Noradrenaline Metabolism in Neocortex and Hippocampus Following Transient Forebrain Ischemia in Rats: Relation to Development of Selective Neuronal Necrosis

Noradrenaline (NA) metabolism in the neocortex and hippocampus was examined in rats at 1, 24, and 48 h following 15 min of reversible forebrain ischemia. As assessed by the ratio of accumulated 3,4-dihydroxyphenylalanine (DOPA) to the tissue NA level after inhibition of DOPA decarboxylase, the NA turnover rates were markedly increased (120-148% above the control) at 1 h postischemia in both the neocortex and hippocampal formation (CA1 and CA3 plus dentate gyrus). The DOPA:NA ratio went back to control levels after longer postischemic survival times. The ratio between levels of the deaminated NA metabolite, 3,4-dihydroxyphenylethyleneglycol (DOPEG), and NA, which gives another measure of NA turnover rate, showed similar changes. In the neocortex and the CA3 plus dentate gyrus, the DOPEG:NA ratio was markedly increased (89-118%) 1 h after the ischemia, but this change had disappeared at 24 and 48 h. Thus, both the DOPA accumulation experiments and the NA and DOPEG measurements indicate that following transient forebrain ischemia, there is an increased NA turnover in the hippocampus and cortex only in the early recirculation period and not after longer postischemic survival times. The degree of neuronal necrosis in the CA1 region was examined light microscopically on celestine blue-acid fuchsin-stained sections at 24, 48, and 96 h following the ischemic insult. The neuronal damage in CA1 was sparse after 24 h of recovery, had increased markedly after 48 h, and was very pronounced at 96 h.(ABSTRACT TRUNCATED AT 250 WORDS)     

The parcellation theory and its relation to interspecific variability in brain organization,evolutionary and ontogenetic development,and neuronal plasticity

Summary Recently discovered neocortical equivalents in anamniotes and certain patterns of interspecific variability in brain organization provide new insights into evolutionary and ontogenetic mechanisms of development. The new data suggest that nervous systems become more complex, not by one system invading another, but by a process of parcellation that involves the selective loss of connections of the newly formed daughter aggregates and subsystems. The parcellation process is reflected in the normal ontogenetic development of the CNS in a given species and can be manipulated, to a certain extent, by deprivation or surgically induced sprouting.The parcellation theory allows certain predictions about the range of variation of a given system at all levels of analysis including the cellular and aggregate levels. For example, the interspecific variability in organization of cortical columns, thalamic nuclei, cortical areas and tectal layers can be explained. The findings, summarized here, suggest that diffuse, undifferentiated systems existed in the beginning of vertebrate evolution and that during the evolution of complex behaviors, and analytical capacities related to these behaviors, a range of patterns of neural systems evolved that relate to these functions. One principle underlying the growth, differentiation and multiplication of neural systems appears to be the process of parcellation as defined by the theory.Presented in part at the meeting of the British Society for Experimental Biology, Belfast, Ireland, July 1979     

Areal and callosal connections in the somatosensory cortex of the star-nosed mole

Star-nosed moles have a well-developed somatosensory cortex with multiple cortical areas representing the behaviorally important tactile star. In each of three cortical representations, the 11 mechanosensory appendages from the contralateral nose are represented in a series of dark cytochrome oxidase modules. Here the connections of this complex cortical network were explored with injections of the neuroanatomical tracer wheat germ agglutinin conjugated to horseradish peroxidase (WGA-H RP). The main goal was to determine the connection patterns of the somatosensory areas that represent the star. Injections of tracer made in or around the primary somatosensory representation (S1) of the star allowed us to determine the topography of local cortical connections and the projection and termination sites of corresponding interhemispheric connections. The results revealed precise topographic corticocortical connections reciprocally interconnecting the S1 star representation with its counterparts in S2 and in a third representation (S3) unique to star-nosed moles. Callosal connections from a widespread area of the contralateral hemisphere terminated primarily in the septa between cytochrome oxidase dark modules and in areas of cortex surrounding the star representations. However, midline structures of the star represented in S1 and S2 exhibited a high level of callosally labeled cells and terminals. This included label both within septa and within the centers of cytochrome oxidase dense modules representing midline appendages.     

