Two methods of catecholamine depletion in kitten visual cortex yield different effects on plasticity

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.

Selective rapid eye movement sleep deprivation affects cell size and number in kitten locus coeruleus

Cells in the locus coeruleus (LC) constitute the sole source of norepinephrine (NE) in the brain and change their discharge rates according to vigilance state. In addition to its well established role in vigilance, NE affects synaptic plasticity in the postnatal critical period (CP) of development. One form of CP synaptic plasticity affected by NE results from monocular occlusion, which leads to physiological and cytoarchitectural alterations in central visual areas. Selective suppression of rapid eye movement sleep (REMS) in the CP kitten enhances the central effects of monocular occlusion. The mechanisms responsible for heightened cortical plasticity following REMS deprivation (REMSD) remain undetermined. One possible mediator of an increase in plasticity is continuous NE outflow, which presumably persists during extended periods of REMSD. Tyrosine hydroxylase (TH) is the rate-limiting enzyme in the synthesis of NE and serves as a marker for NE-producing cells. We selectively suppressed REMS in kittens for 1 week during the CP. The number and size of LC cells expressing immunoreactivity to tyrosine hydroxylase (TH-ir) was assessed in age-matched REMS-deprived (RD)-, treatment–control (TXC)-, and home cage-reared (HCC) animals. Sleep amounts and slow wave activity (SWA) were also examined relative to baseline. Time spent in REMS during the study was lower in RD compared to TXC animals, and RD kittens increased SWA delta power in the latter half of the REMSD period. The estimated total number of TH-ir cells in LC was significantly lower in the RD than in the TXC kittens and numerically lower than in the HCC animals. The size of LC cells expressing TH-ir was greatest in the HCC group. HCC cells were significantly larger than TH-ir cells in the RD kittens. These data are consistent with presumed reduction in NE in forebrain areas, including visual cortex, caused by 1 week of REMSD.

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.

Physiological correlates of perceptual learning in monkey V1 and V2

Performance in visual discrimination tasks improves with practice. Although the psychophysical parameters of these improvements have suggested the involvement of early areas in visual cortex, there has been little direct study of the physiological correlates of such perceptual learning at the level of individual neurons. To examine how neuronal response properties in the early visual system may change with practice, we trained monkeys for more than 6 mo in an orientation discrimination task in which behaviorally relevant stimuli were restricted to a particular retinal location and oriented around a specific orientation. During training the monkeys' discrimination thresholds gradually improved to much better than those of naive monkeys or humans. Although this improvement was specific to the trained orientation, it showed little retinotopic specificity. The receptive field properties of single neurons from regions representing the trained location and a location in the opposite visual hemifield were measured in V1 and V2. In most respects the receptive field properties in the representations of the trained and untrained regions were indistinguishable. However, in the regions of V1 and V2 representing the trained location, there were slightly fewer neurons whose optimal orientation was near the trained orientation. This resulted in a small but significant decrease in the V1 population response to the trained orientation at the trained location. Consequently, the observed neuronal populations did not exhibit any orientation-specific biases sufficient to explain the orientation specificity of the behavioral improvement. Pooling models suggest that the behavioral improvement was accomplished with a task-dependent and orientation-selective pooling of unaltered signals from early visual neurons. These data suggest that, even for training with stimuli suited to the selectivities found in early areas of visual cortex, behavioral improvements can occur in the absence of pronounced changes in the physiology of those areas.

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.

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 of the dendritic fields of layer 3 pyramidal cells in the kitten's visual cortex

