The efferent connections of the orbitofrontal, posterior parietal, and insular cortex of the rat brain.

The orbitofrontal, posterior parietal, and insular cortices are sites of higher-order cognitive processing implicated in a wide range of behaviours, including working memory, attention guiding, decision making, and spatial navigation. To better understand how these regions contribute to such functions, we need detailed knowledge about the underlying structural connectivity. Several tract-tracing studies have investigated specific aspects of orbitofrontal, posterior parietal and insular connectivity, but a digital resource for studying the cortical and subcortical projections from these areas in detail is not available. We here present a comprehensive collection of brightfield and fluorescence microscopic images of serial coronal sections from 49 rat brain tract-tracing experiments, in which discrete injections of the anterograde tracers biotinylated dextran amine and/or Phaseolus vulgaris leucoagglutinin were placed in the orbitofrontal, parietal, or insular cortex. The images are spatially registered to the Waxholm Space Rat brain atlas. The image collection, with corresponding reference atlas maps, is suitable as a reference framework for investigating the brain-wide efferent connectivity of these cortical association areas.

Organization of Posterior Parietal-Frontal Connections in the Rat.

Recent investigations of the rat posterior parietal cortex (PPC) suggest that this region plays a central role in action control together with the frontal cortical areas. Posterior parietal-frontal cortical connections have been described in rats, but little is known about whether these connections are topographically organized as in the primate. Here, we injected retrograde and anterograde tracers into subdivisions of PPC as well as the frontal midline and orbital cortical areas to explore possible topographies within their connections. We found that PPC projects to several frontal cortical areas, largely reciprocating the densest input received from the same areas. All PPC subdivisions are strongly connected with the secondary motor cortex (M2) in a topographically organized manner. The medial subdivision (medial posterior parietal cortex, mPPC) has a dense reciprocal connection with the most caudal portion of M2 (cM2), whereas the lateral subdivision (lateral posterior parietal cortex, lPPC) and the caudolateral subdivision (PtP) are reciprocally connected with the intermediate rostrocaudal portion of M2 (iM2). Sparser reciprocal connections were seen with anterior cingulate area 24b. mPPC connects with rostral, and lPPC and PtP connect with caudal parts of 24b, respectively. There are virtually no connections with area 24a, nor with prelimbic or infralimbic cortex. PPC and orbitofrontal cortices are also connected, showing a gradient such that mPPC entertains reciprocal connections mainly with the ventral orbitofrontal cortex (OFC), whereas lPPC and PtP are preferentially connected with medial and central portions of ventrolateral OFC, respectively. Our results thus indicate that the connections of PPC with frontal cortices are organized in a topographical fashion, supporting functional heterogeneity within PPC and frontal cortices.

Parahippocampal and retrosplenial connections of rat posterior parietal cortex.

The posterior parietal cortex has been implicated in spatial functions, including navigation. The hippocampal and parahippocampal region and the retrosplenial cortex are crucially involved in navigational processes and connections between the parahippocampal/retrosplenial domain and the posterior parietal cortex have been described. However, an integrated account of the organization of these connections is lacking. Here, we investigated parahippocampal connections of each posterior parietal subdivision and the neighboring secondary visual cortex using conventional retrograde and anterograde tracers as well as transsynaptic retrograde tracing with a modified rabies virus. The results show that posterior parietal as well as secondary visual cortex entertain overall sparse connections with the parahippocampal region but not with the hippocampal formation. The medial and lateral dorsal subdivisions of posterior parietal cortex receive sparse input from deep layers of all parahippocampal areas. Conversely, all posterior parietal subdivisions project moderately to dorsal presubiculum, whereas rostral perirhinal cortex, postrhinal cortex, caudal entorhinal cortex and parasubiculum all receive sparse posterior parietal input. This indicated that the presubiculum might be a major liaison between parietal and parahippocampal domains. In view of the close association of the presubiculum with the retrosplenial cortex, we included the latter in our analysis. Our data indicate that posterior parietal cortex is moderately connected with the retrosplenial cortex, particularly with rostral area 30. The relative sparseness of the connectivity with the parahippocampal and retrosplenial domains suggests that posterior parietal cortex is only a modest actor in forming spatial representations underlying navigation and spatial memory in parahippocampal and retrosplenial cortex. © 2017 Wiley Periodicals, Inc.

