Double-labeled metabolic maps of memory

Returning to basic principles to develop more effective treatments for central nervous system disorders

Development of new treatments for diseases of the central nervous system (CNS) is stalled. Of candidate drugs developed through costly preclinical research, 93% fail clinical trials. Hoped-for improvements in diagnosis or treatment from decades of positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) imaging have yet to materialize. To understand what we are doing wrong, I begin with recognition that all aspects of life, including the brain and mind, are physical phenomena consistent with processes described by physicists. Two processes, emergence and entropy, are of particular relevance in complex arrangements of matter that constitute life in general and the brain in particular. The human brain functions through dynamically reconfiguring and hierarchically organized neural functional systems with emergent properties of cognition, emotion, and conscious experience. These systems are shaped and maintained by negentropic environmental input transformed by sensory receptors into neural signals that trigger epigenetic neuroplastic processes. CNS diseases produce clinical disorders by disrupting these systems. As researchers seek appropriate levels of system organization at which to characterize and treat illness, focus has been on medications that impact processes at lower levels or transcranial electric or magnetic stimulation that impact broad contiguous swaths of tissue. Neither align with the brain’s neurosystem organization and therefore lack specificity necessary to be effective and to limit side effects. Digital neurotherapies (DNTs), in contrast, align with neurosystem organization and achieve the needed specificity using the same input pathways and neuroplastic processes that created the neural systems organization to repair it. The omission of DNTs from major systems-based initiatives represents powerful residua of dualist thinking. Interventions based on perceptual and cognitive processes are not thought of as being as physical as drugs or electric or magnetic stimulation through the skull. In fact, they are examples of the most basic processes that create and support life itself.

Movement Retrieval from Long-Term Memory: Physical versus Mental Rehearsal

Despite references in the psychomotor literature regarding the use of rehearsal strategies in recovering movements from longer term memory, no significant difference between physical (overt) and mental (covert) rehearsal strategies for retrieving a complex dance movement over 30 min., 2 days, and 1 wk. were observed for 72 college women in beginning dance classes. Further, no rehearsal was as effective as either physical or mental rehearsal for retrieving dance movements for up to 1 wk. after original learning by these inexperienced dancers. The findings raise the possibility that other forms of retrieval than those already proposed are used to recover complex dance movements from longer term memory.

The "conscious pilot"-dendritic synchrony moves through the brain to mediate consciousness

Cognitive brain functions including sensory processing and control of behavior are understood as "neurocomputation" in axonal-dendritic synaptic networks of "integrate-and-fire" neurons. Cognitive neurocomputation with consciousness is accompanied by 30- to 90-Hz gamma synchrony electroencephalography (EEG), and non-conscious neurocomputation is not. Gamma synchrony EEG derives largely from neuronal groups linked by dendritic-dendritic gap junctions, forming transient syncytia ("dendritic webs") in input/integration layers oriented sideways to axonal-dendritic neurocomputational flow. As gap junctions open and close, a gamma-synchronized dendritic web can rapidly change topology and move through the brain as a spatiotemporal envelope performing collective integration and volitional choices correlating with consciousness. The "conscious pilot" is a metaphorical description for a mobile gamma-synchronized dendritic web as vehicle for a conscious agent/pilot which experiences and assumes control of otherwise non-conscious auto-pilot neurocomputation.

Recurrent affective disorder: Roots in developmental neurobiology and illness progression based on changes in gene expression

Electrophysiological kindling and behavioral sensitization to psychomotor stimulants and stress provide paradigms for understanding how repeated acute events can leave neurobiological residues in gene expression, accounting for the observed long-lasting alterations in behavioral responsivity. Kindling helps conceptualize how repeated electrical stimulation of the brain can progressively evoke increased behavioral and convulsive responsivity, leading to spontaneous seizures in the absence of exogenous stimulation following sufficient stimulations. As kindling unfolds, a complex spatiotemporal cascade of events occurs and includes the induction of immediate early genes (e.g.,c-fos) and late effector genes (including peptides and growth factors) possibly associated with the observed changes in brain microstructure (e.g., synapse formation, axonal and dendritic sprouting, apoptosis). Behavioral sensitization to psychomotor stimulants and stress has also been shown to induce related but different cascades of effects on immediate early and late effector gene expression. These may be associated with the observed long-lasting alterations in behavioral responsivity based on prior experience. If these types of alterations are put into a developmental context, this would provide a paradigm for understanding how early life events could exert profound and behaviorally relevant biochemical and microstructural effects on the central nervous system of the developing organism. The conceptual overview offered by the sensitization and kindling models suggests that environmentally triggered neurobiological processes do not form a single or static residue but, instead, engage processes related to developmental neurobiology and learning and memory and whose substrate is constantly evolving over an organism's lifetime.

