Preservation of binocularity after monocular deprivation in the striate cortex of kittens treated with 6-hydroxydopamine

From pupil to the brain: New insights for studying cortical plasticity through pupillometry

Pupil size variations have been associated with changes in brain activity patterns related with specific cognitive factors, such as arousal, attention, and mental effort. The locus coeruleus (LC), a key hub in the noradrenergic system of the brain, is considered to be a key regulator of cognitive control on pupil size, with changes in pupil diameter corresponding to the release of norepinephrine (NE). Advances in eye-tracking technology and open-source software have facilitated accurate pupil size measurement in various experimental settings, leading to increased interest in using pupillometry to track the nervous system activation state and as a potential biomarker for brain disorders. This review explores pupillometry as a non-invasive and fully translational tool for studying cortical plasticity starting from recent literature suggesting that pupillometry could be a promising technique for estimating the degree of residual plasticity in human subjects. Given that NE is known to be a critical mediator of cortical plasticity and arousal, the review includes data revealing the importance of the LC-NE system in modulating brain plasticity and pupil size. Finally, we will review data suggesting that pupillometry could provide a quantitative and complementary measure of cortical plasticity also in pre-clinical studies.

Regional Downregulation of Dopamine Receptor D1 in Bilateral Dorsal Lateral Geniculate Nucleus of Monocular Form-Deprived Amblyopia Models

Amblyopia is a common eye disease characterized by impaired best-corrected visual acuity. It starts in early childhood and leads to permanent vision reduction if left untreated. Even though many young patients with amblyopia are well treated in clinical practice, the underlying mechanism remains to be elucidated, which limits not only our understanding of this disease but also the therapeutic approach. To investigate the molecular mechanism of amblyopia, primate and rodent models of monocular-deprived amblyopia were created for mRNA screening and confirmation. We obtained 818 differentially expressed genes from the dorsal lateral geniculate nucleus (dLGN) of a primate model of amblyopia. After Gene Ontology and kyoto encyclopedia of genes and genomes (KEGG) enrichment analyses, the main enriched pathways were related to neural development. Interestingly, a particular neurotransmitter pathway, the dopaminergic pathway, was identified. The downregulation of dopamine receptor D1 (DRD1) was confirmed in both monkey and mouse samples. Furthermore, the immunofluorescence staining indicated that DRD1 expression was downregulated in both ventrolateral region of the contralateral dLGN and the dorsomedial region of the ipsilateral dLGN in the mouse model. The regions with downregulated expression of DRD1 were the downstream targets of the visual projection from the amblyopic eye. This study suggested that the downregulation of DRD1 in the LGN may be a cause for amblyopia. This may also be a reason for the failure of some clinical cases of levodopa combined with carbidopa applied to amblyopes.

Ocular dominance plasticity: Molecular mechanisms revisited

Ocular dominance plasticity (ODP) is a type of cortical plasticity operating in visual cortex of mammals that are endowed with binocular vision based on the competition-driven disparity. Earlier, a molecular mechanism was proposed that catecholamines play an important role in the maintenance of ODP in kittens. Having survived the initial test, the hypothesis was further advanced to identify noradrenaline (NA) as a key factor that regulates ODP in the immature cortex. Later, the ODP-promoting effect of NA is extended to the adult with age-related limitations. Following the enhanced NA availability, the chain events downstream lead to the β-adrenoreceptor-induced cAMP accumulation, which in turn activates the protein kinase A. Eventually, the protein kinase translocates to the cell nucleus to activate cAMP responsive element binding protein (CREB). CREB is a cellular transcription factor that controls the transcription of various genes, underpinning neuronal plasticity and long-term memory. In the advent of molecular genetics in that various types of new tools have become available with relative ease, ODP research has lightly adopted in the rodent model the original concepts and methodologies. Here, after briefly tracing the strategic maturation of our quest, the review moves to the later development of the field, with the emphasis placed around the following issues: (a) Are we testing ODP per se? (b) What does monocular deprivation deprive of the immature cortex? (c) The critical importance of binocular competition, (d) What is the adult plasticity? (e) Excitation-Inhibition balance in local circuits, and (f) Species differences in the animal models.

An update on pharmacological treatment options for amblyopia

Amblyopia is a common cause of visual impairment in children and young adults. The cornerstone in the management of this disorder is based on increasing visual stimulation of the amblyopic eye by occlusion, by administering atropine or by causing optical penalization of the dominant eye. All these treatment options have shown some limits in terms of efficacy, due to the suboptimal treatment adherence for the patients and the lack of long-term clinical outcomes. Moreover, although it is well known that clinical efficacy decreases with age, new evidence is suggesting that cortical plasticity can be induced also in older children. For these reasons, new treatment options are being studied, in order to extend the "treatment window" beyond the critical period also in older patients. In this review, we will discuss all the most promising novel pharmacological agents in the management of amblyopia.

