Ford F. Ebner

A COMPARISON OF PRIMITIVE FOREBRAIN ORGANIZATION IN METATHERIAN AND EUTHERIAN MAMMALS

Fossil evidence suggests that both the marsupial opossum and the placental insectivore existed in the Cretaceous period about 100 million years ago (Simpson, 1945). The North American opossum (Didelphis marsupialis virginiana) is an omnivorous, nocturnal, semiarboreal animal that exists as one of the least modified mammals. Insectivores are the only known placental mammals which were present during the Cretaceous period. One existing group, the hedgehogs (our results are from the Pakistanian hedgehog, Paruechinus hypornelus) , are omnivorous, nocturnal animals that also retain some of the earliest mammalian features. A comparison of forebrain organization in the opossum and the hedgehog is of special interest because these two animals are as close, perhaps, as any living form to the archetypical mammals that first showed the multilayered cellular pattern typical of mammalian neocortex. Comparison of the detailed forebrain connections of the opossum and hedgehog with reptiles on the one hand and with more specialized mammals on the other may be one useful approach to clarifying the origin and development of neocortical structures. For the past several years, we have been studying the neocortical organization of the opossum in considerable detail with an emphasis on the connections of the thalamus, basal telencephalic nuclei, and neocortex. More recently, we have been comparing the opossum forebrain with similar connectional and cellular patterns in the hedgehog and turtle forebain in an effort to demonstrate connectional homologies among these existing forms of primitive eutherian, metatherian, and reptilian brains. The techniques employed in all of these studies have been the Nissl and GolgiCox stains for cell configurations and the Nauta-Gygax (1954) and Fink-Heimer ( 1967) modification of the Nauta technique for defining fiber connections.

Visual cortex in a reptile, the turtle (Pseudem ys scripta and Chrysem ys picta)

The afferent and efferent connections of general cortex were studied in two species of turtle,Pseudemys scripta and Chrysemys picta. The thalamic distribution of labeled cells following cortical applications of horseradish peroxidase (HRP) indicated that at least 3 dorsal thalamic nuclei, the dorsal lateral geniculate nucleus, nucleus ventralis and the nucleus dorsolateralis anterior, project to general cortex. When cortical applications of HRP were combined with intraocular injections of tritiated proline in the same animal, autoradiographically labeled retinal terminations were found among the dendrites of lateral geniculate neurons containing the HRP reaction product. These experiments demonstrated that the dorsal lateral geniculate nucleus not only projects to general cortex, but also receives retinal input. Thus, gereral cortex in the turtle is a target of a visual pathway which relays in the dorsal thalamus. Cortical lesions produced anterograde degeneration in the same thalamic nuclei which the HRP experiments demonstrated project to cortex, thereby indicating that the dorsal thalamus and general cortex have reciprocal connections in the turtle. These same experiments with cortical lesions demonstrated that general cortex also sends projections to the optic tectum and tegmentum of the midbrain. These afferent and efferent connections of general cortex in the turtle are compared with the connections of general cortex in other reptilian groups and with those of neocortex in mammals.

A Note on the Termination of Commissural Fibers in the Neocortex

The general arrangement of fibers in the corpus callosum shows a strongly symmetrical (homotopic) pattern characteristic of a true commissure. Connections between non-symmetrical (heterotopic) areas also have been demonstrated for many regions, both by anatomical studies using the Marchi technique13,14,16,24,26,29 and by electrophysiological methods6,18. The contribution of recently developed silver techniques, notably the suppressive Nauta-Gygax method23, has been to emphasize the extent of non-homotopic connections3,8,9,19,21. Some clearly delineated areas of neocortex appear not to receive any commissural fibers1,2,6,8,9,17,18,21,28; outstanding examples of such areas are ‘visual cortex’ or area 17, and the hand and foot portions of somatosensory cortex. It is generally presumed that each cortical area contributes fibers to the corpus callosum, and the sparse histological evidence available indicates that the cells of origin reside in cortical layers III through VI5,12,25.

Connections of he visual cortex in the hedgehog (Paraechinus hypomelas). II. Corticocortical projections

Visual thalamocortical projections were studied in the Pakistani hedgehog with anterograde degeneration techniques following large aspiration lesions and discrete electrolytic lesions of visual thalamic nuclei. After 3‐ to 9‐day survival periods, the brains were processed and stained with the Fink‐Heimer technique, and examined with the light microscope.

Horizontal long-term potentiation of responses in rat somatosensory cortex

The search for mechanisms in neocortex that change synaptic efficacy and produce associative learning through activity-dependent processes has focused on the role of glutamate receptors of the N-methyl-D-aspartate (NMDA) type. NMDA receptor activation is necessary for the induction of long-term potentiation (LTP) in hippocampus and in neocortex. The effect of NMDA receptor activation is modulated in several ways, including Mg2+ block of the NMDA-dependent channel which prevents Ca2+ entry until neurons become partially depolarized. We report that when NMDA receptor activation is facilitated by lowering the extracellular [Mg2+] in the bathing medium, a low-frequency train presented in layer VI induces potentiated responses throughout a wide horizontal extent of layer II/III in neocortical slices. The response amplitudes potentiated by 34-200% over baseline values depending on the intensity of the repetitive conditioning stimulus and the distance of the recording site from the stimulus. At the same time that pre-existing evoked responses were potentiated, horizontal spread of activity in layer II/III was facilitated resulting in responses appearing at sites more than 1 mm from the stimulus. This enhanced transmission of responses persisted for greater than 2 h, and its induction was prevented by selective NMDA receptor antagonists. The results show that the horizontal spread of activity can be increased by altering the conditions of the stimulus presentation. We conclude that the mechanisms supporting LTP could determine the area of neocortex that is activated by a sensory input.

