Development and regulation of beta adrenergic receptors in kitten visual cortex: an immunocytochemical and autoradiographic study

Neuromodulatory transmitter systems in the cortex and their role in cortical plasticity

Cortical neuromodulatory transmitter systems refer to those classical neurotransmitters such as acetylcholine and monoamines, which share a number of common features. For instance, their centers are located in subcortical regions and send long projection axons to innervate the cortex. The same transmitter can either excite or inhibit cortical neurons depending on the composition of postsynaptic transmitter receptor subtypes. The overall functions of these transmitters are believed to serve as chemical bases of arousal, attention and motivation. The anatomy and physiology of neuromodulatory transmitter systems and their innervations in the cerebral cortex have been well characterized. In addition, ample evidence is available indicating that neuromodulatory transmitters also play roles in development and plasticity of the cortex. In this article, the anatomical organization and physiological function of each of the following neuromodulatory transmitters, acetylcholine, noradrenaline, serotonin, dopamine, and histamine, in the cortex will be described. The involvement of these transmitters in cortical plasticity will then be discussed. Available data suggest that neuromodulatory transmitters can modulate the excitability of cortical neurons, enhance the signal-to-noise ratio of cortical responses, and modify the threshold for activity-dependent synaptic modifications. Synaptic transmissions of these neuromodulatory transmitters are mediated via numerous subtype receptors, which are linked to multiple signal transduction mechanisms. Among the neuromodulatory transmitter receptor subtypes, cholinergic M(1), noradrenergic beta(1) and serotonergic 5-HT(2C) receptors appear to be more important than other receptor subtypes for cortical plasticity. In general, the contribution of neuromodulatory transmitter systems to cortical plasticity may be made through a facilitation of NMDA receptor-gated processes.

Noradrenergic modulation of functional selectivity in the cat visual cortex: an in vivo extracellular and intracellular study

In vitro intracellular studies have shown that norepinephrine modulates cellular excitability and synaptic transmission in the cortex. Based on these effects, norepinephrine has been proposed to enhance the signal-to-noise ratio and to improve functional selectivity by potentiating strong synaptic responses and reducing weak ones. Here we have studied the functional effects of iontophoretic applications of norepinephrine during in vivo extracellular and intracellular recordings from neurons of the primary visual cortex of kittens and adult cats. Analysis of extracellular data concentrated on norepinephrine-induced changes in spontaneous and evoked activities, in signal-to-noise ratio, and in orientation and direction selectivity. Analysis of the intracellular data concentrated on actions of norepinephrine on spike firing accommodation, which has been shown to be reduced by norepinephrine in vitro, and on synaptic responses. Application of norepinephrine resulted in a depression of both spontaneous and evoked spiking activity. However, no systematic change in signal-to-noise ratio was observed. The suppressive effect of norepinephrine was exerted with no significant sharpening of direction or orientation selectivity tuning. The overall reduction in visual activity by norepinephrine affected the orientation tuning curves in a way compatible with a divisive effect, that is a normalization or gain control with no change in tuning width. Norepinephrine applied during intracellular recordings reduced the visually evoked depolarizing potentials whereas no change in the responsiveness of the cell to current-induced depolarizations was observed. In conditions of optimal visual stimulation which produced large depolarizations of several hundreds of milliseconds and sustained repetitive firing comparable to that obtained by direct current injection, we were unable to observe a facilitation of the evoked responses by norepinephrine as it would be expected from the well-documented increase in excitability induced by norepinephrine in vitro. In conclusion, from these results we suggest that norepinephrine released in the primary visual cortex primarily reduces the level of cortical activation by afferent signals, without affecting the cortical functional selectivity nor increasing the signal-to-noise ratio.

