Retinal co-mediator acetylcholine evokes muscarinic inhibition of recurrent excitation in frog tectum column.

Acetylcholine receptors contribute to the control of neuronal and neuronal network activity from insects to humans. We have investigated the action of acetylcholine receptors in the optic tectum of Rana temporaria (common frog). Our previous studies have demonstrated that acetylcholine activates presynaptic nicotinic receptors, when released into the frog optic tectum as a co-mediator during firing of a single retinal ganglion cell, and causes: a) potentiation of retinotectal synaptic transmission, and b) facilitation of transition of the tectum column to a higher level of activity. In the present study we have shown that endogenous acetylcholine also activates muscarinic receptors, leading to a delayed inhibition of recurrent excitatory synaptic transmission in the tectum column. The delay of muscarinic inhibition was evaluated to be of ∼80ms, with an extent of inhibition of ∼2 times. The inhibition of the recurrent excitation determines transition of the tectum column back to its resting state, giving a functional sense for the inhibition.

Nicotinic potentiation of frog retinotectal transmission in tectum layer F by α3ß2, α4ß2, α2ß4, α6ß2, or α7 acetylcholine receptor subtypes.

OBJECTIVE: The aim of the study was to explore the effect of semi-specific antagonists and agonists of the nicotinic acetylcholine receptors on the paired-pulse facilitation and nicotinic tonic and phasic potentiation of the frog retinotectal synaptic transmission. MATERIALS AND METHODS: The experiments were performed in vivo on adult frogs, Rana temporaria. An individual retina ganglion cell (or its retinotectal fiber) was stimulated by current pulses delivered through multichannel stimulating electrode positioned on the retina. Responses to a discharge of a single retinal ganglion cell were recorded in the tectum by an extracellular carbon-fiber microelectrode positioned in the terminal arborization of the retinotectal fiber in the tectum layer F. The effect of the antagonists and agonists of the nicotinic acetylcholine receptors on the tectal responses has been tested. RESULTS: We found that the antagonists, MLA and DHßE, and agonists, RJR-2403 and choline, of the nicotinic acetylcholine receptors of the α3ß2, α4ß2, α2ß4, α6ß2 or α7 subtypes have had no effect on the phasic and tonic potentiation of the retinotectal transmission. The paired-pulse facilitation of the retinotectal transmission was not appreciably affected by the antagonists, but the choline, agonist of the α7 subtype receptor, has significantly decreased the paired-pulse facilitation. CONCLUSIONS: The tonic and phasic potentiation of the retinotectal transmission in the tectum layer F were not mediated by the receptors of α3ß2, α4ß2, α2ß4, α6ß2 or α7 subtype. The results suggest that presynaptic nicotinic acetylcholine receptors of the frog optic fibers are different from those of the mammalian optic fibers.

Phasic nicotinic potentiation of frog retinotectal transmission facilitates eliciting of higher activity level of the tectum column.

Nicotinic acetylcholine receptors contribute to the mediation of cholinergic role in attention, vigilance, orienting and detection of behavioral significant stimuli. We have recently demonstrated an increase of the intrinsic recurrent excitatory activity of the tectum column caused by the phasic (after-burst) nicotinic potentiation of a frog single axon retinotectal transmission to the tectum layer F. We have shown in the present study that the phasic nicotinic potentiation facilitates eliciting of higher activity level of the tectum column featured by generation of output signals from the tectum column. Since these signals can lead to an escape from danger reactions, a functional significance of nicotinic modulation of the neural network has been demonstrated. The phasic nicotinic potentiation that facilitates eliciting of higher activity level of the tectum column can be considered as a mechanism of vigilance and cue detection at the level of small neural network.

Inhibition of dendritic L-type calcium current by memantine in frog tectum.

UNLABELLED: The aim of the study was to explore the effects of memantine on responses elicited in the frog tectum by the bursts of spikes of moderate strength of a single retina ganglion cell and to gain an insight about the effect of memantine on the L-type Ca(2+) current. MATERIAL AND METHODS: The experiments were performed in vivo on adult frogs (Rana temporaria). An individual retina ganglion cell (or its retinotectal fiber) was stimulated by current pulses delivered through a multichannel stimulating electrode positioned on the retina. Responses to the discharge of a single retinal ganglion cell were recorded in the tectum by an extracellular carbon-fiber microelectrode positioned in the terminal arborization of the retinotectal fiber in the tectum layer F. The solution of memantine (1-amino-3,5-dimethyladamantane) hydrochloride (30 or 45 µM) was applied onto the surface of the tectum by perfusion at a rate of 0.4 mL/min. RESULTS: Memantine (30-45 µM) largely inhibited the L-type Ca(2+) channel-mediated slow negative wave and late discharges seen in the tectum responses without any effect on fast synaptic retinotectal transmission. CONCLUSIONS: Our results suggest that the neuroprotective effect of memantine could arise not only through the inhibition of the NMDA receptor current but also through the suppression of the L-type Ca(2+) current.

