Connections of the auditory cortex with the claustrum and the endopiriform nucleus in the cat

Exercise Modulates Brain Glucose Utilization Response to Acute Cocaine

Exercise, a proven method of boosting health and wellness, is thought to act as a protective factor against many neurological and psychological diseases. Recent studies on exercise and drug exposure have pinpointed some of the neurological mechanisms that may characterize this protective factor. Using positron emission tomography (PET) imaging techniques and the glucose analog [¹⁸F]-Fluorodeoxyglucose (¹⁸F-FDG), our team sought to identify how chronic aerobic exercise modulates brain glucose metabolism (BGluM) after drug-naïve rats were exposed to an acute dose of cocaine. Using sedentary rats as a control group, we observed significant differences in regional BGluM. Chronic treadmill exercise treatment coupled with acute cocaine exposure induced responses in BGluM activity in the following brain regions: postsubiculum (Post), parasubiculum (PaS), granular and dysgranular insular cortex (GI and DI, respectively), substantia nigra reticular (SNR) and compact part dorsal tier (SNCD), temporal association cortex (TeA), entopenduncular nucleus (EP), and crus 1 of the ansiform lobule (crus 1). Inhibition, characterized by decreased responses due to our exercise, was found in the ventral endopiriform nucleus (VEn). These areas are associated with memory and various motor functions. They also include and share connections with densely dopaminergic areas of the mesolimbic system. In conclusion, these findings suggest that treadmill exercise in rats mediates brain glucose response to an acute dose of cocaine differently as compared to sedentary rats. The modulated brain glucose utilization occurs in brain regions responsible for memory and association, spatial navigation, and motor control as well as corticomesolimbic regions related to reward, emotion, and movement.

Retrosplenial cortex and its role in cue-specific learning and memory

The retrosplenial cortex (RSC) contributes to spatial navigation, as well as contextual learning and memory. However, a growing body of research suggests that the RSC also contributes to learning and memory for discrete cues, such as auditory or visual stimuli. In this review, we summarize and assess the Pavlovian and instrumental conditioning experiments that have examined the role of the RSC in cue-specific learning and memory. We use the term cue-specific to refer to these putatively non-spatial conditioning paradigms that involve discrete cues. Although these paradigms emphasize behavior related to cue presentations, we note that cue-specific learning and memory always takes place against a background of contextual stimuli. We review multiple ways by which contexts can influence responding to discrete cues and suggest that RSC contributions to cue-specific learning and memory are intimately tied to contextual learning and memory. Indeed, although the RSC is involved in several forms of cue-specific learning and memory, we suggest that many of these can be linked to processing of contextual stimuli.

Distribution of GABAergic Neurons and VGluT1 and VGAT Immunoreactive Boutons in the Ferret (Mustela putorius) Piriform Cortex and Endopiriform Nucleus. Comparison With Visual Areas 17, 18 and 19

We studied the cellular organization of the piriform network [comprising the piriform cortex (PC) and endopiriform nucleus (EP)] of the ferret (Mustela putorius)-a highly excitable region prone to seizures-and, more specifically, the distribution and morphology of different types of gamma-aminobutyric acid (GABA)ergic neurons, and the distribution and ratio of glutamatergic and GABAergic boutons, and we compared our findings to those in primary visual area 17, and secondary areas 18 and 19. We accomplished this by using cytochrome oxidase and immunohistochemistry for mature neuronal nuclei (NeuN), GABAergic neurons [glutamic acid decarboxylase-67 (GAD67), calretinin (CR) and parvalbumin (PV)], and for excitatory (vesicular glutamate transporter 1; VGluT1) and inhibitory (vesicular GABA transporter; VGAT) boutons. In the ferret, the cellular organization of the piriform network is similar to that described in other species such as cats, rats and opossums although some differences also exist. GABAergic immunolabeling showed similarities between cortical layers I-III of the PC and visual areas, such as the relative distribution of GABAergic neurons and the density and area of VGluT1- and VGAT-immunoreactive boutons. However, multiple differences between the piriform network and visual areas (layers I-VI) were found, such as the percentage of GABAergic neurons with respect to the total number of neurons and the ratio of VGluT1- and VGAT-immunoreactive boutons. These findings are relevant to better understand the high excitability of the piriform network.

