Visual afferents to norepinephrine-containing neurons in cat locus coeruleus

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.

Activation of mitogen-activated protein kinase in descending pain modulatory system

The descending pain modulatory system is thought to undergo plastic changes following peripheral tissue injury and exerts bidirectional (facilitatory and inhibitory) influence on spinal nociceptive transmission. The mitogen-activated protein kinases (MAPKs) superfamily consists of four main members: the extracellular signal-regulated protein kinase1/2 (ERK1/2), the c-Jun N-terminal kinases (JNKs), the p38 MAPKs, and the ERK5. MAPKs not only regulate cell proliferation and survival but also play important roles in synaptic plasticity and memory formation. Recently, many studies have demonstrated that noxious stimuli activate MAPKs in several brain regions that are components of descending pain modulatory system. They are involved in pain perception and pain-related emotional responses. In addition, psychophysical stress also activates MAPKs in these brain structures. Greater appreciation of the convergence of mechanisms between noxious stimuli- and psychological stress-induced neuroplasticity is likely to lead to the identification of novel targets for a variety of pain syndromes.

Activation of ERK in the locus coeruleus following acute noxious stimulation

In the present study, the activation of extracellular signal-regulated kinase (ERK) in the locus coeruleus (LC) following injection of formalin or complete Freund's adjuvant (CFA) into the rat hindpaw was examined in order to clarify the mechanisms underlying the dynamic changes in the descending pain modulatory system after acute noxious stimulation or chronic inflammation. In naive rats there were few phospho-extracellular signal-regulated kinase-immunoreactive (p-ERK-IR) neurons in the LC. Formalin-, CFA- and saline-injections induced an increase in p-ERK-IR in the LC. The number of p-ERK-IR neurons in the LC in the formalin group was significantly higher than those in all other groups from 5 min to 1 h after the injection (p<0.05). CFA injection induced only a transient significant increase in the number of p-ERK-IR neurons and there was no significant difference in the number of p-ERK-IR neurons between the CFA and saline groups. At 5 min after formalin injection, almost all p-ERK-IR neurons in the LC were tyrosine hydroxylase (TH) -positive. These findings suggest that activation of ERK in the LC is induced by acute noxious stimulation, such as formalin injection, but not by CFA-induced chronic inflammation. The activation of ERK in the LC may be involved in the plasticity of the descending pain modulatory systems following acute noxious stimulation.

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.

Very slow potential oscillations in locus coeruleus and dorsal raphe nucleus under different illumination in freely moving rats

Recent findings have revealed very slow (<0.5 Hz) oscillatory phenomena in the structures of the brain visual system. It has been proposed that very slow brain potentials in an extremely slow domain, less than 0.1 Hz, recorded from the lateral geniculate complex and primary visual cortex are associated with periodic influences originating from the locus coeruleus and dorsal raphe nucleus. The present study was performed to test the hypothesis that extremely slow brain potential oscillatory patterns in the locus coeruleus and dorsal raphe nucleus during several types of visual stimulation--light exposure, darkness, and photostimulation--are similar to those in the primary visual cortex and lateral geniculate complex under the same conditions of illumination. The results support this hypothesis. Specifically, spectral patterns of multisecond oscillations in the range of 0.02-0.04 Hz and fluctuations in the domain of minutes (below 0.002 Hz) were present in both the locus coeruleus and dorsal raphe nucleus and were similar to those found in the primary visual cortex and lateral geniculate complex. Additionally, we detected significant increases in the power spectra of multisecond oscillations in both nuclei in response to photostimulation (P<0.05). Our tentative conclusion is that extremely slow potentials in the locus coeruleus and dorsal raphe nucleus contribute to the regulation of extremely slow activity in the brain visual system.

