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.

Studies on the effect of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on central catecholamine neurons in C57BL/6 mice. Comparison with three other strains of mice

The effect of the parkinsonism-inducing neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on central catecholamine neurons in C57BL/6 mice has been studied employing neuro- and histochemical techniques. The number of dopamine (DA) cell bodies in substantia nigra pars compacta (SNC) was reduced by 70% in MPTP-treated C57BL/6 mice, as demonstrated both by tyrosine hydroxylase (TH) immunohistochemistry and conventional histology (Cresyl violet staining) and an almost complete loss of DA fibers in striatum was also found. A detailed analysis of the effects of MPTP on endogenous catecholamine levels in various brain regions revealed that MPTP caused a severe reduction of endogenous DA in substantia nigra and striatum (35 and 5% of control) which was accompanied by an increase in the 3,4-dihydroxyphenylacetic acid (DOPAC)/DA ratio. There was also a decrease of DA in nucleus accumbens and the olfactory tubercle to 41 and 44% of control, respectively, without any significant change in the DOPAC/DA ratio and density of TH-positive fibers. Small but significant decreases of the noradrenaline (NA) levels in septum, entorhinal cortex and frontal cortex were seen, although the uptake of [3H]NA in frontal cortex was not significantly changed. Minor MPTP-induced decreases of the serotonin levels in frontal cortex, occipital cortex and spinal cord were also seen. The MPTP treatment also induced a 55% increase of adrenaline levels in hypothalamus, while no changes were seen in pons-medulla and spinal cord. Comparing this with 3 other strains of mice, the MPTP-induced reduction of endogenous DA in striatum was most pronounced in C57BL/6, less in N.M.R.I. and CBA/Ca mice, and least in Swiss-Webster. Concerning the effect of MPTP on cortical NA levels, the same relation was at hand except for C57BL/6, where, as mentioned, the effect was merely detectable. No reduction of DA perikarya in SNC was seen in Swiss-Webster mice. These findings show that in mice major differences exist in sensitivity of catecholamine neurons to MPTP between different strains. The data show that MPTP can produce an almost complete, permanent and relatively selective degeneration of the nigrostriatal DA neurons in C57BL/6 mice similar to that seen in primates. This strain may therefore serve as a useful model for studies on various aspects of MPTP-induced parkinsonism.

(+)-8-OH-DPAT and 5-MeODMT induced analgesia is antagonised by noradrenaline depletion

In experiments with both rats and mice the 5-HT agonists 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and 5-methoxy-N,N-dimethyl-tryptamine (5-MeODMT) were shown to produce reliable analgesic effects after acute administration (1 mg/kg SC) in the tail-flick, hot-plate and shock-titration tests of nociception. Prior treatment with the noradrenaline neurotoxin, N-2-chloroethyl-N-ethyl-2-bromobenzylamine (DSP4), systemically administered to both rats and mice abolished the analgesic effects of both the 5-HT agonist compounds in all the tests of nociception used. Intrathecal 6-hydroxydopamine (6-OHDA) treatment also abolished the analgesic effects of 8-OH-DPAT and 5-MeODMT; in the tail-flick test the analgesia induced by 8-OH-DPAT was reversed to an hyperalgesia. Biochemical analyses confirmed notable noradrenaline depletions in the spinal cord. It is concluded that an important interaction between presynaptic noradrenergic terminals and serotonergic receptor sites, possibly 5-HT1A, mediates spinal nociception processes.

Selective attention and place navigation in rats treated prenatally with methylazoxymethanol

Prenatal treatment of rats on gestation day 15 with methylazoxymethanol (MAM) caused forebrain microencephaly. The behavioral analyses included measures of spontaneous motor activity and tests for cognitive deficits, and were performed when the rats had reached adult age. Female MAM-treated rats failed to demonstrate contextual control of latent inhibition, which confirms earlier findings with male rats. Male MAM-treated rats demonstrated a notable impairment of place navigation in a swim-maze, but showed as strong sensory preconditioning as the control animals. Biochemical analyses indicated considerable increases in catecholamine levels in the cerebral cortex, hippocampus and striatum. The cognitive deficits, characterised by the various conditioning (taste-aversion) and instrumental learning (swim-maze) tasks, suggested that the MAM rats are deficient in their capacity to attend selectively to the relevant stimulus in complex arrangements of the stimulus situation.

