Synergistic interaction between serotonin-2 receptor and dopamine D1 receptor stimulation on striatal preprotachykinin mRNA expression in the 6-hydroxydopamine lesioned rat

Serotonergic targets for the treatment of L-DOPA-induced dyskinesia

Dopamine (DA) replacement therapy with L-3,4-dihydroxyphenylalanine (L-DOPA) continues to be the gold-standard treatment for Parkinson's disease (PD). Despite clear symptomatic benefit, long-term L-DOPA use often results in the development of L-DOPA-induced dyskinesia (LID), significantly reducing quality of life and increasing costs for PD patients and their caregivers. Accumulated research has demonstrated that several pre- and post-synaptic mechanisms contribute to LID development and expression. In particular, raphe-striatal hyperinnervation and unregulated DA release from 5-HT terminals is postulated to play a central role in LID manifestation. As such, manipulation of the 5-HT system has garnered considerable attention. Both pre-clinical and clinical research has supported the potential of modulating the 5-HT system for LID prevention and treatment. This review discusses the rationale for continued investigation of several potential anti-dyskinetic strategies including 5-HT stimulation of 5-HT1A and 5-HT1B receptors and blockade of 5-HT2A receptors and SERT. We present the latest findings from experimental and clinical investigations evaluating these 5-HT targets with the goal of identifying those with translational promise and the challenges associated with each.

Deranged NMDAergic cortico-subthalamic transmission underlies parkinsonian motor deficits

Parkinson's disease (PD) is the most prevalent hypokinetic movement disorder, and symptomatic PD pathogenesis has been ascribed to imbalances between the direct and indirect pathways in the basal ganglia circuitry. Here, we applied glutamate receptor blockers to the subthalamic nucleus (STN) of parkinsonian rats and evaluated locomotor behaviors via single-unit and local-field recordings. Using this model, we found that inhibition of NMDAergic cortico-subthalamic transmission ameliorates parkinsonian motor deficits without eliciting any vivid turning behavior and abolishes electrophysiological abnormalities, including excessive subthalamic bursts, cortico-subthalamic synchronization, and in situ beta synchronization in both the motor cortex and STN. Premotor cortex stimulation revealed that cortico-subthalamic transmission is deranged in PD and directly responsible for the excessive stimulation-dependent bursts and time-locked spikes in the STN, explaining the genesis of PD-associated pathological bursts and synchronization, respectively. Moreover, application of a dopaminergic agent via a microinfusion cannula localized the therapeutic effect to the STN, without correcting striatal dopamine deficiency. Finally, optogenetic overactivation and synchronization of cortico-subthalamic transmission alone sufficiently and instantaneously induced parkinsonian-associated locomotor dysfunction in normal mice. In addition to the classic theory emphasizing the direct-indirect pathways, our data suggest that deranged cortico-subthalamic transmission via the NMDA receptor also plays a central role in the pathophysiology of parkinsonian motor deficits.

The 5-HT(2A) Receptor Antagonist M100907 Produces Antiparkinsonian Effects and Decreases Striatal Glutamate

5-HT plays a regulatory role in voluntary movements of the basal ganglia and has a major impact on disorders of the basal ganglia such as Parkinson's disease (PD). Clinical studies have suggested that 5-HT(2) receptor antagonists may be useful in the treatment of the motor symptoms of PD. We hypothesized that 5-HT(2A) receptor antagonists may restore motor function by regulating glutamatergic activity in the striatum. Mice treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) exhibited decreased performance on the beam-walking apparatus. Peripheral administration of the 5-HT(2A) receptor antagonist M100907 improved performance of MPTP-treated mice on the beam-walking apparatus. In vivo microdialysis revealed an increase in striatal extracellular glutamate in MPTP-treated mice and local perfusion of M100907 into the dorsal striatum significantly decreased extracellular glutamate levels in saline and MPTP-treated mice. Our studies suggest that blockade of 5-HT(2A) receptors may represent a novel therapeutic target for the motor symptoms of PD.