Fos and FRA protein expression in rat nucleus paragigantocellularis lateralis during different space flight conditions

The nucleus paragigantocellularis lateralis (LPGi) exerts a prominent excitatory influence over locus coeruleus (LC) neurons, which respond to gravity signals. We investigated whether adult albino rats exposed to different gravitational fields during the NASA Neurolab Mission (STS-90) showed changes in Fos and Fos-related antigen (FRA) protein expression in the LPGi and related cardiovascular, vasomotor, and respiratory areas. Fos and FRA proteins are induced rapidly by external stimuli and return to basal levels within hours (Fos) or days (FRA) after stimulation. Exposure to a light pulse (LP) 1 h prior to sacrifice led to increased Fos expression in subjects maintained for 2 weeks in constant gravity (either at approximately 0 or 1 G). Within 24 h of a gravitational change (launch or landing), the Fos response to LP was abolished. A significant Fos response was also induced by gravitational stimuli during landing, but not during launch. FRA responses to LP showed a mirror image pattern, with significant responses 24 h after launch and landing, but no responses after 2 weeks at approximately 0 or 1 G. There were no direct FRA responses to gravity changes. The juxtafacial and retrofacial parts of the LPGi, which integrate somatosensory/acoustic and autonomic signals, respectively, also showed gravity-related increases in LP-induced FRA expression 24 h after launch and landing. The neighboring nucleus ambiguus (Amb) showed completely different patterns of Fos and FRA expression, demonstrating the anatomical specificity of these results. Immediate early gene expression in the LPGi and related cardiovascular vasomotor and ventral respiratory areas may be directly regulated by excitatory afferents from vestibular gravity receptors. These structures could play an important role in shaping cardiovascular and respiratory function during adaptation to altered gravitational environments encountered during space flight and after return to earth.

Frequenz und mittlere Amplitude spontaner elektrodermaler Fluktuationen sind keine austauschbaren Indikatoren psychischer Prozesse

Zusammenfassung. Die Arbeit ging experimentell der Frage nach, ob zwei zentrale Aspekte spontaner elektrodermaler Fluktuationen, ihre Frequenz und ihre mittlere Amplitude, dieselbe Information über psychische Prozesse liefern. Experimentell variiert wurde dazu mittels Imaginationsmethode das Thema und der Selbstbezug induzierter Vorstellungen. Die experimentellen Manipulationen folgten einem 2×2-faktoriellen Versuchsplan für unabhängige Gruppen (Zellbesetzung: jeweils 24 Personen). Gemessen wurde die Veränderung der elektrodermalen Aktivität von einer Ruhe- zu der Vorstellungsphase. Die Ergebnisse zeigten, daß die beiden Variablen keine austauschbaren Indikatoren psychischer Prozesse sind. Trotz eines substantiellen korrelativen Zusammenhangs reagierten sie sehr eigen auf die experimentellen Manipulationen. Die Variation der Vorstellungsthematik wirkte nur auf die Frequenz, die Variation des Selbstbezugs der Vorstellungen nur auf die mittlere Amplitude der spontanen elektrodermalen Fluktuationen. Die beiden Variablen werden demnach sowohl von wenigstens einem gemeinsamen Faktor als auch von unterschiedlichen Faktoren beeinflußt. Die Frequenz spontaner elektrodermaler Fluktuationen scheint verdeckte Orientierungsprozesse zu reflektieren, während die mittlere Amplitude dieser Fluktuationen die Aktivität eines erfahrungs- und zielabhängigen Bewertungssystems abzubilden scheint. Beide Variablen scheinen darüber hinaus gleichermaßen von einer Art Grundaktiviertheit oder Wachsamkeit abhängig zu sein.

Arousal systems and attentional processes

Unitary concepts of arousal have outlived their usefulness and their psychological fractionation corresponds to a similar chemical differentiation of the reticular formation of the brain. Neurobiological characteristics of the monoaminergic and cholinergic systems can be described in terms of their anatomical, electrophysiological and neurochemical properties. Functional studies suggest that the coeruleo-cortical noradrenergic system, under certain circumstances, is implicated in processes of selective attention, that the mesolimbic and mesostriatal dopaminergic systems contribute to different forms of behavioural activation, and that the cortical cholinergic projections have fundamental roles in the cortical processing of signals, affecting attentional and mnemonic processes. The ascending serotoninergic systems contribute to behavioural inhibition and appear to oppose the functions of the other systems in several ways.

