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Notes: Summary To identify zinc-containing projections to the visual areas, we injected Fluoro-Gold into the occipital cortex of the mouse. Five days later, the mice underwent an intravital selenium-labeling procedure to demonstrate the somata of neurons that give rise to zinc-containing boutons. Numerous double-labeled cells were seen in the ipsi- and contralateral primary (layers II/III and VI), and secondary visual cortices (layers II/III and VI). A few double-labeled cells were apparent in other cortical areas concerned with visual processing: the orbital cortex (layers II and III), the posterior portion of the medial agranular frontal cortex (layer V/VI border), and the temporal cortex (layer VI). The cingulate, retrosplenial, perirhinal, and lateral entorhinal cortices had lamina projecting to the visual cortex and separate lamina harboring zinc-containing cells. A spatial segregation of fluorescent and zinc-containing neurons was also seen in the claustrum. This integration or segregation of projecting and zinc-containing neurons may reflect the function of the cortical areas. N-methyl-d-aspartate receptor function is antagonized by physiological concentrations of zinc in vitro. It is proposed that zinc-positive projections from areas that perform basic visual functions are less likely to be modified by N-methyl-d-aspartate receptor-mediated processes than the zinc-negative connections from associational areas.

Notes: Abstract The purpose of the present study was to examine whether zinc-positive and zinc-negative hippocampal neurons in rats differed with respect to their projections to the septum. By combining retrograde axonal transport of the fluorescent tracer Fluoro-Gold with histochemical demonstration of zinc selenide complexes in zinc-containing neurons after intraperitoneal injection of sodium selenite, we were able to visualize the distribution of retrogradely Fluoro-Gold labeled neurons and zinc-containing neurons in the same sections. After unilateral injection of Fluoro-Gold into the rat septum a few retrogradely labeled cells were observed in layer IV of the ipsilateral medial entorhinal area, and numerous labeled cells were observed mainly in the superficial layers of the ipsilateral subicular areas and throughout the CA1 and CA3 pyramidal cell layers, as well as in the contralateral CA3 pyramidal cell layer. Zinc-containing neurons were observed in layers IV–VI of the medial entorhinal area, layers II and III of the parasubiculum, layers II, III and V of presubiculum, and in the superficial CA1 and deep CA3 pyramidal cell layers. Cells double-labeled with Fluoro-Gold and zinc selenide complexes were primarily located in distal (relative to the area dentata) parts of the superficial CA1 pyramidal cell layer and distal parts of the deep CA3 pyramidal cell layer and in layers II and III of presubiculum. Only a very few double-labeled cells were seen in the contralateral CA3. The result demonstrates that the hippocampo-septal projection of rats is a mixture of zinc-positive and zinc-negative fibers. Where-as zinc-negative fibers originate from neurons throughout the hippocampal and retrohippocampal areas, zinc-positive fibers originate from distinct subgroups of zinc-containing cells in different areas and layers.

Notes: Abstract Denitrification was studied in the sediment of a shallow, Danish estuary in the spring and summer of 1982 and 1983. The acetylene blockage technique was used in undisturbed cores to estimate the in-situ activity of the process. A declining rate of denitrification was observed at the onset of the growth season when the allochthonous input of nitrate levelled off; the denitrification decreased from 5.1 mmol N m-2 d-1 in the early spring (April) to 0.3 mmol N m-2 d-1 in the early summer (June). The activity in the uppermost centimeter comprised more than 70% of the total. In this surface zone, a diurnal pulse of oxygen production from photosynthesis of benthic microalgae was found to affect the denitrification; the later had a distinct minimum in the day when the oxygen production was high and inhibitory. The subsequent, nocturnal stimulation of the denitrification was typically a factor of three or more in the investigated periods. We propose that oxygen is an important control factor for the daily, in-situ denitrification in shallow estuaries where light is adequate to reach the sediment. The availability of nitrate seems more important as a control factor for the seasonal variation of the process.

Notes: Abstract A closed chamber technique was developed to determine the emission of microbially produced N2O from an estuarine sediment. A diurnal variation was observed; maximum emissions of 0.4 to 4.0 μmol N2O−N m-2 h-1 were recorded at night whereas the rates were low or even negative, -0.4 to 0.4 μmol N2O−N m-2 h-1, during the day. The bacterial denitrification located in the uppermost centimeter was apparently the major source of the emitted N2O. The diurnal emission pattern was thus inversely related to the O2 availability at the sediment surface; in the dark, the lack of O2 production by benthic photosynthesis allowed the denitrification to occur closer to the sediment-water interface and was likely to enhance the release of N2O to the water. The daily averages for the emission were about 40 μmol N2O−N m-2 d-1 for three investigation periods in autumn (November), winter (February) and spring (April), whereas no significant emission was recorded in the NO 3 - -depleted sediment in early summer (June). In this estuary, the N2O emissions from the sediment were significant contributions to the overall release of N2O to the atmosphere.