Brain 5-HT(2A) receptors in MPTP monkeys and levodopa-induced dyskinesias

Levodopa-induced dyskinesias (LIDs) are abnormal involuntary movements induced by the chronic use of levodopa (l-Dopa) limiting the quality of life of Parkinson's disease (PD) patients. We evaluated changes of the serotonin 5-HT(2A) receptors in control monkeys, in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned monkeys and in l-Dopa-treated MPTP monkeys, without or with adjunct treatments to inhibit the expression of LID: CI-1041, a selective NR1A/2B subunit antagonist of glutamate N-methyl-d-aspartic acid (NMDA) receptor, or Cabergoline, a long-acting dopamine D(2) receptor agonist. All treatments were administered for 1 month and animals were killed 24 h after the last dose of l-Dopa. Striatal concentrations of serotonin were decreased in all MPTP monkeys investigated, as measured by high-performance liquid chromatography. [(3) H]Ketanserin-specific binding to 5-HT(2A) receptors was measured by autoradiography. l-Dopa treatment that induced dyskinesias increased 5-HT(2A) receptor-specific binding in the caudate nucleus and the anterior cingulate gyrus (AcgG) compared with control monkeys. Moreover, [(3) H]Ketanserin-specific binding was increased in the dorsomedial caudate nucleus in l-Dopa-treated MPTP monkeys compared with saline-treated MPTP monkeys. Nondyskinetic monkeys treated with CI-1041 or Cabergoline showed low 5-HT(2A) -specific binding in the posterior dorsomedial caudate nucleus and the anterior AcgG compared with dyskinetic monkeys. No significant difference in 5-HT(2A) receptor binding was observed in any brain regions examined in saline-treated MPTP monkeys compared with control monkeys. These results confirm the involvement of serotonergic pathways and the glutamate/serotonin interactions in LID. They also support targeting 5-HT(2A) receptors as a potential treatment for LID.

5-HT2A receptor antagonists improve motor impairments in the MPTP mouse model of Parkinson's disease

Clinical observations have suggested that ritanserin, a 5-HT(2A/C) receptor antagonist may reduce motor deficits in persons with Parkinson's Disease (PD). To better understand the potential antiparkinsonian actions of ritanserin, we compared the effects of ritanserin with the selective 5-HT(2A) receptor antagonist M100907 and the selective 5-HT(2C) receptor antagonist SB 206553 on motor impairments in mice treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). MPTP-treated mice exhibited decreased performance on the beam-walking apparatus. These motor deficits were reversed by acute treatment with L-3,4-dihydroxyphenylalanine (levodopa). Both the mixed 5-HT(2A/C) antagonist ritanserin and the selective 5-HT(2A) antagonist M100907 improved motor performance on the beam-walking apparatus. In contrast, SB 206553 was ineffective in improving the motor deficits in MPTP-treated mice. These data suggest that 5-HT(2A) receptor antagonists may represent a novel approach to ameliorate motor symptoms of Parkinson's disease.

Amphetamine and mCPP effects on dopamine and serotonin striatal in vivo microdialysates in an animal model of hyperactivity

In the neonatally 6-hydroxydopamine (6-OHDA)-lesioned rat hyperlocomotor activity, first described in the 1970s, was subsequently found to be increased by an additional lesion with 5,7-dihydroxytryptamine (5,7-DHT) (i.c.v.) in adulthood. The latter animal model (i.e., 134 microg 6-OHDA at 3 d postbirth plus 71 microg 5,7-DHT at 10 weeks; desipramine pretreatments) was used in this study, in an attempt to attribute hyperlocomotor attenuation by D,L-amphetamine sulfate (AMPH) and m-chlorophenylpiperazine di HCl (mCPP), to specific changes in extraneuronal (i.e., in vivo microdialysate) levels of dopamine (DA) and/or serotonin (5-HT). Despite the 98-99% reduction in striatal tissue content of DA, the baseline striatal microdialysate level of DA was reduced by 50% or less at 14 weeks, versus the intact control group. When challenged with AMPH (0.5 mg/kg), the microdialysate level of DA went either unchanged or was slightly reduced over the next 180 min (i.e., 20 min sampling), while in the vehicle group and 5,7-DHT (alone) lesioned group, the microdialysate level was maximally elevated by approximately 225% and approximately 450%, respectively--and over a span of nearly 2 h. Acute challenge with mCPP (1 mg/kg salt form) had little effect on microdialysate levels of DA, DOPAC and 5-HT. Moreover, there was no consistent change in the microdialysate levels of DA, DOPAC, and 5-HT between intact, 5-HT-lesioned rats, and DA-lesioned rats which might reasonably account for an attenuation of hyperlocomotor activity. These findings indicate that there are other important neurochemical changes produced by AMPH- and mCPP-attenuated hyperlocomotor activity, or perhaps a different brain region or multiple brain regional effects are involved in AMPH and mCPP behavioral actions.