Effects of monocular deprivation on the expression pattern of alpha-1 and beta-1 adrenergic receptors in the kitten visual cortex

To examine how adrenergic receptors are regulated by experimental manipulation of sensory afferents, we performed immunohistochemical analysis on alpha1-, and beta1-adrenergic receptors in the brain of kittens. In normal development, these receptors were similarly expressed in both hemispheres of the occipital and frontal cortices. Notably, monocular deprivation during the sensitive period of ocular dominance plasticity significantly increased beta1-adrenergic receptor immunoreactivity in the visual cortex ipsilateral to the deprived eye. No increase in the intensity of the immunoreactivity for beta1-adrenergic receptors following monocular deprivation was found in the frontal and parietal regions of the cerebral cortex and subcortical structures, including the lateral geniculate nucleus and superior colliculus. Furthermore, such hemispheric change was not found in the alpha1-adrenergic receptor immunoreactivity following monocular deprivation. Comparisons of images, obtained by double staining for microtubule-associated protein-2 or glial fibrillary acidic protein, indicated that the increased immunoreactivity was localized on both apical dendrites of deep layer neurons and glial cells. These results indicate that the monocular deprivation during the sensitive period of ocular dominance plasticity modified beta1-adrenergic receptor immunoreactivity, including that in glial cells. Therefore, it was suggested that beta1-adrenergic receptors in the glial cells also play important roles in the regulation of ocular dominance plasticity.

Immunohistochemical localization of dopamine-beta-hydroxylase in human and monkey eyes

PURPOSE

To investigate the pattern of dopamine-beta-hydroxylase (DBH)-containing fibers in human and monkey eyes.

METHODS

DBH-containing fibers were detected by immunohistochemistry. The primary antibody used recognized DBH, the key enzyme in the conversion of dopamine to noradrenaline.

RESULTS

In the anterior segment, DBH immunoreaction product was found in the peripheral corneal endothelium layer, in both the dilator and sphincter muscles of the iris, as well as in the anterior border layer of the iris. The ciliary muscle and the stroma of the ciliary processes were also zones of concentration. In the posterior segment, staining was seen around blood vessels in the choroid, in the vascular walls of the short posterior ciliary arteries and in the ciliary nerves. The retina was also immunopositive, with specific labeling in cones and rods of photoreceptors, inner and outer plexiform layers and ganglion cell layer. There was no significant difference in the distribution of DBH-related immunoreactivity in human and monkey eyes.

CONCLUSIONS

The localization of DBH-related immunoreactivity is generally consistent with the known physiological roles of noradrenaline. However, an apparently high concentration of the enzyme in the anterior border layer of the iris and in retinal photoreceptors raises questions about the possible role of DBH-containing fibers in these structures.

Differential timing for the appearance of neuronal and astrocytic beta-adrenergic receptors in the developing rat visual cortex as revealed by light and electron-microscopic immunocytochemistry

The developing cerebral cortex is likely to exhibit synaptic circuitries differing from those in adulthood, due to the asynchronous maturation of the various neurotransmitter systems. Two antisera directed against mammalian beta-adrenergic receptors (beta AR), beta AR248 and beta AR404, were used to characterize the laminar, cellular, and subcellular distributions of beta AR in postnatally developing visual cortex of rats. The antigenic sites were the receptor's third intracellular loop for beta AR248 and the C-terminus for beta AR404. During week 1, most of the beta AR404- and beta AR248-immunoreactive sites were dendritic. Morphologically identifiable synapses were rare, even in layer 1: yet, semiquantitative analysis revealed that beta AR404-immunoreactive synapses comprise half of those in layer 1. During week 2, the two antisera began to diverge in their immunoreactivity patterns. With beta AR248, there was an overall decline in immunoreactivity, while with beta AR404, there was an increase in immunoreactive sites, primarily due to labeled astrocytic processes that increased 200-fold in areal density by week 3. In contrast, the areal density of synaptic labeling by beta AR404 barely doubled, in spite of the 30-fold increase in areal density of synapses. These results suggest that beta AR undergo conformational changes during early postnatal periods, causing alterations in their relative antigenicity to the two antisera. Furthermore, the first 2 weeks appear to be characterized by modulation of earliest-formed synapses, and the subsequent phase is marked by addition of astrocytic responses that would be more diffuse temporally and spatially. Activation of beta AR is recognized to increase visually evoked activity relative to spontaneous activity. Moreover, astrocytic beta AR are documented to regulate extracellular concentrations of glutamate, ATP, and neurotrophic factors important for the formation of binocular connections. Thus, neuronal and astrocytic responses may, together and in tandem, facilitate strengthening of intracortical synaptic circuitry during early life.

