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The muscarinic antagonists scopolamine and atropine are competitive antagonists at 5-HT 3 receptors

View Article: PubMed Central - PubMed

ABSTRACT

Scopolamine is a high affinity muscarinic antagonist that is used for the prevention of post-operative nausea and vomiting. 5-HT3 receptor antagonists are used for the same purpose and are structurally related to scopolamine. To examine whether 5-HT3 receptors are affected by scopolamine we examined the effects of this drug on the electrophysiological and ligand binding properties of 5-HT3A receptors expressed in Xenopus oocytes and HEK293 cells, respectively. 5-HT3 receptor-responses were reversibly inhibited by scopolamine with an IC50 of 2.09 μM. Competitive antagonism was shown by Schild plot (pA2 = 5.02) and by competition with the 5-HT3 receptor antagonists [3H]granisetron (Ki = 6.76 μM) and G-FL (Ki = 4.90 μM). The related molecule, atropine, similarly inhibited 5-HT evoked responses in oocytes with an IC50 of 1.74 μM, and competed with G-FL with a Ki of 7.94 μM. The reverse experiment revealed that granisetron also competitively bound to muscarinic receptors (Ki = 6.5 μM). In behavioural studies scopolamine is used to block muscarinic receptors and induce a cognitive deficit, and centrally administered concentrations can exceed the IC50 values found here. It is therefore possible that 5-HT3 receptors are also inhibited. Studies that utilise higher concentrations of scopolamine should be mindful of these potential off-target effects.

No MeSH data available.


Effects of atropine on the electrophysiological responses to 5-HT and binding of G-FL. (A) Concentration-inhibition of the 2 μM 5-HT response by co-applied atropine. For each oocyte the responses in the presence of antagonist are normalised to the peak current response to 5-HT alone and data represented as the mean ± S.E.M. for a series of oocytes. Curve fitting yielded a pIC50 of 5.76 ± 0.14 (IC50 = 1.74 μM, n = 5) and Hill Slope of 1.06 ± 0.05. (B) Flow cytometry, showing the competition of 10 nM G-FL (a fluorescent derivative of granisetron; Jack et al., 2015) and varying concentrations of atropine at 5-HT3 receptors expressed on the surface of live HEK 293 cells. The affinity (pKi = 5.10 ± 0.16, Ki = 7.94 μM, n = 5) of atropine calculated from these experiments was similar to that measured using electrophysiology.
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fig5: Effects of atropine on the electrophysiological responses to 5-HT and binding of G-FL. (A) Concentration-inhibition of the 2 μM 5-HT response by co-applied atropine. For each oocyte the responses in the presence of antagonist are normalised to the peak current response to 5-HT alone and data represented as the mean ± S.E.M. for a series of oocytes. Curve fitting yielded a pIC50 of 5.76 ± 0.14 (IC50 = 1.74 μM, n = 5) and Hill Slope of 1.06 ± 0.05. (B) Flow cytometry, showing the competition of 10 nM G-FL (a fluorescent derivative of granisetron; Jack et al., 2015) and varying concentrations of atropine at 5-HT3 receptors expressed on the surface of live HEK 293 cells. The affinity (pKi = 5.10 ± 0.16, Ki = 7.94 μM, n = 5) of atropine calculated from these experiments was similar to that measured using electrophysiology.

Mentions: Atropine is a structurally related muscarinic antagonist (Fig. 1). To test its pharmacological properties we performed measurements using electrophysiology and flow cytometry. In oocytes expressing 5-HT3 receptors, atropine did not elicit a response when applied alone, but it caused concentration-dependent inhibition of the 2 μM 5-HT-evoked response with a pIC50 of 5.76 ± 0.14 (IC50 = 1.74 μM, n = 5) and Hill Slope of 1.06 ± 0.05 (Fig. 5A). This yielded a Kb of 1.89 μM (Eq. (2)). Inhibition was fully reversible after 1 min of washing and was unaltered by pre-application (data not shown).


The muscarinic antagonists scopolamine and atropine are competitive antagonists at 5-HT 3 receptors
Effects of atropine on the electrophysiological responses to 5-HT and binding of G-FL. (A) Concentration-inhibition of the 2 μM 5-HT response by co-applied atropine. For each oocyte the responses in the presence of antagonist are normalised to the peak current response to 5-HT alone and data represented as the mean ± S.E.M. for a series of oocytes. Curve fitting yielded a pIC50 of 5.76 ± 0.14 (IC50 = 1.74 μM, n = 5) and Hill Slope of 1.06 ± 0.05. (B) Flow cytometry, showing the competition of 10 nM G-FL (a fluorescent derivative of granisetron; Jack et al., 2015) and varying concentrations of atropine at 5-HT3 receptors expressed on the surface of live HEK 293 cells. The affinity (pKi = 5.10 ± 0.16, Ki = 7.94 μM, n = 5) of atropine calculated from these experiments was similar to that measured using electrophysiology.
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fig5: Effects of atropine on the electrophysiological responses to 5-HT and binding of G-FL. (A) Concentration-inhibition of the 2 μM 5-HT response by co-applied atropine. For each oocyte the responses in the presence of antagonist are normalised to the peak current response to 5-HT alone and data represented as the mean ± S.E.M. for a series of oocytes. Curve fitting yielded a pIC50 of 5.76 ± 0.14 (IC50 = 1.74 μM, n = 5) and Hill Slope of 1.06 ± 0.05. (B) Flow cytometry, showing the competition of 10 nM G-FL (a fluorescent derivative of granisetron; Jack et al., 2015) and varying concentrations of atropine at 5-HT3 receptors expressed on the surface of live HEK 293 cells. The affinity (pKi = 5.10 ± 0.16, Ki = 7.94 μM, n = 5) of atropine calculated from these experiments was similar to that measured using electrophysiology.
Mentions: Atropine is a structurally related muscarinic antagonist (Fig. 1). To test its pharmacological properties we performed measurements using electrophysiology and flow cytometry. In oocytes expressing 5-HT3 receptors, atropine did not elicit a response when applied alone, but it caused concentration-dependent inhibition of the 2 μM 5-HT-evoked response with a pIC50 of 5.76 ± 0.14 (IC50 = 1.74 μM, n = 5) and Hill Slope of 1.06 ± 0.05 (Fig. 5A). This yielded a Kb of 1.89 μM (Eq. (2)). Inhibition was fully reversible after 1 min of washing and was unaltered by pre-application (data not shown).

View Article: PubMed Central - PubMed

ABSTRACT

Scopolamine is a high affinity muscarinic antagonist that is used for the prevention of post-operative nausea and vomiting. 5-HT3 receptor antagonists are used for the same purpose and are structurally related to scopolamine. To examine whether 5-HT3 receptors are affected by scopolamine we examined the effects of this drug on the electrophysiological and ligand binding properties of 5-HT3A receptors expressed in Xenopus oocytes and HEK293 cells, respectively. 5-HT3 receptor-responses were reversibly inhibited by scopolamine with an IC50 of 2.09 μM. Competitive antagonism was shown by Schild plot (pA2 = 5.02) and by competition with the 5-HT3 receptor antagonists [3H]granisetron (Ki = 6.76 μM) and G-FL (Ki = 4.90 μM). The related molecule, atropine, similarly inhibited 5-HT evoked responses in oocytes with an IC50 of 1.74 μM, and competed with G-FL with a Ki of 7.94 μM. The reverse experiment revealed that granisetron also competitively bound to muscarinic receptors (Ki = 6.5 μM). In behavioural studies scopolamine is used to block muscarinic receptors and induce a cognitive deficit, and centrally administered concentrations can exceed the IC50 values found here. It is therefore possible that 5-HT3 receptors are also inhibited. Studies that utilise higher concentrations of scopolamine should be mindful of these potential off-target effects.

No MeSH data available.