Distribution of spatial and nonspatial information in dorsal hippocampus

The hippocampus in the mammalian brain is required for the encoding of current and the retention of past experience. Previous studies have shown that the hippocampus contains neurons that encode information required to perform spatial and nonspatial short-term memory tasks. A more detailed understanding of the functional anatomy of the hippocampus would provide important insight into how such encoding occurs. Here we show that hippocampal neurons in the rat are distributed anatomically in distinct segments along the length of the hippocampus. Each longitudinal segment contains clusters of neurons that become active when the animal performs a task with spatial attributes. Within these same segments are ordered arrangements of neurons that encode the nonspatial aspects of the task appropriate to those spatial features. Thus, anatomical segregation of spatial information, together with the interleaved representation of nonspatial information, represents a structural framework that may help to resolve conflicting views of hippocampal function.     

Mitochondrial glutamate acts as a messenger in glucose-induced insulin exocytosis

The hormone insulin is stored in secretory granules and released from the pancreatic beta-cells by exocytosis. In the consensus model of glucose-stimulated insulin secretion, ATP is generated by mitochondrial metabolism, promoting closure of ATP-sensitive potassium (KATP) channels, which depolarizes the plasma membrane. Subsequently, opening of voltage-sensitive Ca2+ channels increases the cytosolic Ca2+ concentration ([Ca2+]c) which constitutes the main trigger initiating insulin exocytosis. Nevertheless, the Ca2+ signal alone is not sufficient for sustained secretion. Furthermore, glucose elicits a secretory response under conditions of clamped, elevated [Ca2+]c. A mitochondrial messenger must therefore exist which is distinct from ATP. We have now identified this as glutamate. We show that glucose generates glutamate from beta-cell mitochondria. A membrane-permeant glutamate analogue sensitizes the glucose-evoked secretory response, acting downstream of mitochondrial metabolism. In permeabilized cells, under conditions of fixed [Ca2+]c, added glutamate directly stimulates insulin exocytosis, independently of mitochondrial function. Glutamate uptake by the secretory granules is likely to be involved, as inhibitors of vesicular glutamate transport suppress the glutamate-evoked exocytosis. These results demonstrate that glutamate acts as an intracellular messenger that couples glucose metabolism to insulin secretion.