Galmic, a nonpeptide galanin receptor agonist, affects behaviors in seizure, pain, and forced-swim tests

Abstract

The pharmacological exploitation of the galanin receptors as drug targets for treatment of epilepsy, depression, and pain has been hampered by the lack of workable compounds for medicinal chemists from random screening of large chemical libraries. The present work uses the tripeptidomimetic galnon and displays its presumed pharmacophores on a rigid molecular scaffold. The scaffold is related to marine natural products and presents three functional groups near one another in space, in a manner reminiscent of a protein surface. An active compound, Galmic, was identified from a small synthetic library and tested in vitro and in vivo for its affinity and efficacy at galanin receptors. Galmic has micromolar affinity for GalR1 receptors (Ki = 34.2 microM) and virtually no affinity for GalR2 receptors. In vitro, Galmic, like galanin, suppresses long-term potentiation in the dentate gyrus; it blocks status epilepticus when injected intrahippocampally or administered i.p. Galmic applied i.p. shows antidepressant-like effects in the forced-swim test, and it is a potent inhibitor of flinching behavior in the inflammatory pain model induced by formalin injection. These data further implicate brain and spinal cord galanin receptors as drug targets and provide an example of a systemically active compound based on a scaffold that mimics protein surfaces.

Scheme 1.

Platform structures that present C₃ symmetry. EDCI, 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide; HOBt, hydroxybenzotriazole; DIEA, N,N-diisopropylethylamine; DMF, dimethylformamide; TFA^–, trifluoroacetate; Boc, t-butoxycarbonyl.

Fig. 1.

Calculated structure of Galmic. The side chain amides have been arbitrarily rotated to show the most compact structure. In any conformation, the side chains appear on the same face of the macrocycle.

Fig. 2.

Effects of Galmic on long-term synaptic plasticity in mouse DG slices. Administration of Galmic (1 μM) for 10–15 min before LTP induction attenuated the early and late phases of LTP, compared with the DMSO-treated (0.03%) slices. The bar illustrates the duration of the vehicle (DMSO) or drug administration, and the arrow indicates the time of stimulation with high-frequency trains (tetanus). The effect of Galmic was significantly different from the effect of DMSO on slices at 2–6 min and 17–60 min after LTP induction. Administration of Galmic (1 μM) caused an effect similar to administration of galanin(1–29) (1 μM) in attenuating the LTP in DG. All values are mean ± SEM (t test: ★★, P < 0.005).

Fig. 3.

Effects of Galmic on SSSE. Galmic was injected into the DG ipsilateral to the stimulation site at 10 min (A) or 60 min (B) after PPS. Dose (in nanomoles) is indicated in the legend to the right of the graphs. SE, status epilepticus. (C) Intraperitoneal administration of Galmic 10 min after PPS attenuated SSSE at 2mg/kg, but not at 1 mg/kg. *, P < 0.05 versus control, one-way ANOVA plus Bonferroni t test.

Fig. 4.

Effects of Galmic on formalin-induced flinching. (A) Time course over 60 min of flinch responses after formalin paw injection in mice with i.p. injection of Galmic (9.8 μmol/kg, n = 5) or vehicle (30% DMSO, 10 μl, n = 8). Each point represents the number of flinches per min. (B) Histograms representing the dose–response effects of Galmic (doses in μmol/kg indicated in brackets, n = 5 each dose) on formalin test phase 1 and phase 2 flinching behavior of mice. The bars represent the total number of flinches in each phase: Ph-I, 1–9 min; Ph-II, 10–60 min; Ph-IIA, 10–40 min; and Ph-IIB, 41–60 min. The data are expressed as mean ± SEM. *, P < 0.05 versus the vehicle group, one-way ANOVA followed by Dunnett's tests.

Fig. 5.

Effects of treatment of rats with Galmic and vehicle (DMSO) in forced-swim and open-field tests. (Right) Activity was measured during a 10-min forced-swim test at 40 min after injection. Animals treated with Galmic (15 mg/kg i.p.) showed a significant increase in activity, compared with those treated with vehicle (50% DMSO), consistent with an antidepressant-like profile. (Left) Galmic (15 mg/kg i.p.) and vehicle were administered in the open-field test. Galmic significantly reduced locomotor activity in this task compared with vehicle-treated animals, without any obvious signs of sedation. Values represent group means (±SEM). Independent means t test: *, P < 0.01 versus control.