Phasic nicotinic potentiation of frog retinotectal transmission enhances intrinsic activity of tectum column.

It is well established that cholinergic modulation of functioning of neuronal networks is common in the central nervous system at all scales from neuronal columns to large nuclei. It is involved in various attentional, cognitive and behavioral performances. We have recently demonstrated that a frog retinotectal transmission exhibits after-burst (phasic) potentiation caused by activation of presynaptic nicotinic receptors. We show in the present study that the phasic potentiation of the retinotectal transmission enhances activity of the tectum column by increasing dendritic L-type calcium current, and excitation of recurrent pear-shaped neurons of the column. This enhancement lasts for tens of seconds and may provide the mechanism of animal alertness.

Frog retinal ganglion cells projecting to the tectum layer F release acetylcholine as co-mediator.

Neurons may release more than one classical neurotransmitter (co-mediator). It has been demonstrated in a recent study that a burst of action potentials in frog retina ganglion cells induces an after-burst increase (phasic potentiation) of the retinotectal transmission that lasts tens of seconds. This increase is mediated by presynaptic nicotinic acetylcholine receptors that are activated by the endogenous acetylcholine released during the burst of action potentials of the retinotectal fiber. The objective of the present study was to find out the origin of acetylcholine release. We show that reduction of the retinotectal transmission to the subthreshold level by application of moderate concentrations of kynurenic acid or CNQX had no effect on the phasic nicotinic potentiation of the retinotectal transmission. This demonstrates that the retinotectal terminals are the source of acetylcholine - responsible for the phasic potentiation of retinotectal transmission. The acetylcholine is thus co-released with glutamate.

Presynaptic nicotinic potentiation of a frog retinotectal transmission evoked by discharge of a single retina ganglion cell.

It was demonstrated in our previous studies of the frog retinotectal transmission that retinotectal synaptic potentials are enhanced by a factor of 1.5 due to the tonic presynaptic nicotinic potentiation, caused by the ambient level of the acetylcholine in the frog tectum. Furthermore, the results of those studies have indicated that the mechanism of the nicotinic potentiation is only partially exploited, because the application of the cholinergic agonist had increased the retinotectal transmission more than 2 times above the level of the tonic potentiation. The purpose of the present study was to explore this additional potentiation. We have shown that: (1) Bursts of 4-10 action potentials of a frog retina ganglion cell gave rise to an increase (phasic potentiation) of the retinotectal transmission 1.4-2.2 times, depending on the burst strength, that lasted tens of seconds. (2) This increase has been mediated through the presynaptic nicotinic acetylcholine receptors activated by the endogenous acetylcholine released into the tectum during relatively strong bursts of the retina ganglion cell. (3) Two types of the nicotinic acetylcholine receptors are co-localized in the presynaptic terminals of the individual retinotectal input to the tectum layer F--high-affinity (tonic) and low-affinity (phasic) nicotinic receptors.

Muscarinic inhibition of recurrent glutamatergic excitation in frog tectum column prevents NMDA receptor activation on efferent neuron.

It is widely recognized that neuronal network activity can be modulated via activation of nicotinic and muscarinic acetylcholine receptors located pre- and postsynaptically. It was established in our earlier study that the activation of presynaptic nicotinic receptors greatly facilitates the retinotectal glutamatergic transmission. In the present study, we have determined a transmitter of tectal recurrent excitation and explored the effects of muscarinic acetylcholine receptor activation on the recurrent excitation and the activity of frog tectum column in vivo. Discharge of a single retinal ganglion cell was elicited by a minimal electrical stimulation of the retina. Evoked activity of the tectum column was recorded using the carbon-fiber microelectrode inserted into the tectum layer F. We found the following: 1. The recurrent excitation in the tectum column was not affected by d-tubocurarine (10 µM) and was greatly depressed by the kynurenic acid (500 µM), demonstrating glutamatergic nature of the recurrent excitation. 2. The glutamatergic recurrent excitation was largely reduced by carbamylcholine (100 µM) and oxotremorine-M (10 µM), demonstrating that the activation of muscarinic receptors, located, presumably, on the presynaptic terminals of recurrent pear-shaped neurons, inhibits the recurrent excitation in the tectum column. 3. The muscarinic inhibition of glutamatergic recurrent transmission had critical influence on the activity of the tectum column, preventing the generation of an output signal through suppression of the NMDA receptor activation and establishing necessary conditions for returning of the network to its resting state.

