Rapid tryptophan depletion improves decision-making cognition in healthy humans without affecting reversal learning or set shifting

Rapid tryptophan (Trp) depletion (RTD) has been reported to cause deterioration in the quality of decision making and impaired reversal learning, while leaving attentional set shifting relatively unimpaired. These findings have been attributed to a more powerful neuromodulatory effect of reduced 5-HT on ventral prefrontal cortex (PFC) than on dorsolateral PFC. In view of the limited number of reports, the aim of this study was to independently replicate these findings using the same test paradigms. Healthy human subjects without a personal or family history of affective disorder were assessed using a computerized decision making/gambling task and the CANTAB ID/ED attentional set-shifting task under Trp-depleted (n=17; nine males and eight females) or control (n=15; seven males and eight females) conditions, in a double-blind, randomized, parallel-group design. There was no significant effect of RTD on set shifting, reversal learning, risk taking, impulsivity, or subjective mood. However, RTD significantly altered decision making such that depleted subjects chose the more likely of two possible outcomes significantly more often than controls. This is in direct contrast to the previous report that subjects chose the more likely outcome significantly less often following RTD. In the terminology of that report, our result may be interpreted as improvement in the quality of decision making following RTD. This contrast between studies highlights the variability in the cognitive effects of RTD between apparently similar groups of healthy subjects, and suggests the need for future RTD studies to control for a range of personality, family history, and genetic factors that may be associated with 5-HT function.

What is a reasonable time from decision to delivery by Caesarean section? Evidence from 533 deliveries.

Objective To determine how long it takes from the decision to achieve delivery by non-elective caesarean section (DDI), the influence on this interval, and the impact on neonatal condition at birth. Design Twelve months prospective data collection on all non-elective caesarean sections. Methods Prospective collection of data relating to all caesarean sections in 1996 in a major teaching hospital obstetric unit was conducted, without the knowledge of the other clinicians providing clinical care. Details of the indication for section, the day and time of the decision and the interval till delivery were recorded as well as the seniority of the surgeon, and condition of the baby at birth. Results The mean time from decision-to-delivery for 100 emergency intrapartum caesarean sections was 42.9 minutes for fetal distress and 71.1 minutes for 230 without fetal distress (P<0.0001). For 22 'crash' sections the mean time from decision-to-delivery was 27.4 minutes; for 13 urgent antepartum deliveries for fetal reasons it was 124.7 minutes and for 21 with maternal reasons it was 97.4 minutes. The seniority of the surgeon managing the patient did not appear to influence the interval, nor did the time of day or day of the week when the delivery occurred. Intrapartum sections were quicker the more advanced the labour, and general anaesthesia was associated with shorter intervals than regional anaesthesia for emergency caesarean section for fetal distress (P<0.001). Babies born within one hour of the decision tended to be more acidaemic than those born later, irrespective of the indication for delivery. Babies tended to be in better condition when a time from decision-to-delivery was not recorded than those for whom the information had been recorded. Conclusion Fewer than 40% intrapartum deliveries by caesarean section for fetal distress were achieved within 30 minutes of the decision, despite that being the unit standard. There was, however, no evidence to indicate that overall an interval up to 120 minutes was detrimental to the neonate unless the delivery was a 'crash' caesarean section. These data thus do not provide evidence to sustain the recommendation of a standard of 30 minutes for intrapartum delivery by caesarean section.

Diatom carbon export enhanced by silicate upwelling in the North East Atlantic.

Diatom carbon export enhanced by silicate upwelling in the northeast Atlantic John T. Allen1,2, Louise Brown1,3, Richard Sanders1, C. Mark Moore1, Alexander Mustard1, Sophie Fielding1, Mike Lucas1, Michel Rixen4, Graham Savidge5, Stephanie Henson1 and Dan Mayor1 Top of pageDiatoms are unicellular or chain-forming phytoplankton that use silicon (Si) in cell wall construction. Their survival during periods of apparent nutrient exhaustion enhances carbon sequestration in frontal regions of the northern North Atlantic. These regions may therefore have a more important role in the 'biological pump' than they have previously been attributed1, but how this is achieved is unknown. Diatom growth depends on silicate availability, in addition to nitrate and phosphate2, 3, but northern Atlantic waters are richer in nitrate than silicate4. Following the spring stratification, diatoms are the first phytoplankton to bloom2, 5. Once silicate is exhausted, diatom blooms subside in a major export event6, 7. Here we show that, with nitrate still available for new production, the diatom bloom is prolonged where there is a periodic supply of new silicate: specifically, diatoms thrive by 'mining' deep-water silicate brought to the surface by an unstable ocean front. The mechanism we present here is not limited to silicate fertilization; similar mechanisms could support nitrate-, phosphate- or iron-limited frontal regions in oceans elsewhere.