Effects of DSP-4 on monoamine and monoamine metabolite levels and on beta adrenoceptor binding kinetics in rat brain at different times after administration

The Neurotoxin DSP-4 Dysregulates the Locus Coeruleus-Norepinephrine System and Recapitulates Molecular and Behavioral Aspects of Prodromal Neurodegenerative Disease

The noradrenergic locus coeruleus (LC) is among the earliest sites of tau and α-synuclein pathology in Alzheimer's disease (AD) and Parkinson's disease (PD), respectively. The onset of these pathologies coincides with loss of noradrenergic fibers in LC target regions and the emergence of prodromal symptoms including sleep disturbances and anxiety. Paradoxically, these prodromal symptoms are indicative of a noradrenergic hyperactivity phenotype, rather than the predicted loss of norepinephrine (NE) transmission following LC damage, suggesting the engagement of complex compensatory mechanisms. Because current therapeutic efforts are targeting early disease, interest in the LC has grown, and it is critical to identify the links between pathology and dysfunction. We employed the LC-specific neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4), which preferentially damages LC axons, to model early changes in the LC-NE system pertinent to AD and PD in male and female mice. DSP-4 (two doses of 50 mg/kg, one week apart) induced LC axon degeneration, triggered neuroinflammation and oxidative stress, and reduced tissue NE levels. There was no LC cell death or changes to LC firing, but transcriptomics revealed reduced expression of genes that define noradrenergic identity and other changes relevant to neurodegenerative disease. Despite the dramatic loss of LC fibers, NE turnover and signaling were elevated in terminal regions and were associated with anxiogenic phenotypes in multiple behavioral tests. These results represent a comprehensive analysis of how the LC-NE system responds to axon/terminal damage reminiscent of early AD and PD at the molecular, cellular, systems, and behavioral levels, and provides potential mechanisms underlying prodromal neuropsychiatric symptoms.

Effects of atomoxetine on attention and impulsivity in the five-choice serial reaction time task in rats with lesions of dorsal noradrenergic ascending bundle

Atomoxetine, a noradrenaline reuptake inhibitor (NRI), which is a non-stimulating medicine that is used for the treatment of patients with attention deficit hyperactivity disorder (ADHD), has been found to be effective in reducing behavioral impulsivity in rodents, but its efficacy in a dorsal noradrenergic ascending bundle (DNAB)-lesioned condition has not been examined. The present study aimed to investigate the effects of DNAB lesions on attention and impulsive control in the five-choice serial reaction time task (5-CSRTT) in rats treated with atomoxetine. The drug-induced changes in noradrenaline efflux in the medial prefrontal cortex were also measured. 5-CSRTT-trained rats were included in one of the following groups: N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4)/Atomoxetine, Sham/Atomoxetine, DSP-4/Saline, or Sham/Saline. Acute atomoxetine (0.3 mg/kg) was administered 14 days after the DSP-4 regime. The behavioral testing included manipulations of the inter-trial interval (ITI), stimulation duration and food satiety. In vivo microdialysis of the noradrenaline efflux in the medial prefrontal cortex and the expression of the noradrenaline transporter (NAT) in the DNAB areas were examined. Atomoxetine reduced impulsivity and perseveration in the long-ITI condition with no effects on any other variables. This phenomenon was not influenced by DSP-4 pre-treatment. The DNAB-lesioned rats had lower noradrenaline efflux in the medial prefrontal cortex. DSP-4 caused no change in NAT expression in the DNAB areas. These findings suggested that noradrenaline reuptake may not be exclusively responsible for the atomoxetine effects in adjusting impulsivity. The role of DNAB should also be considered, particularly in conditions requiring greater behavioral inhibition.

Selective loss of noradrenaline exacerbates early cognitive dysfunction and synaptic deficits in APP/PS1 mice

BACKGROUND

Degeneration of the locus coeruleus (LC), the major noradrenergic nucleus in the brain, occurs early and is ubiquitous in Alzheimer's disease (AD). Experimental lesions to the LC exacerbate AD-like neuropathology and cognitive deficits in several transgenic mouse models of AD. Because the LC contains multiple neuromodulators known to affect amyloid β toxicity and cognitive function, the specific role of noradrenaline (NA) in AD is not well understood.

