Development and regulation of alpha adrenoceptors in kitten visual cortex

Comparison of the maturation of the adrenergic and serotonergic neurotransmitter systems in the brain: implications for differential drug effects on juveniles and adults

Our understanding of the development of neurotransmitter systems in the central nervous system has increased greatly over the past three decades and it has become apparent that drug effects on the developing nervous system may differ considerably from effects on the mature nervous system. Recently it has become clear there are significant differences in the effectiveness of antidepressant drug classes in children and adolescents compared to adults. Whereas the selective serotonin reuptake inhibitors are effective in treating all ages from children to adults, the tricyclic antidepressants, many of which inhibit norepinephrine reuptake, have been shown to be ineffective in treating children and adolescents even though they are effective in adults. We review here the development of the noradrenergic and serotonergic nervous systems, both in terms of neurotransmitter system markers and function. Both of these neurotransmitter systems are primary targets of antidepressant medications as well as of central nervous system stimulants. It is clear from a comparison of their development that the serotonin system reaches maturity much earlier than the norepinephrine system. We suggest this may help explain the differences in response to antidepressants in children and adolescents compared to adults. In addition, these differences suggest that drugs acting preferentially on either neurotransmitter system may impact the normal course of CNS development at different time points. Consideration of such differences in the development of neurotransmitter systems may be of significance in optimizing treatments for a variety of centrally mediated disorders.

Alpha-2 adrenergic receptor development in rat CNS: an autoradiographic study

During development norepinephrine plays a role in determining the morphologic organization of the CNS and the density and future responsiveness of adrenergic receptors. alpha-2 Adrenergic receptors, one of three adrenergic receptor types, regulate important adult CNS functions and may have a distinct role during development. We examined alpha-2 receptor distribution and density in the rat brain at postnatal days 1, 5, 10, 15, 21, 28 and in adults using the antagonist [(3)H]RX821002 for autoradiography. Binding kinetics and pharmacology for alpha-2 adrenergic receptors were the same in adults and neonates. There was an overall increase in alpha-2 receptor levels during postnatal development with great variability in pattern and timing of receptor density changes among brain regions. Three major patterns were apparent. First, in many regions receptor density increased during postnatal development, generally reaching adult levels around postnatal day 15. Within this group there was variability in timing between regions and there were several regions with receptor densities higher than adult levels during the postnatal period. Second, there were regions with very high levels of receptors at birth and little or no change in density during the postnatal period. Third, some regions demonstrated decreasing or transient expression of alpha-2 adrenergic receptors in the course of postnatal development, including white matter regions, cerebellum and many brainstem nuclei, suggesting specific roles for alpha-2 receptors during development. This study investigates the development of alpha-2 adrenergic receptors in the rat CNS. It demonstrates there is region-specific regulation of alpha-2 receptor development and identifies brain regions where these receptors may play a specific and critical role in the regulation normal development.

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.

Effects of tetrodotoxin treatment in LGN on neuromodulatory receptor expression in developing visual cortex

The expression and distribution patterns of transmitter receptors change dramatically during pre- and post-natal development of the visual cortex, but the factors that control these processes are largely unknown. We have tested the hypothesis that input activity from the lateral geniculate nucleus (LGN), one major input source to visual cortex, may contribute to the processes underlying transmitter receptor redistributions in the visual cortex during development. We found that a short period of tetrodotoxin (TTX) treatment in LGN retarded the developmental expression and age-dependent reorganization of neuromodulatory receptors, including muscarinic, serotonergic and adrenergic receptors, in kitten primary visual cortex. The visual cortices ipsilateral to the TTX infusion site displayed a 'younger' receptor pattern than that of their contralateral control counterparts in the same animals. The results suggest that active input from LGN regulates the expression profile of a broad range of receptors in the developing visual cortex.

