Descending influences from the infralimbic cortex on vago-vagal reflex control of gastric motor activity in the rat

In experiments on urethane anaesthetised rats the influence of electrical stimulation of ventral areas of the medial prefrontal cortex (mPFC) on spontaneous and vagally-mediated gastric motility were studied. Stimulation of the mPFC resulted in gastric relaxation manifested as a fall in intragastric pressure from a baseline value of 5.0 +/- 0.5 cm H2O. These were most prominent following a short latency when the infralimbic cortex (IL) was stimulated (27.4 +/- 2.5% fall in gastric pressure). Electrical stimulation of the central end of one cervical vagus nerve caused a comparable decrease in gastric pressure (27.1 +/- 2.9%). The cortical mediated relaxation was reduced by atropine and abolished by vagotomy. The cortically induced gastric relaxation followed a shorter latency (5.9 +/- 1.0 s), time to nadir (20.1 +/- 2.7 s) and the half recovery time (21.5 +/- 4.0 s) than vagally mediated-relaxations (9.9 +/- 2.3, 56.0 +/- 5.3 and 83.4 +/- 9.5 s, respectively). Vagally mediated relaxations were inhibited by simultaneous stimulation of the infralimbic cortex. In this case the decrease of gastric pressure, the time to nadir and the half recovery time were significantly decreased in comparison with the gastric relaxatory responses to vagal stimulation alone (P < 0.05). We conclude that one way in which the mPFC influences gastric motility is via corticofugal projections from the infralimbic cortex to the brain-stem which modulate transmission of vago-vagal reflexes.

Opioid-receptor-mediated excitation of rat mesenteric afferent fibres supplying the rat jejunum

The aim of the present study was to examine the sensitivity to opioid-receptor agonists of mesenteric afferents supplying the small intestine and to characterize the subpopulations of any responsive fibres. Mesenteric afferent discharge was recorded electrophysiologically in response to cumulative doses (1-400 microgram kg-1) of the mu-receptor agonist [D-ala,2 N- me-Phe4, Gly5-ol]-enkephalin (DAMGO), the delta-receptor agonist [D-ala,2 D-leu5]-enkephalin (DADLE) and the kappa-receptor agonist U-50488. DAMGO and DADLE, but not U-50488, markedly stimulated whole nerve mesenteric afferent discharge (P < 0.05) that was unrelated to intestinal motor events. Subpopulations of afferent fibres responding to DAMGO were examined using waveform analysis to identify single units from within the whole mesenteric nerve bundles. One population was CCK-sensitive (15/15 fibres) and the other was a subpopulation of mechanosensitive afferents that responded to distension (17/28). 5-HT-sensitive afferents did not respond to DAMGO (0/11). We conclude that specific subpopulations of mesenteric afferents respond to mu- and possibly delta- but not kappa-receptor agonists. This sensitivity to opioids may contribute to the antinociceptive property of vagal afferents.

Prostaglandin EP receptor subtypes have distinctive effects on jejunal afferent sensitivity in the rat

BACKGROUND & AIMS

Tissue levels of prostaglandin (PG) E(2) are increased in inflammatory bowel disease. The aim of this study was to characterize the potential for PGE(2) to modulate the sensitivity of intestinal afferents.

METHODS

Electrophysiologic recordings were obtained from mesenteric afferent supplying the proximal jejunum of anesthetized rats.

RESULTS

PGE(2) evoked a dose-dependent increase in afferent nerve discharge that was biphasic at higher doses. An early response phase, peak discharge frequency of 165.4 +/- 14.3 imp. s(-1), and duration of 20.2 +/- 1.2 seconds were followed by a plateau of elevated afferent nerve discharge lasting several minutes. The increase in afferent nerve discharge was accompanied by an increase in intestinal pressure of 4.4 +/- 0.5 cm H(2)O. Nifedipine (1 mg. kg(-1)) attenuated the pressure response and the plateau phase of afferent discharge, whereas the early component remained unchanged. In contrast, the early phase, but not the plateau phase, was reduced by luminal anesthetic. Experiments with EP receptor-selective agonists and the EP(1)-receptor antagonist AH-6809 (500 microg. kg(-1)) implicate EP1 receptors in the early response, and EP(2) receptors appeared to play a major role in the plateau phase.

