Incidence of humoral sensitization in HLA partially mismatched hematopoietic stem cell transplantation

As part of the 15th International Histocompatibility and Immunogenetics Workshop (IHIWS), seven centers participated in a collaborative project to determine whether any significant humoral sensitization occurred post-transplant among recipients of HLA partially mismatched hematopoietic cell transplants (HCTs). A total of 140 donor/recipient pairs were enrolled with a total of 367 pre-and post-transplant sera analyzed. The majority of the samples (69.1%) were obtained within 30-90 days post-HCT. HLA-specific antibodies were defined using single antigen bead assays on a Luminex platform with a positive cutoff value of 1000 normalized median fluorescence intensity (MFI). There was an overall incidence of post-HCT sensitization toward donor HLA mismatches of 5.7%; however, all cases were among recipients of one HLA haplotype-mismatched grafts under nonmyeloablative, pre-transplant conditioning. Among the one haplotype-mismatched recipients, 15.7% (8/51) developed donor HLA-specific antibodies and 29.4% also had antibodies directed toward third party HLA antigens. Among the donor-specific antibodies, 9.8% were directed toward HLA class I antigens; 7.8% were against class II antigens; and 2.0% had both class I and II specificity. The relative strength of post-transplant antibodies was low with no significant difference in the mean maximum MFI values between third party and donor-specific antibodies. Because only a small number (10.2%) of the post-transplant samples were obtained 180 days or more post-HCT, longer term study is needed to evaluate any clinical relevance of these low-to-moderate levels of donor-specific antibody in one haplotype-mismatched recipients, as well as to determine whether any other antibodies occur at later times.

Identification of T-cell stimulating antigens from Giardia lamblia by using Giardia-specific T-cell hybridomas

T-cell immune response plays an important role in controlling Giardia lamblia infections. Little is known about the G. lamblia-specific antigens that stimulate a cell-mediated immune response. The aim of the present study was to identify T-cell stimulating G. lamblia antigens. For this purpose, we generated a group of Giardia-specific T-cell hybridomas (2F9, 4D5, 6D10, 8B9, 9B10, 10F7 and 10G5). Hybridomas were screened for reactivity with G. lamblia protein extract by the CTLL bioassay. These T-cell hybridomas did not exhibit any significant activation either in the absence of G. lamblia protein extract or in the presence of irrelevant antigen (hen white egg lysozyme). To further characterize the T-cell hybridomas generated, we selected three hybridomas (10G5, 4D5 and 9B10). Giardia lamblia proteins of 90-110, 65-77 and 40-64 kDa showed T-cell stimulating activity for the hybridomas 10G5, 4D5 and 9B10, respectively, in a concentration-dependent manner. Protein extract obtained from different G. lamblia strains (GS/M-83-H7, WB C6 and a clinical isolate (YJJ)) stimulated all T-cell hybridomas, indicating that T-cell-stimulating antigens are expressed among different G. lamblia strains. In conclusion, we identified T-cell stimulating G. lamblia antigens by using Giardia-specific T-cell hybridomas. To our knowledge, these hybridomas are the first-described T-cell hybridomas specific for G. lamblia.

Cytomegalovirus infection in ulcerative colitis: a prospective, comparative study on prevalence and diagnostic strategy

BACKGROUND

Cytomegalovirus (CMV) infection has been reported in ulcerative colitis (UC), especially in severe, steroid-refractory disease. However, its role in steroid-refractoriness remains unknown. Our goals were to evaluate the prevalence of CMV disease in UC, the best diagnostic strategy, and the influence of disease activity and/or treatment in its development.

METHODS

Prospective, observational study including 114 subjects with active UC requiring intravenous steroids, steroid-refractory UC, inactive UC on mesalamine, inactive UC on azathioprine, and healthy controls. CMV antibodies, pp65-antigenemia, and rectal biopsies for hematoxylin and eosin staining, immunohistochemistry, and CMV-pp67 mRNA were performed. These procedures were repeated after medical treatment only in patients with active UC. CMV disease was defined by the presence of inclusion bodies and/or positive immunohistochemistry in colonic biopsies.

RESULTS

CMV disease was found in 6 steroid-refractory, CMV-IgG-positive UC patients but not among controls, inactive UC, or steroid-responding UC patients. In 5 out of the 6 patients, CMV disease was diagnosed after 7-10 days on cyclosporine.

