Takuji Kasamatsu

Local perfusion of noradrenaline maintains visual cortical plasticity

WE have formed a hypothesis which links two important, but so far separate, research areas, the monoaminergic system discovered by the Swedish group1,2 and the phenomenon of critical period plasticity in the visual cortex discovered by Wiesel and Hubel3,4. We propose5 that the widespread system of monoaminergic fibres plays a part in regulating plasticity and that, more specifically, catecholamines are responsible for maintaining the high level of plasticity which is observed in the visual cortex during the critical period4. In an initial test of this hypothesis, we developed a dose and timing regimen of 6-hydroxydopamine (6-OHDA) to produce significant depletion of catecholamines bilaterally in the visual cortex of developing kittens5. The hypothesis was confirmed to the extent that kittens treated with 6-OHDA do not have the usual cortical plasticity, as measured by a change in the ocular dominance of binocular neurones following monocular occlusion5. While all the results we have obtained so far with 6-OHDA are consistent with the view that catecholamines regulate cortical plasticity, other interpretations are possible because of the widespread nature of the changes accompanied by intraventricular 6-OHDA. We now present further evidence in support of the hypothesis from experiments involving microperfusion of catecholamine in localised areas of the visual cortex of animals which would not be expected to show plasticity. These experiments indicate a specific role of noradrenaline (NA) within the cortex because plastic changes are found only in the region of cortex perfused by NA while nearby cortical regions in the same kitten are unaffected.

Single cell responses in cat visual cortex to visual stimulation during iontophoresis of noradrenaline.

SummaryWe studied how iontophoresis of noradrenaline (NA) changes responsiveness of individual cells in the feline visual cortex when their visual receptive fields are stimulated with the appropriate visual stimulus. We found three populations of cortical cells which either increased, decreased or did not change their visual responsiveness during NA iontophoresis. About equal numbers of cells belonged to each of these three groups. In the majority of such cells that changed visual responsiveness during NA iontophoresis and that had measurable amounts of spontaneous activity, the ratio of visually evoked to spontaneous activity (signal-to-noise ratio) improved during NA iontophoresis. This improvement was independent of the direction of changes in the response magnitude to visual stimulation. There was a differential effect of NA on simple and complex visual cortical cells: Although most simple cells (86%) clearly changed their responsiveness during NA iontophoresis, the effects were seen in only one-third of complex cells. Furthermore, the effects on complex cells were usually weak compared to those typically seen in simple cells. In some cases the effects of NA were more complicated than an overall enhancement or suppression of the cortical cells responses to visual stimulation. The possible dual role of NA in the visual cortex is briefly discussed.

Maturation of monoamine neurotransmitters and receptors in cat occipital cortex during postnatal critical period

SummaryThe postnatal development of monoamine levels and receptors in the occipital cortex of the cat has been investigated using neurochemical techniques. The endogenous catecholamines (noradrenaline and dopamine) gradually increased with age, displaying an about 12–13-fold increase in their concentration from the newborn to the adult stage. 3H-dihydroalprendol (β-adrenoceptor ligand) binding showed a rapid increase from the low value (25% of the adult value) at birth, peaking at the age of 7–9 weeks with a value of about 150% of adults. The β-adrenoceptor binding stayed relatively constant at adult value from the age of 11 weeks throughout. Endogenous 5-hydroxytryptamine levels were at birth about 20% of the adult value and thereafter rapidly increased, peaking at the age of 3–5 weeks when it reached the adult value. Between the age of 7–13 weeks the 5-hydroxytryptamine level was about 50–60% of adult. The developmental pattern for 3H-5-hydroxytryptamine binding was similar to that of endogenous 5-hydroxytryptamine, although with certain quantitative differences. The 3H-5-hydroxytryptamine receptor binding showed a steep peak at an age of about 4 weeks when the binding was about 300% of the adult value. Thereafter the binding gradually levelled off in adulthood. Similar results were obtained in the frontal cortex, except for some quantitative differences. The present results thus indicate that both noradrenaline and 5-hydroxytryptamine nerve terminals develop, largely independent of their postsynaptic receptors, probably due to different developmental programs regulating their expression. The development of monoamine receptors appear to precede that of their nerve terminals. The different roles played by β-adrenoceptors and 5-hydroxytryptamine receptors for the maturation of occipital cortex during postnatal critical period were discussed.

Norepinephrine-containing terminals in kitten visual cortex: laminar distribution and ultrastructure.