CONCLUSIONS

PGE(2) has complex actions on intestinal afferent discharge acting by direct and indirect mechanisms and mediated by different receptor subtypes.

5-HT(3) and histamine H(1) receptors mediate afferent nerve sensitivity to intestinal anaphylaxis in rats

BACKGROUND & AIMS

The mechanisms underlying brain stem activation during antigen challenge have not been resolved. Our aim was to characterize afferent nerve responses to intestinal anaphylaxis and determine the mediators involved in afferent activation.

METHODS

Mesenteric afferent discharge was recorded electrophysiologically after intestinal anaphylaxis in anesthetized rats previously sensitized to chicken egg albumin (EA).

RESULTS

Mesenteric afferent nerve discharge increased approximately 1 minute after luminal antigen but not bovine serum albumin (P < 0.001, EA vs. bovine serum albumin). Subsequent administration of antigen had no effect, but systemic EA evoked a marked increase in afferent discharge (P < 0. 05). Afferent responses were unrelated to intestinal motor activity, and the response to luminal antigen was attenuated by luminal anesthetic (1% lidocaine). The 5-HT(3)-receptor antagonist alosetron (30 microg. kg(-1)) and the histamine H(1)-receptor antagonist pyrilamine (5 mg. kg(-1)) markedly attenuated the response to luminal antigen; pretreatment with doxantrazole attenuated responses to both luminal and systemic antigen.

CONCLUSIONS

5-HT(3) and histamine, released from mast cells after intestinal anaphylaxis, stimulate mesenteric afferents via 5-HT(3) and histamine H(1) receptors. Information on intestinal immune status is rapidly relayed to the central nervous system and may play a role in neural reflexes and behavioral responses following activation of the immune system.

Mediators of neuronal activation in the rat brainstem following intestinal anaphylaxis

Brainstem neurones become activated following intestinal antigen challenge but the signalling mechanisms have not been resolved. Our aim was to determine the extent of brainstem activation after intestinal anaphylaxis induced by chicken egg albumin (EA). An increase in Fos-positive neurones in the nucleus tractus solitarius (nTS) was observed following EA (P<0.05). Fos-expression was decreased following pretreatment with pyrilamine and ondansetron i.p. and to a similar extent when both antagonists were administered together (all P<0.05 vs. control). Indomethacin had no effect on Fos-expression after antigen challenge. 5-HT and histamine but not prostanoids, released following intestinal anaphylaxis, induce nTS activation via histamine H(1)- and 5-HT(3) receptors. Information on the intestinal inflammatory status is relayed centrally and may play a role in reflexes and behavioural responses to activation of the immune system.

The effect of fentanyl, DNQX and MK-801 on dorsal horn neurones responsive to colorectal distension in the anaesthetized rat

Certain dorsal horn neurones respond in a graded manner to noxious colorectal distension (CRD). Morphine inhibits these responses in the spinalized rat, but the role of excitatory amino acids in baseline visceral nociceptive transmission is less clear. This study examines the effect of the mu-opiate receptor agonist fentanyl, and the non-NMDA and NMDA antagonists DNQX and MK-801, respectively, on such responses to CRD in the sodium pentobarbitone-anaesthetized rat. Male rats were prepared for extracellular recording from the lumbosacral spinal cord. 90 neurones responsive to CRD, located throughout the dorsal horn, were classified according to their response duration and latency to 60 mmHg distension, as SL-A (short latency-abrupt; 59%), SL-S (short latency-sustained; 23%), L-L (long-latency; 10%) and Inhib (inhibited; 8%). Convergent cutaneous receptive fields were mapped for 79/90 neurones and classified as LT (low threshold), WDR (wide dynamic range) or HT (high threshold). CRD (20-100 mm Hg) elicited graded responses in most neurones. In 6/6 SL-S neurones, fentanyl (1-8 microg kg-1) dose-dependently inhibited the response to 60 mm Hg CRD, in a naloxone-sensitive manner, with an ID50 value (+/-95% confidence limits) of 2.48 (1.7-3. 7) microg kg-1. In 6/6 SL-A neurones, fentanyl had no significant effect on the response to CRD. DNQX (0.03-3 mg kg-1) produced a dose-dependent inhibition of the response to CRD in 5/5 SL-A neurones, with an ID50 value of 0.32 (0.01-41.1) mg kg-1. MK-801 (0. 03-0.3 mg kg-1) had no significant effect on responses to CRD in 6/6 SL-A neurones. The differential inhibitory effects of fentanyl on two neuronal subtypes may indicate functional differences. In SL-A neurones AMPA/kainate, but not NMDA receptors are involved in mediating baseline nociceptive neurotransmission.