The cat's visual cortex is immature at birth and undergoes extensive postnatal development. For example, cells of layers 2 and 3 do not complete migration until about 3 weeks after birth. Despite the importance of dendritic growth for synaptic and functional development, there have been few studies of dendritic development in the cat's visual cortex to correlate with numerous studies of functional and synaptic development. Accordingly, we used the Golgi method to study the development of the dendrites of layer 3 pyramidal cells in the visual cortex of a series of cats ranging in age from 2 days to 3 years. Blocks of visual cortex were impregnated by the Golgi-Kopsch method and sectioned in the tangential plane. Layer 3 pyramidal cells were drawn with a camera lucida and analyzed by Sholl diagrams and vector addition. In kittens < 1 week old, these cells were very immature, with only an apical dendrite and no basal dendrites. Basal dendrites appeared during the second week. By 2 weeks, all of the basal dendrites had emerged from the soma, but they had few branches and were tipped with growth cones. By 4 weeks, they had finished branching but continued to grow in length until, by 5 weeks, they reached their adult size. Examination of the basal dendritic fields in the tangential plane revealed that their dendritic fields were more elongated at 2 weeks than at later ages, perhaps because of their smaller size. The distribution of dendritic field orientations was uniform at all ages except 3 and 4 weeks, when there was a preponderance of fields oriented in the rostrocaudal direction. Because dendritic growth and branching occurred very rapidly over a period that precedes and overlaps with the peak periods of synaptogenesis and of sensitivity to the effects of early visual experience, they may depend on afferent visual activity. The early emergence of primary dendrites, however, suggests that this process is independent of afferent activity. The coincident timing of dendritic branching with the presence of dendritic growth cones suggests that branching may occur at growth cones.

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.

Changing patterns of binocular visual connections in the intertectal system during development of the frog, Xenopus laevis. III. Modifications following early eye rotation

In the frog, Xenopus laevis, a system of intertectal connections underlies the visual projection from an eye to its ipsilateral tectal lobe and is involved in the topographic representation of binocular visual space. Rotation of one eye in early life may be followed by a radical rearrangement of the connections in this system. The modified pattern which later emerges is that which keeps the visual projection through the ipsilateral eye in topographic registration with the direct visual projection from the contralateral eye to the same tectal lobe. This plasticity requires visual experience. In this paper we describe the time-course and sequence of events by which this plasticity is effected. Following rotation of one eye in larval animals or in animals undergoing metamorphic climax, the earliest evidence of intertectal modification was found 3-4 weeks after metamorphosis. With increasing intervals after metamorphosis an increasing proportion of animals displayed modified intertectal systems. At intermediate intervals many animals showed partial modifications, which were interpreted as transitional stages in the modification process. Analysis of these transitional stages indicated that the sequence of events involved in the elaboration of a modified intertectal system following the experimental alteration of eye alignment exhibits features in common with rearrangements of the system that occur during normal development in response to growth-related alterations in eye alignment.

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.

Effects of NMDA antagonists on developmental plasticity in kitten visual cortex

The existence of Hebb synapses in the visual cortex of young kittens has long been postulated. A mechanism for the correlation of activity in simultaneously active pre- and postsynaptic neurons could be provided by the properties of the N-methyl-D-aspartate (NMDA) receptor and its associated Ca2+ channel, which opens in a transmitter- and voltage-dependent manner. We have studied the effects on cortical plasticity of blocking NMDA receptors in different ways with competitive and non-competitive NMDA antagonists. In our first approach, the non-competitive NMDA antagonist ketamine, a short-acting dissociative anaesthetic, was injected systemically after each of a series of brief monocular exposures. This procedure prevented the development of an ocular dominance shift towards the experienced eye in the visual cortex. Other short-acting anaesthetics, such as xylazine or methohexital, while providing the same depth of anaesthesia, did not have the same effect on ocular dominance plasticity. We conclude, therefore, that ketamine quite specifically interferes with synaptic consolidation in the visual cortex. In order to establish a role of NMDA receptors for cortical plasticity directly in the visual cortex, we performed another series of experiments: 2-amino-5-phosphono-valerate (APV), a competitive NMDA antagonist, was infused intracortically by means of implanted osmotic minipumps in kittens, which were monocularly deprived for 1-2 weeks. Within a radius of 4-5 mm, the expected ocular dominance shift was prevented or reduced. In addition, however, physiologically determined cell density and responsiveness to visual stimuli were grossly abnormal around the infusion site, and histological cell density was also reduced. Similar effects were found when MK801 (a non-competitive NMDA antagonist) was used in the same type of experiment. The outcome of both experimental approaches makes it very likely that NMDA antagonists somehow interfere with cortical plasticity. Their mode of action, however, remains ambiguous. Although it is quite possible that blocking of the NMDA channel prevents the Hebbian correlative process necessary for synaptic consolidation, more complex effects, such as an interference with a neurotrophic action normally exerted via the NMDA receptor, may have to be taken into account as well.

