as the non-competitive NMDA antagonist MK-801, inhibited spontaneous activity occurring in slices of rat cerebral cortex exposed to Mg2(+)-free medium. D-CPP-ene (SDZ EAA 494) was the most active competitive antagonist with a threshold concentration of 10 nM and an ED50 of 39 nM. The inhibitory effects of all competitive antagonists were reversible, whereas reversibility following MK-801 (ED50 = 33 nM) was incomplete and slow. D-CPP-ene was also the most potent competitive antagonist against NMDA-evoked depolarizations, having an apparent pA2 value of 6.8; its action was specific to the NMDA type of excitatory amino acid receptor.

Single-unit study of lateral line cells in the optic tectum of Xenopus laevis: evidence for bimodal lateral line/optic units

Responses of single units in the Xenopus tectum to stimulation of the contralateral anterior lateral line nerve (cALLN) and optic nerve were studied. Cells responded to cALLN stimulation with a phasic burst of spikes that was repeatable between trials; latencies ranged from 4 to 23 msec. The most excitable cells were located in layer 6 of the ventrolateral tectum. Cells responding to stimulation of the ipsilateral ALLN were far less numerous and robust, and showed latencies 3-10 msec greater than those of contralateral responses. Tectal cell responses to cALLN nerve stimulation grew progressively to saturation with stimulus voltage and paralleled the growth of the ALLN compound action potential; cells responded to stimulation of either supra- or infraorbital branches of cALLN. These observations indicate convergence of primary lateral line afferents in the medulla and/or tectum. Lateral line tectal cells showed strong habituation at interstimulus times less than 8-20 seconds. Experiments on bimodal cells revealed facilitatory and inhibitory interactions between optic and lateral line inputs. Some cells responded to stimulation of either lateral line or optic nerves, with combined stimulation producing responses exceeding the sum of responses to separate nerve stimulation. In other cells the response to optic nerve stimulation was markedly increased by lateral line nerve stimulation, despite the absence of response to lateral line nerve stimulation alone. Facilitation was also evident in cells that responded only to combined stimulation of lateral line and optic nerves. Some cells exhibited an early (5-10 msec) and late (20-40 msec) response to optic nerve stimulation; lateral line nerve stimulation, despite eliciting no response itself, produced strong facilitation of the early but almost complete suppression of the late optic nerve response.

Organisation of lateral line and auditory areas in the midbrain of Xenopus laevis

Lateral line areas in the midbrain of Xenopus laevis were identified by recording evoked potentials and neural activity elicited by stimulating anterior and posterior lateral line nerves. Spike activity was found in the lateral half of the optic tectum, ventrolateral tectum, and torus semicircularis. Contra- and ipsilateral lateral line pathways to these regions were identified. Spike discharge was associated with an evoked potential (EP) consisting of a large negative-positive wave sometimes preceded by a small positive-negative deflection. EP depth profiles varied according to electrode position within the lateral line midbrain projection field. In the middle of the field a dramatic increase in EP growth occurred as the electrode passed through the torus semicircularis, with peak amplitudes being achieved 900-1,100 micron from the surface within nucleus principalis and magnocellularis. Tracks at the lateral edge of the field showed a steady growth of EP, with peak amplitudes around 600 micron as the electrode passed through ventrolateral tectum. Auditory responses to tone pips were found in the nucleus laminaris and principalis in caudomedial regions of the torus semicircularis, in areas lying medial to the main centers of lateral line evoked activity; this is a similar organisation to that found in teleost fish. The results indicate the torus semicircularis and deep layers of the lateral tectum to be involved in lateral line processing Some topographic separation of the representation of anterior and posterior lateral line systems is indicated. The possible involvement of these areas in lateral line stimulus localisation is discussed.