Relationship between increasing duodenal lipid doses, gastric perception, and plasma hormone levels in humans

Duodenal lipid causes gastric relaxation, CCK secretion, and nausea. Vasopressin has been implicated in motion sickness-related nausea. We hypothesized that increasing doses of lipid enhance gastric relaxation and CCK-vasopressin secretion, resulting in a dose-related exacerbation of nausea. Nine healthy subjects received isotonic saline or lipid (1, 2, or 3 kcal/min, L1, L2, L3) duodenally. Changes in gastric volume, sensations, and plasma hormone levels were assessed during infusions and isobaric gastric distensions. Lipid infusions increased gastric volume, plasma CCK (but not vasopressin) levels, and gastric compliance during distensions, compared with saline. Plasma CCK levels were related to the dose of lipid administered [CCK levels at 30 min (pmol/l), saline: 1.1 +/- 0.2, L1: 1.8 +/- 0.2, L2: 3.0 +/- 0.2, L3: 4.3 +/- 0.6]. During distensions, nausea increased in intensity with increasing doses of lipid [score (where 0 is no sensation and 100 is strongest sensation), saline: 7 +/- 4, L1: 19 +/- 7, L2: 44 +/- 7, L3: 66 +/- 8]; however, no further rise in plasma CCK occurred. Because neither lipid nor distension alone induced significant nausea, we conclude that the interaction between these stimuli together with a modulation by CCK is responsible for the effects observed. Vasopressin is not involved in lipid- and distension-induced nausea.

The 5-HT(3) receptor antagonist alosetron inhibits the colorectal distention induced depressor response and spinal c-fos expression in the anaesthetised rat

BACKGROUND

Noxious intestinal distention elicits a reflex depressor response in the sodium pentobarbitone anaesthetised rat, which can be used as an index of visceral nociception. 5-HT(3) receptor antagonists inhibit this reflex. Repeated colorectal distention (CRD) induces Fos like immunoreactivity (Fos-LI) in the rat spinal cord.

AIMS

To examine the effect of the 5-HT(3) receptor antagonist alosetron on the depressor response to CRD, and on Fos expression in the lumbosacral spinal cord.

METHODS

Male rats were anaesthetised with sodium pentobarbitone, and mean arterial blood pressure monitored during repeated colorectal balloon inflation before and after treatment with alosetron or saline. Rats anaesthetised with urethane and treated with alosetron or saline underwent a repeated CRD paradigm, after which the lumbosacral spinal cord was removed and processed for visualisation of Fos-LI.

RESULTS

CRD elicited reproducible, volume dependent falls in arterial blood pressure, and repeated distention-effect curves were constructed. Alosetron (1-100 microg/kg intravenously) inhibited the depressor response to CRD in a dose related manner, with an ID(50) value of 3.0 microg/kg. Following repeated CRD, numbers of Fos-LI neurones were significantly increased to 1246 (total in 12 sections at 120 microm intervals from L6 to S1) compared with 49 in sham distended animals. Pretreatment with alosetron (100 microg/kg) significantly reduced numbers of Fos-LI neurones to 479.8.