Developmental expression and regulation of alpha 1 adrenergic receptors in cultured cortical neurons

The expression and distribution of alpha 1 receptors in cultured neurons derived from rat visual cortex were studied with confocal microscopy using the fluorescently labeled alpha 1 adrenergic receptor selective antagonist BODIPY FL prazosin. The receptors were found to be clustered on neuronal somata and on proximal dendrites. We found that expression of the alpha 1 receptor is regulated both by neuronal excitability and by the usage of the receptor itself. The specificity of receptor regulation to the blockade of a particular receptor class was also studied. We approached this by comparing the effects of treatment with the alpha 1 adrenoreceptor antagonist prazosin and M1 muscarinic receptor antagonist pirenzepine on the expression of alpha 1 and M1 receptors in cultured cortical neurons. The results showed that blockade of muscarinic receptors with pirenzepine up-regulated muscarinic receptor expression selectively without changing alpha 1 receptor expression. Conversely blockade of alpha 1 receptors up-regulated alpha 1 expression but not muscarinic receptor expression. This implies that the expression levels of M1 and alpha 1 receptors are both regulated through specific signal transduction pathways. The interactions between neuronal activity and receptor activation (or blockade) on receptor expression were studied as well. In these experiments we compared the effects of high K+, tetrodotoxin (TTX), prazosin, and noradrenaline on the expression of alpha 1 and M1 receptors in cultured neurons. The results show that high K+ exposure increased both alpha 1 and M1 receptor expression regardless of the presence of receptor agonists or antagonists. On the other hand, TTX exposure reduced both alpha 1 and M1 receptor expression regardless of the presence of the receptor agonists or antagonists. This implies that regulation of receptor number is predominantly regulated by neuronal activity rather than by receptor occupancy.

Transient expression of alpha-1B adrenoceptor messenger ribonucleic acids in the rat superior cervical ganglion during postnatal development

We have examined the developmental profile of the alpha-1 and alpha-2 adrenergic receptor messenger ribonucleic acids expression in the rat superior cervical ganglion. The expression of the six messenger ribonucleic acids was studied using reverse transcription-polymerase chain reaction. At four weeks, the dominant messenger ribonucleic acids transcripts in this sympathetic ganglion were alpha-1C, alpha-2A, alpha-2B and alpha-2C. The expression of alpha-1 genes in the superior cervical ganglion appears to be regulated during postnatal development in that two alpha-1 (alpha-1B, alpha-1C) genes were expressed at birth, three, seven and 14 days postnatal but no amplified product for alpha-1B was detected at 28 days and in the aged animals, while the alpha-1C transcript continued to be expressed. No amplified product for alpha-1D was detected in superior cervical ganglion at any of the ages studied. While all three alpha-2 genes were expressed in the superior cervical ganglion at four weeks the dominant alpha-2 messenger ribonucleic acids transcript expressed in the superior cervical ganglion was alpha-2A. This pattern of alpha-2 adrenoceptor gene expression was maintained from birth, throughout development and into old age. These results suggest that the expression of alpha-1 adrenergic receptors in the superior cervical ganglion is regulated developmentally while the expression of alpha-2 genes remains unchanged.

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.

The alpha 2-adrenergic antagonist idazoxan enhances the frequency selectivity and increases the threshold of auditory cortex neurons

Idazoxan (IDA), an alpha 2 antagonist of adrenoceptors, has been shown to increase cortical release of norepinephrine (NE) by an action mediated primarily by the alpha 2 autoreceptors located on the NE terminals. In the present experiment, IDA application was used to increase the cortial concentration of NE. Single unit activity (n = 107) was recorded in the rat auditory cortex, and the neurons' frequency receptive fields (FRF) were determined before and after systemic (intraperitoneal or intravenous) or local application of IDA. In the whole population (n = 107) there was a decrease in spontaneous activity and/or evoked activity for 84% of the recordings (90/107 cells). Decreased tone-evoked responses were obtained after systemic injections (n = 39), as well as after local applications (n = 68) of IDA. These effects were not observed after either systemic injections (n = 13) or local applications (n = 9) of saline. The signal-to-noise ratio (the mean evoked responses divided by the spontaneous activity) was slightly decreased after systemic injections and slightly increased after local applications. However, after both systemic and local injections the frequency selectivity of the neuronal responses was increased. For a group of neurons (n = 27), testing the FRF at three intensities indicated that this increased selectivity can be expressed at high or middle range intensity but not at low intensity. For 37 cells, the intensity function was tested at the best frequency before and after IDA application, and the threshold for excitatory responses was determined in 28 cases. An increased threshold was observed in 16 of 28 cases after IDA application. Thus, using a pharmacological procedure to increase the extracellular concentration of NE, the dominant inhibitory effect on the auditory cortex neurons led to an enhancement of the frequency selectivity, but also an increase in the threshold of these neurons.