The relation between soma position and fibre trajectory of neurons in the mesencephalic trigeminal nucleus of Xenopus laevis

In adult Xenopus laevis the mandibular and ophthalmic branches of the trigeminal nerve were backfilled with CoCl2 or cobaltous lysine and whole brains silver intensified to reveal neurons of the mesencephalic Vth nucleus (mes. V). The nucleus contains about 100 cells arranged in a band extending arch-like from the ventrolateral margin of the optic tectum to the midline. Many cells possess a small number of short dendritic processes that arborize in the tectal neuropil; in some cells one dendrite terminates within the ependyma or ventricle. A single axon arises from each cell and courses in layer 7 to the margin of the tectum. Axon collaterals arise close to the cell body to terminate principally within layer 6, but occasionally also in layers 8 and 9. Collaterals occurring more caudally terminate in layer 6. These findings suggest that mes. V cells acts as tectal interneurons as well as conveying somatosensory information to the tectum from the mouth region. In the dorsal roof of the tectum the trajectory of a fibre is related to the distance of the soma from the midline. Mes. V cells located at the lateral end of the nucleus possess axons that course initially in a mediolateral direction before turning along the ventrolateral margin of the tectum. Cells positioned close to the midline have axons that project rostrocaudally the entire length of the tectum. The axons of cells located at intermediate positions within the nucleus course at correspondingly oblique angles through the dorsal roof of the tectum. Thus in this area there is a more or less 90 degrees rang in the orientation of mes. V fibres to the longitudinal axis. It is proposed that this topographical relationship between soma position and axon trajectory arises through a developmental mechanism, in which mes. V fibres grow during larval life sequentially into the medial zone of tectal growth and become subsequently displaced rostrolaterally, owing to the further addition of tectal tissue medially, through an angle dependent upon the parent cell's date of birth.

The central projections of lateral line and cutaneous sensory fibres (VII and X) in Xenopus laevis

The central terminal fields of anterior (VII) and posterior (X) lateral line nerves, as well as the somatosensory fibres of VII and X, have been studied in Xenopus laevis by CoCl2 backfilling of appropriate nerves followed by Timms intensification of whole brains. Lateral line fibres terminate in a dorsomedial region of the medulla that extends from the auricular lobe of the cerebellum to the obex. There is great overlap in the terminal fields of lateral line fibres from nerves innervating different groups of stitches. The technique also reveals second order cells within the lateral line nucleus, just ventral and medial to the zone of fibre termination. Lateral line efferent cell bodies lie within the medullary motor areas and their ventrally descending dendrites ramify profusely within the reticular formation. Somatosensory fibres of VII and X project into a totally separate medullary-spinal pathway, the tractus descendens trigemini. Collaterals of these fibres innervate an area of neuropil considered to be the most posterior portion of the trigeminal sensory nucleus. At the region of the calamus scriptus further collaterals ramify within distinct transversely arranged plexi and may decussate to innervate similar plexi on the contralateral side. The functional significance of these anatomical findings is discussed.

The calcium antagonistic effects of cyproheptadine on contraction, membrane electrical events and calcium influx in the guinea-pig taenia coli

1 The ability of cyproheptadine (Cph) to inhibit membrane translocation of calcium in smooth muscle was investigated by studying the drug's action on contraction, electrical activity and calcium influx in the guinea-pig taenia coli.2 Cph >/= 10(-6)M reduced the amplitude of normal spontaneous contractions and concurrently decreased the number of action potentials occurring with each slow-wave of depolarization (sucrose-gap recordings). These inhibitory effects of Cph were antagonized by increasing the medium [Ca] three fold to 7.68 mM.3 Intracellular recordings showed that Cph >/= 2 x 10(-6)M decreased the amplitude and extended the duration of the action potential. These effects were only partially reversible in normal medium whereas large overshooting action potentials were again seen in 7.68 mM Ca medium.4 High frequency mechanical activity was produced by inclusion of veratridine 5 x 10(-6)M in the perfusate. Low concentrations of Cph (>/= 10(-7)M) reduced the amplitude of such contractions at a faster rate than they did normal spontaneous contractions.5 At concentrations between 10(-7) and 10(-6)M, Cph fully reduced the tonic component of contractions elicited in 112 mM isotonic KCl whilst having little or no effect on either (i) the initial phasic KCl contraction or (ii) the ;repolarization contracture' normally produced on wash-out of the KCl or (iii) the spontaneous contractions before and after KCl treatment. In contrast, at Cph 2 x 10(-6)M, the repolarization contracture, as well as the isotonic KCl contraction, was totally blocked whereas spontaneous contractions were still unaffected. Progressively higher Cph concentrations inhibited all components of this contractile cycle.6 Dose-response curves for the rate of drug-induced relaxation of tonic contractures produced in hypertonic 42.7 mM high-potassium medium, showed the calcium antagonistic potency of Cph to be intermediate between that of chlorpromazine and D600. The minimum Cph concentration for effect lay between 1 and 5 x 10(-7)M, and the effects of Cph 2 x 10(-6)M (approximately the ID(50)) were totally antagonized by 12.8 mM Ca.7 By means of a lanthanum wash procedure, Cph >/= 2 x 10(-6)M was found to decrease the (45)Ca uptake occurring into strips of taenia coli in normal medium, although the maximum effect (at Cph 10(-5)M) amounted to only 25% inhibition of the uptake occurring into control strips (also found with D600). The increased uptake occurring in hypertonic 44.7 mM high-potassium medium was inhibited in a dose-dependent manner by Cph 1 x 10(-7)M.8 The results are consistent with an action of Cph in reducing the flow of Ca(2+) through voltage-dependent Ca channels in the smooth muscle cell membrane. It is suggested that the interaction of Cph molecules with such sites is dependent upon membrane potential as well as drug concentration.