Two Different Types of Thalamocortical Projections to a Single Cortical Area in Mammals; pp. 156–169

There are two distinct types of thalamocortical projections to parietal cortex in the opossum and the hedgehog. Both projections arise from the posterior portion of the ventral nuclear group of the do

Development of cholinergic markers in mouse forebrain. II. Muscarinic receptor binding in cortex

The distribution of muscarinic receptor sites throughout the ontogeny of cerebral cortex in the BALB/c mouse have been labeled, placing special emphasis on binding site development in parietal neocortex and hippocampus. We describe a new procedure for the use of [3H]propylbenzilylcholine mustard as a muscarinic cholinergic ligand in an in vitro binding assay on brain sections. Muscarinic binding sites, as visualized by autoradiography, can be seen in cortex as early as embryonic day 18. They achieve maximal labeling density and adult distribution in neocortex by the end of the first postnatal month. The adult distribution pattern in hippocampus is reached by the second postnatal week, but the maximal density of label is not achieved until 4 weeks of age. Changes in the receptor binding pattern are illustrated at 5 different ages between birth and adulthood. We conclude that muscarinic cholinergic receptors develop late in cortical ontogeny as do other cholinergic markers. The distribution pattern of muscarinic binding sites in mouse cortex is puzzling because it does not correspond to the reported distribution of cells physiologically responsive to applied acetylcholine. These results are compared to the onset of choline acetyltransferase activity and acetylcholine esterase staining. The ontogenesis of the cortical cholinergic system is compared with other features of general cortical morphogenesis.

Thalamocortical projections from the mediodorsal nucleus in the virginia opossum

Abstract The projections from the mediodorsal nucleus to cortex were studied in the opossum by the Fink-Heimer stain for anterograde axonal degeneration. The results show that lesions of the mediodorsal nucleus produce fiber degeneration in the head of the caudate nucleus and in layers IV and I of preorbital cortex; this cortical area lies on the dorsolateral convexity of the hemisphere just rostral to the orbital sulcus. Preorbital cortex shows distinct lamination, with a well developed stratum of large pyramidal cells in layer V. In contrast to somatic sensory cortex, layer VI neurons in preorbital cortex are mostly stellate cells and do not possess prominent apical dendrites that traverse the overlying cortical layers.

Basal forebrain lesions facilitate adult host fiber ingrowth into neocortical transplants

The ability of mature host thalamic neurons to innervate embryonic (E19) cortex when implanted into the cortex of adult hosts was compared in normal and basal forebrain lesioned mice. The ingrowth of mature horseradish peroxidase-labeled thalamic axons into the transplants is facilitated by prior basal forebrain lesions. We discuss the possible reasons for the lesion-induced enhancement of axonal ingrowth, including the possibility that the enhanced ingrowth of thalamic fiber systems may be related to the loss of cortical innervation by extrathalamic brainstem inputs, especially cholinergic afferent fibers. The results support the interpretation that extrathalamic inputs to cortex play a modulatory role in regulating the growth and connections of specific sensory fiber systems during brain responses to injury.

Induction of high frequency activity in the somatosensory thalamus of rats in vivo results in long-term potentiation of responses in SI cortex

SummaryExtracellular single-unit techniques were employed to record unitary activity simultaneously from the thalamic ventral posterior medial (VPM) nucleus and the ipsilateral primary somatosensory cortex of adult rats. Cross-correlation analysis triggered by the spontaneous firing of thalamocortical relay neurons in VPM and the discharge of layer IV neurons in the corresponding ipsilateral cortical barrel indicated that the paired-units included in this study were strongly correlated in their activity. The baseline responses of highly correlated cortical/thalamic pairs to a 10 ms deflection of a vibrissa on the contralateral side were measured using poststimulus time histograms. After establishing the baseline response, high frequency activity in VPM was induced in one of two ways: i) direct electrical stimulation of thalamic neurons or ii) whisker stimulation in the presence of bicuculline methiodide (BIC) released near the thalamic neurons. Both methods resulted in a conditioning stimulus (CS) paradigm consisting of “bursts” of high-frequency activity (50–100 Hz) with an inter-burst interval of 150 ms (∼7 Hz). Almost immediately following the presentation of the CS, the response of layer IV cortical neurons to vibrissa stimulation increased by 37–62% over baseline values, which was maintained after the effects of BIC had worn off in VPM. This enhancement in the response of the cortical neurons was not accompanied by a concomitant increase in the thalamic responses. Thus, these results strongly suggest that the potentiation first occurred at the thalamocortical synapse.