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In two turtle species--Emys orbicularis and Testudo horsfieldi--by the method of anterograde and retrograde traicing method at the light and electron microscopy level, the existence is proven of direct descending projections from the thalamic nucleus of the tectofugal visual system n. rotunds (Rot) to the optic tectum. After injection of tracers into Rot alone and into Rot with involvement of the tectothalamic tract (Trtth), occasional labeled fibers with varicosities and terminals are revealed predominantly in the deep sublayers of SGFS of the rostral optic tectum, while in the lower amount in other tectal layers. After the tracer injections into the optic tectum, a few retrogradely labeled neurons were found mainly in the Rot ventral parts and within Trtth. Their localization coincides with that of GABA-immunoreactive cells. Electron microscopy showed the existence of many retrogradely labeled dendrites throughout the whole Rot; a few labeled cell bodies were also present there, some of them being also GABA-immunoreactive. These results allow us to conclude about the existence of reciprocal connections between the optic tectum and Rot in turtles, these connections being able to affect processing of visual information in tectum. We suggest that reciprocity of tectothalamic connections might be the ancestral feature of the vertebrate brain; in the course of amniote evolution the functional significance of this feature can be decreased and even lost in parallel with a rise of the role of direct corticotectal projections.

The nucleus rotundus of the turtles Emys orbicularis and Testudo horsfieldi was analysed by axonal tracing methods and post-embedding GABA immunocytochemistry. After injections of horseradish peroxidase or biotinylated dextran amine into the optic tectum, electron microscopic observations showed that the vast majority of ipsilateral tectorotundal axon terminals were small in size, had smooth contours and contained small, round, densely packed synaptic vesicles. These terminals were GABA-immunonegative, often gathered in clusters, and established asymmetrical synaptic contacts with either small- or medium-sized GABA-negative dendritic profiles and with GABA-immunoreactive (GABA-ir) dendrites, which did not contain synaptic vesicles. Occasional GABA-ir-labelled axon terminals were observed; these may arise from the rare GABAergic neurons in the central tectal layer, or from neurons in the ventral pretectal nucleus, which projects both to the optic tectum and nucleus rotundus. In addition to tracer-labelled axon terminals, we observed both GABA-negative and GABA-ir cell bodies and dendrites also labelled by the tracer. No GABA-ir presynaptic dendritic profiles containing synaptic vesicles were observed. The existence in reptiles of reciprocal connections between the nucleus rotundus and the optic tectum as a phylogenetically ancient feedback system is discussed.

Extrinsic sources of calcium-binding proteins involved in immunoreactive innervation of the visual thalamic nuclei Rot and GLd in turtles (Testudo horsfieldi and Emys orbicularis) were studied using HRP tracing method and immunohistochemistry. In 1.5-4.5 months after monocular enucleation calbindin (Calb)-, parvalbumin (Parv)- and calretinin (Calr)-labeling was found in fragments of degenerated retinal fibers in the contralateral optic tract and in some retinorecipient structures (optic tectum, GLd and GLv). Changes in GLd were detected in its neuropil part. in 2.0-3.5 months after unilateral ablation of tectum and pretectum, the densities of Parv-, Calb- and Aclr-immunoreactivity terminals and fibers were diminisched in the ipsilateral n. Rot, with the maximum effect seen in Parv. Following HRP injection into the visual thalamus (Rot and GLd), retrogradely labeled cells with Parv label only, were revealed in the ventrothalamic nucleus Enta, pretectal nucleus Ptv, and in all types of Ca-binding proteins (CaBPr) in separately labeled cells of the optic tectum. Thus, it has been shown that thalamic visual centers in turtles have multiple extrinsic cells, which serve as sources of CaBPr projections. The present data suggest that organization of CaBPr inputs to visual thalamus in reptiles (turtle) and higher amniotes are fundamentally similar.

