Abstract
It is well established that serotonin (5-HT; 5-hydroxytryptamine) plays a role in mammalian regulation of the hypothalamic–pituitary–adrenal (HPA) axis via the 5-HT receptor subtype 1A (5-HT
1A). To date, there has not been a comprehensive investigation of the molecular, pharmacological and physiological aspects of the 5-HT
1A receptor and its role in the activation of the hypothalamic–pituitary–interrenal (HPI) axis in teleost fish. The 5-HT
1A receptor of the Gulf toadfish (
Opsanus beta) was cloned and sequenced, showing 67.5% amino acid similarity to the human homologue. The 5-HT
1A receptor was distributed throughout the brain, with the whole brain containing significantly higher levels of 5-HT
1A mRNA compared to all other tissues and the midbrain/diencephalon region containing significantly higher levels of transcript than any other brain region. Substantial levels of transcript were also found in the pituitary, while very low levels were in the kidney that contains the interrenal cells.
Xenopus oocytes injected with toadfish 5-HT
1A receptor cRNA displayed significantly higher binding of [
3H]5-HT that was abolished by the mammalian 5-HT
1A receptor agonist, 8-OH-DPAT, indicating a conserved binding site of the toadfish 5-HT
1A receptor and a high specificity for the agonist. Supporting this, binding of [
3H]5-HT was not affected by the mammalian 5-HT
1B receptor agonist, 5-nonyloxytryptamine, the 5-HT
7 receptor antagonist, SB269970, or the 5-HT
2 receptor agonist, α-methylserotonin. Confirming these molecular and pharmacological findings, intravenous injection of 8-OH-DPAT stimulated the HPI axis to cause a 2-fold increase in circulating levels of cortisol. The present study of the 5-HT
1A receptor in a single teleost species illustrates the high conservation of this 5-HT receptor amongst vertebrates.
