Direct comparison of cognitive facilitation by physostigmine and tetrahydroaminoacridine in two primate models

Cognitive facilitation by physostigmine and tetrahydroaminoacridine (THA) was compared in two primate models. Disruption of spatial delayed response performance by scopolamine (0.03 mg/kg) was fully reversed by coadministration of 5 doses of physostigmine in the range 0.03–0.08 mg/kg, but by only one dose (4.0 mg/kg) of THA; partial reversal of some effects of scopolamine was observed at 1 and 3 mg/kg of THA. Visual recognition memory was enhanced following treatment with 4 doses of physostigmine in the range 0.001–0.03 mg/kg. The effect of THA across the group of animals was not significant but performance tended to improve using a dose of 0.8 mg/kg. Our findings indicate that THA does not have a superior profile to physostigmine as a cognitive enhancer in primates.     

DAU 6215, a novel 5-HT3 receptor antagonist, improves performance in the aged rat in the Morris water maze task

The effects of the new 5-HT3 receptor antagonist, DAU 6215, on aged rats' cognition were assessed in the Morris water maze task. Task performance of aged animals that received acutely the dose of 10 μg/kg IP was not different than that of their aged controls treated with the vehicle. Conversely, a repeated IP administration of 10 μg/kg DAU 6215 for 3 weeks significantly improved task performance of the aged animals as compared to that displayed by the old rats treated with the vehicle.     

Age-related working memory deficits in the allocentric place discrimination task: possible involvement in cholinergic dysfunction

It is well known that learning and memory ability declines with aging. Age-related long-term changes in learning and memory ability in rats were investigated with the place navigation task and the allocentric place discrimination task (APDT) in a water maze using the same animals for each task. In a working memory place navigation task, aged animals could learn the location of the platform as well as when they were young, although strategy shifts were observed. In contrast, accuracy in the APDT significantly declined from 90% to 65% with aging. This impairment was ameliorated by an acetylcholine esterase inhibitor physostigmine at 22–23 months old. No amelioration was, however, detected in the same animals tested when they further aged to 26–27 months old. These results suggest that the APDT performance is sensitive to age-related memory deficits and that this may be due to the cholinergic dysfunction.     

Enhanced passive avoidance retention following posttrain serotonergic receptor antagonist administration in middle-aged and aged rats

The experiments examined the ability of posttrain administration of serotonergic receptor antagonists to attenuate the age-related deficits in avoidance retention normally exhibited by middle-aged (12 months) and aged (22 months) rats. Ketanserin (0, 1.0, 10.0 mg/kg) produced a significant dose-dependent increase in test step-through latencies in both age groups. The suppression of responding did not appear to be due to generalized learned aversion as nonshocked rats, injected with the highest dose of ketanserin, did not exhibit similar elevations in test latencies. In order to determine whether the ketanserin-induced effect could be generalized to other serotonergic antagonists, middle-aged and aged animals were injected with a single dose of mianserin (10.0 mg/kg). This antagonist also significantly increased step-through latencies in both age groups, while not affecting the performance of nonshocked animals. The results provide additional evidence for a role of the serotonergic nervous system in memory, and may have important implications in the development of effective treatment strategies for geriatric-related cognitive disorders.     

Modulation of morphine state-dependent learning by muscarinic cholinergic receptors of the ventral tegmental area

In the present study, the effects of bilateral intra-ventral tegmental area (intra-VTA) injections of an anticholinesterase, physostigmine and/or muscarinic acetylcholine receptor antagonist, atropine on memory retention and morphine state-dependent learning were examined in adult male Wistar rats. As a model of learning, a step-through passive avoidance task was used. Post-training subcutaneous administration of morphine (0.5, 2.5 and 5 mg/kg) dose-dependently impaired memory retrieval on the test day. Pre-test administration of morphine (2.5 and 5 mg/kg) induced state-dependent retrieval of the memory acquired under post-training morphine influence. Pre-test intra-VTA microinjection of physostigmine (0.5, 1 and 2 microg/rat) or atropine (1, 2 and 3 microg/rat) alone cannot affect memory retention. Interestingly, pre-test intra-VTA administration of physostigmine (1 and 2 microg/rat) reversed post-training morphine (5 mg/kg, s.c.)-induced retrieval impairment. Furthermore, pre-test intra-VTA microinjection of physostigmine (1 and 2 mug/rat) with an ineffective dose of morphine (0.5 mg/kg), synergistically improved memory performance impaired by post-training morphine. On the other hand, pre-test intra-VTA microinjection of atropine (2 and 3 microg/rat) 5 min before the administration of morphine (5 mg/kg, s.c.) dose-dependently inhibited morphine state-dependent memory. Pre-test atropine microinjection also reversed the influence of physostigmine on morphine response. It may be concluded that the muscarinic acetylcholine receptors of the VTA play an important role in morphine-induced recovery of memory, on the test day.     

