Effects of peripherally restricted κ opioid receptor agonists on pain-related stimulation and depression of behavior in rats

Abstract

κ opioid receptor agonists that do not readily cross the blood-brain barrier are peripherally restricted and distribute poorly to the central nervous system after systemic administration. Peripherally restricted κ agonists have promise as candidate analgesics, because they may produce antinociception mediated by peripheral κ receptors more potently than they produce undesirable sedative and psychotomimetic effects mediated by central κ receptors. The present study used assays of pain-related stimulation and depression of behavior in rats to compare effects of 1) two peripherally restricted κ agonists [the tetrapeptide D-Phe-D-Phe-D-Ile-D-Arg-NH(2) (ffir) and the nonpeptidic compound ((R,S)-N-[2-(N-methyl-3,4-dichlorophenylacetamido)-2-(3-carboxyphenyl)-ethyl]pyrrolidine hydrochloride (ICI204448)], 2) a centrally penetrating κ agonist (salvinorin A), and 3) several reference drugs, including a nonsteroidal anti-inflammatory drug (NSAID; ketoprofen). Intraperitoneal injection of dilute lactic acid served as a noxious stimulus to stimulate a stretching response and depress intracranial self-stimulation (ICSS) maintained by the delivery of electrical brain stimulation to the medial forebrain bundle. Acid-stimulated stretching was blocked by ketoprofen, the peripherally restricted κ agonists, and salvinorin A. However, acid-induced depression of ICSS was blocked only by ketoprofen. The peripherally restricted κ agonists had little effect, and salvinorin A exacerbated acid-induced depression of ICSS. These results suggest that peripherally restricted κ agonists may be safer than centrally penetrating κ agonists but less efficacious than NSAIDS or μ opioid receptor agonists to block pain-related depression of behavior; however, the peripheral selectivity of ffir and ICI204448 is limited, and future studies with κ agonists capable of greater peripheral selectivity are warranted.

Fig. 1.

Structures of peripherally restricted κ agonists. Top, ffir. Bottom, ICI204448.

Fig. 2.

Effects of the κ agonists ffir (a, c, and e) and salvinorin A (b, d, and f) on acid-stimulated stretching in rats. a and b, abscissae, dose in milligram/kilogram of ffir (30 min pretreatment) or salvinorin A (10 min pretreatment). c and d, abscissae, time in minutes after administration of 3.2 mg/kg ffir or 3.2 mg/kg salvinorin A. e and f, abscissae, dose of norBNI in milligram/kilogram administered 24 h before 3.2 mg/kg ffir (30-min pretreatment) or 3.2 mg/kg salvinorin A (10-min pretreatment). Ordinates: percentage of vehicle baseline number of stretches observed after treatment with κ agonist vehicle + lactic acid (baseline ± S.E.M. = 31.0 ± 3.3 for ffir and 28.4 ± 4.2 for salvinorin A). * indicates significantly different from vehicle (Veh), and # indicates significantly different from 10 min (c and d) or 0 mg/kg norBNI (e and f) as determined by a significant one-factor ANOVA followed by Dunnett's (a and b) or Neuman-Keuls (c–f) post hoc test (p < 0.05). All bars show mean + S.E.M. for six rats.

Fig. 3.

Depression of ICSS by lactic acid. a, abscissa, brain stimulation frequency in log Hz. Ordinate, rate of ICSS expressed as %MCR. Filled points indicate significantly different from lactic acid vehicle as determined by a significant two-factor ANOVA followed by the Holm-Sidak post hoc test (p < 0.05). Average baseline data collected before testing are shown by the gray line, but these data were not included in statistical analysis. b, abscissa, treatment with lactic acid vehicle (LA Veh) or 1.8% lactic acid (1.8% LA). Ordinate, percentage of baseline number of stimulations per component. # indicates significantly different from LA Veh as determined by paired t test (p < 0.05). All data show mean ± S.E.M. from 34 rats used in the study (n = 6 for ffir, salvinorin A, ketoprofen, and flupenthixol; n = 5 for ICI204448 and cocaine).

Fig. 4.

Effects of ffir (a, c, and e) and salvinorin A (b, d, and f) on ICSS. a to d, κ agonist effects on full frequency-rate curves. Abscissae, brain stimulation frequency in log Hz. Ordinates, rate of ICSS expressed as %MCR. a and b, data for κ agonists administered as a pretreatment to lactic acid vehicle (control ICSS). c and d, data for κ agonists administered as a pretreatment to 1.8% lactic acid (acid-depressed ICSS). Filled points indicate significantly different from κ agonist vehicle as determined by a significant two-factor ANOVA followed by the Holm-Sidak post hoc test (p < 0.05). Error bars are not shown for clarity. e and f, summary data for each κ agonist. Abscissae, dose in milligram/kilogram. Ordinates, percentage of baseline number of stimulations per component (baseline ± S.E.M. = 275 ± 14.1 for ffir and 325 ± 23.5 for salvinorin A). * indicates significantly different from Veh as determined by a significant one-factor ANOVA followed by Dunnett's post hoc test (p < 0.05). # indicates significantly different from lactic acid vehicle as determined by a paired t test (p < 0.05). All data show results from six rats, and error bars in e and f show S.E.M.

Fig. 5.

Effects of ICI204448 on acid-induced stimulation of stretching (a) and acid-induced depression of ICSS (b–d). a and b, summary data for stretching and ICSS. Abscissae, dose of ICI204448 (mg/kg). a, ordinate, percentage of vehicle baseline number of stretches (baseline ± S.E.M. = 39.2 ± 2.9 stretches). b, ordinate, percentage of baseline number of stimulations per component (baseline ± S.E.M. = 305.8 ± 29.4). * indicates significantly different from Veh as determined by a significant one-factor ANOVA followed by Dunnett's post hoc test (p < 0.05). # indicates significantly different from lactic acid vehicle as determined by a paired t test (p < 0.05). c and d, full frequency-rate curves. Abscissae, brain stimulation frequency in log Hz. Ordinates, rate of ICSS expressed as %MCR. c, data for ICI204448 administered as a pretreatment to lactic acid vehicle (control ICSS). d, data for ICI204448 administered as a pretreatment to 1.8% lactic acid (acid-depressed ICSS). Filled points indicate significantly different from ICI204448 vehicle as determined by a significant two-factor ANOVA followed by the Holm-Sidak post hoc test (p < 0.05). Error bars are not shown for clarity. All data show mean results from six rats (stretching) or five rats (ICSS), and error bars in a and b show S.E.M.

Fig. 6.

Effects of ketoprofen (a and d), cocaine (b and e), and flupenthixol (c and f) on acid-induced stimulation of stretching (a–c) and acid-induced depression of ICSS (d–f). Abscissae, dose drug in milligram/kilogram. a to c, ordinate, percentage of vehicle baseline number of stretches [baseline ± S.E.M. = 29.1 ± 3.1 for ketoprofen (n = 5), 30.0 ± 7.1 for cocaine (n = 11), and 34.4 ± 8.7 for flupenthixol (n = 5)]. d to f, ordinate, percentage of baseline number of stimulations per component [baseline ± S.E.M. = 296.8 ± 25.5 for ketoprofen (n = 6), 347 ± 55.1 for cocaine (n = 5), and 308.6 ± 26.9 for flupenthixol (n = 6)]. * indicates significantly different from Veh as determined by a significant one-factor ANOVA followed by Dunnett's post hoc test (p < 0.05). # indicates significantly different from lactic acid vehicle in the right panels as determined by a paired t test (p < 0.05).