METHODS

To determine the consequences of selective NA deficiency in an AD mouse model, we crossed dopamine β-hydroxylase (DBH) knockout mice with amyloid precursor protein (APP)/presenilin-1 (PS1) mice overexpressing mutant APP and PS1. Dopamine β-hydroxylase (-/-) mice are unable to synthesize NA but otherwise have normal LC neurons and co-transmitters. Spatial memory, hippocampal long-term potentiation, and synaptic protein levels were assessed.

RESULTS

The modest impairments in spatial memory and hippocampal long-term potentiation displayed by young APP/PS1 or DBH (-/-) single mutant mice were augmented in DBH (-/-)/APP/PS1 double mutant mice. Deficits were associated with reduced levels of total calcium/calmodulin-dependent protein kinase II and N-methyl-D-aspartate receptor 2A and increased N-methyl-D-aspartate receptor 2B levels and were independent of amyloid β accumulation. Spatial memory performance was partly improved by treatment with the NA precursor drug L-threo-dihydroxyphenylserine.

CONCLUSIONS

These results indicate that early LC degeneration and subsequent NA deficiency in AD may contribute to cognitive deficits via altered levels of calcium/calmodulin-dependent protein kinase II and N-methyl-D-aspartate receptors and suggest that NA supplementation could be beneficial in early AD.

Sequential Loss of LC Noradrenergic and Dopaminergic Neurons Results in a Correlation of Dopaminergic Neuronal Number to Striatal Dopamine Concentration

Noradrenergic neurons in the locus coeruleus (LC) are significantly reduced in Parkinson’s disease (PD) and the LC exhibits neuropathological changes early in the disease process. It has been suggested that a loss of LC neurons can enhance the susceptibility of dopaminergic neurons to damage. To determine if LC noradrenergic innervation protects dopaminergic neurons from damage, the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) was administered to adult male C57Bl/6 mice 3 days after bilateral LC administration of 6-hydroxydopamine (6OHDA), a time when there is a significant reduction in LC neuronal number and innervation to forebrain regions. To assess if LC loss can affect dopaminergic loss four groups of animals were studied: control, 6OHDA, MPTP, and 6OHDA + MPTP; animals sacrificed 3 weeks after MPTP administration. The number of dopaminergic neurons in the substantia nigra (SN) and ventral tegmental area (VTA), and noradrenergic neurons in the LC were determined. Catecholamine levels in striatum were measured by high-pressure liquid chromatography. The loss of LC neurons did not affect the number of dopaminergic neurons in the SN and VTA compared to control; however, LC 6OHDA significantly reduced striatal dopamine (DA; 29% reduced) but not norepinephrine (NE) concentration. MPTP significantly reduced SN and VTA neuronal number and DA concentration in the striatum compared to control; however, there was not a correlation of striatal DA concentration with SN or VTA neuronal number. Administration of 6OHDA prior to MPTP did not enhance MPTP-induced damage despite an effect of LC loss on striatal DA concentration. However, the loss of LC neurons before MPTP resulted now in a correlation between SN and VTA neuronal number to striatal DA concentration. These results demonstrate that the loss of either LC or DA neurons can affect the function of each others systems, indicating the importance of both the noradrenergic and dopaminergic system in PD.

Lesioning noradrenergic neurons of the locus coeruleus in C57Bl/6 mice with unilateral 6-hydroxydopamine injection, to assess molecular, electrophysiological and biochemical changes in noradrenergic signaling

The locus coeruleus (LC) is the major loci of noradrenergic innervation to the forebrain. Due to the extensive central nervous system innervation of the LC noradrenergic system, a reduction in the number of LC neurons could result in significant changes in noradrenergic function in many forebrain regions. LC noradrenergic neurons were lesioned in adult male C57Bl/6 mice with the unilateral administration of 6-hydroxydopamine (6OHDA) (vehicle on the alternate side). Noradrenergic markers were measured 3 weeks later to determine the consequence of LC loss in the forebrain. Direct administration of 6OHDA into the LC results in the specific reduction of noradrenergic neurons in the LC (as measured by electrophysiology, immunoreactivity and in situ hybridization), the lateral tegmental neurons and dopaminergic neurons in the substantia nigra (SN) and ventral tegmental region were unaffected. The loss of LC noradrenergic neurons did not result in compensatory changes in the expression of mRNA for norepinephrine (NE)-synthesizing enzymes. The loss of LC noradrenergic neurons is associated with reduced NE tissue concentration and NE transporter (NET) binding sites in the frontal cortex and hippocampus, as well as other forebrain regions such as the amygdala and SN. Adrenoreceptor (AR) binding sites (α(1)- and α(2)-AR) were not significantly affected on the 6OHDA-treated side compared to the vehicle-treated side, although there is a reduction of AR binding sites on both the vehicle- and 6OHDA-treated side in specific forebrain regions. These studies indicate that unilateral stereotaxic injection of 6OHDA into mice reduces noradrenergic LC neurons and reduces noradrenergic innervation to many forebrain regions, including the contralateral side.