The autistic syndrome and endogenous ion cyclotron resonance: state of the art

The autistic syndrome is a multigenic disease whose expression is different according to the level of involvement of different structures in the central nervous system. The pathogenesis is unknown. No completely effective medical therapy has yet been demonstrated. Accepting the request of the families of eight autistic children in Lomazzo, Milan and Naples, we used ion cyclotron resonance (Seqex therapy) therapeutic support after many other therapies had been already carried out on these patients. After regimens consisting of 20-30 treatments with ICR, improvements were noted in all cases.

The sometimes pernicious role of theory in science

The role of theory in science is discussed in the context of understanding brain function. Historically, theories of brain functions have oscillated between localization and anti-localization beliefs. In the last 50 years, the important discoveries of the ascending reticular activating system (ARAS), feature extracting neurons and synaptic growth led many to orthodoxy. Research became more and more focused upon the elements comprising the nervous system and their interconnections. The mainstream belief became that many brain functions including consciousness were localized, certain kinds of brain injuries produced irreversible functional deficits. Contrary scientific challenges were discouraged by the omnipresence of such theory. Examples of theoretical "Einstellungen" in the areas of ARAS, coma, treatment of brain injuries and consciousness are given, as well as signs that the pendulum is swinging back to an approach to the system as a whole rather than a focus on its parts.

A quantum approach to visual consciousness

A theoretical approach relying on quantum computation in microtubules within neurons can potentially resolve the enigmatic features of visual consciousness, but raises other questions. For example, how can delicate quantum states, which in the technological realm demand extreme cold and isolation to avoid environmental 'decoherence', manage to survive in the warm, wet brain? And if such states could survive within neuronal cell interiors, how could quantum states grow to encompass the whole brain? We present a physiological model for visual consciousness that can accommodate brain-wide quantum computation according to the Penrose-Hameroff 'Orch OR' model. In this view, visual consciousness occurs as a series of several-hundred-millisecond epochs, each comprising 'crescendo sequences' of quantum computations occurring at approximately 40 Hz.

Consciousness, the brain, and spacetime geometry

What is consciousness? Conventional approaches see it as an emergent property of complex interactions among individual neurons; however these approaches fail to address enigmatic features of consciousness. Accordingly, some philosophers have contended that "qualia," or an experiential medium from which consciousness is derived, exists as a fundamental component of reality. Whitehead, for example, described the universe as being composed of "occasions of experience." To examine this possibility scientifically, the very nature of physical reality must be re-examined. We must come to terms with the physics of spacetime--as described by Einstein's general theory of relativity, and its relation to the fundamental theory of matter--as described by quantum theory. Roger Penrose has proposed a new physics of objective reduction: "OR," which appeals to a form of quantum gravity to provide a useful description of fundamental processes at the quantum/classical borderline. Within the OR scheme, we consider that consciousness occurs if an appropriately organized system is able to develop and maintain quantum coherent superposition until a specific "objective" criterion (a threshold related to quantum gravity) is reached; the coherent system then self-reduces (objective reduction: OR). We contend that this type of objective self-collapse introduces non-computability, an essential feature of consciousness which distinguishes our minds from classical computers. Each OR is taken as an instantaneous event--the climax of a self-organizing process in fundamental spacetime--and a candidate for a conscious Whitehead "occasion of experience." How could an OR process occur in the brain, be coupled to neural activities, and account for other features of consciousness? We nominate a quantum computational OR process with the requisite characteristics to be occurring in cytoskeletal micro-tubules within the brain's neurons. In this model, quantum-superposed states develop in microtubule subunit proteins ("tubulins") within certain brain neurons, remain coherent, and recruit more superposed tubulins until a mass-time-energy threshold (related to quantum gravity) is reached. At that point, self-collapse, or objective reduction (OR), abruptly occurs. We equate the pre-reduction, coherent superposition ("quantum computing") phase with pre-conscious processes, and each instantaneous (and non-computable) OR, or self-collapse, with a discrete conscious event. Sequences of OR events give rise to a "stream" of consciousness. Microtubule-associated proteins can "tune" the quantum oscillations of the coherent superposed states; the OR is thus self-organized, or "orchestrated" ("Orch OR"). Each Orch OR event selects (non-computably) microtubule subunit states which regulate synaptic/neural functions using classical signaling. The quantum gravity threshold for self-collapse is relevant to consciousness, according to our arguments, because macroscopic superposed quantum states each have their own spacetime geometries. These geometries are also superposed, and in some way "separated," but when sufficiently separated, the superposition of spacetime geometries becomes significantly unstable and reduces to a single universe state. Quantum gravity determines the limits of the instability; we contend that the actual choice of state made by Nature is non-computable. Thus each Orch OR event is a self-selection of spacetime geometry, coupled to the brain through microtubules and other biomolecules. If conscious experience is intimately connected with the very physics underlying spacetime structure, then Orch OR in microtubules indeed provides us with a completely new and uniquely promising perspective on the difficult problems of consciousness.