A Re-evaluation of the Anatomy of the Claustrum in Rodents and Primates-Analyzing the Effect of Pallial Expansion

The components of the claustrum have been identified by gene expression in mice, but there is still uncertainty about the location of homologous components in primates. To aid interpretation of homologous elements between rodents and primates, we used a current understanding of pallial topology, species-specific telencephalic deformation, and gene expression data. In both rodents and primates, pallial areas maintain conserved topological relationships regardless of relative differences in pallial expansion. The components of the claustrum in primates can, therefore, be identified on the basis of their conserved topological relationships and patterns of gene expression. In rodents, a fairly straight telencephalic long axis runs between the early septopreoptic and amygdalar poles of the pallium. In primates, however, the remarkable dorsal pallial expansion causes this axis to be distorted to form a C shape. This has resulted in a number of errors in the interpretation of the location of claustral components. These errors are likely to have resulted from the unexpected topographical positioning of claustral components due to the bent telencephalic axis. We argue that, once the telencephalic distortion has been accounted for, both rodents and primates have homologous claustral components, and that the topological relationships of these components are conserved regardless of differences in the relative expansion of pallial areas.

Organization of the connections between claustrum and cortex in the mouse

The connections between the claustrum and the cortex in mouse are systematically investigated with adeno-associated virus (AAV), an anterograde viral tracer. We first define the boundary and the three-dimensional structure of the claustrum based on a variety of molecular and anatomical data. From AAV injections into 42 neocortical and allocortical areas, we conclude that most cortical areas send bilateral projections to the claustrum, the majority being denser on the ipsilateral side. This includes prelimbic, infralimbic, medial, ventrolateral and lateral orbital, ventral retrosplenial, dorsal and posterior agranular insular, visceral, temporal association, dorsal and ventral auditory, ectorhinal, perirhinal, lateral entorhinal, and anteromedial, posteromedial, lateroposterior, laterointermediate, and postrhinal visual areas. In contrast, the cingulate and the secondary motor areas send denser projections to the contralateral claustrum than to the ipsilateral one. The gustatory, primary auditory, primary visual, rostrolateral visual, and medial entorhinal cortices send projections only to the ipsilateral claustrum. Primary motor, primary somatosensory and subicular areas barely send projections to either ipsi- or contralateral claustrum. Corticoclaustral projections are organized in a rough topographic manner, with variable projection strengths. We find that the claustrum, in turn, sends widespread projections preferentially to ipsilateral cortical areas with different projection strengths and laminar distribution patterns and to certain contralateral cortical areas. Our quantitative results show that the claustrum has strong reciprocal and bilateral connections with prefrontal and cingulate areas as well as strong reciprocal connections with the ipsilateral temporal and retrohippocampal areas, suggesting that it may play a crucial role in a variety of cognitive processes. J. Comp. Neurol. 525:1317-1346, 2017. © 2016 Wiley Periodicals, Inc.

Retrosplenial cortex is required for the retrieval of remote memory for auditory cues

The restrosplenial cortex (RSC) has a well-established role in contextual and spatial learning and memory, consistent with its known connectivity with visuo-spatial association areas. In contrast, RSC appears to have little involvement with delay fear conditioning to an auditory cue. However, all previous studies have examined the contribution of the RSC to recently acquired auditory fear memories. Since neocortical regions have been implicated in the permanent storage of remote memories, we examined the contribution of the RSC to remotely acquired auditory fear memories. In Experiment 1, retrieval of a remotely acquired auditory fear memory was impaired when permanent lesions (either electrolytic or neurotoxic) were made several weeks after initial conditioning. In Experiment 2, using a chemogenetic approach, we observed impairments in the retrieval of remote memory for an auditory cue when the RSC was temporarily inactivated during testing. In Experiment 3, after injection of a retrograde tracer into the RSC, we observed labeled cells in primary and secondary auditory cortices, as well as the claustrum, indicating that the RSC receives direct projections from auditory regions. Overall our results indicate the RSC has a critical role in the retrieval of remotely acquired auditory fear memories, and we suggest this is related to the quality of the memory, with less precise memories being RSC dependent.