Ionic mechanisms underlying excitatory effects of serotonin on embryonic rat motoneurons in long-term culture

The actions of serotonin were investigated on motoneurons isolated from embryonic day 14 rat spinal cord and enriched by metrizamide density gradient centrifugation. Trophic support was provided by a spinal cord glial monolayer, ciliary neurotrophic factor and heat-inactivated serum. Cultures were maintained for 17–83 days and investigated using whole-cell patch-clamp recording. Serotonin evoked slow depolarizations (6.2±0.7 or 9.3±1.3 mV in the presence of 6-cyano-7-nitroquinoxaline-2,3-dione and strychnine, ₅₀ 8.2 nM), which were reversibly blocked by 0.1 μM ketanserin. Serotonin generated synaptic potentials in motoneurons, lowered the threshold for repetitive firing and changed the slope of the current intensity–firing frequency relationship. The inward current evoked by serotonin (−147±15.2 pA) was ascribed to a complex ionic mechanism, which varied amongst neurons in the sampled population. It was due to closure of barium-sensitive potassium channels, effects on I_h and increase in a separate mixed cation current which comprised both transient voltage-sensitive and sustained components.

We conclude that serotonergic responses develop in motoneurons cultured under these conditions in the absence of serotonergic input, sensory neurons or many interneurons.     

Frontal cortex as the site of action of physostigmine in nbM-lesioned rats

The administration of a variety of cholinomimetic agents to nucleus basalis of Meynert-lesioned rats has been shown to alleviate their lesion-induced memory deficits. This experiment attempted to determine whether the frontal cortex was the site of the memory enhancing action of the cholinomimetic physostigmine. Different groups of rats received excitotoxic lesions of the basal forebrain, the frontal cortex or both. Immediately after one trial passive avoidance training, these rats were injected with either saline or a 0.06 mg/kg dose of physostigmine. Physostigmine enhanced the 72-hour retention test performance of sham-operated and basal forebrain-lesioned rats, but failed to affect the performance of rats with cortical lesions. These data were interpreted as consistent with the hypothesis that the memory-enhancing effects of physostigmine are at least partially mediated by the frontal cortex.     

Perceptual bisection in rats: the effects of physostigmine, scopolamine and pirenzepine.

The effects of cholinergic drugs on three different perceptual bisection tasks were studied in rats. Physostigmine (0.056-0.56 mg/kg), a reversible anticholinesterase, produced dose-dependent decrements in discriminability (A'), but did not affect the bisection point (BP) in visual duration, auditory duration, and auditory intensity bisection tasks. This finding is consistent with results previously obtained in an auditory duration bisection task with an irreversible anticholinesterase, diisopropyl phosphofluoridate. Scopolamine (0.075-0.422 mg/kg), a muscarinic cholinergic-receptor antagonist, produced dose-dependent decrements in both A' and BP in visual and auditory duration bisection tasks. The behavioral antagonism between physostigmine (0.56 mg/kg) and scopolamine (0.075-0.237 mg/kg) was studied in the visual and auditory duration bisection tasks. The BP was not affected by physostigmine alone or in combination with scopolamine, except at the largest dose of scopolamine, which produced a reliable decrement in the BP. A', however, was equally decreased by physostigmine alone and all combinations of physostigmine and scopolamine. Pirenzepine (1, 3 and 10 mg/kg), a selective high-affinity M1 muscarinic antagonist, had no effect on A' or the BP in the duration bisection tasks, suggesting changes in perception produced by muscarinic antagonists do not involve the M1 receptor subtype. The similar drug effects in different sensory modalities (visual and auditory) and perceptual systems (subjective duration and loudness) suggest that cholinergic drugs may affect perceptual mechanisms responsible for sensory coding, such as the output of a neural generator.     

Fluoxetine and 8-OH-DPAT in the lateral septum enhances and impairs retention of an inhibitory avoidance response in rats.