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.

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.

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.

Forebrain norepinephrine and neurobehavioral plasticity: neonatal 6-hydroxydopamine eliminates enriched-impoverished experience effects on maze performance

Newborn male rats were depleted of forebrain norepinephrine (NE) by systemic 6-hydroxydopamine injection and then reared from 25 to 60 days under either isolated or enriched conditions. They were subsequently tested for acquisition of either the Lashley III maze or the Hebb-Williams maze problems. Isolated rearing impaired Lashley maze performance of the controls but not the 6-OHDA injected rats. Similarly, for the Hebb-Williams maze, the isolation-reared controls made more errors than their enriched-reared counterparts while no differences were observed between the isolated and enriched reared, 6-OHDA injected rats. These results are consistent with the hypothesis that forebrain NE is permissive to the deleterious behavioral consequences of restricted experience during maturation.

Increased levels of testosterone have little effect on visual cortex plasticity in the kitten

We tested the hypothesis that increased levels of sex steroids preceding puberty are an important factor in the termination of the critical period for monocular deprivation. Male kittens were injected with Depo-testosterone in order to elevate plasma testosterone to a higher level than that in normal prepubertal male kittens. Control animals did not receive testosterone injections. All kittens were monocularly deprived for 7-18 days, then cells in the visual cortex were examined electrophysiologically, and an ocular dominance histogram was constructed. Treated animals showed an increase in plasma testosterone (1.82-15.16 ng/mL) when compared with the control animals (0.80 +/- 0.25 ng/mL). The fraction of cells driven exclusively by the experienced eye was slightly lower in the treated animals, and there was a slight increase in the dominance of cells by both eyes. However, in both groups of animals, the majority of cells were dominated by the experienced eye, with no significant difference in the weighted parameter used to describe this dominance. In summary, although there is a slight difference between treated and control animals, the results do not support the hypothesis that elevated levels of sex steroids play a crucial role in the termination of the critical period.

Timing of 6-hydroxydopamine administration influences its effects on visual cortical plasticity

We recorded from the visual cortex of 4 groups of monocularly deprived kittens. Three groups were treated with intraventricular 6-hydroxydopamine (6-OHDA) at different times relative to monocular deprivation (MD). One group received only vehicle solution and MD. 6-OHDA caused the greatest decrease in plasticity in the kittens receiving 6-OHDA throughout the deprivation period; that is, these kittens were the least affected by MD. 6-OHDA caused a smaller decrease in plasticity in kittens receiving 6-OHDA just prior to eyelid suture and a still smaller decrease in kittens waiting a week between 6-OHDA treatment and eyelid suture. The kittens in all groups receiving 6-OHDA were equally depleted of norepinephrine (NE). We conclude that 6-OHDA decreases plasticity in the visual cortex; however, the time course of this decrease is better related to the time course of the 6-OHDA treatment than to the time course of NE depletion.

The cAMP cascade in the nervous system: molecular sites of action and possible relevance to neuronal plasticity

Many intercellular messages regulate the activity of their target cells by altering the intracellular level of cAMP and, as a consequence, the phosphorylation state of proteins which serve as substrates for cAMP-dependent protein kinase. Such regulation plays a crucial role in neuronal development, neuronal function, and neuronal plasticity (e.g., elementary learning mechanisms). Ample information has been accumulated in recent years on the enzymes that regulate the level of cAMP or respond to it, on the regulation of cAMP synthesis by neurohormones, neurotransmitters, ions, and toxins, on neuronal-specific substrate proteins that are phosphorylated by the cAMP-dependent kinase, and on the interaction of the cAMP-cascade with other second-messenger systems within neurons. Such data, obtained by a combination of molecular-biological, biochemical, and cellular approaches, shed light on the detailed mechanisms by which modulation of a ubiquitous molecular cascade leads to a great variety of short-term as well as long-term specific neuronal responses and alterations.

