Serotonin content is elevated in the dopamine deficient striatum of the weaver mutant mouse

What Gene Mutations Affect Serotonin in Mice?

Although serotonin neurotransmission has been implicated in several neurodevelopmental and psychological disorders, the factors that drive dysfunction of the serotonin system are poorly understood. Current research regarding the serotonin system revolves around its dysfunction in neuropsychiatric disorders, but there is no database collating genetic mutations that result in serotonin abnormalities. To bridge this gap, we developed a list of genes in mice that, when perturbed, result in altered levels of serotonin either in brain or blood. Due to the intrinsic limitations of search, the current list should be considered a preliminary subset of all relevant cases. Nevertheless, it offered an opportunity to gain insight into what types of genes have the potential to impact serotonin by using gene ontology (GO). This analysis found that genes associated with monoamine metabolism were more often associated with increases in brain serotonin than decreases. Speculatively, this could be because several pathways (and therefore many genes) are responsible for the clearance and metabolism of serotonin whereas only one pathway (and therefore fewer genes) is directly involved in the synthesis of serotonin. Another contributor could be cross talk between monoamine systems such as dopamine. In contrast, genes that were associated with decreases in brain serotonin were more likely linked to a developmental process. Sensitivity of serotonin neurons to developmental perturbations could be due to their complicated neuroanatomy or possibly they may be negatively regulated by dysfunction of their innervation targets. Thus, these observations suggest hypotheses regarding the mechanisms underlying the vulnerability of brain serotonin neurotransmission.

Deficits in the mid-brain raphe nuclei and striatum of the AS/AGU rat, a protein kinase C-γ mutant

The AS/AGU rat carries a recessive mutation (agu) in the gene coding for the gamma isoform of protein kinase C. The rat is characterized by disordered locomotion and progressive dysfunction of the nigrostriatal dopaminergic (DA) system. This dysfunction begins with a failure to release DA within the striatum and culminates in cell loss within the substantia nigra pars compacta. The present study examines another midbrain aminergic system with input to the basal ganglia, the serotonergic (5-HT) raphe-striatal system originating in the dorsal raphe nucleus. By 3 months after birth, there is a very substantial reduction in the extracellular levels of 5-HT in the dorsal caudate-putamen of the mutants compared with controls (c. 70%). This is accompanied by a proportional increase in the levels of the 5-HT metabolite 5-hydroxyindole acetic acid (5-HIAA). At a later age, there are reductions in whole tissue 5-HT (and increases in 5-HIAA) in both the striatum and the region containing the dorsal raphe nucleus, as well as numbers of 5-HT-immunoreactive cells in the dorsal raphe nucleus. The median raphe appears to be unaffected. The results are seen in terms of an initial dysfunction in transmitter release leading to cell death, perhaps through the formation of free radicals or neurotoxins.

Serotonin fibre densities in subcortical areas: differential effects of isolated rearing and methamphetamine

Serotoninergic neurons interact with dopaminergic cells on all levels and are physiologically affected by both isolated rearing (IR) and a single early methamphetamine (MA) injection. We therefore checked for anatomical effects of both interventions by immunohistochemically staining serotonin fibres and assessing fibre densities in the caudate-putamen (CPu), nucleus accumbens (NAc) and amygdala of Mongolian gerbils. IR led to significantly increased 5-HT fibre densities in the dorsal part of the CPu and in the central and basolateral amygdala. No effects were seen in the ventral CPu, in the NAc and in the lateral amygdala. The early MA injection resulted in a denser 5-HT innervation in the dorsomedial and ventromedial CPu, in the NAc shell of animals reared in an enriched environment and in the NAc core of both rearing conditions, leaving the lateral CPu and the amygdala unaffected. Thus, the single pharmacological versus the environmental challenge exerts an almost complementary effect on the 5-HT innervation in different areas of the brain, which demonstrates that systemic interactions, e.g. with dopaminergic and glutamatergic afferents, must be taken into account when the seemingly uniform 5-HT projections are investigated.