CONCLUSION

The 5-HT(3) receptor antagonist alosetron inhibits the depressor response to CRD in a potent and dose dependent manner. It also inhibits CRD induced Fos-LI in the spinal cord. These results suggest that 5-HT(3) receptors are involved in visceral nociceptive transmission, perhaps located on primary afferent or spinal neurones.

Cilansetron acts at its site of absorption to antagonize the sensitivity of mesenteric afferent fibres to 5-hydroxytryptamine in the rat jejunum

The present study compares the efficacy of cilansetron, a 5-hydroxytryptamine (5-HT3)-receptor antagonist, delivered via intravenous and intraluminal routes, on the sensitivity of mesenteric afferent fibres supplying the proximal jejunum. Waveform analysis was performed to extract 5-HT sensitive single units from electrophysiological recordings of whole afferent nerve discharge. Dose effects of intravenous cilansetron (0.2-20 microg/kg) on the afferent response to 5-HT (10 microg) were examined to determine the threshold dose of cilansetron (2 microg/kg). This dose applied intraluminally to the region of jejunum innervated by the afferents, resulted in a greater degree of antagonism of the 5-HT response than intravenous administration (47.8+/-7.9 vs. 76.9+/-4.7%, P = 0.008). We concluded that cilansetron is active at its site of absorption to antagonize 5-HT3 receptors on vagal mucosal afferent terminals.

Modulation of gastrointestinal afferent sensitivity by a novel substituted benzamide (ecabapide)

The effects of ecabapide, a novel substituted benzamide compound (3-[2-(3,4-dimethoxyphenyl)ethylcarbamoylmethyl]amino-N-methylb enzamide) that has gastrointestinal prokinetic action, were examined on the discharge of extrinsic afferent nerves supplying the stomach and jejunum in anaesthetized rats. Ecabapide (60 and 180 microg kg(-1), i.v.) had no effect on the baseline discharge of vagal gastric distension-sensitive afferents or the stimulus-response profile to gastric distension. Ecabapide also had no effect on either spontaneous jejunal mesenteric afferent nerve discharge or responses to intestinal distension. Ecabapide (180 microg kg(-1)) significantly inhibited the maximum discharge of jejunal afferents induced by cholecystokinin (CCK8; 50 pmol, i.v.), whereas it failed to inhibit the excitatory action of 2-methyl-5-hydroxytryptamine (2Me-5-HT; 10 microg, i.v.), a selective 5-HT3 receptor agonist. A model of acute focal intestinal ischaemia was used to evaluate the actions of ecabapide on the discharge of activated jejunal afferents. Ischaemia produced a substantial increase in afferent discharge which was reproducible when the duration of ischaemia was limited to less than 10 min and repeated every 15 min. Ecabapide at doses of 60 and 180 microg kg(-1) significantly reduced ischaemia-induced increases in afferent discharge. In addition to its therapeutic efficacy as a gastrointestinal prokinetic agent, these findings show also that ecabapide may also have an inhibitory action on the discharge of intestinal afferents activated by ischaemia.

Sensitization of visceral afferents to bradykinin in rat jejunum in vitro

1. We have investigated the effects of inflammatory mediators on visceral afferent discharge and afferent responses to bradykinin (BK) in rat jejunum using a novel in vitro technique. 2. Prostaglandin E2 (1 microM) augmented responses to BK without affecting basal firing, while histamine (100 microM) and adenosine (100 microM) activated basal discharge and enhanced BK responses. In contrast, 5-HT (100 microM) increased basal discharge without influencing responses to BK. 3. Afferent discharge induced by histamine was inhibited by both H1 (pyrilamine) and H3 (thioperamide) but not H2 (ranitidine) receptor antagonists at 10 microM. In contrast, sensitization to BK induced by histamine was inhibited by ranitidine (10 microM). 4. Afferent discharge induced by adenosine was blocked by the A1 receptor antagonist DPCPX (10 microM) but remained unaffected by A2A receptor blockade with ZM241385 (10 microM). In contrast, sensitization of BK responses by adenosine was unaffected by both antagonists. Basal discharge and BK-induced responses were unaffected by the A3 receptor agonist IB-MECA (1 microM). While involvement of A2B receptors is not excluded, adenosine may activate afferent discharge through A1 receptors, while sensitization to BK could involve a receptor other than A1, A2A or A3, possibly the A2B receptor. 5. Inhibition of cyclo-oxygenase with naproxen (10 microM) prevented sensitization after histamine but not adenosine. 6. Sensitization was mimicked by dibutyryl cAMP. This occurred without changes in basal firing and was unaffected by naproxen. 7. In conclusion, afferent discharge induced by BK is augmented by histamine, adenosine and PGE2, but not by 5-HT. Evidence suggests that sensitization involves separate mechanisms from afferent activation. Sensitization may be mediated by increases in cAMP following direct activation by mediators at the nerve terminal or through indirect pathways such as the release of prostaglandins.