CONCLUSIONS

CMV disease in UC only affects seropositive, steroid-refractory UC patients. Steroid/cyclosporine treatment together with disease activity may predispose to latent colonic CMV reactivation. The impact of antiviral therapy on the clinical outcome of these patients remains to be elucidated.

[Pyoderma gangrenosum peristomal in ulcerative colitis; first report of a case in Mexico]

Pyoderma gangrenosum is one of the most severe extraintestinal manifestations in patients with ulcerative colitis (UC) and Crohn s disease. This lesion is frequently located on the lower extremities and the torso. Peristomal pyoderma gangrenosum (PPG) is extremely rare. We report the first published patient with PPG and UC in Mexico. PPG occurred six weeks after restorative proctocolectomy. Diagnosis was performed by clinical presentation and biopsy. Ulcer resolution was achieved with oral steroids and local wound care. Patient did not show any recurrence at one year follow-up. We suggest suspecting this illness in all patients with UC who had a restorative proctocolectomy and present difficult management peristomal ulcers.

Hospital admission is a relevant source of hepatitis C virus acquisition in Spain

BACKGROUND/AIMS

Isolated cases of acute hepatitis C, as well as hepatitis C outbreaks transmitted by health-care related procedures, have drawn attention to nosocomial transmission of HCV. The aim of this study was to investigate the current relevance of nosocomial HCV infection.

METHODS

For this purpose, we performed a retrospective epidemiological analysis of all cases of acute hepatitis C diagnosed in 18 Spanish hospitals. Between 1998 and 2005, 109 cases were documented.

RESULTS

The most relevant risk factors registered during the 6-month period preceding the diagnosis of acute hepatitis C were: hospital admission in 73 (67%) cases, intravenous drug use in 9 (8%), accidental needlestick injury in 7 (6%) and sexual contact in 6 (5%). Among the 73 patients in whom hospital admission was the only risk factor, 33 underwent surgery and 24 were admitted to a medical emergency unit or a medical ward; the remaining 16 patients underwent an invasive diagnostic or therapeutic procedure. Sixty two patients underwent antiviral therapy and 51 (82%) achieved a sustained virological response. In 47 patients treatment was not indicated (in 24 due to spontaneous resolution of HCV infection).

CONCLUSIONS

In most patients with acute hepatitis C the only documented risk factor associated with the infection is hospital admission. These results stress the need for strict adherence to universal precaution measures. Fortunately, most cases of acute hepatitis C either resolve spontaneously or after antiviral therapy.

CD20-positive infiltrates in renal allograft biopsies with acute cellular rejection are not associated with worse graft survival

We examined rejection outcome and graft survival in 58 adult patients with acute cellular rejection Banff type I (ARI) or II (ARII), within 1 year after transplantation, with or without CD20-positive infiltrates. Antibody-mediated rejection was not examined. Of the 74 allograft biopsies, performed from 1999 to 2001, 40 biopsies showed ARI and 34 biopsies showed ARII; 30% of all the biopsies showed CD20-positive clusters with more than 100 cells, 9% with more than 200 cells and 5% with more than 275 cells. Patients with B cell-rich (>100 or >200/HPF CD20-positive cells) and B cell-poor biopsies (<50 CD20-positive cells/HPF) were compared. Serum creatinine and eGFR of B cell-rich (CD20 > 100/HPF) and B cell-poor were not significantly different at rejection, or at 1, 3, 6 and 12 months, and during additional 3 years follow-up after rejection, although higher creatinine at 1 year was noted in the >200/HPF group. Graft survival was also not different between B cell-rich and B cell-poor groups (p = 0.8 for >100/HPF, p = 0.9 for >200/HPF CD20-positive cells). Our data do not support association of B cell-rich infiltrates in allograft biopsies and worse outcome in acute rejection type I or II, but do not exclude the possible contribution of B cells to allograft rejection.

Reversible, high-voltage square-wave pulse generator for triggering spark gaps

A design is presented for a reversible, square-pulse generator that employs coaxial cables for charge storage and pulse formation and a thyratron as the switch. The generator has a nominal output voltage of 5-30 kV and a pulse duration determined by the cable's physical length. Two variations are presented: (1) a single-stage one consisting of cable that is charged via its shield on one end and discharged with a thyratron on the opposite end and (2) a two-stage one having an inverting circuit that uses a coaxial cable to reverse the polarity of the pulse. The generator operates with "flying shields," i.e., high-voltage pulses also propagate on the outside of the cables; this calls for a dedicated insulation that avoids breakdown between sections of the cable's shield. The rise time obtained is mostly dictated by the switching time of the thyratron; with the one we used in the tests, rise times in the range of 30-40 ns were obtained. We present the results obtained in the implementation of the generators as well as its application to fire a large Marx generator.