Pharmacological therapy for amblyopia

Amblyopia is the most common cause of preventable blindness in children and young adults. Most of the amblyopic visual loss is reversible if detected and treated at appropriate time. It affects 1.0 to 5.0% of the general population. Various treatment modalities have been tried like refractive correction, patching (both full time and part time), penalization and pharmacological therapy. Refractive correction alone improves visual acuity in one third of patients with anisometropic amblyopia. Various drugs have also been tried of which carbidopa & levodopa have been popular. Most of these agents are still in experimental stage, though levodopa-carbidopa combination therapy has been widely studied in human amblyopes with good outcomes. Levodopa therapy may be considered in cases with residual amblyopia, although occlusion therapy remains the initial treatment choice. Regression of effect after stoppage of therapy remains a concern. Further studies are therefore needed to evaluate the full efficacy and side effect profile of these agents.

Layer- and area-specific actions of norepinephrine on cortical synaptic transmission

The cerebral cortex is a critical target of the central noradrenergic system. The importance of norepinephrine (NE) in the regulation of cortical activity is underscored by clinical findings that involve this catecholamine and its receptor subtypes in the regulation of a large number of emotional and cognitive functions and illnesses. In this review, we highlight diverse effects of the LC/NE system in the mammalian cortex. Indeed, electrophysiological, pharmacological, and behavioral studies in the last few decades reveal that NE elicits a mixed repertoire of excitatory, inhibitory, and biphasic effects on the firing activity and transmitter release of cortical neurons. At the intrinsic cellular level, NE can produce a series of effects similar to those elicited by other monoamines or acetylcholine, associated with systemic arousal. At the synaptic level, NE induces numerous acute changes in synaptic function, and ׳gates' the induction of long-term plasticity of glutamatergic synapses, consisting in an enhancement of engaged and relevant cortical synapses and/or depression of unengaged synapses. Equally important in shaping cortical function, in many cortical areas NE promotes a characteristic, most often reversible, increase in the gain of local inhibitory synapses, whose extent and temporal properties vary between different areas and sometimes even between cortical layers of the same area. While we are still a long way from a comprehensive theory of the function of the LC/NE system, its cellular, synaptic, and plastic effects are consistent with the hypothesis that noradrenergic modulation is critical in coordinating the activity of cortical and subcortical circuits for the integration of sensory activity and working memory. This article is part of a Special Issue entitled SI: Noradrenergic System.

Amblyopia

Amblyopia refers to unilateral or bilateral reduction in best corrected visual acuity, not directly attributed to structural abnormality of the eye or posterior visual pathways. Early detection of amblyopia is crucial to obtaining the best response to treatment. Amblyopia responds best to treatment in the first few years of life. In the past several years a series of studies undertaken by the Pediatric Eye Disease Investigator Group (PEDIG) have been designed to evaluate traditional methods for treating amblyopia and provide evidence on which to base treatment decisions. This article summarizes and discusses the findings of the PEDIG studies to date.

c-Fos activity mapping reveals differential effects of noradrenaline and serotonin depletion on the regulation of ocular dominance plasticity in rats

The roles of the central noradrenergic and serotonergic system in the activity-dependent regulation of ocular dominance plasticity have been a contentious issue. Using c-Fos activity mapping, we have developed a new, straightforward method to measure the strength of ocular dominance plasticity: the number of c-Fos-immunopositive cells in layer IV of rat visual cortex (Oc1B), ipsilateral to the stimulated eye, is a sensitive and reliable measure of the effects of monocular deprivation. Applying this new method, here we studied the unique modification of the degree of c-Fos expression induced in the visual cortex, in that endogenous noradrenaline (NA) and serotonin (5HT) in the cortex were significantly reduced, respectively by specific pharmacological agents. Intraperitoneal injections of N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4) and p-chlorophenylalanine (pCPA) selectively impair NA- and 5HT-containing nerve terminals and fibers, respectively. In the visual cortex with strongly reduced NA, the number of c-Fos-immunopositive cells was found remaining significantly decreased in response to stimulation of the deprived eye, while by open eye stimulation the expected increase in c-Fos-immunoreactivity was strongly suppressed, showing values not different from those obtained by monocular stimulation in the normal rats. In contrast, in the visual cortex with strongly reduced 5HT no expected decrease was found in response to stimulation of the deprived eye, while, as is usually the case for the normal animals, a significant increase was still induced in response to open eye stimulation. These findings suggest that the noradrenergic and serotonergic system regulate ocular dominance (OD) plasticity differently: in the NA-depleted cortex the expected increase in c-Fos expression by open eye stimulation was not seen due to strong suppression, whereas in 5HT-depletion, the expected decrease in c-Fos expression was not materialized due to strong suppression. The present findings with c-Fos activity mapping method indicated a novel possibility of the differential regulation of OD plasticity by two types of common monoaminergic systems.