The anatomy of the cerebral cortex of the echidna (Tachyglossus aculeatus)

The cerebral cortex of the echidna is notable for its extensive folding and the positioning of major functional areas towards its caudal extremity. The gyrification of the echidna cortex is comparable in magnitude to prosimians and cortical thickness and neuronal density are similar to that seen in rodents and carnivores. On the other hand, many pyramidal neurons in the cerebral cortex of the echidna are atypical with inverted somata and short or branching apical dendrites. All other broad classes of neurons noted in therian cortex are also present in the echidna, suggesting that the major classes of cortical neurons evolved prior to the divergence of proto- and eutherian lineages. Dendritic spine density on dendrites of echidna pyramidal neurons in somatosensory cortex and apical dendrites of motor cortex pyramidal neurons is lower than that found in eutheria. On the other hand, synaptic morphology, density and distribution in somatosensory cortex are similar to that in eutheria. In summary, although the echidna cerebral cortex displays some structural features, which may limit its functional capacities (e.g. lower spine density on pyramidal neurons), in most structural parameters (e.g. gyrification, cortical area and thickness, neuronal density and types, synaptic morphology and density), it is comparable to eutheria.     

5-HT<Subscript>1A</Subscript> receptor expression in pyramidal neurons of cortical and limbic brain regions

We studied expression of the 5-HT_1A receptor in cortical and limbic areas of the brain of the tree shrew. In situ hybridization with a receptor-specific probe and immunocytochemistry with various antibodies was used to identify distinct neurons expressing the receptor. In vitro receptor autoradiography with ³H-8-OH-DPAT (³H-8-hydroxy-2-[di-n-propylamino]tetralin) was performed to visualize receptor-binding sites. In the prefrontal, insular, and occipital cortex, 5-HT_1A receptor mRNA was expressed in pyramidal neurons of layer 2, whereas ³H-8-OH-DPAT labeled layers 1 and 2 generating a columnar-like pattern in the prefrontal and occipital cortex. In the striate and ventral occipital cortex, receptor mRNA was present within layers 5 and 6 in pyramidal neurons and Meynert cells. Pyramid-like neurons in the claustrum and anterior olfactory nucleus also expressed the receptor. Principal neurons in hippocampal region CA1 expressed 5-HT_1A receptor mRNA, and ³H-8-OH-DPAT labeled both the stratum oriens and stratum radiatum. CA3 pyramidal neurons displayed low 5-HT_1A receptor expression, whereas granule neurons in the dentate gyrus revealed moderate expression of this receptor. In the amygdala, large pyramid-like neurons in the basal magnocellular nucleus strongly expressed the receptor. Immunocytochemistry with antibodies against parvalbumin, calbindin, and gamma aminobutyric acid (GABA) provided no evidence for 5-HT_1A receptor expression in GABAergic neurons in cortical and limbic brain areas. Our data agree with previous findings showing that the 5-HT_1A receptor mediates the modulation of glutamatergic neurons. Expression in the limbic and cortical areas suggested an involvement of 5-HT_1A receptors in emotional and cognitive processes.This work was supported by the German Science Foundation (SFB 406; C4 to G.F.).     

Developmental downregulation of GABAergic drive parallels formation of functional synapses in cultured mouse neocortical networks

