fMRI and MEG analysis of visceral pain in healthy volunteers

BACKGROUND

Although many studies of painful rectal stimulation have found activation in the insula, cingulate, somatosensory, prefrontal cortices and thalamus, there is considerable variability when comparing functional magnetic resonance imaging (fMRI) results. Multiple factors may be responsible, including the model used in fMRI data analysis. Here, we assess the temporal response of activity to rectal barostat distension using novel fMRI and magnetoencephalography (MEG) analysis.

METHODS

Liminal and painful rectal barostat balloon inflation thresholds were assessed in 14 female healthy volunteers. Subliminal, liminal and painful 40s periods of distension were applied in a pseudo-randomized paradigm during fMRI and MEG neuroimaging. Functional MRI data analysis was performed comparing standard box-car models of the full 40s of stimulus (Block) with models of the inflation (Ramp-On) and deflation (Ramp-Off) of the barostat. Similar models were used in MEG analysis of oscillatory activity.

KEY RESULTS

Modeling the data using a standard Block analysis failed to detect areas of interest found to be active using Ramp-On and Ramp-Off models. Ramp-On generated activity in anterior insula and cingulate regions and other pain-matrix associated areas. Ramp-Off demonstrated activity of a network of posterior insula, SII and posterior cingulate. Active areas were consistent with those identified from MEG data.

CONCLUSIONS & INFERENCES

In studies of visceral pain, fMRI model design strongly influences the detected activity and must be accounted for to effectively explore the fMRI data in healthy subjects and within patient groups. In particular a strong cortical response is detected to inflation and deflation of the barostat, rather than to its absolute volume.

Contrasting nutrient effects on the plasma levels of an amino acid-like antiepileptic agent from jejunal administration in dogs

The absorption of gabapentin was investigated by monitoring drug plasma levels as a function of time following midjejunal administration in mongrel dogs. From previous work, dose-dependent absorption had been postulated to be a consequence of carrier-mediated transport and a paracellular pathway had been postulated to contribute to the passive absorption component in mammalian small intestine. The potential for amino acid inhibition of the carrier-mediated absorption component was investigated by drug coinfusion with leucine and phenylalanine. The potential for monosaccharide-enhanced increases in drug absorption was studied by drug coinfusion with D-glucose and 3-O-methylglucose. While lower drug plasma levels were observed with amino acid coinfusion versus controls in each of the dogs studied, mean area under the plasma level time curves (AUC) were not statistically significantly different (p < or = 0.07). Monosaccharide coinfusion significantly increased gabapentin AUC over control studies (p < or = 0.014) and over coinfusion with L-system amino acids (p < or = 0.0025). Implications for the mechanisms of intestinal absorption of this amino acid-like antiepileptic drug in this canine model are discussed.

Restricting a familiar name in response to learning a new one: evidence for the mutual exclusivity bias in young two-year-olds

Children under 2 1/2 years old tend to interpret novel words in accordance with the Mutual Exclusivity Principle, but tend not to reinterpret familiar words this way. Because alternative principles have been proposed that only predict the novel word effects, and because tests of the familiar word effects may have been flawed, a new test was administered. In Experiment 1 (N = 32), 24- to 25-month-olds heard stories in which a novel noun was used for an atypical exemplar of a familiar noun. When asked to select exemplars of the familiar noun, they showed a small but reliable tendency to avoid the object from the story. In Experiment 2 (N = 16), the novel nouns in the stories were replaced by pronouns and proper names, and the children did not avoid the story object in the test of the familiar noun. Thus, the aversion to this object that was observed in Experiment 1 was not due to its greater exposure or its being referenced immediately before testing, but to toddlers' Mutual Exclusivity bias. Their bias is hypothesized to be a form of implicit probabilistic knowledge that derives from the competitive nature of category retrieval.

Colonic absorption of antiepileptic agents

PURPOSE

To evaluate a canine intestinal accessport model to study colonic absorption of drugs. The antiepileptic drugs phenytoin and gabapentin were chosen to study absorption of a lipophilic and hydrophilic compound, respectively.

METHODS

Drug plasma level-time plots were generated subsequent to small intestinal and colonic drug administration of both drugs. The poorly water-soluble phenytoin was administered in two doses to evaluate the impact of dissolution rate limits on colonic absorption. Maximal plasma concentration (Cmax) and area under the plasma level-time curve (AUC) were used to assess the relative contribution of colonic absorption to plasma levels.

RESULTS

Whereas colonic gabapentin AUC and Cmax were only 0.25 and 0.15 of those seen after small intestinal administration, colonic phenytoin AUC and Cmax were one half and equivalent to, respectively, those observed for small intestinal administration. Furthermore, colonic administration of a higher phenytoin dose showed secondary maxima and continued increases in drug plasma levels with time.

CONCLUSIONS

Colonic gabapentin absorption is poor compared with upper intestinal absorption, consistent with membrane transport rate limits to the absorption of this hydrophilic AED. Peak phenytoin plasma levels from colonic and small intestinal administration are comparable, indicating membrane transport does not limit absorption of this lipophilic agent. Continued plasma-level increases from higher phenytoin doses are consistent with dissolution-rate control of drug absorption in the colon. We suggest that colonic absorption provides a greater potential for toxicity from phenytoin overdose as a function of continued drug dissolution than for gabapentin overdose.

Emergence of multiply resistant pneumococci

Multiple antimicrobial resistance in pneumococci was detected in Johannesburg in July, 1977, and prompted an investigation of the prevalence of resistant strains in two hospitals. Carriers of Types 6A and 19A penicillin-resistant pneumococci, resistant to antibiotic concentrations ranging between 0.12 and 4 microgram per milliliter were found in 29 per cent of 543 pediatric patients and 2 per cent of 434 hospital staff members. Multiply resistant Type 19A strains, resistant to beta-lactam antibiotics, erythromycin, clindamycin, tetracycline and chloramphenicol, were isolated from 128 carriers, and were responsible for bacteremia in four patients. Isolates from 40 other carriers were resistant to penicillin alone or to penicillin and chloramphenicol or to penicillin, chloramphenicol and tetracycline. Pneumococci can be screened for penicillin resistance with a modified Kirby--Bauer technic; the strains with zones of less than 35 mm around 6-microgram penicillin disks or less than 25 mm around 5-microgram methicillin disks should be tested for sensitivity to penicillin by measurements of minimum inhibitory concentration.