Mechanism by which cyproheptadine inhibits insulin secretion

1 Isolated islets of Langerhans from the rat have been used in studies designed to elucidate the mechanism by which cyproheptadine inhibits insulin secretion. 2 D-Glucose and tolbutamide, both of which require extracellular Ca2+ to produce insulin release, failed to evoke a secretory response from islets pretreated with cyproheptadine. Conversely veratridine, the calcium ionophore A23187 and theophylline, all of which are capable of mobilizing sufficient intracellular Ca2+ to evoke insulin secretion in the absence of extracellular Ca2+, produced similar responses from cyproheptadine pretreated and control islets. 3 Cyproheptadine completely inhibited Ca2+ uptake induced by D-glucose and high Ko+, two agents which depolarize the islet beta-cell membrane, whilst Ca2+ uptake elicited by removal of extracellular Na+ (i.e. Na+-Ca2+ counter transport) was only slightly reduced. 4 A significant increase in Na+ uptake produced by veratridine was sensitive to tetrodoxin but only partially reduced by cyproheptadine. 5 These results suggest that cyproheptadine inhibits depolarization-dependent calcium entry into pancreatic beta-cells.

Morphological changes in the cat cerebral cortex produced by superfusion of ouabain

The morphological changes following superfusion of the cat cerebral cortex with ouabain were studied. Autoradiography of [3H]ouabain was performed to study drug distribution. The resulting lesion consists of an upper vacuolated layer which is distinctly separated from an underlying region containing dark neurones. Ouabain is confined to the vacuolated layer. Swelling of apical dendrites and many presynaptic terminals are the main morphological changes occurring in the vacuolar layer. Depletion of synaptic vesicles, clustering of vesicles around the synaptic membrane, and the production of coated vesicles and cisternae are further changes found within presynaptic endings. Whilst swelling of fibrous astrocytes within the glialimitans occurs, this is not true of astrocytic processes elsewhere in ouabain exposed regions. Regions containing dark neurones are characterised by a general swelling of astroglial processes. The results strongly suggest that apical dendrites and presynaptic endings possess high activities of Na+, K+-ATPase whereas the activity on astroglial processes within the neuropil is relatively low. Astroglial swelling in areas of dark neurones is produced by some change in the chemical milieu surrounding the processes which appears unrelated to Na+, K+-ATPase inhibition.

