Electrophysiological analysis of retrieval of conditioned taste aversion in rats. Unit activity changes in critical brain regions.

Gustatory discrimination testing shows that rats with an overtrained conditioned taste aversion (CTA) to isotonic LiCl stop salt intake after 1 to 2 licks at the LiCl spout and move to the adjacent water spout within 0.7 s. Activity of 526 neurones from the nucleus of the solitary tract, gustatory thalamus, gustatory cortex, lateral and ventromedial thalamus, and amygdala was recorded in naive or CTA trained rats during the above gustatory discrimination. Post-stimulus histograms (PSH) triggered by water or salt licks or by spout switching were plotted for single units. Population responses of various regions were obtained by integration of the statistically significant excitatory and inhibitory intervals in the individual PSHs. Lick related changes of unit activity were orserved in 52% and 65% of neurones in control and CTA trained rats, respectively. The CTA training increased the incidence of units in which salt licking influenced the activity less than water licking. Presentation of the aversive fluid induced inhibition of unit activity in the gustatory cortex, ventromedial hypothalamus, and amygdala and excitation in the lateral hypothalamus. The changes started 100 to 150 ms after spout switching and culminated 100 ms later. Activity of the solitary tract nucleus and gustatory thalamus was affected less consistently. The results indicate that the gustatory cortex, amygdala and hypothalamus participate in CTA retrieval but a more specific identification of the electrical correlates of memory readout and of drinking control was not possible.     

Investment in higher order central processing regions is not constrained by brain size in social insects

The extent to which size constrains the evolution of brain organization and the genesis of complex behaviour is a central, unanswered question in evolutionary neuroscience. Advanced cognition has long been linked to the expansion of specific brain compartments, such as the neocortex in vertebrates and the mushroom bodies in insects. Scaling constraints that limit the size of these brain regions in small animals may therefore be particularly significant to behavioural evolution. Recent findings from studies of paper wasps suggest miniaturization constrains the size of central sensory processing brain centres (mushroom body calyces) in favour of peripheral, sensory input centres (antennal and optic lobes). We tested the generality of this hypothesis in diverse eusocial hymenopteran species (ants, bees and wasps) exhibiting striking variation in body size and thus brain size. Combining multiple neuroanatomical datasets from these three taxa, we found no universal size constraint on brain organization within or among species. In fact, small-bodied ants with miniscule brains had mushroom body calyces proportionally as large as or larger than those of wasps and bees with brains orders of magnitude larger. Our comparative analyses suggest that brain organization in ants is shaped more by natural selection imposed by visual demands than intrinsic design limitations.     

Inferring causality in brain images: a perturbation approach

When engaged by a stimulus, different nodes of a neural circuit respond in a coordinated fashion. We often ask whether there is a cause and effect in such interregional interactions. This paper proposes that we can infer causality in functional connectivity by employing a 'perturb and measure' approach. In the human brain, this has been achieved by combining transcranial magnetic stimulation (TMS) with positron emission tomography (PET), functional magnetic resonance imaging or electroencephalography. Here, I will illustrate this approach by reviewing some of our TMS/PET work, and will conclude by discussing a few methodological and theoretical challenges facing those studying neural connectivity using a perturbation.     

Synaptic plasticity, memory and the hippocampus: a neural network approach to causality

Two facts about the hippocampus have been common currency among neuroscientists for several decades. First, lesions of the hippocampus in humans prevent the acquisition of new episodic memories; second, activity-dependent synaptic plasticity is a prominent feature of hippocampal synapses. Given this background, the hypothesis that hippocampus-dependent memory is mediated, at least in part, by hippocampal synaptic plasticity has seemed as cogent in theory as it has been difficult to prove in practice. Here we argue that the recent development of transgenic molecular devices will encourage a shift from mechanistic investigations of synaptic plasticity in single neurons towards an analysis of how networks of neurons encode and represent memory, and we suggest ways in which this might be achieved. In the process, the hypothesis that synaptic plasticity is necessary and sufficient for information storage in the brain may finally be validated.     