Survival and dependence on home parenteral nutrition: experience over a 25-year period in a UK referral centre

BACKGROUND

Home parenteral nutrition (HPN) is the standard treatment for severe intestinal failure in the United Kingdom.

AIM

To review long-term survival and ongoing HPN dependence of patients receiving HPN treated at a specialist UK referral centre.

METHODS

Medical records of patients commenced on HPN between 1979 and 2003 were reviewed retrospectively. Regression analysis was employed to identify factors associated with poor prognosis.

RESULTS

Case notes of 188 patients were reviewed. Overall probability of survival was 86%, 77%, 73% and 71% at 1, 3, 5 and 10 years after starting treatment. In multivariate analysis, association was seen between mechanism of intestinal failure and survival: short bowel syndrome associated with a favourable prognosis, and intestinal dysfunction, dysmotility and obstruction with poorer prognoses. There was an association between increasing age and poor prognosis, but increased mortality was also seen in the youngest age groups. Only 9% of deaths were due to complications of HPN. Continued HPN dependence was 89%, 87%, 84% and 84% at 1, 3, 5 and 10 years in survivors.

CONCLUSIONS

Long-term survival of patients receiving HPN remains better than that reported after intestinal transplantation. Mortality predominantly relates to underlying disease rather than complications of HPN.

In vitro quantitative determination of ophthalmic irritancy by the chorioallantoic membrane test with trypan blue staining as alternative to eye irritation test

The damage provoked by some substances on the chicken egg's chorioallantoic membrane (CAM) is used as an alternative assay to determine ocular irritation. There is good prediction of the eye irritation when compared to the in vivo Draize method. Nevertheless, this assay has some limitations, such as subjectivity. Hagino et al. developed an objective evaluation technique using the amount of trypan blue absorbed at the site of treatment as an indicator of injury to the CAM. The present work was aimed at the determination of ocular irritation of 21 substances (chemicals and cosmetics). We used the spectrophotometric quantification by trypan blue staining of the damage produced on CAM, of fertile chicken eggs. Results were compared with the values obtained by the traditional Draize assay. We observed a good correlation (r=0.835; p<0.0001) between the amount of dye absorbed by the CAM and the Draize eye irritation test score. The r values were 0.688; p<0.05 for cosmetics and 0.925; p<0.0001 for chemicals. Three chemicals turned as false positive and one cosmetic substance as false negative. The CAM-TBS assay is inexpensive, simple and provides an in vitro alternative method to predict the damage that chemical substances or cosmetics can cause to the ocular structures.

[Ion channels that are sensitive to the extracellular concentration of protons: their structure, function, pharmacology and pathophysiology]

AIM

Acid sensing ion channels (ASIC) members of the ENaC degenerine channel family, have been shown to participate in various sensorial pathways including nociception, also they have been shown to participate in synaptic transmission, learning and memory processes and in the physiopathology of the ischemic stroke.

DEVELOPMENT

The proton concentration in the organism is strictly regulated by distinct buffer systems. Drastic changes of pH are generated only by pathological conditions as is the ischemia; however, some physiological processes may produce local changes in the extracellular pH. Recently, a new family of proton receptors known as ASIC has been cloned. These are ionic channels inactivated at physiological pH (7.4) and activated with a pH fall (increase in H+ concentration). ASICs are permeable to sodium ions and in a lesser degree to calcium ions, activation of these channels leads to an increase in cell excitability. The ASICs are distributed widely in the central and peripheral nervous system, and in specialized epithelia. In the past few years they have become a focus of interest due to its role in nociception, taste perception, long term potentation and the physiopathology of ischemic stroke.

CONCLUSIONS

In this review we address the most relevant molecular, physiological and pharmacological aspects of the ASICs, its participation in some pathological process, and the perspectives of basic and clinic investigation in this arising research field.

Comparative study of red blood cell method in rat and calves blood as alternatives of Draize eye irritation test

INTRODUCTION

Red blood cell assay (RBC) is used to estimate potential irritation of tensioactive agents and detergents. Cell membrane lysis and cell protein denaturation are measured photometrically. This study was aimed to determine if rat blood cells can be used to predict eye potential irritation in the same way of calves blood cells in RBC assay.

