Journal of Physiology

The diversity of cellular targets of direct current stimulation (DCS), including somas, dendrites and axon terminals, determine the modulation of synaptic efficacy.

Abstract  Transcranial direct current stimulation (tDCS) is a non‐invasive brain stimulation technique to modulate cortical excitability. Although increased/decreased excitability under the anode/cathode electrode is nominally associated with membrane depolarization/hyperpolarization, which cellular compartments (somas, dendrites, axons and their terminals) mediate changes in cortical excitability remains unaddressed. Here we consider the acute effects of DCS on excitatory synaptic efficacy. Using multi‐scale computational models and rat cortical brain slices, we show the following. (1) Typical tDCS montages produce predominantly tangential (relative to the cortical surface) direction currents (4–12 times radial direction currents), even directly under electrodes. (2) Radial current flow (parallel to the somatodendritic axis) modulates synaptic efficacy consistent with somatic polarization, with depolarization facilitating synaptic efficacy. (3) Tangential current flow (perpendicular to the somatodendritic axis) modulates synaptic efficacy acutely (during stimulation) in an afferent pathway‐specific manner that is consistent with terminal polarization, with hyperpolarization facilitating synaptic efficacy. (4) Maximal polarization during uniform DCS is expected at distal (the branch length is more than three times the membrane length constant) synaptic terminals, independent of and two–three times more susceptible than pyramidal neuron somas. We conclude that during acute DCS the cellular targets responsible for modulation of synaptic efficacy are concurrently somata and axon terminals, with the direction of cortical current flow determining the relative influence.

1. Cholinergic drugs were infused into the retinal circulation of the rabbit while we analysed the receptive field properties of directionally sensitive retinal ganglion cells. Physostigmine eliminated the trigger feature, directional specificity, of both types (on‐centre and on—off) of these cells. In this respect the action of physostigmine (an ACh potentiator) was very like that of picrotoxin (a GABA antagonist). Therefore, a detailed analysis of the receptive field properties of directionally sensitive ganglion cells was made to analyse the effects of physostigmine and picrotoxin.

2. Size specificity and radial grating inhibition were not abolished by physostigmine, but were often affected by picrotoxin. The optimal velocity in the preferred direction (as measured by maximum firing frequency) was not much changed by physostigmine, but was higher during infusion of picrotoxin. Infusion of nicotine, a depolarizing ACh agonist which increases the activity of retinal ganglion cells, revealed the presence of inhibition to movement in the null direction. The null direction response during picrotoxin started slightly later than this inhibition. The null direction response during physostigmine was weaker and started later still. Mecamylamine and dihydro‐β‐erythroidine, nicrotinic receptor antagonists, totally blocked the effect of physostigmine and reduced the control light response by about half.

3. From this analysis, it appears that on—off ACh release onto directionally sensitive cells provides a substantial excitation which, when potentiated by physostigmine, overcomes or outlasts the null direction GABA inhibition within the receptive field. The spatial extent of GABA inhibition is asymmetric to and larger than the spatial extent of ACh excitation. Similar pathways appear to be involved in both the on‐centre and on—off directionally sensitive ganglion cells, yet the on‐centre cell pathway may receive an additional input which suppresses the ACh excitation at light offset. Possible schemes for the cellular mechanism of directional sensitivity are discussed in light of these results and recent anatomical and pharmacological findings.

1. The iontophoretic application of bicuculline, an antagonist of GABA, the putative inhibitory transmitter in the visual cortex, has been used to examine the contribution of post‐synaptic inhibitory processes to the directional selectivity of simple, complex and hypercomplex cells in the cat's striate cortex.

2. The directional selectivity of simple cells was significantly reduced or eliminated during the iontophoretic application of bicuculline. This supports the view that the selectivity is derived from the action of a GABA‐mediated post‐synaptic inhibitory input modifying their response to a non‐directionally specific excitatory input.

3. Complex cells were subdivided into three categories on the basis of the action of iontophoretically applied bicuculline on their directional selectivity, receptive field characteristics and distribution in terms of cortical layer. They are referred to as type ‘1’, ‘2’ and ‘3’ complex cells.

4. The directional specificity of type ‘1’ complex cells was eliminated during the iontophoretic application of bicuculline. It seems likely, therefore, that they receive a non‐directionally specific excitatory input and that, as for simple cells, the directional specificity derives from the action of a GABA‐mediated post‐synaptic inhibitory input. No type ‘1’ complex cells were recorded below layer IV.