Uptake inhibition protects nigro-striatal dopamine neurons from the neurotoxicity of 1-methyl-4-phenylpyridine (MPP+) in mice

Intracerebroventricular administration of MPP+ to C57 BL/6 mice caused a pronounced depletion of striatal levels of dopamine and 3,4-dihydroxyphenyl-acetic acid ipsilaterally, and a less marked depletion contralaterally. The MPP+-induced reductions were clearly diminished by pretreatment with the dopamine uptake inhibitors mazindol and nomifensine. Similar results were obtained from determinations using tyrosine hydroxylase immunohistochemistry. These results are consistent with the hypothesis that MPTP neurotoxicity is related to the formation of MPP+ from MPTP outside the dopamine neurons and that subsequent uptake of MPP+ into these neurons initiates degeneration.

Selective lesioning of forebrain noradrenaline neurons at birth abolishes the improved maze learning performance induced by rearing in complex environment

The effect of selective destruction of forebrain noradrenaline (NA) neurons induced by 6-hydroxydopamine (6-OHDA) at Day 1 after birth on Hebb-Williams maze performance was investigated in adult rats housed after weaning in a complex environment (EC) or an isolated (IC) environment for 35 days. Saline treated control rats raised in the EC made fewer errors than those raised in the IC. This effect of EC was completely abolished in 6-OHDA treated rats; for these animals no improved performance due to the housing condition was obtained. Protection of the NA neurons against 6-OHDA neurotoxicity by pretreatment with desipramine (DMI) resulted in an effect of EC identical to that seen in saline-treated controls. Postweaning housing in the IC led to an increased locomotion as compared to housing in EC, but this effect was not affected by neonatal 6-OHDA and/or DMI treatment. Neurochemical analysis confirmed cortical NA and metabolite depletion as well as a good protection by the DMI pretreatment. The present results indicate that central NA neurons are involved critically in mediating mainly the cognitive components of behavioral alterations induced by EC.

5-Methoxy-N,N-dimethyltryptamine-induced analgesia is blocked by alpha-adrenoceptor antagonists in rats

The effects of the alpha-adrenoceptor antagonists prazosin, phentolamine and yohimbine upon 5-methoxy-N,N-dimethyltryptamine (5-MeODMT)-induced analgesia were tested in the hot-plate, tail-flick and shock-titration tests of nociception with rats. Intrathecally injected yohimbine and phentolamine blocked or attenuated the analgesia produced by systemic administration of 5-MeODMT in all three nociceptive tests. Intrathecally administered prazosin attenuated the analgesic effects of 5-MeODMT in the hot-plate and tail-flick tests, but not in the shock titration test. Intrathecal yohimbine showed a dose-related lowering of pain thresholds in saline and 5-MeODMT-treated animals. Phentolamine and prazosin produced normal dose-related curves in the hot-plate test and biphasic effects in the shock titration and tail-flick tests. These results demonstrate a functional interaction between alpha 2-adrenoceptors and 5-HT agonist-induced analgesia at a spinal level in rats.

Effect of MPTP and its pyridinium metabolites on monoamine uptake and on central catecholamine neurons in mice

The effect of MPTP and its pyridinium metabolites MPDP+ and MPP+ on the in vitro [3H]monoamine uptake in synaptosomal preparations from mouse striatum and cerebral cortex was investigated. All compounds inhibited [3H]monoamine uptake in a dose-dependent manner in both regions analysed. MPP+ had the highest affinity to dopamine and noradrenaline uptake sites, while MPTP had the highest affinity to serotonin uptake sites. The results indicate that the affinity of MPP+ to different monoamine uptake sites appears to be better correlated to MPTP neurotoxicity as expressed in vivo than MPTP and MPDP+. Intracerebral injection of MPP+ into substantia nigra produced an almost complete disappearance of dopamine in striatum and noradrenaline in cerebral cortex, while injection of MPTP or MPDP+ had no or only moderate catecholamine-depleting effects. The MPP+-induced catecholamine depletion could be partially reversed by pretreatment with the catecholamine uptake blocker nomifensine. Histological analysis disclosed that MPP+ was a potent generally cytotoxic agent, while MPDP+ less and MPTP least so. The present results are compatible with the view that an interaction with the catecholamine uptake mechanism, probably through an uptake and accumulation of extraneuronally formed MPP+, is most likely the explanation for neuron-specific neurotoxic action on catecholamine neurons following MPTP administration.