Serotonin 2A receptor antagonist treatment reduces dopamine D1 receptor-mediated rotational behavior but not L-DOPA-induced abnormal involuntary movements in the unilateral dopamine-depleted rat

Previous experiments have demonstrated that serotonin (5-HT) 2A receptor antagonists suppress hyperkinetic behaviors associated with dopamine (DA) D1 receptor supersensitivity in rats with 6-hydroxydopamine (6-OHDA) lesions. Since l-DOPA induced dyskinesia (LID) may be mediated by over-sensitive D1-mediated signaling, the present study examined the effects of the selective 5-HT2A antagonist M100907 on LID behaviors in DA-depleted rats. Adult male Sprague-Dawley rats with unilateral 6-OHDA lesions received daily l-DOPA treatments to produce dyskinetic behaviors as measured by abnormal involuntary movements (AIMs) testing. In these animals, M100907 (0.01, 0.1 or 1.0mg/kg, ip) given 30 min before l-DOPA did not alter the appearance or intensity of AIMs behaviors. Because l-DOPA induced AIMs in rats are dependent upon D1 and D2 receptor activation, a second study was performed to determine if M100907 could suppress D1- or D2-mediated rotational behaviors. Contralateral rotations induced by the D1 agonist SKF82958 were significantly reduced by pre-treatment with M100907. However, M100907 was ineffective in reducing rotations induced by the D2 agonist quinpirole. The finding that M100907 suppresses rotations induced by D1, but not D2, agonists may provide a partial explanation for the lack of effect of a selective 5-HT2A antagonist on l-DOPA-induced AIMs behaviors.

Blockade of stimulant-induced preprodynorphin mRNA expression in the striatal matrix by serotonin depletion

Cocaine and methamphetamine (METH) induce preprodynorphin (PPD) mRNA expression in the striatum. Cocaine induces PPD expression in both the patch and matrix compartments of the rostral striatum, whereas METH induces PPD expression in the patch compartment of the rostral striatum. In middle striatum, both stimulants increase PPD expression in the patch and matrix compartments. METH and cocaine treatment also increase extracellular serotonin (5-HT). Several studies have shown that 5-HT receptors are present on striatonigral neurons that express PPD mRNA, and that 5-HT is a positive regulator of striatal neuropeptide expression. The current study examined whether 5-HT plays a role in the patch/matrix expression of PPD mRNA induced by cocaine and METH in striatum. Male Sprague-Dawley rats were treated with p-chloroamphetamine (PCA; 8 mg/kg, i.p), a serotonin neurotoxin, 1 week prior to cocaine (30 mg/kg, i.p) and METH (15 mg/kg, s.c.) treatment. The 80% loss of 5-HT induced by PCA-pretreatment blocked cocaine-induced PPD expression in the rostral matrix compartment. Cocaine- and METH-induced PPD expression in the rostral patch compartment was unaffected by PCA-pretreatment. PCA-pretreatment also decreased both cocaine- and METH-induced PPD expression in the matrix, but not patch of middle striatum. PCA-induced 5-HT depletion did not affect stimulant-induced increases in PPT mRNA expression in the striatum. These data suggest that 5-HT plays a role in stimulant-induced PPD expression in the matrix compartment of rostral and middle striatum. Thus, 5-HT innervation may play a critical role in basal ganglia function.