Electrophysiological analysis of the registration, storage and retrieval of information in delayed matching from samples

Brain processes of registration, storage in working memory and retrieval of different kinds of information were studied by analysis of EEG and ERP activity recorded during two delayed matches from sample tasks: (1) matching the digits in two series of six numbers, and (2) matching the sums of the same two series of six numbers. each trial was composed of six intervals continuing six equally spaced visual stimuli: (1) control--observing a series of six fixation points, P1, on a computer monitor; (2) priming--viewing a series, S1, of six numbers; (3) delay--observing a second series of six fixation points, P2; (4) matching--viewing a second series, S2, of six numbers; (5) response selection--selecting the left button to press if S1 contained all the items in S2 or the right button if any item appeared only in S2, while observing six fixation points; (6) feedback--six color coded fixation points indicate correct or error. Each interval was 4 s in duration and 20 trials were presented in each task. During each interval the visual field flickered at a tracer frequency of 1.5/s, whether numbers or fixation points were on the monitor screen. Very narrow band power spectra (VNB), ERPs elicited by presentation of S1 or S2 information items, and non-contingent probes (NCP) elicited by presentation of fixation points were used to trace the processing of information by neural populations activated by the visual stimulation. Global field power maxima identified latencies at which functional landscapes were analyzed. VNB, ERP, NCP and landscape differences were found between digits and sums. However, though these differences were highly significant within each subject (p < 0.001), no consistency was found across individuals for the electrophysiological changes during the tasks. This suggests that utilization of brain resources in cognition varies greatly with individual cognitive styles and strategies.

Differential temporal evolution of post-training changes in regional brain glucose metabolism induced by repeated spatial discrimination training in mice: visualization of the memory consolidation process?

The present study analyses the effects of the stage of learning on the spatial patterns and time-course of [14C]glucose uptake in BALB/c mice brain regions produced by spatial discrimination training in an eight-arm radial maze. Our particular approach was designed to follow, during the post-training period, the level of functional activity in individual brain areas which may underlie the memory consolidation process. Regional mapping of relative [14C]glucose uptake was assessed at three post-training time intervals (5 min, 1 and 3 h) after either the first (Day 1), the fourth (Day 4) or the last (Day 9) daily training session of the discrimination task and compared with sham-conditioned animals placed in the same experimental environment. The results indicated that numerous subcortical and cortical brain regions exhibit metabolic alterations following the acquisition of the spatial discrimination task. These alterations, which were specifically related to learning since they did not appear in sham-conditioned animals, were functions both of the post-training interval studied and of the degree of mastery of the task. On Day 1, a progressive, time-dependent and sequential increase in labelling was found from subcortical (5 min post-training) to cortical regions (3 h post-training). On Day 4, a peak of cortical metabolic activation was identified at 1 h post-training. In contrast, on Day 9, maximum labelling was found 5 min post-training in all subcortical and cortical regions followed by a general monotonic decline at 1 and 3 h post-training. These findings, which show widely distributed changes of metabolic activity in the brain, are consistent with the hypothesis that learning involves distributed neural networks. The sequential activation from subcortical to cortical regions seems to indicate a general mechanism whose function would ultimately be to store cortical memory representations. The acquisition-dependent shifts in the patterns of post-training metabolic labelling observed as a function of task mastery may be taken to represent a visualization of the spatio-temporal evolution of the networks of brain structures actively engaged in the memory consolidation process. In particular, the present data suggest that the duration of post-acquisition memory processing is a function of the quantity of new information which has to be dealt with by the central nervous system.

Spatial working memory over long retention intervals: dependence on sustained cholinergic activation in the septohippocampal or nucleus basalis magnocellularis-cortical pathways?

