Selective Regulation of 5-HT1B Serotonin Receptor Expression in the Striatum by Dopamine Depletion and Repeated L-DOPA Treatment: Relationship to L-DOPA-Induced Dyskinesias

Abstract

Dopamine and serotonin in the basal ganglia interact in a bidirectional manner. On the one hand, serotonin (5-HT) receptors regulate the effects of dopamine agonists on several levels, ranging from molecular signaling to behavior. These interactions include 5-HT receptor-mediated facilitation of dopamine receptor-induced gene regulation in striatal output pathways, which involves the 5-HT1B receptor and others. Conversely, there is evidence that dopamine action by psychostimulants regulates 5-HT1B receptor expression in the striatum. To further investigate the effects of dopamine and agonists on 5-HT receptors, we assessed the expression of 5-HT1B and other serotonin receptor subtypes in the striatum after unilateral dopamine depletion by 6-OHDA and subsequent treatment with L-DOPA (5 mg/kg; 4 weeks). Neither dopamine depletion nor L-DOPA treatment produced significant changes in 5-HT2C, 5-HT4 or 5-HT6 receptor expression in the striatum. In contrast, the 6-OHDA lesion caused a (modest) increase in 5-HT1B mRNA levels throughout the striatum. Moreover, repeated L-DOPA treatment markedly further elevated 5-HT1B expression in the dopamine-depleted striatum, an effect that was most robust in the sensorimotor striatum. A minor L-DOPA-induced increase in 5-HT1B expression was also seen in the intact striatum. These changes in 5-HT1B expression mimicked changes in the expression of neuropeptide markers (dynorphin, enkephalin mRNA) in striatal projection neurons. After repeated L-DOPA treatment, the severity of L-DOPA-induced dyskinesias and turning behavior was positively correlated with the increase in 5-HT1B expression in the associative, but not sensorimotor, striatum ipsilateral to the lesion, suggesting that associative striatal 5-HT1B receptors may play a role in L-DOPA-induced behavioral abnormalities.

The Multimodal Serotonergic Agent Vilazodone Inhibits L-DOPA-Induced Gene Regulation in Striatal Projection Neurons and Associated Dyskinesia in an Animal Model of Parkinson’s Disease

“…; in 0.9% saline; Sigma-Aldrich, St Louis, MO, USA) 30 min prior to surgery. After being deeply anesthetized with isoflurane vapors (2–5%), they received an infusion of either 6-OHDA (6-OHDA HBr, Sigma-Aldrich; 8 μg/4 μL, in saline containing 0.1% ascorbic acid) or saline/ascorbic acid (sham lesion) into the right medial forebrain bundle, as previously described [ 33 ]. The coordinates used were (in mm): AP, −4.3 (from bregma); ML, 1.6; DV, −8.3 (from dura) [ 54 ].…”

“…Coronal sections (12 µm) were thaw-mounted onto glass slides (Superfrost/Plus, Daigger, Wheeling, IL, USA), dried on a slide warmer, and stored at −30 °C. In situ hybridization histochemistry was performed, as described before [ 33 , 60 ]. Oligonucleotide probes (48-mers; Invitrogen, Rockville, MD, USA) were labeled with [35S]-dATP.…”

“…The probes had the following sequence: dynorphin, complementary to bases 862–909, GenBank accession number M10088; enkephalin, bases 436–483, M28263; zif268, bases 352–399, M18416; 5-HT1B (Htr1b), bases 62–109, NM 022225. Hybridization and washing procedures were as reported [ 33 , 61 ]. The sections were apposed to X-ray film (BioMax MR-2, Kodak) for 3–11 days.…”

“…These neuronal changes comprise “supersensitive” postsynaptic (D1) dopamine receptors in the direct pathway (striatonigral) MSNs (dMSNs) [ 22 , 23 , 24 ], which, in the presence of supraphysiological dopamine levels, produce aberrant second messenger signaling [ 9 , 24 , 25 ], resulting in facilitated neuronal activity (e.g., [ 26 ]) and enhanced transmitter release (e.g., [ 13 , 27 ]), as well as dysregulated gene expression [ 8 , 9 ] in these neurons. Studies have shown that L-DOPA treatment after dopamine lesions produces changes in the regulation of thousands of genes in MSNs (e.g., [ 9 , 28 , 29 , 30 ]), including those encoding various neuropeptides, receptors, and transcription factors, and several of these have been directly related to the occurrence or severity of L-DOPA-induced dyskinesia (e.g., [ 31 , 32 , 33 , 34 ]; [ 9 ], for review).…”

“…For one, we focused on the expression of two neuropeptide markers—dynorphin and enkephalin—which are selectively expressed by dMSNs and indirect pathway (striatopallidal) MSNs (iMSNs), respectively [ 53 ], as a means to determine the projection neuron subtypes impacted by this treatment. We also assessed the effects of these drugs on the expression of a transcription factor, zif268, and the 5-HT1B serotonin receptor [ 33 ], both of which are regulated by dopamine agonists. Changes in gene regulation were mapped throughout the striatum in order to identify the functional domains affected.…”