L-type Ca2+ current in frog tectal recurrent neurons determines the NMDA receptor activation on efferent neuron.

The activity patterns of a neuronal network originate from the intrinsic properties and synaptic interactions of the constituent neurons. Our recent studies support this view, showing that the discharge of a single frog retina ganglion cell brings an elementary neuronal network of the tectum (tectum column) to a suprathreshold activity of two distinct levels that are related to the activation of the slow L-type calcium current in dendrites of the recurrent pear-shaped neurons (lower level) and the NMDA receptors in neurons (higher level) of the tectum column. We show in the present study that the dendritic slow L-type calcium current is necessary for the NMDA receptor activation in the tectum column. A small decrease of this current prevents the NMDA receptor activation and, hence, the transition of the network to the higher activity level, at which the efferent neuron of the network fires. So, the activity of the frog tectum column can be effectively controlled through the intrinsic properties of the recurrent pear-shaped neurons of the column.

Single retinal ganglion cell evokes the activation of L-type Ca(2+)-mediated slow inward current in frog tectal pear-shaped neurons.

The dendrites of neurons from many regions of the nervous system contain voltage-sensitive channels that generate persistent inward currents. We have recently suggested that a slow negative wave (sNW), extracellularly observed in the frog tectum during the burst discharge of a single retinal ganglion cell, can be generated as a result of the persistent inward current in dendrites of tectal pear-shaped neurons. The aim of this study is to substantiate this hypothesis by simulation using a quasi-reconstructed pear-shaped neuron with bistable dendrites and experimental investigation of the sNW. In the experiments, the discharge of a single retinal ganglion cell was elicited by an electrical stimulation of the retina. The evoked electrical activity of the tectum was recorded using a carbon-fiber microelectrode inserted into tectum layer F. We found the following: (1) Slow inward current or plateau potential in bistable dendrites is reflected in the extracellular space as a sNW. (2) The sNW evoked by the burst discharge of a single retinal ganglion cell projecting to frog tectum layer F is generated by the activation of L-type calcium channels in the dendrites of pear-shaped neurons. (3) A few pear-shaped neurons may be suprathresholdly excited during the development of the sNW.

Single retinal changing contrast (third) detector elicits NMDA receptor response and higher activity level of frog tectum neuron network.

The present study was designed to explore whether a discharge of a certain type of frog retinal ganglion cell [likely changing contrast (third) detector] can evoke NMDA response in frog tectum neurons and higher level of activity of tectal neuron network. Discharge of a single retinal ganglion cell was elicited by electrical stimulation of the retina. Evoked electrical activity of the tectum was recorded by the carbon-fiber microelectrode brought into the optic fiber layer F. We show that: (1) strong discharge of a frog individual retinal ganglion cell (third detector) has evoked NMDA response of tectal neurons and higher level of tectal neuron network activity characterized by prominent suprathreshold excitation of efferent neurons. Consequently, the firing of only one retinal ganglion cell (third detector) could lead to the activation of the tectobulbospinal tract and motor reaction. (2) The excitation of a retinotectal fiber of the first kind (axon of third detector) gave rise to the same effects as activation of a retinotectal fiber of the second kind (axon of fifth detector): the suprathreshold excitation of recurrent and efferent tectal neurons, the slow depolarizing potential (seen as the sNW), and the NMDA receptor activation were observed. However, stronger excitation (longer bursts of action potentials) was needed to evoke those effects in the considered case of the retinotectal input of the first kind. This difference could be attributed to the lower quantal size of neurotransmitter release in synapses of the retinotectal input of the first than second kind.

Non-NMDA and NMDA receptors are involved in suprathreshold excitation of network of frog tectal neurons by a single retinal ganglion cell.

