The firing activity of pyramidal neurons in medial prefrontal cortex and their response to 5-hydroxytryptamine-1A receptor stimulation in a rat model of Parkinson's disease

Pramipexole Hyperactivates the External Globus Pallidus and Impairs Decision-Making in a Mouse Model of Parkinson's Disease

In patients with Parkinson's disease (PD), dopamine replacement therapy with dopamine D2/D3 receptor agonists induces impairments in decision-making, including pathological gambling. The neurobiological mechanisms underlying these adverse effects remain elusive. Here, in a mouse model of PD, we investigated the effects of the dopamine D3 receptor (D3R)-preferring agonist pramipexole (PPX) on decision-making. PD model mice were generated using a bilateral injection of the toxin 6-hydroxydopamine into the dorsolateral striatum. Subsequent treatment with PPX increased disadvantageous choices characterized by a high-risk/high-reward in the touchscreen-based Iowa Gambling Task. This effect was blocked by treatment with the selective D3R antagonist PG-01037. In model mice treated with PPX, the number of c-Fos-positive cells was increased in the external globus pallidus (GPe), indicating dysregulation of the indirect pathway in the corticothalamic-basal ganglia circuitry. In accordance, chemogenetic inhibition of the GPe restored normal c-Fos activation and rescued PPX-induced disadvantageous choices. These findings demonstrate that the hyperactivation of GPe neurons in the indirect pathway impairs decision-making in PD model mice. The results provide a candidate mechanism and therapeutic target for pathological gambling observed during D2/D3 receptor pharmacotherapy in PD patients.

5-HT1B receptors in the basolateral amygdaloid nucleus regulate anxiety-like behaviors through AC-PKA signal pathway in a rat model of Parkinson's disease

BACKGROUND

Parkinson's disease (PD) is commonly accompanied with anxiety, multiple studies indicate that the basolateral amygdaloid nucleus (BLA) is closely related to modulation of anxiety and expresses serotonin_1B (5-HT_1B) receptors, however, effects of BLA 5-HT_1B receptors on anxiety-like behaviors are unclear, particularly in PD-related anxiety.

METHODS

The open-field and elevated plus maze tests were used to examine anxiety-like behaviors. In vivo electrophysiology and microdialysis were performed to observe the firing activity of BLA neurons and GABA, glutamate, dopamine (DA) and 5-HT release in the BLA, respectively. Western blotting was used to analyze protein expression of 5-HT_1B receptors, adenylate cyclase (AC) and phosphorylated protein kinase A at threonine 197 site (p-PKA-Thr197) in the BLA.

RESULTS

Intra-BLA injection of 5-HT_1B receptor agonist CP93129 produced anxiety-like effects and antagonist SB216641 induced anxiolytic-like responses in sham-operated and 6-hydroxydopamine-lesioned rats. Further, pretreatment with AC inhibitor SQ22536 and PKA inhibitor KT5720 blocked the behavioral effects of CP93129, respectively. Intra-BLA injection of CP93129 increased the firing rate of BLA glutamate neurons and decreased GABA/glutamate ratio and DA and 5-HT levels in the BLA of sham-operated and the lesioned rats, while SB216641 induced the opposite effects. Compared with sham-operated rats, effects of CP93129 and SB216641 on behaviors, electrophysiology and microdialysis were decreased in the lesioned rats, which were associated with decreased expression of 5-HT_1B receptors, AC and p-PKA-Thr197 in the BLA.

CONCLUSION

These findings suggest that 5-HT_1B receptor-AC-PKA signal pathway in the BLA is involved in the regulation of PD-related anxiety.

Motor cortico-nigral and cortico-entopeduncular information transmission and its modulation by buspirone in control and after dopaminergic denervation

Cortical information is transferred to the substantia nigra pars reticulata (SNr) and the entopeduncular nucleus (EP), the output structures of the basal ganglia (BG), through three different pathways: the hyperdirect trans-subthalamic and the direct and indirect trans-striatal pathways. The nigrostriatal dopamine (DA) and the activation of 5-HT1A receptors, distributed all along the BG, may modulate cortical information transmission. We aimed to investigate the effect of buspirone (5-HT1A receptor partial agonist) and WAY-100635 (5-HT1A receptor antagonist) on cortico-nigral and cortico-entopeduncular transmission in normal and DA loss conditions. Herein, simultaneous electrical stimulation of the motor cortex and single-unit extracellular recordings of SNr or EP neurons were conducted in urethane-anesthetized sham and 6-hydroxydopamine (6-OHDA)-lesioned rats before and after drug administrations. Motor cortex stimulation evoked monophasic, biphasic, or triphasic responses, combination of an early excitation, an inhibition, and a late excitation in both the SNr and EP, while an altered pattern of evoked response was observed in the SNr after 6-OHDA lesion. Systemic buspirone potentiated the direct cortico-SNr and cortico-EP transmission in sham animals since increased duration of the inhibitory response was observed. In DA denervated animals, buspirone administration enhanced early excitation amplitude in the cortico-SNr transmission. In both cases, the observed effects were mediated via a 5-HT1A-dependent mechanism as WAY-100635 administration blocked buspirone's effect. These findings suggest that in control condition, buspirone potentiates direct pathway transmission and DA loss modulates responses related to the hyperdirect pathway. Overall, the results may contribute to understanding the role of 5-HT1A receptors and DA in motor cortico-BG circuitry functionality.