Topographic organization of rat locus coeruleus and dorsal raphe nuclei: distribution of cells projecting to visual system structures

Previous reports from this laboratory and elsewhere have provided evidence that the locus coeruleus (LC) and dorsal raphe (DR) nuclei are topographically organized with respect to their efferent targets. Whereas most of these previous studies have focused on relationships between these monoamine-containing brainstem nuclei and cerebral cortex, basal ganglia, and limbic structures, they have not systematically examined the distribution of LC and DR cells that project to multiple structures with common sensory or motor functions. The goal of the present study was to characterize and compare the distributions of LC and DR cells which project to different visual areas of the rat central nervous system. Long-Evans hooded rats received unilateral pressure injections of the retrograde tracer wheat germ agglutinin-horseradish peroxidase in either the dorsal lateral geniculate, ventral lateral geniculate, or lateral posterior nucleus of thalamus; superior colliculus, cortical area 17, cortical area 18a/b; cerebellar vermis (lobules VI and VII); or paraflocculus. Transverse sections through the midbrain and pons were examined by light microscopy after performing routine tetramethyl benzidine histochemical procedures. For all cases studied, retrogradely labeled cells were observed throughout the rostrocaudal extent of the LC and DR; however, labeling patterns which were distinctive for different injection sites were noted in each of these brainstem nuclei. The major conclusion drawn from this work is that subsets of LC and DR cells which project to different target structures within the rat visual system are found in overlapping but not necessarily coextensive zones within these nuclei. These studies provide further evidence of a rough topographic ordering within both the LC and DR nuclei, as well as support a new hypothesis that the outputs from each of these nuclei are organized with respect to the sensory related functions of their efferent targets.

Beta-adrenergic receptors: astrocytic localization in the adult visual cortex and their relation to catecholamine axon terminals as revealed by electron microscopic immunocytochemistry

It has long been recognized that noradrenaline, the most abundant catecholamine within the visual cortex, plays important roles in modulating the sensitivity of cortical neurons to visual stimuli. However, whether or not these noradrenaline effects are confined to a discrete synaptic specialization or mediated by diffuse modulation of a group of synapses has remained an issue open for debate. The aim of this study was to examine the cellular basis for noradrenaline action within the visual cortex of adult rats and cats. To this end, I used electron microscopic immunocytochemistry to examine the relationship between (1) catecholamine axon terminals and beta-adrenergic receptors (beta AR), which, together, may define the effective sphere of noradrenaline modulation; and then (2) these putative sites for catecholamine modulation and axospinous asymmetric junctions where excitatory neurotransmission is likely to dominate. Antibodies against beta AR were used at light and electron microscopic levels on the visual cortex of rat and cat. Rat visual cortex was also labeled simultaneously for beta AR and the catecholamine-synthesizing enzyme, tyrosine hydroxylase (TH), to determine the ultrastructural relationships between catecholamine terminals and beta AR. Immunoperoxidase labeling revealed that beta AR404, a polyclonal antibody directed against the C-terminal tail of hamster lung beta AR (beta 2-type), recognized astrocytic processes predominantly. In contrast, beta AR248, a polyclonal antibody directed against the third cytoplasmic loop, recognized neuronal perikarya as observed in previous studies. Dual labeling for beta AR404 and TH revealed that catecholamine axon terminals that contained numerous vesicles formed direct contacts with astrocytic processes exhibiting beta AR404 immunoreactivity. However, some catecholamine axon terminals that lacked dense clusters of vesicles were positioned away from beta AR404-immunoreactive astrocytes. Frequently, beta AR-immunoreactive astrocytic processes surrounded asymmetric axospinous junctions while also contacting catecholamine axon terminals. These observations support the possibility that, through activation of astrocytic beta AR, noradrenaline modulates astrocytic uptake mechanism for excitatory amino acids, such as L-glutamate. Astrocytic beta AR might also define the effective sphere of catecholamine modulation through alterations in the morphology of distal astrocytic processes and the permeability of gap junctions formed between astrocytes.

