Pre & postsynaptic tuning of action potential timing by spontaneous GABAergic activity

Frequency and timing of action potential discharge are key elements for coding and transfer of information between neurons. The nature and location of the synaptic contacts, the biophysical parameters of the receptor-operated channels and their kinetics of activation are major determinants of the firing behaviour of each individual neuron. Ultimately the intrinsic excitability of each neuron determines the input-output function. Here we evaluate the influence of spontaneous GABAergic synaptic activity on the timing of action potentials in Layer 2/3 pyramidal neurones in acute brain slices from the somatosensory cortex of young rats. Somatic dynamic current injection to mimic synaptic input events was employed, together with a simple computational model that reproduce subthreshold membrane properties. Besides the well-documented control of neuronal excitability, spontaneous background GABAergic activity has a major detrimental effect on spike timing. In fact, GABA(A) receptors tune the relationship between the excitability and fidelity of pyramidal neurons via a postsynaptic (the reversal potential for GABA(A) activity) and a presynaptic (the frequency of spontaneous activity) mechanism. GABAergic activity can decrease or increase the excitability of pyramidal neurones, depending on the difference between the reversal potential for GABA(A) receptors and the threshold for action potential. In contrast, spike time jitter can only be increased proportionally to the difference between these two membrane potentials. Changes in excitability by background GABAergic activity can therefore only be associated with deterioration of the reliability of spike timing.     

Convergence of mechanosensory inputs onto neuromodulatory serotonergic neurons in the leech

By the frequency-dependent release of serotonin, Retzius neurons in the leech modulate diverse behavioral responses of the animal. However, little is known about how their firing pattern is produced. Here we have analyzed the effects of mechanical stimulation of the skin and intracellular stimulation of mechanosensory neurons on the electrical activity of Retzius neurons. We recorded the electrical activity of neurons in ganglia attached to their corresponding skin segment by segmental nerve roots, or in isolated ganglia. Mechanosensory stimulation of the skin induced excitatory synaptic potentials (EPSPs) and action potentials in both Retzius neurons in a ganglion. The frequency and duration of responses depended on the strength and duration of the skin stimulation. Retzius cells responded after T and P cells, but before N cells, and their sustained responses correlated with the activity of P cells. Trains of five impulses at 10 Hz in every individual T, P, or N cell in isolated ganglia produced EPSPs and action potentials in Retzius neurons. Responses to T cell stimulation appeared after the first impulse. In contrast, the responses to P or N cell stimulation appeared after two or more presynaptic impulses and facilitated afterward. The polysynaptic nature of all the synaptic inputs was shown by blocking them with a high calcium/magnesium external solution. The rise time distribution of EPSPs produced by the different mechanosensory neurons suggested that several interneurons participate in this pathway. Our results suggest that sensory stimulation provides a mechanism for regulating serotonin-mediated modulation in the leech.     

Postactivation inhibition of spontaneously active neurosecretory neurons in the medicinal leech

Spontaneously active neurosecretory neurons in vertebrate and invertebrate nervous systems share similarities in firing frequencies, spike shapes, inhibition by the transmitters they themselves release and postactivation inhibition, an intensity-dependent period of suppressed spontaneous generation of action potentials following phases of high-frequency activity. High-frequency activation of spontaneously active serotonin-containing Retzius cells in isolated ganglia of the leech Hirudo medicinalis induced prolonged membrane hyperpolarisations causing periods of postactivation inhibition of up to 33 s. The duration of the inhibitory periods was directly related to both the number and rate of spikes during activation and was inversely proportional to a cell’s spontaneous firing frequency. The periods of postactivation inhibition remained unaffected by both serotonin depletion through repeated injections of 5,7-dihydroxytryptamine and suppressing the afterhyperpolarisation following each action potential with tetraethylammonium (TEA), iberiotoxin or charybdotoxin, suggesting that neither autoinhibition by synaptic release of serotonin nor calcium-activated potassium channels contribute to the underlying mechanism. In contrast, the postactivation inhibitory period was significantly affected both by differential electrical stimulation of the same Retzius cells via microelectrodes filled with molar concentrations of either Na⁺-acetate or K⁺-acetate, and by partial inhibition of Na⁺/K⁺-ATPase with ouabain. Thus, postactivation inhibition in Retzius cells results from prolonged hyperpolarising activity of Na⁺/K⁺-ATPase stimulated by the accumulation of cytosolic Na⁺ during phases of high-frequency spike activity.     

