5-HT 4 receptor antagonism potentiates inhibition of intestinal allodynia by 5-HT 3 receptor antagonism in conscious rats M.I. Smith * , S.E. Banner, G.J. Sanger SmithKline Beecham Pharmaceuticals, New Frontiers Science Park, Third Avenue, Harlow, Essex, CM19 5AW, UK Received 9 March 1999; received in revised form 23 June 1999; accepted 25 June 1999 Abstract Acute levels of distension were applied by balloon to the colo-rectal region in conscious rats and visceromotor responses observed as abdominal muscle contraction; the threshold was typically between 10±40 mmHg. In saline- pretreated rats, the selective 5-HT 3 (granisetron) and 5-HT 4 (SB-207266) receptor antagonists had no effects on the visceromotor thresholds. 5-Hydroxytryptophan 10 mg/kg, subcutaneously (s.c.) decreased the distension threshold, indicating mechanical allodynia. This increased sensitivity was dose-dependently inhibited by granisetron but was unaffected by SB-207266 100 mg/kg, s.c., a dose which maximally and selectively antagonizes at 5-HT 4 receptors. However, this dose of SB-207266 potentiated the inhibitory activity of submaximally-effective doses of granisetron, reducing the ED 50 from 0.83 to 0.02 mg/kg, s.c., but without changing the maximum response or the bell-shaped nature of the dose-response curve for granisetron. These data suggest that 5-HT 4 receptor activation enhances the ability of 5- HT 3 receptor activation to induce intestinal allodynia. q 1999 Elsevier Science Ireland Ltd. All rights reserved. Keywords: 5-Hydroxytryptamine; Intestinal allodynia; SB-207266; Rats 5-Hydroxytryptamine (5-HT) acting at 5-HT 3 receptors has little-or-no role to play in normal visceral or somatic nociceptive processing but this receptor may be involved in pathological visceral hyperalgesia, as indicated by the abil- ity of 5-HT to evoke mechanical allodynia within the intes- tine [1,8]. Similarly, 5-HT 4 receptor antagonism does not affect normal pseudoaffective or visceromotor re¯exes evoked by noxious levels of colo-rectal distension in anaes- thetized or conscious rats [7,10], but may exert anti-hyper- algesic/ -allodynic activity in at least some pathological conditions. Thus, antagonism at this receptor has been shown to reduce nociceptive behaviours in models of rat paw in¯ammation (acute administration of formalin [3]), mouse writhing (intraperitoneal administration of acetic acid [4]) and rat intestinal hypersensitivity (increased sensi- tivity to colo-rectal distension by restraint stress or by acute colo-rectal irritation with acetic acid [10]). These data may be explained, at least in part, if there is a positive linkage between the 5-HT 4 receptor and tetrodotoxin-insensitive Na 1 currents in type 2, capsaicin-sensitive dorsal root gang- lion cells [2]. In an attempt to reproduce and study this type of data, we looked for an ability of selective 5-HT 4 receptor antagonism (SB-207266 [15]) to inhibit the hypersensitivity to colo- rectal distension caused by 5-hydroxytryptophan (5-HTP), in the absence and presence of selective 5-HT 3 receptor antagonism (granisetron [11]). This model of visceral mechanical allodynia is sensitive to inhibition by granise- tron and certain other 5-HT 3 receptor antagonists [1]. However, in contrast to the reports of antinociceptive activ- ity associated with 5-HT 4 receptor antagonism (see above), no evidence of such activity was found in this model. Instead, we found evidence for a synergistic interaction between 5-HT 4 and 5-HT 3 receptor antagonism, suggesting that 5-HT 4 receptor activation can enhance the ability of 5- HT 3 receptor activation to induce intestinal allodynia. Male Wistar rats (180±450 g), housed individually and fasted overnight, were used in these studies. Experiments were conducted in compliance with the Home Of®ce guidance on the Operation of the Animals (Scienti®c Proce- dures) Act 1986 and was reviewed and approved by the SmithKline Beecham procedures review panel. Under halothane (4±5%) anaesthesia, a 6±7 cm long latex balloon was inserted intra-anally to a position approximately 1cm beyond the ano-rectal verge. The cannula from the balloon was taped to the tail to prevent its expulsion and was connected to a pressure transducer linked to a pen recorder via a three-way tap and a peristaltic pump. Throughout the Neuroscience Letters 271 (1999) 61±64 0304-3940/99/$ - see front matter q 1999 Elsevier Science Ireland Ltd. All rights reserved. PII: S0304-3940(99)00513-3 * Corresponding author.