3,4-Methylenedioxymethamphetamine increases neuropeptide messenger RNA expression in rat striatum

The amphetamine analog 3,4-methylenedioxymethamphetamine (MDMA) is also known as the recreational drug of abuse, Ecstasy. Several neuropeptides are found in striatal neurons postsynaptic to dopamine and serotonin nerve terminals, and changes in neuropeptide neurotransmission may be important for behavioral effects of 3,4-methylenedioxymethamphetamine. This study used in situ hybridization to characterize the effects of 3,4-methylenedioxymethamphetamine on four neuropeptide mRNAs: preprodynorphin, preprotachykinin, neurotensin/neuromedin N, and preproenkephalin. Male, Sprague-Dawley rats received a single administration of 10 mg/kg 3,4-methylenedioxymethamphetamine and were sacrificed 30 min or 3 h later. Three hours after administration, 3,4-methylenedioxymethamphetamine increased preprodynorphin, preprotachykinin, and neurotensin/neuromedin N mRNAs. These increases were most prominent in ventral and medial aspects of the rostral-middle striatum, and then became more dorsally restricted in the caudal striatum. At the 30-minute time point, MDMA significantly decreased the signal for preproenkephalin mRNA in a general manner but did not affect the signal for the other neuropeptide precursors. These data suggest that 3,4-methylenedioxymethamphetamine has a generalized, transient, inhibitory effect on striatopallidal neuron gene expression, and then preferentially influences striatonigral neuropeptide systems at the later time point in a regionally selective manner.

Dopamine Uptake Inhibitor-Induced Rotation in 6-Hydroxydopamine-Lesioned Rats Involves Both D1 and D2 Receptors but Is Modulated through 5-Hydroxytryptamine and Noradrenaline Receptors

Dopamine uptake inhibitors may provide a means of sustaining endogenous and exogenous striatal dopamine levels in Parkinson's disease, but most are not selective and also inhibit the noradrenaline and 5-hydroxytryptamine (5-HT) transporters. To determine the involvement of the individual monoamine transporters in the production of motor activity, the effect of the nonselective monoamine uptake inhibitor BTS 74 398 1-([1-(3,4-dichlorophenyl)cyclobutyl]-2-(3-diaminethylaminopropylthio) ethanone monocitrate) and the selective dopamine, GBR 12909 [1-(2-(bis-(4-fluorphenyl)-methyl)ethyl)-4-(3-phenylpropyl)piperazine) dihydrochloride], noradrenaline (nisoxetine), and 5-HT (fluvoxamine) reuptake inhibitors on circling in the unilateral 6-hydroxydopamine-lesioned rat was investigated. GBR 12909 induced ipsilateral circling, but fluvoxamine and nisoxetine were without effect. However, when administered with GBR 12909, fluvoxamine enhanced rotation, whereas nisoxetine had no effect. The results suggest that 5-HT, but not noradrenaline, reuptake inhibition facilitates dopamine-mediated motor activity. To test this hypothesis, BTS 74 398 was administered in combination with selective dopamine, 5-HT, and noradrenaline receptor antagonists. Both D(1) and D(2) receptor antagonists, SCH 23390 [R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine] and raclopride, inhibited BTS 74 398-induced circling. In contrast, the 5-HT(1A) 5-HT(1A/B) antagonists, WAY 100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-cyclohexane-carboxamide maleate) and pindolol, and the 5-HT(2A) antagonist, ketanserin, had no effect. The nonspecific 5-HT((1/2)) antagonists, methysergide and metergoline, and the specific 5-HT(2C) antagonist, N-desmethylclozapine, enhanced BTS 74 398-induced circling, as did the alpha(2)-adrenoceptor antagonist idazoxan. Overall, the data suggest that inhibition of the 5-HT and noradrenaline transporters modulate dopamine uptake inhibitor-mediated motor activity. However, the mechanism of this interaction is complex, involving opposing effects of noradrenaline and 5-HT agonism and antagonism.

