Demonstration of afferent connections of the forebrain in Emys orbicularis turtles by the peroxidase method

The transport of horseradish peroxidase (HRP) out of the injection site in the dorsal ventricular ridge was studied in turtles Emys orbicularis. Labeled cells in the forebrain were observed in the paleostriatum among fibers of the lateral forebrain bundle. In the thalamus most of cells containing the granular HRP reaction product were located in the n. rotundus, n. reuniens and perirotundal nuclei (n. dorso-medialis anterior, n. magnocellularis thalami, n (centralis) lateralis, n. dorso-medialis). Fewer labeled cells were revealed in the n. anterior and n. ventralis. The density of labeled cells in the majority of all thalamic nuclei increased if the HRP was extended from the dorsal ventricular ridge into the neostriatum and the pallial thickening with adjacent general cortex. HRP positive cells in the pretectal area, nuclei of the posterior commissura and mesencephalic ventro-lateral tegmentum were observed only in cases when the enzyme was diffused from the injection site into the neostriatum, while the HRP retrograde transport to n. geniculatus lateralis, pars dorsalis was revealed only when HRP was extended into the pallial thickening and adjacent general cortex. Ascending connections of the paleostriatum, thalamic nuclei and mesencephalic tegmentum with telencephalic structures, mainly with the dorsal ventricular ridge, were discussed.     

Corticalization of two divisions of the visual system during vertebrate evolution

Data on the evolution of the visual system in vertebrate phylogeny are described. Visual projections are demonstrated in the telencephalon of cyclostomata (lampreys). The existence of a retino-thalamo-telencephalic pathway is demonstrated in elasmobranchs (skates). Two visual pathways are present in amphibians (frogs) and reptiles (turtles): retino-thalamo-telencephalic and retino-tecto-thalamo-telencephalic, and these overlap partly at the thalamic level in the lateral geniculate nucleus and completely in the telencephalon. In turtles the earliest visual and tectal impulses relay on their way to the telencephalon in the lateral geniculate body, and later impulses relay in the nucleus rotundus. In mammals (rats) visual tecto-cortical connections are seen; judging from the latent period of potentials arising in the visual cortex in response to stimulation of the superior colliculi these connections have one synaptic relay in the thalamus. The much shorter latent periods of visual evoked potentials recorded in the tectum of the monkey than in turtles (under identical chronic experimental conditions) confirm the views of morphologists on the progressive development of the tectal division of the visual system in vertebrate phylogeny. It is concluded that corticalization of both divisions of the visual system, i.e., the existence of telencephalic representation, appears in the early stages of vertebrate evolution.     

Comparative electrophysiological characteristics of afferent representation in the cortical and striatal divisions of the turtle forebrain

In experiments on immobilized, lightly anesthetized turtles the presence of visual and somatic representation was established in the subcortical striatal division of the forebrain — the pallial thickening, the dorsal ventricular ridge, and the putamen. In their physiological characteristics they are similar to the corresponding representation in the general cortex. The absence of significant differences between the latent periods of cortical and striatal evoked potentials to flashes and to stimulation of the dorsal thalamus indicates that visual projection fibers (from the lateral geniculate body) terminate at both cortical and striatal levels. Differences in the distribution of latent periods of unit responses in the cortex to visual and thalamic stimulation are due to the presence of a rotundo-telencephalic visual channel, with direct connections with the striatal and polysynaptic connections with the general cortex, as well as the geniculo-telencephalic tract. Considerable differences between the latent periods of the evoked potentials and also between unit responses to electrodermal stimulation in the cortical and striatal structures indicate that somatic projection fibers relay in the striatum on their path to the general cortex. Consequently, the somatosensory system of turtles is less corticalized than the visual system. Comparison of the results described with those obtained by workers studying other vertebrates suggests that the afferent supply of the striatum may be reorganized in the transition from premammals to mammals.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 7, No. 2, pp. 184–193, March–April, 1973.     

Calcium-binding proteins and cytochrome oxidase activity in the turtle optic tectum with special reference to the tectofugal visual pathway

Using immunohistochemistry and a tracer technique we investigated the distribution in the optic tectum of turtles (Emys orbicularis and Testudo horsfieldi) of the calcium-binding proteins (CaBPr) parvalbumin (PV), calbindin (CB) and calretinin (CR) before and after labeling of the nucleus rotundus (Rot) with horseradish peroxidase. The optic tectum activity of the cytochrome oxidase (CO) was studied in parallel. In the principal link of the tectofugal visual pathway (central gray layer, SGC) in both chelonian species, the sparse PV-ir as well as CB- and CR-ir neurons were found significantly varying both in number and the intensity of immunoreactivity of their bodies and dendrites. In contrast, the superficial (SGFS) and deeper periventricular (SGP) tectal layers comprised numerous cells immunoreactive to all three CaBPr in different proportions. Only few retrogradely labeled tectorotundal SGC neurons expressed PV, CB or CR. The very large PV-ir neurons in SGC and SAC were not retrogradely labeled; morphologically they matched the efferent neurons with descending projections. SGC neurons of two chelonian species differed in the level of CO activity. Intense immunoreactivity to all three CaBPr and high CO activity were detected in both species in SGFS neuropil with some differences in sublaminar distribution patterns. The peculiarities of the CaBPr and CO activity distribution patterns in different segments of SGC neurons are discussed as related to the laminar organization of the turtle tectum and its retinal innervation. It is suggested that in the projection tectorotundal SGC neurons the CaBPr are concentrated mainly in their distal dendrites that contact retinal afferents in the superficial retinorecipient tectal layer.     