The present study investigated the role of lateral septal serotonin (5HT) in memory consolidation and the subtype of 5HT receptors involved in this process. Rats with cannulae implanted bilaterally into the lateral septum were trained in an inhibitory avoidance task. Immediately after training, the septal serotonergic function was manipulated by pharmacological agents selectively blocking 5HT reuptake (fluoxetine and zimelidine), antagonizing 5HT2 receptors (ketanserin and ritanserin), or activating 5HT1A receptors, respectively. Results indicated that direct fluoxetine infusions into the lateral septum at a dose of 6 micrograms/0.5 microliter and zimelidine at a dose of 5 micrograms/0.5 microliter both markedly enhanced memory. Intralateral septal injections of ketanserin (0.3 microgram/0.5 microliter and 0.5 microgram/0.5 microliter) and ritanserin (0.3 microgram/0.5 microliter and 0.6 microgram/0.5 microliter) did not have a significant effect by themselves on memory, and neither did they attenuate the memory-facilitating effect of fluoxetine in the same area. Intralateral septal infusions of 8-hydroxy-2-(di-n-propylamino)tetralin at 5 micrograms/0.5 microliter significantly impaired memory retention. These findings altogether support the notion that the lateral septal nuclei of rats are involved in the memory processes of inhibitory avoidance learning. Furthermore, postsynaptic 5HT receptor activation (not the 5HT2 receptor subtype) probably exerts a facilitatory effect while presynaptic 5HT1A receptor activation exerts an impairing effect on the memory consolidation process, probably due to autoreceptor inhibition of 5HT release.     

Well-maintained responses of acetylcholine release and blood flow in the cerebral cortex to focal electrical stimulation of the nucleus basalis of Meynert in aged rats

Responses of extracellular acetylcholine (ACh) release and blood flow in the cerebral cortex in the parietal lobe following focal electrical stimulation of the nucleus basalis of Meynert (NBM) ipsilateral to the parietal cortex were compared in healthy adult (6–8 months) and aged (27–28 months) Fischer-344 rats anesthetized with halothane. The focal electrical stimulation of the NBM produced an increase in the ACh release and blood flow in the parietal cortex in both the adult and aged rats. The increased responses of ACh and blood flow observed in the healthy, adult rats were well maintained in the aged rats.     

Intracellular studies on the dopamine-induced firing inhibition of neostriatal neurons in vitro: evidence for D1 receptor involvement

Intracellular recordings were obtained from rat neostriatal slices. Bath-applied dopamine (1–10 μM) produced a reversible inhibition of the action potentials evoked by direct stimulation and a decrease in the amplitude of the intrastriatally evoked depolarizing postsynaptic potentials. No change in membrane potential was detected during the application of 1–10 μM dopamine. Dopamine application also produced a decrease in anomalous rectification in the depolarizing direction. This subthreshold inward rectification was abolished by tetrodotoxin, but not by calcium-free and cadmium (0.1–1 mM)-containing solutions. The dopamine-induced decrease in excitatory postsynaptic potential amplitude was evident at resting membrane potential or at more positive levels, but was absent at hyperpolarized values of the membrane potential. Addition of bicuculline (50–500 μM) to the medium did not affect the inhibitory action of dopamine. The inhibitory action of dopamina also persisted in calcium-free and cadmium-containing solutions. The adenosine 3′,5′-cyclic monophosphate analogue, 8-bromo-adenosine 3′,5′-cyclic monophosphate (0.1–1 mM), mimicked the effects produced by D1 receptor activation. Bath application of 2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine (SKF 38393) (1–10 μM), a selective D1 dopaminergic agonist, mimicked the effects of micromolar concentrations of dopamine. The D2 dopaminergic agonists,4,4a,5,6,7,8,8a,9-octahydro-5-n-propyl-2H-pyrazolo-3,4-g-quinoline (LY 171555) and bromocriptine (both at 10 nM-10 μM), had no effects on neostriatal cells. The inhibition induced by micromolar doses of dopamine or SKF 38393 was antagonized by bath applications of R-( + )-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin-7-ol (SCH 23390; 0.1–10 μM), a D1-selective antagonist, but not by sulpiride (10nM–10μM), a D2 antagonist.

We conclude that the inhibitory effect of dopamine on rat striatal neurons is postsynaptically mediated by the activation of D1 dopaminergic receptors via the reduction of a voltage-dependent tetrodotoxin sensitive inward conductance.     