Resistance to striatal dopamine depletion induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in mice expressing human mutant Cu,Zn superoxide dismutase

Recent data indicate that overexpression of the enzyme Cu,Zn superoxide dismutase (SOD1) in mice confers neuroprotection against various dopamine neurotoxins like 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), methamphetamine, 6-hydroxydopamine and methylenedioxymethamphetamine. In the present study we investigated whether a mutant form of SOD1 (G93A), occurring in humans affected by amyotrophic lateral sclerosis, leads to a differential vulnerability of nigrostriatal dopaminergic neurons to the chronic dopamine depletion induced by the selective neurotoxin MPTP. Our results indicate that overexpression of both wild-type and human mutant SOD1 induces comparable neuroprotective effects against striatal dopaminergic depletion.

Topology of ionotropic glutamate receptors in brains of heterozygous and homozygous weaver mutant mice

In weaver mice, mutation of a G-protein inwardly rectifying K(+) channel leads to a cerebellar developmental anomaly characterized by granule and Purkinje cell loss and, in addition, degeneration of dopaminergic neurons. To evaluate other deficits, ionotropic glutamate receptors sensitive to N-methyl-D-aspartate (NMDA), amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), and kainic acid (KA) were examined by autoradiography with [(3)H]MK-801, [(3)H]AMPA, and [(3)H]KA. These surveys were carried out in selected areas of cerebral cortex, hippocampus and related limbic regions, basal ganglia, thalamus, hypothalamus, brainstem, and cerebellum from heterozygous (wv/+) and homozygous (wv/wv) weaver mutants, and compared to wild-type (+/+) mice. In wv/+ and wv/wv mutants, NMDA receptor levels were lower in cortical areas, septum, hippocampus, subiculum, neostriatum, nucleus accumbens, superior colliculus, and in the cerebellar granular layer. Densities of KA receptors were lower in cortical areas, hippocampus, limbic system structures, neostriatum, nucleus accumbens, thalamus and hypothalamus, superior and inferior colliculi, and cerebellar cortex of wv/wv mutants. Levels of AMPA receptors in the weaver were higher than in +/+ mice, particularly in somatosensory and piriform cortices and periaqueductal gray of wv/+, and in somatosensory cortex, CA1 field of Ammon's horn and cerebellar granular layer of wv/wv. Abnormal developmental signals, aberrant cellular responses, or a distorted balance between neurotransmitter interactions may underlie such widespread and reciprocal glutamate receptor alterations, while in the case of cerebellar cortex, NMDA receptors are lacking due to a massive disappearance of cerebellar granule cells and some loss of Purkinje neurons.

Distribution of serotonin, its metabolites and 5-HT transporters in the neostriatum of Lurcher and weaver mutant mice

Serotonin (5-HT) uptake sites, or transporters, were measured in the neostriatum (caudate putamen) of wild type (+/+) mice and heterozygous (wv/+) and homozygous (wv/wv) weaver, as well as in heterozygous Lurcher (Lc/+) mutants. These topological surveys were carried out by quantitative ligand binding autoradiography using the uptake site antagonist [3H]-citalopram as a probe of innervation densities in four quadrants of the rostral neostriatum and in two halves of the caudal neostriatum. In addition, tissue concentrations of 5-HT, 5-hydroxyindole-3-acetic acid and 5-hydroxytryptophol were measured by high-performance liquid chromatography with electrochemical detection in these neostriatal divisions. In +/+ mice and in Lc/+ mutants there was a dorso-ventral gradient of increasing 5-HT levels, and they exhibited a similar heterogeneity of [3H]-citalopram labeling. In contrast, the gradients of 5-HT concentrations and [3H]-citalopram binding disappeared in the weaver mutants, suggesting a rearrangement of the 5-HT innervation. This reorganization of the 5-HT system in the neostriatum was more obvious in the wv/wv and is compatible with the hypothesis that the postnatal dopaminergic deficiencies that characterize weaver mutants lead to a sprouting of fibers and thus constitute a genetic model of dopaminergic denervation that leads to a 5-HT hyperinnervation.

Alterations in serotonin receptors in the neostriatum of weaver mutant mice

Mice that carry the autosomal recessive gene weaver show a distinctive loss of nigrostriatal dopamine innervation, with the greatest deficits in the dorsal caudate-putamen and almost complete sparing in the nucleus accumbens and ventral caudate. In addition to loss of dopamine in this model, it has recently been shown that markers of serotonin (5-hydroxytryptamine, 5-HT) innervation including 5-HT content, synaptosomal uptake of [3H]5-HT and [3H]citalopram binding were elevated in the dorsal neostriatum of the weaver mutant mouse. Using quantitative autoradiography of specific ligands for dopamine and 5-HT uptake sites as well as serotonin 5-HT1 and 5-HT2A receptors, we found an increased density of 5-HT uptake sites and 5-HT1 receptors restricted to the dorsal portion of the neostriatum of the weaver mouse. In contrast, 5-HT2A receptors were increased in both the dorsal and ventral portions of the rostral neostriatum as well as the nucleus accumbens. The behavioural and functional relevance of these receptor changes is unclear, although, adaptations in 5-HT may play a role in certain aspects of spontaneous behaviour in the weaver mutant mouse.

I. Serotonin (5-HT) within dopamine reward circuits signals open-field behavior. II. Basis for 5-HT--DA interaction in cocaine dysfunctional behavior

Light microscopic immunocytochemical studies, using a sensitive silver intensification procedure, show that dopamine (DA) and serotonin (5-HT) axons terminate on neurons in the nucleus accumbens (NAcc) (A10) terminals and also in dorsal striatum (DSTr) (A9) terminals. The data demonstrate a prominent endogenous anatomic interaction at these distal presynaptic sites between the neurotransmitters 5-HT and DA; the pattern of the 5-HT-DA interaction differs between A10 and A9 terminals. Moreover, in distinction to the variance shown anatomically between 5-HT--DA interactions at distal A9 and A10 sites, the 5-HT--DA interactions at the level of DA somatodendrites, the proximal site, are similar, i.e. 5-HT terminals in the midbrain tegmentum are profuse and have a massive overlap with DA neurons in both ventral tegmental area (VTA) and substantia nigra pars compacta (SNpc). We suggest with reference to the DA neurons of A10 and A9 pathways, inclusive of somatodendrites (sites of proximal presynaptic interactions in the midbrain) and axons (sites of distal presynaptic interactions), that 5-HT--DA interactions in A10 terminals are more likely to exceed those in the DStr arrangement. Furthermore, our neuroanatomic data show that axonally released DA at A10 terminals may originate from proximal 5-HT somatodendrites, i.e. dorsal raphe (DR) or the proximal DA somatodendrites, VTA. In vivo microvoltammetric studies were done with highly sensitive temporal and spatial resolution; the studies demonstrate basal (endogenous) real time 5-HT release at distal A10 and distal A9 terminal fields and real time 5-HT release at proximal A10 VTA somatodendrites. In vivo microvoltammetric studies were performed concurrently and on line with studies of DA release, also at distal A10 and distal A9 terminal fields and at proximal A10 somatodendrites. Serotonin release was detected in a separate voltammetric peak from the DA voltammetric peak. The electrochemical signal for 5-HT release was detected within 10-12 s and that for DA release within 12-15 s, after each biogenic amine diffused through the synaptic environment onto the microelectrode surface. The electrochemical signal for 5-HT and a separate electrochemical signal for DA are detected on the same voltammogram within 22-27 s; each electrochemical signal represents current changes in picoamperes, within seconds of detection time. The amplitude of each electrochemical signal reflects the changes in diffusion of each biogenic amine to the microelectrode surface. Each neurotransmitter has a distinct potential at which oxidation occurs; this results in a recording which has a distinct peak for a specific neurotransmitter. The concentration of each neurotransmitter within the synaptic environment is directly related to the electrochemical signal detected via the Cottrell equation. Voltammograms were recorded every 5 min. At the time that basal 5-HT release and basal DA release were recorded within same animal control, open-field behavioral studies were performed, also concurrently, by infrared photocell beams. The frequency of each behavioral parameter was monitored every 100 ms; the number of behavioral events, were summated every 5 min during the time course of study. Thus, the detection of neurotransmitters occurs in real time, while simultaneously monitoring the animal's behavior by infrared photocell beams. The results from the in vivo microvoltammetric and behavioral data from this study show that basal 5-HT release at distal A10 and A9 terminals dramatically increased with DA release. Moreover, each increase in basal 5-HT release, at both A10 and at A9 terminal fields occurred consistently and at the same time as each increase in open-field locomotion and stereotypy occurred naturally during the animal's exploration in a novel chamber. Thus, the terminology 'synchronous and simultaneous' describes aptly the correlation between 5-HT release at distal A10 and A9 terminal fields and open-field locomo

Morphological and biochemical adaptations to unilateral dopamine denervation of the neostriatum in newborn rats

Basal ganglia of adult rats were examined for morphological and biochemical changes resulting from neonatal unilateral dopamine denervation of the striatum with increasing doses of 6-hydroxydopamine (4, 12 and 20 microg). Rotational behaviour induced by apomorphine (0.1 mg/kg) was observed in all rats injected with the high dose (20 microg) and totally absent in those injected with the low dose (4 microg). As assessed with tyrosine hydroxylase immunocytochemistry, the extent of dopamine denervation within the injected striatum was clearly related to the dose injected. In the mesencephalon, losses of tyrosine hydroxylase-immunoreactive cell bodies were proportional to the dose injected and the extent of neostriatal dopamine denervation. This retrograde cell loss predominated in the ventromedial and lateral parts of the substantia nigra pars compacta, with relative sparing of the ventral tegmental area. After the injection of the intermediate (12 microg) and the high (20 microg) doses, a network of thin tyrosine hydroxylase-immunoreactive fibres was visualized in the ventral part of the pars reticulata ipsilateral to the injected striatum, suggesting a neoinnervation of this structure by dopamine axons. After the high dose, the density of serotonin-immunoreactive fibres was enhanced in the anterior half of the lesioned striatum. Associated changes in dopamine and serotonin content and turnover were also documented on both sides, in the striatum and in two output structures of the basal ganglia, the globus pallidus and the substantia nigra. Dopamine content was decreased only on the injected side. After the low dose, equal reductions (-60%) were observed in the anterior striatum and the substantia nigra, whereas a more marked decrease was measured in the anterior striatum (-93%) than in the substantia nigra (-60% to -74%) after the intermediate and high doses. In the globus pallidus, dopamine tissue content was decreased (-51%) only after the high dose. Dopamine turnover was unchanged after the low dose in all structures examined and was increased in the striatum, on the lesioned side only, after the intermediate and high doses. Serotonin content was increased only on the injected side in the anterior striatum (+50% after the low and +92% after the high dose). Serotonin turnover was unchanged on the injected side but increased by +118% and by +81% in the contralateral anterior striatum after the low and high doses, respectively. It was also increased in both substantia nigra after the high dose. In conclusion, morphological changes similar to those described after a bilateral neonatal lesion were observed on the injected side in the model of the unilateral neonatal nigrostriatal dopamine denervation. Biochemical changes were, however, not restricted to the lesioned side. Notably, changes in serotonin turnover developed on the contralateral side. These morphological and biochemical adaptative changes need to be taken into account in considering the mechanisms implicated in the rotional behaviour measured in these animals.

Endogenous serotonin release from the dopamine-deficient striatum of the weaver mutant mouse

In addition to an altered dopaminergic input, the striatum of the weaver mutant mouse (wv/wv) has increased serotonin tissue content and uptake compared to the wild-type mouse (+/+). To gain information regarding the functional status of serotonergic inputs to the wv/wv striatum, endogenous serotonin release from wv/wv and +/+ striatum was measured under basal conditions as well as in the presence of fenfluramine or elevated concentrations of potassium (K+). Fractional basal release of serotonin from the +/+ striatum was significantly greater than that from the wv/wv striatum. In the presence of K+, evoked release (stimulated release minus basal release) was greater from the +/+ striatum than from the wv/wv striatum. In the presence of fenfluramine, evoked serotonin release was greater from the wv/wv striatum compared to the +/+ striatum. These data are consistent with the involvement of an additional transmitter(s) in modulating serotonin release to a greater extent in the wv/wv than the +/+ striatum. The data on fenfluramine-stimulated serotonin release suggest that the additional serotonin content found in the wv/wv striatum is in a releasable pool but that striatal serotonin release might be attenuated more in wv/wv than in +/+ mice.

The weaver mutant mouse as a model of nigrostriatal dysfunction

The weaver mutant mouse has a genetic defect that results in the loss of dopamine neurons in the nigrostriatal pathway. Striatal tyrosine hydroxylase and dopamine content are reduced by 60-70%, and dopamine uptake is reduced by as much as 95%. Deficits in all three of these striatal dopamine markers are seen as early as postnatal d 3. The striatal dopamine systems in the weaver apparently have the ability to compensate for this dopamine deficit. Thus, in the weaver, in vitro resting release, as well as amphetamine-evoked fractional release of endogenous dopamine are increased. An additional change seen in the weaver striatum is an elevated serotonin content. These alterations may play an adaptive role in attempting to compensate for the dopamine loss. In summary, the weaver mutant mouse has dramatic deficits in the nigrostriatal pathway, but also seems to develop certain adaptive mechanisms in dopaminergic and other transmitter systems that may compensate functionally for the dopamine deficit. Thus, the weaver mouse provides a unique animal model for studying naturally induced neuronal degeneration that complements those models using surgical and pharmacological protocols.

Sparing of the dopaminergic neurons containing calbindin-D28k and of the dopaminergic mesocortical projections in weaver mutant mice

In mice carrying the weaver mutation there is a spontaneous degeneration of dopaminergic neurons that is heterogeneous among cell groups: nigrostriatal neurons are more affected than mesolimbic neurons, while involvement of the mesocortical system is controversial. We questioned whether the pattern of cell loss in mesencephalon and fiber depletion in telencephalon could be related to the differential content of Calbindin-D28k in dopaminergic cells. The mesencephalon of seven-month-old mutants was serially sectioned and alternate series were immunostained with tyrosine hydroxylase and Calbindin-D28k. Cell counts indicated a 40% loss for the ensemble of dopamine mesencephalic neurons. However, double-immunostained preparations revealed that this cell loss was restricted to the neurons that lacked Calbindin-D28k, which were reduced by 72%, while the dopaminergic neurons containing Calbindin-D28k were completely spared. Calbindin-D28k was present in both the cytoplasm and nucleus of the dopaminergic cells. This nuclear localization was confirmed at the ultrastructural level. In the telencephalon of weaver mutants, areas receiving projections from the Calbindin-D28k-positive dopaminergic neurons, such as the cerebral cortex, contained normal densities of fibers, while areas harboring projections from the non-Calbindin-D28k dopaminergic neurons, such as the dorsal striatum, had reduced amounts of fibers. The vulnerability pattern in the mesencephalon of weaver mutants bears similarities to that described in idiopathic Parkinson's disease or in N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced Parkinsonism: Calbindin-D28k may thus delimit a group of dopaminergic neurons resistant to cell death in different conditions. On the other hand, the vulnerability pattern of dopaminergic fibers in weaver differs from that of Parkinson's disease, since there is a complete sparing of the dopaminergic mesocortical projection in weaver, contrasting with the damage of these projections in Parkinson's disease.

