Functional characterization of a 5‐HT₃ receptor which modulates the release of 5‐HT in the guinea‐pig brain

The aims of the present study were to confirm the modulation by 5‐HT 3 receptors of the electrically evoked release of tritium from slices preloaded with [ 3 H]‐5‐HT of guinea‐pig frontal cortex, hippocampus and hypothalamus, and to assess their functional role in 5‐HT release. The selective 5‐HT 3 agonist, 2‐methyl‐5‐HT, introduced 8 min before the electrical stimulation, enhanced in a concentration‐dependent manner the evoked release of [ 3 H]‐5‐HT in the three brain regions studied. The 5‐HT 3 agonists, phenylbiguanide and m ‐chlorophenyl‐biguanide, did not enhance the release of tritium in frontal cortex and hypothalamus slices. In hypothalamus slices, this response was lost when 2‐methyl‐5‐HT was introduced 20 min before the stimulation, thus indicating that these 5‐HT 3 receptors desensitize rapidly. When 2‐methyl‐5‐HT was added 20‐min before the first stimulation period to desensitize the 5‐HT 3 receptors, removed for 24 min, and then re‐introduced 8 min before the second stimulation period, the enhancing effect of 2‐methyl‐5‐HT was restored, thus indicating that these 5‐HT 3 receptors can rapidly regain normal sensitivity. The enhancing effect of 2‐methyl‐5‐HT was attenuated by the 5‐HT 3 receptor antagonists m ‐chloro‐phenylpiperazine = quipazine = ondansetron ≥ ICS 205–930 = BRL 24924 > MDL 72222 = zacopride. The 5‐HT reuptake blocker, paroxetine, enhanced the electrically evoked release of tritium when introduced 8 min before stimulation; this effect of paroxetine was blocked by ICS 205–930, thus indicating that these 5‐HT 3 receptors can be activated by endogenous 5‐HT. In the absence of electrical stimulation, 2‐methyl‐5‐HT (10 μ m ) produced a marked enhancement of the basal release of [ 3 H]‐5‐HT which was calcium‐dependent and blocked by S‐zacopride but not by paroxetine. The enhancing effect of 2‐methyl‐5‐HT was dependent both on the frequency of stimulation, as indicated by the attenuated effect of 120 stimulations delivered at 1 Hz instead of 5 Hz, and on the duration of the stimulation, as indicated by the more pronounced effect of pulses delivered at 5 Hz for 24 s instead of 72 s or 120 s.