Intracellular activity of morphologically identified neurons of the grass frog's optic tectum in response to moving configurational visual stimuli

Intracellular and current source density analysis of pretectal input to the optic tectum of the frog

OBJECTIVE

Electrophysiological examination of the ipsilateral pretectotectal projection has proved that pretectal cells elicit strong suppressive responses to the ipsilateral tectum. However, the neural mechanisms underlying the contralateral pretectotectal prejection are still obscure. The present study aimed to examine the synaptic nature of pretectal nuclei and contralateral tectal cells, and to demonstrate the spatiotemporal pattern of neuronal activity in the 2 main brain structures.

METHODS

Intracellular recording and current source density (CSD) analysis were used to test the complexity of neuronal mechanism of pretectotectal information transfer.

RESULTS

The pretectal stimulation elicited only one type of response on the contralateral tectum, the inhibitory postsynaptic potential (IPSP). The majority of contra-induced IPSPs were assumed to be polysynaptically driven. In the CSD analysis, only one sink with short latency was observed in each profile. The ipsilateral projection produced a prominent monosynaptic sink in layer 8 of tectum. Recipient neurons were located in layers 6 and 7 of tectum. The result confirmed former findings from ipsilateral intracellular recordings.

CONCLUSION

These results suggest the following neuronal circuit: afferents from the pretectal nuclei broadly inhibit both tectal neuron, and since no second sink occurs in tectal layers, the pretectotectal excitatory afferents probably do not extend over the whole tectum, but are within limited state. The results of intracellular recording and CSD analysis further provide evidence of how pretectal afferent activity flows within the tectal laminae.

Neuroethology of releasing mechanisms: Prey-catching in toads

“Sign stimuli” elicit specific patterns of behavior when an organism's motivation is appropriate. In the toad, visually released prey-catching involves orienting toward the prey, approaching, fixating, and snapping. For these action patterns to be selected and released, the prey must be recognized and localized in space. Toads discriminate prey from nonprey by certain spatiotemporal stimulus features. The stimulus-response relations are mediated by innate releasing mechanisms (RMs) with recognition properties partly modifiable by experience. Striato-pretecto-tectal connectivity determines the RM's recognition and localization properties, whereas medialpallio-thalamo-tectal circuitry makes the system sensitive to changes in internal state and to prior history of exposure to stimuli. RMs encode the diverse stimulus conditions referring to the same prey object through different combinations of “specialized” tectal neurons, involving cells selectively tuned to prey features. The prey-selective neurons express the outcome of information processing in functional units consisting of interconnected cells. Excitatory and inhibitory interactions among feature-sensitive tectal and pretectal neurons specify the perceptual operations involved in distinguishing the prey from its background, selecting its features, and discriminating it from predators. Other connections indicate stimulus location. The results of these analyses are transmitted by specialized neurons projecting from the tectum to bulbar/spinal motor systems, providing a sensorimotor interface. Specific combinations of such projective neurons – mediating feature- and space-related messages – form “command releasing systems” that activate corresponding motor pattern generators for appropriate prey-catching action patterns.

Current source density analysis of contra- and ipsilateral isthmotectal connections of the frog

The nucleus isthmi (NI) of the frog receives input from the ipsilateral optic tectum and projects back to both optic tecta. After ablation of NI, frogs display no visually elicited prey-catching or threat avoidance behavior. Neural mechanisms that underlie the loss of such important behavior have not been solved. Electrophysiological examination of the contralateral isthmotectal projection has proved that it contributes to binocular vision. On the other hand, there are very few physiological investigations of the ipsilateral isthmotectal projection. In this study, current source density (CSD) analysis was applied to contra- and ipsilateral isthmotectal projections. The contralateral projection produced monosynaptic sinks in superficial layers and in layer 8. The results confirmed former findings obtained by single unit recordings. The ipsilateral projection elicited a prominent monosynaptic sink in layer 8. Recipient neurons were located in layers 6–7. These results, combined with those from the former intracellular study, led to the following neuronal circuit. Afferents from the ipsilateral NI inhibit non-efferent pear shaped neurons in the superficial layers, and strongly excite large ganglionic neurons projecting to the descending motor regions. Thus feedback to the output neurons strengthens the visually elicited responses.