A comprehensive analysis of the effect of DSP4 on the locus coeruleus noradrenergic system in the rat

Degeneration of the noradrenergic neurons in the locus coeruleus (LC) is a major component of Alzheimer's (AD) and Parkinson's disease (PD), but the consequence of noradrenergic neuronal loss has different effects on the surviving neurons in the two disorders. Therefore, understanding the consequence of noradrenergic neuronal loss is important in determining the role of this neurotransmitter in these neurodegenerative disorders. The goal of the study was to determine if the neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4) could be used as a model for either (or both) AD or PD. Rats were administered DSP4 and sacrificed 3 days 2 weeks and 3 months later. DSP4-treatment resulted in a rapid, though transient reduction in norepinephrine (NE) and NE transporter (NET) in many brain regions receiving variable innervation from the LC. Alpha(1)-adrenoreceptors binding site concentrations were unchanged in all brain regions at all three time points. However, an increase in alpha(2)-AR was observed in many different brain regions 2 weeks and 3 months after DSP4. These changes observed in forebrain regions occurred without a loss in LC noradrenergic neurons. Expression of synthesizing enzymes or NET did not change in amount of expression/neuron despite the reduction in NE tissue content and NET binding site concentrations at early time points, suggesting no compensatory response. In addition, DSP4 did not affect basal activity of LC at any time point in anesthetized animals, but 2 weeks after DSP4 there is a significant increase in irregular firing of noradrenergic neurons. These data indicate that DSP4 is not a selective LC noradrenergic neurotoxin, but does affect noradrenergic neuron terminals locally, as evident by the changes in transmitter and markers at terminal regions. However, since DSP4 did not result in a loss of noradrenergic neurons, it is not considered an adequate model for noradrenergic neuronal loss observed in AD and PD.

Norepinephrine depletion facilitates recovery of function after focal ischemia in the rat

Previous studies have suggested that increased norepinephrine plays an important role in recovery of function after brain injury; however, the majority of these studies used drugs that are known to also affect other monoamines to increase or decrease norepinephrine. The purpose of the present study was to determine if norepinephrine is required to promote recovery after ischemia. A form of enriched rehabilitation was used to rehabilitate animals after ischemia and the neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine was used to selectively destroy norepinephrine projections from the locus coeruleus. Three sensorimotor tests were used to evaluate the recovery of the animals. Depletion of norepinephrine improved sensorimotor recovery in standard-housed animals and did not impede recovery in the rehabilitation groups. Dopamine beta hydroxylase staining was used to confirm N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine-depleted terminal norepinephrine levels. The amount of norepinephrine terminal staining negatively correlated with recovery of function in the staircase test after ischemia. In addition, enriched rehabilitation increased, but depletion of norepinephrine had no effect on, brain-derived neurotrophic factor protein levels, which have also been linked to improved recovery of function. Together the above findings question the previously postulated role of norepinephrine in recovery of function after stroke.

N-(2-Chloroethyl)-N-ethyl-2-bromobenzylamine reduces intracellular calcium response to noradrenaline in rat visual cortex

Using the fluorescent indicator Fura-2, we investigated the effects of N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4), a noradrenergic neurotoxin, on intracellular calcium responses to noradrenaline, N-methyl-D-aspartate, and carbamylcholine chloride in brain slices of the rat visual cortex. Noradrenergic depletion in the visual cortex of young rats was induced by DSP-4, and its selectivity was confirmed by two different methods, i.e., immunostaining with anti-dopamine-beta-hydroxylase antibody and biochemical analysis by high-performance liquid chromatography. The treatment with DSP-4 (25 mg/kg i.p., x2) caused disruption of noradrenergic fibers throughout all cortical layers, and reduced the content of noradrenaline to 6.4% of that in the normal control. In the normal cortex, bath-applied noradrenaline (100 microM) increased the intracellular calcium to 123% of the control in terms of the F(340)/F(380) ratio of Fura-2 fluorescence. Quantitative analysis of the F(340)/F(380) ratio was performed in layers II to IV, since the increase was mainly observed in these layers. The intracellular calcium response to noradrenaline was significantly (P<0.0001) reduced in the DSP-4-treated animals to 63.2% of that in the normal control. The response to N-methyl-D-aspartate (100 microM) was also reduced, whereas the response to carbamylcholine chloride, a muscarinic cholinergic agonist (100 microM), was not affected by the DSP-4 treatment. From these findings we suggest that noradrenergic denervation by DSP-4 reduces the intracellular calcium response to noradrenaline through changes in the intracellular signal transduction.