Activity-dependent suppression of spontaneous spike generation in the Retzius neurons of the leech Hirudo medicinalis L.

We report on factors affecting the spontaneous firing pattern of the identified serotonin-containing Retzius neurons of the medicinal leech. Increased firing activity induced by intracellular current injection is followed by a ‘post-stimulus-depression’ (PSD) without spiking for up to 23 s. PSD duration depends both on the duration and the amplitude of the injected current and correlates inversely with the spontaneous spiking activity. In contrast to serotonin-containing neurons in mammals, serotonin release from the Retzius cells presumably does not mediate the observed spike suppression in a self-inhibitory manner since robust PSD persists after synaptic isolation. Moreover, single additional spikes elicited at specific delays after spontaneously occurring action potentials are sufficient to significantly alter the firing pattern. Since sub-threshold current injections do not affect the ongoing spiking pattern and PSD persists in synaptically isolated preparations our data suggest that PSD reflects an endogenous and ‘spike-dependent’ mechanism controlling the spiking activity of Retzius cells in a use-dependent way.     

Segmental specialization of neuronal connectivity in the leech

1. Every segmental ganglion of the leech Hirudo medicinalis contains two serotonergic Retzius cells. However, Retzius cells in the two segmental ganglia associated with reproductive function are morphologically distinct from Retzius cells elsewhere. This suggested that these Retzius cells might be physiologically distinct as well. 2. The degree of electrical coupling between Retzius cells distinguishes the reproductive Retzius cells; all Retzius cells are coupled in a non-rectifying manner, but reproductive Retzius cells are less strongly coupled. 3. Retzius cells in standard ganglia depolarize following swim motor pattern initiation or mechanosensory stimulation while Retzius cells in reproductive ganglia either do not respond or hyperpolarize. 4. In standard Retzius cells the depolarizing response caused by pressure mechanosensory neurons has fixed latency and one-to-one correspondence between the mechanosensory neuron action potentials and Retzius cell EPSPs. However, the latency is longer than for most known monosynaptic connections in the leech. 5. Raising the concentration of divalent cations in the bathing solution to increase thresholds abolishes the mechanosensory neuron-evoked EPSP in standard Retzius cells. This suggests that generation of action potentials in an interneuron is required for production of the EPSP, and therefore that the pathway from mechanosensory neuron to Retzius cell is polysynaptic. 6. P cells in reproductive segments have opposite effects on reproductive Retzius cells and standard Retzius cells in adjacent ganglia. Thus the difference in the pathway from P to Retzius is not localized specifically in the P cell, but elsewhere in the pathway, possibly in the type of receptor expressed by the Retzius cells.     

Effects of Nimodipine,Calcium-Free Medium and Colchicine on Electrogenesis of Neurosecretory Retzius Cells in the Leech <Emphasis Type="Italic">Hirudo medicinalis</Emphasis>

Retzius neuron (RN) in the medicinal leech is known to function as a typical neurosecretory cell. This study addresses the relationship between its two functions, neural and paracrine. It was shown that synaptic activation of RN at 710-Hz frequencies causes the neurophysiological process of habituation. Experiments conducted in calcium-free solution and in that containing nimodipine and colchicine (which block somatic exocytosis of serotonin under these experimental conditions) demonstrate alterations in the electrophysiological characteristics of RN: the rate of spontaneous impulse activity (IA), action potential (AP) amplitude, and synaptic stimulation threshold. Under these conditions, RN generates AP to every stimulus even at frequencies of 7 to 10 Hz. Thus, while somatic exocytosis of serotonin is blocked, habituation does not develop. It is suggested that habituation of RN to high-frequency synaptic stimulation is mediated by the concurrent effects of two factors — stimulatory (via synaptic activation) and inhibitory (via autoinhibition).     