Distribution of calcium-binding proteins,parvalbumin and calbindin,in the midbrain auditory center (MLd) of a pigeon

Immunohistochemical distribution of calcium-binding proteins, parvalbumin (PV) and calbindin (CB), has been studied in the mesencephalic auditory center (MLd) of pigeon (Columba livia). In the central region of the MLd (core, ICC), an overlap in distribution of the PVand CB-immunopositive (ip) neurons and neuropil has been observed, with different patterns in the central and peripheral parts. In the peripheral region of the MLd (belt, ICS, and ICX), both neurons and neuropil contained only CB. A selective CB chemospecificity of the belt, ICS, and ICX is an evolutionary conserved feature characteristic of all avian species. Interspecies differences in the distribution of PV and CB immunoreactivity in the ICC are a result of adaptive functional specialization, which provides specific processing of different aspects of the auditory information.     

Components of the pigeon tectothalamic visual pathway, revealed with aid of study of cytochrome oxidase and immunoreactivity to calcium-binding proteins

Distribution of activity of cytochrome oxidase (CO) and immunoreactivity to parvalbumin (Pv) and calbindin (Cb) was studied in the optic tectum of the pigeon (Columba livia). In the first link of the tectofugal pathway in the central gray layer (SGC = layer 13), small amounts of the CO-active and Pv-immunoreactive (Pv-ir) cellular bodies were revealed in its internal part (sublayer 13b). Some of these neurons located along the SGC lower boundary had long dendritic processes ascending into the superficial tectum layer (SGF). In the retinorecipient SGF sublayers and particularly in neuropil of the sublayers 4 and 7, the high CO activity correlating with Pv-immunoreactivity was found. It is suggested that a great contribution to metabolic activity of these sublayers is made by the largely branching dendritic processes of Pv-ir neurons of sublayer 13b. The projectional neurons SGC located in its external part (sublayer 13a) were CO-inactive and contained Cb. They sent long dendrites into sublayer 5b; in its neuropil, the high density of Cb-immunoreactivity and a moderate CO activity were detected. Thus, the tectal link of the pigeon tectofugal visual channel consists of two components--the Pv-specific, highly metabolically active and the Cb-specific, metabolically less active ones that transduce visual information from different retinorecipient SGF sublayers. The absence of the significant amount of CO-positive bodies of projectional neurons in SGC can be due to that metabolically more active are their dendritic arborizations in the SGF sublayers.     

The level of metabolic activity (cytochrome oxidase) as an index of functional significance of tectofugal and thalamofugal channels of the reptilian visual system

Using histochemical determination of activity of the mitochondrial oxidative enzyme cytochrome oxidase (CO) in brain structures, metabolic activity both in turtles and in lizards has been shown to be higher in centers of the tectofugal channel (the tectal stratum griseum centrale, SGC; nucleus pretectalis ventralis, Ptv; thalamic nucleus rotundus, Rot; telencephalic visual area of the anterior dorsal ventricular ridge, Advr) than in the thalamofugal channel centers (the thalamic nucleus geniculatus lateralis pars dorsalis, GLd; cortex dorsolateralis, Cxdl; and pallial thickening, Path) of the visual system. Some interspecies differences in distribution of the CO activity in the tectal, thalamic, and telencephalic visual centers between terrestrial and pond turtles and lizards were revealed. The obtained data confirm the idea on the dominating role of the tectofugal channel over the thalamofugal channel of the visual system in information processing and organization of the day-to-day behavior of reptiles.     

Neurochemical characteristics of the turtle optic tectum: Comparison with other reptilian species and birds

Data on distribution of biologically active substances in the turtle optic tectum are compared with results of similar experiments on other reptilian as well as on avian species. In two turtle species (Testudo horsfield and Emys orbicularis), immunoreactivity to monoamines (5-HT and TH), NPY, as well as NADPH-d activity were similarly distributed in neuropil of the SGFS retinorecipient part and in that of the SGP/SAP periventricular layers. Immunoreactivity to neuropeptides SP and m-Enk was maximal in neuropil of the SGFS non-retinorecipient part. The periventricular layers were characterized by the abundant radial SP- and mENK-ir as well as the NADPH-d-positive neurons. Diffusely dispersed ChAT-ir elements and many ir fibers perpenducilar to the tectal surface were observed in the SGFS retinorecipient part; the SGFS non-retinorecipient part contained a dense plexus of thick ir fibers and diffusely distributed ir terminals. The GABA ir cells were the most numerous in the tectum; they were spread in all tectal layers. Thus, various biologically active substances located in superficial retinorecipient tectal sublayers could affect processing and transmission of information via ascending dendrites of neurons in deeper layers. The cells containing SP, m-Enk, and NADPH-d had laminar organization in SGP; via the system of ascending and descending axons, they are able to affect other structures within and outside of the optic tectum. Putative sources of tectal modulatory innervation are discussed. In all studied reptilian and avian species, the principal similarity is revealed in the neurochemical organization. Some differences might be explained by the level of tectal differentiation due to factors of phylogenetic evolution and/or adaptive specialization.