Novel effects of CRF on visuomotor behavior and autonomic function in anuran amphibians

Administration of corticotropin-releasing factor (CRF) or exposure to stressors inhibits feeding in anuran amphibians. Since most amphibians rely on visual cues for feeding, these findings have led to the hypothesis that CRF may modulate visuomotor pathways involved in prey detection and predator avoidance. The inhibitory effects of CRF on feeding and prey capture are rapid, and do not appear to require the pituitary-adrenal axis in the short term. CRF neurons are located in key visuomotor processing areas of the anuran brain. Corticotropin-releasing factor also has potent stimulatory effects on sympathetic nervous system activity, a key regulatory system involved in both prey capture and predator avoidance. In this review I will discuss the unique model that amphibian species provide for investigating CRF effects on visual perception and visuomotor processing, and will summarize the data suggesting a role for CRF in visuomotor behavior and autonomic function in amphibians.

Postsynaptic potentials of tectal neurons evoked by electrical stimulation of the pretectal nuclei in bullfrogs (Rana catesbeiana)

Postsynaptic responses of the tectal cells to electrical stimulation of pretectal (Lpd/P) nuclei were intracellularly recorded in the bullfrog (Rana catesbeiana). The pretectal stimulation elicited mainly two types of responses in the ipsilateral tectum: an EPSP followed by an IPSP and a pure IPSP. The latter predominates in the tectal cells responding to ipsilateral pretectal stimulation. In a few cells, biphasic hyperpolarization appeared under stronger stimulus intensities. Only one type of response was found in the contralateral tectum, a pure IPSP. The antidromically invaded tecto-pretectal projecting cells were recorded in both tecta, which revealed reciprocal connections between the tectum and particular pretectal nuclei. This paper demonstrates the synaptic nature underlying pretectotectal information transfer. EPSPs with short latencies were concluded to be monosynaptic. Most IPSPs were generated through polysynaptic paths, but monosynaptic IPSPs were also recorded in both optic tecta. Nearly 98% of impaled tectal cells (except for intra-axonally recorded and antidromically invaded cells) showed inhibitory responses to pretectal stimulation. The results provide strong evidence that pretectal cells broadly inhibit tectal neurons as suggested by behavioral and extracellular recording studies.

Firing properties of frog tectal neurons in vitro

Whole-cell recordings from frog tectal slices revealed different types of neuronal firing patterns in response to prolonged current injection. The patterns included regular spiking without adaptation, accelerating firing, adapting spiking, repetitive bursting and phasic response with only one spike. The observed firing patterns are similar to those found in the mammalian superior colliculus. The frog tectum could be a useful preparation in elucidating the relationship between neuronal function and membrane properties.

Intracellular analysis of directional sensitivity of tectal neurons of the frog

The directional sensitivity of tectal neurons of the frog was examined by means of in vivo whole cell recording technique. Three kinds of stimulus were applied; (1) diffuse light 'on-off', (2) moving dark spot and (3) light spot given at one dimensional grid points. The first stimulus revealed whether or not retinal 'on-off' (R3) or 'off' (R4) fibers contribute to the response. As reported earlier, the following patterns were found for both light 'on' and light 'off': EPSPs only, IPSPs only or a combination of EPSPs and IPSPs. Four directionally sensitive neurons and three non-directionally sensitive neurons were found using the second stimulus. Using the third stimulus, responses at up to 11 positions separated by 2 degrees or 4 degrees were recorded. By measuring the amplitudes of 'on' and 'off' responses at different times, spatio-temporal receptive fields were composed. Two types of directional sensitivity were found. The response of the first type was composed of exclusively excitatory potentials, but the second type was composed of a combination of excitatory and inhibitory potentials. The spatio-temporal receptive field of the second type showed spatially separated excitatory and inhibitory regions with constant latencies. Such simple spatio-temporal receptive field organization was not found for directional sensitive neurons of the cat visual cortex. The spatio-temporal receptive field organization of the second type of directionally sensitive neuron in the present study is in agreement with striated receptive field found in some of the T5 neurons classified by extracellular unit recording [Frog Neurobiology (1976) 297].