Alterations in dopamine and serotonin uptake systems in the striatum of the weaver mutant mouse

In the striatum of the homozygous weaver mutant mouse (wv/wv), dopamine content, uptake and tyrosine hydroxylase activity are decreased compared to wild-type (+/+) mice. In mice heterozygous for the weaver gene (wv/+), these dopaminergic parameters exhibit only minor reductions compared to +/+ mice. The wv/wv striatum has recently been shown to have an increase in serotonin content. In the present study, the serotonin uptake system of the weaver striatum was investigated. Synaptosomal uptake of [3H] serotonin was determined in the dorsal portion of wv/wv and +/+ striatum, and serotonin uptake sites were examined by the binding of [3H] citalopram in the striatum of wv/wv, wv/+ and +/+ mice. The dopamine uptake system was also investigated in all three genotypes via the binding of [3H] mazindol. Synaptosomal uptake of [3H] serotonin was increased by 79% in the dorsal portion of the wv/wv striatum compared to that seen in the +/+ striatum. The binding of [3H] citalopram was increased by 62% in the dorsolateral and by 111% in the dorsomedial portions of the wv/wv striatum compared to +/+. [3H] Citalopram binding in the wv/+ striatum was also higher than +/+, but this increase did not reach statistical significance. Within the wv/wv striatum, [3H] mazindol binding was almost completely absent (88-89% reduction) in the dorsal portion and severely reduced in the other striatal areas. These data support the notion that the dorsal portion of the wv/wv striatum, which has the severest reduction in dopamine uptake, is hyperinnervated by serotonin fibers.

In vivo electrochemical studies of gradient effects of (SC) cocaine on dopamine and serotonin release in dorsal striatum of conscious rats

Cocaine (20 mg/kg) was administered subcutaneously (SC) to conscious male Sprague-Dawley rats after exploration in a novel chamber. (SC) cocaine was studied for its influence on in vivo dopamine (DA) and serotonin (5-HT) release in dorsal striatum (STr), with a further study of an anterior-posterior dorsal subdivision in a range of +/- 400 microns. Semiderivative voltammetry, a circuit for in vivo electrochemical biotechnologies, was used in combination with a stearate microelectrode to concurrently detect in separate electrochemical signals the electroactive species for DA and 5-HT in dorsal STr. The temporal resolution for detection was in the order of seconds. Concomitantly, cocaine-induced psychostimulant behaviors were studied with infrared photo beam detection. Psychostimulant behaviors classically thought to depend on DA--that is, hyperactivity (increased locomotor activity or ambulations), rearing, and finally stereotypy (fine movements of grooming and head bob)--and a 5-HT-ergic behavior, central ambulations, were monitored. The results showed that (SC) cocaine significantly (p < 0.0001) increased DA release in dorsal STr, whereas the overall effect of (SC) cocaine on 5-HT release was a significant increase (p < 0.0001) followed by an overall small (13%) but statistically significant decrease (p < 0.05). A dramatic cocaine-induced gradient effect on 5-HT release was seen in anterior-posterior dorsal STr, where 5-HT release was significantly (p < 0.0001) increased throughout the entire time period of study. Classically DA-dependent behaviors were significantly and positively correlated with increased DA release in dorsal STr and anterior-posterior dorsal STr (p < 0.001) in the 4-h period of study. However, 5-HT release after cocaine in the anterior-posterior dorsal STr was significantly and positively correlated with the classically DA-dependent behaviors as well (p < 0.001), implicating a role for 5-HT in the effectuation of cocaine-induced psychostimulant behavior. Generally, the 5-HT-ergic response to cocaine was enhanced before the DA-ergic response. Therefore, the data show that 5-HT as well as DA plays a role in the underlying mechanism of action of cocaine in dorsal STr. The data suggest that 5-HT may play a compensatory or adaptive role in the modulation of cocaine-induced nigrostriatal DA-ergic regulation.