Down-regulation of beta-adrenergic receptor following long-term monocular deprivation in cat visual cortex

To examine how adrenergic receptor binding is modified by experimental manipulation of sensory afferent, we carried out binding experiments (membrane fraction and in vitro autoradiography) for both alpha 2- and beta-adrenergic receptors in the brain of cats which had been deprived of vision in one eye. In the cerebral cortex of control animals, beta-adrenergic receptor (beta-AR) binding was found to be higher in the occipital regions than in other regions, while alpha 2-AR binding was relatively uniform. Monocular deprivation throughout the postnatal sensitive period (1-7 month of age) significantly decreased beta-AR binding in the visual cortex and lateral geniculate nucleus. Scatchard plot analysis in the visual cortex showed ca. 50% reduction in Bmax and little change in Kd. No significant difference was found in alpha 2-AR binding following monocular deprivation. Similar extent of down-regulation in beta-AR binding was confirmed in all layers of visual cortex using autoradiography.

Synergistic interactions between noradrenaline and glutamate in cytosolic calcium influx in cultured visual cortical neurons

In primary neuronal cultures derived from the visual cortex of embryonic day 16-18 rats, intracellular free calcium concentration, [Ca2+]i, was increased by bath application of glutamate in a dose-dependent manner. Noradrenaline applied alone had relatively small effects. However, when glutamate concentrations eliciting modest increases in [Ca2+]i were applied together with 1 microM noradrenaline, the increase in [Ca2+]i could be enhanced by a factor of up to eight. The synergistic effect was seen in 147 neurons out of a total of 215 cells observed in 54 experiments. The observed enhancement was much more obvious at low doses of glutamate than with higher doses, augmenting all submaximal calcium responses to similar asymptotic levels. 2-Amino-5-phosphonovalerate (APV), the NMDA receptor antagonist, completely blocked the adrenergic enhancing effect (29/29 cells in 8 experiments). Among the antagonists specific to alpha 1, alpha 2 and beta subtypes of adrenoceptors, the beta antagonist propranolol most completely blocked the enhancing effect (13/14 cells in 4 experiments, reducing the effect by an amplitude of 90%). The involvement of the beta receptor pathway was further supported by the ability of a cAMP analog to mimic the enhancing effect of noradrenaline. On the other hand, an alpha 1 blocker showed no effect and an alpha 2 blocker showed only a relatively small effect. These results suggest that receptors for noradrenaline and glutamate colocalize on postsynaptic cortical cells and that adrenergic modulation of glutamate induced calcium influx most likely operate through the beta receptor pathway. It is further postulated that cortical ocular dominance plasticity may be at least partially implemented via a calcium dependent cascade.

Postnatal development and laminar distribution of noradrenergic fibers in cat visual cortex

Previous studies have indicated that adrenergic receptors show significant changes either in laminar distribution or in number during the critical period of kitten visual cortex development. In order to further investigate the postnatal development of this neurotransmitter system, especially in relation to the critical period, we used a polyclonal antibody against dopamine-beta-hydroxylase to localize noradrenaline-containing afferents in visual cortex of kittens of various ages from birth to adulthood. In young kittens, less than 2 weeks of age, noradrenergic fibers were sparse, short and randomly oriented, and were concentrated in layer I and in deep cortical layers V and VI. By postnatal day 40, the fibers were present throughout all cortical layers and exhibited higher densities in layers I, II, III, V and VI, with a band of lower staining in layer IV. While tangential fibers predominated in layers I, V and VI, relatively straight radial fibers traversed layers II and III. After postnatal day 40, we did not find major changes in the laminar distribution of adrenergic fibers. This developmental laminar distribution pattern of adrenergic fibers resembles that of the beta-adrenergic receptors that we and others have studied in kitten visual cortex, but differs from that of alpha-adrenergic receptors.