Concentration-dependent suppression by beta-adrenergic antagonists of the shift in ocular dominance following monocular deprivation in kitten visual cortex

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.

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.

Activation of NMDA receptors is necessary for the recovery of cortical binocularity

Classic experiments have indicated that monocular deprivation (MD) for a few days during a critical period of development results in a decrease in the strength of connections mediating responses to the deprived eye, leading to a dramatic breakdown of cortical neuron binocularity. Despite the substantial functional change in the visual cortex, recovery from the effects of MD can be obtained if binocular vision is promptly restored. While great efforts have been made to elucidate the mechanisms regulating loss of deprived eye function, the mechanisms that underlie the recovery of cortical binocularity are poorly understood. Here, we examined whether activation of the N-methyl-d-aspartate receptor (NMDAR) is required for the recovery of cortical binocularity by pharmacologically blocking the NMDAR using d,l-2-amino-5-phosphonopentanoic (APV). Ferrets (n = 10) were monocularly deprived for 6 days, and osmotic minipumps, filled with APV (5.6 mg/ml) or saline, were surgically implanted into the primary visual cortex. One day after surgery, the deprived eye was reopened, and the animals were allowed 24 h of binocular vision. Extracellular recordings showed that intracortical infusion of the NMDAR antagonist, APV, prevented recovery of cortical binocularity while preserving neuronal responsiveness. These findings provide an important new insight for a specific role of NMDARs in the recovery of cortical binocularity from the effects of MD.

Neural plasticity maintained high by activation of cyclic AMP-dependent protein kinase: an age-independent, general mechanism in cat striate cortex

Adult cats lack ocular dominance plasticity, showing little change in the ocular dominance distribution following monocular deprivation. Ocular dominance plasticity is also lost in kitten visual cortex that has been continuously infused with either catecholaminergic neurotoxin, beta-adrenoreceptor blocker, or inhibitor of cyclic AMP-dependent protein kinase (protein kinase A). Complementarily, in adult cats we showed earlier that pharmacological activation of protein kinase A, albeit partially, restored ocular dominance plasticity. In the present study, we first asked whether, mediated by protein kinase A activation, the same molecular mechanisms could restore ocular dominance plasticity to kitten cortex that once lost the expression of plasticity due to prior pharmacological treatments. Concurrently with monocular deprivation, two kinds of cyclic AMP-related drugs (cholera toxin A-subunit or dibutyryl cyclic AMP) were directly infused in two types of aplastic kitten cortex pretreated with either 6-hydroxydopamine or propranolol. The combined treatment resulted in clear ocular dominance shift to the non-deprived eye, indicating that cortical plasticity was fully restored to aplastic kitten cortex. Next, to directly prove the sensitivity difference in protein kinase A activation between the immature and mature cortex, we compared the thus-obtained data in kittens with the published data derived from adult cats under the comparable experimental paradigm. The extent of ocular dominance changes following monocular deprivation was compared at different drug concentrations in the two preparations: the shifted ocular dominance distribution in aplastic kitten cortex infused with dibutyryl cyclic AMP at the lowest concentration tested and the W-shaped distribution in similarly treated adult cortex at a thousandfold-higher drug concentration that induced nearly maximal changes. We conclude that, irrespective of the animal's age, activation of protein kinase A cascades is a general mechanism to maintain ocular dominance plasticity high, their sensitivity being substantially higher in the immature than mature cortex.

Muscimol and baclofen differentially suppress retinotopic and nonretinotopic responses in visual cortex

This study relates to local field potentials and single-unit responses in cat visual cortex elicited by contrast reversal of bar gratings that were presented in single, double, or multiple discrete patch (es) of the visual field. Concurrent stimulation of many patches by means of the pseudorandom, binary m-sequence technique revealed interactions between their respective responses. An analysis identified two distinct components of local field potentials: a fast local component (FLC) and a slow distributed component (SDC). The FLC is thought to be a primarily postsynaptic response, as judged by its relatively short latency. It is directly generated by thalamocortical volleys following retinotopic stimulation of receptive fields of a small cluster of single cells, combined with responses to recurrent excitation and inhibition derived from the cells under study and immediately neighboring cells. In contrast, the SDC is thought to be an aggregate of dendritic potentials related to the long-range lateral connections (i.e. long-range coupling). We compared the suppressive effects of a GABAA-receptor agonist, muscimol, on the FLC and SDC with those of a GABAB-receptor agonist, baclofen, and found that muscimol more strongly suppressed the FLC than the SDC, and that the reverse was the case for baclofen. The differential suppression of the FLC and SDC found in the present study is consistent with the notion that intracortical electrical signals related to the FLC terminate on the somata and proximal/basal dendrites, while those related to the SDC terminate on distal dendrites.