Paternal deprivation alters the development of catecholaminergic innervation in the prefrontal cortex and related limbic brain regions

The impact of paternal care on the development of catecholaminergic fiber innervations in the prefrontal cortex, nucleus accumbens, hippocampus and the amygdala was quantitatively investigated in the biparental Octodon degus. Two age (juvenile, adult) and rearing groups: (1) degus reared without father and (2) degus raised by both parents were compared. Juvenile father-deprived animals showed significantly elevated densities of TH-immunoreactive fibers in all analyzed regions, except in the orbitofrontal cortex, as compared to biparentally reared animals. This difference between the two rearing groups was still evident in adulthood in the prelimbic and infralimbic cortices and in the hippocampal formation. Interestingly, the elevated TH fiber density in both nucleus accumbens subregions was reversed in adulthood, i.e. adult father-deprived animals showed strongly reduced TH fiber densities as compared to biparentally reared animals. We show here that paternal care plays a critical role in the functional maturation of catecholaminergic innervation patterns in prefrontal and limbic brain circuits.

New concepts of molecular communication among neurons

Recently a number of complex electrophysiological responses to neurotransmitters have been observed that cannot be described as simple excitation or inhibition. These responses are often characterized as modulatory, although there is no consensus on what defines modulation. Morphological studies reveal certain neurotransmitters stored in what might be release sites without synaptic contact. There is no direct evidence for nonsynaptic release from CNS sites, although such release does occur in the periphery and in invertebrates. Nonsynaptic release might provide a basis for diffuse one-cell-to-many communication, but it might also simply be a means of sending the transmitter to a broader area of a single neuron than occurs in typical synapses. Several kinds of macromolecules have been found to be transported in a retrograde direction – and in some cases transsynaptically. There have been suggestions that some neurons may release more than one type of transmitter. Particularly intriguing is the possibility of release of substances that modulate actions of a primary transmitter. Taken together this range of evidence suggests that neurons may use a variety of forms of molecular communication in addition to traditionally described synaptic transmission.

Several authors have suggested modes of communication distinct from classical synaptic transmission and have classified released substances using terms such as neurohumor, neurohormone, neuroregulator, and modulator. These suggestions have the heuristic value of drawing together diverse kinds of data, but it remains to be established that the pieces fit together in that fashion – for example, that complex electrophysiological effects are associated with substances released nonsynaptically. In order to reduce confusion, a flexible, generic approach to nomenclature for substances released from neurons and for hypothetical modes of communication is recommended. Some behavioral implications of nonconventional transmission are considered.

Unique neural circuitry for neonatal olfactory learning

Imprinting ensures that the infant forms the caregiver attachment necessary for altricial species survival. In our mammalian model of imprinting, neonatal rats rapidly learn the odor-based maternal attachment. This rapid learning requires reward-evoked locus ceruleus (LC) release of copious amounts of norepinephrine (NE) into the olfactory bulb. This imprinting ends at postnatal day 10 (P10) and is associated with a dramatic reduction in reward-evoked LC NE release. Here we assess whether the functional emergence of LC alpha2 inhibitory autoreceptors and the downregulation of LC alpha1 excitatory autoreceptors underlie the dramatic reduction in NE release associated with termination of the sensitive period. Postsensitive period pups (P12) were implanted with either LC or olfactory bulb cannulas, classically conditioned with intracranial drug infusions (P14), and tested for an odor preference (P15). During conditioning, a novel odor was paired with either olfactory bulb infusion of abeta-receptor agonist (isoproterenol) to assess the target effects of NE or direct LC cholinergic stimulation combined with alpha2 antagonists and alpha1 agonists in a mixture to reinstate neonatal levels of LC autoreceptor activity to assess the source of NE. Pups learned an odor preference when the odor was paired with either olfactory bulb isoproterenol infusion or reinstatement of neonatal LC receptor activity. These results suggest that LC autoreceptor functional changes rather than olfactory bulb changes underlie sensitive period termination.

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.

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.

Singular subsets of locus coeruleus neurons may recover tyrosine hydroxylase phenotype transiently expressed during development

The number of tyrosine hydroxylase (TH)-expressing neurons appears to be precisely determined in basal conditions within the noradrenergic pontine nucleus locus coeruleus (LC). However, additional neurons exhibiting TH phenotype have been observed in the adult rat LC following a single administration of RU 24722, a potent inducer of TH expression specific to the LC. The neurons acquiring TH phenotype following treatment had a topographical localization similar to that of the neurons, which transiently expressed TH during postnatal development and lost TH phenotype during the third postnatal week. The idea that the fluctuation of TH phenotype in singular subsets of LC neurons during development may be selectively restored in adults is of particular interest. The present study attempted to determine whether the cells in which TH expression was repressed during the third postnatal week could correspond to those which exhibited TH phenotype in response to RU 24722 treatment in adults. We first verified that no massive cell death occurred in the LC during the period ranging from days 13 to 30. Then, we observed that both cell populations exhibited the same altered steady-state concentration of TH-mRNA as compared to cells that permanently expressed TH. Finally, we demonstrated the presence of TH-negative neurons expressing the homeodomain transcription factor Phox2a, specific for the determination of noradrenergic phenotype, providing further evidence that "resting-noradrenergic" neurons exist in the adult rat LC under basal conditions. These neurons provide interesting prospective for gain of noradrenergic function when classical noradrenergic LC neurons are impaired.

Role of the basal forebrain cholinergic projection in somatosensory cortical plasticity

Trimming all but two whiskers in adult rats produces a predictable change in cortical cell-evoked responses characterized by increased responsiveness to the two intact whiskers and decreased responsiveness to the trimmed whiskers. This type of synaptic plasticity in rat somatic sensory cortex, called "whisker pairing plasticity," first appears in cells above and below the layer IV barrels. These are also the cortical layers that receive the densest cholinergic inputs from the nucleus basalis. The present study assesses whether the cholinergic inputs to cortex have a role in regulating whisker pairing plasticity. To do this, cholinergic basal forebrain fibers were eliminated using an immunotoxin specific for these fibers. A monoclonal antibody to the low-affinity nerve growth factor receptor 192 IgG, conjugated to the cytotoxin saporin, was injected into cortex to eliminate cholinergic fibers in the barrel field. The immunotoxin reduces acetylcholine esterase (AChE)-positive fibers in S1 cortex by >90% by 3 wk after injection. Sham-depleted animals in which either saporin alone or saporin unconjugated to 192 IgG is injected into the cortex produces no decrease in AChE-positive fibers in cortex. Sham-depleted animals show the expected plasticity in barrel column neurons. In contrast, no plasticity develops in the ACh-depleted, 7-day whisker-paired animals. These results support the conclusion that the basal forebrain cholinergic projection to cortex is an important facilitator of synaptic plasticity in mature cortex.

Environmental enrichment results in higher levels of nerve growth factor mRNA in the rat visual cortex and hippocampus

Evidence for structural modifications in the brain following environmental changes have been provided during the last decades. The most pronounced alterations following environmental manipulations have been found in the visual cortex. These plastic changes are supposed to reflect reorganization of neuronal connections involved in postnatal development and adult adjustments of connections involved in sensori-perceptual processing and learning. Potential candidates to mediate these changes are neurotrophins. Nerve growth factor (NGF) has been associated with cognitive functions and shown to improve the performance of aged rats in spatial learning and memory task. In the central nervous system, NGF is of importance for development and maintenance of cholinergic neurons and atrophy of cholinergic neurons is strongly correlated with learning and memory impairments. Exposure to enriched environmental conditions improves learning and problem-solving ability and results in plastic changes in the brain. This study examined the effect of environmental enrichment on expression of NGF mRNA in the rat visual cortex and hippocampus. Rats housed in groups in a stimulus-rich environment for 30 days had significantly higher levels of NGF mRNA than rats housed individually in single cages without stimulus-enrichment. We have recently presented results showing higher levels of neurotrophin-3 (NT-3) mRNA and improved spatial learning following environmental enrichment, and suggest that an interplay involving the neurotrophins NGF and NT-3 may be mediating experience-induced structural changes.

Recovery of function is associated with increased spine density in cortical pyramidal cells after frontal lesions and/or noradrenaline depletion in neonatal rats

Rats were given medial frontal lesions at 7 days of age and were tested as adults on tests of forelimb use, forelimb tactile sensitivity, tongue use, hindleg use, and in a spatial navigation task. The brains were processed with a modified Golgi-Cox procedure and dendritic arborization and spine density was measured. The animals showed recovery only on the spatial task and this was associated with an increase in the number of spines per unit length of dendrite. We also reanalyzed Golgi-Cox stained material from an experiment in which animals were depleted of cortical noradrenaline (NA) in infancy and then given frontal lesions on day 7. The NA depletion blocked the recovery from frontal lesions. Analysis of dendritic morphology showed that in otherwise intact rats, NA depletion decreased dendritic arbor but increased spine density to the level of frontal operates. Depleted frontal-operates showed no additional increase in spine density and also showed a decrease in dendritic arborization. These results suggest that recovery from neonatal cortical injury and from neonatal noradrenaline depletion may be supported by changes in both the dendritic arborization and the spine density in the remaining cortex.

Controlled targeting of tyrosine hydroxylase protein toward processes of locus coeruleus neurons during postnatal development

Dendrites of locus coeruleus (LC) neurons laying within the pericoerulean neuropil (PCA) organize the major site where tyrosine hydroxylase (TH) is present throughout postnatal development. Those dendrites constitute the neuronal compartment in which TH levels increase beyond postnatal day (P) 21 or after RU24722-induced TH expression. Distal LC dendrites are present in the PCA by at least P20 but are devoid of TH and can rapidly accumulate TH protein when gene induction is triggered. Contrasting with the increase in TH levels within LC perikarya and dendrites, TH-mRNA concentration remains constant in LC perikarya from P4 to P42. Thus, supposing TH synthesis and degradation are also constant, any change in TH levels targeted toward axons might be balanced by a shift in the TH deposition within LC dendrites. This mechanism may be crucial in functions that the different processes of LC neurons have at critical steps of postnatal ontogeny.

Postnatal development of functional properties of visual cortical cells in rats with excitotoxic lesions of basal forebrain cholinergic neurons

In the rat, visual cortical cells develop their functional properties during a period termed as critical period, which is included between eye opening, i.e. postnatal day (PD) 15, and PD40. The present investigation was aimed at studying the influence of cortical cholinergic afferents from the basal forebrain (BF) on the development of functional properties of visual cortical neurons. At PD15, rats were unilaterally deprived of the cholinergic input to the visual cortex by stereotaxic injections of quisqualic acid in BF cholinergic nuclei projecting to the visual cortex. Cortical cell functional properties, such as ocular dominance, orientation selectivity, receptive-field size, and cell responsiveness were then assessed by extracellular recordings in the visual cortex ipsilateral to the lesioned BF both during the critical period (PD30) and after its end (PD45). After the recording session, the rats were sacrificed and the extent of both cholinergic lesion in BF and cholinergic depletion in the visual cortex was determined. Our results show that lesion of BF cholinergic nuclei transiently alters the ocular dominance of visual cortical cells while it does not affect the other functional properties tested. In particular, in lesioned animals recorded during the critical period, a higher percentage of visual cortical cells was driven by the contralateral eye with respect to normal animals. After the end of the critical period, the ocular dominance distribution of animals with cholinergic deafferentation was not significantly different from that of controls. Our results suggest the possibility that lesions of BF cholinergic neurons performed during postnatal development only transiently interfere with cortical competitive processes.

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.

Development and plasticity of cortical processing architectures

One of the basic functions of the cerebral cortex is the analysis and representation of relations among the components of sensory and motor patterns. It is proposed that the cortex applies two complementary strategies to cope with the combinatorial problem posed by the astronomical number of possible relations: (i) the analysis and representation of frequently occurring, behaviorally relevant relations by groups of cells with fixed but broadly tuned response properties; and (ii) the dynamic association of these cells into functionally coherent assemblies. Feedforward connections and reciprocal associative connections, respectively, are thought to underlie these two operations. The architectures of both types of connections are susceptible to experience-dependent modifications during development, but they become fixed in the adult. As development proceeds, feedforward connections also appear to lose much of their functional plasticity, whereas the synapses of the associative connections retain a high susceptibility to use-dependent modifications. The reduced plasticity of feedforward connections is probably responsible for the invariance of cognitive categories acquired early in development. The persistent adaptivity of reciprocal connections is a likely substrate for the ability to generate representations for new perceptual objects and motor patterns throughout life.

Involvement of nerve growth factor in visual cortex plasticity

The physiological role of nerve growth factor (NGF), the prototype member of the neurotrophin family, has been widely studied. NGF has been shown to promote survival, sprouting and differentiation of sympathetic ganglion cells and sensory neurons in the peripheral nervous system; it has also been shown to support survival and regeneration of cholinergic neurons in the central nervous system. Recent evidence indicates that NGF is also involved in the neuronal plasticity of the visual cortex. Exogenous supplies of NGF have been shown to interfere with normal processes underlying activity- and age-dependent synaptic modifications in both developing and adult visual cortex. In parallel to these physiological effects, numerous neuronal markers in the visual cortex have been found to be influenced by NGF. Several proposals have been introduced to explain the physiological role of NGF in visual cortex plasticity. Although the mechanisms underlying NGF effects in the visual cortex are still under active investigation, current evidence implies that NGF, and perhaps other neurotrophins as well, may be useful for preventing or correcting inappropriate or anomalous connections in the visual cortex, and thus for treating visual dysfunctions such as amblyopia and strabismus.

Development and regulation of alpha adrenoceptors in kitten visual cortex

Alpha-1 and alpha-2 adrenergic receptors were localized in developing cat visual cortex by using [3H]prazosin and [3H]rauwolscine, respectively as selective ligands. The effects of neuronal input on the development of the two receptor subtypes were also studied in animals with lesions at various sites within the central visual pathways. Binding densities for both ligands increased during the first few postnatal weeks and declined thereafter. For both receptor subtypes, the highest concentration of binding sites was found in the subplate zone of the cortex in neonatal animals. Both ligands showed their highest concentrations in cortical layer IV beginning at postnatal day 30 and in the superficial cortical layers in adulthood. However, the developmental redistribution of alpha-1 receptors began at earlier ages than that of the alpha-2 sites. The alpha-1 sites were still concentrated in the subplate zone up to 60 days postnatal, while the alpha-2 sites in this region disappeared much earlier. Receptor binding densities were also examined in animals with quinolinic acid lesions within cortex, lesions of the lateral geniculate nucleus and lesions of the optic tract. The results indicate that both alpha-adrenoceptor subtypes were mainly located on cortical cells, and that the absence of neuronal activity during development resulted in a reduction of the binding density for both subtypes in the visual cortex. An additional major reduction in alpha-2 but not alpha-1 binding sites was observed following the lateral geniculate nucleus lesion, suggesting that the development of alpha-2 receptors is also dependent on input from the lateral geniculate nucleus. Removal of the lateral geniculate nucleus early in life resulted in a significant increase in alpha-1 receptors in the subplate region, indicating that receptor densities in this zone may be negatively regulated by the lateral geniculate nucleus afferents. These results show that adrenergic receptors reorganize during postnatal cortical development with a strong temporary concentration in the subplate zone. The reorganization process is heavily influenced by cortical inputs.

The origin of synaptic neuroplasticity: crucial molecules or a dynamical cascade?

What is neuroplasticity and what are its origins? These questions have been the subject of intense theoretical and experimental research in the neurosciences for decades. Basically, the term neuroplasticity refers to the ability of neurons to alter some functional property in response to alterations in input. Traditional definitions, however, are often imprecise and restricted to particular 'model' neural systems. In the present article we will consider several of the most widely studied models of synaptic-level neuroplasticity including alterations in response properties of two types of invertebrate sensory neurons, long-term potentiation (LTP) in mammalian hippocampus and cortex, and ocular dominance shifts in cat visual cortex. While many other forms of neuroplasticity have been studied, these examples typify the diversity of the subject, as well as illustrate our contention that no unitary model of the phenomena is possible for all conditions. This last point is of particular importance for the mammalian literature, since many hypotheses concerning the mechanism(s) underlying the initiation of neuroplasticity have proposed a single crucial molecular element as the primary causal agent. A closer examination of these various hypotheses, in concert to several examples from the invertebrate literature, leads, however, to the conclusion that synaptic neuroplasticity must arise from a series of inter-related molecular events of a particular form, a cascade, in which individual elements may differ radically from system to system. We next provide an overview of our studies of age-dependent regulation of excitatory and inhibitory ionotropic neurotransmitter receptor populations in cortex in response to agonist and depolarizing stimulation. We provide evidence that such regulation for ionotropic receptors is under the control of ionically driven receptor kinase and phosphatase activity which is also age-dependent in function. These data provide the basis for a cascade model of receptor regulation. Based on this qualitative model, we describe a quantitative computer simulation of certain age-dependent stages in the receptor regulatory cascade which may interact to produce LTP-like effects. While such a model is not exclusive, it nevertheless provides a demonstration that elements in the proposed cascade may comprise the necessary and sufficient conditions for some forms of neuroplasticity. We also propose some of the principles underlying our model as a means of unifying much of the diverse phenomenology reported in the literature. Finally, we make a series of explicit predictions which are testable with current experimental techniques.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional postnatal development of the rat primary visual cortex and the role of visual experience: dark rearing and monocular deprivation

Postnatal development of rat visual cortical functions was studied by recording extracellularly from the primary visual cortex of 22 animals ranging in age from postnatal day 17 (P17) to P45. We found that in the youngest animals (P17-P19) all visual cortical functions tested were immature. Selectivity for orientation and movement direction of visual stimuli was almost absent, most cells received binocular input and their mean receptive field size was 5-6 times the adult size. Visual acuity was half its adult value. These functional properties developed gradually during the following weeks and by P45 they were all adult-like. This functional development is affected by manipulations of the visual input such as dark rearing (DR) and monocular deprivation (MD). DR prevented the normal postnatal maturation of visual cortical functions: in P60 rats, dark reared from birth, their visual cortical functions resembled those of P19-P21 rats. MD from P15 to P45 resulted in a dramatic shift of the ocular dominance distribution (ODD) in favour of the open eye and in a loss of visual acuity for the deprived eye. To determine the sensitive period of rat visual cortex to MD (critical period) we evaluated the shift in ODD of visual cortical neurones in rats that were subjected to the progressive delay of the onset of fixed MD period (10 days). Our results show that the critical period begins around the end of the third postnatal week, peaks between the fourth and fifth week and starts to decline from the end of the fifth week.

Monoaminergic afferents to cortex modulate structural plasticity in the barrelfield of the mouse

Electrolytic lesions of the follicles of a set of mystacial vibrissae, and their innervation, of the mouse placed during the early postnatal period result in a modification in appearance of the corresponding and of adjacent barrels in the somatosensory cortex of the adult animal. These changes can be evoked during the first 6 days of postnatal life--the so-called critical period. The pattern of these modifications varies with the age of the animal at which the lesion was placed. In order to evaluate the contribution of the monoaminergic cortical input to this type of plasticity, the noradrenergic and/or serotonergic afferents to the cerebral cortex of newborn mice were destroyed by systemic administration of various selective neurotoxic drugs (6-hydroxydopamine, 5,7-dihydroxytryptamine, N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine). The animals were then subjected, on postnatal day 3 (P3; P0 = day of birth), to a lesion of the follicles of the large, caudal mystacial vibrissae of row C. Control animals were injected with vehicle solution only but had the same follicles lesioned. Compared with animals with intact monoaminergic afferents, those treated with neurotoxins showed a different changed barrel pattern, i.e. one that corresponded to a pattern normally obtained after a lesion placed at an earlier stage of development, i.e. at P2 or P1. Thus, monoaminergic depletion of the cortex results in a retardation of the maturation of the parietal cortex as defined by its plastic response to peripheral nerve injury.(ABSTRACT TRUNCATED AT 250 WORDS)

Regeneration of norepinephrine-containing fibers in occipital cortex of adult cats

Regeneration of norepinephrine (NE)-containing nerve fibers in occipital cortex of adult cats was studied using morphological and biochemical methods. Initially, degeneration of cortical NE fibers was induced by direct infusion of 6-hydroxydopamine (6-OHDA) for a week. Cortical reinnervation by NE fibers continuously proceeded throughout 52 weeks, the longest survival period studied, after stopping 6-OHDA infusion. The rate of the reinnervation was slower in mature cortex than that obtained earlier in immature cortex. The present results indicate that the regenerative ability of the central NE neurons is universal, not limited to the immature brain. It is implied that the central NE neurons are equipped with a transmitter-specific repair mechanism throughout life.

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

The developmental pattern and laminar distribution of beta 1 and beta 2 adrenergic receptor subtypes were studied in cat visual cortex with autoradiography using [125I]iodocyanopindolol as a ligand and also with immunocytochemistry using a monoclonal antibody directed against beta adrenergic receptors. In the primary visual cortex of adult cats, the laminar distributions of both beta 1 and beta 2 adrenergic receptors revealed by autoradiography were very similar, with concentrations in layers I, II, III and VI. In young kittens (postnatal days 1 and 10), fewer beta adrenergic receptors were present, and they were concentrated in the deep cortical layers (V-VI) and subcortical white matter. Between postnatal days 15 and 40, beta adrenergic receptors increased in density more quickly in the superficial layers than they did in the deep and middle cortical layers. By postnatal day 40, the adult pattern was achieved, with two bands of intense binding in the superficial and deep cortical layers and a lower density in layer IV. Immunocytochemical techniques applied to adult cat cortex showed that beta adrenergic receptor-like immunoreactivity was found in different populations of neurons and glial cells. The immunoreactive neural cells were most dense in layers II, III and VI. About 50% of these immunoreactive neural cells were glial cells, primarily astrocytes. Immunoreactive pyramidal cells were mostly located in layers III and V. In layer IV, many stellate cells were stained. Immunoreactive astrocytes in the subplate and white matter progressively increased in number during development until adulthood. The pattern of laminar distribution and the developmental process was not affected by interrupting noradrenergic innervation from locus coeruleus either before or after the critical period. However, when visual input was interrupted by lesions of the lateral geniculate nucleus in young kittens (postnatal day 10), the density of both beta adrenergic receptor subtypes decreased significantly in the deep cortical layers. Lateral geniculate nucleus lesions in adult cats resulted in a pronounced decrease in beta adrenergic receptor density in layer IV.

Development of dopamine-beta-hydroxylase-positive fiber innervation of the rat hippocampus

Development of the noradrenergic fiber innervation of the rat hippocampus by the locus coeruleus was examined immunohistochemically in fixed tissue from animals aged 4 days through 55 days postnatal. The presence of tyrosine hydroxylase (TH) and dopamine-beta-hydroxylase (DBH) immunoreactive cells and fibers was evaluated in sections of hippocampus and locus coeruleus. Large, multipolar TH- and DBH-positive cells with long beaded fibers were visible within locus coeruleus at all ages; no immunopositive cell bodies were found in hippocampus. In hippocampal sections from mature animals (PN55), the highest density of DBH-stained fibers was found in stratum lucidum of CA3 and in the hilus and inner molecular layer of the dentate gyrus. Whereas similar patterns of fiber positivity were found at PN21 and PN10 (although with somewhat reduced density of immunopositive fibers), the pattern was quite different at PN4. Although fiber staining was relatively sparse at PN4, relative density of DBH fibers was highest in stratum radiatum of CA1 and subiculum. This change in staining pattern suggests that noradrenergic function in hippocampus may change as the rat matures. Double immunofluorescence techniques showed an overlap of DBH and TH positive fibers in all hippocampal regions at all ages. DBH immunostaining appeared to be somewhat more sensitive than the TH staining. These data made it impossible to confirm the presence of significant numbers of nonnoradrenergic, catecholamine-containing fibers in hippocampus.

Does the destruction of catecholaminergic neurons in newborn rats influence the modulator function of the cholinergic system?

The influence of the destruction of the catecholaminergic (CA) system on the reactions of neurons of the somatosensory zone of the cortex elicited by stimulation of the sciatic nerve, and on the features of the modulation of these reactions following the stimulation of the region of the basal nuclei (source of cholinergic innervation of the neocortex) and the microiontophoretic application of acetylcholine (ACh), was investigated in mongrel infant rats, 21-31 days of age. It was demonstrated that destruction of the CA system in newborn rats increases the reactivity of neurons somatosensory cortex to sensory stimulation, has no influence on the modulating effect of the cholinergic system, of the forebrain, and leads to intensification of the modulating influence of applied ACh [2].

Alpha 1-adrenoceptors in the adult rat barrel field: effects of deafferentation and norepinephrine removal

Norepinephrine (NE), acting on brain adrenoceptors, plays an important role in barrel field neuronal activity and plasticity. For this reason, the distribution of alpha 1- and alpha 2-adrenoceptors in the somatosensory cortex barrel field was studied by autoradiographic techniques in rats undergoing plastic change or NE depletion. In layers IV and V of the cortex, the pattern of alpha 1-adrenoceptors (assessed by [3H]prazosin binding) varied across the barrel field. There was relatively low binding within the barrels themselves, with 21% higher binding in the surrounding septa. alpha 2-Adrenoceptor binding (assessed with [3H]paraminoclonidine) was almost homogeneous across the entire barrel field. Two weeks after noradrenergic deafferentation by unilateral lesioning of the locus coeruleus, there was a 16% upregulation of [3H]prazosin binding. This then returned to control levels of by 8 weeks. Peripheral deafferentation of sensory input to the barrel field produced the opposite effect on alpha 1-adrenoceptors. Unilateral removal of all but the central (C3) vibrissa (which induces plastic changes in the cortical representation of the spared virbrissa) caused a 12% decrease in [3H]prazosin binding in the whole barrel field at 2 weeks after surgery which returned to normal by 8 weeks. Therefore, alpha 1-adrenoceptors in the barrel field of the rat are affected in opposite ways by changes in NE content and afferent sensory input. We hypothesize that alpha 1-adrenoceptor levels are modulated after vibrissectomy through either an indirect reaction to reduced cortical gamma-aminobutyric acid levels, or by a reordering of metabolic priorities during plastic change of the cortical neuronal network.

The ontogeny of repetitive firing and its modulation by norepinephrine in rat neocortical neurons

The postnatal ontogeny of electrical properties was studied in rat sensorimotor cortical neurons (P6 to adult) using intracellular recording in an in vitro slice preparation. Many action potential properties and input resistance changed during the first 4 postnatal weeks. Repetitive firing behavior also changed during the first postnatal month. Spike-frequency adaptation was much stronger in immature neurons. At 1 week postnatal, the majority of cortical neurons would only fire for less than 200 ms regardless of the intensity of long depolarizing current injections. These cells were normal in other parameters and could fire throughout a depolarizing current injection in the presence of inorganic calcium channel blockers or norepinephrine (NE), suggesting that the inability to fire was not due to injury. The frequency with which we encountered cells with this extreme adaptation decreased with age. Spike-frequency adaptation in immature neurons appears to be primarily controlled by Ca-dependent K+ conductances as in mature neurons. In mature and immature neurons, three afterhyperpolarizations (AHPs) could be distinguished by their rate of decline. The fast AHP followed repolarization of a single spike and was only partially Ca- and K-dependent. The medium duration AHP was Ca-dependent and apamin-sensitive and the slow AHP was partially Ca-dependent and not blocked by apamin. NE decreased the slow Ca-dependent AHP via beta-adrenergic receptors. This effect of NE on AHPs appeared qualitatively similar throughout postnatal development. NE had a proportionately greater effect in younger neurons, however, due to their relatively larger slow AHP. The quantitative differences of NE's action on the slow AHP (sAHP) led to a qualitative difference in NE's effect on firing behavior. The effects of NE on firing behavior may therefore be greater during times critical for cortical maturation.