Destruction of noradrenergic cell bodies by intracerebral 6-hydroxydopamine in the newborn rat

Restoration of ascending noradrenergic projections by residual locus coeruleus neurons: compensatory response to neurotoxin-induced cell death in the adult rat brain

There is clinical and experimental evidence that monoamine neurons respond to lesions with a wide range of compensatory adaptations aimed at preserving their functional integrity. Neurotoxin-induced lesions are followed by increased synthesis and release of transmitter from residual monoamine fibers and by axonal sprouting. However, the fate of lesioned neurons after long survival periods remains largely unknown. Whether regenerative sprouting may contribute significantly to recovery of function following lesions which induce cell loss has been questioned. We have previously analyzed the response of locus coeruleus (LC) neurons to systemic administration of the noradrenergic (NE) neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) to adult rats. This drug causes ablation of nearly all LC axon terminals within 2 weeks after administration, followed by a profound loss of LC cell bodies 6 months later. The present study was conducted to determine the fate of surviving LC neurons and to characterize their potential for regenerative sprouting during a 16 month period after DSP-4 treatment. The time-course and extent of LC neuron degeneration were analyzed quantitatively in Nissl-stained sections, and the regenerative response of residual neurons was characterized by dopamine-beta-hydroxylase immunohistochemistry. The results document that LC neurons degenerate gradually after DSP-4 treatment, cell loss reaching on average 57% after 1 year. LC neurons which survive the lesion exhibit a vigorous regenerative response, even in those animals in which cell loss exceeds 60-70%. This regenerative process leads progressively to restoration of the NE innervation pattern in the forebrain, with some regions becoming markedly hyperinnervated. In stark contrast to the forebrain, very little reinnervation takes place in the brainstem, cerebellum and spinal cord. These findings suggest that regenerative sprouting of residual neurons is an important compensatory mechanism by which the LC may regain much of its functional integrity in the presence of extensive cell loss. Furthermore, regeneration of LC axons after DSP-4 treatment is region-specific, suggesting that the pattern of reinnervation is controlled by target areas. Elucidation of the factors underlying recovery of LC neurons after DSP-4 treatment may provide insights into the compensatory mechanisms of central neurons after injury and in disease states.

Adaptive changes of beta-adrenergic receptors after neonatal locus coeruleus lesion: regulation of serotoninergic unit activity

Spontaneous activity of 5-hydroxytryptamine (5-HT) neurons in the dorsal raphe nucleus (DRN) was recorded in adult rats that had undergone a bilateral locus coeruleus (LC) lesion during the neonatal period. The susceptibility of this neuronal firing to beta-adrenergic manipulation was tested. Microiontophoretic application of the beta-blockers d,l-propranolol and acebutolol inhibited the firing of DRN cells in lesioned rats but not in control animals. This effect was specific to beta-receptors since the effects of pharmacological manipulation of other receptors--5-HT, gamma-aminobutyric acid (GABA), alpha-adrenoceptors--were identical in lesioned and control animals. The present data demonstrate that a neonatal noradrenergic lesion allowed the persistence of a beta-regulation of DRN neuronal firing, which in young rats is normally only transient.

Cellular localization of adrenergic receptors in rat and human brain

The localization of adrenergic receptors in the central nervous system was studied in two physiological conditions of noradrenergic denervation, a 6-hydroxydopamine-induced lesion of the locus coeruleus in newborn rat, and a pathological related degeneration of the locus coeruleus in man, Parkinson's disease. The localization of these receptors in the synapse has been studied with the technique of subcellular fractionation by differential centrifugation. In lesioned rats, an increase in the density of alpha 1 and beta 1 receptors was observed in several brain regions, in contrast to alpha 2 receptors which were not modified. Subcellular fractionation in lesioned rats showed an increase in alpha 1 and beta 1 receptors in synaptosomal fractions. Similar results were found in parkinsonian patients: alpha 1 receptors increased in the synaptosomal fraction; beta receptors increased in synaptosomal and microsomal fractions. These results suggest that alpha 1 and beta 1 receptors may be located on non-noradrenergic nerve terminals in mammalian brain. alpha 2 and beta 2 receptors may be situated on glial cells or neuronal elements unrelated to noradrenergic input.

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.