METHODS

We evaluated 20 cosmetic formulations using rat and calves blood according to INVITOX protocol No 37. Data of media hemolysis concentration, denaturation index and the ratio of both parameters were compared with in vivo data of eye irritancy.

RESULTS

There was a significant difference (p<0.01) between H50 value when evaluated the standard SDS with red blood cell method in rat and calves blood. According to the exact probability of Fisher taking as approach the acceptance or rejection of the substance there are no significant differences between in vitro assay with calves blood and in vivo results. Not happening the same way for the RBC assay with rat blood where significant differences were obtained (p<0.01) among the classification of in vitro and in vivo test.

DISCUSSION

The RBC assay using calves blood showed better results. Several test substances were false negatives with rat blood. This high false negative rate would be correctly identified by the animal test but it may also lead to increased animal consumption. For that RBC assay with calf blood cells is preferable to the employment of rat blood as screening method with a reduction and refinement strategy.

[Brain vasculitis associated with drugs]

INTRODUCTION

Vasculitis or angiitis is the term used to define the inflammation of the vessels, either blood or lymphatic, which causes may be primary related to immunological disorders, or secondary, with great variety of causal factors.

CASE REPORT

We present a very uncommon case of a 36 year old man with brain vasculitis associated with drugs with very peculiar characteristics in the imaging studies and with definite diagnosis through histopathology obtained by biopsy, which differs from the few reports in the literature until our days.

CONCLUSIONS

Our report is an uncommon case of cerebral vasculitis whose clinical features were confusing with impressing neuroimaging studies that showed possible lesions due to vasculitis that was confirmed through a cerebral biopsy and considering that the immunological tests were negative for a primary vasculitic process, we concluded that it was a vasculitis secondary to drug abuse which represents a special interest in view of the few existing reports in literature with definite diagnostic methods, such as cerebral biopsy or autopsy.

Efficacy of oral rofecoxib versus intravenous ketoprofen as an adjuvant to PCA morphine after urologic surgery

BACKGROUND

Adjunctive use of nonsteroidal anti-inflammatory drugs has become increasingly popular in the perioperative period because of their opioid-sparing effects. This randomized, controlled, double-dummy study was designed to evaluate the cost-effectiveness of using oral rofecoxib as an alternative to intravenous ketoprofen for pain management in patients undergoing urologic surgery.

METHODS

Seventy patients were randomly assigned to receive either a placebo (Control) or rofecoxib 50 mg po (Rofecoxib) 1 h prior to surgery. After a standardized spinal anesthetic, patients in the Control group received ketoprofen 100 mg IV q 8 h for 24 h, while the Rofecoxib group received an equivolume of saline at 8-h intervals for 24 h. Both groups were allowed to self-administer morphine (1 mg IV boluses) using a PCA delivery system. The need for 'rescue' analgesic medication, as well as pain scores [using an 11-point verbal rating scale (VRS) (0 = none to 10-severe)], were recorded at 1, 2, 6, 12, and 24-h intervals after surgery. In addition, the incidences of side-effects were recorded at the end of the study period.

RESULTS

Total amount of morphine required in the initial 24-h postoperative period was nonsignificantly reduced in the Rofecoxib group (29 +/- 2 vs. 37 +/- 4 mg). More importantly, the percentage of patients reporting moderate-to-severe pain (VRS score > or =4) during the study period was lower in the Rofecoxib group (12 vs. 22%, P < 0.05). The daily cost of rofecoxib (USD 1.14 for 50-mg dose) was also significantly less than ketoprofen (USD 3.06 for three 100-mg doses).

CONCLUSION

Premedication with oral rofecoxib (50 mg) is a cost-effective alternative to the parenteral nonselective NSAID, ketoprofen (100 mg q 8 h), when used as an adjuvant to PCA morphine for pain management after urologic surgery.

A mathematical model of the response of semicircular canal and otolith to head rotation under gravity

The mathematical model of the system composed of two sensors: semicircular canal and sacculus, is presented. The system is described by three series of blocks: biomechanical block, mechanoelectrical transduction mechanism and hair cell ionic currents and membrane potential dynamics. The response of the aforecited system to various stimuli (head rotation under gravity and falling) was investigated. The identification of the model parameters was fulfilled for the experimental data, obtained for the axolotle (Ambystoma tigrinum) in Institute of Physiology, Autonomous University of Puebla, Mexico. The comparative analysis of canal and sacculus membrane potential was realized.