Noradrenergic lesion antagonizes desipramine-induced adaptation of NMDA receptors

Repeated administration of the tricyclic antidepressant, desipramine, for 28 days to mice effected a decrease in the potency of glycine to displace [3H]5,7-dichlorokynurenic acid (5,7-DCKA) in mouse cortical homogenates. Pre-treatment with the noradrenergic neurotoxin DSP-4, while having no effect alone, attenuated the desipramine-induced effect. The present findings support a norepinephrine-dependent adaptation of the NMDA receptor complex in vivo following chronic desipramine treatment. The inter-relationship of norepinephrine and glutamate transmission may provide insight into the mechanism underlying the action of antidepressant drugs.

Chronic mild unpredictable stress after noradrenergic denervation: attenuation of behavioural and biochemical effects of DSP-4 treatment

Chronic mild unpredictable stress, which reduces rewarded behaviour in rats, is becoming increasingly popular as an animal model of depression. The effect of chronic mild stress (applied to animals housed five per cage for 15 days) on forced swimming and open field behaviour, and on beta-adrenoceptor binding was studied in naive rats and after the denervation of the locus coeruleus projections by DSP-4 (50 mg kg(-1)) treatment. In the forced swimming test, chronic mild stress reduced the immobility time on the second day of testing in both vehicle- and DSP-4-treated rats, indicating rather an antidepressant-like effect. This antidepressant-like effect of chronic mild stress in the forced swimming test was not present in individually housed rats which suggests that this paradigm is sensitive to housing conditions. Stress had no clear effect on the open field locomotion in naive animals (but caused a reduction in defecations), but completely blocked the DSP-4-induced decrease in the exploratory activity. As measured by 3H-dihydroalprenolol binding, DSP-4 treatment increased the beta-adrenoceptor affinity in the frontal cortex and the number of binding sites in the hippocampus and in the cerebral cortex (total-frontal cortex). Stress had no effect on the beta-adrenoceptor binding in the frontal cortex and cerebral cortex, but prevented the increase in affinity caused by DSP-4 treatment in the frontal cortex. In the hippocampus, chronic mild stress and DSP-4 treatment increased the number of beta-adrenoceptor binding sites. Neither chronic mild stress nor DSP-4 treatment had any effect on CCK(B) receptor binding in the cerebral cortex and striatum. These results show that chronic mild stress applied to group-housed rats can prevent the development of certain behavioural and biochemical changes caused by the denervation of the locus coeruleus projection areas.

Noradrenaline neurotoxin DSP-4 effects on sleep and brain temperature in the rat

N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) has a selective degenerative effect on noradrenergic fibers originating from locus coeruleus (LC) neurons. In the present study, we studied its effect on vigilance states and brain temperature by continuous recordings for periods of 1-5 days and 2-4 weeks following DSP-4 treatment. On the first day, paradoxical sleep duration was significantly decreased (-67%, P < 0.05), slow-wave sleep (SWS) duration increased (+16%, P < 0.05) up to 48 h after DSP-4 treatment (+8%, P < 0.05) and the wake period decreased (-8%, P < 0.05). The vigilance states returned to control values 4-5 days later. The brain temperature was decreased during the first night (-2 degrees C) and then recovered the control values. Two and 4 weeks after DSP-4 treatment, paradoxical sleep was still decreased (-18% and -23%, respectively, P < 0.05), while SWS was significantly increased only at night during the fourth week (+23%, P < 0.05). These results therefore provide evidence for a differential involvement of the noradrenergic LC system in sleep mechanisms depending on the light-dark cycle. Different hypotheses are proposed.