The functional significance of sodium channels in pancreatic beta-cell membranes

1. The existence and functional significance of Na channels in pancreatic beta-cell membranes were investigated by studying the effects of the plant alkaloid veratridine on the temporal release of insulin from perfused isolated rat islets of Langerhans.2. 100 muM veratridine evoked a sustained threefold increase in insulin release which was almost completely inhibited by 3 muM tetrodotoxin (TTX). This action of TTX was rapidly reversible.3. The simultaneous presence of 100 muM propranolol, 100 muM phenoxy-benzamine and 10 muM atropine did not alter the magnitude of the response to 100 muM veratridine, indicating that the action of veratridine on the beta-cells was direct and was not mediated via the release of neurotrans-mitters from nerve endings within the islets.4. (45)Ca uptake by isolated islets in static incubation was increased almost threefold by 100 muM veratridine. This increase was completely inhibited by the simultaneous presence of 3 muM TTX.5. Replacement of Na(o) by choline caused a transient fourfold increase in insulin release which was associated with an increase in the uptake of (45)Ca from the extracellular space of similar magnitude. Subsequent exposure of islets to 100 muM veratridine still evoked some insulin release but this only achieved 32% of that secreted by islets exposed to veratridine in medium of normal [Na](o).6. The addition of 2.5 mM CoCl(2) to the medium caused a 62.5% inhibition of veratridine-mediated insulin release.7. In Ca-free medium supplemented with 1 mM EGTA, 100 muM veratridine evoked insulin release of equal magnitude and of similar temporal relationship to that obtained in the presence of normal [Ca](o).8. A twofold increase in insulin release that occurred in the 15 min period immediately following exposure to 1 mM ouabain was completely independent of [Ca](o). Subsequent ouabain-evoked release became increasingly dependent on [Ca](o).9. Tetrodotoxin (3 muM) inhibited the first phase of insulin release evoked by 16.7 mMd-glucose by 37% and the second phase by 20%.10. Both Na and Ca appear capable of entering through Na channels opened in the beta-cell membrane by veratridine. The increase in [Na](i), resulting from the veratridine mediated increase in P(Na+), causes depolarization of the beta-cell membrane with a consequent opening of voltage-sensitive, Co(2+)-blockable channels for additional Ca entry. An increase in [Na](i) also increases [Ca](i) by altering the equilibria of intracellular Ca-sequestering mechanisms. The small but significant reduction of glucose-mediated insulin release by TTX indicates that glucose has a rather weak action on the Na channel and a more pronounced effect on the voltage-dependent Co(2+)-blockable Ca channel.

Biosynthesis of chloramphenicol in Streptomyces species 3022a. Isotope incorporation experiments with (G-14C) chorismic, (G-14C) prephenic, and (G-14C, 6-3H) shikimic acids

When chloramphenicol-producing cultures of Streptomyces species 3022a were administered [G-14C] chorismic, [G-14C] prephenic, and [G-14C, 6-3H] shikimic acids, radiochemical yields in the antibiotic were very low (0.01–0.1%). [G-14C] chorismic and prephenic acids labeled chloramphenicol to about the same specific activity as tyrosine, aspartic acid, and glutamic acid from mycelial protein, whereas protein phenylalanine was 100 times more active. We conclude that these two substrates are unable to penetrate the cell membrane and the radioactivity incorporated enters as [G-14C] phenylpyruvic acid. Labeled shikimic acid, on the other hand, was directly incorporated into the cells. The 14C: 3H ratio in chloramphenicol isolated after feeding the [G-14C, 6R-3H] and [G-14C, 6S-3H] labeled forms indicated that the pro-6R-hydrogen was eliminated during ring aromatization. Antibiotic labeled from [G-14C, 6S-3H] shikimic acid was chemically degraded to 2,4,6-tribromoaniline without change in the 14C: 3H ratio thus establishing the specific incorporation of shikimic acid into the phenyl ring, and locating the tritium at positions ortho to the propanoid substituent.

The fine structure of the PD proprioceptor of Cancer pagurus. II. The position sensitive cells

The cell bodies of the position sensitive units form a row distal to the movement sensitive cells and their dendrites run in pairs in a narrow neck of tissue on the ventral side of the receptor strand. The scolopidia share the features of elongation and relaxation sensitive movement units. Thus the canal cell is absent, but there is some scolopale material in the enveloping cells. Also the scolopale is apposed by a mixture of strand cells and collagen. The more distal scolopidia are found in a region of large haemocoelic lacunae. The physiological differences between movement and position sensitive units could be explained in terms of how well the dendrites are anchored into the tube; with the position cells possibly being held at different levels with respect to their maximum sensitivity. On the other hand, the dendrites of both types of unit may behave identically and, if so, then the necessary physiological differences could occur in the transduction and/or impulse initiation sites. These alternative explanations are discussed.

The fine structure of the PD proprioceptor of Cancer pagurus. I. The receptor stand and the movement sensitive cells