NMDA receptors play an important functional role in the neuron excitability and plasticity. The conditions and consequences of their activation are of interest for many neuroscientists. This investigation was designed to explore an activation of the NMDA receptors of frog tectal neurons in vivo by a burst of spikes of individual retinotectal fiber. We show that: (1) the NMDA receptors of tectal neurons can be activated by an intense burst discharge of an individual ganglion cell (likely darkness detector) at physiological conditions. (2) Activation of the NMDA receptors is achieved, primarily, due to temporal summation and frequency facilitation of the fast non-NMDA synaptic potentials. However, it is very likely that spatial summation of the fast retinotectal synaptic potentials with excitatory synaptic potentials of recurrent connections contributes to elicit the NMDA response. (3) The activation of NMDA receptors causes a higher level of activity of tectal neuron network. The suprathreshold excitation of efferent tectal neurons is characteristic for this level. Therefore, the burst discharge of only single retinal ganglion cell can activate the tectobulbospinal tract and lead to the motor reaction.

Suprathreshold excitation of network of frog tectal neurons by discharging of single retina moving-edge detector.

OBJECTIVE: It has been shown that discharge of single darkness detector in the frog retina can lead to suprathreshold excitation of the tectal neurons. The present study was designed to explore whether a suprathreshold excitation of frog tectal neurons can be elicited by the discharge of single moving-edge detector. MATERIAL AND METHODS: The discharge of a single retina ganglion cell was elicited by the electrical stimulation. The evoked electrical activity of the tectal neurons was recorded by the carbon-fiber microelectrode brought into the optic fiber layer F. RESULTS: The obtained data have suggested that a discharge of a single retinal moving-edge detector elicits a suprathreshold excitation of tectal neurons. The suprathreshold excitation of the tectal neurons is achieved due to the frequency facilitation of the fast retinotectal synaptic potentials. CONCLUSIONS: Results of the present study suggest that activation of moving-edge detector gives rise to the same effects as the activation of the darkness one. However, the stronger excitation (the longer volleys of action potentials) for the moving-edge detector is needed to evoke suprathreshold excitation of tectal neurons compared to the darkness one. This difference could be caused by a lower quantal size of neurotransmitter release in synapses of the retinotectal input from the moving-edge detector than from the darkness one.

Suprathreshold excitation of frog tectal neurons by short spike trains of single retinal ganglion cell.

It has been established that coincident inputs from multiple presynaptic axons are required to achieve a suprathreshold level of excitation for the most of central neurons. The present study, however, was designed to determine whether a train of spikes of an individual retinal ganglion cell (that is, input from a single presynaptic axon) targeting a frog tectum layer F could evoke suprathreshold excitation of tectal neurons. The lungs of immobilized frog were artificially ventilated during experiments. An individual ganglion cell was electrically stimulated in the retina through a multi-channel electrode. Responses evoked in the tectum by the stimulation were recorded extracellularly from a terminal arborization of the retinotectal fiber using the carbon-fiber microelectrode. Negative and negative-positive spikes (referred to as first type population responses) and polyphasic spikes followed by excitatory synaptic potentials (referred to as second type population responses) were observed in the recordings of retinotectal activity. Usually, the population responses have ensued after the frequency facilitated first and/or second testing individual retinotectal synaptic potential and disappeared in a threshold manner with a reduction of retinotectal transmission by an application of kynurenic acid. These observations have suggested that the population responses were a consequence of a suprathreshold excitation of tectal neurons and, therefore, could serve as the sign for such an excitation. Recordings have also demonstrated that sources of the first type population responses (likely, the hillocks of axons or somas of postsynaptic neurons) lie deeper than the optic fiber layer F of the tectum, whereas sources of the second type population responses (likely, axon terminal arborizations of these postsynaptic neurons) are scattered throughout the optic fiber layers. The findings have suggested: 1) a short train of action potentials of an individual retinal ganglion cell (likely darkness, also known as 5th, detector) can excite tectal neurons to suprathreshold level; 2) tectal and perhaps, nucleus isthmi neurons that make up recurrent connection circuits to the optic fiber layers of the tectum are also activated; 3) a suprathreshold level for an individual retinotectal input is achieved primarily due to the frequency facilitation of synaptic potentials; and 4) an artificial ventilation of the lungs of immobilized frog favors the eliciting of a suprathreshold excitation of tectal neurons, demonstrating that the ventilation certainly improves the physiological condition of a frog.