An immunocytochemical and histochemical study was undertaken of the torus semicircularis and nucleus reuniens, the mesencephalic and diencephalic auditory centres, in two chelonian species, Testudo horsfieldi and Emys orbicularis. The nucleus centralis of the torus semicircularis receives few 5-HT-, TH-, substance P-, and menkephalin-immunoreactive fibres and terminals, in marked contrast to the external nucleus laminaris of the torus semicircularis, in which 5-HT-, TH-, substance P-, and menkephalin-immunoreactive elements and cell bodies show a laminar distribution. Dense NPY-positive terminal-like profiles and cell bodies were observed in both the nuclei centralis and laminaris, and many NADPH-d-positive cell bodies were observed in the cell layers of the latter. In the nucleus reuniens, the distribution of 5-HT-, TH-, substance P-, and menkephalin-immunolabelling resembles that seen in the torus semicircularis, but at a lower density. The dorsorostral regions of the nucleus reuniens, as in the nucleus centralis, is insignificantly labelled, in contrast to the ventrocaudal regions in which labelled elements abound. NPY-positive elements are uniformly distributed throughout the nucleus, but no labelled cell bodies were observed. NADPH-d-positive fibres and terminals were observed in both dorsal and ventral regions of the nucleus reuniens, but the few labelled cell bodies to be observed were located in the peripheral regions of the nucleus. These findings are discussed in terms of the evolution of the core-and-belt organisation of sensory nuclei observed in other vertebrate species.

In immobilized, weakly anesthetized with chloralose turtles Emys orbicularis the focus of evoked potentials and the largest number of single unit responses to the stimulation of the mesencephalic tegmentum were revealed in the ventral thalamic region (a heterosensory zone with dominating somatic modality). An incomplete separation of latero- and mediotegmental projections was observed in dorsal and ventral subdivisions of the ventral thalamic region, respectively. The tegmental stimulation coinciding or preceding the electrical stimulation of the limb skin or light flash produced mainly a suppressing effect on somatic reactions of single units, the influence on visual reactions being much less significant. A possibility to compare tegmento-thalamic systems in reptiles and mammals is discussed.

It turtles, Testudo horsfieldi (Gray) connections of anterior dorsomedial and dorsolateral thalamic nuclei have been investigated by means of horseradish peroxidase, injected ionophoretically. Retrogradely labelled neurons are predominantly revealed ipsilaterally in the cerebral structures belonging to the limbic system: in the forebrain--basal parts of the hemisphere, septum, adjoining nucleus, nuclei of the anterior and hippocampal commissures, hippocampal cortex, preoptic area; in the diencephalon--in the subthalamus (suprapeduncular nucleus), in some hypothalamic structures (para- and periventricular nuclei, posterior nucleus, lateral hypothalamic area, mamillary complex); in the brain stem--ventral tegmental area, superior nucleus of the suture. Less vast connections are with nonlimbic cerebral formations: projections to the striatum, afferents from the laminar nucleus of the acoustic torus, nuclei of the posterior commissure. Similarity and difference of the nuclei investigated in the turtles with the thalamic anterior nuclei in lizards, with the anterior and intralaminar nuclei in Mammalia are discussed. An idea is suggested on functional heterogeneity of the anterior nuclei in reptiles and on their role for ensuring limbic functions at the thalamic level.

Acute electrophysiological experiments on lizards Ophisaurus apodus show that stimulation of both thalamic n. dorsolateralis anterior and medial forebrain bundle elicited short latency responses in the hippocampal cortex resembling in topic and patterns responses evoked by sensory stimuli in the same structure. Ample lesion of n. dorsolateralis anterior and of medial forebrain bundle suppressed up to a complete block conduction of sensory (visual, somatic, audiovibratory) and tectal impulses to the hippocampal cortex. A conclusion is made that n. dorsolateralis anterior and medial forebrain bundle are the main, if not the only route for conducting these sensory impulses to the hippocampal cortex in lizards.

Experiments with unanesthetized tubocurarine-immobilized turtles show that the auditory representation is localized in the n. reuniens which contains monomodal (auditory) and bimodal (auditory and somatosensory) units. The somatosensory system is represented wider, overlapping the auditory system projections in the lateral parts of n. reuniens. The focus of the somatosensory representation is localized in the n. ventralis where monomodal somatic units were recorded. A predominance of contralateral somatic projections is revealed as well. A range of optimal frequencies was 200-400 Hz. It is shown that the thalamic neurons have both wide and limited receptive fields to auditory and somatosensory stimulations.