Opioid receptors mediate a postsynaptic facilitation and a presynaptic inhibition at the afferent synapse of axolotl vestibular hair cells

This study was designed to determine the effects of opiate drugs on the electrical activity of afferent neurons and on the ionic currents of hair cells from semicircular canals. Experiments were done on larval axolotls (Ambystoma tigrinum). The multiunit spike activity of afferent neurons was recorded in the isolated inner ear under both resting conditions and mechanical stimulation. Ionic currents were recorded using voltage clamp of hair cells isolated from the semicircular canal. In the isolated inner-ear preparation, microperfusion of either non-specific opioid receptor antagonist naloxone (10 nM to 1 mM), mu receptor agonist [D-Ala(2), N-Me-Phe(4),Gly(5)-ol]-enkephalin (1 pM to 10 microM), or kappa receptor antagonist nor-binaltorphimine (10 nM to 100 microM) elicited a dose-dependent long-lasting (>5 min) increase of the electrical discharge of afferent neurons. The mu receptor agonist funaltrexamine (1 nM to 100 microM) and the kappa receptor agonist U-50488 (1 nM to 10 microM) diminished the basal spike discharge of vestibular afferents. The delta receptor agonist D-Pen(2)-D-Pen(5)-enkephalin (1 nM to 10 mM) and the antagonist naltrindole (1 nM to 10 mM) were without a significant effect. The only drug that displayed a significant action on hair-cell ionic currents was trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]-cyclohexyl) benzeneacetamide methanesulfonate (U-50488) that reduced the Ca(2+) current in a dose-dependent fashion. On its own, mu receptor agonist [D-Ala(2), N-Me-Phe(4),Gly(5)-ol]-enkephalin (0.01 and 10 microM) significantly potentiated the response of afferent neurons to the excitatory amino acid agonist (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (0.1 microM), while synaptic transmission was blocked by the use of high-Mg(2+), low-Ca(2+) solutions. Our data indicate that the activity of vestibular afferent neurons may be regulated in a complex fashion by opioid receptors: mu opioid receptors mediating an excitatory, postsynaptic modulatory input to afferent neurons, and kappa receptors mediating an inhibitory, presynaptic input to hair cells.

Outbreak of nosocomial hepatitis C virus infection resolved by genetic analysis of HCV RNA

In July 2000, symptomatic acute hepatitis C was diagnosed in five patients who had attended the emergency room of a municipal hospital on the same day, about 6 weeks before. Investigation of the remaining 65 patients visited at the emergency room on that day disclosed that 8 patients had a positive anti-hepatitis C virus (anti-HCV) test and 4 of them had biochemical evidence of acute anicteric hepatitis. HCV RNA was detected in 12 of the 13 anti-HCV-positive patients. Phylogenetic analysis of the nonstructural 5A (NS5A) and E2 regions showed that 10 patients, including all 9 with acute hepatitis, were infected with a closely related HCV strain, while the remaining 2 patients harbored unrelated strains. Flushing of intravenous catheters with heparin retrieved from a multidose heparin solution in saline was carried out for all the patients involved in the hepatitis outbreak but in only 1 of 23 (4%) matched controls recruited among HCV-noninfected patients attending the emergency room on the same day, and this was the only significant difference concerning risk factors for HCV infection between patients and controls. Thus, accidental contamination of a multidose heparin solution with blood from an unrecognized HCV carrier was identified as the source of this nosocomial outbreak of hepatitis C.

Betahistine produces post-synaptic inhibition of the excitability of the primary afferent neurons in the vestibular endorgans

Betahistine has been used to treat several vestibular disorders of both central and peripheral origin. The objective of this work was to study the action of betahistine in the vestibular endorgans. Experiments were done in wild larval axolotl (Ambystoma tigrinum). Multiunit extracellular recordings were obtained from the semicircular canal nerve using a suction electrode. Betahistine (10 microM to 10 mM; n = 32) inhibited the basal spike discharge of the vestibular afferent neurons with an IC50 of 600 microM. To define the site of action of betahistine, its interactions with the nitric oxide synthase inhibitor NG-nitro-L-arginine (3 microM) and with the cholinergic antagonists atropine (10 microM; n = 3) and d-tubocurarine (10 microM; n = 3) were studied. The action of betahistine when co-administered with these drugs was the same as that in control experiments, indicating that its effects did not include nitric oxide production or the activation of cholinergic receptors. In contrast, 0.01-1 mM betahistine reduced the excitatory action of kainic acid (10 microM; n = 6) and quiscualic acid (1 microM; n = 13). These results indicate that the action of betahistine on the spike discharge of afferent neurons seems to be due to a post-synaptic inhibitory action on the primary afferent neuron response to the hair cell neurotransmitter.

