Ambient and laboratory observations of organic ammonium salts in PM1.

Ambient measurements of PM1 aerosol chemical composition at Cabauw, the Netherlands, implicate higher ammonium concentrations than explained by the formation of inorganic ammonium salts. This additional particulate ammonium is called excess ammonium (eNH4). Height profiles over the Cabauw Experimental Site for Atmospheric Research (CESAR) tower, of combined ground based and airborne aerosol mass spectrometric (AMS) measurements on a Zeppelin airship show higher concentrations of eNH4 at higher altitudes compared to the ground. Through flights across the Netherlands, the Zeppelin based measurements furthermore substantiate eNH4 as a regional phenomenon in the planetary boundary layer. The excess ammonium correlates with mass spectral signatures of (di-)carboxylic acids, making a heterogeneous acid-base reaction the likely process of NH3 uptake. We show that this excess ammonium was neutralized by the organic fraction forming particulate organic ammonium salts. We discuss the significance of such organic ammonium salts for atmospheric aerosols and suggest that NH3 emission control will have benefits for particulate matter control beyond the reduction of inorganic ammonium salts.

Stage-specific activity patterns affect motoneuron axonal retraction and outgrowth during the metamorphosis of Manduca sexta.

During the metamorphosis of holometabolous insects, most larval muscles and sensory neurons are replaced by new adult elements, whereas most motoneurons persist and are remodelled to serve new adult functions. In Manduca sexta, the formation of the anlagen of the adult dorsal longitudinal flight muscle (DLM) is characterized by retraction of axonal terminals and dendrites of persisting larval motoneurons, partial target muscle degeneration and myoblast accumulation during late larval life. Most of these structural changes have been attributed to hormonal control, not only because ecdysteroids govern metamorphosis, but also because motoneurons express ecdysteroid receptors and experimental manipulations of ecdysteroid titres perturb normal development. To test whether activity-dependent mechanisms also came into play, chronic extracellular recordings were conducted in vivo from the five future DLM motoneurons throughout the last 3 days of larval life. Motoneuron activity is regulated developmentally. The types of motoneurons recruited, the number of motor spikes and the duration of bursts change in a stereotypical fashion during different stages, indicating an internal control of motor activity. A characteristic cessation in the activity of the five future DLM motoneurons coincides in time with the retraction of their dendrites and their terminal arborizations, whereas their activation during ecdysis coincides with the onset of new outgrowth. Inducing advanced activity by stimulating the motoneurons selectively with ecdysis-like patterns results in significant outgrowth of their terminal arborizations. Therefore, steroids might act in concert with activity-dependent mechanisms during the postembryonic modifications of neuromuscular systems.

Distribution and activation of different types of octopaminergic DUM neurons in the locust.

The first part of this study describes the distribution of all different types of octopaminergic, efferent dorsal unpaired median (DUM) neurons in the first two thoracic ganglia by immunocytochemistry, retrograde labeling, and intracellular staining. The prothoracic ganglion contains five different types of 10 DUM neurons. The mesothoracic ganglion has 21 octopaminergic somata in the DUM neuron cluster. Retrograde labeling and intracellular staining show that 19 of these 21 somata belong to five different types of efferent DUM neurons. In both ganglia, the number and the distribution of all types of DUM neurons are completely described. Differences in the distribution of efferent DUM neurons between the thoracic ganglia are discussed as functional segmental specializations. In the second part, we show that, in contrast to previous suggestions, DUM neurons are not recruited as a homogeneous population mediating general arousal but differentially, thus forming subpopulations of specific types. The existence or the absence of commonly occurring postsynaptic potentials in paired recordings clearly shows that only specific types of DUM neurons are targeted by the same presynaptic pathways. Within the thoracic ganglia, different subpopulations of DUM neurons can be distinguished by their different local inputs. Furthermore, only specific subpopulations of DUM neurons receive common intersegmental drive and inputs from the subesophageal ganglion. As a result of all our recordings, we propose a scheme for the differential activation of efferent DUM neurons. This scheme is sufficient to explain DUM neuron activity during principal motor programs.