The propodite-dactylopodite (PD) proprioceptor ofCancer pagurusconsists of a receptor strand in which are embedded the dendrites of up to 80 bipolar nerve cells. The strand is composed of numerous cells and collagen and is surrounded by a thin layer of amorphous connective tissue. Beneath the latter there are often haemocoelic lacunae and individual haemocytes are found surrounded by strand tissue. The sensory cells can be divided into a proximal group with large cell bodies (movement sensitive cells) and a distal row with smaller cell bodies (position sensitive cells). The former can be further divided on the basis of the insertion of their dendrites into the strand. Thus elongation sensitive movement cells (e. s. m. cs) insert into the anterior surface and run in the ventral region of the receptor strand, while relaxation sensitive movement cells (r. s. m. cs) insert into the dorsal surface and run dorsally through the strand. The dendrites run in pairs, the members of each pair being either e. s. m. cs or r. s. m. cs. Each pair is surrounded distally by a scolopidium, consisting of a scolopale cell, two enveloping cells, a tube of extracellular material and, in e. s. m. cs, a canal cell. The scolopale cell contains a scolopale consisting of electron dense material laid down around a matrix of longitudinally oriented microtubules. In e. s. m. cs the enveloping cells also contain aggregations of a similar material. The dendrites make desmosome contacts with the scolopale cell and the corresponding thickenings of the scolopale cell membrane are juxtaposed by scolopale material. Beyond the scolopale the dendrites enter and terminate within a tube of extracellular material. In e. s. m. cs the scolopidium is surrounded mainly by strand cells, but in r. s. m. cs it is surrounded by collagen fibres. Where the collagen fibres meet the sheathing cells there are often contact zones. Other differences between e. s. m. cs and r. s. m. cs occur in the fine structure of the dendrites and these are described. It is suggested that in e. s. m. cs, since the scolopale is apposed by strand cells, stretching of the strand will lead to elongation of the scolopale and hence of the dendritic terminals. In contrast, in r. s. m. cs, it is possible that the scolopidium is not stretched during elongation of the strand, since there is little cellular contact between the two, but that during strand relaxation the energy stored in the collagen fibres forces the proximal end of the scolopale away from the more distal regions and hence stretches the dendritic terminations. Previous theories are discussed and compared with this idea. Also the relative merits of mechanical and physiological explanations for the phasic nature of these units are discussed.

Regulation of chloramphenicol synthesis in Streptomyces sp. 3022a. Branch-point enzymes of the shikimic acid pathway

The pattern of induction and regulatory properties of the enzymes chorismate mutase, prephenate dehydratase, and anthranilate synthetase were investigated in a chloramphenicol-producing Streptomycete under a variety of environmental conditions. The enzymes were not subject to repression by intermediates or end products of the shikimic acid pathway. Throughout purification procedures chorismate mutase and prephenate dehydratase activities were each represented by single enzyme proteins of molecular weights 75 000 and 220 000, respectively. Chorismate mutase activity was not inhibited by end products or intermediates of the shikimic acid pathway. Prephenate dehydratase activity was controlled solely by feedback inhibition by L-phenylalanine, and anthranilate synthetase activity was only inhibited by L-tryptophan.

An Analysis of the Types of Sensory Unit Present in the PD Proprioceptor of Decapod Crustaceans

1. The PD organs of various decapod crustaceans were stimulated by elongation and relaxation of the strand, and single-unit recordings were made from the different types of sensory unit.

2. The sensory units can all be placed in one or other of three categories: unidirectional movement cells, position cells and intermediate cells.

3. Unidirectional movement cells are large, phasic units which respond either to elongation or relaxation of the strand. Some are pure movement-sensitive units, which reach saturation at very low movement velocities and fire over most or all of the range of movement. Other units show velocity sensitivity and differential range sensitivity in varying degrees.

4. Position cells are small tonic units and are usually maximally sensitive at one end of the range of movement. Their firing pattern is asymmetric in that the frequency of impulses is dependent on the direction of movement by which a given position is achieved.

5. Intermediate cells have large phasic and tonic components which may or may not show a direct relationship to each other.

6. The importance of the detailed information produced by the PD organ is discussed and suggestions made as to how it could be utilized by the central nervous system.

Regulation of chloramphenicol synthesis in Streptomyces sp. 3022a. 3-Deoxy-D-arabino-heptulosonate 7-phosphate synthetase

The repression and end-product inhibition of 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthetase were studied in a chloramphenicol-producing Streptomycetes. Synthesis of DAHP synthetase was repressed by p-hydroxybenzoate, and enzyme activity was inhibited competitively by sugar phosphates, especially D-ribose 5-phosphate. The presence of chloramphenicol, aromatic amino acids, or shikimic acid pathway intermediates did not repress enzyme synthesis nor inhibit enzyme activity. Chloramphenicol production by growing cultures was not affected by the intermediates or end products of the shikimic acid pathway nor by the repression of DAHP synthetase. Purification of DAHP synthetase activity indicated the presence of a single enzyme protein with a molecular weight of 88 000.