5-HT agonist induced analgesia modulated by central but not peripheral noradrenaline depletion in rats

The antinociceptive effect elicited by the 5-hydroxytryptamine (5-HT) agonist 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) was reversed or blocked in animals which had previously sustained severe spinal noradrenaline (NA) depletion via either systemic N-2-chlorethyl-N-ethyl-2-bromobenzylamine hydrochloride (DSP 4), neonatal 6-hydroxydopamine (neon. 6-OHDA), or intrathecal 6-OHDA treatment. Biochemical analysis of the lumbar spinal cord samples confirmed severe central NA depletions. Animals were tested with nondamaging heat pain (tail-flick test, hot-plate test) and electric footshock titration to determine the amount of antinociception or nociception. Peripheral NA depletion following intravenous (i.v.) 6-OHDA injection to adult rats had no effect on the antinociception induced by 5-MeODMT, but did cause severe NA depletions in the left heart atrium. These results suggest a modulatory effect of central and not peripheral noradrenergic system upon 5-HT agonist induced analgesia, and also give evidence that this effect is spinally mediated.

Astrocyte responses to dopaminergic denervations by 6-hydroxydopamine and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine as evidenced by glial fibrillary acidic protein immunohistochemistry

Astrocytic responses to dopaminergic denervation by two widely used dopamine neurotoxins, 6-hydroxydopamine (6-OHDA) and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) were monitored using immunofluorescence with antibodies against glial fibrillary acidic protein (GFA) while neurofilament (NF) antibodies were used to monitor neuronal disturbances. Following stereotaxic injection of 6-OHDA into the nigrostriatal dopamine bundle in rats, an increased amount of GFA-immunoreactivity in striatum was detectable after 24 hours and remained after one month. Retrograde degeneration of nigral neurons led to gliosis in the cell body area. At the site of injection, astrocytes were destroyed and NF-immunoreactivity increased. New astrocytes invaded the injection area during the first month after injection. MPTP given systemically to mice in a dose that causes marked dopaminergic denervation of striatum also caused marked increases of GFA-immunoreactivity in striatum. These changes were larger in C57 BL/6 mice, known to be more sensitive to MPTP, than in N.M.R.I. mice, which are less sensitive to MPTP. The glial responses to MPTP-induced dopaminergic denervation did not occur when the dopamine neurotoxic effects were prevented by pretreatment with nomifensine or pargyline. It is concluded that dopaminergic denervation by neurotoxins causes rapid and profound changes in striatal astrocytes characterized by increased GFA-immunoreactivity. These changes remained up to a month after denervation and should be taken into account when functional consequences of dopaminergic denervations are discussed.

Regional distribution and extracellular levels of amino acids in rat central nervous system

The extracellular levels of aspartate, glutamate, serine, glutamine, glycine, alanine and GABA were studied in vivo with the microdialysis technique in 15 different regions of the rat brain. The effect of high K+ on the overflow of these amino acids was also studied. These results were compared with those from a regional dissection of 17 brain regions in which the tissue content of the same amino acids was determined. The in vivo data showed an unevenly distributed KCl response of aspartate, glutamate, taurine and GABA, all of which are putative neurotransmitters. It was not possible to predict the response to high K+ from the magnitude of the unstimulated overflow. Glutamine overflow was inversely related to that of glutamate during the high K+ stimulus, which is consistent with glutamine being the main precursor of glutamate. Only for GABA and alanine was overflow proportional to the tissue level in the different regions studied.

Noradrenaline-serotonin interactions in the control of sexual behavior in the male rat: DSP4-induced noradrenaline depletion antagonizes the facilitatory effect of serotonin receptor agonists, 5-MeODMT and lisuride

The present communication reports how depletion of central noradrenaline neurons of DSP4 treatment antagonizes the facilitatory actions of 5-MeODMT and lisuride on male rat sexual behavior. In males with intact noradrenaline, 5-MeODMT facilitated sexual behavior by reducing the number of intromissions required for ejaculation; inhibitory actions were also noted, since 5-MeODMT prolonged intromission and ejaculation latencies. In DSP4-pretreated animals the inhibitory effect of 5-MeODMT remained unchanged, whereas its facilitatory action was abolished. Consistent with previous research, lisuride also reduced intromission frequency prior to ejaculation. This facilitation of sexual behavior was not observed in DSP4-treated animals. In the male rat, ejaculations following the first have a lower latency and are preceded by a lower number of intromissions. This naturally occurring facilitation of sexual behavior was not prevented by DSP4-induced noradrenaline depletion. Our results suggest that serotonin and noradrenaline interact in the control of sexual behavior in the male rat.