Animal models of Parkinson's disease: a guide to selecting the optimal model for your research

Parkinson's disease (PD) is a complex, multisystem disorder characterised by α-synuclein (SNCA) pathology, degeneration of nigrostriatal dopaminergic neurons, multifactorial pathogenetic mechanisms and expression of a plethora of motor and non-motor symptoms. Animal models of PD have already been instructive in helping us unravel some of these aspects. However, much remains to be discovered, requiring continued interrogation by the research community. In contrast with the situation for many neurological disorders, PD benefits from of a wide range of available animal models (pharmacological, toxin, genetic and α-synuclein) but this makes selection of the optimal one for a given study difficult. This is especially so when a study demands a model that displays a specific combination of features. While many excellent reviews of animal models already exist, this review takes a different approach with the intention of more readily informing this decision-making process. We have considered each feature of PD in turn - aetiology, pathology, pathogenesis, motor dysfunctions and non-motor symptoms (NMS) - highlighting those animal models that replicate each. By compiling easily accessible tables and a summary figure, we aim to provide the reader with a simple, go-to resource for selecting the optimal animal model of PD to suit their research needs.

Activation and Blockade of Serotonin-4 Receptors in the Lateral Habenula Produce Antidepressant Effects in the Hemiparkinsonian Rat

BACKGROUND

The 5-hydroxytryptamine (5-HT) neurotransmitter system and lateral habenula (LHb) are involved in the regulation of depression, while the mechanisms remain to be clarified.

OBJECTIVES

The effects and possible mecha-nism underlying activation or blockade of 5-HT4 receptors (5-HT4Rs) in the LHb in depression were investigated by behavioral and neurochemical methods based on a Parkinson's disease (PD) rat model.

METHOD

6-Hydroxydopamine (6-OHDA) was injected unilaterally into the substantia nigra pars compacta to establish the PD rat model. The depressive-like behaviors were measured by the forced swimming test (FST) and sucrose preference test (SPT). The concentrations of dopamine (DA), noradrenaline (NA) and 5-HT in the related brain regions were measured by a neurochemical method.

RESULTS

The 6-OHDA lesions increased the immobility time in the FST and decreased the sucrose consumption in the SPT, suggesting the induction of depressive-like behaviors. Intra-LHb injection of BIMU-8 (5-HT4R agonist) or GR113808 (5-HT4R antagonist) produced antidepressant effects in the lesioned rats. Intra-LHb injection of BIMU-8 significantly increased the DA levels in the medial prefrontal cortex (mPFC) and ventral hippocampus (vHip), increased the 5-HT level in the mPFC and decreased the NA level in the vHip only in the lesioned rats, while intra-LHb injection of GR113808 changed DA, NA and 5-HT levels in the mPFC, LHb and vHip in both sham and the lesioned rats.

CONCLUSIONS

All these results suggest that activation or blockade of the LHb 5-HT4Rs produce antidepressant effects in the 6-OHDA-lesioned rats, which are related to the changes of monoamines in the limbic and limbic-related regions.

6-Hydroxydopamine lesion and levodopa treatment modify the effect of buspirone in the substantia nigra pars reticulata

BACKGROUND AND PURPOSE

l-DOPA-induced dyskinesia (LID) is considered a major complication in the treatment of Parkinson's disease (PD). Buspirone (5-HT1A partial agonist) have shown promising results in the treatment of PD and LID, however no 5-HT-based treatment has been approved in PD. The present study was aimed to investigate how the substantia nigra pars reticulata (SNr) is affected by buspirone and whether it is a good target to study 5-HT antidyskinetic treatments.

EXPERIMENTAL APPROACH

Buspirone was studied using in vivo single-unit, electrocorticogram, local field potential recordings along with microdialysis and immunohistochemistry in naïve/sham, 6-hydroxydopamine (6-OHDA)-lesioned or 6-OHDA-lesioned and l-DOPA-treated (6-OHDA/l-DOPA) rats.

KEY RESULTS

Local buspirone inhibited SNr neuron activity in all groups. However, systemic buspirone reduced burst activity in 6-OHDA-lesioned rats (with or without l-DOPA treatment), whereas 8-OH-DPAT, a full 5-HT1A agonist induced larger inhibitory effects in sham animals. Neither buspirone nor 8-OH-DPAT markedly modified the low-frequency oscillatory activity in the SNr or synchronization within the SNr with the cortex. In addition, local perfusion of buspirone increased GABA and glutamate release in the SNr of naïve and 6-OHDA-lesioned rats but no effect in 6-OHDA/l-DOPA rats. In the 6-OHDA/l-DOPA group, increased 5-HT transporter and decreased 5-HT1A receptor expression was found.

CONCLUSIONS AND IMPLICATIONS

The effects of buspirone in SNr are influenced by dopamine loss and l-DOPA treatment. The present results suggest that the regulation of burst activity of the SNr induced by DA loss may be a good target to test new drugs for the treatment of PD and LID.

Medial prefrontal cortex in neurological diseases

The medial prefrontal cortex (mPFC) is a crucial cortical region that integrates information from numerous cortical and subcortical areas and converges updated information to output structures. It plays essential roles in the cognitive process, regulation of emotion, motivation, and sociability. Dysfunction of the mPFC has been found in various neurological and psychiatric disorders, such as depression, anxiety disorders, schizophrenia, autism spectrum disorders, Alzheimer's disease, Parkinson's disease, and addiction. In the present review, we summarize the preclinical and clinical studies to illustrate the role of the mPFC in these neurological diseases.

Involvement of prelimbic 5-HT7 receptors in the regulation of anxiety-like behaviors in hemiparkinsonian rats

OBJECTIVE

At present, little is known about the role of serotonin₇ (5-HT₇) receptor in anxiety, particularly in Parkinson's disease-related anxiety. Here, we tested whether 5-HT₇ receptors in the prelimbic (PrL) cortex are involved in the regulation of anxiety-like behaviors in sham-operated rats and rats with unilateral 6-hydroxydopamine lesions of the medial forebrain bundle (MFB).

METHODS

The open field and elevated plus maze (EPM) tests were performed to study the influence of MFB lesion and intra-PrL injection of 5-HT₇ agonist AS19 (0.5, 1 or 2 μg/rat) and antagonist SB269970 (1.5, 3 or 6 μg/rat) on anxiety-like behaviors. Additionally, changes in monoamine levels in limbic and limbic-related brain regions were observed after intra-PrL injection of AS19 (2 μg/rat) and SB269970 (6 μg/rat).

RESULTS

The MFB lesion induced anxiety-like behaviors compared to sham-operated rats. Intra-PrL injection of AS19 showed anxiolytic effects by the open field and EPM tests in two groups of rats, and administration of SB269970 showed anxiogenic responses. However, the doses producing these effects in the lesioned rats were higher than those in sham-operated rats. Neurochemical results showed that intra-PrL injection of AS19 increased dopamine, 5-HT and noradrenaline (NA) levels in the medial prefrontal cortex, ventral hippocampus and amygdala in two groups of rats, whereas SB269970 decreased 5-HT and NA levels in these brain regions.

DISCUSSION

5-HT₇ receptors in the PrL are involved in the regulation of anxiety-like behaviors, which is attributable to changes in dopamine, 5-HT and NA levels in the limbic and limbic-related brain regions after activation and blockade of 5-HT₇ receptors.

ABBREVIATIONS

6-OHDA: 6-hydroxydopamine; DMSO: dimethyl sulfoxide; DA: dopamine; EPM: elevated plus maze; MFB: medial forebrain bundlem; PFC: medial prefrontal cortex; NA: noradrenaline; PD: Pakinson's disease; PrL: prelimbic; 5-HT: serotonin; vHip: ventral hippocampus.

Levodopa treatment and dendritic spine pathology

Parkinson's disease (PD) is a neurodegenerative disorder associated with the progressive loss of nigrostriatal dopaminergic neurons. Levodopa is the most effective treatment for the motor symptoms of PD. However, chronic oral levodopa treatment can lead to various motor and nonmotor complications because of nonphysiological pulsatile dopaminergic stimulation in the brain. Examinations of autopsy cases with PD have revealed a decreased number of dendritic spines of striatal neurons. Animal models of PD have revealed altered density and morphology of dendritic spines of neurons in various brain regions after dopaminergic denervation or dopaminergic denervation plus levodopa treatment, indicating altered synaptic transmission. Recent studies using rodent models have reported dendritic spine head enlargement in the caudate‐putamen, nucleus accumbens, primary motor cortex, and prefrontal cortex in cases where chronic levodopa treatment following dopaminergic denervation induced dyskinesia‐like abnormal involuntary movement. Hypertrophy of spines results from insertion of alpha‐amino‐2,3‐dihydro‐5‐methyl‐3‐oxo‐4‐isoxazolepropanoic acid receptors into the postsynaptic membrane. Such spine enlargement indicates hypersensitivity of the synapse to excitatory inputs and is compatible with a lack of depotentiation, which is an electrophysiological hallmark of levodopa‐induced dyskinesia found in the corticostriatal synapses of dyskinetic animals and the motor cortex of dyskinetic PD patients. This synaptic plasticity may be one of the mechanisms underlying the priming of levodopa‐induced complications such as levodopa‐induced dyskinesia and dopamine dysregulation syndrome. Drugs that could potentially prevent spine enlargement, such as calcium channel blockers, N‐methyl‐D‐aspartate receptor antagonists, alpha‐amino‐2,3‐dihydro‐5‐methyl‐3‐oxo‐4‐isoxazolepropanoic acid receptor antagonists, and metabotropic glutamate receptor antagonists, are candidates for treatment of levodopa‐induced complications in PD. © 2017 The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society.

