Pharmacology zyxwvutsrqpo & Toxicology zyxwvutsrqpo 1989, 65, 189-191. zyxwvutsr Both Single-Dose and Repeated Administration of Clomipramine Reduces the Behavioural Response to Intrathecal Capsaicin in Mice Jan Henrik Rosland, Steinar Hunskaar and KjeU Hole Department of Physiology, University of Bergen, Arstadveien 19, N-5009 Bergen, Norway (Received December 12, 1988; Accepted April 5, 1989) Abstract: Capsaicin injected intrathecally releases substance P from primary sensory nerve endings, and induces a pain related behaviour in mice consisting of licking, biting and scratching directed to the distal part of the body. This behavioural response was reduced by approximately 35% after intraperitoneal administration of clomipramine, 10 mg/ kg, 1 hr in advance, and by 38% after repeated administration (10 mg/kg/day for 9 days). Twenty-four hr after the last repeated clomipramine injection, the response after capsaicin was still reduced by 36%, indicating a long-lasting effect of repeated treatment. The results indicate that clomipramine has analgesic properties against pain of central origin both after single-dose and repeated administration. Tricyclic antidepressants (TCAs) may induce antinocicep- tion both in humans (Magni et zyxwvutsrqp al. 1987; Moore 1980) and in animals (De Felipe et al. 1986; Reichenberg et al. 1985). The mechanisms of the antinociceptive action are, however, still unclear. While some investigators claim that the anal- gesia is caused by interference with the emotional compo- nent of pain (Moore 1980; Walsh 1983), others have found a direct interaction with nociceptive systems in the CNS (Baraldi et al. 1983; Biegon & Samuel 1980). It has been suggested that TCAs may influence nocicep- tion both in the CNS and peripherally (Fasmer et al. 1987a). Using intrathecal injection of capsaicin as a nociceptive stimulus, it may be possible to test for central effects of analgesic drugs (Hunskaar et al. 1985). Capsaicin releases substance P (SP), and both drugs induce a similar pain related behaviour (Hunskaar et al. 1986). It is thus possible to mimic stimulation of SP containing primary sensory neurones by intrathecal injection of capsaicin. In this study we have investigated the antinociceptive effect of single-dose and repeated clomipramine administra- tion in mice on intrathecal capsaicin induced nociceptive behaviour. Materials and Methods Animals. Male albino mice (Bom: NMRI, Ry, Denmark) weighing 25-35 g were used in the experiments. The animals were housed in colony cages (8-10 mice in each) with free access to food and water prior to the experiments. They were maintained in climate and light controlled rooms (23+0.5", 12/12 hr dark/light cycle with lights on at 6 a.m.), and testing took place between 9 a.m. and noon. The animals were brought to the test room the day before testing, and were thus adapted to the testing environment for at least 18 hr. Two hr before testing, the animals were placed individually in standard macrolone cages (30 x 12 x 13 zyxwvuts cm), which also served as observation chambers after injection of capsaicin. Drugs and administration routes. Clomipramine HCl (Ciba-Geigy) was dissolved in 0.9% NaCl and injected intraperitoneally in a volume of 10 ml/kg. An equal volume of 0.9% NaCl was injected in control animals. Repeated injections were given at 10 a.m. every day. The dose was 10 mg/kg (calculated as salt) for the single-dose treatment, the repeated treatment was zyxw 10 mg/kg/dag for 9 days. In order to avoid interferenceof different drug treatments, mice housed in the same colony cage received the same repeated treatment. Capsaicin (Sigma, St. Louis, MO, U.S.A.) was dissolved in 0.9% NaCl containing 0.4% ethanol to a final concentration of 8 mg/ml. This vehicle has previously been shown not to have any detectable behavioural effect injected intrathecally (Hunskaar et al. 1986). A volume of 7.5 pI(60 ng capsaicin) was injected intrathecallybetween z L, and L6 using a 30 G needle connected to a microsyringe with a polyetylene tubing. Nociceptive testing. Intrathecal injection of capsaicin elicited a cau- dally directed licking, biting and scratchingbehaviour (Hunskaar et al. 1986). The total number of responses (licks, bites and scratches) directed to the body distal to the site of injection was counted during the first 120 zyxw sec. after injection. Animals receiving repeated clomipramine treatment were tested either 1 hr or 24 hr after the last injection, single-dose treated animals were tested 1 hr after clomipramine injection. All groups of animal were handled in the same way during the injection period. The control animals and the single-dose treated animals received injections with 0.9% NaCl when the other animals had their clomipramine injection. The ani- mals which were to be tested 24 hr after the last injection, had their first clomipramine injection one day before the other repeated clomipramine group (table 1). Thus all groups could be tested on the same day using a single control group. Each mouse was used on one occasion only, and testing was performed by an observer unaware of the drug treatment. Statistics. Because of marked differences between variances for the different treatments (Fmax(,,,=9.6, P < 0.001, test for differences among several independent variances), the data were analysed by a non-parametric test, the Mann-Whitney U-test, the level of signifi- cance was set to 5%. Results Single-dose administration of clomipramine reduced the mean number of responses induced by intrathecal capsaicin by 35% compared to controls (U=27, Z=2.40, P<O.O2)