Adenosine A2A Receptor Modulates the Activity of Globus Pallidus Neurons in Rats

Adenosinergic Pathway in Parkinson's Disease: Recent Advances and Therapeutic Perspective

Parkinson's disease (PD) is a neurodegenerative disease characterized pathologically by α-synuclein (α-syn) aggregation. In PD, the current mainstay of symptomatic treatment is levodopa (L-DOPA)-based dopamine (DA) replacement therapy. However, the development of dyskinesia and/or motor fluctuations which is relevant to levodopa is restricting its long-term utility. Given that the ability of which is to modulate the striato-thalamo-cortical loops and function to modulate basal ganglia output, the adenosinergic pathway (AP) is qualified as a potential promising non-DA target. As an indispensable component of energy production pathways, AP modulates cellular metabolism and gene regulation in both neurons and neuroglia cells through the recognition and degradation of extracellular adenosine. In addition, AP is geared to the initiation, evolution, and resolution of inflammation as well. Besides the above-mentioned crosstalk between the adenosine and dopamine signaling pathways, the functions of adenosine receptors (A₁R, A_2AR, A_2BR, and A₃R) and metabolism enzymes in modulating PD pathological process have been extensively investigated in recent decades. Here we reviewed the emerging findings focused on the function of adenosine receptors, adenosine formation, and metabolism in the brain and discussed its potential roles in PD pathological process. We also recapitulated clinical studies and the preclinical evidence for the medical strategies targeting the Ado signaling pathway to improve motor dysfunction and alleviate pathogenic process in PD. We hope that further clinical studies should consider this pathway in their monotherapy and combination therapy, which would open new vistas to more targeted therapeutic approaches.

Intrapallidal injection of cannabidiol or a selective GPR55 antagonist decreases motor asymmetry and improves fine motor skills in hemiparkinsonian rats

Cannabidiol (CBD) presents antiparkinsonian properties and neuromodulatory effects, possibly due to the pleiotropic activity caused at multiple molecular targets. Recently, the GPR55 receptor has emerged as a molecular target of CBD. Interestingly, GPR55 mRNA is expressed in the external globus pallidus (GPe) and striatum, hence, it has been suggested that its activity is linked to motor dysfunction in Parkinson’s disease (PD). The present study aimed to evaluate the effect of the intrapallidal injection of both CBD and a selective GPR55 antagonist (CID16020046) on motor asymmetry, fine motor skills, and GAD-67 expression in hemiparkinsonian rats. The hemiparkinsonian animal model applied involved the induction of a lesion in male Wistar rats via the infusion of the neurotoxin 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle via stereotaxic surgery. After a period of twenty days, a second surgical procedure was performed to implant a guide cannula into the GPe. Seven days later, lysophosphatidylinositol (LPI), CBD, or CID16020046 were injected once a day for three consecutive days (from the 28th to the 30th day post-lesion). Amphetamine-induced turning behavior was evaluated on the 14th and 30th days post-injury. The staircase test and fine motor skills were evaluated as follows: the rats were subject to a ten-day training period prior to the 6-OHDA injury; from the 15th to the 19th days post-lesion, the motor skills alterations were evaluated under basal conditions; and, from the 28th to the 30th day post-lesion, the pharmacological effects of the drugs administered were evaluated. The results obtained show that the administration of LPI or CBD generated lower levels of motor asymmetry in the turning behavior of hemiparkinsonian rats. It was also found that the injection of CBD or CID16020046, but not LPI, in the hemiparkinsonian rats generated significantly superior performance in the staircase test, in terms of the use of the forelimb contralateral to the 6-OHDA-induced lesion, when evaluated from the 28th to the 30th day post-lesion. Similar results were also observed for superior fine motor skills performance for pronation, grasp, and supination. Finally, the immunoreactivity levels were found to decrease for the GAD-67 enzyme in the striatum and the ipsilateral GPe of the rats injected with CBD and CID16020046, in contrast with those lesioned with 6-OHDA. The results obtained suggest that the inhibitory effects of CBD and CID16020046 on GPR55 in the GPe could be related to GABAergic overactivation in hemiparkinsonism, thus opening new perspectives to explain, at a cellular level, the reversal of the motor impairment observed in PD models.

In vivo Bidirectional Modulation of Cannabinoid on the Activity of Globus Pallidus in Rats

Endocannabinoids are bioactive substances which participate in central motor control. The globus pallidus (GP) is a major nucleus in the basal ganglia circuit, which plays an important function in movement regulation. Both cannabinoid receptor type 1 (CB₁R) and cannabinoid receptor type 2 (CB₂R) are expressed in the GP suggesting GP as a main action area of endocannabinoids. To investigate the direct electrophysiological and behavioral effects of cannabinoids in GP, in vivo single unit extracellular recordings and behavioral tests were performed in rats. Administration of WIN 55,212-2 exerted three neuronal response patterns from all sampled neurons of GP, including (1) increase of the firing rate; (2) decrease of the firing rate; (3) increase and then decrease of the firing rate. Selectively blocking CB₁R by AM 251 decreased the firing rate and increased the firing rate. Selectively blocking CB₂R by AM 630 did not change the firing rate significantly, which suggested that endocannabinoids modulated the spontaneous firing activity of pallidal neurons mainly via CB₁R. Furthermore, co-application of AM 251, but not AM 630, blocked WIN 55,212-2-induced modulation of firing activity of pallidal neurons. Finally, both haloperidol-induced postural behavioral test and elevated body swing test (EBST) showed that unilateral microinjection of WIN 55,212-2 mainly induced contralateral-biased swing and deflection behaviors. Meanwhile, AM 251 produced opposite effect. The present in vivo study revealed that cannabinoids produced complicated electrophysiological and behavioral effects in the GP, which further demonstrated that the GP is a major functional region of endocannabinoid.

Cannabidiol as a Therapeutic Target: Evidence of its Neuroprotective and Neuromodulatory Function in Parkinson's Disease

The phytocannabinoids of Cannabis sativa L. have, since ancient times, been proposed as a pharmacological alternative for treating various central nervous system (CNS) disorders. Interestingly, cannabinoid receptors (CBRs) are highly expressed in the basal ganglia (BG) circuit of both animals and humans. The BG are subcortical structures that regulate the initiation, execution, and orientation of movement. CBRs regulate dopaminergic transmission in the nigro-striatal pathway and, thus, the BG circuit also. The functioning of the BG is affected in pathologies related to movement disorders, especially those occurring in Parkinson’s disease (PD), which produces motor and non-motor symptoms that involving GABAergic, glutamatergic, and dopaminergic neural networks. To date, the most effective medication for PD is levodopa (l-DOPA); however, long-term levodopa treatment causes a type of long-term dyskinesias, l-DOPA-induced dyskinesias (LIDs). With neuromodulation offering a novel treatment strategy for PD patients, research has focused on the endocannabinoid system (ECS), as it participates in the physiological neuromodulation of the BG in order to control movement. CBRs have been shown to inhibit neurotransmitter release, while endocannabinoids (eCBs) play a key role in the synaptic regulation of the BG. In the past decade, cannabidiol (CBD), a non-psychotropic phytocannabinoid, has been shown to have compensatory effects both on the ECS and as a neuromodulator and neuroprotector in models such as 6-hydroxydopamine (6-OHDA), 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), and reserpine, as well as other PD models. Although the CBD-induced neuroprotection observed in animal models of PD has been attributed to the activation of the CB1 receptor, recent research conducted at a molecular level has proposed that CBD is capable of activating other receptors, such as CB2 and the TRPV-1 receptor, both of which are expressed in the dopaminergic neurons of the nigro-striatal pathway. These findings open new lines of scientific inquiry into the effects of CBD at the level of neural communication. Cannabidiol activates the PPARγ, GPR55, GPR3, GPR6, GPR12, and GPR18 receptors, causing a variety of biochemical, molecular, and behavioral effects due to the broad range of receptors it activates in the CNS. Given the low number of pharmacological treatment alternatives for PD currently available, the search for molecules with the therapeutic potential to improve neuronal communication is crucial. Therefore, the investigation of CBD and the mechanisms involved in its function is required in order to ascertain whether receptor activation could be a treatment alternative for both PD and LID.

Apremilast regulates acute effects of ethanol and other GABAergic drugs via protein kinase A-dependent signaling

Phosphodiesterase type 4 (PDE4) inhibitors prevent hydrolysis of cyclic adenosine monophosphate and increase protein kinase A (PKA)-mediated phosphorylation. PDE4 inhibitors also regulate responses to ethanol and GABAergic drugs. We investigated mechanisms by which the PDE4 inhibitor, apremilast, regulates acute effects of ethanol and GABAergic drugs in male and female mice. Apremilast prolonged the sedative-hypnotic effects of gaboxadol, zolpidem, and propofol but did not alter etomidate effects, and unexpectedly shortened the sedative-hypnotic effects of diazepam. Apremilast prolonged rotarod ataxia induced by zolpidem, propofol, and loreclezole, shortened recovery from diazepam, but had no effect on ataxia induced by gaboxadol or etomidate. The PKA inhibitor H-89 blocked apremilast's ability to prolong the sedative-hypnotic effects of ethanol, gaboxadol, and propofol and to prolong ethanol- and propofol-induced ataxia. H-89 also blocked apremilast's ability to shorten the sedative-hypnotic and ataxic effects of diazepam. The β1-specific antagonist, salicylidene salicylhydrazide (SCS), produced faster recovery from ethanol- and diazepam-induced ataxia, but did not alter propofol- or etomidate-induced ataxia. SCS shortened the sedative-hypnotic effects of ethanol and diazepam but not of propofol. In Xenopus oocytes, a phosphomimetic (aspartate) mutation at the PKA phosphorylation site in β1 subunits decreased the maximal GABA current in receptors containing α1 or α3, but not α2 subunits. In contrast, phosphomimetic mutations at PKA sites in β3 subunits increased the maximal GABA current in receptors containing α1 or α2, but not α3 subunits. The GABA potency and allosteric modulation by ethanol, propofol, etomidate, zolpidem, flunitrazepam, or diazepam were not altered by these mutations. We propose a model whereby apremilast increases PKA-mediated phosphorylation of β1-and β3-containing GABA_A receptors and selectively alters acute tolerance to ethanol and GABAergic drugs.

Orexins alleviate motor deficits via increasing firing activity of pallidal neurons in a mouse model of Parkinson's disease

Orexin is a peptide neurotransmitter released in the globus pallidus. Morphological evidence reveals that both orexin 1 receptor (OX₁R) and orexin 2 receptor (OX₂R) exist in the globus pallidus. Here we showed that bilateral microinjection of both orexin-A and orexin-B into the globus pallidus alleviated motor deficits in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced parkinsonian mice. Further in vivo extracellular single-unit recording revealed that the basal spontaneous firing rate of the globus pallidus neurons in MPTP parkinsonian mice was slower than that of normal mice. Application of orexin-A or orexin-B significantly increased the spontaneous firing rate of pallidal neurons. The influx of Ca²⁺ through the L-type Ca²⁺ channel is the major mechanism involved in orexin-induced excitation in the globus pallidus. Orexin-A-induced increase in firing rate of pallidal neurons in MPTP parkinsonian mice was stronger than that of normal mice. Orexin-A exerted both electrophysiological and behavioral effects mainly via OX₁R, and orexin-B exerted the effects via OX₂R. Endogenous orexins modulated the excitability of globus pallidus neurons mainly through OX₁R. The present behavioral and electrophysiological results suggest that orexins ameliorate parkinsonian motor deficits through increasing the spontaneous firing of globus pallidus neurons.

Adenosine A2A and histamine H3 receptors interact at the cAMP/PKA pathway to modulate depolarization-evoked [3H]-GABA release from rat striato-pallidal terminals

We previously reported that the activation of histamine H₃ receptors (H₃Rs) selectively counteracts the facilitatory action of adenosine A_2A receptors (A_2ARs) on GABA release from rat globus pallidus (GP) isolated nerve terminals (synaptosomes). In this work, we examined the mechanisms likely to underlie this functional interaction. Three possibilities were explored: (a) changes in receptor affinity for agonists induced by physical A_2AR/H₃R interaction, (b) opposite actions of A_2ARs and H₃Rs on depolarization-induced Ca²⁺ entry, and (c) an A_2AR/H₃R interaction at the level of adenosine 3',5'-cyclic monophosphate (cAMP) formation. In GP synaptosomal membranes, H₃R activation with immepip reduced A_2AR affinity for the agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride hydrate (CGS-21680) (K_i control 4.53 nM; + immepip 9.32 nM), whereas A_2AR activation increased H₃R affinity for immepip (K_i control 0.63 nM; + CGS-21680 0.26 nM). Neither A_2AR activation nor H₃R stimulation modified calcium entry through voltage-gated calcium channels in GP synaptosomes, as evaluated by microfluorometry. A_2AR-mediated facilitation of depolarization-evoked [2,3-³H]-γ-aminobutyric acid ([³H]-GABA) release from GP synaptosomes (130.4 ± 3.6% of control values) was prevented by the PKA inhibitor H-89 and mimicked by the adenylyl cyclase activator forskolin or by 8-Bromo-cAMP, a membrane permeant cAMP analogue (169.5 ± 17.3 and 149.5 ± 14.5% of controls). H₃R activation failed to reduce the facilitation of [³H]-GABA release induced by 8-Bromo-cAMP. In GP slices, A_2AR activation stimulated cAMP accumulation (290% of basal) and this effect was reduced (- 75%) by H₃R activation. These results indicate that in striato-pallidal nerve terminals, A_2ARs and H₃Rs interact at the level of cAMP formation to modulate PKA activity and thus GABA release.