NMDA receptor blockade attenuates locomotion elicited by intrastriatal dopamine D1-receptor stimulation

Previous behavioral studies suggest that the striatum mediates a hyperactive response to systemic NMDA receptor antagonism in combination with systemic D1 receptor stimulation. However, many experiments conducted at the cellular level suggest that inhibition of NMDA receptors should block D1 receptor-mediated locomotor activity. Therefore, we investigated the consequences of NMDA receptor blockade on the ability of striatal D1 receptors to elicit locomotor activity using systemic and intrastriatal injections of the NMDA antagonist MK-801 combined with intrastriatal injections of the D1 full agonist SKF 82958. Following drug treatment locomotor activity was measured via computerized activity monitors designed to quantify multiple parameters of rodent open-field behavior. Both systemic (0.1 mg/kg) and intrastriatal (1.0 microg) MK-801 pretreatments completely blocked locomotor and stereotypic activity elicited by 10 microg of SKF 82958 directly infused into the striatum. Further, increased activity triggered by intrastriatal SKF 82958 was attenuated by a posttreatment with intrastriatal infusion of 1 microg MK-801. These data suggest that D1-stimulated locomotor behaviors controlled by the striatum require functional NMDA channels.

Combined intrastriatal dopamine D1 and serotonin 5-HT2 receptor stimulation reveals a mechanism for hyperlocomotion in 6-hydroxydopamine-lesioned rats

Loss of dopaminergic innervation to the striatum increases the sensitivity of dopamine (DA) D1 and serotonin (5-HT) 5-HT2 receptor signaling. Previous work from our laboratory has shown that systemic co-administration of D1 and 5-HT2 receptor agonists leads to the synergistic overexpression of striatal preprotachykinin mRNA levels in the DA-depleted, but not intact animals. In the present study, we examined this mechanism as related to locomotor behavior. Adult male Sprague-Dawley rats were subject to bilateral i.c.v. 6-hydroxydopamine (6-OHDA; 200 microg in 10 microl/side) or vehicle (0.9% saline and 0.1% ascorbic acid). After 3 weeks, rats were tested for locomotor responses to bilateral intrastriatal infusions of vehicle (0.9% NaCl), the D1 agonist SKF82958 [(+/-)6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetra-hydro-(1H)-3-benzazepine hydrobromide; 0.1, 1.0 or 10.0 microg/side], the 5-HT2 agonist DOI [(+/-)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane; 0.1, 1.0 or 10.0 microg/side] or subthreshold doses of DOI and SKF82958 (0.1 microg+0.1 microg in 0.8 microl/side). Rats with DA loss demonstrated supersensitive locomotor responses to SKF82958, but not DOI. Combined administration of subthreshold SKF82958 and DOI doses (0.1 microg+0.1 microg) synergistically increased locomotor behavior only in 6-OHDA-lesioned rats. These effects were blocked by either the D1 antagonist SCH23390 3-methyl-1-phenyl-2,3,4,5-tetrahydro-7-chloro-8-hydroxy-(1H)-3-benzazepine or the 5-HT2 antagonist ritanserin (each 1.0 microg in 0.8 microl/side). The results of this study suggest that the behavioral synergy induced by local co-stimulation of D1 and 5-HT2 receptors within the 6-OHDA-lesioned striatum may lead to hyperkinesias that can occur with continued pharmacological treatment of Parkinson's disease.