Effects of neurotoxin-induced brainstem lesions on the respiratory responses of conscious rats

Respiration was recorded in unanaesthetized rats breathing air, or CO2 in air, 4-8 days after injection into the rostral brainstem at the level of the dorsal raphe nucleus of either a catecholaminergic neurotoxin, a serotoninergic neurotoxin, or the vehicle (0.5% ascorbic acid) of these drugs. Some rats were pretreated with the noradrenergic neurone protecting agent, desmethylimipramine (DMI). Vehicle injection resulted in an increase in the frequency (f) sensitivity to CO2. In two of four cases, injection of 6-hydroxydopamine caused a similar response but in the remaining cases injections caused increases in inspiratory duration (TI), expiratory duration (TE) and tidal volume (VT) during air-breathing. The serotonergic neurotoxin, 5,7-dihydroxytryptamine, reduced the frequency sensitivity to CO2 significantly. This reduction was not sustained. In all cases air-breathing f was decreased due to a prolongation of TI; in most cases air-breathing VT was increased. The characteristic effect of midline 5,7-dihydroxytryptamine lesions was an increase in the VT sensitivity to CO2. On the basis of the results, it is proposed that noradrenaline is essential to the processes determining basic respiratory rhythm, whereas dopamine and serotonin are important inhibitors in chemoreflex ventilatory patterns.

Dopamine modulates S-potential amplitude and dye-coupling between external horizontal cells in carp retina

Horizontal cells in the fish retina are electrically coupled and possess gap junctions so that intracellularly injected dye normally diffuses freely to neighbouring cells. Applied dopamine (DA) alters the spatial properties of the horizontal cell responses to light, increasing the amplitude of photopic L-type S-potentials but decreasing their lateral spread. These effects have been attributed to the action of DA on horizontal cell membrane resistance, particularly at the gap junctions, and our present study on the carp retina agrees with this in showing that DA also restricts intracellular Lucifer yellow (LY) to single injected horizontal cells, an effect, like those of DA on the S-potentials, which is antagonized by the dopamine blocker haloperidol. In addition, we present evidence that dopaminergic interplexiform cells in fish normally function to regulate the spatial properties of responses in horizontal cells, possibly acting on their junctional resistance via a DA-receptor-mediated mechanism. Previous destruction of the interplexiform cells with 6-hydroxydopamine (6-OHDA) resulted in much reduced L-type S-potentials to centred lights but wider lateral spread of these responses, while the dye injected spread extensively to neighbouring cells. After 6-OHDA treatment, however, applied DA retained its normal activity, restoring large-amplitude, narrow receptive-field S-potentials and restricting LY to the injected cells, effects which were both closely mimicked by dibutyryl cyclic AMP.

Development of the dopaminergic innervation of the rat cerebral cortex. A light microscopic immunocytochemical study using anti-tyrosine hydroxylase antibodies

A precise topographical analysis of the distribution of tyrosine hydroxylase-like immunoreactive processes was performed in the frontal, cingular and parietal cortex of the rat during late embryonic and early postnatal life. Until birth, labeled processes were only observed in the restricted cortical areas known to receive a dopaminergic innervation in the adult brain. Their distribution differed markedly from that of noradrenergic fibers as identified by their dopamine-beta-hydroxylase-like immunoreactivity. Thus we considered TH-like immunoreactivity to be a selective marker of the cortical dopaminergic innervation during late fetal life, at least with the antibody we used. With this marker, dopaminergic fibers were first detected in the anterior frontal cortex at day 16 of embryonic life (E16). They developed as two bundles passing medially and laterally to the ventricular layer without penetrating it. From E20 on, the terminal fields extended to the cingular and rhinal cortex, still being restricted to the intermediate zone. No fibers were visible in the lateral and dorsal frontal cortex at this time, nor in the cortical plate and molecular layer in any cortical area. At E21, rare labeled fibers were seen in the molecular layer of the medial frontal and cingular cortex. After birth, the terminal fields of the TH-containing fibers extended further caudally in the cingular cortex and also superficially in the cortical plate. Moreover, labeled axons now also appeared in the lateral frontal and parietal cortex where their density gradually increased. At P14, two different patterns of distribution were observed: a high density of TH-positive fibers in the cortical areas known to receive a dopaminergic innervation; a low density of fibers in the other cortical areas which represented noradrenergic fibers. Indeed these TH-containing presumed noradrenergic fibers were absent at P14 following a bilateral destruction of the locus coeruleus in 4-day-old pups.

Regulation of sleep after neonatal locus coeruleus lesion: functional evidence of beta-adrenergic supersensitivity

In the present report we analyzed the long-term effect of neonatal noradrenergic denervation on the regulation of paradoxical sleep in the rat. The locus coeruleus was destroyed bilaterally at 4 days of age by direct infusion of 6-hydroxydopamine into the nuclei. After they reached adulthood, the rats received either i.p. injections of a beta-blocker, propranolol, or the same treatment combined with intraventricular infusion of a beta-agonist, isoproterenol. Several doses of each drug were tested. The effects of propranolol, alone and together with isoproterenol, on paradoxical sleep (a decrease for the former drug and a restoration for the latter combination) were significantly more pronounced in the lesioned group than in age-paired controls. These data illustrate a functional aspect of the denervation supersensitivity phenomenon in beta-receptors.