Changes in fos expression in the rat brain after unilateral lesions of the anterior thalamic nuclei

Activity of the immediate early gene c-fos was compared across hemispheres in rats with unilateral anterior thalamic lesions. Fos protein was quantified after rats performed a spatial working memory test in the radial-arm maze, a task that is sensitive to bilateral lesions of the anterior thalamic nuclei. Unilateral anterior thalamic lesions produced evidence of a widespread hippocampal hypoactivity, as there were significant reductions in Fos counts in a range of regions within the ipsilateral hippocampal formation (rostral CA1, rostral dentate gyrus, 'dorsal' hippocampus, presubiculum and postsubiculum). A decrease in Fos levels was also found in the rostral and caudal retrosplenial cortex but not in the parahippocampal cortices or anterior cingulate cortices. The Fos changes seem most closely linked to sites that are also required for successful task performance, supporting the notion that the anterior thalamus, retrosplenial cortex and hippocampus form key components of an interdependent neuronal network involved in spatial mnemonic processing.

Fos imaging reveals that lesions of the anterior thalamic nuclei produce widespread limbic hypoactivity in rats

Activity of the immediate early gene c-fos was compared in rats with neurotoxic lesions of the anterior thalamic nuclei and in surgical controls. Fos levels were measured after rats had been placed in a novel room and allowed to run up and down preselected arms of a radial maze. An additional control group showed that in normal rats, this exposure to a novel room leads to a Fos increase in a number of structures, including the anterior thalamic nuclei and hippocampus. In contrast, rats with anterior thalamic lesions were found to have significantly less Fos-positive cells in an array of sites, including the hippocampus (dorsal and ventral), retrosplenial cortex, anterior cingulate cortex, and prelimbic cortex. These results show that anterior thalamic lesions disrupt multiple limbic brain regions, producing hypoactivity in sites associated in rats with spatial memory. Because many of the same sites are implicated in memory processes in humans (e.g., the hippocampus and retrosplenial cortex), this hypoactivity might contribute to diencephalic amnesia.

Photometry of cometary nuclei: Rotation rates, colours and a comparison with Kuiper Belt Objects

We present time-series data on Jupiter Family Comets (JFCs) 17P/Holmes, 47P/Ashbrook-Jackson and 137P/Shoemaker-Levy 2. In addition we also present results from `snap-shot' observations of comets 43P/Wolf-Harrington, 44P/Reinmuth 2, 103P/Hartley 2 and 104P/Kowal 2 taken during the same run. The comets were at heliocentric distances of between 3 and 7 au at this time. We present measurements of size and activity levels for the snap-shot targets. The time-series data allow us to constrain rotation periods and shapes, and thus bulk densities. We also measure colour indices (V - R) and (R - I) and reliable radii for these comets. We compare all of our findings to date with similar results for other comets and Kuiper Belt Objects (KBOs). We find that the rotational properties of nuclei and KBOs are very similar, that there is evidence for a cut-off in bulk densities at ~0.6 g cm-3 in both populations, and the colours of the two populations show similar correlations. For JFCs, there is no observational evidence for the optical colours being dependent on either position in the orbit or orbital parameters.

The nuclei of comets 7P/Pons-Winnecke, 14P/Wolf and 92P/Sanguin

Jupiter Family comets (JFCs) are short period comets which have recently entered the inner solar system, having previously orbited in the Kuiper Belt since the formation of the planets. We used two nights on the 3.6 m New Technology Telescope (NTT) at the European Southern Observatory, to obtain VRI photometry of three JFCs; 7P/Pons-Winnecke, 14P/Wolf and 92P/Sanguin. These were observed to be stellar in appearance. We find mean effective radii of 2.24 ± 0.02 km for 7P, 3.16 ± 0.01 km for 14P and 2.08 ± 0.01 km for 92P, assuming a geometric albedo of 0.04. From light-curves for each comet we find rotation periods of 7.53 ± 0.10 and 6.22 ± 0.05 h for 14P and 92P respectively. 7P exhibits brightness variations which imply a rotation period of 6.8 = Prot = 9.5 h. Assuming the nuclei to be ellipsoidal the measured brightness variations imply minimum axial ratios a/b of 1.3 ± 0.1 for 7P and 1.7 ± 0.1 for both 14P and 92P. This in turn implies minimum densities of 0.23 ± 0.08 g cm-3 for 7P, 0.32 ± 0.02 g cm-3 for 14P and 0.49 ± 0.06 g cm-3 for 92P. Finally, we measure colour indices of (V-R) = 0.40 ± 0.05 and (R-I) = 0.41 ± 0.06 for 7P/Pons-Winnecke, (V-R) = 0.57 ± 0.07 and (R-I) = 0.51 ± 0.06 for 14P/Wolf, and (V-R) = 0.54 ± 0.04 and (R-I) = 0.54 ± 0.04 for 92P/Sanguin.

Nonlinear excitations in electron-positron-ion plasmas in accretion disks of active galactic nuclei

The propagation of acoustic nonlinear excitations in an electron-positron-ion (e-p-i) plasma composed of warm electrons and positrons, as well as hot ions, has been investigated by adopting a two-dimensional cylindrical geometry. The electrons and positrons are modeled by hydrodynamic fluid equations, while the ions are assumed to follow a temperature-parametrized Boltzmann distribution (the fixed ion model is recovered in the appropriate limit). This situation applies in the accretion disk near a black hole in active galactic nuclei, where the ion temperature may be as high as 3 to 300 times that of the electrons. Using a reductive perturbation technique, a cylindrical Kadomtsev-Petviashvili equation is derived and its exact soliton solutions are presented. Furthermore, real situations in which the strength of the nonlinearity may be weak are considered, so that higher-order nonlinearity plays an important role. Accordingly, an extended cylindrical Kadomtsev-Petviashvili equation is derived, which admits both soliton and double-layer solutions. The characteristics of the nonlinear excitations obtained are investigated in detail

The size distribution of Jupiter Family comet nuclei

We present an updated cumulative size distribution (CSD) for Jupiter Family comet (JFC) nuclei, including a rigorous assessment of the uncertainty on the slope of the CSD. The CSD is expressed as a power law, N(>rN) ?r-qN, where rN is the radius of the nuclei and q is the slope. We include a large number of optical observations published by us and others since the comprehensive review in the Comets II book, and make use of an improved fitting method. We assess the uncertainty on the CSD due to all of the unknowns and uncertainties involved (photometric uncertainty, assumed phase function, albedo and shape of the nucleus) by means of Monte Carlo simulations. In order to do this we also briefly review the current measurements of these parameters for JFCs. Our final CSD has a slope q= 1.92 ± 0.20 for nuclei with radius rN= 1.25 km.