Developmental Sex Differences in Nicotinic Currents of Prefrontal Layer VI Neurons in Mice and Rats

There is a large sex difference in the prevalence of attention deficit disorder; yet, relatively little is known about sex differences in the development of prefrontal attention circuitry. In male rats, nicotinic acetylcholine receptors excite corticothalamic neurons in layer VI, which are thought to play an important role in attention by gating the sensitivity of thalamic neurons to incoming stimuli. These nicotinic currents in male rats are significantly larger during the first postnatal month when prefrontal circuitry is maturing. The present study was undertaken to investigate whether there are sex differences in the nicotinic currents in prefrontal layer VI neurons during development.Using whole cell recording in prefrontal brain slice, we examined the inward currents elicited by nicotinic stimulation in male and female rats and two strains of mice. We found a prominent sex difference in the currents during the first postnatal month when males had significantly greater nicotinic currents in layer VI neurons compared to females. These differences were apparent with three agonists: acetylcholine, carbachol, and nicotine. Furthermore, the developmental sex difference in nicotinic currents occurred despite male and female rodents displaying a similar pattern and proportion of layer VI neurons possessing a key nicotinic receptor subunit.This is the first illustration at a cellular level that prefrontal attention circuitry is differently affected by nicotinic receptor stimulation in males and females during development. This transient sex difference may help to define the cellular and circuit mechanisms that underlie vulnerability to attention deficit disorder

Combining Nitrous Oxide with Carbon Dioxide Decreases the Time to Loss of Consciousness during Euthanasia in Mice — Refinement of Animal Welfare?

Carbon dioxide (CO2) is the most commonly used euthanasia agent for rodents despite potentially causing pain and distress. Nitrous oxide is used in man to speed induction of anaesthesia with volatile anaesthetics, via a mechanism referred to as the “second gas” effect. We therefore evaluated the addition of Nitrous Oxide (N2O) to a rising CO2 concentration could be used as a welfare refinement of the euthanasia process in mice, by shortening the duration of conscious exposure to CO2. Firstly, to assess the effect of N2O on the induction of anaesthesia in mice, 12 female C57Bl/6 mice were anaesthetized in a crossover protocol with the following combinations: Isoflurane (5%)+O2 (95%); Isoflurane (5%)+N2O (75%)+O2 (25%) and N2O (75%)+O2 (25%) with a total flow rate of 3l/min (into a 7l induction chamber). The addition of N2O to isoflurane reduced the time to loss of the righting reflex by 17.6%. Secondly, 18 C57Bl/6 and 18 CD1 mice were individually euthanized by gradually filling the induction chamber with either: CO2 (20% of the chamber volume.min−1); CO2+N2O (20 and 60% of the chamber volume.min−1 respectively); or CO2+Nitrogen (N2) (20 and 60% of the chamber volume.min−1). Arterial partial pressure (Pa) of O2 and CO2 were measured as well as blood pH and lactate. When compared to the gradually rising CO2 euthanasia, addition of a high concentration of N2O to CO2 lowered the time to loss of righting reflex by 10.3% (P<0.001), lead to a lower PaO2 (12.55±3.67 mmHg, P<0.001), a higher lactataemia (4.64±1.04 mmol.l−1, P = 0.026), without any behaviour indicative of distress. Nitrous oxide reduces the time of conscious exposure to gradually rising CO2 during euthanasia and hence may reduce the duration of any stress or distress to which mice are exposed during euthanasia