Non-serotonergic potentiation by (-)-pindolol of DOI-induced forward locomotion in rats: possible involvement of beta-adrenoceptors?

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.

Decreased social behaviour following 3,4-methylenedioxymethamphetamine (MDMA) is accompanied by changes in 5-HT2A receptor responsivity

This study examined the involvement of the 5-HT(2A) receptor in the long-term anxiogenic effect of a brief exposure of young rats to 3,4-methylenedioxymethamphetamine (MDMA) using the social interaction and elevated plus-maze paradigms. Wistar rats (post-natal day (PND) 28) received either MDMA (5 mg/kg i.p.) or saline (1 ml/kg i.p.) hourly for 4 h on 2 consecutive days. Locomotor activity was measured for 60 min after the first injection and core body temperature was recorded at regular intervals over 4 h. On PND 84, without further drug administration, social interaction was assessed between treatment-matched rat pairs derived from separate litters. On PND 86, rats received either the 5-HT(2A/2C) receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI, 1 mg/kg i.p.) or saline and locomotor activity, wet-dog shakes and back muscle contractions were monitored. The change in elevated plus-maze behaviour was assessed following the same injection on PND 87. Acutely, MDMA produced a significant hyperlocomotion and hyperthermia (p<0.01). Following 55 days of abstinence, social interaction was reduced by 27% in MDMA pre-treated rats compared with that in controls (p<0.01). On the elevated plus-maze, pre-treatment with MDMA prevented the anxiogenic effect of DOI. On PND 92, hippocampal, frontal cortical and striatal 5-hydroxytryptamine (5-HT) was significantly reduced in MDMA pre-treated rats by between 16% and 22%, without any accompanying change in [(3)H]paroxetine binding in cortical homogenates. In conclusion, exposure of young rats to repeated MDMA caused serotonin depletion and induced 'anxiety-like' behaviour in the social interaction test accompanied by a long-lasting reduction in specific 5-HT(2A) receptor mediated behaviour.