Spinal and locus coeruleus noradrenergic lesions abolish the analgesic effects of 5-methoxy-N,N-dimethyltryptamine

Two experiments were performed on Sprague-Dawley rats to study the effects of noradrenaline and 5-hydroxytryptamine depletion upon the antinociceptive effects of acute 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) administration. 6-Hydroxydopamine-induced lesions following microinjections to either the locus coeruleus or the spinal cord (lumbar) abolished completely 5-MeODMT-induced analgesia in the tail-flick, hot-plate, and shock titration tests whereas 5,7-dihydroxytryptamine-induced lesions of the nucleus raphe magnus and the lumbar spinal cord attenuated 5-MeODMT analgesia in the tail-flick and shock titration tests. Thus, the experiments serve to demonstrate an important interaction between descending noradrenergic and serotonergic pathways, possibly at a spinal locus.

The catecholamine uptake blocker nomifensine protects against MPTP-induced parkinsonism in monkeys

Administration of MPTP (1-methyl-1,2,3,6-tetrahydropyridine) to Macaca fascicularis monkeys produced severe parkinsonism (hypokinesia, tremor, rigidity, aphagia, adipsia) and more than 90% loss of nigral dopamine neurons, striatal dopamine content and striatal 3H-mazindol binding. Treatment with the catecholamine uptake blocker nomifensine counteracted the behavioral, histological and neurochemical effects induced by MPTP. For obtaining best protection, nomifensine had to be administered for weeks after MPTP. The results suggest that the selective target-directed neurotoxic action of MPTP on dopamine neurons in monkeys is mediated via the dopamine uptake mechanism.

DSP4-induced noradrenergic lesions increase beta-adrenergic receptors and hippocampal electrophysiological responsiveness

Following profound (greater than 90%) depletions of norepinephrine (NE) by the noradrenergic neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4), the numbers of beta-adrenergic receptors were significantly increased (20-25%) in rat hippocampal and somatosensory cortical membranes; however, the numbers of alpha 1-adrenergic receptors and the affinities of both types of receptors were unaffected. This selective up-regulation of beta-adrenergic receptors was evident 1 week after DSP4 administration and was maintained for at least 2 more weeks. In electrophysiological experiments in the hippocampal slice preparation, responses to threshold as well as maximal concentrations of isoproterenol were enhanced 150% and 33%, respectively, in the DSP4-lesioned animals. The results demonstrate that nearly complete depletion of brain NE produced by administration of DSP4, like that produced by 6-hydroxydopamine, results in increased numbers of beta- but not alpha-adrenergic receptors, and suggest that the density of the former are regulated by afferent noradrenergic fibers. Furthermore, the functional significance of the increased number of hippocampal beta-adrenergic receptors is directly manifested in a greater electrophysiological responsiveness to an exogenously administered beta-adrenergic receptor agonist.

[Locus ceruleus: the regulation of hemato-encephalic barrier function normally and under emotional stress]

To study the role of locus ceruleus in maintaining the blood-brain barrier function in conditions of emotional stress, the blood-brain barrier permeability was estimated after locus ceruleus damaging with DSP4 (N-2-chloroethyl-N-ethyl-2-bromobenzylamine). The importance of locus ceruleus for the integrity of blood-brain barrier functions in control animals and especially in rats exposed to emotional stress was revealed. The data obtained prove the homeostatic role of locus ceruleus, which is the most prominent in conditions of emotional stress.

Attenuation of sensory preconditioning by noradrenaline depletion in the rat

In order to investigate the effect of noradrenaline (NA) depletion upon an associative learning phenomenon, sensory preconditioning, rats were inflicted with either 6-hydroxydopamine-induced lesions of the dorsal noradrenergic bundle (DNAB) or the locus coeruleus (LC), or with systemic injections of the NA neurotoxin, N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4). Using appropriate controls (the UP groups) sensory preconditioning was demonstrated clearly in the non-lesion conditions (Sham or saline), but was blocked or strongly attenuated in the DNAB and DSP4 conditions. LC lesions did not affect sensory preconditioning. These findings suggest that the loss of central NA may cause some disruption of some aspects of complex associative learning. The role of NA in sensory preconditioning may be relevant to current notions of NA function.

Impaired selective attention in methylazoxymethanol-induced microencephalic rats

Prenatal treatment of rats on gestation day 15 with methylazoxymethanol (MAM) caused forebrain microencephaly. Several behavioral tests were performed when the rats had reached an adult age. MAM treated rats were hyperactive, and were severely impaired in the acquisition of successive position reversal in a T-maze. The microencephalic rats failed also to demonstrate contextual control of latent inhibition (the stimulus preexposure effect) in taste-aversion conditioning. These results indicate that MAM treatment disrupts attentional processes and that this may account for the learning impairment.