Innate visual object recognition in vertebrates: some proposed pathways and mechanisms

Almost all vertebrates are capable of recognizing biologically relevant stimuli at or shortly after birth, and in some phylogenetically ancient species visual object recognition is exclusively innate. Extensive and detailed studies of the anuran visual system have resulted in the determination of the neural structures and pathways involved in innate prey and predator recognition in these species [Behav. Brain Sci. 10 (1987) 337; Comp. Biochem. Physiol. A 128 (2001) 417]. The structures involved include the optic tectum, pretectal nuclei and an area within the mesencephalic tegmentum. Here we investigate the structures and pathways involved in innate stimulus recognition in avian, rodent and primate species. We discuss innate stimulus preferences in maternal imprinting in chicks and argue that these preferences are due to innate visual recognition of conspecifics, entirely mediated by subtelencephalic structures. In rodent species, brainstem structures largely homologous to the components of the anuran subcortical visual system mediate innate visual object recognition. The primary components of the mammalian subcortical visual system are the superior colliculus, nucleus of the optic tract, anterior and posterior pretectal nuclei, nucleus of the posterior commissure, and an area within the mesopontine reticular formation that includes parts of the cuneiform, subcuneiform and pedunculopontine nuclei. We argue that in rodent species the innate sensory recognition systems function throughout ontogeny, acting in parallel with cortical sensory and recognition systems. In primates the structures involved in innate stimulus recognition are essentially the same as those in rodents, but overt innate recognition is only present in very early ontogeny, and after a transition period gives way to learned object recognition mediated by cortical structures. After the transition period, primate subcortical sensory systems still function to provide implicit innate stimulus recognition, and this recognition can still generate orienting, neuroendocrine and emotional responses to biologically relevant stimuli.

Neural modulation of visuomotor functions underlying prey-catching behaviour in anurans: perception, attention, motor performance, learning

The present review points out that visuomotor functions in anurans are modifiable and provides neurophysiological data which suggest modulatory forebrain functions. The retino-tecto/tegmento-bulbar/spinal serial processing streams are sufficient for stimulus-response mediation in prey-catching behaviour. Without its modulatory connections to forebrain structures, however, these processing streams cannot manage perceptual tasks, directed attention, learning performances, and motor skills. (1) Visual prey/non-prey discrimination is based on the interaction of this processing stream with the pretectal thalamus involving the neurotransmitter neuropeptide-Y. (2) Experiments applying the dopamine agonist apomorphine in combination with 2DG mapping and single neurone recording suggest that prey-catching strategies in terms of hunting prey and waiting for prey depend on dose dependent dopaminergic adjustments in the neural macronetwork in which retinal, pretecto-tectal, basal ganglionic, limbic, and mesolimbic structures participate. (3) Visual response properties of striatal efferent neurones support the concept that ventral striatum is involved in directed attention. (4) Various modulatory loops involving the ventral medial pallium modify prey-recognition in the course of visual or visual-olfactory learning (associative learning) or are responsible for stimulus-specific habituation (non-associative learning). (5) The circuits suggested to underlie modulatory forebrain functions are accentuated in standard schemes of the neural macronetwork. These provide concepts suitable for future decisive experiments.

Current source density analysis of ON/OFF channels in the frog optic tectum

In the vertebrate retina, it is well known that an ON/OFF dichotomy is present. In other words, ON-center and OFF-center cells participate in segregated pathways morphologically and physiologically. However, there is no doubt that integration of both channels is necessary to generate the complicated response properties of visual neurons in higher optic centers. So far, functional organization of the ON and OFF channels in the optic centers has not been demonstrated at the level of neuronal populations. In this review article, we summarize our experimental approaches to demonstrate functional organization of the ON and OFF channels using current source density (CSD) analysis in the frog optic tectum. First, we show that one-dimensional CSD analysis, assuming constant conductivity, is applicable in the tectal laminated structure. The CSD depth profile of a response to electrical stimulation of the optic tract is composed of three current sinks (A, B, and D) in the retinorecipient layers and two current sinks (C and E) below those layers. This result is in agreement with previous morphological and physiological findings, and shows that CSD analysis is very useful to demonstrate the flow of visual information processing. Second, CSD analysis of tectal responses evoked by diffuse light ON and OFF stimuli reveals obviously different distributions of synaptic activity in the laminar structure. Two or three current sinks (I, II and III) are generated in response to ON stimulation only in the retinorecipient layers, while up to six current sinks (IV, V, VI, VII, VIII and IX) to OFF stimulation throughout the tectal layers. Based on well known properties of retinal ganglion cells of the frog, possible neuronal mechanisms underlying each current sinks and their functional roles in visually guided behavior are considered.