Noradrenergic mechanisms in neurodegenerative diseases: a theory

A deficiency in the noradrenergic system of the brain, originating largely from cells in the locus coeruleus (LC), is theorized to play a critical role in the progression of a family of neurodegenerative disorders that includes Parkinson's disease (PD) and Alzheimer's disease (AD). Consideration is given here to evidence that several neurodegenerative diseases and syndromes share common elements, including profound LC cell loss, and may in fact be different manifestations of a common pathophysiological process. Findings in animal models of PD indicate that the modification of LC-noradrenergic activity alters electrophysiological, neurochemical and behavioral indices of neurotransmission in the nigrostriatal dopaminergic system, and influences the response of this system to experimental lesions. In models related to AD, noradrenergic mechanisms appear to play important roles in modulating the activity of the basalocortical cholinergic system and its response to injury, and to modify cognitive functions including memory and attention. Mechanisms by which noradrenaline may protect or promote recovery from neural damage are reviewed, including effects on neuroplasticity, neurotrophic factors, neurogenesis, inflammation, cellular energy metabolism and excitotoxicity, and oxidative stress. Based on evidence for facilitatory effects on transmitter release, motor function, memory, neuroprotection and recovery of function after brain injury, a rationale for the potential of noradrenergic-based approaches, specifically alpha2-adrenoceptor antagonists, in the treatment of central neurodegenerative diseases is presented.

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.

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.

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.

Restoration of ocular dominance plasticity mediated by adenosine 3',5'-monophosphate in adult visual cortex

Noradrenaline (NA)-stimulated beta-adrenoreceptors activate adenylate cyclase via excitatory G-proteins (Gs). Activated adenylate cyclase in turn promotes the production of cAMP. Critical roles of cAMP-dependent protein kinase A (PKA) in divergent cellular functions have been shown, including memory, learning and neural plasticity. Ocular dominance plasticity (ODP) is strongly expressed in early postnatal life and usually absent in the mature visual cortex. Here, we asked whether the activation of cAMP-dependent PKA could restore ODP to the aplastic visual cortex of adult cats. Concurrent with brief monocular deprivation, each of the following cAMP-related drugs was directly and continuously infused in the adult visual cortex: cholera toxin (a Gs-protein stimulant), forskolin (a Gs-protein-independent activator of adenylate cyclase) and dibutyryl cAMP (a cAMP analogue). We found that the ocular dominance distribution became W-shaped, the proportion of binocular cells being significantly lower than that in respective controls. We concluded that the activation of cAMP cascades rapidly restores ODP to the adult visual cortex, though moderately. The finding further extends the original hypothesis that the NA-beta-adrenoreceptors system is a neurochemical mechanism of cortical plasticity.

Roles of N-methyl-D-aspartate receptors in ocular dominance plasticity in developing visual cortex: re-evaluation

We have re-examined whether N-methyl-D-aspartate receptors play a specific role in experience-dependent plasticity in kitten visual cortex. A specific antagonist of this glutamate receptor subtype, D,L-2-amino-5-phosphonovaleric acid, was directly and continuously infused into kitten striate cortex for one week concurrently with monocular lid suture. In the hemisphere infused with 50 mM antagonist, we found the usual shift in ocular dominance toward the open eye with only a few binocular cells remaining. The changes were accompanied by an extremely high incidence (38%) of abnormal cells lacking orientation selectivity across different ocular dominance groups. In kitten cortex infused with 10 mM antagonist concurrently with monocular deprivation for a week, recording from a drug-affected region near the infusion centre, we again found the U-shaped ocular dominance distribution with the high incidence of non-selective cells. In antagonist-infused, otherwise normal striate cortex of adult cats, we found that the proportion of binocular cells decreased by one-half in two cellular populations: one recorded during the continuous infusion of 10 mM antagonist under general anaesthesia and paralysis, and the other about two days after stopping the infusion. We also established that in vivo concentrations of chronically infused 10 mM antagonist decreased, not near-exponentially, but linearly with increasing distance from the infusion site. Thus, the effects of a directly and continuously infused, concentrated antagonist of N-methyl-D-aspartate receptors on receptive-field properties of visuocortical cells are complex. The present findings strongly suggest that the antagonist effects in the developing cortex may be due primarily to blockade of normal synaptic transmission rather than specific disruption of an experience-dependent mechanism underlying ocular dominance plasticity.