Claustrum: a case for directional, excitatory, intrinsic connectivity in the rat

Claustrum, a gray matter structure that underlies the neocortex, is reciprocally connected with many neocortical and limbic cortical areas. This connectivity positions claustrum ideally for the integration or coordination of widespread cortical activity. In anatomical studies using multiple planes of section, claustrum has distinct subregions based on latexin immunohistochemistry, and an approximately rostro-caudal alignment of fusiform cells supporting a laminar intrinsic organization. Physiological studies of claustral connectivity in disinhibited brain slices demonstrate (1) intrinsic connectivity sufficient to generate spontaneous synchronized burst discharges, (2) activity spread within the oblique laminae that contained the principal cellular axis, and (3) segregation of activity as evidenced by the absence of spread within coronal planes. Activity spread depended on glutamatergic synaptic transmission, and activity restrictions did not depend on inhibitory circuits. We conclude that the claustrum has an intrinsic excitatory connectivity that is constrained in approximately rostro-caudal laminae, with minimal cross-communication between laminae. Further, claustrum has the intrinsic capability of generating synchronized population activity and facilitating its spread within laminae, a feature that may contribute to seizure generation and spread.

Hypotheses relating to the function of the claustrum

This paper present a new hypothesis as to the function of the claustrum. Our basic premise is that the claustrum functions as a detector and integrator of synchrony in the axonal trains in its afferent inputs. In the first place an unexpected stimulus sets up a processed signal to the sensory cortex that initiates a focus of synchronized gamma oscillations therein. This focus may then interact with a general alerting signal conveyed from the reticular formation via cholinergic mechanisms, and with other salient activations set up by the stimulus in other sensory pathways that are relayed to the cortex. This activity is relayed from the cortex to the claustrum, which then processes these several inputs by means of multiple competitive intraclaustral synchronized oscillations at different frequencies. Finally it modulates the synchronized outputs that the claustrum distributes to most cortical and many subcortical structures, including the motor cortex. In this way, during multicenter perceptual and cognitive operations, reverberating claustro-cortical loops potentiate weak intracortical synchronizations by means of connected strong intraclaustral synchronizations. These may also occur without a salient stimulus. By this mechanism, the claustrum may play a strong role in the control of interactive processes in different parts of the brain, and in the control of voluntary behavior. These may include the neural correlates of consciousness. We also consider the role of GABAergic mechanisms and deafferentation plasticity.

On the classification of pathways in the auditory midbrain, thalamus, and cortex

Auditory forebrain pathways exhibit several morphological and physiological properties that underlie their specific neurobiological roles in auditory processing. Anatomically, such projections can be distinguished by their terminal size, arborization patterns, and postsynaptic dendritic locations. These structural features correlate with several postsynaptic physiological properties, such as EPSP amplitude, short-term plasticity, and postsynaptic receptor types. Altogether, these synaptic properties segregate into two main classes that are associated with either primarily information-bearing (Class 1) or modulatory (Class 2) roles, and have been used to delineate the principle routes of information flow through the auditory midbrain, thalamus, and cortex. Moreover, these synaptic properties engender as yet unexplored issues regarding the neuronal processing of auditory information, such as the convergent integration and long-term plasticity of auditory forebrain inputs.

Non-sensory cortical and subcortical connections of the primary auditory cortex in Mongolian gerbils: bottom-up and top-down processing of neuronal information via field AI