When two become one: the limits of causality analysis of brain dynamics

Biological systems often consist of multiple interacting subsystems, the brain being a prominent example. To understand the functions of such systems it is important to analyze if and how the subsystems interact and to describe the effect of these interactions. In this work we investigate the extent to which the cause-and-effect framework is applicable to such interacting subsystems. We base our work on a standard notion of causal effects and define a new concept called natural causal effect. This new concept takes into account that when studying interactions in biological systems, one is often not interested in the effect of perturbations that alter the dynamics. The interest is instead in how the causal connections participate in the generation of the observed natural dynamics. We identify the constraints on the structure of the causal connections that determine the existence of natural causal effects. In particular, we show that the influence of the causal connections on the natural dynamics of the system often cannot be analyzed in terms of the causal effect of one subsystem on another. Only when the causing subsystem is autonomous with respect to the rest can this interpretation be made. We note that subsystems in the brain are often bidirectionally connected, which means that interactions rarely should be quantified in terms of cause-and-effect. We furthermore introduce a framework for how natural causal effects can be characterized when they exist. Our work also has important consequences for the interpretation of other approaches commonly applied to study causality in the brain. Specifically, we discuss how the notion of natural causal effects can be combined with Granger causality and Dynamic Causal Modeling (DCM). Our results are generic and the concept of natural causal effects is relevant in all areas where the effects of interactions between subsystems are of interest.     

Altered connectivity pattern of hubs in default-mode network with Alzheimer's disease: an Granger causality modeling approach

Background

Evidences from normal subjects suggest that the default-mode network (DMN) has posterior cingulate cortex (PCC), medial prefrontal cortex (MPFC) and inferior parietal cortex (IPC) as its hubs; meanwhile, these DMN nodes are often found to be abnormally recruited in Alzheimer''s disease (AD) patients. The issues on how these hubs interact to each other, with the rest nodes of the DMN and the altered pattern of hubs with respect to AD, are still on going discussion for eventual final clarification.

Principal Findings

To address these issues, we investigated the causal influences between any pair of nodes within the DMN using Granger causality analysis and graph-theoretic methods on resting-state fMRI data of 12 young subjects, 16 old normal controls and 15 AD patients respectively. We found that: (1) PCC/MPFC/IPC, especially the PCC, showed the widest and distinctive causal effects on the DMN dynamics in young group; (2) the pattern of DMN hubs was abnormal in AD patients compared to old control: MPFC and IPC had obvious causal interaction disruption with other nodes; the PCC showed outstanding performance for it was the only region having causal relation with all other nodes significantly; (3) the altered relation between hubs and other DMN nodes held potential as a noninvasive biomarker of AD.

Conclusions

Our study, to the best of our knowledge, is the first to support the hub configuration of the DMN from the perspective of causal relationship, and reveal abnormal pattern of the DMN hubs in AD. Findings from young subjects provide additional evidence for the role of PCC/MPFC/IPC acting as hubs in the DMN. Compared to old control, MPFC and IPC lost their roles as hubs owing to the obvious causal interaction disruption, and PCC was preserved as the only hub showing significant causal relations with all other nodes.     

Functional connections between auditory cortical fields in humans revealed by Granger causality analysis of intra-cranial evoked potentials to sounds: comparison of two methods

Knowledge of neural interactions amongst cortical sites is important for understanding higher brain function. We studied such interactions using Granger causality (GC) to analyze auditory event-related potentials (ERPs) recorded directly and simultaneously from two physiologically identified and functionally interconnected auditory areas of cerebral cortex in human neurosurgical patients. Two methods of GC analysis were used and the results compared. Both approaches involved adaptive autoregressive modeling but differed from each other in other ways. Results obtained by using the two methods also differed. Fewer false-positive results were obtained using the method that suppressed the ERP non-stationarity and that expressed the GC as the sum of model coefficients, which suggests that this is the more appropriate approach for analyzing ERPs recorded directly from the human cortex.     