Differential time course of protection by monoamine oxidase inhibition and uptake inhibition against MPTP neurotoxicity on central catecholamine neurons in mice

Pretreatment with the monoamine oxidase inhibitor pargyline or the catecholamine uptake blocker nomifensine both protected central catecholamine neurons against MPTP-induced neurotoxicity as monitored by analyzing catecholamine levels and [3H]mazindol binding. Post-treatment with the inhibitors showed that the administration of nomifensine could be delayed longer than that of pargyline in order to achieve a protective effect. The results are compatible with the view that the monoamine oxidase-catalyzed conversion of MPTP to a toxic metabolite MPP+ occurs mainly extraneuronally. MPP+ is subsequently taken up and accumulated selectively in catecholamine neurons, initiating degeneration.

Thyrotropin-releasing hormone (TRH) counteracts neuronal damage induced by a substance P antagonist

Intrathecal administration of the substance P antagonist Spantide caused marked necrotic changes of the gray matter of the spinal cord extending several segments from the injection site. Intravenous treatment with several doses of thyrotropin releasing hormone before and after Spantide injection completely prevented the necrotic lesion.

Spinal noradrenergic neurotransmission and the analgesia induced by brief footshock

Antinociception induced by brief footshock as well as by 5-methoxy-N,N-dimethyltryptamine was antagonized by lesions of the descending bulbospinal noradrenergic (NA) pathways by intrathecal injections of 6-hydroxydopamine. The alpha 2-adrenoceptor antagonist, yohimbine, injected intrathecally also blocked both types of nociceptive effects in the tail-flick and hot-plate tests. 5-Methoxy-N,N-dimethyltryptamine (1 mg/kg) potentiated shock-induced antinociception and this potentiation was also antagonized by decreased NA neurotransmission. These findings suggest an important role for spinal NA innervation, and possibly alpha 2-adrenoceptors in antinociception induced by brief footshock and serotonergic receptor stimulation induced analgesia in rats.

Noradrenergic-serotonergic interactions and nociception in the rat

Spinal noradrenaline (NA) depletion in rats, via either systemic N-2-chloroethyl-N-ethyl-2-bromobenzylamine (DSP4) or intrathecal 6-hydroxydopamine (6-OHDA), reversed and/or abolished the analgesic effects of the 5-hydroxytryptamine (5-HT) agonists, 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) and p-chloroamphetamine (PCA), in shock titration, hot-plate and tail-flick measures of pain sensitivity. Spinal NA depletion also abolished the analgesic effects of 5-HT itself, administered intrathecally, in all three nociception tests and potentiated the analgesic effects of intrathecal NA, a demonstration of receptor supersensitivity. Spinal 5-HT depletion, via intrathecal 5,7-dihydroxytryptamine (5,7-DHT), only attenuated 5-MeODMT-induced analgesia in the tail-flick test but potentiated the 5-MeODMT effect in the hot-plate test. Intrathecal 5,7-DHT treatment caused a drastic potentiation of NA-induced analgesia in the shock titration and tail-flick tests but not in the hot-plate test. Biochemical analyses confirmed the NA and 5-HT depletion. The spinal noradrenergic system appears to be an important tonic factor modulating the function of the descending 5-hydroxytryptaminergic pathway.

GM1 ganglioside counteracts selective neurotoxin-induced lesion of developing serotonin neurons in rat spinal cord

The effect of exogenous monosialoganglioside GM1 on neurotoxin-induced lesioning of bulbo-spinal serotonergic neurons of newborn rats was studied by means of biochemical and immunocytochemical techniques. 5,7-dihydroxytryptamine (5,7-HT, a selective serotonin neurotoxin) treatment of newborn rats caused a pronounced reduction of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) levels in the thoracic and lumbar spinal cord, while an increase of 5-HT and 5-HIAA was found in the pons medulla. These biochemical alterations were regionally correlated with similar changes in 5-HT nerve terminal density analyzed by image analysis. GM1 administration (30 mg/kg for 4 consecutive days) antagonized the reduction of 5-HT and 5-HIAA levels induced by 5,7-HT treatment in the lumbar spinal cord of 2-month-old rats, as well as the decrease of 5-HT nerve terminal density in both thoracic and lumbar spinal cord of 1- and 2-month-old rats. A minor counteracting effect of GM1 was found in the pons medulla where the neurotoxin induced an increase of 5-HT and 5-HIAA levels. These data support the hypothesis that GM1 may have a preventing action on retrograde degenerative processes following chemical lesion and/or a growth-stimulating effect on injured 5-HT neurons.