Intrastriatal serotonin 5-HT2 receptors mediate dopamine D1-induced hyperlocomotion in 6-hydroxydopamine-lesioned rats

Striatal dopamine (DA) and serotonin (5-HT) functions are altered following DA denervation. Previous research indicates that intrastriatal coadministration of D1 and 5-HT2 receptor agonists synergistically increase locomotor behavior in DA-depleted rats. In the present study, we examined whether striatal 5-HT2 mechanisms also account for supersensitive D1-mediated locomotor behavior following DA denervation. Adult male Sprague-Dawley rats were subjected to bilateral striatal cannulation and then received either intracerebroventricular (i.c.v.) or intrastriatal 6-hydroxydopamine (6-OHDA; 200 microg or 20 microg/side, respectively). After at least 3 weeks, i.c.v.-lesioned rats received intrastriatal infusions of the 5-HT2 receptor antagonist ritanserin (2.0 microg/side) or its vehicle (DMSO) followed by systemic SKF 82958, a D1 agonist (1.0 mg/kg, i.p.) and locomotor activity was monitored. In another experiment, intrastriatal sham and 6-OHDA-lesioned rats received bilateral intrastriatal infusions of ritanserin (2.0 microg/side) or its vehicle (DMSO) followed by intrastriatal infusions of SKF 82958 (5.0 microg/side) or vehicle (0.9% saline). Rats with DA loss demonstrated supersensitive locomotor responses to both systemic and intrastriatal SKF 82958. Ritanserin pretreatment blunted systemic SKF 82958-induced hyperlocomotion and returned intrastriatal D1-mediated hyperactivity to sham lesion levels. The results of this study suggest that striatal 5-HT2 receptors contribute to D1-mediated hyperkinesias resulting from DA loss and suggest a pharmacological target for the alleviation of dyskinesia that can develop with continued DA replacement therapy.

Striatal tachykinin and enkephalin mRNAs are normalized by serotonin2 and NMDA manipulation following dopamine depletion

We have examined the effects of serotonin2 (5-HT2) stimulation and NMDA antagonism on preprotachykinin (PPT) and preproenkephalin (PPE) gene regulation in the dopamine (DA) depleted striatum. Following DA lesions, PPT mRNA expression was reduced (dorsomedial (DM) 44 +/- 9%, dorsolateral (DL) 40 +/- 4%), whereas PPE message levels were elevated (DM 207+/-28%, DL 198+/-25%). Within this state of dysregulated gene activity, DOI (5-HT2 agonist) increased PPT message levels (174 +/- 5%, DM; 153 +/- 13%, DL) without affecting PPE gene expression. In addition, MK-801 (NMDA antagonist) decreased PPE message levels (DM 59 +/- 10%, DL 52 +/- 7%) without significantly altering PPT mRNA expression. Combined application of DOI and MK-801 resulted in normalization of both PPTand PPE message. Statistical analysis revealed no drug interactions in this paradigm suggesting independent mechanisms for 5-HT2 and NMDA receptors in controlling neuropeptide production following DA depletion.

Neonatal dopamine depletion reveals a synergistic mechanism of mRNA regulation that is mediated by dopamine(D1) and serotonin(2) receptors and is targeted to tachykinin neurons of the dorsomedial striatum