The molecule-group schema of memory: storage, recognition and retrieval

A new schema, the molecule-group schema, explains memory storage, recognition and retrieval. The schema consists of three postulates about molecular specificity, grouping, and diffusion. In the schema, the physical memory trace consists of a stable group of different kinds of highly specific molecules. The schema is intended to provide an alternative to the widely known synaptic-change schema, in which it is assumed that changes of synaptic efficacies constitute the memory trace. The new schema is used to develop a particular model of memory. In the model, recognition occurs when specific intracellular "endotransmitters" react with complementary "endoreceptors" in the same cell. Retrieval, modelled as the process whereby memory causes the recurrence of a previously experienced pattern of neural activity, occurs when a group of pools of endotransmitters, located within an intracellular memory organelle, is released, allowing the endotransmitters to diffuse to the periphery of the cell body. The model suffices to explain long-term and short-term memory of events as well as innate memory.     

Forgetting, reminding, and remembering: the retrieval of lost spatial memory

Retrograde amnesia can occur after brain damage because this disrupts sites of storage, interrupts memory consolidation, or interferes with memory retrieval. While the retrieval failure account has been considered in several animal studies, recent work has focused mainly on memory consolidation, and the neural mechanisms responsible for reactivating memory from stored traces remain poorly understood. We now describe a new retrieval phenomenon in which rats' memory for a spatial location in a watermaze was first weakened by partial lesions of the hippocampus to a level at which it could not be detected. The animals were then reminded by the provision of incomplete and potentially misleading information—an escape platform in a novel location. Paradoxically, both incorrect and correct place information reactivated dormant memory traces equally, such that the previously trained spatial memory was now expressed. It was also established that the reminding procedure could not itself generate new learning in either the original environment, or in a new training situation. The key finding is the development of a protocol that definitively distinguishes reminding from new place learning and thereby reveals that a failure of memory during watermaze testing can arise, at least in part, from a disruption of memory retrieval.     

Isolation, characterization, and primary structure of a base non-specific and adenylic acid preferential ribonuclease with higher specific activity from Trichoderma viride.

In order to elucidate the structure-function relationship of RNases belonging to the RNase T2 family (base non-specific and adenylic acid-preferential RNase), an RNase of this family was purified from Trichoderma viride (RNase Trv) to give three closely adjacent bands with RNase activity on slab-gel electrophoresis in a yield of 20%. The three RNases gave single band with the same mobility on slab-gel electrophoresis after endoglycosidase F digestion. The enzymatic properties including base specificity of RNase Trv were very similar to those of typical T2-family RNases such as RNase T2 from Aspergillus oryzae and RNase M from A. saitoi. The specific activity of RNase Trv towards yeast RNA was about 13-fold higher than that of RNase M. The complete primary structure of RNase Trv was determined by analyses of the peptides generated by digestion of reduced and carboxymethylated RNase Trv with Staphylococcus aureus V8 protease, lysylendopeptidase and alpha-chymotrypsin. The molecular weight of the protein moiety deduced from the sequence was 25,883. The locations of 10 half-cystine residues were almost superimposable upon those of other RNases of this family. The homologies between RNase Trv and RNase T2, RNase M, and RNase Rh (Rhizopus niveus) were 124, 132, and 92 residues, respectively. The sequences around three histidine residues, His52, His109, and His114, were highly conserved in these 4 RNases.     

Adaptation of brain regions to habitat complexity: a comparative analysis in bats (Chiroptera)

Vertebrate brains are organized in modules which process information from sensory inputs selectively. Therefore they are probably under different evolutionary pressures. We investigated the impact of environmental influences on specific brain centres in bats. We showed in a phylogenetically independent contrast analysis that the wing area of a species corrected for body size correlated with estimates of habitat complexity. We subsequently compared wing area, as an indirect measure of habitat complexity, with the size of regions associated with hearing, olfaction and spatial memory, while controlling for phylogeny and body mass. The inferior colliculi, the largest sub-cortical auditory centre, showed a strong positive correlation with wing area in echolocating bats. The size of the main olfactory bulb did not increase with wing area, suggesting that the need for olfaction may not increase during the localization of food and orientation in denser habitat. As expected, a larger wing area was linked to a larger hippocampus in all bats. Our results suggest that morphological adaptations related to flight and neuronal capabilities as reflected by the sizes of brain regions coevolved under similar ecological pressures. Thus, habitat complexity presumably influenced and shaped sensory abilities in this mammalian order independently of each other.     