Registration of neural maps through value-dependent learning: modeling the alignment of auditory and visual maps in the barn owl's optic tectum

In the optic tectum (OT) of the barn owl, visual and auditory maps of space are found in close alignment with each other. Experiments in which such alignment has been disrupted have shown a considerable degree of plasticity in the auditory map. The external nucleus of the inferior colliculus (ICx), an auditory center that projects massively to the tectum, is the main site of plasticity; however, it is unclear by what mechanisms the alignment between the auditory map in the ICx and the visual map in the tectum is established and maintained. In this paper, we propose that such map alignment occurs through a process of value-dependent learning. According to this paradigm, value systems, identifiable with neuromodulatory systems having diffuse projections, respond to innate or acquired salient cues and modulate changes in synaptic efficacy in many brain regions. To test the self-consistency of this proposal, we have developed a computer model of the principal neural structures involved in the process of auditory localization in the barn owl. This is complemented by simulations of aspects of the barn owl phenotype and of the experimental environment. In the model, a value system is activated whenever the owl carries out a foveation toward an auditory stimulus. A term representing the diffuse release of a neuromodulator interacts with local pre- and postsynaptic events to determine synaptic changes in the ICx. Through large-scale simulations, we have replicated a number of experimental observations on the development of spatial alignment between the auditory and visual maps during normal visual experience, after the retinal image is shifted through prismatic goggles, and after the reestablishment of normal visual input. The results suggest that value-dependent learning is sufficient to account for the registration of auditory and visual maps of space in the OT of the barn owl, and they lead to a number of experimental predictions.

Continuous and direct infusion of drug solutions in the brain of awake animals: implementation, strengths and pitfalls

One of the best strategies for understanding an animal's behavior is to study the function of the brain by experimentally modifying brain chemistry temporarily or on a long-term basis. This can be achieved by direct manipulation of neurochemistry of a targeted brain area with various drugs whose in vitro specificity and sensitivity are known. We assume that an animal's behavior is primarily controlled by the integrated performance of neural networks, rather than the action of a "superstar" single neuron which has narrowly tuned selectivity, in a specified brain region. Therefore, the former must be regulated by a large number of combinations of various transmitter/modulator receptors, hormones, growth factors, and other biochemically identifiable and yet unidentified substances. Under certain conditions, the activation of receptor-bound second messenger systems is thought to cause the enhanced expression of particular genes. Given the wide possibilities in manipulating brain chemistry, which may otherwise result in a variety of consequences, it is crucial to have a dependable means of sustaining the steady-state action of a drug for a sufficiently long time period at a targeted area in the brain of behaving animals. In most cases the continuous application of a drug is necessary to counteract its secondary mitigating effect, which is set in action through negative feedback loops and which in effect reduces the primary action of the drug in use. We have developed a technique to answer this need, using the Alzet osmotic minipump as the source of the continuous infusion force. A drug solution is continuously and directly infused, guided through a chronically implanted cannula, into a targeted area in the brain of behaving animals. The consequences of such an infusion are assessed, during as well as after the infusion, using various types of measurements in behavior, biochemistry, neurophysiology, pharmacology and morphology. The method has been successfully applied, for example, to the study of developmentally regulated neural plasticity in cat visual cortex. A few preconditions should be satisfied for the method to be properly applied to the brains of live animals. Those are: (1) manufacturing a suitable guide system, i.e., cannula-minipump assembly, for the infusion solution; (2) stereotaxic implantation of a cannula-minipump assembly into a selected brain region; and (3) estimating the concentration gradient of the continuously infused solution. This is crucial to assess the specificity and sensitivity of a drug for its assumed effects in vivo.

GABAB receptors, monoamine receptors, and postsynaptic inositol trisphosphate-induced Ca2+ release are involved in the induction of long-term potentiation at visual cortical inhibitory synapses

gamma-Aminobutyric acid (GABA)A receptor-mediated inhibitory synaptic transmission in visual cortex undergoes long-term potentiation (LTP), which is input-specific and associative. The present study, conducted under a blockade of ionotropic glutamate receptors, demonstrates an induction mechanism of LTP considerably different from those of associative LTP at excitatory synapses. Inhibitory responses of layer V cells evoked by layer IV stimulation were studied in developing rat visual cortex slices by using intracellular and whole-cell recording methods. LTP induction was prevented by the application of an antagonist for GABAB receptors but not for GABAA or metabotropic glutamate receptors. Inhibition of postsynaptic G-proteins, phospholipase C, inositol trisphosphate (IP3) receptors, or Ca2+ increase prevented the generation of LTP, as did the blockade of GABAB receptors. In rat cerebral cortex, GABAB receptor activation is not known to affect the IP3 level by itself. However, it facilitates IP3 formation induced by the activation of alpha 1 adrenoceptors, which are believed to be located postsynaptically. Accordingly, I examined the involvement of these and other amine receptors, including histamine H1, muscarinic acetylcholine, and serotonin 5-HT2 receptors, all of which are coupled to IP3 formation. Only the blockade of alpha 1 adrenoceptors or serotonin 5-HT2 receptors prevented LTP induction in most, but not all, of the cells. These results suggest that LTP induction requires the activation of postsynaptic GABAB receptors and that its effect is mediated at least partly by facilitation of the monoamine-induced IP3 formation, which then causes Ca2+ release from the internal stores in postsynaptic cells.

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.

Postnatal development of adrenergic terminals in rat locus coeruleus, with special reference to growth of noradrenergic neurons

The postnatal development of noradrenergic (NA) neurons and adrenergic (AD) terminals in the rat locus coeruleus (LC) was studied immunohistochemically. Cell body size was measured after staining of NA neurons with anti-tyrosine hydroxylase (TH) serum, and AD terminals were visualized with anti-phenylethanolamine N-methyltransferase serum. NA neurons in the LC were strongly TH-immunoreactive throughout the postnatal period. At birth, their mean cell body volume was 660 +/- 30 microns 3. It reached a maximum of 2580 +/- 230 microns 3 at postnatal day (PD) 14, and decreased thereafter to 930 +/- 50 microns 3 at PD 60. This transient enlargement of NA neurons may be closely related to the development of the cerebral cortex. AD afferents to the LC had terminals forming predominantly asymmetric junctions at birth (about 96% of all junctions). They occasionally made axo-somatic contact, suggesting that AD input already modulated the activity of LC neurons at this stage. AD terminals making axo-spinous synapses increased in number until PD 31, but still represented a minor proportion of these LC terminals, since there were more than 80% in contact with dendritic shafts at all ages examined.

The action of neurotrophins in the development and plasticity of the visual cortex

Nerve growth factor (NGF) and the other members of the NGF gene family have been extensively characterized as neurotrophic factors. Recently a modulatory action of these neurotrophic factors on synapse efficacy has emerged. The developing visual system has provided a convenient model to test the role of neurotrophins on neural plasticity in vivo.

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.

Involvement of serotonin in developmental plasticity of kitten visual cortex

During a critical period of postnatal development, neuronal connections in the kitten visual cortex are susceptible to experience-dependent modifications. These modifications are facilitated by the neuromodulators noradrenaline and acetylcholine. To address the question of whether serotonin (5-hydroxytryptamine; 5-HT), the other major neuromodulator in the cerebral cortex, also plays a role in developmental plasticity, we investigated whether interference with serotoninergic transmission in the kitten visual cortex affects ocular dominance (OD) plasticity. The serotonin neurotoxin 5,7-dihydroxytryptamine or the serotonin receptor blockers ketanserin and methysergide were infused into the visual cortex of kittens undergoing monocular deprivation. We found that both methods of disrupting serotoninergic transmission reduced OD plasticity. However, to be effective, the receptor blockers ketanserin and methysergide had to be applied in combination, suggesting that coactivation of serotonin receptor subtypes of both the 5-HT1 and 5-HT2 families have a permissive function in OD plasticity. Since activation of 5-HT2 receptors stimulates phosphoinositide hydrolysis, our data suggest that second messengers from the phospholipid pathway may play an important role in developmental plasticity of visual cortex.

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.

Effects of intracortical infusion of anticholinergic drugs on neuronal plasticity in kitten striate cortex

During a critical period of postnatal development the mammalian visual cortex is highly susceptible to experience-dependent alterations of neuronal response properties. These modifications are facilitated by the neuromodulators noradrenaline and acetylcholine. To identify the cholinergic mechanisms responsible for this facilitation, muscarinic and nicotinic antagonists were infused into the visual cortex of kittens while the animals were subject to monocular deprivation. Subsequently the ocular dominance of cortical cells was assessed by single-unit recording. Ocular dominance changes were suppressed by scopolamine and pirenzepine but not by gallamine, hexamethonium and mecamylamine. This blocking effect was concentration-dependent, and control experiments revealed that it was not due to suppression of neuronal responses to light. It is concluded from these results that acetylcholine facilitates neuronal plasticity in the visual cortex through mechanisms activated by muscarinic M1 receptors.

Gliotoxin-induced suppression of ocular dominance plasticity in kitten visual cortex

We studied the role of astrocytes in the regulation of ocular dominance plasticity. A small quantity of 10 microM fluorocitrate (0.2 nmol in 20 microliters) was pressure-injected into the visual cortex of 7-9-week-old kittens (subcortical depth: 0-5 mm, 20 microliters/10 min). Immediately after injection, 1 eye contralateral to the injected cortex was closed for 3 days. Single-unit recordings revealed that the proportion of binocular cells was significantly higher in a region close (approximately 1 mm) to the fluorocitrate injection site than that in a remote region (> 4 mm) within the same hemisphere and that in the opposite hemisphere. The results suggest that reduction of glial functions by fluorocitrate retarded the usual process of shift in ocular dominance of visual cells following monocular deprivation.

Morphology and distribution of neurons and glial cells expressing beta-adrenergic receptors in developing kitten visual cortex

The morphology and distribution of cells expressing beta-adrenergic receptors has been studied in developing kitten visual cortex using a monoclonal antibody which recognizes both beta-1 and beta-2 adrenergic receptors. We found specific populations of neurons and glial cells which express beta-adrenergic receptor immunoreactivity in the kitten visual cortex. In adult animals, the receptors are most concentrated in the superficial and deep cortical layers (layers I, II, III and VI). About 50% of the stained neural cells in adult cat visual cortex are glial cells. Most of the immunoreactive neurons in layers III and V are pyramidal cells while those in layers II and IV are more likely to be nonpyramidal cells. In neonatal kittens, staining is weaker than that in adult cats and it appears to be concentrated in neurons of the deep cortical layers and in the subcortical plate and white matter. Only a few immunoreactive glial cells were found at this age. Receptor numbers increase after birth and by 24 days of age, the laminar distribution of beta-adrenergic receptors approaches that of adult animals. Immunoreactive glial cells in the white matter show a progressive increase in number throughout postnatal development.

A role for glial cells in activity-dependent central nervous plasticity? Review and hypothesis

Activity-dependent plasticity relies on changes in neuronal transmission that are controlled by coincidence or noncoincidence of presynaptic and postsynaptic activity. These changes may rely on modulation of neural transmission or on structural changes in neuronal circuitry. The present overview summarizes experimental data that support the involvement of glial cells in central nervous activity-dependent plasticity. A role for glial cells in plastic changes of synaptic transmission may be based on modulation of transmitter uptake or on regulation of the extracellular ion composition. Both mechanisms can be initiated via neuronal-glial information transfer by potassium ions, transmitters, or other diffusible factor originating from active neurons. In addition, the importance of changes in neuronal circuitry in many model systems of activity-dependent plasticity is summarized. Structural changes in neuronal connectivity can be influenced or mediated by glial cells via release of growth or growth permissive factors on neuronal activation, and by active displacement and subsequent elimination of axonal boutons. A unifying hypothesis that integrates these possibilities into a model of activity-dependent plasticity is proposed. In this model glial cells interact with neurons to establish plastic changes; while glial cells have a global effect on plasticity, neuronal mechanisms underlie the induction and local specificity of the plastic change. The proposed hypothesis not only explains conventional findings on activity-dependent plastic changes, but offers an intriguing possibility to explain several paradoxical findings from studies on CNS plasticity that are not yet fully understood. Although the accumulated data seem to support the proposed role for glial cells in plasticity, it has to be emphasized that several steps in the proposed cascades of events require further detailed investigation, and several "missing links" have to be addressed by experimental work. Because of the increasing evidence for glial heterogeneity (for review see Wilkin et al., 1990) it seems to be of great importance to relate findings on glial populations to the developmental stage and topographical origin of the studied cells. The present overview is intended to serve as a guideline for future studies and to expand the view of "neuro" physiologists interested in activity-dependent plasticity. Key questions that have to be addressed relate to the mechanisms of release of growth and growth-permissive factors from glial cells and neuronal-glial information transfer. It is said that every complex problem has a simple, logical, wrong solution. Future studies will reveal the contribution of the proposed simple and logical solution to the understanding of central nervous plasticity.

Lithium reduces ocular dominance plasticity in kitten visual cortex

Activity-dependent processes within the visual pathway play a crucial role in the expression of ocular dominance plasticity in immature visual cortex. The necessity of non-retinal, modulatory afferents to the regulation of ocular dominance plasticity has been recognized. Among a few chemically defined signaling systems, the noradrenaline-activated beta-adrenoreceptors seem to have a prime role in this matter. The involvement of acetylcholine afferents was also proven. We looked for plausible molecular mechanisms which integrate the contribution of the two neuromodulator systems to ocular dominance plasticity. At the first step, based on the rich literature on psychotropic action of lithium and the recent advancement in understanding of its molecular mechanisms, we physiologically studied visual cortex of kittens which had been repeatedly injected with the lithium solution intraperitoneally or the cortex directly infused with it. We found (1) that ocular dominance plasticity was significantly reduced in lithium-injected kittens, (2) that the decrease was directly correlated with plasma concentrations of lithium (i.e. the higher the lithium concentration, the lower the plasticity), and (3) that the comparable decrease in the plasticity was obtained from kitten visual cortex which had been directly infused with the lithium solution. The present results suggest that lithium-sensitive processes, through most likely reduced production of second messengers, underlie the regulation of ocular dominance plasticity.

Adrenergic regulation of visuocortical plasticity: a role of the locus coeruleus system

Noradrenaline-beta-adrenoceptor-mediated neural plasticity in cat visual cortex exemplifies clearly established roles of the locus coeruleus system in brain function. The prime role of the noradrenaline-beta-adrenoceptor system in the regulation of ocular dominance plasticity is discussed in this chapter and includes a newly invented paradigm of ocular dominance changes under anesthesia and paralysis without benefit of visual attention. Based on our recent findings, we have sought to integrate positive contributions of muscarinic cholinergic receptors to the beta-adrenoceptor-mediated regulatory processes. The issue of "activity dependency" is important and we recognize the necessity of designing new studies in which relationships between activity dependency within the visual pathway and global neurochemical/cellular factors can be tested directly. Further, we critically reviewed the involvement of gamma-aminobutyric acidA type receptors and N-methyl-D-aspartate receptors in the regulation of ocular dominance plasticity.

Developmental aspects of the locus coeruleus-noradrenaline system

The locus coeruleus-noradrenaline (LC-NA) system exhibits an early developmental pattern, so that its nerve terminals are present in target areas before formation of most synapses. Several properties of the source neurons in the LC change substantially during early postnatal periods: spontaneous activity patterns, responsiveness to sensory stimulation, and responsiveness to NA. The effect may be to confer enhanced responsiveness of LC neurons, and an enhanced release of NA in target areas, during early postnatal development. Developmental changes in density of adrenoceptors or adrenergic responsiveness in target areas have also been documented. The usual pattern is a progressive increase in adrenergic ligand binding, with some reduction during later phases of development. However, there are a number of examples of receptor subtypes and region-specific transient binding during the first few weeks of postnatal life, followed by reductions to very low levels. These observations may reflect developmentally transient adrenergic responsiveness in certain target areas. NA and the LC-NA system have been implicated in the control of morphological and functional properties of neurons in target areas, and in the control of developmentally important biochemical systems (ornithine decarboxylase). NA, as well as other neurotransmitters, may individually, or in cooperation, exert important trophic influences during a restricted developmental period.

Evidence for the Involvement of Rat Olfactory Bulb in Processes Supporting Long-Term Olfactory Memory

Current advances in the neurobiology of learning and memory suggest the existence of experience-induced plasticity in sensorial pathways conveying relevant information to higher integrative brain structures. For instance, olfactory learning is known to induce long-lasting modifications of neural activity at the level of the first relay structure of the olfactory system, the olfactory bulb. The observed forms of plasticity depend on the action exerted during learning by ascending neuromodulatory systems, such as the noradrenergic (NA) system originating from the locus ceruleus. This study was aimed at investigating the importance of olfactory bulb plasticity in learning and retention of an olfactory task. In a daily training schedule animals had to learn to use multi-site electrical stimulation patterns of the olfactory bulb as discriminative cues for choosing between a palatable and a nonpalatable solution. We first examined the effects of a continuous intrabulbar infusion of propranolol (a beta-NA receptor antagonist) carried out during the learning period. We found that this treatment neither impaired the retention of a previously learned task nor the learning of a new task. However, the animals presented a severe deficit in long-term retention (>5 days) of the task learned under perfusion. Unexpectedly, this effect cannot be ascribed to a selective blockade of beta-NA receptors since infusion of the drug vehicle (saline-ascorbate) produced exactly the same deficit while a saline solution remained without effect. A final experiment showed that the selective deficit in long-term retention was not observed when the infusion of the saline-ascorbate solution started on the day following completion of learning. Taken together, these results suggest that ascorbate-sensitive neural processes occurring within the olfactory bulb during learning are of functional importance for long-term storage of olfactory information.

Interaction of noradrenergic and cholinergic systems in regulation of ocular dominance plasticity

We studied interactions among the noradrenergic (NA) and the muscarinic cholinergic (ACh) systems in the regulation of ocular dominance plasticity in kitten visual cortex. The cortex was bilaterally infused with 6-hydroxydopamine (6-OHDA) for a week. Upon termination of the 6-OHDA infusion, one hemisphere was infused with a muscarinic ACh agonist, bethanechol, through the same, chronically implanted cannula for the second week together with monocular lid suture. The other hemisphere received an infusion of the vehicle solution alone. (1) Only in the hemisphere infused with bethanechol at relatively high concentrations did we obtain a clear shift in ocular dominance. We also found that the effect of bethanechol was concentration-dependent. (2) By comparing necessary concentrations of bethanechol and NA for the respective maximal effects, we noted that the former was at least 100-fold less effective than the latter in restoring the plasticity. (3) The cortical infusion of bethanechol did not restore the plasticity to the propranolol-pretreated cortex; the ocular dominance distribution remained virtually unchanged. This result was interpreted as suggesting that functioning beta-adrenoreceptors are needed for the cortical effect of activating the muscarinic ACh receptors to become detectable. (4) The expected shift in ocular dominance following monocular deprivation was partially suppressed, when highly concentrated scopolamine, a muscarinic ACh antagonist, was used, indicating that the involvement of the ACh system in this matter was indirect. The concentration of scopolamine needed for the half-maximum effect was 172-fold higher than that of propranolol. We thus conclude that the involvement of the muscarinic ACh system in ocular dominance plasticity is secondary to that of the NA-beta-adrenoreceptor system.

Noradrenergic control of ocular dominance plasticity in the visual cortex of dark-reared cats

In the visual cortex of cats which had been dark-reared for several months since the time before natural eye opening, a cortical infusion of 6-hydroxydopamine (6-OHDA), a noradrenaline (NA)-related neurotoxin, partially suppressed a usual shift in ocular dominance following brief monocular lid suture, causing a significant loss of binocular cells. This partial shift in ocular dominance (U-shaped histogram) was also observed typically in the control hemisphere of cats which were subjected to dark-rearing for more than a year. Furthermore, the expected shift in ocular dominance following monocular deprivation was blocked by a direct cortical infusion of D,L-metoprolol, a selective beta 1-adrenergic receptor antagonist. The blockade was not obtained by D-metoprolol, a biologically inert stereo-isomer, under the comparable condition. In contrast, exogenous L-NA gave rise to an obvious shift in ocular dominance toward the non-deprived eye. The present results suggest that the NA-beta 1 adrenoreceptor system was necessary to maintain the ocular dominance plasticity in the visual cortex of dark-reared cats.

The role of muscarinic acetylcholine receptors in ocular dominance plasticity

During a critical period of postnatal development neuronal connections in the visual cortex are susceptible to experience-dependent modifications. In normally reared kittens the majority of neurons respond to visual stimulation of either eye. A few days of monocular deprivation, however, are sufficient to render most cortical neurons unresponsive to visual stimuli presented to the deprived eye. Among other factors the cholinergic projection to striate cortex has been identified as having a permissive role in this use-dependent modification of synaptic transmission. In order to analyze further the influence of acetylcholine in cortical plasticity, we tested whether the blockade of muscarinic or nicotinic receptors interfered with ocular dominance plasticity. At four weeks of age kittens had one eyelid sutured closed and osmotic minipumps implanted, which delivered scopolamine (1 nmol/h) or hexamethonium (1 or 10 nmol/h) into the striate cortex of one hemisphere and vehicle solution (saline) into the other. After one week, ocular dominance distributions were determined in area 17 with single unit recording. In the control hemispheres, most neurons became unresponsive to the deprived eye, while in the scopolamine-treated hemispheres most neurons remained binocular. In contrast to the effects of scopolamine, the intracortical infusion of hexamethonium had no effect on ocular dominance plasticity. These results demonstrate that blockade of muscarinic, but not nicotinic receptors renders kitten striate cortex resistant to the effects of monocular deprivation.

Acutely induced shift in ocular dominance during brief monocular exposure: effects of cortical noradrenaline infusion

Acutely anesthetized and paralysed kittens were monocularly exposed to TV shows for approximately 20 h, throughout which the visual cortex received a direct infusion of (-)-noradrenaline (NA) or (+)-NA. The ocular dominance distribution was found shifted toward the exposed eye in the (-)-NA-infused cortex, but not in control cortex, i.e. the drug-free kitten cortex, the kitten cortex infused with (+)-NA, and the (-)-NA-infused adult cortex. Noise patterns on the same TV screen also failed to induce a shift in the (-)-NA-infused kitten cortex. It is concluded that shift in ocular dominance takes place in kittens, even under general anesthesia and paralysis, when cortical (-)-NA infusion and pattern vision through one eye are combined.

Norepinephrinergic reinnervation of cat occipital cortex following localized lesions with 6-hydroxydopamine

We studied biochemical and morphological changes in central catecholamine (CA) terminals in the kitten visual cortex following direct infusion with 4 mM 6-hydroxydopamine (6-OHDA) for a week. Two zones may be distinguished within the cortical area affected by 6-OHDA (a radius of approximately 10 mm). In the primary lesion zone (a radius of approximately 5 mm) near the center of the 6-OHDA infusion, excluding an area of non-specific damage left by cannulation (a radius of less than 1.5 mm), we found: (1) absence of fluorescent CA terminals by histochemistry; (2) very low desipramine-sensitive uptake of tritiated norepinephrine (NE) by cortical slices (desipramine-resistant NE uptake stayed high); (3) a 50% increase in beta-adrenoreceptor binding sites by densitometry of light microscopic autoradiograms; and (4) low levels (less than 20% of control) of endogenous NE and low to moderate levels (10-70%) of endogenous dopamine (DA). In the surrounding zone (about 5-10 mm from the infusion center), however, none of the above changes were observed, except for a moderate to substantial reduction (50-80% of control) in endogenous NE and a small (10-20%) reduction in endogenous DA. Within two weeks after the end of the cortical 6-OHDA infusion, the dimensions of the cortical area devoid of CA terminals became substantially smaller than those found earlier. Fluorescent CA terminals were seen virtually everywhere in the cortex by 4 weeks, including the scar left by placement of the infusion cannula. In 24 weeks CA terminals in the occipital cortex appeared close to normal in density as well as in fluorescence intensity. Biochemical assays also revealed the recovery trend of CA contents. A steady increase in the NE content was obtained in the surrounding zone, with the stronger trend at its periphery, eventually attaining full recovery in 23 weeks. The recovery was slow in the primary lesion zone, especially near the infusion center, though there was a continual increase in endogenous DA toward control even at the infusion center.