It has been hypothesized that dopamine(D1) and serotonin(2) receptors become sensitized to agonist-mediated regulation of gene expression following loss of dopaminergic innervation to the striatum. We have previously demonstrated that the combined administration of dopamine(D1) and serotonin(2) receptor agonists to dopamine-depleted adult rats induced preprotachykinin mRNA expression within the periventricular rostral striatum to levels which were significantly different than what could be elicited by either agonist alone. In the present study, we have determined that this phenomenon is revealed only after dopamine depletion. In addition, it is targeted primarily to tachykinin producing neurons of the dorsomedial striatum and is dependent on both dopamine(D1) and serotonin(2) receptor activation. Preprotachykinin mRNA levels in the intact striatum were unaltered 4 h following an i.p. injection of either SKF-38393 (1 mg/kg, dopamine(D1) partial agonist) or (+/-)-1-(4-Iodo-2,5-dimethoxyphenyl)-2-aminopropane (DOI 1 mg/kg, serotonin(2) agonist). However, the combined application of both agonists increased (+44%) preprotachykinin message levels, but these changes were restricted to the dorsomedial striatum. In adult animals depleted of dopamine as neonates, striatal preprotachykinin mRNA expression was reduced by approximately 50%. From this lowered level of basal expression, DOI or SKF-38393 raised preprotachykinin mRNA levels within the dorsomedial, but not the dorsolateral striatum. Furthermore, co-stimulation of dopamine(D1) and serotonin(2) receptors produced a nearly four-fold induction of preprotachykinin message levels in the dorsomedial striatum that was significantly greater than either agonist alone. Application of both agonists also elevated preprotachykinin mRNA expression within the dorsolateral striatum, but to a lesser extent. All increases in preprotachykinin mRNA resulting from co-application of SKF-38393 and DOI were prevented by pretreatment with either SCH-23390 (1 mg/kg, dopamine(D1) antagonist) or ritanserin (1 mg/kg, serotonin(2) antagonist). Alternately, preproenkephalin mRNA expression was unaffected by dopamine(D1) receptor stimulation, but was slightly elevated by DOI or both agonists together (42-58%) in intact animals. However, neither agonist treatment in this experiment significantly altered preproenkephalin mRNA expression in the dopamine-depleted striatum which was elevated in response to dopamine lesion alone. Dopamine depletion appears to promote a synergistic interaction between dopamine(D1) and serotonin(2) receptors that leads to enhanced expression of striatal preprotachykinin mRNA levels. The localization of this phenomenon to tachykinin neurons of the direct striatonigral pathway specifically within the dorsomedial regions of the rostral striatum may be relevant to the problem of dyskinetic behaviors which arise during the pharmacological treatment of movement disorders.

NMDA receptor antagonism modifies the synergistic regulation of striatal tachykinin gene expression induced by dopamine D(1) and serotonin(2) receptor stimulation following neonatal dopamine depletion

Co-application of SKF-38393 (dopamine D(1) agonist; 1 mg/kg) and DOI (serotonin(2) agonist; 1 mg/kg) induced a synergistic increase in striatal preprotachykinin (PPT) mRNA levels in adult rats 60 days after neonatal intracerebroventricular injection of 6-hydroxydopamine. This magnitude of response was not observed in intact (vehicle-injected) rats and was restricted to the dorsomedial (DM, 333+/-25% of lesion) subregion of the anterior striatum, with smaller increases observed in the dorsolateral striatum (DL, 206+/-26% of lesion). A single i.p. injection of MK-801 (NMDA antagonist; 0.1 mg/kg) administered prior to dopamine D(1) (D(1)) and serotonin(2) (5-HT(2)) receptor co-stimulation suppressed the synergistic regulation of PPT mRNA expression in the DM striatum, but also produced a large increase in PPT message levels within the DL striatum (321+/-17% of lesion). These data suggest that the synergistic regulation of PPT mRNA within the DM striatum induced by D(1)/5-HT(2) receptor co-stimulation in the dopamine lesioned rat is dependent on NMDA receptor activity. However, MK-801 may simultaneously potentiate striatal PPT mRNA expression by a separate mechanism due to the changed environment of the dopamine-depleted basal ganglia.

Serotonin 2A and 2C receptor biosynthesis in the rodent striatum during postnatal development: mRNA expression and functional linkage to neuropeptide gene regulation