Developmental patterns of galactosyltransferase activity in various regions of rat brain

Abstract: The developmental pattern of glycoprotein-galactosyltransferase activity was determined in the microsomal fractions of three regions of the embryonic rat brain and in parts of the visual system and the cerebellum postnatally. It could be shown that the enzyme activity was highest in the embryonic brain, where regional differences were apparent, and decreased progressively after birth. The enzyme profile in the cerebellum showed no marked postnatal changes.     

Non-canonical base pairs and higher order structures in nucleic acids: crystal structure database analysis

Non-canonical base pairs, mostly present in the RNA, often play a prominent role towards maintaining their structural diversity. Higher order structures like base triples are also important in defining and stabilizing the tertiary folded structure of RNA. We have developed a new program BPFIND to analyze different types of canonical and non-canonical base pairs and base triples involving at least two direct hydrogen bonds formed between polar atoms of the bases or sugar O2' only. We considered 104 possible types of base pairs, out of which examples of 87 base pair types are found to occur in the available RNA crystal structures. Analysis indicates that approximately 32.7% base pairs in the functional RNA structures are non-canonical, which include different types of GA and GU Wobble base pairs apart from a wide range of base pair possibilities. We further noticed that more than 10.4% of these base pairs are involved in triplet formation, most of which play important role in maintaining long-range tertiary contacts in the three-dimensional folded structure of RNA. Apart from detection, the program also gives a quantitative estimate of the conformational deformation of detected base pairs in comparison to an ideal planar base pair. This helps us to gain insight into the extent of their structural variations and thus assists in understanding their specific role towards structural and functional diversity.     

Higher environmental temperature-induced change in synaptosomal acetylcholinesterase activity of brain regions

Expcsure of adult male albino rats to higher environmental temperature (HET) at 35° for 2–12 hr or at 45° for 1–2 hr increases hypothalamic synaptosomal acetylcholinesterase (AChE) activity. Synaptosomal AChE activity in cerebral cortex of rats exposed to 35° for 12 hr and in cerebral cortex and pons-medulla of rats exposed to 45° for 1–2 hr are also activated. AChE activity of synaptosomes prepared from normal rat brain regions incubated in-vitro at 39° or 41° for 0.5 hr increases significantly in cerebral cortex and hypothalamus. The activation of AChE in ponsmedulla is also observed when this brain region is incubated at 41° for 0.5 hr. Increase of (a) the duration of incubation at 41° and (b) the incubation temperature to 43° under in-vitro condition decreases the synaptosomal AChE activity. Lioneweaver-Burk plots indicate that (a) in-vivo and invitro HET-induced increases of brain regional synaptosomal AChE activity are coupled with an increase ofV _max without any change inK _m (b) very high temperature (43° under in-vitro condition) causes a decrease inV _max with an increase inK _m of AChE activity irrespective of brain regions. Arrhenius plots show that there is a decrease in transition temperature in hypothalamus of rats exposed to either 35° or 45°; whereas such a decrease in transition temperature of the pons-medulla and cerebral cortex regions are observed only after exposure to 45°. These results suggests that heat exposure increases the lipid fluidity of synaptosomal membrane depending on the brain region which may expose the catalytic site of the enzyme (AChE) and hence activate the synaptosomal membrane bound AChE activity in brain regions. Further the in-vitro higher temperature (43°C)-induced inhibition of synaptosomal AChE activity irrespective of brain regions may be the cause iof partial proteolysis/disaggregation of AChE oligomers and/or solubilization of this membrane-bound enzyme.To whom to address reprint requests: