The ontogeny of tolerance to the 5-HT1A agonist 8-OH-DPAT: a study in the rat

Effects of repeated treatment with the 5-HT1A and 5-HT1B agonists (R)-( +)-8-hydroxy-DPAT and CP-94253 on the locomotor activity and axillary temperatures of preweanling rats: evidence of tolerance and behavioral sensitization

RATIONALE

Drugs that stimulate 5-HT_1A/1B receptors produce both tolerance and behavioral sensitization in adult rats and mice, yet it is unknown whether the same types of plasticity are evident during earlier ontogenetic periods.

OBJECTIVE

The purpose of this study was to determine whether repeated treatment with selective 5-HT_1A and/or 5-HT_1B agonists cause tolerance and/or sensitization in preweanling rats.

METHODS

In Experiments 1 and 2, male and female preweanling rats were given a single pretreatment injection of saline, the 5-HT_1A agonist (R)-( +)-8-hydroxy-DPAT (8-OH-DPAT), or the 5-HT_1B agonist CP-94253 on PD 20. After 48 h, rats received a challenge injection of 8-OH-DPAT or CP-94253, respectively. In Experiment 3, rats were pretreated with saline or DPAT + CP on PD 20 and challenged with the same drug cocktail on PD 22. In Experiment 4, the tolerance- or sensitization-inducing properties of 8-OH-DPAT, CP-94253, or DPAT + CP were tested after a 4-day pretreatment regimen on PD 17-20.

RESULTS

On the first pretreatment day, 8-OH-DPAT, CP-94253, and DPAT + CP increased locomotion and caused hypothermia. Repeated treatment with 8-OH-DPAT (2 or 8 mg/kg) or DPAT + CP caused locomotor sensitization in preweanling rats. In contrast, tolerance to the hypothermic effects of 8-OH-DPAT (8 mg/kg), CP-94253 (5-20 mg/kg), or DPAT + CP was evident after repeated drug treatment.

CONCLUSIONS

During the preweanling period, a single injection of a selective 5-HT_1A or 5-HT_1B agonist is capable of producing drug-induced plasticity. A pretreatment administration of 8-OH-DPAT causes both tolerance (hypothermia) and behavioral sensitization (locomotor activity) in preweanling rats, whereas repeated CP-94253 treatment results in tolerance.

Age differences in the sensitivity to clomipramine in an animal model of obsessive-compulsive disorder

RATIONALE

Subtypes of obsessive-compulsive disorder (OCD) related to age could determine differential response to treatment.

OBJECTIVES

To explore possible age differences in the effect of clomipramine in an animal model of OCD.

METHODS

The deficits on spontaneous alternation produced by 8-OH-DPAT and the preventing actions of clomipramine, desipramine and WAY 100635 were compared between young and adult rats.

RESULTS

No age differences were found in spontaneous alternation. The 5-HT(1A) agonist, 8-OH-DPAT (0.031, 0.125, 0.5 and 2.0 mg/kg, -15 min) produced perseveration in young and adult rats. However, young rats were sensitive to a lower dose of 8-OH-DPAT. Clomipramine (10 mg/kg per three administrations) completely prevented the action of 8-OH-DPAT (0.5 mg/kg) in adult rats. However, this treatment as well as higher doses (15 mg/kg 3 administrations) or injected for longer periods (10 mg/kg 5 administrations) produced weak protective effects (versus 0.125 mg/kg 8-OH-DPAT) or had no action (versus 0.5 mg/kg 8-OH-DPAT) in young animals. WAY 100 635 (0.5 mg/kg) blocked the action of 8-OH-DPAT (0.5 mg/kg) in both young and adult rats. Desipramine (10 mg/kg/3 administrations) lacked of a preventive effect on the 8-OH-DPAT (0.5 mg/kg) action. This result indicated that the 5-HT(1A) receptor is involved in the deficits on spontaneous alternation produced by 8-OH-DPAT.

CONCLUSIONS

The present data shows important age differences in the effect of clomipramine in a model of OCD. Such differences could be relevant for the age variations in the response to treatment in clinical practice.

Effect of the serotonin agonist 8-OH-DPAT on the sensorimotor system of the rat

8-Hydroxy-2-(di-n-propylamino)-tetralin hydrobromide (8-OH-DPAT, 2 mg/kg) is used to induce perseverative behavior in rats in a T-maze as a model for obsessive-compulsive disorder (OCD). Using the open-field test, radiant heat test, and the test with von Frey filaments, we examined whether alterations in sensorimotor functioning could contribute to the perseverative tendencies in this model by measuring differences in left versus right hind paw reactions after 8-OH-DPAT administration (2 mg/kg, sc). Also, the effect of repeated 8-OH-DPAT administration on sensorimotor functioning was tested every third day. 8-OH-DPAT administration induced a significantly decreased sensorimotor performance in the open-field test, an increased threshold for noxious thermal stimulation (increased withdrawal latency, WL, and decreased elevation time, ET) in the radiant heat test, and a decreased nociceptive threshold for mechanical stimulation in the test with von Frey filaments. All changes in sensorimotor functioning were similar for left and right hind paws suggesting that, these changes as measured with the tests in the present study, are not likely to contribute to the perseverative behavior of rats in a T-maze. Further, repeated administration of 8-OH-DPAT had no effect in the radiant heat test and the test with the Frey filaments, but produced a tolerance effect in the open-field test.