Locus coeruleus control of spinal motor output

Using electrophysiological techniques, we investigated the functional properties of the coeruleospinal system for regulating the somatomotor outflow at lumbar cord levels. Many of the fast-conducting, antidromically activated coeruleospinal units were shown to exhibit the alpha 2-receptor response common to noradrenergic locus coeruleus (LC) neurons. Electrically activating the coeruleospinal system potentiated the lumbar monosynaptic reflex and depolarized hindlimb flexor and extensor motoneurons via an alpha 1-receptor mechanism. The latter synaptically induced membrane depolarization was mimicked by norepinephrine applied iontophoretically to motoneurons. That LC inhibited Renshaw cell activity and induced a positive dorsal root potential at the lumbar cord also reinforced LC's action on motor excitation. We conclude that LC augments the somatomotor output, at least in part, via an alpha 1-adrenoceptor-mediated excitation of ventral horn motoneurons. Such process is being strengthened by LC's suppression of the recurrent inhibition pathway as well as by its presynaptic facilitation of afferent impulse transmission at the spinal cord level.

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.

Neurophysiology and neurochemistry of drug dependence: a review

To understand the neurophysiological and neurochemical mechanisms of drug dependence, the functional significance of dopamine, noradrenaline and endogenous opioid peptides in the mediation of natural, self-stimulation and pharmacological reinforcement are discussed. Data on search of system(s), mediator(s) and neurons of reinforcement as well as my own notions on reinforcement as a critical element in organization and regulation of the organism's adaptive activity in variable environments are presented. The role of chronic drug-induced stable modification of central neurochemical systems' functioning as a basis for the alteration of endogenous reinforcement processes and raising drug dependence are examined in detail for main addictive drugs, opiates and psychomotor stimulants.

Catecholamines and the covert orientation of attention in humans

The role of brain catecholamines in covert orienting was tested in normal subjects using a cued reaction time paradigm which measures the directional engagement, disengagement and movement of attention. Droperidol and clonidine were administered intravenously to suppress central dopamine and noradrenaline transmission. Both drugs produced reductions in the cost of invalid cueing without change in the benefit of valid cueing suggesting that both noradrenaline and dopamine are involved in facilitating the disengagement of attention. These results are relevant to the slowed disengagement observed with parietal lesions in monkeys and humans since this region of the cortex associated with visuospatial analysis receives a dense innervation by both dopamine and noradrenaline projections in primates.

Labyrinthine influences on locus coeruleus neurons

The locus coeruleus (LC) complex, located in the dorsolateral pontine tegmentum, is composed principally of noradrenergic neurons, which project to broad regions of the CNS, including the spinal cord. Experiments were performed in precollicular decerebrate cats to ascertain whether units histologically identified within the LC complex, and having the physiological characteristics of noradrenergic neurons, would respond to sinusoidal stimulation of labyrinth receptors. Among 141 LC complex neurons, 16 of which could be activated antidromically by stimulation of the spinal cord at T12-L1, 80 (i.e. 56.7%) responded to roll tilt of the animal at 0.15 Hz, +/- 10 degrees. The responses were particularly related to the extreme animal displacements, thus being attributed to stimulation of macular utricular receptors. The proportion of responsive units, and also the average gain of the responses, were higher in the LCd and the subcoerular (subLC) area than in the LCa. Moreover in the same structures the majority of units showed a beta-pattern of response (excitation during side-up tilt), which contrasted with the predominant alpha-pattern (excitation during side-down tilt) displayed by the previously recorded vestibulospinal neurons projecting to the same segments of the spinal cord. The role that the noradrenergic coeruleospinal neurons exert in the dynamic control of posture during the vestibulospinal reflexes is discussed.