Inspiratory neuron activity in the ventrolateral medulla of the dog

The purpose of this study was to describe the distribution and activity pattern of respiratory neurons located in the ventrolateral medulla (VLM) of the dog. Spike activity of 129 respiratory neurons was recorded in 23 ketamine-anesthetized spontaneously breathing dogs. Pontamine blue dye was used to mark the location of each neuron. Most VLM neurons displaying respiratory related spike patterns were located in a column related closely to ambigual and retroambigual nuclei. Both inspiratory and expiratory neurons were present with inspiratory units being grouped more rostrally. The predominant inspiratory neuron firing pattern was "late" inspiratory, although eight "early" types were located. All expiratory firing patterns were the late expiratory variety. Each neuron burst pattern was characterized by determining burst duration (BD), spikes per burst (S/B), peak frequency (PF), time to peak frequency (TPF), rate of rise to peak frequency (PF/TPF), and mean frequency. CO2-induced minute ventilation increases were associated with decreases in BD and TPF and increases in PF, S/B, and PF/TPF. In 11 experiments the relative influences of vagotomy and tracheal occlusion on late inspiratory units were compared. Tracheal occlusion increased late inspiratory BD and S/B but did not alter PF/TPF. Vagotomy increased BD and S/B beyond those obtained by tracheal occlusion and, in some neurons, decreased the PF/TPF. We conclude that the location of respiratory units in the VLM of the dog is similar to that in other species, the discharge pattern of VLM respiratory units is similar to those in cat VLM, and vagotomy and tracheal occlusion affect discharge patterns differently.     

Sensitivity of the resting sodium conductivity of leech Retzius nerve cells to tetrodotoxin

Simultaneously, the effect of sodium-free medium and tetrodotoxin (3 X 10(-8) M/ml) were investigated on some passive electrophysiological properties of leech Retzius nerve cells. Complete replacement of Na+ with Tris or addition of tetrodotoxin to the leech Ringer was followed by an increase of input resistance in contrast to the cell-to-cell interaction which was not affected by such a procedure. At the same time tetrodotoxin was not able to block repetitive spike activity. The data imply the existence of two types of Na+ channel in leech Retzius nerve cells.     

Neuronal activity of the monkey striatum points at the integration of successive actions into functional blocks

In order to study the role of the striatum in generation of multistage behavior, the spike activity of 148 cells was recorded in the monkey brain putamen. Two kinds of neuron responses were observed. Phasic response involved activity during only one stage of the behavior program, and tonic response involved activity during more than one sequential stage. The tonic responses were recorded in 132 neurons out of 148, 11 neurons responding only as tonic. Other 121 cells show under different conditions both tonic and phasic responses. Beginnings and ends of "tonic" responses as a rule corresponded to the start and completion of the nearest behavioral aim. The obtained data suggest that the neuron activity of striatum is related not only to the control of individual movements but also to the whole structure of behavior.     

Calcium influx into dendrites of the leech Retzius neuron evoked by 5-hydroxytryptamine

5-Hydroxytryptamine (5-HT) is a ubiquitous neurotransmitter and neuromodulator that affects neural circuits and behaviours in vertebrates and invertebrates. In the present study, we have investigated 5-HT-induced Ca(2+) transients in subcellular compartments of Retzius neurons in the leech central nervous system using confocal laser scanning microscopy, and studied the effect of 5-HT on the electrical coupling between the Retzius neurons. Bath application of 5-HT (50mM) induced a Ca(2+) transient in axon, dendrites and cell body of the Retzius neuron. This Ca(2+) transient was significantly faster and larger in dendrites than in axon and cell body, and was half-maximal at a 5-HT concentration of 5-12mM. The Ca(2+) transient was suppressed in the absence of extracellular Ca(2+) and by methysergide (100mM), a non-specific antagonist of metabotropic 5-HT receptors, and was strongly reduced by bath application of the Ca(2+) channel blocker Co(2+) (2mM). Injection of the non-hydrolysable GTP analogue GTPgammaS increased and prolonged the dendritic 5-HT-induced Ca(2+) transient. The non-selective protein kinase inhibitor H7 (100mM) and the adenylate cyclase inhibitor SQ22536 (500 mM) did not affect the Ca(2+) transient, and the membrane-permeable cAMP analogue dibutyryl-cAMP (500 mM) did not mimic the effect of 5-HT application. 5-HT reduced the apparent electrical coupling between the two Retzius neurons, whereas suppression of the Ca(2+) influx by removal of external Ca(2+) improved the transmission of action potentials at the electrical synapses which are located between the dendrites of the adjacent Retzius neurons. The results indicate that 5-HT induces a Ca(2+) influx through calcium channels located primarily in the dendrites, and presumably activated by a G protein-coupled 5-HT receptor. The dendritic Ca(2+) increase appears to modulate the excitability of, and the synchronization between, the two Retzius neurons.     

Serotonin release from the neuronal cell body and its long-lasting effects on the nervous system

Serotonin, a modulator of multiple functions in the nervous system, is released predominantly extrasynaptically from neuronal cell bodies, axons and dendrites. This paper describes how serotonin is released from cell bodies of Retzius neurons in the central nervous system (CNS) of the leech, and how it affects neighbouring glia and neurons. The large Retzius neurons contain serotonin packed in electrodense vesicles. Electrical stimulation with 10 impulses at 1 Hz fails to evoke exocytosis from the cell body, but the same number of impulses at 20 Hz promotes exocytosis via a multistep process. Calcium entry into the neuron triggers calcium-induced calcium release, which activates the transport of vesicle clusters to the plasma membrane. Exocytosis occurs there for several minutes. Serotonin that has been released activates autoreceptors that induce an inositol trisphosphate-dependent calcium increase, which produces further exocytosis. This positive feedback loop subsides when the last vesicles in the cluster fuse and calcium returns to basal levels. Serotonin released from the cell body is taken up by glia and released elsewhere in the CNS. Synchronous bursts of neuronal electrical activity appear minutes later and continue for hours. In this way, a brief train of impulses is translated into a long-term modulation in the nervous system.     

Analysis of spontaneous unit activity of the cat caudate nucleus

Spontaneous unit activity recorded extracellularly from the caudate nucleus in acute experiments on cats was analyzed. A graph of the sliding mean frequency, an interspike interval histogram, correlogram, intensity function, and histogram of correlation between adjacent intervals were plotted for the spontaneous activity of each neuron. The spontaneous activity of neurons of the caudate nucleus showed considerable variability in time and its mean frequency varied for different neurons from 0.5 to 20 spikes/sec. Depending on the temporal pattern of the spikes and also on the statistical indices, spontaneous unit activity in the caudate nucleus was conventionally divided into two types: single and grouped. A switch from one type of activity to the other was observed for the same neuron. On the basis of the data as a whole it is impossible to regard the spontaneous unit activity of the caudate nucleus as a simple random (Poissonian) spike train.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 9, No. 4, pp. 369–376, July–August, 1977.     

Initiation of swimming activity by trigger neurons in the leech subesophageal ganglion

The aim of this study was to identify neurons in the subesophageal ganglion of the medicinal leech which initiate swimming activity and to determine their output connections. We found two bilaterally symmetrical pairs of interneurons, Tr1 and Tr2, located in the first division of the subesophageal ganglion which initiate swimming activity in the isolated nervous system when depolarized with brief (1-3 s) current pulses. Tr1 and Tr2 are considered trigger neurons because elicited swimming episodes outlast the stimulus duration, and because the length of elicited swim episodes is nearly independent of the intensity with which Tr1 and Tr2 are stimulated. Tr1 and Tr2 have similar morphologies. The neurites of both cells cross contralaterally in the subesophageal ganglion, project posteriorly, and exit the subesophageal ganglion in the contralateral connective. The axons of Tr1 and Tr2 extend as far posterior as segmental ganglion 18 of the ventral nerve cord. Tr1 provides direct excitatory drive to three groups of segmental neurons which are capable of initiating swimming: swim-initiating interneurons (cells 204 and 205), serotonin-containing interneurons (cells 61 and 21), and the serotonergic Retzius cells. In addition, all Retzius cells in the subesophageal ganglion are excited directly by Tr1. These three groups of neurons are excited even if Tr1 stimulation is subthreshold for swim initiation. In contrast to Tr1, Tr2 stimulation evokes transient inhibition in swim-initiating and serotonin-containing interneurons, and has little immediate effect on Retzius cells. In addition, Tr2 indirectly inhibits several oscillator neurons, including cells 208, 33, and 60. When Tr1 is stimulated during a swimming episode the swim period decreases for several cycles, while stimulation of Tr2 during swimming episodes reliably resets the ongoing swimming rhythm. Our findings indicate that Tr1 and Tr2 are trigger neurons which initiate swimming activity by different pathways. These neurons also have functional interactions with the swim oscillator network since either Tr1 or Tr2 stimulation during swimming can modulate the ongoing swimming rhythm.     