Role of the locus coeruleus in the sleep rebound following two different sleep deprivation methods in the rat

The aim of the present study was to assess the involvement of the locus coeruleus in the paradoxical sleep rebound following sleep deprivation in the rat. Animals were sleep-deprived for 10 h before, and after, specific N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) lesioning of the noradrenergic-locus coeruleus system. Sleep deprivation was produced using either an instrumental (water tank) or pharmacological (methylamphetamine) method. After lesioning, the rats submitted to the instrumental method showed a significant decrease in the paradoxical and slow-wave sleep rebounds (-54% and -78%, respectively), while animals receiving metamphetamine did not. Our results suggest that the noradrenergic system of the locus coeruleus is a relevant component of the sleep rebound mechanisms. However, the extent of involvement is dependent on the sleep deprivation method used.

Acute and chronic noradrenergic regulation of neurotrophin messenger RNA expression in rat hippocampus: evidence from lesions and organotypic cultures

Noradrenergic neurons from the locus coeruleus innervate several brain regions, such as hippocampus and cortex. The hippocampus exhibits the highest concentration of the neurotrophins nerve growth factor, brain-derived neurotrophic factor and neurotrophin-3 in the brain. To study the role of the noradrenergic system in the chronic regulation of neurotrophin messenger RNA expression, chemical [N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine, 6-hydroxydopamine] and mechanical (knife-cut axotomy) lesions were performed, in the rat, and neurotrophin messenger RNAs analysed after 14 and 35 days. The intensity of the lesion was verified by characterization of the noradrenergic system using immunohistochemistry and in situ hybridization for dopamine-beta-hydroxylase and the measurement of noradrenaline tissue levels. To study the acute regulation, hippocampal organotypic slice cultures were prepared and neurotrophin messenger RNAs analysed after incubation in different concentrations of noradrenaline. We report that the noradrenergic N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine depletion significantly increased nerve growth factor and brain-derived neurotrophic factor messenger RNAs but not neurotrophin-3 messenger RNA in hippocampal areas 35 days after the lesion, while the knife-cut axotomy had a less pronounced effect and the 6-hydroxydopamine lesion did not change the neurotrophins. When incubating the organotypic hippocampal cultures with different concentrations of noradrenaline, nerve growth factor and brain-derived neurotrophic factor messenger RNAs but not neurotrophin-3 messenger RNA were significantly reduced in the dentate gyrus. We conclude that nerve growth factor and brain-derived neurotrophic factor but not neurotrophin-3 expression are inhibited by noradrenaline, arising from the locus coeruleus.

Sleep increase after immobilization stress: role of the noradrenergic locus coeruleus system in the rat

In a preliminary study we showed that the sleep rebound occurring after sleep deprivation is decreased in rats treated with N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4), a neurotoxic agent specific for the noradrenergic cells of the locus coeruleus (LC). Sleep deprivation methods not only involve sleep loss, but also stress, which per se may induce an increase in sleep duration. Extensive research showed that the locus coeruleus is involved in stress. To evaluate the participation of LC in this mechanism, the effect of DSP-4 treatment was studied on sleep duration following a short intense stress in the absence of sleep loss. The results showed that the augmentation of sleep after 1 h of immobilization stress is lower in DSP-4-treated rats (slow-wave sleep duration, -24%; paradoxical sleep duration, -52%). These findings suggest that the increase in sleep induced by such a stressor is mediated, at least in part, by the noradrenergic LC.

Laminar distribution and sources of catecholaminergic input to the optic tectum of the pigeon (Columbia livia)

A combined immunohistochemical and retrograde tracing approach was used to characterize the catecholaminergic innervation of the optic tectum (TeO), the major target of retinal projections in many avian species. Giemsa counterstaining was employed to determine precisely the laminar localization of immunoreactive fibers and presumptive terminals. The TeO of the pigeon is densely innervated by fibers immunoreactive for tyrosine hydroxylase (TH), which are most heavily distributed to the superficial layers of its dorsal and anterior portions. Within the dorsal-anterior tectum, TH-immunoreactive processes are particularly dense in retinorecipient layers 4 and 7 and in layer 5a. As in the mammalian superior colliculus, the bulk of the catecholaminergic innervation of the pigeon TeO reflects inputs, presumably noradrenergic, originating in the locus coeruleus and nucleus subcoeruleus. However, the catecholaminergic innervation of the pigeon TeO shows several features distinct from those reported for the mammalian superior colliculus. These include an input from a pretectal TH-positive cell group unknown in mammals and the presence of residual TH immunoreactivity after administration of the noradrenergic neurotoxin DSP-4. Moreover, the pattern of TH-immunoreactive fibers in pigeon TeO indicates more laminar and regional specialization within this structure than has been reported for the catecholaminergic innervation of the superior colliculus in mammals.