Central noradrenaline depletion during development and its effect on behaviour

Although early depletion of noradrenaline is known to affect the morphological development of various structures in the brain, it is not clear what implications this has for adult behaviour. In the present study, 6-hydroxydopamine (6OHDA) was injected into the lateral ventricles of 12 day old rats, permanently destroying most of the noradrenergic innervation of the spinal cord and of all the brain areas examined except for the pons/medulla, and reducing the dopamine content of the cerebral cortex considerably. The noradrenaline content of the heart, as well as general developmental parameters such as food intake and body weight, were unaffected. Despite the extensive noradrenaline depletion during development, these rats' spatial memory--as determined in a radial maze task--was no worse than that of controls. In a lever-pressing task the 6OHDA treated rats made no more errors than did controls, but performed more slowly. The results indicate that at least some aspects of learning, memory and sensory-motor ability can develop normally when the noradrenergic innervation of the brain is largely destroyed.

Reduction of cerebellar norepinephrine alters climbing fiber enhancement of mossy fiber input to the Purkinje cell

Extra-cellular simple and complex spike activity from 58 Purkinje cells were recorded in cats that previously received an intracisternal injection of 6-OHDA which depletes brain catecholamines. The severest catecholamine depletion was noted for cerebellar norepinephrine (21.1% of controls). Less depletion occurred in the brainstem and the visual cortex. Past studies have shown that in normal non-depleted cats, somatosensory stimuli (forepaw tap) evoke both complex and simple spike responses. On those trials where complex spike or climbing fiber responses are evoked, there is an enhancement or increase in responsiveness in the majority of excitatory and inhibitory simple spike responses. In the norepinephrine depleted animal, there is a significant decrease in this climbing fiber enhancement only for the excitatory response components. Furthermore, on those trials where no complex spikes are evoked, there is a significant decrease in the excitatory but not in the inhibitory response amplitude. A slight but non-significant increase in Purkinje cell background firing rate is also observed in the depleted animals. Thus, depletion of norepinephrine is associated with a reduction of both response amplitude and climbing fiber induced enhancement of excitatory simple spike responses. The inhibitory responses in these same cells are unchanged when compared to those recorded in the normal non-depleted animals.

Effects of neonatal forebrain noradrenaline depletion on recovery from brain damage: performance on a spatial navigation task as a function of age of surgery and postsurgical housing

The experiments examined the contributions of forebrain noradrenaline and environmental enrichment to recovery of place navigation ability in rats after hemidecortication in infancy or adulthood. Noradrenaline depletion did not affect recovery from neonatal hemidecortication, although the early hemidecortications did allow sparing of function relative to adult operates. Noradrenaline depletion also failed to attenuate the positive effects of enriched housing on otherwise normal rats. Noradrenaline depletion did retard recovery of adult hemidecorticate rats housed in standard laboratory cages, but it did not retard recovery of adult hemidecorticate rats housed in enriched environments. The results suggest that noradrenaline is importantly involved in enhancing recovery from brain damage when other sources of compensation (e.g., neonatal injury, enriched environment) are absent.

Acetylcholine-rich neuronal grafts in the forebrain of rats: effects of environmental enrichment, neonatal noradrenaline depletion, host transplantation site and regional source of embryonic donor cells on graft size and acetylcholinesterase-positive fibre outgrowth

The importance of several factors influencing the survival of cholinergic-rich embryonic tissue transplanted to the adult rat forebrain and the extent of acetylcholinesterase-positive fibre innervation of the host brain was investigated in 3 experiments. In the first two experiments, embryonic ventral forebrain tissue was grafted to the neocortex of rats in which the intrinsic cortical cholinergic innervation had been removed by nucleus basalis lesions. Housing the host rats in an enriched environment produced a temporary enhancement of fibre outgrowth 4 weeks after transplantation, but this was not maintained after 10 weeks. Fibre outgrowth was greater when the grafts were transplanted to the noradrenaline-depleted neocortex than to the intact neocortex. Neither environmental enrichment nor noradrenaline depletion influenced graft survival or size. In the third experiment, the embryonic donor tissue was dissected to separate regions containing precursors of the nucleus basalis cholinergic cells from regions containing precursors of the septal cholinergic cells, and transplanted to either the neocortex following nucleus basalis lesions or to the hippocampus following fimbria-fornix lesions. Nucleus basalis grafts showed greater growth in size than septal grafts, and grafts placed into the hippocampus showed greater growth in size than grafts placed into the neocortex. More interestingly, the extent of fibre outgrowth depended on the appropriateness of the donor tissue to the host transplantation site: nucleus basalis tissue showed greater acetylcholinesterase-positive outgrowth than septal tissue in the neocortex, whereas septal tissue showed greater outgrowth than nucleus basalis tissue in the hippocampus.

The ontogeny of the laminar distribution of beta-adrenergic receptors in the visual cortex of cats, normally reared and dark-reared

Patterns of distribution of beta 1 and beta 2 adrenergic receptors were examined autoradiographically in slide-mounted sections from the visual cortical areas of 22 developing cat brains, using [125I]iodopindolol as the ligand in combination with displacers specific for beta 1 and beta 2 subtypes of adrenergic receptors. Within visual cortical areas 17 and 18 of adult brains, the density of beta 1 and beta 2 adrenergic receptors was highest in laminae I-III, lowest in lamina IV, and intermediate in laminae V-VI. For beta 1 adrenergic receptors, this laminar distribution was also seen in visual area 19 as well as in the non-visual area 7 that is lateral to area 19. By contrast, the distribution of beta 2 adrenergic receptors varied across cortical areas, such that its density was more homogeneous across the laminae in area 19, and decreased in all laminae in area 7. This pattern of distribution in adult brains was already formed at the beginning of the critical period and was not disturbed by dark-rearing.

Modulation of visual cortical plasticity by acetylcholine and noradrenaline

During a critical period of postnatal development, the temporary closure of one eye in kittens will permanently shift the ocular dominance (OD) of neurones in the striate cortex to the eye that remains open. The OD plasticity can be substantially reduced if the cortex is infused continuously with the catecholamine neurotoxin 6-hydroxydopamine (6-OHDA) during the period of monocular deprivation, an effect that has been attributed to selective depletion of cortical noradrenaline. However, several other methods causing noradrenaline (NA) depletion leave the plasticity intact. Here we present a possible explanation for the conflicting results. Combined destruction of the cortical noradrenergic and cholinergic innervations reduces the physiological response to monocular deprivation although lesions of either system alone are ineffective. We also find that 6-OHDA can interfere directly with the action of acetylcholine (ACh) on cortical neurones. Taken together, our results suggest that intracortical 6-OHDA disrupts plasticity by interfering with both cholinergic and noradrenergic transmission and raise the possibility that ACh and NA facilitate synaptic modifications in the striate cortex by a common molecular mechanism.

The brain as a self-organizing system

Clinical evidence and numerous results from animal experimentation indicate that cognitive functions have to be learned. Brain structures subserving these functions require sensory experience for their maturation. Genetic instructions are in principle not sufficient to specify neuronal connections with sufficient precision. Self-organization processes are implemented in addition which allow to optimize genetically determined blue prints of connectivity by making use of functional criteria. Thus, neuronal activity becomes an important shaping factor in the development of the structural and functional architecture of the forebrain. To the extent that this neuronal activity is modulated by sensory signals, environmental factors can influence the development of neuronal networks. Recent experiments indicate that these shaping processes are additionally controlled by modulatory systems. Both, the noradrenergic projection from the locus coeruleus and the cholinergic projection from the basal forebrain facilitate activity-dependent long-term changes of neuronal connections during development. The activity of these modulatory systems in turn depends on central states such as arousal, attention, and perhaps also motivation. It is inferred from this evidence that experience-dependent self-organization should not be considered as a passive imprinting process but rather as an active dialogue between the brain and its environment. The hypothesis is discussed that many developmental disturbances which are commonly attributed to deprivation are in fact due to defaults of the CNS which either lead to the formulation of wrong questions or to the reduction of exploratory drive.

Visual behavior of monocularly deprived kittens treated with 6-hydroxydopamine

Several investigators have reported that treating the visual cortex with 6-hydroxydopamine (6-OHDA) preserves the ability of a monocularly deprived eye to drive cells in the visual cortex. If 6-OHDA provides useful protection from the effects of monocular deprivation, it should also prevent the behavioral blindness that normally accompanies monocular deprivation. To test this prediction we compared the visual behavior of monocularly deprived kittens pretreated with 6-OHDA with that of kittens similarly deprived, but not drug-treated. Kittens were trained on a visual discrimination task before drug treatment or suture. Starting at about 5 weeks of age the kittens were given 6-OHDA via ventricular cannula, given vehicle solution, or given no treatment at all. At about 6 weeks of age all kittens were monocularly deprived for one week. When the deprived eye was opened at 7 weeks of age, most kittens not receiving 6-OHDA were blind when tested with the deprived eye. In contrast, none of the kittens receiving 6-OHDA intraventricularly were blind when tested with the deprived eye. 6-OHDA had no effect on performance with the non-deprived eye. We conclude that 6-OHDA protects vision through the monocularly deprived eye without impairing vision through the non-deprived eye.

Clonidine and cortical plasticity: possible evidence for noradrenergic involvement

In order to test the hypothesis that noradrenergic transmission modulates ocular dominance plasticity in kitten visual cortex, we monocularly deprived kittens while administering the alpha-2 adrenergic agonist clonidine (CLON). To avoid bias in testing the hypothesis, we included, with a single blind technique, saline-treated control kittens in the series. First, using high-pressure liquid chromatography, we demonstrated that CLON treatments resulted in an average decline in cerebrospinal fluid levels of the norepinephrine metabolite, 3-methoxy-4-hydroxy phenylethylene glyolol (MHPG) of 44%. Then, single-unit recording in area 17 revealed the expected ocular dominance (OD) shift in monocularly deprived saline controls, but recording failed to find a significant shift in CLON-treated kittens. Our results support the notion that CLON treatment interferes with ocular dominance plasticity by inhibiting noradrenergic transmission in visual cortex. We discuss side effects of CLON, concluding that CLON's sedative effect may contribute to the lack of OD shift.

Noradrenaline and functional plasticity in kitten visual cortex: a re-examination

A quantitative re-examination was made of the influence of noradrenergic depletion on the epigenesis of kitten visual cortex. Two methods were used to deplete noradrenaline at the cortical level: stereotaxically controlled injection of 6-hydroxydopamine (6-OHDA) in the coeruleus complex, from which the noradrenergic input to visual cortex arises; intraventricular injection of 6-OHDA. The latter chemical lesion also depleted dopamine levels in the brain. Lesion of the noradrenergic or catecholaminergic systems was performed neonatally or at an age of 3-4 weeks in kittens submitted to five different rearing procedures: normal rearing, dark rearing, monocular rearing, monocular exposure following dark rearing and monocular deprivation following normal rearing. Forty-two kittens between 3 and 12 weeks of age were used for this biochemical and electrophysiological study. Noradrenaline and dopamine levels were measured by a radioenzymatic method in the primary visual cortex of twenty-six kittens. A total of 1263 cells were recorded in area 17 of twenty-six kittens. Combined biochemical and electrophysiological data were obtained in ten 6-OHDA-lesioned kittens. Whatever the mode of chemical lesion used, cortical noradrenergic depletion failed to block either maturation or vision-dependent processes which are known to affect orientation selectivity and/or ocular dominance during the critical period. However, in some cases, the amplitude of the epigenetic functional modifications was slightly reduced in 6-OHDA-treated kittens. The cortical effects of monocular deprivation starting from the age of 5 weeks were studied quantitatively both in lesioned and intact kittens. Disappearance of noradrenaline in area 17 did not prevent the loss of binocularity in cortical cells. However, even when monocular occlusion had been maintained for 2 or 3 weeks in 6-OHDA-treated kittens, ocular dominance shifts were limited to a stage equivalent to that observed in the intact kitten after 5-8 days of monocular occlusion. The amplitude of this partial protective effect was found to be unrelated either to the delay following the chemical lesion, or to the level of noradrenaline remaining in lesioned kitten cortex. Although a putative gating role of noradrenaline cannot be excluded in the development of the intact animal, this report shows that its presence is not required for functional plasticity to occur in kitten area 17.

Development of the A1 adenosine receptors in the visual cortex of cats, dark-reared and normally reared

The ontogeny of the distribution of the binding sites for [3H]chlorohydroxyladenosine, an A1 adenosine receptor-specific ligand, was visualized autoradiographically within coronal sections of the visual cortical areas of developing cats. In adults, the A1 adenosine receptors were found in all lamina except for lamina IV, and in particularly high concentration within laminas I-III. In brains of kittens 2 months old and younger who were within the critical period for the development of visual neural function, the receptor distribution was less defined and sparser, except that in contrast to adults, it was found in relatively high concentration within lamina VI. Animals dark-reared from birth, so that the critical period was postponed, exhibited an ontogenetic pattern identical to that of the normally reared animals. These results indicate that, at least with respect to ocular dominance determination, A1 adenosine receptors are probably not involved in determining the state of plasticity that is seen during the critical period.

Growth and plasticity of rat cerebral cortex after central noradrenaline depletion

Male Wistar rats (aged 31 to 33 days) received bilateral intraventricular injections of saline or 6-hydroxydopamine and were subjected to either "standard" or "enriched" rearing conditions for 42 days. The treatment reduced cerebral cortical noradrenaline by 80% and decreased the growth of the cerebral cortex. It did not, however, prevent experience in an enriched environment from enhancing the growth of the cerebral cortex.

Plasticity in cat visual cortex restored by electrical stimulation of the locus coeruleus

It has been proposed that the presence of noradrenaline (NA)-containing terminals and NA-related receptors within the visual cortex is necessary to maintain the high level of neuronal plasticity in the immature visual cortex of kittens. In the present study we wanted to show whether electrical stimulation of the locus coeruleus (LC), which contains the somata of these cortical NA fibers, can restore neuronal plasticity to the normally aplastic visual cortex of juvenile and adult cats. We consistently found a significant loss of binocular cells in the visual cortex of mature animals which had monocular vision for only 12 h dispersed over 6 days (2 h a day, otherwise kept in the dark) in combination with concurrent LC stimulation. This result was interpreted as indicating that endogenous NA released from NA terminals restored susceptibility to monocular vision in the mature visual cortex. We next examined how long the restored plasticity lasts in the same animals after the LC stimulation was ended. The animals revived from the first recording session were either returned to the same daily schedule of brief monocular exposure (light/dark = 2/22 h) as before, or subjected to the usual monocular lid suture and kept in a cat colony environment (light/dark = 16/8 h). The LC electrodes had been removed and no more electrical stimulation was delivered at this stage. In the animals subjected to reiteration of brief monocular exposure, the state of reduced binocularity gradually returned to normal over a period of 2-3 weeks after stopping LC stimulation. We calculated that the revived plasticity disappeared at an average rate of a 22% loss every 7 days. This result sharply contrasted with the result obtained in the animals subjected to usual monocular lid suture. In this test the state of reduced binocularity continued for at least the next 3 weeks, suggesting that the restored plasticity was sustained throughout a period of 3 weeks (longest term tested). The different results obtained in the two paradigms may be explained by the different strength of binocular imbalance in the two tests imposed on the visual cortex in which neuronal plasticity was restored partially.

Intertectal neuronal plasticity in Xenopus laevis: persistence despite catecholamine depletion

In normal Xenopus, the tectum receives a direct projection from the contralateral retina and an indirect projection, via the intertectal system, from the ipsilateral eye. The two maps of binocular visual space, at each tectum, are in register. If one eye is rotated during larval development, the ipsilateral visuotectal projection compensates by changing its orientation. Rearrangement of the intertectal system brings the ipsilateral map back into register with the contralateral map. We sought to determine whether this intertectal plasticity required normal levels of brain monoamines. Animals received an eye rotation between stages 55-63 of larval life and were then placed in one of 3 groups. A first control group received no further treatment. A second control group was given intraventricular injections of ascorbate vehicle. The experimental group was given intraventricular injections of 6-hydroxydopamine in ascorbate vehicle. Two to 3 months after metamorphosis, visuotectal projections were mapped electrophysiologically and the brains were assayed for monoamines. Intertectal plasticity occurred in all 3 groups of animals, including animals in which brain catecholamine levels were severely reduced. We conclude that normal levels of brain catecholamines are not required for this form of neural plasticity.