In Vivo Effect of a 5-HT7 Receptor Agonist on 5-HT Neurons and GABA Interneurons in the Dorsal Raphe Nuclei of Sham and PD Rats

The 5-hydroxytryptamine (5-HT; serotonin) neurotransmission is severely affected by the degeneration of nigrostriatal dopaminergic neurons. Here, we report the effects of the systemic administration of the 5-HT7 receptor agonist AS-19. In sham rats, the mean response of the 5-HT neurons in the dorsal raphe nucleus (DRN) to systemic AS-19 was excitatory and the mean response of the γ-aminobutyric acid (GABA) interneurons was inhibitory. In Parkinson disease (PD) rats, the same dose did not affect the 5-HT neurons and only high doses (640 μg/kg intravenous) were able to the increase GABA interneuron activity. These results indicate that DRN 5-HT neurons and GABA interneurons are regulated by the activation of 5-HT7 receptors and that the degeneration of the nigrostriatal pathway leads to decreased responses of these neurons to AS-19, which in turn suggests that the 5-HT7 receptors on 5-HT neurons and GABA interneurons in PD rats are dysfunctional and downregulated.

A heuristic model for working memory deficit in schizophrenia

BACKGROUND

The life of schizophrenia patients is severely affected by deficits in working memory. In various brain regions, the reciprocal interactions between excitatory glutamatergic neurons and inhibitory GABAergic neurons are crucial. Other neurotransmitters, in particular dopamine, serotonin, acetylcholine, and norepinephrine, modulate the local balance between glutamate and GABA and therefore regulate the function of brain regions. Persistent alterations in the balances between the neurotransmitters can result in working memory deficits.

METHODS

Here we present a heuristic computational model that accounts for interactions among neurotransmitters across various brain regions. The model is based on the concept of a neurochemical interaction matrix at the biochemical level and combines this matrix with a mobile model representing physiological dynamic balances among neurotransmitter systems associated with working memory.

RESULTS

The comparison of clinical and simulation results demonstrates that the model output is qualitatively very consistent with the available data. In addition, the model captured how perturbations migrated through different neurotransmitters and brain regions. Results showed that chronic administration of ketamine can cause a variety of imbalances, and application of an antagonist of the D2 receptor in PFC can also induce imbalances but in a very different manner.

CONCLUSIONS

The heuristic computational model permits a variety of assessments of genetic, biochemical, and pharmacological perturbations and serves as an intuitive tool for explaining clinical and biological observations.

GENERAL SIGNIFICANCE

The heuristic model is more intuitive than biophysically detailed models. It can serve as an important tool for interdisciplinary communication and even for psychiatric education of patients and relatives. This article is part of a Special Issue entitled "System Genetics" Guest Editor: Dr. Yudong Cai and Dr. Tao Huang.

Nigrostriatal Dopamine Acting on Globus Pallidus Regulates Sleep

Lesions of the globus pallidus externa (GPe) produce a profound sleep loss (∼45%) in rats, suggesting that GPe neurons promote sleep. As GPe neuronal activity is enhanced by dopamine (DA) from the substantia nigra pars compacta (SNc), we hypothesized that SNc DA via the GPe promotes sleep. To test this hypothesis, we selectively destroyed the DA afferents to the caudoputamen (CPu) using 6-hydroxydopamine and examined changes in sleep-wake profiles in rats. Rats with 80-90% loss of SNc neurons displayed a significant 33.7% increase in wakefulness (or sleep reduction). This increase significantly correlated with the extent of SNc DA neuron loss. Furthermore, these animals exhibited sleep-wake fragmentation and reduced diurnal variability of sleep. We then optogenetic-stimulated SNc DA terminals in the CPu and found that 20-Hz stimulation from 9 to 10 PM increased total sleep by 69% with high electroencephalograph (EEG) delta power. We finally directly optogenetic-stimulated GPe neurons and found that 20-Hz stimulation of the GPe from 9 to 10 PM increased total sleep by 66% and significantly increased EEG delta power. These findings elucidate a novel circuit for DA control of sleep and the mechanisms of abnormal sleep in BG disorders such as Parkinson's disease and Huntington's disease.

Serotonergic modulation of the activity of mesencephalic dopaminergic systems: Therapeutic implications

Since their discovery in the mammalian brain, it has been apparent that serotonin (5-HT) and dopamine (DA) interactions play a key role in normal and abnormal behavior. Therefore, disclosure of this interaction could reveal important insights into the pathogenesis of various neuropsychiatric diseases including schizophrenia, depression and drug addiction or neurological conditions such as Parkinson's disease and Tourette's syndrome. Unfortunately, this interaction remains difficult to study for many reasons, including the rich and widespread innervations of 5-HT and DA in the brain, the plethora of 5-HT receptors and the release of co-transmitters by 5-HT and DA neurons. The purpose of this review is to present electrophysiological and biochemical data showing that endogenous 5-HT and pharmacological 5-HT ligands modify the mesencephalic DA systems' activity. 5-HT receptors may control DA neuron activity in a state-dependent and region-dependent manner. 5-HT controls the activity of DA neurons in a phasic and excitatory manner, except for the control exerted by 5-HT_2C receptors which appears to also be tonically and/or constitutively inhibitory. The functional interaction between the two monoamines will also be discussed in view of the mechanism of action of antidepressants, antipsychotics, anti-Parkinsonians and drugs of abuse.

Activation of 5-HT₁A receptors in the medial subdivision of the central nucleus of the amygdala produces anxiolytic effects in a rat model of Parkinson's disease

Although the medial subdivision of the central nucleus of the amygdala (CeM) and serotonin-1A (5-HT1A) receptors are involved in the regulation of anxiety, their roles in Parkinson's disease (PD)-associated anxiety are still unknown. Here we assessed the importance of CeM 5-HT1A receptors for anxiety in rats with unilateral 6-hydroxydopamine (6-OHDA) lesion of the medial forebrain bundle (MFB). The lesion induced anxiety-like behaviors, increased the firing rate and burst-firing pattern of CeM γ-aminobutyric acid (GABA) neurons, as well as decreased dopamine (DA) levels in the striatum, medial prefrontal cortex (mPFC), amygdala and ventral part of hippocampus (vHip). Intra-CeM injection of the selective 5-HT1A receptor agonist 8-OH-DPAT produced anxiolytic effects in the lesioned rats, and decreased the firing rate of CeM GABAergic neurons in two groups of rats. Compared to sham-operated rats, the duration of the inhibitory effect on the firing rate of GABAergic neurons was shortened in the lesioned rats. The injection increased DA levels in the mPFC and amygdala in two groups of rats and the vHip in the lesioned rats, and increased 5-HT level in the lesioned rats, whereas it decreased NA levels in the mPFC in two groups of rats and the vHip in the lesioned rats. Moreover, the mean density of 5-HT1A receptor and GABA double-labeled neurons in the CeM was reduced after the lesioning. These results suggest that activation of CeM 5-HT1A receptor produces anxiolytic effects in the 6-OHDA-lesioned rats, which involves decreased firing rate of the GABAergic neurons, and changed monoamine levels in the limbic and limbic-related brain regions.

Serotonergic modulation of glutamate neurotransmission as a strategy for treating depression and cognitive dysfunction

Monoamine-based treatments for depression have evolved greatly over the past several years, but shortcomings such as suboptimal efficacy, treatment lag, and residual cognitive dysfunction are still significant. Preclinical and clinical studies using compounds directly targeting glutamatergic neurotransmission present new opportunities for antidepressant treatment, with ketamine having a surprisingly rapid and sustained antidepressant effect that is presumably mediated through glutamate-dependent mechanisms. While direct modulation of glutamate transmission for antidepressant and cognition-enhancing actions may be hampered by nonspecific effects, indirect modulation through the serotonin (5-HT) system may be a viable alternative approach. Based on localization and function, 5-HT can modulate glutamate neurotransmission at least through the 5-HT1A, 5-HT1B, 5-HT3, and 5-HT7 receptors, which presents a rational pharmacological opportunity for modulating glutamatergic transmission without the direct use of glutamatergic compounds. Combining one or more of these glutamate-modulating 5-HT targets with 5-HT transporter inhibition may offer new therapeutic opportunities. The multimodal compounds vortioxetine and vilazodone are examples of this approach with diverse mechanisms, and their different clinical effects will provide valuable insights into serotonergic modulation of glutamate transmission for the potential treatment of depression and associated cognitive dysfunction.

Activation of prelimbic 5-HT1A receptors produces antidepressant-like effects in a unilateral rat model of Parkinson's disease

Depression is a common symptom in Parkinson's disease (PD), but its pathophysiology remains unclear. Several lines of studies have revealed that the prelimbic (PrL) sub-region of the ventral medial prefrontal cortex and 5-HT1A receptors are involved in the regulation of depression. In this study, we examined whether complete unilateral lesions of the medial forebrain bundle (MFB) using 6-hydroxydopamine in rats are able to induce depressive-like behaviors, the role of PrL 5-HT1A receptors in the regulation of these behaviors, and co-localization of 5-HT1A receptor and neuronal glutamate transporter EAAC1-immunoreactive (EAAC1-ir) neurons in the PrL. The MFB lesions induced depressive-like responses as measured by the sucrose preference and forced swim tests when compared to sham-operated rats. The intra-PrL injection of 5HT1A receptor agonist 8-OH-DPAT (50, 100, and 500ng/rat) increased sucrose consumption, and decreased immobility time in both sham-operated and the lesioned rats, indicating the induction of antidepressant effects. Furthermore, the intra-PrL injection of 5HT1A receptor antagonist WAY-100635 (60, 120, and 240ng/rat) showed a decrease in sucrose consumption, and an increase in immobility time, indicating the induction of depressive-like responses. However, the effective doses in the lesioned rats were higher than those in sham-operated rats, which attribute to down-regulation of 5-HT1A receptor expression on EAAC1-ir neurons in the PrL of the lesioned rats. These findings suggest that unilateral lesions of the MFB in rats may induce depressive-like behaviors, and 5-HT1A receptors of the PrL play an important role in the regulation of these behaviors.

Unilateral lesion of the nigrostriatal pathway decreases the response of GABA interneurons in the dorsal raphe nucleus to 5-HT(1A) receptor stimulation in the rat

This study examined the firing rate and pattern of electrophysiologically and chemically identified GABA interneurons in the dorsal raphe nucleus (DRN), and role of 5-HT(1A) receptor agonist 8-OH-DPAT and the medial prefrontal cortex (mPFC) in the firing activity in rats with 6-hydroxydopamine lesions of the substantia nigra pars compacta (SNc). The interneurons in rats with lesions of the SNc showed a more burst-firing, while having no change in the firing rate; the mPFC and combined mPFC and SNc lesions in rats decreased the firing rate of the interneurons and firing pattern shifted towards a more burst-firing compared to rats with sham lesions of the SNc, respectively. In rats with sham lesions of the SNc, administration of 8-OH-DPAT (1-243 μg/kg, i.v.) produced excitatory-inhibitory, excitatory and inhibitory effects in the firing rate of individual interneurons. However, when these effects were averaged over the group, 8-OH-DPAT had no significant effect on firing rate. In rats with lesions of the SNc, mPFC and the paired lesions, 8-OH-DPAT, at the same doses, inhibited all interneurons tested, respectively. Cumulative doses producing inhibition in rats with the paired lesions were higher than that of rats with lesions of the mPFC. In contrast to rats with sham lesions of the SNc, SNc lesion reduced expression of 5-HT(1A) receptor on parvalbumin positive neurons in the DRN, a subpopulation of GABA interneurons. Our results indicate that the SNc and mPFC regulate the firing activity of GABA interneurons in the DRN. Furthermore, response of likely GABA interneurons to systemic administration of 8-OH-DPAT is altered by lesion of the SNc and mPFC.

Acute and chronic methylphenidate alters prefrontal cortex neuronal activity recorded from freely behaving rats

Today's students around the world are striking deals to buy and sell the drug methylphenidate (MPD) for cognitive enhancement. Our knowledge on the effects of MPD on the brain is very limited. The present study was designed to investigate the acute and chronic effect of MPD on the prefrontal cortex (PFC) neurons. On experimental day 1 (ED1) recordings were obtained following saline injections and after 2.5 mg/kg MPD. On ED2 through ED6, daily single 2.5 mg/kg MPD was given followed by 3 washout days (ED7 to 9). On ED10, neuronal recordings were resumed from the same animal after saline and MPD injection similar to that obtained at ED1. Ninety PFC units were recorded, all responded to the initial MPD injection, 66 units (73%) increased their activity at ED10. Recordings were resumed for the 66 units that increased their firing rate at ED1, and following MPD injection 54 units (82%) exhibited significant increases in their baseline firing rates compared to ED1 baseline. When these 54 units were rechallenged (chronic effect) with MPD, 39/54 (72%) exhibited reduction in their firing rate which can be interpreted as tolerance. From the 24 (27%) units that responded to MPD at ED1 by decreasing their activity, 14 units (58%) exhibited a decrease in their baseline firing rates at ED10 compared to ED1 baseline. However, following MPD rechallenge of these 14 units, 11 units (79%) exhibited an increase in their firing rate which is interpreted as sensitization. In conclusion, all PFC units modified their neural baseline activity.

In vivo effect of 5-HT₇ receptor agonist on pyramidal neurons in medial frontal cortex of normal and 6-hydroxydopamine-lesioned rats: an electrophysiological study

The 5-hydroxytryptamine (5-HT)-7 receptor began to be cloned and pharmacologically characterized close to 20 years ago. It couples positively via G-proteins to adenylyl cyclase and activation of this receptor increases neuronal excitability, and several studies have shown that degeneration of the nigrostriatal pathway leads to an impairment of 5-HT system. Here we reported that systemic and local administration of 5-HT₇ receptor agonist AS 19 produced excitation, inhibition and no change in the firing rate of pyramidal neurons in medial prefrontal cortex (mPFC) of normal and 6-hydroxydopamine-lesioned rats. In normal rats, the mean response of the pyramidal neurons to AS 19 by systemic and local administration in mPFC was excitatory. The inhibitory effect by systemic administration of AS 19 was reversed by GABA(A) receptor antagonist picrotoxinin. Systemic administration of picrotoxinin excited all the neurons examined in normal rats, and after treatment with picrotoxinin, the local administration of AS 19 further increased the firing rate of the neurons. In the lesioned rats, systemic administration of AS 19, at the same doses, also increased the mean firing rate of the pyramidal neurons. However, cumulative dose producing excitation in the lesioned rats was higher than that of normal rats. Systemic administration of AS 19 produced inhibitory effect in the lesioned rats, which was partially reversed by picrotoxinin. The local administration of AS 19, at the same dose, did not change the firing rate of the neurons in the lesioned rats. Systemic administration of picrotoxinin and the local administration of AS 19 did not affect the firing rate of the neurons in the lesioned rats. These results indicate that activity of mPFC pyramidal neurons is regulated through activation of 5-HT₇ receptor by direct or indirect action, and degeneration of the nigrostriatal pathway leads to decreased response of these neurons to AS 19, suggesting dysfunction and/or down-regulation of 5-HT₇ receptor on the pyramidal neurons and GABA interneurons in the lesioned rats.