Notes: Summary Fetal rat neocortex grafted into lesion cavities made in the newborn rat neocortex can exchange multiple axonal connections with the host brain. Most previous studies demonstrating efferent transplant-tohost brain connections have used fluorescent retrograde tracers injected into the host brain (Castro et al. 1985, 1987; Floeter and Jones 1984; O'Leary and Stanfield 1989). Other studies have used anterograde axonal tracing with either tritium-labelled amino acids impregnating the transplant and its efferents (Floeter and Jones 1985) or horseradish peroxidase injected into the transplants (Chang et al. 1984, 1986). In the present study we used the anterograde axonal tracer Phaseolus vulgaris — leucoagglutinin (PHA-L) to examine in detail the course and termination of the efferent neocortical graft fibers. Twenty-six newborn rats had the right frontal cortex forepaw area removed by vacuum aspiration, while anesthetized by hypothermia. A piece of fetal frontal cortex 14–16 embryonic days old (E14–16) was immediately thereafter placed in the lesion, and the recipient rats allowed to survive for 5–7 months. At this time the rats were reoperated under sodium pentobarbital (Nembutal) anesthesia and the transplants iontophoretically injected with PHA-L. Two weeks later the animals were again anesthetized, perfused, and processed for PHA-L immunocytochemistry and routine histology. Analysis of acetylcholinesterase- (AChE) and Nissl-stained sections showed graft survival in 19 of the 26 animals used in this study. When these 19 brains were processed for PHA-L immunocytochemistry, 5 of them were found with certainty to have the PHA-L injection confined to the transplant. Based on these cases PHA-L-reactive fibers arising from labelled transplant neurons were traced into the ipsilateral host neocortex adjacent to the transplant and found to project through the subcortical white matter to the ipsilateral parietal neocortical area 1, and claustrum. Callosal fibers were traced to the contralateral frontal neocortical forelimb and parietal areas. Transplant fibers were also observed to descend through the caudate putamen in the dispersed fiber bundles of the internal capsule to distribute as terminal branches and varicose fibers within the mesencephalic periaqueductal gray, red nucleus, deep mesencephalic nucleus, and intermediate gray of the superior colliculus, as well as in the pontine gray. Similar fibers and terminations were present in the caudate putamen, the reticular, ventrobasal, centrolateral, posterior, and parafascicular thalamic nuclei. On the side contralateral to the transplant, fewer fibers were observed in the caudate putamen, the ventrobasal, centrolateral, and posterior thalamic nuclei, as well as more caudally in the deep mesencephalic nucleus and the intermediate gray of the superior colliculus. Our findings demonstrate that homotopic grafts of fetal rat frontal neocortex can project to the developing host brain in a manner which for most projections corresponds to the normal rat neocortical parietal area 1–2 and forelimb area. The density of these transplant-to-host projections is, however, less than in the normal rat corticofugal pathways.

Notes: Abstract Thalamic neuronal degeneration after neocortical lesions involve both anterograde and retrograde components. This study deals with the thalamic microglial response after neocortical aspiration lesions, using fluorogold fluorescent prelabeling, to identify retrogradely degenerating thalamocortical neurons, combined with histochemical or immunohistochemical staining of microglial cells. Adult male Wistar rats were injected with the retrograde fluorescent tracer fluorogold, in the right sensorimotor cortex (forepaw area) in order to retrogradely label thalamic neurons projecting to this area. After 1 week, the fluorogold injection site was removed by aspiration, axotomizing at the same time the thalamic projection neurons now retrogradely labeled with fluorogold. After 3, 7, 14, and 28 days the animals were killed and processed for nucleoside diphosphatase histochemistry or complement type 3 receptor immunohistochemistry and class I and II major histocompatibility complex immunohistochemistry using OX42, OX18, and OX6 antibodies. The histological analysis showed a prominent and progressive nucleoside diphosphatase-,OX42-, and OX6-positive microglial cell response in the ventrolateral, posterior, and ventrobasal thalamic nuclei with ongoing retrograde and anterograde neuronal degeneration. Initially the reactive microglia had a bushy morphology and were succeeded by ameboid microglia and microglial cluster cells as the reaction progressed. However, in the reticular thalamic nucleus, which suffered exclusively anterograde neuronal degeneration, a different picture was seen with only bushy microglia. The neurons undergoing retrograde degeneration in the ventrolateral, posterior, and ventrobasal thalamic nuclei were retrogradely labeled by the fluorogold tracer. Individual nucleoside diphosphatase-, OX42-, or OX6-positive microglial cells extended long cytoplasmic processes surrounding fluorogold-labeled neurons and had in some cases apparently phagocytized these. Several microglial cells were thus double-labeled with nucleoside diphosphatase or OX42 and fluorogold. In addition, small nucleoside diphosphatase-positive, fluorogold-labeled perivascular cells were observed in the neocortex near the fluorogold-injected and ablated neocortical areas and in the ipsilateral thalamus. This study demonstrates: (1) that the microglial response to thalamic degeneration after neocortical lesion is graded with a limited reaction to the well-known massive anterograde axonal degeneration and a more extended reaction to the axotomy-induced retrograde cell death; and (2) that also perivascular cells and possibly macrophages may contribute to this reaction, as seen by uptake of fluorogold from axotomized neurons in the degenerating thalamic nuclei.