Effects of choline and other nicotinic agonists on the tectum of juvenile and adult Xenopus frogs: a patch-clamp study

We have used anatomical methods and whole-cell patch-clamp recording to assess the distribution of nicotinic receptors in the tectum of Xenopus frogs and to measure effects of nicotinic ligands (carbachol, cytisine and nicotine) on glutamatergic spontaneous miniature excitatory postsynaptic currents. Our results confirm that retinotectal axons account for the majority of nicotinic receptors in the tectum and that nicotinic agonists exert presynaptic effects that increase the rate of transmitter release on to tectal cells. The nicotinic blockers mecamylamine and methyllycaconitine reduced responses to carbachol and cytisine. A small percentage of cells also showed postsynaptic responses. We have assessed whether there are developmental changes in the frequency of occurrence of spontaneous miniature excitatory postsynaptic currents. The first three months post-metamorphosis fall within the critical period for the dramatic plasticity displayed by binocular inputs during development in Xenopus. During this period, visual activity governs the formation of orderly maps relayed from the ipsilateral eye via the cholinergic projection from the nucleus isthmi to the tectum. In this study, we have found that critical-period tecta (two to 12 weeks postmetamorphosis) tend to have higher spontaneous activity than do older tecta (two to 69 weeks postmetamorphosis), and that nicotinic agonists increase that activity in both groups, with the result that the peak rates in response to nicotinic agonists are higher during the critical period than later. We also investigated the possible role of choline as an agonist of nicotinic receptors in the tectum. We have found that choline, as well as carbachol and cytisine, can cause a reversible increase in the rate of miniature excitatory postsynaptic currents. This result may help to explain how the isthmotectal projection, which accounts for the overwhelming majority of cholinergic input to the tectum, can exert effects on retinotectal terminals even though there are no morphologically identifiable synapses between the two populations. We have examined the morphology of cells filled with biocytin during the patch-clamp experiments, and we find that cells with dendrites in the stratum zonale, a layer with particularly dense input from the contralateral nucleus isthmi, have higher spontaneous activity than cells with dendrites that do not extend into that layer. Nicotinic agonists increased the activity recorded in both classes of cells. In addition, four pretectal cells were identified. Nicotinic agonists increased the rate of spontaneous activity recorded in that population. The results indicate that retinotectal transmission in the superior colliculus can be increased presynaptically by activity of the cholinergic projections of the nucleus isthmi. This modulation may be the basis for observations that blocking of cholinergic input disrupts the formation of topographic retinotectal projections. Moreover, the ability of choline to activate these receptors suggests that this metabolite of acetylcholine may permit paracrine activation of presynaptic receptors even though the tectum contains high acetylcholinesterase activity.

Morphology, axonal projection pattern, and response types of tectal neurons in plethodontid salamanders. II: intracellular recording and labeling experiments

In the plethodontid salamanders Plethodon jordani and P. glutinosus, the morphology and axonal projections of 140 tectal neurons and their responses to electrical optic nerve stimulation were determined by intracellular recording and biocytin labeling. Six types of neurons are distinguished morphologically. TO1 neurons have wide dendritic trees that arborize mainly in tectal layers 1 and 3; they project bilaterally to the tegmentum and contralaterally to the medulla oblongata. TO2 neurons have very wide dendritic trees that arborize mainly in layers 2 and 3; axons project bilaterally or unilaterally to the pretectum and thalamus and ipsilaterally to the medulla oblongata. TO3 neurons have very wide and flat dendritic trees confined to layers 3-5; some have the same axonal projection as TO2 neurons, whereas others have descending axons that reach only the level of the cerebellum. TO4 neurons have narrower dendritic trees that arborize in layers 2 and 3; they project to the ipsilateral pretectum, thalamus, and medulla oblongata. TO5 neurons have dendritic trees that arborize in layers 1 and 2 or 1-3 and project bilaterally or unilaterally to the pretectum and thalamus. TO-IN are interneurons, with a number of subtypes with respect to variations in dendritic arborization pattern. TO1-TO5 neurons generally have short latencies of 2-16 ms (average = 8.4 ms) at electrical optic nerve stimulation; first responses are always excitatory, often followed by inhibition. They are likely to be mono- or oligosynaptically driven by retinal afferents. TO-IN interneurons have long latencies of 20-80 ms (average = 38.6 ms) and appear to receive no direct retinal input. With their specific dendritic arborization, consequent dominant retinal input, specific axonal projections, the different types of tectal projection neurons constitute separate ascending and descending visual pathways. Hypotheses are presented regarding the nature of the information processed by these pathways.