A critical period of the development of beta-adrenergic receptor binding in the visual system of rat during visual deprivation

The density of [3H]dihydroalprenolol binding to beta-adrenergic receptors in the visual structures (visual cortex, superior colliculus and lateral geniculate nucleus) of rats raised under a normal 12 hr light-dark cycle was compared to those of rats visually deprived at different postnatal ages. Unilateral eyelid suture from postnatal days 10 or 16 to 3 months resulted in an increased [3H]dihydroalprenolol bilateral binding in the lateral geniculate nucleus compared to control animals. Monocular deprivation from postnatal days 25, 40, 60 and 90 had no effect on the density of [3H]dihydroalprenolol binding. After re-opening of the eyelid, which was sutured on postnatal day 10, at postnatal day 25 no changes in beta-adrenergic receptor binding in the lateral geniculate nucleus of the adult animal could be detected. After re-opening of the sutured eyelid on day 90, followed by examination of the adrenoceptor density 4 weeks later, the [3H]dihydroalprenolol binding in both lateral geniculate nuclei remained elevated as was also found in corresponding regions of monocular deprived animals. Binocular visual deprivation from postnatal day 10 until the age of 3 months had no effect on [3H]dihydroalprenolol binding in the visual centres in comparison to corresponding control animals. The data suggest that there exists a critical period for the ontogenetic development of beta-adrenergic receptors binding in the visual system of rats during which permanent alterations of receptor binding can be induced by monocular but not binocular visual deprivation.

Catecholamines and attention. I: Animal and clinical studies

One important function of the catecholamine innervation of the cerebral cortex may be the control of attention. Of particular interest are the catecholamine projections to the cerebral cortex from the reticular formation, namely the dopamine neurons of the ventral tegmentum of the midbrain and the noradrenergic neurons of the locus coeruleus in the upper pons. Animal studies implicate noradrenaline and dopamine in a wide range of attention-related behaviours involving search and exploratory activity, distractibility, response rate, discriminability and the switching of attention. Most human studies come from the clinical literature relating to schizophrenia, Parkinson's disease and attention deficit disorder. An association has been claimed in each of these conditions between abnormal catecholamine activity (in particular dopamine) and attentional dysfunction. In particular, difficulty with the attachment of appropriate responses to environmental stimuli, akin to those observed in animals with lesions to central dopamine pathways, indicates a role for dopamine in response selection processes. Overall, the animal and human studies reviewed indicate a role for central noradrenaline and dopamine in the early and late processing of information, respectively.

Regenerative catecholamine-containing terminals in kitten visual cortex: an ultrastructural study

Ultrastructure of catecholaminergic (CA) terminals was studied in the kitten visual cortex which had been continuously infused with 6-hydroxydopamine (6-OHDA) for one week. Two methods were used to identify CA terminals: (1) cardiac perfusion with glyoxylic acid (GA) followed by potassium permanganate (KMnO4), a fixation method which leads to the precipitation of a dense core in CA-containing synaptic vesicles; and (2) induction by 6-OHDA of terminal degeneration in glutaraldehyde-fixed material. In material which was obtained from 3-week-survival animals and treated with GA-KMnO4, the 6-OHDA-infused hemisphere contained large terminal boutons (1-3 microns in diameter) having large dense-cored vesicles. Usual CA terminal boutons with small dense-cored vesicles were found only in the opposite control hemisphere. The unusual terminal boutons with large dense-cored vesicles were interpreted as "regenerative" CA terminals. They were no longer found in animals which survived for 33 weeks. Instead, the return of usual CA terminals filled with small dense-cored vesicles was noted. The above interpretation was further supported from the results of an additional two kittens in the 3-week group which received a single injection of 6-OHDA prior to their sacrifice (in addition to the initial 6-OHDA infusion 3 weeks before). In the glutaraldehyde-fixed sections, there were found terminal boutons filled with the accumulation of abnormal electron-dense material, suggesting the presence of ("regenerative") CA terminals in the 6-OHDA-infused cortex. The present results have thus provided further evidence for the presence of "regenerative" CA terminals in a cortical area in which CA terminals have been previously destroyed by cortical 6-OHDA infusion.