Previous direct neurochemical studies of the temporal dynamics of cholinergic activation in the septohippocampal and nucleus basalis magnocellularis-cortical pathways at various stages during repeated testing of mice with selective spatial reference or working memory protocols [Durkin and Toumane (1992), Behav. Brain Res. 50, 43-52] showed that the post-test durations of cholinergic activation in each pathway varied as a function of the type of memory tested and the level of task mastery. Since (i) the hippocampal formation is considered to constitute a critical component of a temporary memory buffer, and (ii) working memory items are not thought to be submitted to consolidation and permanent storage, we postulated that the duration of testing-induced cholinergic activation in the septohippocampal pathway may govern the maintenance of the working memory trace over the retention interval. In order to test directly this hypothesis C57 B1/6 mice were extensively trained (one trial/day, 25-30 days) on an identical selective working memory task to attain high levels of retention (> 80% correct), but using either 5 min (Group 1), or 60 min (Group 2) retention intervals. At various times (30 s-75 min) following the initial acquisition phase of the test, cholinergic activity in the hippocampus and frontal cortex was quantified using measures of high-affinity choline uptake. Whereas cholinergic activation was observed in both pathways at 30 s post-acquisition and throughout the 5 min retention interval in Group 1, the situation in Group 2 is different, activation of the septohippocampal pathway being maintained for only 15 min, while activation in the nucleus basalis magnocellularis-cortical pathway is maintained for the totality of the 1 h retention interval. The nucleus basalis magnocellularis-cortical cholinergic pathway, in addition to its role in long-term reference memory storage processes may, thus, via an intervention in the temporal encoding of information, also subsume a complementary intermediate-term buffer storage role in working memory situations requiring retention intervals in excess of 15 min in mice. This secondary, "backup", function of the nucleus basalis magnocellularis-cortical pathway would thus liberate the septohippocampal complex from its primary active role in the temporary maintenance and/or accessibility of the working memory trace in these particular cases requiring long retention intervals.

Lateralization of membrane-associated protein kinase C in rat piriform cortex: specific to operant training cues in the olfactory modality

Rats were trained on an olfactory and a control modality (auditory or visual) discrimination task and brain membrane-associated protein kinase C (mPKC) was subsequently assessed using quantitative autoradiography of radiolabelled phorbol ester binding. In rats which received olfactory-cued training, mPKC showed a highly significant lateralization in the piriform cortex but not in the hippocampus. Both olfactory-trained rats and control modality rats showed a significant increase in mPKC in the hippocampus when compared to naive rats. Thus, while behavioral training procedures appeared to result in a hippocampal increase in the activated state of this enzyme as has been reported elsewhere, only olfactory learning produced an piriform cortex lateralization in the activated state of the enzyme. While the functional significance of such a change in the distribution of protein kinase C is still unclear, it does suggest that the monitoring of this enzyme's activational state may prove to be a useful tool in the study of memory formation in a wide variety of behavioral contexts.

Molecules and cognition: the latterday lessons of levels, language, and lac. Evolutionary overview of brain structure and function in some vertebrates and invertebrates

The characteristics of the nervous systems of a number of organisms in different phyla are examined at the recombinant DNA, protein, neuroanatomic, neurophysiological, and cognitive levels. Among the invertebrates, special attention is paid to the advantages as well as the shortcomings of the fly Drosophila melanogaster, the worm Caenorhabditis elegans, the honey bee Apis mellifera, the sea hare Aplysia californica, the octopus Octopus vulgaris, and the squid Loligo pealei. Among vertebrates, the focus is on Homo sapiens, the mouse Mus musculus, the rat Rattus norvegicus, the cat Felis catus, the macaque monkey Macaca fascicularis, the barn owl Tyto alba, and the zebrafish Brachydanio rerio. Vertebrate nervous systems have also been compared in fossil vs. extant organisms. I conclude that complex nervous systems arose in the Early Cambrian via a big bang that was underpinned by a modular method of construction involving massive pleiotropy of gene circuits. This rapidity of construction had enormous implications for the degrees of freedom that were subsequently available to evolving nervous systems. I also conclude that at the level of neuronal populations and interactions of neuropiles there is no model system between phyla except at the basic macromolecular level. Further, I argue that to achieve a significant understanding of the functions of extant nervous systems we need to concentrate on fewer organisms in greater depth and manipulate genomes via transgenic technologies to understand the behavioral outputs that are possible from an organism. Finally, I analyze the concepts of "perceptual categorization" and "information processing" and the difficulties involved in the extrapolation of computer analogies to sophisticated nervous systems.