Excitatory effect of P2X receptor activation on mesenteric afferent nerves in the anaesthetised rat

1. We examined the effects of P2X purinoceptor agonists and P2 purinoceptor antagonists on mesenteric afferent nerves supplying the jejunum in the pentobarbitone sodium-anaesthetised rat. 2. ATP (0. 01-10 mg kg-1, i.a.) and alpha,beta-methylene-ATP (1-30 microg kg-1, i.a.) each induced dose-dependent increases in afferent nerve discharge and intrajejunal pressure. The effect on afferent nerves comprised an early (< 2 s after administration) intense burst of activity followed by a later increase (> 2 s after administration), less pronounced in comparison, which coincided with elevated intrajejunal pressure. 3. Pyridoxalphosphate-6-azophenyl-2', 4'-disulphonic acid (20 mg kg-1, i.v.) and suramin (80 mg kg-1, i.v. ) each antagonised both the early and later increases in afferent nerve discharge elicited by alpha,beta-methylene-ATP (30 microg kg-1, i.a.). 4. Co-administration of omega-conotoxin MVIIA and omega-conotoxin SVIB (each at 25 microg kg-1, i.v.), or treatment with the selective 5-HT3 receptor antagonist alosetron (30 microg kg-1, i.v.), did not affect the rapid burst of afferent nerve activity elicited by alpha,beta-methylene-ATP (30 microg kg-1, i.a.). However, toxin treatment did attenuate the elevations in intrajejunal pressure and the corresponding later phases of evoked afferent discharge, while alosetron inhibited basal afferent nerve activity. 5. In summary, ATP and alpha,beta-methylene-ATP each evoke excitation of mesenteric afferent nerves in the anaesthetised rat. We propose that the early increase in mesenteric afferent nerve activity represents a direct effect on the nerve ending, mediated by P2X receptors, whereas the later increase reflects activation of mechanosensitive fibres secondary to elevated intrajejunal pressure.

Plasticity in the mesenteric afferent response to cisplatin following vagotomy in the rat

The aim of this study was to investigate the actions of the cytotoxic drug cisplatin on populations of mesenteric afferents supplying the rat jejunum. Extracellular whole mesenteric nerve discharge was monitored and the activity of individual single afferent units determined using waveform discriminator software. Baseline whole nerve discharge was 21.5 +/- 3.8 impulses s(-1). Nerve discharge began to increase approximately 10 min after cisplatin administration, reached a plateau around 30 min, and remained elevated at 60 min (35.3 +/- 5.7 impulses s(-1), p < 0.01). Granisetron reversed the increase in nerve activity indicating that the response to cisplatin was mediated by the release of endogenous 5-HT acting on 5-HT3 receptors. Single afferent units, selected by waveform analysis on the basis of their response to exogenous 5-HT, showed a similar time course of activation following cisplatin. In contrast, the discharge frequency of afferent units that were insensitive to 5-HT was unaffected by cisplatin or granisetron. The sensitivity of mesenteric afferent bundles to exogenous 5-HT was absent in chronically vagotomized animals. However, cisplatin elicited an increase in nerve discharge in vagotomized animals that was not different from control (34.6 +/- 8.9 impulses s(-1)) but this increase was unaffected by treatment with granisetron. Thus, after vagotomy there is a switch from 5-HT3 mediated activation of vagal afferents to a 5-HT3-independent activation of non-vagal (possibly splanchnic) afferents. Since this later mechanism of activation is absent in control animals, it appears that there is plasticity in the gastrointestinal afferent sensitivity to cisplatin.