Immunohistochemical localization and electrophysiological action of nociceptin/orphanin-FQ in the snail (Helix aspersa) neurons

We report the existence and anatomical distribution to nociceptin/orphanin-FQ (N/O FQ)-like immunoreactivity in neurons and fibers in the perioesophageal ganglia of the snail (Helix aspersa). Intracellular recordings from perioesophageal ganglion neurons showed that the application of 10 microM N/O FQ produced an excitatory action in 22% of the neurons studied and an inhibitory action in 33% of the neurons regardless of their origin (cerebral or parietal ganglion). Our result provides evidence that N/O FQ-like peptide is located in whole perioesophageal ganglia (mainly in the cerebral one), and that it may serve as a neuromodulator of the neuronal spike discharge. These data support the idea that the N/O FQ opioid system has an early phylogenetic origin and a functional continuity during the course of evolution.

Nitric oxide in the afferent synaptic transmission of the axolotl vestibular system

This study was performed using intracellular and multiunit extracellular recording techniques in order to characterize the role of nitric oxide in the afferent synaptic transmission of the vestibular system of the axolotl (Ambystoma tigrinum). Bath application of nitric oxide synthase inhibitors N(G)-nitro-L-arginine (0.01microM to 10microM) and N-nitro-L-arginine methyl ester hydrochloride (0.1microM to 1000microM) elicited a dose-dependent decrease in the basal discharge of the semicircular canal afferent fibers. N(G)-Nitro-L-arginine also diminished the response to mechanical stimuli. Moreover, N(G)-nitro-L-arginine (1microM) produced a hyperpolarization associated with a decrease in the spike discharge and diminished the frequency of the excitatory postsynaptic potentials on afferent fibers recorded intracellularly. Nitric oxide donors were also tested: (i) S-nitroso-N-acetyl-DL-penicillamine (0.1microM to 100microM) increased the basal discharge and the response to mechanical stimuli. At the maximum effective concentration (100microM) this drug affected neither the amplitude nor the frequency of the excitatory postsynaptic potentials. However, it slightly depolarized the afferent neurons and decreased their input resistance. (ii) 3-Morpholino-sydnonimine hydrochloride did not significantly affect the basal discharge or the mechanically evoked peak response of afferent neurons at any of the concentrations used (1microM to 1000microM). However, after 10min of perfusion in the bath, 1microM and 10microM 3-morpholino-sydnonimine hydrochloride significantly modified the baseline of the mechanically evoked response, producing an increase in the mean spike discharge of the afferent fibers. These results indicate that nitric oxide may have a facilitatory role on the basal discharge and on the response to mechanical stimuli of the vestibular afferent fibers. Thus, nitric oxide probably participates in the sensory coding and adaptative changes of vestibular input in normal and pathological conditions.

Action mechanism of betahistine in the vestibular end organs

Betahistine has been used to treat several vestibular disorders of both central and peripheral origin. The objective of this work was to study the betahistine action mechanism at the vestibular end organs. Experiments were carried out in wild larval axolotl (Ambystoma tigrinum). Multiunit extracellular recordings were obtained from the semicircular canal nerve using a suction electrode. Betahistine (10 microM to 10 mM, n = 32) inhibited the basal spike discharge of the vestibular afferent neurons with an IC50 of 600 microM. To define the site of action of betahistine, its interactions with antagonists of nitric oxide sintethizing enzyme, cholinergic drugs, and excitatory amino acids were studied. Betahistine 1 mM (n = 5) was coadministered with NG-nitro-L-arginine 3 microM. The action of betahistine remained as in control experiments. Betahistine 1 mM reduced the excitatory action of carbachol (200 microM, n = 5) in a 30 +/- 3.4%. Cholinergic antagonists atropine (10 microM, n = 3) and d-tubocurarine (10 microM, n = 3) did not modify betahistine actions. Betahistine 1 mM also reduced kainic acid (10 microM, n = 4) excitatory action in 45.5 +/- 9.8%. These results corroborate that betahistine has a peripheral inhibitory action in the spike discharge of the afferent neurons in the vestibule. This action seems to involve neither NO production nor modifications in the release of acetylcholine from the efferent fibers. The inhibitory action of betahistine seems to be due to a postsynaptic binding site on the afferent neurons.