The effect of substance P on the neuronal activity of the antinociceptive system

The experiments on rats showed that the 1 micrograms substance P injection to dorsal raphe nucleus caused prolonged (24 hours of study) analgetic effect--it enhances the reaction latent period to thermal nociceptive stimulation, intensifies the background impulse activity, rises the middle frequency of neuron discharges and creates high-frequency neurons as well as the neurons with burst impulse activity. The supposition is being confirmed that the mechanism of antinociceptive structures activation leads to analgesia caused by substance P.     

Electrical activity of neurons in the cat cortex during cooling

Changes of the activity of cortical neurons were studied in the posterior crucial gyrus and in the middle parts of the suprasylvian and ectosylvian gyri on cooling the brain to 18°C and below. In exact experiments it was noted that cooling the cortex to 18.8–21.8° causes a complete cessation of neuron activity. The kinetics of the change of activity under these conditions follows a definite order: first an increase of the frequency of spike discharges is observed (31–27°), then a decrease of their amplitude (at 25–22°), and finally a complete disappearance of neuron activity (at 21.8–18.8°). Discontinuation of the cooling leads to restoration of the activity of the nerve cells in inverse order: low-amplitude high-frequency discharges manifest (at 23–26°), the amplitude of the spikes increases (at 29–31°) and then the initial activity is restored (at 31–32°). The decrease of neuron activity depends on the rate of temperature drop in the cortex. The faster the cortex is cooled, the lower is the temperature at which the neurons cease to function. And conversely, slow cooling of the cortex causes an inactivation of the spike potentials at a higher temperature.S. M. Kirov Gorki State Medical Institute. Translated from Neirofiziologiya, Vol. 2, No. 1, pp. 59–63, January–February, 1970.     

Ionic mechanisms underlying spontaneous CA1 neuronal firing in Ca2+-free solution

Hippocampal CA1 neurons exposed to zero-[Ca(2+)] solutions can generate periodic spontaneous synchronized activity in the absence of synaptic function. Experiments using hippocampal slices showed that, after exposure to zero-[Ca(2+)](0) solution, CA1 pyramidal cells depolarized 5-10 mV and started firing spontaneous action potentials. Spontaneous single neuron activity appeared in singlets or was grouped into bursts of two or three action potentials. A 16-compartment, 23-variable cable model of a CA1 pyramidal neuron was developed to study mechanisms of spontaneous neuronal bursting in a calcium-free extracellular solution. In the model, five active currents (a fast sodium current, a persistent sodium current, an A-type transient potassium current, a delayed rectifier potassium current, and a muscarinic potassium current) are included in the somatic compartment. The model simulates the spontaneous bursting behavior of neurons in calcium-free solutions. The mechanisms underlying several aspects of bursting are studied, including the generation of triplet bursts, spike duration, burst termination, after-depolarization behavior, and the prolonged inactive period between bursts. We show that the small persistent sodium current can play a key role in spontaneous CA1 activity in zero-calcium solutions. In particular, it is necessary for the generation of an after-depolarizing potential and prolongs both individual bursts and the interburst interval.     