5. The directional specificity of type ‘2’ complex cells was unaffected by the iontophoretic application of bicuculline, despite increases in response magnitude, a block of the action of iontophoretically applied GABA and, in some cases, changes in other receptive field properties. It is suggested that these cells receive a directionally specific excitatory input. The type ‘2’ complex cells were found both superficial and deep to layer IV with the majority in layer V.

6. Type ‘3’ complex cells appear to have very similar receptive field properties to those of the cells described by other workers as projecting to the superior colliculus. They were found predominantly in layer V. Their directional specificity was not eliminated by the iontophoretic application of bicuculline. However, they exhibited a powerful suppression of the resting discharge in response to stimulus motion in the non‐preferred direction. Iontophoretic application of ammonium ions revealed a small excitatory response in place of the suppression. It appears from these observations that the directional specificity of the type ‘3’ complex cells could be determined, at least in part, by an inhibitory process which is not GABA‐mediated.

7. The directional specificity of hypercomplex cells found in layers II and III was unaffected by the iontophoretic application of bicuculline, and they showed no suppression of their background discharge level in response to stimulus motion in the non‐preferred direction. This evidence is consistent with the view that they receive a directionally specific excitatory input.

1. 284 single cortical neurones were studied in area seventeen of twenty‐five normal kittens and of fifteen kittens, binocularly deprived, whose first visual experience had been delayed until the experiment by bilateral lid‐suture. Both normal and binocularly deprived kittens ranged in age from 1 to 6 weeks.

2. The optimal, binocularly presented, visual stimulus and receptive fields were determined for each neurone by varying target configuration, speed and direction of movement and the prism‐induced alignment of both eyes. Repetitive, controlled stimulation in eighty‐four cases allowed quantitative estimates to be made of the response selectivity for the target configuration (spot vs. line), the direction of target motion and the prism‐induced disparity between the retinal images of the binocular target.

3. Before the fourth post‐natal week neurones from both normal and binocularly deprived cortex showed similar properties: selectivity for direction of target motion was present in both preparations but both lacked binocular specificity and dependence on target configuration.

4. After the fourth week, normal kittens had increasing numbers of neurones with selective responses which were dependent upon target configuration and the degree of binocular misalignment. The proportion of selective neurones approached the adult value after the fifth week.

5. The cortex of binocularly deprived kittens failed to show an increase of selectivity with age, and of 150 neurones, sixty‐two were visually unresponsive, two showed selectivity which was dependent upon target configuration and none showed selectivity for prism‐induced retinal disparity.

6. The data are not consistent with the hypothesis that the highly specific response properties of visual cortical neurones can develop without appropriate visual experience. Innate mechanisms appear to be sufficient for the development of the excitatory connexions producing motion sensitivity and receptive field location on both retinas, but patterned visual experience is necessary for the ‘fine‐tuning’ which vetoes responses to stimuli with non‐optimal configuration or binocular disparity.

1. Earlier experiments rearing kittens with one eye closed and reversing the closure after a certain age, or rearing kittens in a rotating drum and reversing the direction after a certain age, suggest that the critical periods for ocular dominance and directional sensitivity may differ. Since these results were obtained by different investigators in different laboratories, we have made a direct comparison of the two types of visual deprivation. 2. Four pairs of litter‐mate kittens (matched in weight) were reared. One animal in each pair was monocularly deprived with subsequent eye reversal; the other animal was directionally deprived with reversal of drum direction. All reversals took place at age 5 weeks. Both kittens in a given pair were either 'left first' (left eye open first or left direction first) or both were right first. One died prematurely. 3. Recordings were made from the visual cortex at some age after 4 months. Some recordings were made in the left cortex and some in the right. In all cases of monocular deprivation, the majority of cells were driven by the eye that was open last (i.e. open after 5 weeks of age). In all cases of directional deprivation, the majority of the cells preferred movement in the first direction of exposure (i.e. the direction before 5 weeks of age). 4. We conclude that the critical period for directional deprivation terminates earlier than the critical period for monocular deprivation.