Functional comparison of neuronal properties in the primate posterior hippocampus and parahippocampus (area TF/TH) during different behavioural paradigms involving memory and selective attention

Monkeys were trained on a delayed match-to-sample (DMS) task. In addition a standardized behavioural trial was performed which involved an experimenter approaching the animal in certain sequence and presenting edible or other objects ('raisin trial'). Neuronal activity of 617 units was recorded in the posterior parahippocampus (PH) and in the posterior hippocampus (H). In many cases, we compared the activity of the same neuron in different tasks. 32.7% of the 455 PH neurons and 28.5% of the 130 H cells responded during the presentation of the visual stimuli in the DMS task. These responses were only mildly influenced by the physical dimensions of the visual stimulus, but often depended on the context in which the stimuli were presented. There was no differential response to the second stimulus that clearly depended on the nature of the first stimulus. 6.2% of the PH units, but none in H, responded in relation to the reward. 4.4% of the PH neurons, but none in H, showed a mild response during the interstimulus interval. 38.1% of 215 PH neurons and 37.8% of 45 H cells responded during one or more phases of the raisin trial. These responses were not related to the physical dimensions of the sensory stimuli. 210 PH and 41 H units were investigated during the DMS task as well as during the raisin trial. 18.1% (PH) and 12.2% (H) of the units responded during the DMS task, but not during the raisin trial; 17.1% (PH) and 36.6% (H) responded vice versa. A response in both trials was found in 17.1% of the PH neurons, but in none of the H cells. There were also other PH unit types showing responses during different aspects of the DMS task and even in other control paradigms, while no such overlap was encountered in H. Our results suggest a function of H and PH in the evaluation of the behavioural significance of sensory information. It may be this aspect which leads to anterograde memory disturbances after lesion of these areas. Since representation of neuronal information was found to be more specific in H, a possible function as an 'evaluation index' is discussed.

Preparation and execution of movement: Parallels between insect and mammalian motor systems

1. The organization of the motor systems underlying locomotion in insects and mammals is surprisingly similar. There are also parallels between the insect motor system and the system underlying reaching and the occulomotor system in primates. 2. The movements generated by all these systems are planned or prepared before their execution and there is a partial separation of circuits for preparation and execution. 3. These circuits consist of multiple descending pathways interconnected to form overlapping loops which work co-operatively to determine the motor output. Thus, both insect and mammalian motor systems can be treated as parallel distributed (PDP) systems. 4. This enables a comparison of functional levels of processing in the different systems and also provides a basis for modelling motor systems with attractor neural networks.

Analysis of 2-DG autoradiograms using image-averaging and image-differencing procedures for systems-level description of neurobehavioral function

Computer assisted 2-deoxyglucose (2-DG) autoradiography has been used to provide functional maps of areas of altered neural activity related to changes in an animal's behavior or state. The standard procedure for comparison of autoradiograms between different treatment groups has been to take measurement samples from predefined neuroanatomical regions and to average these across brains to attain statistical sensitivity for detecting treatment effects. Unfortunately, when sampling is restricted to predefined areas, important topographic information is lost along with the ability to reveal an unexpected change in neural activity. To preserve the rich topographical detail of metabolic information and to enhance the capacity to uncover novel areas of altered metabolic activity, we have developed a system for averaging entire images from 2-DG autoradiograms and for comparing the average images from two experimental groups by creating an image of differences. This procedure does not rely on sampling only preselected regions, but still allows statistical comparisons between experimental groups. The procedures we describe can be readily and inexpensively adapted for use in individual laboratories and are based on modifications of preexisting image analysis software. We show that, when average and difference images are created using standardized protocols for sectioning brain tissue and editing section images, they are impressively resolved and realistic and can serve as effective topographic descriptions of group differences in neural activity of functional and behavioral relevance.

Nonhuman primates in behavioral toxicology: issues of validity, ethics and public health

The small, but vital, niche of nonhuman primates in neurotoxicology is examined. Several models of sensory and cognitive function have been especially useful with primates. Their sensitivity to low doses is clear. The validity of data from these models is indicated by their high correlation with data from intoxicated and normal humans, by the degree to which they approximate job functions and other vital human performances, and by their ability to document specific changes in behavioral function which correlate well with morphological and biochemical effects. The use of primates for this research is justified by the absence of adequate alternatives using nonprimate species, in vitro tests or computer programs. A series of experiments on the effects of methylmercury is used to illustrate ethical and scientific issues concerning research with primates. Recent trends are illustrated by data with trimethyltin.

An analysis of the correlation of lesion size, localization and behavioral effects in 283 published studies of cortical and subcortical lesions in old-world monkeys

The present article evaluates the quality and magnitude of the effects of lesion size and location and their interaction, on the behavioral performance of old world monkeys by a quantitative comparison of 283 published studies. The results indicate that lesion size alone is a poor predictor of the behavioral performance of monkeys, as opposed to Lashley's work in rats. Lesion location is a reliable predictor of the behavioral performance for brain regions thought to be primarily involved in a specific behavior; however, similar behavioral effects, although less reliable, can be observed for many different lesion loci, suggesting a specialized and a holistic brain functioning to be working at the same time. