Cytisine-based nicotinic partial agonists as novel antidepressant compounds

Abstract

Nicotine and other nicotinic agents are thought to regulate mood in human subjects and have antidepressant-like properties in animal models. Recent studies have demonstrated that blockade of nicotinic acetylcholine receptors (nAChRs) including those containing the beta2 subunit (beta2(*)), results in antidepressant-like effects. Previous studies have shown that cytisine, a partial agonist at alpha4/beta2(*) nAChRs, and a full agonist at alpha3/beta4(*) and alpha7 nAChRs, has antidepressant-like properties in several rodent models of antidepressant efficacy; however, it is not clear whether more selective partial agonists will also be effective in these models. We tested cytisine and two derivatives, 5-bromo-cytisine (5-Br-Cyt) and 3-(pyridin-3'-yl)-cytisine (3-pyr-Cyt) for their ability to act as a partial agonist of different nAChR subtypes and to show antidepressant-like activity in C57/BL6 mice in the tail suspension, the forced-swim, and the novelty-suppressed feeding tests. 3-pyr-Cyt was a partial agonist with very low efficacy at alpha4/beta2(*) nAChRS but had no agonist effects at other nAChRs normally targeted by cytisine, and it was effective in mouse models of antidepressant efficacy. Animals showed dose-dependent antidepressant-like effects in all three behavioral paradigms. 5-Br-Cyt was not effective in behavioral tests when administered peripherally, probably because of its inability to penetrate the blood-brain barrier, because it efficiently reduced immobility in the tail suspension test when administered intraventricularly. These results suggest that novel nicotinic partial agonists may provide new possibilities for development of drugs to treat mood disorders.

Fig. 1.

Partial agonist properties of cytisine and derivatives at high- and low-sensitivity α4β4 nAChRs. Structure and activity of the compounds used in this study: cytisine (A and B), 3-pyr-Cyt (C and D), and 5-Br-Cyt (E and F). K_i values have been published previously and were determined by competition for [³H]epibatidine and [³H]methyllycaconitine binding sites using radioligand binding in rat forebrain (α4/β2^*, α7^*), pig adrenals (α3/β4^*), and T. californica electroplax [(α1)₂β1γδ] membrane fractions (Gündisch et al., 1999; Mukhin et al., 2000; Gohlke et al., 2002). The high-sensitivity form of α4β2[α4(2)β2(3)], was generated by the coexpression of RNA coding the β2–6-α4 concatamer along with monomeric β2 (Zhou et al., 2003). The low-sensitivity form of α4β2[α4(3)β2(2)] was generated by the coexpression of RNA coding the β2–6-α4 concatamer along with monomeric α4 (Zhou et al., 2003). The data plotted (B, D, and F) represent the average responses (±S.E.M.) from at least four oocytes at each concentration and have been normalized relative to the maximum ACh-evoked responses for each receptor subtype (see Materials and Methods).

Fig. 2.

Effects of 5-Br-Cyt and 3-pyr-Cyt on α4β2, α3β4, and α7 nAChR subtypes. 5-Br-Cyt (A) and 3-pyr-Cyt (B) were tested for their ability to activate α3β4 and α7 type nAChRs expressed in X. laevis oocytes and compared with activity at α4β2 nAChRs (Papke and Heinemann, 1994; Papke and Porter Papke, 2002; Papke et al., 2007). The data plotted represent the average responses (±S.E.M.) from at least four oocytes at each concentration and have been normalized relative to the ACh maximal responses for each receptor subtype (see Materials and Methods). C, effects of 3-pyr-Cyt on α4β2 receptors were also studied in coapplication experiments with ACh. Oocytes were tested for their responses to applications of 30 μM ACh, and these responses were compared with the responses evoked by 30 μM ACh coapplied with increasing concentrations of 3-pyr-Cyt. The coapplication responses of at least four oocytes (±S.E.M.) are plotted, normalized to the responses of the same oocytes to ACh alone. Note that in these experiments (A–C) the α4β2 receptors were formed from the coexpression of α4 and β2 monomers and so represent a mixed population of the high- and low-sensitivity subtypes.

Fig. 3.

Effects of cytisine, 3-pyr-Cyt, 5-Br-Cyt, nicotine, and fluoxetine in the tail suspension test. Total time spent immobile in the tail suspension test by C57BL/6J male mice treated with various doses of cytisine (A), 3-pyr-Cyt (B), 5-Br-Cyt (C), and nicotine and fluoxetine (D). n = 10/treatment group. The x-axis values indicate the dose injected in milligrams per kilogram. Error bars represent S.E.M.. *, p < 0.05; **, p < 0.01; and ***, p < 0.001.

Fig. 4.

Effects of cytisine, 3-pyr-Cyt, and 5-Br-Cyt in the forced swim test. Total time spent immobile in the forced swim test by C57BL/6J male mice treated with various doses of cytisine (A), 3-pyr-Cyt (B), 5-Br-Cyt (C), and nicotine and fluoxetine (D). n = 10/treatment group. The x-axis values indicate the dose injected in milligrams per kilogram. Error bars represent S.E.M.. *, p < 0.05; **, p < 0.01; ***, p < 0.001.

Fig. 5.

Effects of acute cytisine, nicotine, 3-pyr-Cyt, and 5-Br-Cyt on locomotor activity. Horizontal activity was measured as total number of beam breaks in C57BL/6J male mice 20 min after cytisine, 3-pyr-Cyt, 5-Br-Cyt, or nicotine. n = 10/treatment group. Error bars represent S.E.M.

Fig. 6.

Effects of cytisine, 3-pyr-Cyt, and 5-Br-Cyt in the novelty suppressed feeding test. Time required for C57BL/6J male mice to initiate a feeding episode in the novelty suppressed feeding test after chronic injection (15 days) of various doses of cytisine (A), 3-pyr-Cyt (B), and 5-Br-Cyt (C). n = 10/treatment group. The x-axis values indicate the dose injected in milligrams per kilogram. Error bars represent S.E.M. *, p < 0.05; **, p < 0.01; and ***, p < 0.001.

Fig. 7.

Effects of centrally administered 5-Br-Cyt in the tail suspension test. Total time spent immobile in the tail suspension test by C57BL/6J male mice after 50 ng i.c.v. 5-Br-Cyt. n = 8/treatment group. Error bars represent S.E.M. *, p < 0.05.