Posterior parietal cortex of the rat: Architectural delineation and thalamic differentiation.

This study refines the characterization of the rat parietal cortical domain in terms of cyto- and chemoarchitecture as well as thalamic connectivity. We recognize three subdivisions of the posterior parietal cortex (PPC), which are architectonically distinct from the neighboring somatosensory and visual cortices. Furthermore, we show that the different parietal areas are differently connected with thalamic nuclei. The medial portion of PPC (mPPC) is connected primarily with the medial portion of the lateral posterior nucleus (LP), whereas the lateral portion (lPPC) connects with the posterior complex (Po). The more caudolateral part of PPC (PtP) also projects to Po but can be distinguished from lPPC based on architectonic criteria. The primary somatic and visual cortices, neighboring PPC, are preferentially connected with the primary ventral posterior and dorsolateral geniculate nuclei, respectively, and less with the associational Po and LP. Particularly the border between the secondary visual cortex and the PPC has been a matter of controversy, but here we show that, although PPC subareas are connected with Po and medial LP, the medial and lateral secondary visual cortices are connected with lateral LP and a portion of medial LP different from that connected with PPC. The resulting delineations and specifications of connectivity with thalamic nuclei together with upcoming studies of cortical connectivity will facilitate detailed studies on the role of the subdivisions of PPC in the rat as diverse, higher order associative cortical areas, comparable to those described in the primate.for J. Comp. Neurol. 524:3774-3809, 2016. © 2016 Wiley Periodicals, Inc.

Glutamate: Where does it come from and where does it go?

Pyruvate carboxylation, the anaplerotic reaction in the brain, has been demonstrated in astrocytes but not neurons. Since anaplerosis cannot proceed without cataplerosis in a closed system such as the brain, there have to be mechanisms to degrade molecules such as glutamate, glutamine, GABA and aspartate which have more carbon atoms than pyruvate. Pyruvate recycling is a cataplerotic process which is very active in liver. It has also been demonstrated in the brain and has been shown to proceed both in astrocytes and neurons. Increasing recycling as a consequence of increasing glutamate concentration in medium has been shown in astrocytes. In the present study cerebellar granule neurons were incubated with medium containing 0.1, 0.25 or 0.5 mM [U-(13)C]glutamate or [U-(13)C]aspartate and pyruvate recycling in combination with tricarboxylic acid (TCA) cycle metabolism was analysed in glutamate, aspartate and malate using mass spectrometry. It could be shown that pyruvate recycling of TCA cycle intermediates as seen in glutamate increased with increasing [U-(13)C]glutamate but not [U-(13)C]aspartate concentration confirming compartmentation of glutamate metabolism and the importance of glutamate in cataplerosis. Partial pyruvate recycling (lactate production from the TCA cycle) was more active in astrocytes than neurons in line with the astrocytes' greater capacity for glutamate uptake.

Pyruvate recycling in cultured neurons from cerebellum.

Pyruvate recycling is a pathway for complete oxidation of glutamate. The cellular location and the physiological significance of such recycling has been debated during the last decade. The present study was aimed at elucidating whether recycling takes place in neuron-enriched cultures of dissociated cerebella, consisting mainly of glutamatergic granule cells, some GABAergic neurons, and few astrocytes. These cultures and cultures of astrocytes from cerebellum were incubated in medium containing [U-(13)C]glutamate, and cell extracts were analyzed by gas chromatography and mass spectrometry. Additionally, in the case of the neuron-enriched cultures, a magnetic resonance (MR) spectrum was obtained. It could be shown that the atom percentage excess of the isotopomer representing pyruvate recycling in glutamate (M + 4) was similar for astrocytes and neuron-enriched cultures. However, the latter showed more recycling in glutamine (synthesized in the small fraction of astrocytes) than the pure astrocyte cultures, whereas the reverse was the case for aspartate. In fact, the atom percentage excess of the isotopomer representing pyruvate recycling in glutamine was slightly but significantly higher than that in glutamate in the neuron-enriched cultures. It can be concluded that pyruvate recycling is clearly present in neurons, and this was verified by MR spectroscopy.