Rat hippocampal interneurons express diverse subtypes of functional nicotinic acetylcholine receptors (nAChRs), including α7‐containing receptors that have properties unlike those expected for homomeric α7 nAChRs. We previously reported a strong correlation between expression of the α7 and of the β2 subunits in individual neurons. To explore whether co‐assembly of the α7 and β2 subunits might occur, these subunits were co‐expressed in Xenopus oocytes and the functional properties of heterologously expressed nAChRs were characterized by two‐electrode voltage clamp. Co‐expression of the β2 subunit, both wild‐type and mutant forms, with the α7 subunit significantly slowed the rate of nAChR desensitization and altered the pharmacological properties. Whereas ACh, carbachol and choline were full or near‐full agonists for homomeric α7 receptor channels, both carbachol and choline were only partial agonists in oocytes expressing both α7 and β2 subunits. In addition the EC₅₀ values for all three agonists significantly increased when the β2 subunit was co‐expressed with the α7 subunit. Co‐expression with the β2 subunit did not result in any significant change in the current‐voltage curve. Biochemical evidence for the co‐assembly of the α7 and β2 subunits was obtained by co‐immunoprecipitation of these subunits from transiently transfected human embryonic kidney (TSA201) cells. These data provide direct biophysical and molecular evidence that the nAChR α7 and β2 subunits co‐assemble to form a functional heteromeric nAChR with functional and pharmacological properties different from those of homomeric α7 channels. This co‐assembly may help to explain nAChR channel diversity in rat hippocampal interneurons, and perhaps in other areas of the nervous system.

Abstract  The hippocampal network comprises a large variety of locally connected GABAergic interneurons exerting a powerful control on network excitability and which are responsible for the oscillatory behaviour crucial for information processing. GABAergic interneurons receive an important cholinergic innervation from the medial septum‐diagonal band complex of the basal forebrain and are endowed with a variety of muscarinic and nicotinic acetylcholine receptors (mAChRs and nAChRs) that regulate their activity. Deficits in the cholinergic system lead to the impairment of high cognitive functions, which are particularly relevant in neurodegenerative pathologies such as Alzheimer's and Parkinson's diseases as well as in schizophrenia. Here, we highlight some recent advances in the mechanisms by which cholinergic signalling via nAChRs regulates local inhibitory circuits in the hippocampus, early in postnatal life and in adulthood. We also discuss recent findings concerning the functional role of nAChRs in controlling short‐ and long‐term modifications of synaptic efficacy. Insights into these processes may provide new targets for the therapeutic control of pathological conditions associated with cholinergic dysfunctions.

Our aim is to describe the acute effects of catecholamines/β‐adrenergic agonists on contraction of non‐fatigued skeletal muscle in animals and humans, and explain the mechanisms involved. Adrenaline/β‐agonists (0.1–30 μm) generally augment peak force across animal species (positive inotropic effect) and abbreviate relaxation of slow‐twitch muscles (positive lusitropic effect). A peak force reduction also occurs in slow‐twitch muscles in some conditions. β₂‐Adrenoceptor stimulation activates distinct cyclic AMP‐dependent protein kinases to phosphorylate multiple target proteins. β‐Agonists modulate sarcolemmal processes (increased resting membrane potential and action potential amplitude) via enhanced Na⁺–K⁺ pump and Na⁺–K⁺–2Cl⁻ cotransporter function, but this does not increase force. Myofibrillar Ca²⁺ sensitivity and maximum Ca²⁺‐activated force are unchanged. All force potentiation involves amplified myoplasmic Ca²⁺ transients consequent to increased Ca²⁺ release from sarcoplasmic reticulum (SR). This unequivocally requires phosphorylation of SR Ca²⁺ release channels/ryanodine receptors (RyR1) which sensitize the Ca²⁺‐induced Ca²⁺ release mechanism. Enhanced trans‐sarcolemmal Ca²⁺ influx through phosphorylated voltage‐activated Ca²⁺ channels contributes to force potentiation in diaphragm and amphibian muscle, but not mammalian limb muscle. Phosphorylation of phospholamban increases SR Ca²⁺ pump activity in slow‐twitch fibres but does not augment force; this process accelerates relaxation and may depress force. Greater Ca²⁺ loading of SR may assist force potentiation in fast‐twitch muscle. Some human studies show no significant force potentiation which appears to be related to the β‐agonist concentration used. Indeed high‐dose β‐agonists (∼0.1 μm) enhance SR Ca²⁺‐release rates, maximum voluntary contraction strength and peak Wingate power in trained humans. The combined findings can explain how adrenaline/β‐agonists influence muscle performance during exercise/stress in humans. image