In the present study, we will provide further anatomical evidence that the primary auditory cortex (field AI) is not only involved in sensory processing of its own modality, but also in complex bottom-up and top-down processing of multimodal information. We have recently shown that AI in the Mongolian gerbil (Meriones unguiculatus) has substantial connections with non-auditory sensory and multisensory brain structures [Budinger, E., Heil, P., Hess, A., Scheich, H., 2006. Multisensory processing via early cortical stages: Connections of the primary auditory cortical field with other sensory systems. Neuroscience 143, 1065-1083]. Here we will report about the direct connections of AI with non-sensory cortical areas and subcortical structures. We approached this issue by means of the axonal transport of the sensitive bidirectional neuronal tracers fluorescein-labelled (FD) and tetramethylrhodamine-labelled dextran (TMRD), which were simultaneously injected into different frequency regions of the gerbil's AI. Of the total number of retrogradely labelled cell bodies found in non-sensory brain areas, which identify cells of origin of direct projections to AI, approximately 24% were in cortical areas and 76% in subcortical structures. Of the cell bodies in the cortical areas, about 4.4% were located in the orbital, 11.1% in the infralimbic medial prefrontal (areas DPC, IL), 18.2% in the cingulate (3.2% in CG1, 2.9% in CG2, 12.1% in CG3), 9.5% in the frontal association (area Fr2), 12.0% in the insular (areas AI, DI), 10.8% in the retrosplenial, and 34.0% in the perirhinal cortex. The cortical regions with retrogradely labelled cells, as well as the entorhinal cortex, also contained anterogradely labelled axons and their terminations, which means that they are also target areas of direct projections from AI. The laminar pattern of corticocortical connections indicates that AI receives primarily cortical feedback-type inputs and projects in a feedforward manner to its target areas. The high number of double-labelled somata, the non-topographic distribution of single FD- and TMRD-labelled somata, and the overlapping spatial distribution of FD- and TMRD-labelled axonal elements suggest rather non-tonotopic connections between AI and the multimodal cortices. Of the labelled cell bodies in the subcortical structures, about 38.8% were located in the ipsilateral basal forebrain (10.6% in the lateral amygdala LA, 11.5% in the globus pallidus GP, 3.7% in the ventral pallidum VPa, 13.0% in the nucleus basalis NB), 13.1% in the ipsi- and contralateral diencephalon (6.4% in the posterior paraventricular thalamic nuclei, 6.7% in the hypothalamic area), and 48.1% in the midbrain (20.0% in the ipsilateral substantia nigra, 9.8% in the ipsi- and contralateral ventral tegmental area, 5.0% in the ipsi- and contralateral locus coeruleus, 13.3% the ipsi- and contralateral dorsal raphe nuclei). Thus, the majority of subcortical inputs to AI was related to different neurotransmitter systems. Anterograde labelling was only found in some ipsilateral basal forebrain structures, namely, the LA, basolateral amygdala, GP, VPa, and NB. As for the cortex, the proportion and spatial distribution of single FD-, TMRD-, and double-labelled neuronal elements suggests rather non-tonotopic connections between AI and the neuromodulatory subcortical structures.

Multisensory convergence in auditory cortex, I. Cortical connections of the caudal superior temporal plane in macaque monkeys

The caudal medial auditory area (CM) has anatomical and physiological features consistent with its role as a first-stage (or "belt") auditory association cortex. It is also a site of multisensory convergence, with robust somatosensory and auditory responses. In this study, we investigated the cerebral cortical sources of somatosensory and auditory inputs to CM by injecting retrograde tracers in macaque monkeys. A companion paper describes the thalamic connections of CM (Hackett et al., J. Comp. Neurol. [this issue]). The likely cortical sources of somatosensory input to CM were the adjacent retroinsular cortex (area Ri) and granular insula (Ig). In addition, CM had reliable connections with areas Tpt and TPO, which are sites of multisensory integration. CM also had topographic connections with other auditory areas. As expected, connections with adjacent caudal auditory areas were stronger than connections with rostral areas. Surprisingly, the connections with the core were concentrated along its medial side, suggesting that there may be a medial-lateral division of function within the core. Additional injections into caudal lateral auditory area (CL) and Tpt showed similar connections with Ri, Ig, and TPO. In contrast to CM injections, these lateral injections had inputs from parietal area 7a and had a preferential connection with the lateral (gyral) part of Tpt. Taken together, the findings indicate that CM may receive somatosensory input from nearby areas along the fundus of the lateral sulcus. The differential connections of CM compared with adjacent areas provide additional evidence for the functional specialization of the individual auditory belt areas.

