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

Emergence of Selectivity to Looming Stimuli in a Spiking Network Model of the Optic Tectum

The neural circuits in the optic tectum of Xenopus tadpoles are selectively responsive to looming visual stimuli that resemble objects approaching the animal at a collision trajectory. This selectivity is required for adaptive collision avoidance behavior in this species, but its underlying mechanisms are not known. In particular, it is still unclear how the balance between the recurrent spontaneous network activity and the newly arriving sensory flow is set in this structure, and to what degree this balance is important for collision detection. Also, despite the clear indication for the presence of strong recurrent excitation and spontaneous activity, the exact topology of recurrent feedback circuits in the tectum remains elusive. In this study we take advantage of recently published detailed cell-level data from tadpole tectum to build an informed computational model of it, and investigate whether dynamic activation in excitatory recurrent retinotopic networks may on its own underlie collision detection. We consider several possible recurrent connectivity configurations and compare their performance for collision detection under different levels of spontaneous neural activity. We show that even in the absence of inhibition, a retinotopic network of quickly inactivating spiking neurons is naturally selective for looming stimuli, but this selectivity is not robust to neuronal noise, and is sensitive to the balance between direct and recurrent inputs. We also describe how homeostatic modulation of intrinsic properties of individual tectal cells can change selectivity thresholds in this network, and qualitatively verify our predictions in a behavioral experiment in freely swimming tadpoles.

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.

Visual Stimuli Evoked Action Potentials Trigger Rapidly Propagating Dendritic Calcium Transients in the Frog Optic Tectum Layer 6 Neurons

The superior colliculus in mammals or the optic tectum in amphibians is a major visual information processing center responsible for generation of orientating responses such as saccades in monkeys or prey catching avoidance behavior in frogs. The conserved structure function of the superior colliculus the optic tectum across distant species such as frogs, birds monkeys permits to draw rather general conclusions after studying a single species. We chose the frog optic tectum because we are able to perform whole-cell voltage-clamp recordings fluorescence imaging of tectal neurons while they respond to a visual stimulus. In the optic tectum of amphibians most visual information is processed by pear-shaped neurons possessing long dendritic branches, which receive the majority of synapses originating from the retinal ganglion cells. Since the first step of the retinal input integration is performed on these dendrites, it is important to know whether this integration is enhanced by active dendritic properties. We demonstrate that rapid calcium transients coinciding with the visual stimulus evoked action potentials in the somatic recordings can be readily detected up to the fine branches of these dendrites. These transients were blocked by calcium channel blockers nifedipine CdCl₂ indicating that calcium entered dendrites via voltage-activated L-type calcium channels. The high speed of calcium transient propagation, >300 μm in <10 ms, is consistent with the notion that action potentials, actively propagating along dendrites, open voltage-gated L-type calcium channels causing rapid calcium concentration transients in the dendrites. We conclude that such activation by somatic action potentials of the dendritic voltage gated calcium channels in the close vicinity to the synapses formed by axons of the retinal ganglion cells may facilitate visual information processing in the principal neurons of the frog optic tectum.

Excitation and inhibition in recurrent networks mediate collision avoidance in Xenopus tadpoles

Information processing in the vertebrate brain is thought to be mediated through distributed neural networks, but it is still unclear how sensory stimuli are encoded and detected by these networks, and what role synaptic inhibition plays in this process. Here we used a collision avoidance behavior in Xenopus tadpoles as a model for stimulus discrimination and recognition. We showed that the visual system of the tadpole is selective for behaviorally relevant looming stimuli, and that the detection of these stimuli first occurs in the optic tectum. By comparing visually guided behavior, optic nerve recordings, excitatory and inhibitory synaptic currents, and the spike output of tectal neurons, we showed that collision detection in the tadpole relies on the emergent properties of distributed recurrent networks within the tectum. We found that synaptic inhibition was temporally correlated with excitation, and did not actively sculpt stimulus selectivity, but rather it regulated the amount of integration between direct inputs from the retina and recurrent inputs from the tectum. Both pharmacological suppression and enhancement of synaptic inhibition disrupted emergent selectivity for looming stimuli. Taken together these findings suggested that, by regulating the amount of network activity, inhibition plays a critical role in maintaining selective sensitivity to behaviorally-relevant visual stimuli.

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.

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.

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.

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.