The present study was designed to determine if there are region-specific differences in serotonin (5-HT) neurotransmission and 5-HT receptor expression that may limit the stimulatory effects of the 5-HT releaser p-chloroamphetamine (pCA) on striatal neuropeptide gene expression to the posterior striatum (P-STR) during postnatal maturation. Sprague-Dawley rat brains from postnatal days (PND) 1-35 were processed for 5-HT(2A) and 5-HT(2C) receptor mRNA expression by in situ hybridization and monoamine analysis by HPLC. Within the P-STR, 5-HT(2A) receptor mRNA expression reached young adult (PND 35) levels by PND 3, while levels in the A-STR were significantly less (range: 1.43 +/- 0.219-6. 36 +/- 0.478) than P-STR (5.36 +/- 0.854-12.11 +/- 1.08) at each respective age throughout the time course. 5-HT(2C) receptor mRNA expression reached young adult levels at PND 7 in the A-STR and by PND 3 in the P-STR. At each PND age 5-HT(2C) receptor mRNA levels within the P-STR were significantly less (6.23 +/- 1.02-12.32 +/- 0.427) than the A-STR (7.31 +/- 1.65-26.84 +/- 2.24). 5-HT content increased across the developmental time course within the P-STR (5.01 +/- 0.327-15.7 +/- 1.03 ng/mg protein) and A-STR (2.97 +/- 0. 223-11.2 +/- 0.701 ng/mg protein). Four hours following injection (i. p.) of pCA (10 mg/kg), preprotachykinin (PPT) mRNA levels increased 89% in the P-STR but not the anterior (A-STR) striatum of the 3-week-old rat, which were prevented by preinjection (30 min, i.p.) of the 5-HT(2) receptor antagonist ritanserin (1 mg/kg). Together, these data suggest that faster maturity of 5-HT(2A) receptor expression in the P-STR may be sufficient to convey the region-specific acute stimulatory effects of pCA on PPT mRNA transcription in the developing rodent striatum. These results provide further evidence that the influence of 5-HT on neuropeptide gene expression is far stronger in caudal vs. rostral striatal regions during postnatal development.

Serotonin-2 receptor stimulation normalizes striatal preprotachykinin messenger RNA in an animal model of Parkinson's disease

Dopamine and serotonin neurotransmission regulate striatal preprotachykinin messenger RNA levels. In the present study, we investigated serotonin 2A/2C receptor-mediated regulation of preprotachykinin messenger RNA expression in the rat striatum after adult dopamine depletion produced with 6-hydroxydopamine. Significant reductions (46-61% of control values) in preprotachykinin messenger RNA levels were detected by in situ hybridization in rostral, central and caudal regions of the striatum after >85% dopamine depletion. Repeated administration of the specific serotonin2A/2C receptor agonist, (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrobromide, to dopamine-depleted rats completely reversed the reduction in preprotachykinin messenger RNA levels in rostral, central and dorsal-caudal striatal regions. In unlesioned (vehicle-injected) control animals, repeated administration of (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrobromide did not affect preprotachykinin messenger RNA expression in rostral, central and ventral-caudal striatal regions, but decreased preprotachykinin messenger RNA levels in the dorsal-caudal striatal subregion. In addition, serotonin turnover in the dopamine-depleted rostral striatum was significantly increased by 35-45% which is consistent with serotonin hyperinnervation after 6-hydroxydopamine lesions. These data show that the decrease in striatal preprotachykinin messenger RNA after dopamine depletion can be normalized with repeated serotonin2A/2C receptor stimulation. We hypothesize that this serotonin2A/2C receptor regulation of preprotachykinin messenger RNA expression after 6-hydroxydopamine is a consequence of serotonin hyperinnervation, which may include increased striatal serotonin2A/2C receptors, induced by dopamine depletion. We also propose that the serotonin system could be pharmacologically targeted to restore the direct striatal tachykinin pathway in Parkinson's disease.

Serotonin 2A receptor mRNA levels in the neonatal dopamine-depleted rat striatum remain upregulated following suppression of serotonin hyperinnervation

Sixty days after bilateral dopamine (DA) depletion (>98%) with 6-hydroxydopamine (6-OHDA) in neonatal rats, serotonin (5-HT) content doubled and 5-HT(2A) receptor mRNA expression rose 54% within the rostral striatum. To determine if striatal 5-HT(2A) receptor mRNA upregulation is dependent on increased 5-HT levels following DA depletion, neonatal rats received dual injections of 6-OHDA and 5,7-dihydroxytryptamine (5,7-DHT) which suppressed 5-HT content by approximately 90%. In these 6-OHDA/5,7-DHT-treated rats, striatal 5-HT(2A) receptor mRNA expression was still elevated (87% above vehicle controls). Comparative analysis of 5-HT(2C) receptor mRNA expression yielded no significant changes in any experimental group. These results demonstrate that upregulated 5-HT(2A) receptor biosynthesis in the DA-depleted rat is not dependent on subsequent 5-HT hyperinnervation.