The distributed auditory cortex

A synthesis of cat auditory cortex (AC) organization is presented in which the extrinsic and intrinsic connections interact to derive a unified profile of the auditory stream and use it to direct and modify cortical and subcortical information flow. Thus, the thalamocortical input provides essential sensory information about peripheral stimulus events, which AC redirects locally for feature extraction, and then conveys to parallel auditory, multisensory, premotor, limbic, and cognitive centers for further analysis. The corticofugal output influences areas as remote as the pons and the cochlear nucleus, structures whose effects upon AC are entirely indirect, and it has diverse roles in the transmission of information through the medial geniculate body and inferior colliculus. The distributed AC is thus construed as a functional network in which the auditory percept is assembled for subsequent redistribution in sensory, premotor, and cognitive streams contingent on the derived interpretation of the acoustic events. The confluence of auditory and multisensory streams likely precedes cognitive processing of sound. The distributed AC constitutes the largest and arguably the most complete representation of the auditory world. Many facets of this scheme may apply in rodent and primate AC as well. We propose that the distributed auditory cortex contributes to local processing regimes in regions as disparate as the frontal pole and the cochlear nucleus to construct the acoustic percept.

Focal projections of cat auditory cortex to the pontine nuclei

The pontine nuclei (PN) receive projections from the auditory cortex (AC) and they are a major source of mossy fibers to the cerebellum. However, they have not been studied in detail using sensitive neuroanatomical tracers, and whether all AC areas contribute to the corticopontine (CP) system is unknown. We characterized the projection patterns of 11 AC areas with WGA-HRP. We also compared them with their corticothalamic and corticocollicular counterparts. A third objective was to analyze the structure of the CP axons and their terminals with BDA. Both tracers confirm that all AC areas projected to lateral, central, and medial ipsilateral pontine divisions. The strongest CP projections were from nontonotopic and polymodal association areas. Preterminal fibers formed single terminal fields having many boutons en passant as well as terminal endings, and there was a specific morphological pattern for each pontine target, irrespective of their areal origin. Thus, axons in the medial division had a simpler terminal architecture (type 1 terminal plexus); both the central and lateral pons received more complex endings (type 2 terminal plexus). Auditory CP topographical distribution resembled visual and somatosensory CP projections, which preserve retinotopy and somatotopy in the pons, respectively. However, the absence of pontine tonotopy suggests that the AC projection topography is unrelated to tonotopy. CP input to the medial and central pons coincides with the somatosensory and visual cortical inputs, respectively, and such overlap might subserve convergence in the cerebellum. In contrast, lateral pontine input may be exclusively auditory.

Multisensory processing via early cortical stages: Connections of the primary auditory cortical field with other sensory systems

It is still a popular view that primary sensory cortices are unimodal, but recent physiological studies have shown that under certain behavioral conditions primary sensory cortices can also be activated by multiple other modalities. Here, we investigate the anatomical substrate, which may underlie multisensory processes at the level of the primary auditory cortex (field AI), and which may, in turn, enable AI to influence other sensory systems. We approached this issue by means of the axonal transport of the sensitive bidirectional neuronal tracer fluorescein-labeled dextran which was injected into AI of Mongolian gerbils (Meriones unguiculatus). Of the total number of retrogradely labeled cell bodies (i.e. cells of origin of direct projections to AI) found in non-auditory sensory and multisensory brain areas, approximately 40% were in cortical areas and 60% in subcortical structures. Of the cell bodies in the cortical areas about 82% were located in multisensory cortex, viz., the dorsoposterior and ventroposterior, posterior parietal cortex, the claustrum, and the endopiriform nucleus, 10% were located in the primary somatosensory cortex (hindlimb and trunk region), and 8% in secondary visual cortex. The cortical regions with retrogradely labeled cells also contained anterogradely labeled axons and their terminations, i.e. they are also target areas of direct projections from AI. In addition, the primary olfactory cortex was identified as a target area of projections from AI. The laminar pattern of corticocortical connections suggests that AI receives primarily cortical feedback-type inputs and projects in a feedforward manner to its target areas. Of the labeled cell bodies in the subcortical structures, approximately 90% were located in multisensory thalamic, 4% in visual thalamic, and 6% in multisensory lower brainstem structures. At subcortical levels, we observed a similar correspondence of retrogradely labeled cells and anterogradely labeled axons and terminals in visual (posterior limitans thalamic nucleus) and multisensory thalamic nuclei (dorsal and medial division of the medial geniculate body, suprageniculate nucleus, posterior thalamic cell group, zona incerta), and in the multisensory nucleus of the brachium of the inferior colliculus. Retrograde, but not anterograde, labeling was found in the multisensory pontine reticular formation, particularly in the reticulotegmental nucleus of the pons. Conversely, anterograde, but no retrograde, labeling was found in the visual laterodorsal and lateroposterior thalamic nuclei, in the multisensory peripeduncular, posterior intralaminar, and reticular thalamic nuclei, as well as in the multisensory superior and pericentral inferior colliculi (including cuneiform and sagulum nucleus), pontine nuclei, and periaqueductal gray. Our study supports the notion that AI is not merely involved in the analysis of auditory stimulus properties but also in processing of other sensory and multisensory information. Since AI is directly connected to other primary sensory cortices (viz. the somatosensory and olfactory ones) multisensory information is probably also processed in these cortices. This suggests more generally, that primary sensory cortices may not be unimodal.

Decoding the auditory corticofugal systems

The status of the organization of the auditory corticofugal systems is summarized. These are among the largest pathways in the brain, with descending connections to auditory and non-auditory thalamic, midbrain, and medullary regions. Auditory corticofugal influence thus reaches sites immediately presynaptic to the cortex, sites remote from the cortex, as in periolivary regions that may have a centrifugal role, and to the cochlear nucleus, which could influence early central events in hearing. Other targets include the striatum (possible premotor functions), the amygdala and central gray (prospective limbic and motivational roles), and the pontine nuclei (for precerebellar control). The size, specificity, laminar origins, and morphologic diversity of auditory corticofugal axons is consonant with an interpretation of multiple roles in parallel descending systems.

Decoding the auditory corticofugal systems

The status of the organization of the auditory corticofugal systems is summarized. These are among the largest pathways in the brain, with descending connections to auditory and non-auditory thalamic, midbrain, and medullary regions. Auditory corticofugal influence thus reaches sites immediately presynaptic to the cortex, sites remote from the cortex, as in perolivary regions that may have a centrifugal role, and to the cochlear nucleus, which could influence early central events in hearing. Other targets include the striatum (possible premotor functions), the amygdala and central gray (prospective limbic and motivational roles), and the pontine nuclei (for precerebellar control). The size, specificity, laminar origins, and morphologic diversity of auditory corticofugal axons is consonant with an interpretation of multiple roles in parallel descending systems.

Strong expression of NETRIN-G2 in the monkey claustrum

The claustrum is a phylogenetically conserved structure, with extensive reciprocal connections with cortical regions, and has thus been considered important for sensory, motor, emotional, and mnemonic coordination or integration. Here, we show by in situ hybridization that the adult monkey claustrum is strongly positive for NETRIN-G2, a gene encoding a glycosyl phosphatidyl-inositol-linked membrane protein, which constitutes a subfamily with NETRIN-G1 within the netrin/UNC6 family. There is a conspicuous dorsal/ventral differentiation, where the label is stronger in the ventral claustrum. NETRIN-G2 positive neurons are not GABAergic, but rather correspond to claustrocortical projection neurons, as demonstrated by retrograde transport of Fast Blue from cortical injections and by double in situ hybridization for NETRIN-G2 and GAD67. Since NETRIN-G2 is known to be preferentially expressed in cortex, in contrast with the thalamically expressed NETRIN-G1, these results raise the possibility of some functional similarity in regulation of excitatory neural transmission in the claustrum and cortex.

Interaction of corticosterone and nicotine in regulation of prepulse inhibition in mice

The aim of the present study was to investigate if different levels of circulating corticosterone (CORT) modulate the effect of nicotine on prepulse inhibition (PPI), a measure of sensorimotor gating that is disrupted in schizophrenia and other mental illnesses. Four groups of mice were investigated: sham-operated, adrenalectomized (ADX) and implanted with a cholesterol pellet, ADX and implanted with a 10 mg CORT pellet, or ADX and 50 mg of CORT. Different CORT levels or doses of nicotine did not significantly affect startle responses. Baseline PPI was significantly reduced in mice implanted with the highest dose of CORT. In ADX mice implanted with cholesterol, nicotine treatment influenced PPI depending on the prepulse intensity. In ADX mice implanted with 50 mg of CORT, treatment with 10 mg/kg of nicotine caused a significant increase in PPI at all prepulse intensities. Binding studies showed that corticosterone treatment had significantly affected nicotinic acetylcholine receptor (nAChR) density in the mouse brain. Treatment with 50 mg CORT decreased 125I-epibatidine binding in the globus pallidus and 125I-alpha-bungarotoxin binding in the claustrum. These results suggest a possible interaction of corticosterone and nicotine at the level of the alpha4- and alpha7-type nAChR in the regulation of PPI. In situations of high circulating levels of corticosterone, nicotine may be beneficial to restore disruption of PPI.

Distinct types of nitric oxide-producing neurons in the developing and adult mouse claustrum

We studied at the light and electron microscopic levels the nitric oxide-producing neurons in the mouse claustrum. Nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase histochemistry and neuronal nitric oxide synthase (nNOS) immunohistochemical staining were used to reveal putative nitrergic neurons. We also analyzed colocalization of nNOS with the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) as well as the ontogenesis of the nNOS-immunoreactive neurons, providing evidence for different populations of nitrergic neurons in the mouse claustrum. The general staining pattern was similar for the histochemical and the immunohistochemical methods, resulting in neuron and neuropil staining throughout the whole claustrum. We described two populations of nitric oxide-producing neurons in the mouse claustrum on the basis of a different level of nNOS expression. Densely nNOS-stained neurons were mostly GABA immunoreactive, displayed ultrastructural features typically seen in aspiny neurons, and may originate in the subpallium; they were first seen in the claustrum at embryonic stage 17.5 and probably represent local inhibitory interneurons. Densely stained cells were found from rostral to caudal levels throughout the dorsal claustrum and the endopiriform nucleus. Lightly nNOS-stained neurons, on the other hand, were more numerous than densely stained ones, especially in the dorsal claustrum. These claustral lightly stained cells, barely observed in the NADPH-diaphorase reacted sections, were mostly non-GABAergic, and appeared earlier during ontogenesis than densely stained cells (at embryonic stages 15.5-16.5). We suggest that these neurons are probably projection neurons.

FoxO6, a novel member of the FoxO class of transcription factors with distinct shuttling dynamics

Forkhead transcription factors of the FoxO-group are associated with cellular processes like cell cycle progression and DNA-repair. FoxO function is regulated by protein kinase B (PKB) via the phosphatidylinositol 3-kinase/PKB survival pathway. Phosphorylation of serine and threonine residues in specific PKB phosphorylation motifs leads to exclusion of FoxO-proteins from the nucleus, which excludes them from exerting transactivating activity. Members of the FoxO-group have three highly conserved regions containing a PKB phosphorylation motif. This study describes the cloning and characterization of a novel forkhead domain gene from mouse that appeared to be highly related to the FoxO group of transcription factors and was therefore designated FoxO6. The FoxO6 gene was mapped in region D1 on mouse chromosome 4. In humans, FOXO6 is located on chromosomal region 1p34.1. Embryonic expression of FoxO6 is most apparent in the developing brain, and FoxO6 is expressed in a specific temporal and spatial pattern. Therefore it is probably involved in regulation of specific cellular differentiation. In the adult animal FoxO6 expression is maintained in areas of the nucleus accumbens, cingulate cortex, parts of the amygdala, and in the hippocampus. Structure function analysis of FoxO6 compared with its group members shows that the overall homology is high, but surprisingly a highly conserved region containing multiple phosphorylation sites is lacking. In transfection studies, FoxO6 coupled to GFP showed an unexpected high nuclear localization after stimulation with growth factors, in contrast to the predominant cytosolic localization of FoxO1 and FoxO3. We also show that nuclear export of FoxO6 is mediated through the phosphatidylinositol 3-kinase/PKB pathway. Furthermore, we show using a chimeric approach that we can fully restore the ability of FoxO6 to shuttle between nucleus and cytosol. In conclusion, the data presented here gives a new view on regulation of FoxO-function through multiple phosphorylation events and other mechanisms involved in the nuclear exclusion of FoxO-proteins.