The role of prostaglandins in the bradykinin-induced activation of serosal afferents of the rat jejunum in vitro

1. This study was performed to elucidate the role of prostaglandins in the action of bradykinin on serosal afferent neurones supplying the rat jejunum. Extracellular recordings of multi-unit activity were made from serosal afferents in isolation, using a novel in vitro preparation. The discharge of single afferents within the multi-unit recording was monitored using waveform discrimination software. 2. All afferents tested were both mechano- and capsaicin sensitive. Application of bradykinin elicited increases in whole nerve discharge in a concentration-dependent manner. The agonist potency estimate (EC50) was 0.62 +/- 0.12 microM and is consistent with an interaction at the B2 receptor subtype. 3. The stimulatory effect of bradykinin on serosal afferents was antagonized by a specific antagonist of the B2 receptor, HOE140. In contrast, a selective B1 receptor antagonist, [des-Arg10]HOE140, had no effect. The IC50 estimate obtained for HOE140 was 1.6 nM and again consistent with an interaction at B2 receptors. 4. The response to a submaximal concentration of bradykinin (1 microM) was significantly reduced to 24.4 +/- 54.9 % of control following blockade of cyclo-oxygenase activity with naproxen (10 microM). The addition of 1 microM prostaglandin E2 (PGE2), in the presence of naproxen, had no direct effect on afferent activity, but fully restored the response to bradykinin in 15 single afferents. 5. In summary, bradykinin stimulates serosal afferents by a direct action on kinin B2 receptors that are present on serosal afferent terminals. The response to bradykinin is dependent on the presence of prostaglandins, particularly PGE2. We suggest that bradykinin has a self-sensitizing action, whereby it stimulates the release of PGE2, which in turn sensitizes the endings of serosal afferent neurones responsive to bradykinin.

Mesenteric afferent sensitivity to cholecystokinin and 5-hydroxytryptamine

The present electrophysiological investigation examines the effect of CCK and 5-hydroxytryptamine on gastrointestinal afferent fibre discharge. 5-HT markedly stimulated mesenteric afferents. The response was transient (< 10s) and mediated by 5-HT3 receptors as demonstrated by the action of 2-methyl-5-HT and antagonism by granisetron. CCK was also a potent stimulus to mesenteric afferents causing a long-lasting (> 30s) increase in excitability. The response to CCK was mediated via the CCKA receptor as shown by the antagonistic action of devazepide. At doses of granisetron and devazepide which completely block the response to exogenous 5-HT and CCK, the afferent fibres still responded to both mechanical and chemical stimulation of the mucosa. Thus products of enteroendocrine cells can have profound effects on mucosal afferent sensitivity but do not play an obligatory role in afferent signal transduction.

Characterization of adenosine receptors evoking excitation of mesenteric afferents in the rat