Activation of μ-opioid receptor modulates GABAA receptor-mediated currents in isolated spinal dorsal horn neurons

Whole-cell voltage-clamp technique was used to examine the effects of a μ-opioid receptor agonist DAGO (Tyr-D-Ala-Gly-Me-Phe-Gly-ol-enkephalin) on GABA-induced currents in acutely isolated spinal dorsal horn (DH) neurons from laminae I-IV of young rats. We found that a bicuculline-sensitive GABA-induced current was potentiated by DAGO (0.5–500 nM), in a dose-dependent manner, in 62% of the tested cells. The elevated GABA responses outlasted the period of DAGO application, and either recovered within 10 min after the removal of the peptide or persisted for up to 50 min. The potentiating effect of DAGO was reduced or prevented by naloxone and the μ-opioid receptor-selective antagonist β-funaltrexamine. A similar enhancing effect on the membrane currents activated by administration of muscimol, a GABA_A receptor-specific agonist, was produced by DAGO. In addition, a transient depression of GABA responses was observed in 25% of the cells tested. These results indicate that the μ-opioid agonist DAGO modulates the sensitivity of postsynaptic GABA_A receptors in a large proportion of spinal neurons from laminae I–IV, with the major effect being facilitation. The DAGO action could contribute to the regulation of the strength of primary afferent neurotransmission, including nociception.     

低剂量木黄酮降低缺血后神经元损伤并改善学习记忆功能

目的探讨染料木黄酮(GEN)对全脑缺血(GCI)大鼠海马CA1区神经元的神经保护作用及其可能的机制。方法建立大鼠4动脉结扎全脑缺血模型,实验动物随机分为假手术组(sham)、缺血再灌注组(I/R)、GEN处理组、ICI 182,780组和溶剂对照组。采用Fluoro-Jade B和神经元特异性核蛋白(NeuN)染色观察海马CA1区神经元存活情况,TUNEL技术观察海马CA1区神经元的凋亡。Morris水迷宫观察大鼠的空间学习和记忆功能。结果 GEN发挥神经保护作用的最佳剂量为1.0 mg/kg;与sham组相比,I/R组和溶剂对照组海马CA1区TUNEL阳性神经元数量显著增多(P0.01),而1.0 mg/kg GEN可显著降低缺血后TUNEL阳性神经元数量(P0.01);与I/R组相比,GEN能明显改善缺血后大鼠的空间学习和记忆能力。缺血前侧脑室给予ICI 182,780可显著降低GEN的神经保护作用(P0.01)。结论低剂量(1.0mg/kg)GEN可显著降低缺血后大鼠海马CA1区神经元损伤,改善认知功能,其分子机制可能与雌激素受体活性密切相关。     

Role of lipid peroxidation in protection of rats by rebamipide against gastric mucosal lesions induced by stress plus indomethacin

The roles of lipid peroxidation and prostaglandins (PGs) in the protective effect of rebamipide against gastric lesions in rats induced by stress plus indomethacin were investigated. Lesions of the gastric mucosa were induced within 3 h by restraint in water (23°C) plus intraperitoneal injection of indomethacin (20 mg/kg) but not by either of these treatments alone. Lipid peroxidation, measured as TBA reactive substances, was increased by stress + indomethacin treatment and the increase in TBA reactive substances was closely related to the lesion score (P < 0.01). This treatment also reduced the release of PGE₂ from the gastric mucosa, the reduction being related with the lesion score (P < 0.01). Teprenone, an inducer of endogenous PGE₂, did not inhibit the induction of gastric lesions by stress + indomethacin. Rebamipide injected subcutaneously dose-dependently inhibited the induction of lesions at doses of 3–30 mg/kg. At a dose of 30 mg/kg, it inhibited the increase of TBA reactants in the gastric mucosa, but did not completely restore PGE₂ release. In the rebamipide-treated group, a significant correlation (P < 0.01) was found between the lesion score and the changes in TBA reactants, but not the PG levels. These results indicate that inhibition of lipid peroxidation may be an important factor in the protective effect of rebamipide against gastric lesions in rats induced by stress + indomethacin.     

Senktide, a selective neurokinin B-like agonist, elicits serotonin-mediated behaviour following intracisternal administration in the mouse

Behavioural responses to tachykinins were observed following intracisternal administration in mice. The synthetic NK-3 agonists senktide and L-363,851 caused behaviour typically associated with serotonergic stimulation, including head twitches, reciprocal forepaw treading and hindlimb splaying. Neurokinin B produced some features of the serotonin (5-HT) syndrome, while substance P, neurokinin A and eledoisin failed to elicit any such behaviours. Senktide-induced head twitches were prevented by pretreatment with the 5-HT₂ antagonists ketanserin and ritanserin, while forepaw treading was attenuated by the 5-HT₁ antagonists (−)-pindolol and methysergide. These data suggest that NK-3 agonists interact with central 5-HT mechanisms.     

Chronic methylphenidate treatment enhances water maze performance following traumatic brain injury in rats

Methylphenidate (MPH), a central nervous system stimulant with dopaminergic activity, facilitates neurobehavioral outcome following cortical suction ablation injury, but its potential efficacy following experimental traumatic brain injury (TBI) is unknown. Thus, beginning 24 h after controlled cortical impact injury or sham surgery, male Sprague–Dawley rats were injected (i.p.) once daily for 18 days with either MPH (5 mg/kg) or saline vehicle (VEH) and motor function assessed on post-operative days 1–4, followed by Morris water maze training to find a hidden platform on days 14–18. The MPH treatment regimen was ineffective in accelerating beam-balance or beam-walk recovery, but did significantly decrease swim latencies when compared to VEH-treated controls. The results are consistent with published studies showing improved outcome with MPH therapy. Furthermore, this positive finding with delayed treatment suggests that strategies that enhance catecholamine neurotransmission during the chronic post injury phase may be a useful adjunct in ameliorating some of the neurobehavioral sequelae following TBI in humans.     

Nicotinic cholinoceptor-mediated excitation in ambigual motoneurons of the rat

The purpose of this study was to determine if ambigual oesophageal motoneurons of the rat possess functional nicotinic cholinoceptors. In urethane anaesthetized rats, acetylcholine (20–50 pmol) delivered micropneumophoretically from multibarrelled pipettes to the compact formation of the nucleus ambiguus produced either synchronous or propulsive oesophageal contractions which were fully and reversibly blocked by dihydro-β-erythroidine (8–10 pmol) but were resistant tod-tubocurarine and hexamethonium (10–20 pmol). 1,1-Dimethyl-4-phenyl-piperazinium but not muscarine (8 pmol) exerted an analogous agonist action. Ejection of glutamate at the same sites produced similar oesophageal responses which were, however, resistant to dihydro-β-erythroidine. Acetylcholine applied 5–15 s prior to glutamate transiently facilitated the glutamate-evoked response. The facilitatory effect of acetylcholine was replicated by 1,1-dimethyl-4-phenyl-piperazinium but not muscarine and inhibited by dihydro-β-ery-throidine. Physostigmine, applied either intra-ambigually (10–20 pmol) or by intravenous injection (0.15–0.3 μmol/kg), enhanced both acetylcholine and glutamate-evoked responses. In brainstem transverse slices, application of acetylcholine and glutamate to quiescent ambigual neurons of the compact formation resulted in a rapid membrane depolarization associated with an increased membrane conductance and spiking. Under voltage clamp, both acetylcholine and glutamate elicited a net inward current. The depolarizing response of these neurons to acetylcholine was blocked by dihydro-β-erythroidine (0.5–2 pmol), hexamethonium (0.2 mM) andd-tubocurarine (10 μM) and persisted in the presence of tetrodotoxin (10⁻⁶ M) or Mn²⁺ (5 mM) in the bathing medium.

It is concluded that functional postsynaptic nicotinic cholinoceptors are present on ambigual oesophageal motoneurons, and that nicotinic receptor-mediated transmission may play a role in oesophageal peristalsis at the motoneuronal level.     

Prostaglandins are necessary and sufficient to induce contextual fear learning impairments after interleukin-1 beta injections into the dorsal hippocampus

The intra-hippocampal administration of interleukin-1β (IL-1β) as well as the induction of elevated but physiological levels of IL-1β within the hippocampus interferes with the formation of long-term memory. There is evidence suggesting that the induction of prostaglandin (PG) formation by IL-1β is involved in impairments in working and spatial memory following IL-1β. The present experiments extend these findings by showing that PGs are responsible for memory deficits in contextual fear conditioning that occur following IL-1β injection into the dorsal hippocampus of Sprague-Dawley rats. Cyclooxygenase (COX) inhibition blocked the disruption in contextual fear conditioning produced by IL-1β and COX inhibition alone also disrupted contextual memory, suggesting an inverted U-shaped relationship between PG levels and memory. In addition to demonstrating the necessity of PGs in IL-1β-mediated memory deficits, we also show that PGs injected directly into the dorsal hippocampus are sufficient to impair context memory and significantly reduce post-conditioning levels of BDNF within the hippocampus, suggesting a possible mechanism for the memory-impairing effects of PGs.     

Potential role of endothelin-1 in the pathogenesis of acute hypertensive vascular disease in the rat

Endothelin-1 (ET-1) is a potent and long-acting vasoactive peptide often implicated in the pathogenesis of hypertension. However, little is known about its role in the pathogenesis of hypertensive vascular disease. This study compares the vascular changes produced in vivo by pressor or subpressor doses of ET-1 and angiotensin II (ANG II) in rat intestinal arterioles. In one experiment, equivalent pressor doses of either ET-1 (0.2 μg/kg/min) or ANG II (2 μg/kg/min) were given. In a second experiment, subpressor doses of ET-1 (10 ng/kg/min) were either infused alone or given in combination with pressor doses of ANG II (2 μg/kg/min). Normal saline infusions (0.02 mL/min) served as controls. All infusions were continued for 60 minutes, interrupted for 30 minutes, and reinstituted for 30 minutes. Changes in vascular permeability were assessed using ferritin as a tracer; vascular lesions were assessed by computerized morphometry and electron microscopy. Results from the first experiment showed that ET-1 and ANG II produced approximately equivalent increases in blood pressure but that ET-1 had more prolonged pressor effects when infusions were interrupted. Pressor doses of either ET-1 or ANG II caused a segmental pattern of vasoconstriction and dilatation, an increase in ferritin permeability, and severe vascular smooth muscle damage. However, vessels from ET-1-treated animals had fewer damaged smooth muscle cells (p < 0.05) and smaller areas of vessel wall damage (p < 0.05) than did those from the ANG II-treated group. Subpressor doses of ET-1 in the second experiment produced no vascular lesions, but combined subpressor ET-1/pressor ANG II infusion caused greater hypertension (p < 0.05) and more extensive damage (p < 0.01) to greater numbers of vascular smooth muscle cells (p < 0.01) than did pressor infusions of ET-1 or ANG II alone. The results indicate that subpressor concentrations of ET-1 clearly potentiate the vascular damage caused by ANG II-induced hypertension. On the other hand, pressor doses of ET-1 do not cause more severe vascular injury than do equipresor doses of ANG II, though the hypertension induced by ET-1 has longer-lasting hypertensive effects.     

Involvement of 5-hydroxytryptamine and prostaglandin E2 in the intestinal secretory action of Escherichia coli heat-stable enterotoxin B.

The intestinal secretory action of Escherichia coli heat-stable enterotoxin B (STb) is poorly defined. Previous work indicates that STb causes loss of intestinal fluid and electrolytes by a mechanism independent of elevated levels of cyclic nucleotides, the hallmark of other E. coli cytotonic enterotoxins. In the work described in this report, we observed that treatment of ligated rat intestinal loops with purified STb of E. coli resulted in a dose-dependent rise in intestinal secretion concomitant with dose-related increases in levels of serotonin (5-hydroxytryptamine [5-HT]) and prostaglandin E2 (PGE2). Treatment of rats with the 5-HT2 receptor antagonist ketanserin prior to STb challenge resulted in significant (P < 0.05) reduction in intestinal secretion. Blockage of 5-HT2 receptors with ketanserin also reduced (P < 0.05) the level of PGE2 observed following STb treatment, indicating that at least a portion of the PGE2 was formed in response to 5-HT2 receptor stimulation. In a similar fashion, indomethacin, an inhibitor of cyclooxygenase activity, significantly reduced the level of secretion (P < 0.05) observed following STb treatment yet had no effect on 5-HT levels. Treatment of rats with both ketanserin and indomethacin further reduced STb-mediated secretion to a level not attained by either drug alone. Taken together, our data suggest that secretion due to STb involves both 5-HT and PGE2 as intestinal secretagogues. Furthermore, PGE2 formation appears to arise through both 5-HT-dependent and 5-HT-independent pathways.