Unilateral lesion of the nigrostriatal pathway decreases the response of fast-spiking interneurons in the medial prefrontal cortex to 5-HT1A receptor agonist and expression of the receptor in parvalbumin-positive neurons in the rat

5-Hydroxytryptamine(1A) (5-HT(1A)) receptors are expressed in the prefrontal cortical interneurons. Among these interneurons, calcium-binding protein parvalbumin (PV)-positive fast spiking (FS) interneurons play an important role in regulatory function of the prefrontal cortex. In the present study, the response of medial prefrontal cortex (mPFC) FS interneurons to the selective 5-HT(1A) receptor agonist 8-OH-DPAT and change in expression of 5-HT(1A) receptor on PV-positive neurons were examined in rats with 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra pars compacta (SNc) by using extracellular recording and double-labeling immunofluorescence histochemistry. Systemic administration of 8-OH-DPAT (1-243 μg/kg, i.v.) dose-dependently inhibited the mean firing rate of the FS interneurons in sham-operated and the lesioned rats, respectively. The cumulative doses producing inhibition in the lesioned rats (243 μg/kg) was significantly higher than that of sham-operated rats (27 μg/kg). Furthermore, the local application of 8-OH-DPAT (0.01 μg) in the mPFC inhibited the FS interneurons in sham-operated rats, while having no effect on firing rate of the FS interneurons in the lesioned rats. In contrast to sham-operated rats, the lesion of the SNc in rats did not cause the change of PV-positive neurons in the prelimbic prefrontal cortex, a subregion of the mPFC, whereas the lesion of the SNc markedly reduced in percentage of PV-positive neurons expressing 5-HT(1A) receptors. Our results indicate that degeneration of the nigrostriatal pathway results in the decreased response of FS interneurons in the mPFC to 5-HT(1A) receptor stimulation, which attributes to down-regulation of 5-HT(1A) receptor expression in these interneurons.

Alterations of emotion, cognition and firing activity of the basolateral nucleus of the amygdala after partial bilateral lesions of the nigrostriatal pathway in rats

Although increasing evidence indicates that psychiatric symptoms are crucial characteristic of the early stage of Parkinson's disease (PD) and precede motor impairments, the neuronal firing activity of the basolateral nucleus of the amygdala (BLA) in the psychiatric symptom of PD and the involved mechanism are still unclear. In the present study, we examined the changes in emotional and cognitive tests not focused on motor fluency and firing activity of projection neurons in the BLA rats with 6-hydroxydopamine (6-OHDA) injected bilaterally into dorsal striatum, and the effects of apomorphine and the medial prefrontal cortex (mPFC) on these changes. Injection of 6-OHDA (10.5 μg) into the dorsal striatum produced 18-22% and 26-30% loss of tyrosine hydroxylase immunoreactive neurons in the ventral tegmental area and substantia nigra pars compacta of rats, respectively. The striatal lesions induced anxiety-like responses in the rats but did not result in depressive-like behavior or cognitive impairments. In the lesioned rats, the firing rate of BLA projection neurons decreased significantly compared with sham-operated rats, and the firing pattern of BLA projection neurons was not changed. No significant differences were observed either in behaviors or firing activity of BLA projection neurons by further ibotenic acid lesions of the mPFC in the lesioned rats. Systemic administration of cumulative apomorphine (10-160 μg/kg) inhibited the firing rate of BLA projection neurons in sham-operated, 6-OHDA-lesioned and combined 6-OHDA- and mPFC-lesioned rats, but the latter needed more apomorphine stimulation. These data suggest that the anxiety in early stage of PD is possibly related to the decrease in firing activity of BLA projection neurons, which may be regulated by the activation of dopamine receptor in the mPFC.

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.

The pyramidal neurons in the medial prefrontal cortex show decreased response to 5-hydroxytryptamine-3 receptor stimulation in a rodent model of Parkinson's disease

In the present study, effect of SR 57227A, a selective 5-hydroxytryptamine-3 (5-HT(3)) receptor agonist, on the firing activity of pyramidal neurons in the medial prefrontal cortex (mPFC) was studied in normal rats and rats with 6-hydroxydopamine lesions of the substantia nigra pars compacta by using extracellular recording. Systemic administration of SR 57227A (40-640 μg/kg, i.v.) decreased the mean firing rate of pyramidal neurons in normal and the lesioned rats. This inhibition was significant only at doses higher than 320 μg/kg and 640 μg/kg in normal and the lesioned rats, respectively, and was reversed by i.v. administration of 5-HT(3) receptor antagonist tropisetron or GABA(A) receptor antagonist bicuculline. Furthermore, local application of SR 57227A (0.01 μg) in the mPFC inhibited the firing rate of pyramidal neurons in normal rats while having no effect on firing rate in the lesioned rats. The i.v. administration of bicuculline excited the pyramidal neurons in normal rats, and then local application of SR 57227A did not alter the mean firing rate of these neurons. However, these two drugs did not affect the activity of the pyramidal neurons in the lesioned rats. We conclude that activation of 5-HT(3) receptors inhibited pyramidal neurons in the mPFC of normal rats via GABAergic interneurons, and degeneration of the nigrostriatal pathway decreased response of the pyramidal neurons to SR 57227A, suggesting the dysfunction of 5-HT(3) receptors and/or down-regulation of the expression on GABAergic interneurons in the lesioned rats.

A combined solenoid-surface RF coil for high-resolution whole-brain rat imaging on a 3.0 Tesla clinical MR scanner

Rat brain models effectively simulate a multitude of human neurological disorders. Improvements in coil design have facilitated the wider utilization of rat brain models by enabling the utilization of clinical MR scanners for image acquisition. In this study, a novel coil design, subsequently referred to as the rat brain coil, is described that exploits and combines the strengths of both solenoids and surface coils into a simple, multichannel, receive-only coil dedicated to whole-brain rat imaging on a 3.0 T clinical MR scanner. Compared with a multiturn solenoid mouse body coil, a 3-cm surface coil, a modified Helmholtz coil, and a phased-array surface coil, the rat brain coil improved signal-to-noise ratio by approximately 72, 61, 78, and 242%, respectively. Effects of the rat brain coil on amplitudes of static field and radiofrequency field uniformity were similar to each of the other coils. In vivo, whole-brain images of an adult male rat were acquired with a T(2)-weighted spin-echo sequence using an isotropic acquisition resolution of 0.25 x 0.25 x 0.25 mm(3) in 60.6 min. Multiplanar images of the in vivo rat brain with identification of anatomic structures are presented. Improvement in signal-to-noise ratio afforded by the rat brain coil may broaden experiments that utilize clinical MR scanners for in vivo image acquisition.

Chronic, systemic treatment with a metabotropic glutamate receptor 5 antagonist produces anxiolytic-like effects and reverses abnormal firing activity of projection neurons in the basolateral nucleus of the amygdala in rats with bilateral 6-OHDA lesions

Although 2-methyl-6-(phenylethynyl)-pyridine (MPEP), a selective metabotropic glutamate receptor 5 antagonist, improves the motor symptoms of Parkinson's disease (PD), the effects of MPEP on the psychiatric symptom of PD and the mechanism involved are still unclear. In the present study, we examined the effects of MPEP in anxiolytic-like behavior and firing activity of projection neurons in the basolateral nucleus of the amygdala (BLA) in rats with 6-hydroxydopamine (6-OHDA) injected bilaterally into dorsal striatum. Rats were divided into three groups, sham-operated group, 6-OHDA lesion with vehicle treatment group and 6-OHDA lesion with MPEP treatment group. Injection of 6-OHDA (10.5 μg) into the dorsal striatum produced 31.5% loss of tyrosine hydroxylase immunoreactive (TH-ir) neurons in the SNpc. The 6-OHDA-lesioned rats showed anxiety behavior and the firing rate of BLA projection neurons decreased significantly compared with sham-operated rats, and no difference was found in the firing pattern of these neurons. Whereas chronic, systemic treatment of MPEP (3 mg/kg/day, i.p.; 14 days) attenuated loss of TH-ir neurons, produced anxiolytic-like effect and normalized the abnormal firing rate of projection neurons of the BLA in rats with the bilateral lesions. Systemic administration of cumulative apomorphine (10-160 μg/kg, i.v.) inhibited the firing rate of BLA projection neurons in sham-operated, 6-OHDA lesion with vehicle-treated and MPEP-treated rats, but the 6-OHDA lesion decreased the response of BLA projection neurons to apomorphine stimulation, while MPEP reversed the reactivity of these neurons. These data demonstrate that the partial lesion of the nigrostriatal pathway causes anxiety symptom and decreases firing rate of BLA projection neurons in the rat. Furthermore, chronic, systemic MPEP treatment has the neuroprotective and anxiolytic-like effects, and reverses the abnormal firing rate of BLA projection neurons, suggesting that MPEP has important implication for the treatment of PD.

Unilateral lesion of the nigrostriatal pathway decreases the response of interneurons in medial prefrontal cortex to 5-HT 2A/2C receptor stimulation in the rat

The aim of the present study was to investigate changes in the firing rate and pattern of interneurons in the medial prefrontal cortex (mPFC), and effects of 5-HT(2A/2C) receptor agonist DOI and antagonist ritanserin, and the selective 5-HT(2C) receptor antagonist SB 242084 on the neuronal firing in rats with 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra pars compacta (SNc) by extracellular recording in vivo. The lesion of the SNc decreased the firing rate of the interneurons compared to sham-lesioned rats, and firing pattern of these interneurons changed toward a more burst-firing. Administration of DOI (20-320 microg/kg, i.v.) dose-dependently increased the firing rate of all interneurons examined in sham-lesioned and the 6-OHDA-lesioned rats. The excitation was significant at doses higher than 40 microg/kg and 320 microg/kg in sham-lesioned and the 6-OHDA-lesioned rats, respectively. This dose, which produced marked effect in the 6-OHDA-lesioned rats, was much higher than that of sham-lesioned rats. The local application of DOI (5 microg) in mPFC increased the firing rate of the interneurons in sham-lesioned rats, while having no effect on the firing rate in the 6-OHDA-lesioned rats. The excitatory effect of DOI in sham-lesioned and the 6-OHDA-lesioned rats was completely or partially reversed by ritanserin or SB 242084. The results of our study show that lesion of the SNc leads to a decrease in the firing rate of interneurons in mPFC and fire with a more burst pattern, and decreased response of the interneurons to DOI in rat.