Adaptation-dependent synchronous activity contributes to receptive field size change of bullfrog retinal ganglion cell

Nearby retinal ganglion cells of similar functional subtype have a tendency to discharge spikes in synchrony. The synchronized activity is involved in encoding some aspects of visual input. On the other hand, neurons always continuously adjust their activities in adaptation to some features of visual stimulation, including mean ambient light, contrast level, etc. Previous studies on adaptation were primarily focused on single neuronal activity, however, it is also intriguing to investigate the adaptation process in population neuronal activities. In the present study, by using multi-electrode recording system, we simultaneously recorded spike discharges from a group of dimming detectors (OFF-sustained type ganglion cells) in bullfrog retina. The changes in receptive field properties and synchronization strength during contrast adaptation were analyzed. It was found that, when perfused using normal Ringer's solution, single neuronal receptive field size was reduced during contrast adaptation, which was accompanied by weakening in synchronization strength between adjacent neurons' activities. When dopamine (1 μM) was applied, the adaptation-related receptive field area shrinkage and synchronization weakening were both eliminated. The activation of D1 receptor was involved in the adaptation-related modulation of synchronization and receptive field. Our results thus suggest that the size of single neuron's receptive field is positively related to the strength of its synchronized activity with its neighboring neurons, and the dopaminergic pathway is responsible for the modulation of receptive field property and synchronous activity of the ganglion cells during the adaptation process.     

Protective action of serotonin on changes in the ultrastructure of the Retzius neuron induced by acetylcholine

A combined action of acetylcholine and serotonin is demonstrated to produce, in ultrastructure of the Retzius neuron of the leech, changes similar to those resulted from synaptic activation. Nevertheless, acetylcholine alone produces much deeper morphological shifts. A conclusion is made that serotonin not only retards impulse activity of the neuron, but it "slows down" development of rather great changes in its ultrastructure.     

Synchronization and sensitivity enhancement of the Hodgkin-Huxley neurons due to inhibitory inputs

Recent experimental results imply that inhibitory postsynaptic potentials can play a functional role in realizing synchronization of neuronal firing in the brain. In order to examine the relation between inhibition and synchronous firing of neurons theoretically, we analyze possible effects of synchronization and sensitivity enhancement caused by inhibitory inputs to neurons with a biologically realistic model of the Hodgkin-Huxley equations. The result shows that, after an inhibitory spike, the firing probability of a single postsynaptic neuron exposed to random excitatory background activity oscillates with time. The oscillation of the firing probability can be related to synchronous firing of neurons receiving an inhibitory spike simultaneously. Further, we show that when an inhibitory spike input precedes an excitatory spike input, the presence of such preceding inhibition raises the firing probability peak of the neuron after the excitatory input. The result indicates that an inhibitory spike input can enhance the sensitivity of the postsynaptic neuron to the following excitatory spike input. Two neural network models based on these effects on postsynaptic neurons caused by inhibitory inputs are proposed to demonstrate possible mechanisms of detecting particular spatiotemporal spike patterns. Received: 15 April 1999 /Accepted in revised form: 25 November 1999     

Responses of caudate neurons to direct electrical stimulation of the medial geniculate body in cats

Activity of 124 neurons of the caudate nucleus during stimulation of the medial geniculate by infrequent (0.5 Hz) square electrical stimuli 0.3 msec in duration and ranging in intensity from 50 µA to 1 mA was investigated extracellularly in chronic experiments on cats. Responses were recorded from 54 neurons (43%). The main types of neuronal responses were phasic activation in the form of a single spike or spike discharge, initial activation followed by inhibition, and primary inhibition of unit activity. Responses of excitatory character predominated (81% of all responses). Their latent period varied in different neurons from 2.7 to 64 msec. Latent periods of responses of the same neuron always showed great variability, so that all the responses recorded could be considered to be orthodromic. The mode of the histogram of latent periods of excitatory responses lay between 9 and 12 msec. The latent period of the inhibitory response varied from 12 to 130 msec, and in most neurons with this type of response it was 40–60 msec. An increase in the strength of stimulation was accompanied by an increase in the regularity of the responses, an increase in the number of spikes in them, and shortening of their latent period. The character and structure of the response of the same caudate neuron to stimulation of the medial geniculate body and to presentation of clicks were usually identical. The latent period of responses to clicks was longer. The particular features of the functional connection of the medial geniculate body with the caudate nucleus as a polymodal nonspecific structure of the forebrain are discussed.