Networks of cortical neurons in vitro spontaneously develop synchronous oscillatory electrical activity at around the second week in culture. However, the underlying mechanisms and in particular the role of GABAergic interneurons in initiation and synchronization of oscillatory activity in developing cortical networks remain elusive. Here, we examined the intrinsic properties and the development of GABAergic and glutamatergic input onto presumed projection neurons (PNs) and large interneurons (L-INs) in cortical cultures of GAD67-GFP mice. Cultures developed spontaneous synchronous activity already at 5-7 days in vitro (DIV), as revealed by imaging transient changes in Fluo-3 fluorescence. Concurrently, spontaneous glutamate-mediated and GABA(A)-mediated postsynaptic currents (sPSCs) occured at 5 DIV. For both types of neurons the frequency of glutamatergic and GABAergic sPSCs increased with DIV, whereas the charge transfer of glutamatergic sPSCs increased and the charge transfer of GABAergic sPSCs decreased with cultivation time. The ratio between GABAergic and the overall charge transfer was significantly reduced with DIV for L-INs and PNs, indicating an overall reduction in GABAergic synaptic drive with maturation of the network. In contrast, analysis of miniature PSCs (mPSCs) revealed no significant changes of charge transfer with DIV for both types of neurons, indicating that the reduction in GABAergic drive was not due to a decreased number of functional synapses. Our data suggest that the global reduction in GABAergic synaptic drive together with more synaptic input to PNs and L-INs during maturation may enhance rhythmogenesis of the network and increase the synchronization at the level of population bursts.     

Slow oscillations in neural networks with facilitating synapses

The synchronous oscillatory activity characterizing many neurons in a network is often considered to be a mechanism for representing, binding, conveying, and organizing information. A number of models have been proposed to explain high-frequency oscillations, but the mechanisms that underlie slow oscillations are still unclear. Here, we show by means of analytical solutions and simulations that facilitating excitatory (E _f) synapses onto interneurons in a neural network play a fundamental role, not only in shaping the frequency of slow oscillations, but also in determining the form of the up and down states observed in electrophysiological measurements. Short time constants and strong E _f synapse-connectivity were found to induce rapid alternations between up and down states, whereas long time constants and weak E _f synapse connectivity prolonged the time between up states and increased the up state duration. These results suggest a novel role for facilitating excitatory synapses onto interneurons in controlling the form and frequency of slow oscillations in neuronal circuits.     

Bicuculline-induced blockade of neocortical rapid kindling suggesting facilitative GABAergic action on seizure development

Summary To investigate how GABAergic function affects seizure development, the effects of a GABA antagonist, bicuculline, on neocortical and hippocampal kindling were examined in chronically prepared rabbits. Kindling-inducing stimulations consisted of stimulus trains repeated at 5-min interstimulus intervals to produce so-called rapid kindling. The changes in after-discharge (AD) durations induced by each of 15 trials of stimulus trains per session were compared before and 30 min after i.p. injection of bicuculline solution (2 mg/kg) in each of three kindling groups consisting of 5 rabbits each, i.e. visual cortical, motor cortical and hippocampal kindling groups. In the visual cortex and to a less extent, the motor cortex kindling groups, the AD durations were shortened after bicuculline injection and did not show the progressive prolongation seen before the injection. In contrast, the hippocampal kindling group showed a further marked prolongation of the AD durations after the injection. The bicuculline-induced blockade of neocortical kindling suggests facilitative GABAergic action on seizure development, while the drug-induced enhancement of hippocampal kindling reflects the known inhibitory GABAergic action.     

Characterization of the Dihydropyridine Binding Sites of Rat Neocortical Synaptosomes and Microvessels

The dihydropyridine binding sites associated with rat neocortical synaptosomes and microvessels were compared using an in vitro [3H]PN 200-110 [(+)-[methyl-3H]-isopropyl 4-(2,1,3-benzoxadiazol-4-yl)-1,4-dihydro-2,6-dimethyl-5- methoxycarbonylpyridine-3-carboxylate] binding assay. Saturation experiments yielded similar KD values (approximately 70 pM) and Bmax values (approximately 400 fmol/mg of protein) for the two membrane preparations. Interaction experiments with [3H]PN 200-110 and various calcium-modulating substances provided further evidence for the practically identical nature of the synaptosomal and microvascular dihydropyridine binding sites. These findings predict that lipophilic dihydropyridines, simultaneously occupying the two central binding sites, have the dual effect of altering neuronal function and local blood flow.