Targeting muscarinic receptor subtypes as a therapeutic approach in dystonia

D Klisko, P Jenner, S Rose. King's College London, London, UK

Anticholinergics are effective in non-levodpoa-responsive dystonia, particularly in juveniles (1), however, they elicit peripheral and central side effects which reduce compliance due to lack of selectivity (1). M4 muscarinic receptors are specifically located in the striatum, and are therefore, a target for the treatment of dystonia with a reduced side effects profile (2). This study aimed to investigate whether selective M4 receptor antagonists can reduce dystonia without inducing the M1/M3-mediated peripheral side effects.

All experiments complied with the UK Animals (Scientific Procedure) Act 1986. Male Wistar rats (150 – 300 g; n=6 – 8) were treated with pilocarpine (0.1 – 32 mg/kg ip) and the number of purposeless perioral movements (PPM) was recorded for 1 min every 10 min (3). Additionally, saliva secretion was measured following administration of pilocarpine (0.1 – 8 mg/kg ip) by oral introduction of a cotton bud for 10 s (4). Subsequently, non-selective and subtype selective antimuscarinics were administered ip or ICV (Table 1), 30 min prior to pilocarpine (ED50=3.4 mg/kg ip) and PPM and salivation measured as stated above. For ICV pirenzepine administration, a stainless steel cannula was stereotaxically implanted into the right lateral ventricle (coordinates AP: -0.8; ML: -1.4; DV: -2.8 mm relative to bregma) (3) under isoflurane anaesthesia. Injections were made using a microinjection pump (2 µl at a rate 1 µl/min). Data are expressed as mean ± SEM and analysed by non-linear regression (ED50 and ID50), Friedman test followed by Dunn’s test for PPM and one-way ANOVA followed by Dunnett’s test for salivation.

Systemic administration of pilocarpine (0.1 – 32 mg/kg ip) dose-dependently induced PPM (EC50=3.4 mg/kg). PPM was significantly inhibited in a dose-dependent manner, by peripheral and central administration of antimuscarinics (Table 1). Finally, pilocarpine-stimulated sialorrhea was also reduced by all antimuscarinics, except pirenzepine (Table 1).

Table 1 ID50 values for antimuscarinics on pilocarpine-induced perioral movements and sialorrhea

All peripherally administered antimuscarinics, regardless of their relative selectivity, inhibited pilocarpine-induced PPM and reduced saliva production. Trihexyphenidyl and benztropine were more potent in preventing chewing than other compounds (Table 1). Systemic administration of M3 antagonist almost completely inhibited sialorrhea, whereas ICV administration of pirenzepine inhibited PPM but did not suppress salivation, suggesting the involvement of centrally located muscarinic receptor in perioral movements but not saliva production. However, since none of the compounds were highly selective for the M4 receptor, further investigation with more selective compounds is needed to elucidate the role of M4 muscarinic receptor in production of involuntary movements.

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