Promotion of dendritic growth by CPG15, an activity-induced signaling molecule

Activity-independent and activity-dependent mechanisms work in concert to regulate neuronal growth, ensuring the formation of accurate synaptic connections. CPG15, a protein regulated by synaptic activity, functions as a cell-surface growth-promoting molecule in vivo. In Xenopus laevis, CPG15 enhanced dendritic arbor growth in projection neurons, with no effect on interneurons. CPG15 controlled growth of neighboring neurons through an intercellular signaling mechanism that requires its glycosylphosphatidylinositol link. CPG15 may represent a new class of activity-regulated, membrane-bound, growth-promoting proteins that permit exquisite spatial and temporal control of neuronal structure.

Serotonin modulates induced synaptic activity in the optic tectum of the frog

The extent to which retinal signals are modulated at central sites is unknown. We sought to determine the effects of serotonin, a neurotransmitter present in the retinorecipient layers of the frog tectum, on retinotectal transmission. Acute electrical stimulation delivered to the retinorecipient layer of optic tectum brain slices was used to model the activation of tectal neurons by visual inputs. This stimulation evoked either a monosynaptic or a polysynaptic current response in patch-clamped tectal neurons. External application of serotonin blocked both of these induced currents as did 5-carbotryptamine (5-CT), a nonselective agonist of 5-HT1 receptors. Alpha-methylserotonin, a nonselective agonist of 5-HT2 receptors, also blocked polysynaptic responses but was less effective than either serotonin or 5-CT in blocking monosynaptic ones. Lateral synaptic interactions between tectal cells, modeled by acute electrical stimulation in the main cellular layer of the tectum, were also blocked by serotonin, 5-CT or alpha-methylserotonin. The presented data suggest that endogenous serotonin may strongly affect visual signal processing by modulating synaptic transmission between both the retina and the tectum as well as between tectal neurons. This modulation is likely to be due, at least in part, to a demonstrated outward current induced by serotonin in a subpopulation of tectal cells.

Principal neuronal organization in the frog optic tectum revealed by a current source density analysis

In the frog optic tectum, the spatiotemporal pattern of neuronal activity evoked by electrical stimulation of the optic tract was examined by means of a current source density (CSD) analysis. The CSD depth profile was highly reproducible in different experiments. In all seven CSD profiles, three current sinks A, B, and D were observed in the retinorecipient layers. Four out of the seven profiles show additional two sinks C and E below the retinorecipient layers. Very small and short lasting sinks related to afferent fiber activities precede sinks A and B by about 1 ms, which could be accounted for by monosynaptic delay, in the corresponding depth region. The earliest prominent sink A at the bottom of the retinorecipient layers reflects only excitatory monosynaptic activities derived from R3 and/or R4 retinal ganglion cells. The second prominent sink B in the superficial retinorecipient layer is composed partly of excitatory monosynaptic activity from medium-sized myelinated optic fibers. It may involve excitatory monosynaptic activity from unmyelinated optic fibers and further polysynaptic activity. The fourth prominent sink D in the intermediate retinorecipient layer partially reflects excitatory monosynaptic activity derived from unmyelinated optic fibers. It may also involve further polysynaptic activity. In contrast with these three sinks, the third prominent sink C and fifth sink E exclusively reflect intratectal polysynaptic activity that has not been reported in any previous CSD studies in the frog optic tectum. These sinks almost overlap spatially in the tectal layer. We also measured the intratectal resistance changes and computed inhomogeneous CSD depth profiles to show that the results from homogeneous CSD computation assuming constant conductivity are valid for our present study. Finally, we compared the present results with previously reported CSD studies on the frog optic tectum and discuss consistencies and discrepancies among these experiments.