Membrane excitability changes in hindlimb motoneurons induced by stimulation of the locus coeruleus in cats

The present analysis describes the cellular mechanisms underlying the heightened membrane excitability of hindlimb flexor and extensor motoneurons upon stimulation of the locus coeruleus (LC) in unanesthetized, decerebrate cats. In a total of 73 cells, brief train stimuli to the LC at 50-300 microA intensity evoked one of 4 patterns of motoneuronal responses: a simple excitatory postsynaptic potential (EPSP) with weak trailing depolarization, a double-peak EPSP, an EPSP succeeded by a weak hyperpolarization, or a slow rising EPSP. As the initial dominant EPSP was a consistent finding among all cells and the ensuing potentials were variable in polarity, quantitative characterization was focused on the initial EPSP only. In all cells tested (n = 11), the LC-EPSP was accompanied by a decrease in input resistance. The excitatory LC action was further demonstrated by the consistent (n = 25 cells) motoneuron rheobase decrease when the latter was measured coincident with the summit of an LC-EPSP. Furthermore, the time course of the single-spike afterhyperpolarization became shortened during the LC conditioning stimuli (n = 16 motoneurons). Our data show that the descending LC action on motoneurons is typified by an EPSP accompanied by a net decrease in input resistance as well as a concurrent increase in motoneuron electrical excitability.

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.

Impulse conduction properties of noradrenergic locus coeruleus axons projecting to monkey cerebrocortex

Antidromically driven action potentials were recorded from norepinephrine-containing locus coeruleus neurons in response to electrical stimulation of cerebrocortical and thalamic areas in anesthetized squirrel monkeys. These cells reliably conducted impulses from cortical sites of distances up to 100 mm from locus coeruleus. Monkey locus coeruleus neurons were found to exhibit several properties previously described for these cells in rat, including slow spontaneous discharge rates, characteristic impulse waveforms, antidromic activation from many target areas, a period of suppressed activity following either antidromic or orthodromic driving and responsiveness to noxious stimuli presented as subcutaneous electrical stimulation of a rear foot. However, a large population of monkey locus coeruleus neurons was found to exhibit more rapid conduction velocities than previously found for rat (e.g. approximately 34% were greater than 1 m/s), resulting in similar conduction latencies to distant target areas in the two species. This indicates that the conduction times required for locus coeruleus impulses to reach distant target areas may be conserved across different species and sizes of brains, suggesting that these latencies play an important role in the general function of the locus coeruleus system in brain and behavioral processes.

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.

The role of noradrenaline in tuning and dopamine in switching between signals in the CNS

Neuronal catecholaminergic activity modulates central nervous function. Specifically noradrenaline can exert a tuning or biassing function whereby the signal to noise ratio is altered. Dopamine activity may promote switching between inputs and outputs of information to specific brain regions. It has been ten years since evidence for a tuning function was advanced for noradrenaline and in the last 5 years the switching hypothesis for dopamine has been tentatively put forward. Recent studies are reviewed to show that while catecholamine activity contributes to neural interactions in separate brain regions that give rise to the organization of different functions, their working principles may be common between species and independent of the nucleus of origin. Behavioral examples are discussed and an attempt is made to integrate this with evidence from intracellular recording studies. It is suggested that the tuning principle in noradrenergic systems is particularly important for the formation of associations and neural plasticity (interference control) and that the switching principle of dopaminergic systems modulates the timing, time-sharing and initiation of responses (program-control).

Physiological properties of ascending locus coeruleus axons in the squirrel monkey

Discharge activity was recorded extracellularly from individual neurons of the nucleus locus coeruleus in anesthetized squirrel monkeys. These cells exhibited long-duration (2-3 ms) action potentials and discharged spontaneously in a slow (0.2-2 Hz) irregular fashion. Stimulation of the lateral hypothalamus evoked antidromic responses at latencies of 10-20 ms, indicating conduction velocities of over 1 m/s in some cases. The mean refractory period for these axons was 2.6 ms. When the rate of hypothalamic stimulation was increased from 1 to 10 Hz there was a 15-20% increase in antidromic latencies. These properties are similar to those previously observed for rat LC neurons, except that conduction velocities are higher in monkey.