Anticataleptic effects of 5-HT(1B) receptors in the globus pallidus

Simultaneous 5-HT1BR agonist/5-HT6R antagonist action as a potential treatment of Parkinson's disease and its comorbidities

Parkinson's disease (PD) treatment focuses mainly on the augmentation of dopamine transmission, but to alleviate adverse motor effects accompanying L-DOPA use, additional treatments with serotonergic (5-HT) medications may be considered. We propose a novel concept based on the simultaneous activation of 5-HT1BR and 5-HT6R blockade as a putative therapeutic option for PD treatment. We have developed PZKKN-94, a dual human 5-HT1BR agonist (EC50 = 39 nM) and human 5-HT6R antagonist (KB = 7.7 nM), with selectivity over 43 targets, favorable drug-like properties, and brain penetration. Importantly, PZKKN-94 potency was increased or retained at rat 5-HT1B and 5-HT6 orthologs but not at mouse 5-HT6. Therefore, PZKKN-94 was tested in 2 rat disease models: haloperidol-induced catalepsy and 6-hydroxydopamine-induced sensorimotor deficits in rats, showing antiparkinsonian-like effects in both. Of note, PZKKN-94 did not affect the therapeutic effects of L-DOPA and attenuated L-DOPA-induced motor fluctuation ("on-off" phenomena) in the stepping and vibrissae tests. PZKKN-94 had no effect on L-DOPA-induced contralateral rotation, suggesting no impact on dopamine-mimetic medication effects. In addition, PZKKN-94 reversed scopolamine-, phencyclidine-, and aged-induced learning deficits in the rat novel object recognition test, increased cognitive flexibility in the attention set-shifting task, and displayed antidepressant-like actions in the forced swim test in rats. Our data suggest that dual-acting 5-HT1BR agonists/5-HT6R antagonists provide a novel therapeutic approach to alleviate both motor symptoms and accompanying cognitive and depression comorbidities in PD. Our present findings highlight the dual 5-HT1BR agonist/5-HT6R antagonist strategy to simultaneously spare L-DOPA's action and alleviate motor fluctuations related to L-DOPA treatment. SIGNIFICANCE STATEMENT: The commonly used L-DOPA-based medications for Parkinson's disease, though effective in alleviating initial disease states, are limited in long-term use due to the motor (dyskinesia and on-off phenomena) and nonmotor (psychotic-like) side effects. A novel nondopaminergic strategy for treatment of Parkinson's disease based on simultaneous activation of the 5-HT1B receptor and blockade of the 5-HT6 receptor is proposed. The compound PZKKN-94 produces an antiparkinsonian-like effect and attenuates motor fluctuations, preserving the efficacy of L-DOPA. In addition, PZKKN-94 demonstrates procognitive and antidepressant-like properties.

Angiotensin II increases the firing activity of pallidal neurons and participates in motor control in rats

The globus pallidus has emerged as a crucial node in the basal ganglia motor control circuit under both healthy and parkinsonian states. Previous studies have shown that angiotensin II (Ang II) and angiotensin subtype 1 receptor (AT1R) are closely related to Parkinson's disease (PD). Recent morphological study revealed the expression of AT1R in the globus pallidus of mice. To investigate the functions of Ang II/AT1R on the globus pallidus neurons of both normal and parkinsonian rats, electrophysiological recordings and behavioral tests were performed in the present study. Electrophysiological recordings showed that exogenous and endogenous Ang II mainly excited the globus pallidus neurons through AT1R. Behavioral tests further demonstrated that unilateral microinjection of Ang II into the globus pallidus induced significantly contralateral-biased swing in elevated body swing test (EBST), and bilateral microinjection of Ang II into the globus pallidus alleviated catalepsy and akinesia caused by haloperidol. AT1R was involved in Ang II-induced behavioral effects. Immunostaining showed that AT1R was expressed in the globus pallidus of rats. On the basis of the present findings, we concluded that pallidal Ang II/AT1R alleviated parkinsonian motor deficits through activating globus pallidus neurons, which will provide a rationale for further investigations into the potential of Ang II in the treatment of motor disorders originating from the basal ganglia.

Interaction between the 5-HT system and the basal ganglia: functional implication and therapeutic perspective in Parkinson's disease

The neurotransmitter serotonin (5-HT) has a multifaceted function in the modulation of information processing through the activation of multiple receptor families, including G-protein-coupled receptor subtypes (5-HT1, 5-HT2, 5-HT4-7) and ligand-gated ion channels (5-HT3). The largest population of serotonergic neurons is located in the midbrain, specifically in the raphe nuclei. Although the medial and dorsal raphe nucleus (DRN) share common projecting areas, in the basal ganglia (BG) nuclei serotonergic innervations come mainly from the DRN. The BG are a highly organized network of subcortical nuclei composed of the striatum (caudate and putamen), subthalamic nucleus (STN), internal and external globus pallidus (or entopeduncular nucleus in rodents, GPi/EP and GPe) and substantia nigra (pars compacta, SNc, and pars reticulata, SNr). The BG are part of the cortico-BG-thalamic circuits, which play a role in many functions like motor control, emotion, and cognition and are critically involved in diseases such as Parkinson's disease (PD). This review provides an overview of serotonergic modulation of the BG at the functional level and a discussion of how this interaction may be relevant to treating PD and the motor complications induced by chronic treatment with L-DOPA.