Reduced nociceptive effects of intravenous serotonin (5–HT) in the spontaneously hypertensive rat

Abstract

This study compared the cutaneous (baseline tail-flick (TF) latencies) and visceral nociceptive responses (5-HT-induced inhibition of the TF reflex and pseudaffective responses) in lightly pentobarbital-anesthetized 16 week old Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). SHR were significantly less sensitive to the nociceptive effect of i.v. 5–HT compared to WKY rats. That is, SHR had a greater quantal ED50 for inhibition of the TF reflex than WKY rats (50.0 and 15.1 μg/kg, respectively). SHR also showed the presence of distinct pseudaffective responses (flattening of the ears, closure of the eyes to slits and contraction of the facial musculature) at much greater doses than in WKY rats, thereby paralleling the same changes in sensitivity observed the 5-HT-induced inhibition of the TF reflex. 5-HT-induced similar cardiovascular responses in WKY and SHR except that the SHR showed a significantly greater pressor response compared to WKY rats. These results demonstrate that the SHR show a marked reduction in sensitivity to the nociceptive, but not the cardiovascular, responses to i.v. 5-HT.

Key Words:

• rat
• serotonin
• 5-HT
• visceral
• nociception
• WKY
• SHR
• Sprague-Dawley
• tail-flick reflex