Synaptic connection patterns between frog retinal ganglion cells and tectal neurons revealed by whole-cell recordings in vivo

The synaptic connections between frog retinal ganglion cells and tectal neurons were examined by whole-cell recordings in vivo combined with nystatin perforated patch-clamp technique and pulse-triggered averaging analysis. In five tectal neurons, monosynaptic EPSPs from seven R3 retinal ganglion cells were observed. One tectal neuron was found to receive EPSPs from both R1/2 and R3 retinal ganglion cell simultaneously. Thus we could provide the first evidence for convergence of multiple types of retinal axons upon a single tectal neuron.

Distribution of GABA immunoreactivity in the optic tectum of the frog: a light and electron microscopic study

GABA immunoreactivity was studied in the optic tectum of the frog, Rana esculenta, by postembedding immunohistochemical methods at the light and electron microscopic levels. Nearly one-third of the total population of tectal cells appeared to be GABA-immunoreactive. The proportion of stained neurons was highest in layer 9 (61%), and they occurred less frequently in layers 7 (21%) and 6 (27%). Stained perikarya represented a population of small neurons with a diameter of 8-10 microns. Large cell bodies in layer 7 or at the top of layer 6, and cells of origin of the mesencephalic trigeminal tract in layer 2, were devoid of labelling. Axon terminals and dendrites displaying immunoreactivity for GABA were observed in all of the plexiform layers. On the basis of ultrastructural characteristics two types of GABA-positive axon terminals and two variations of GABA-immunoreactive dendrites were distinguished. Synaptic relations of GABA-immunoreactive and GABA-negative axons as well as dendrites were also studied. Besides a wide variety of axodendritic synapses, dendrodendritic synaptic appositions were also revealed. The results suggest that various inhibitory mechanisms are involved in tectal circuits, which have to be incorporated into future neuronal models concerning visual information processing in the optic tectum of the frog.

Postsynaptic potentials and morphology of tectal cells responding to electrical stimulation of the bullfrog nucleus isthmi

Postsynaptic responses of tectal cells in the bullfrog (Rana catesbeiana) were intracellularly recorded following electrical stimulation of the optic tract and the nucleus isthmi, and fluorescent dye, Lucifer yellow, was injected into some of the impaled cells to show their morphologies. Two main response types were found: The first type was an EPSP followed by an IPSP, and the second type was single IPSP. The first type predominates in cells responding to the optic tract stimulation and the second type prevails in cells responding to the isthmic stimulation. Fifteen cells stained with Lucifer yellow were localized in layer 6 (11 cells), layer 7 (1 cell), and layer 8 (3 cells). They were mainly identified as pear-shaped cells, large ganglionic cells, and stellate cells. Three injections demonstrated "dye-coupling," which labeled up to six cells following one injection. Comparisons of postsynaptic potentials with cellular morphologies suggested that the nucleus isthmi could directly excite large ganglionic neurons in layer 6. Synaptic mechanisms for strong isthmic inhibition on the tectal neurons remain unknown.

Modelling the response of auditory midbrain neurons in the grassfrog to temporally structured monaural stimuli

In a previous paper [Van Stokkum and Gielen, Hear. Res. 41, 71-86, 1989] a model was presented to describe the processing of monaural stimuli by the auditory periphery of the grassfrog. The main components of this model were: a middle ear filter, transduction and tuning of the haircell, short-term adaptation, action potential (event) generation with refractory properties, and spatiotemporal integration of converging inputs. The model is now extended to model auditory midbrain neurons as third order neurons. The mechanisms that generate selectivity for temporal characteristics of sound are adaptation, coincidence detection of second order neurons, temporal integration of third order neurons, and most important, event generation of the first, second and third order model neurons. Variation of the parameters of the model successfully reproduces the range of response patterns which have been obtained from eighth nerve fibres, dorsal medullary nucleus neurons, and torus semicircularis neurons without inhibition. With a single set of parameters the output of the model in response to a set of spectrally and temporally structured stimuli qualitatively resembles the responses of a single neuron to all these stimuli. In this way the responses to the different stimuli are synthesized into a framework, which functionally describes the neuron.