Abstract The laminar distribution of monoamine-containing terminals in the visual cortex (areas 17 and 18) of 6- to 8-week-old kittens was studied with a modified glyoxylic acid histofluorescence method. The ultrastructure of monoamine-containing terminal boutons in cortical tissues fixed with glyoxylic acid perfusion followed by immersion in potassium permanganate was also studied in serial sections. In both coronal and sagittal planes, rich innervation of catecholamine-containing fibers and terminals was found throughout all 6 cortical layers. Intensely fluorescent stem fibers, which are preterminal fibers in the white matter, entered layer VI from the white matter after veering abruptly towards the cortical surface. While the stem fibers ran towards the surface, many collateral terminal fibers branched out in all directions in the middle layers; a dense plexus of catecholamine fibers and terminals was found mostly in layers II and III. Reaching the superficial layer, the stem fibers gave off terminal fibers which ran parallel to the surface. Monoamine-containing terminal boutons were identified by electron microscopy as small bulges which contained dense-cored vesicles. About 10% of the monoamine-containing boutons showed the usual synaptic membrane specialization suggesting synapse formation. Another 10% of the monoamine boutons showed only some characteristic features such as widening of the associated intermembrane space, and aggregation and contact of cored vesicles with the presynaptic membrane. The remaining 80% of the monoamine boutons failed to show any of the above described properties. The 2 types of monoamine boutons with classical synaptic features were seen in all layers, particularly II and III, but not in layer VI. The most frequent target of monoamine boutons in all 6 layers seemed to be dendritic shafts. The few axo-spiny and axo-somatic contacts observed occurred in the superficial and deep layers, respectively. The circumstantial evidence suggested that the monoamine visualized in the current study of either histofluorescence or electron microscopy was in fact norepinephrine rather than dopamine and serotonin. The rich innervation of all cortical layers with catecholamine-containing boutons is consistent with noradrenaline being a factor in modifying the plasticity of neurons in the visual 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.

Visual afferents to norepinephrine-containing neurons in cat locus coeruleus.

SummaryA total of 208 single neurons were extracellularly recorded in the locus coeruleus (LC) of 11 cats. In later histofluorescence studies, greenish fluorescent LC neurons, from which we believed to have recorded well-isolated action potentials, were always found in or close to the center of red fluorescent halo due to an injected dye which marked the recording site.One hundred twelve of these 208 neurons were further subjected to electrical stimulation of the dorsal bundle of ascending axons originating from the norepinephrine (NE)-containing LC neurons and thus activated antidromically with a mean latency of 8.9 ms (the remaining neurons were lost before this examination). The mean conduction velocity was 1.2 m/s. Furthermore, it is suggested that 22% of thus antidromically identified NE neurons in the cat LC had an ascending axon of the conduction velocity faster than 2.4 m/s. This finding may be related with an electron-microscopic observation which indicated the presence of myelinated catecholamine (CA) axons, though not many in number, in the cat visual cortex.Responses by the NE-containing LC neurons to various natural visual stimuli, such as flashlight, moving and stationary light-slit, multiple spots, and gratings were examined. It turned out, however, that flash alone was effective to activate LC neurons. The latency of flash evoked activity was between 48 and 96 ms (N = 12; mean: 60 ms). Furthermore, the following areas in the central visual pathway were electrically stimulated to activate LC neurons orthodromically: the optic chiasm (OX), the dorsal lateral geniculate nucleus (LGN), the superior colliculus (SC), and the visual cortex (VC). The range and the mean of the latency for orthodromic responses were as follows: OX (N = 36, 8.4–42 ms; mean: 21 ms); LGN (N = 17, 6.0–17 ms; mean: 8.1 ms); SC (N = 12, 3.6–12 ms; mean: 5.6 ms); VC (N = 10, 7.8–40 ms; mean: 16.4 ms).The long latency of these orthodromic responses and its wide distribution suggest that afferents to the LC from the above-mentioned visual structures are most likely polysynaptic in nature. The extensive input convergence, including acoustic and nociceptive afferents, and the polysynaptic connection in each afferent pathway indicated a strong similarity between the afferent connectivity of NE-containing LC neurons revealed in the present study and that known for reticular formation neurons. Then, we would like to suggest that visual signals from the eyes impinge upon the NE-containing LC neurons via the reticular formation and that the afferents from the LGN, the SC, and the VC also join this common path through the reticular formation to reach the LC.

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

Summary 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 ∼10 mm). In the primary lesion zone (a radius of ∼5 mm) near the center of the 6-OHDA infusion, excluding an area of non-specific damage left by cannulation (a radius of β -adrenoreceptor binding sites by densitometry of light microscopic autoradiograms; and (4) low levels (