Developmental regulation of intracellular calcium by N-methyl-D-aspartate and noradrenaline in rat visual cortex

Molecular mechanisms of experience-dependent plasticity in visual cortex

A remarkable amount of our current knowledge of mechanisms underlying experience-dependent plasticity during cortical development comes from study of the mammalian visual cortex. Recent advances in high-resolution cellular imaging, combined with genetic manipulations in mice, novel fluorescent recombinant probes, and large-scale screens of gene expression, have revealed multiple molecular mechanisms that underlie structural and functional plasticity in visual cortex. We situate these mechanisms in the context of a new conceptual framework of feed-forward and feedback regulation for understanding how neurons of the visual cortex reorganize their connections in response to changes in sensory inputs. Such conceptual advances have important implications for understanding not only normal development but also pathological conditions that afflict the central nervous system.

Activation of alpha1-adrenoceptors increases firing frequency through protein kinase C in pyramidal neurons of rat visual cortex

Properties of repetitive firing, including spike adaptation, are considered to play an essential role in controlling neural excitability in the central nervous system. Noradrenaline is one of major neurotranmitters that modulate repetitive firing in the cerebral cortex. Although activation of beta-adrenoceptors increases firing frequency similarly to noradrenaline, it is still controversial whether alpha(1)-adrenoceptor activation influences repetitive firing. In the present study, we examined the effects of adrenoceptor agonists on firing properties and the intracellular mechanism for alpha(1)-adrenoceptor-dependent modulation of firing in pyramidal neurons of rat cerebral cortex. In agreement with previous reports, bath application of 100microM isoproterenol, a beta-adrenoceptor agonist, increased firing frequency in response to a long intracellular depolarizing current injection. Phenylephrine (100microM), an alpha(1)-adrenoceptor agonist, also increased firing rate, which was inhibited by 100microM prazosin, an alpha1-adrenoceptor antagonist. The extent of increment in firing rate is comparable to that induced by isoproterenol. Furthermore, phenylephrine's effects on firing properties were mimicked by 2-5microM phorbol ester, a protein kinase C (PKC) activator, and pre-application of 10microM chelerythrine, a PKC inhibitor, prevented phenylephrine-induced facilitation of repetitive firing. These results suggest that phenylephrine has a facilitatory effect on repetitive firing through PKC activation.

Cellular and subcellular localization of alpha-1 adrenoceptors in the rat visual cortex

Noradrenaline is thought to play modulatory roles in a number of physiological, behavioral, and cellular processes. Although many of these modulatory effects are mediated through alpha-1 adrenoceptors, basic knowledge of the cellular and subcellular distributions of these receptors is limited. We investigated the laminar distribution pattern of alpha-1 adrenoceptors in rat visual cortex, using immunohistochemistry at both light and electron microscopic levels. Affinity-purified anti-alpha-1 antibody was confirmed to react only with a single band of about 70-80 kDa in total proteins prepared from rat visual cortex. Alpha-1 adrenoceptors were widely distributed though all cortical layers, but relatively high in density in layers I, II/III, and V. Immunoreactivity was observed in both neuronal perikarya and processes including apical dendrites. In double-labeling experiments with anti-microtubule-associated protein 2, anti-neurofilament, anti-glial fibrillary acidic protein, anti-glutamic acid decarboxylase 65/67, anti-2-3-cyclic nucleotide 3-phosphodiesterase, and anti-tyrosine hydroxylase antibodies, alpha-1 adrenoceptors were found mainly in dendrites and somata of microtubule-associated protein 2-immunopositive neurons. About 20% of alpha-1 adrenoceptors were in GABAergic neurons. A small number of alpha-1 adrenoceptors were also distributed in axons of excitatory neurons, astrocytes, oligodendrocytes and noradrenergic fibers. Using an immunoelectron microscopic technique, numerous regions of alpha-1 adrenoceptor immunoreactivity were found in cell somata, on membranes of dendrites, and in postsynaptic regions. Moreover, a small number of immunoreaction products were also detected in axons and presynaptic sites. These findings provide the first quantitative evidence regarding the cellular and subcellular localization of alpha-1 adrenoceptor immunoreactivity in visual cortex. Moreover, the ultrastructural distribution of alpha-1 adrenoceptor immunoreactivity suggests that alpha-1 adrenoceptors are transported mainly into dendrites and that they exert effects at postsynaptic sites of neurons.

Effect of neonatal isolation on the noradrenergic transduction system in the rat hippocampal slice

Numerous studies suggest that early adverse experiences induce neurochemical, morphological, and functional changes in the hippocampus in adolescence and adulthood. The aim of this study was to identify the influence of neonatal isolation (NI) on noradrenaline (NA)-mediated intracellular calcium ([Ca(2+)](i)) mobilization. To measure [Ca(2+)](i), we used the Ca(2+)-sensitive dye fura-2 and analysis by fluorescence microscopy. First, we examined the contributions of adrenergic receptor subtypes to the NA-stimulated increase in [Ca(2+)](i) in the granule cell layers of the dentate gyrus (DG) and in the pyramidal cell layers of the CA3 in the hippocampus. Second, we found that the NA-stimulated [Ca(2+)](i) increment was significantly decreased in response to NI in these hippocampal regions. In addition, we examined the influence of environmental enrichment (EE) after weaning on the decrease in the NA-stimulated [Ca(2+)](i) increment induced by NI. The administration of EE reversed the influence of NI on the NA-stimulated [Ca(2+)](i) increment in the CA3 pyramidal cell layer but not in the DG granular cell layer in the hippocampus. These findings suggest that NI and EE after weaning may modulate hippocampal function by altering adrenergic receptor-mediated signal transduction during adolescence.

N-(2-Chloroethyl)-N-ethyl-2-bromobenzylamine reduces intracellular calcium response to noradrenaline in rat visual cortex

Using the fluorescent indicator Fura-2, we investigated the effects of N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4), a noradrenergic neurotoxin, on intracellular calcium responses to noradrenaline, N-methyl-D-aspartate, and carbamylcholine chloride in brain slices of the rat visual cortex. Noradrenergic depletion in the visual cortex of young rats was induced by DSP-4, and its selectivity was confirmed by two different methods, i.e., immunostaining with anti-dopamine-beta-hydroxylase antibody and biochemical analysis by high-performance liquid chromatography. The treatment with DSP-4 (25 mg/kg i.p., x2) caused disruption of noradrenergic fibers throughout all cortical layers, and reduced the content of noradrenaline to 6.4% of that in the normal control. In the normal cortex, bath-applied noradrenaline (100 microM) increased the intracellular calcium to 123% of the control in terms of the F(340)/F(380) ratio of Fura-2 fluorescence. Quantitative analysis of the F(340)/F(380) ratio was performed in layers II to IV, since the increase was mainly observed in these layers. The intracellular calcium response to noradrenaline was significantly (P<0.0001) reduced in the DSP-4-treated animals to 63.2% of that in the normal control. The response to N-methyl-D-aspartate (100 microM) was also reduced, whereas the response to carbamylcholine chloride, a muscarinic cholinergic agonist (100 microM), was not affected by the DSP-4 treatment. From these findings we suggest that noradrenergic denervation by DSP-4 reduces the intracellular calcium response to noradrenaline through changes in the intracellular signal transduction.

Effect of partial sensory deprivation on monoaminergic neuromodulators in striate cortex of adult cat

The role of monoaminergic neuromodulators in the reorganization of cortical topography following limited sensory deprivation in the adult cat was investigated. The total concentrations of dopamine, noradrenaline, serotonin and their major metabolites were measured in the visual cortex of both normal control and experimental animals using microbore high-performance liquid chromatography coupled with electrochemical detection. The experimental animals were subjected to a binocular retinal lesion corresponding to the central 10 degrees of vision and killed two weeks post-lesion. The sensory deprivation was confirmed in area 17 by measuring immediate-early gene zif-268 messenger RNA expression. Following the retinal lesion, the total concentrations of noradrenaline and dopamine were significantly higher in the non-deprived cortex of retinal lesion cats than in the deprived cortex of retinal lesion cats and the cortex of normal animals. This pattern follows the release of the excitatory neurotransmitter glutamate under the same conditions. Serotonin levels were significantly lower in the deprived cortex, and its metabolite 5-hydroxyindole-3-acetic acid was significantly higher in the non-deprived cortex than in deprived cortex and normal cortex. From these results, we suggest that the modulation of noradrenaline, dopamine and serotonin is regulated by visual afferent activity.

Selective suppression of horizontal propagation in rat visual cortex by norepinephrine

The release of norepinephrine in the cerebral cortex from axon terminals of locus coeruleus neurons was suggested to be involved in the control of attention. Accumulating data indicate that the responses of cortical neurons are varied when norepinephrine is applied iontophoretically in the vicinity of the cells being recorded. However, it is not known how the pattern of excitatory propagation is modified when norepinephrine is applied over a wide area in the visual cortex. By applying optical imaging to rat visuocortical slices, we found a new mode of norepinephrine action; a prominent suppression of the horizontal propagation in layers II/III. This action of norepinephrine was confirmed by the simultaneous recording of field potentials from multiple sites by use of a multi-electrode dish. Furthermore, our electrophysiological recordings showed that this norepinephrine action is exerted through suppression of excitatory neural transmission and enhancement of inhibitory transmission to the pyramidal neurons in these layers. Because the release of norepinephrine in the visual cortex is regulated by the level of attention, the neural basis of visual attention may relate partially to the suppression of the integration of visual information by norepinephrine resulting in a state-dependent restructuring of the receptive field.