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

Circuits for Action and Cognition: A View from the Superior Colliculus

The superior colliculus is one of the most well-studied structures in the brain, and with each new report, its proposed role in behavior seems to increase in complexity. Forty years of evidence show that the colliculus is critical for reorienting an organism toward objects of interest. In monkeys, this involves saccadic eye movements. Recent work in the monkey colliculus and in the homologous optic tectum of the bird extends our understanding of the role of the colliculus in higher mental functions, such as attention and decision making. In this review, we highlight some of these recent results, as well as those capitalizing on circuit-based methodologies using transgenic mice models, to understand the contribution of the colliculus to attention and decision making. The wealth of information we have about the colliculus, together with new tools, provides a unique opportunity to obtain a detailed accounting of the neurons, circuits, and computations that underlie complex behavior.

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 α3β2, α4β2, α2β4, α6β2, 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 DHβE, and agonists, RJR-2403 and choline, of the nicotinic acetylcholine receptors of the α3β2, α4β2, α2β4, α6β2 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 α3β2, α4β2, α2β4, α6β2 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.

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.