We examined the effects of adenosine receptor agonists and antagonists on the discharge of mesenteric afferent nerves supplying the jejunum in pentobarbitone sodium-anaesthetized rats. Adenosine (0.03-10 mg kg(-1), i.v.), NECA (0.3-300 microg kg(-1), i.v.) and the A1 receptor agonist, GR79236 (0.3-1000 microg kg(-1), i.v.), each induced dose-dependent increases in afferent nerve activity and intrajejunal pressure, hypotension and bradycardia. The A1 receptor antagonist, DPCPX (3 mg kg(-1), i.v.), antagonized all the effects of GR79236 but only the haemodynamic effects of adenosine and NECA. The A2A receptor antagonist, ZM241385 (3 mg kg(-1), i.v.), antagonized the hypotensive effect of NECA but none of the effects of GR79236. The A2A receptor agonist, CGS21680 (0.3-300 microg kg(-1), i.v.), and the A3 receptor agonist, IB-MECA (0.3-300 microg kg(-1), i.v.), each induced only a dose-dependent hypotension. Subsequent administration of adenosine (3 mg kg(-1), i.v.) induced increases in afferent nerve activity and intrajejunal pressure and bradycardia. ZM241385 (3 mg kg(-1), i.v.) antagonized the hypotensive effect of CGS21680 but not the effects of adenosine. Bethanechol (300 microg kg(-1), i.v.) evoked increases in afferent nerve activity and intrajejunal pressure, hypotension and bradycardia. However, adenosine (3 mg kg(-1), i.v.) evoked greater increases in afferent nerve activity than bethanechol despite inducing smaller increases in intrajejunal pressure. In summary, A1 and A2B and/or A2B-like receptors evoke adenosine-induced increases in mesenteric afferent nerve activity and intrajejunal pressure in the anaesthetized rat. Furthermore, elevations in intrajejunal pressure do not wholly account for adenosine-evoked excitation of mesenteric afferent nerves.

Serotonin and cholecystokinin activate different populations of rat mesenteric vagal afferents

This electrophysiological study was performed to elucidate the interactions of serotonin (5-hydroxytryptamine, 5-HT) and cholecystokinin (CCK) on mesenteric afferents supplying the rat jejunum. 5-HT and CCK produced characteristic responses in multi-unit recordings of mesenteric afferents. Waveform analysis to extract single units from the whole nerve recording identified populations of single afferents that were sensitive to either 5-HT or CCK, but not both. Furthermore, devazepide (0.5 mg/kg) completely abolished the response to CCK without altering the response to 5-HT while granisetron (0.5 mg/kg) abolished the response to 5-HT with no effect on the response to CCK. These results suggest that there are discrete, noninteractive populations of jejunal afferents that possess either 5-HT3 or CCK-A receptors but not both.

The role of endogenous cholecystokinin in the sensory transduction of luminal nutrient signals in the rat jejunum

Some vagal afferent fibres are exquisitely sensitive to exogenous administration of cholecystokinin (CCK) but their sensitivity to endogenous CCK released by luminal stimuli has not been demonstrated directly, although implied from reflex and behavioural studies. We have therefore utilised electrophysiological techniques to record afferent discharge in mesenteric nerve bundles supplying the rat jejunum in response to luminal application of casein acid hydrolysate (CAH). CAH stimulated whole nerve afferent discharge in both in vivo and in vitro preparations (P < 0.01) while single unit analysis revealed that fibres sensitive to CAH also responded to exogenous CCK. The responses to both CCK and CAH were abolished by the CCKA antagonist devazepide. This study therefore supports the hypothesis that a functional relationship exists between CCK-containing enteroendocrine cells and the afferent fibres whose terminals lie within close proximity.

Histamine sensitivity of mesenteric afferent nerves in the rat jejunum

The concept of functional interaction between mast cells and intestinal afferents is gaining support. We have therefore characterized the action of histamine on jejunal afferent discharge in the anesthetized rat. Whole nerve mesenteric afferent discharge was recorded in conjunction with intestinal pressure in response to a range of histamine agonists and antagonists. Histamine at 2, 4, and 8 micromol/kg (iv) evoked a dose-dependent biphasic increase in afferent discharge together with a biphasic rise in intestinal pressure. However, these two events were mediated independently, since nifedipine (1 mg/kg) substantially reduced the intestinal pressure increase but not the afferent discharge. These responses were completely inhibited by pyrilamine (5 mg/kg) but unaffected by ranitidine (5 mg/kg) or thioperamide (2 mg/kg). Neither the selective H2 receptor agonist dimaprit nor the selective H3 receptor agonist R-alpha-methylhistamine caused any modulation of afferent discharge. We conclude that histamine stimulates an H1 receptor-mediated increase in mesenteric afferent discharge that is independent of intestinal motor events. This suggests that histamine potentially acts as a mediator in mast cell-to-afferent nerve communication in the small intestine.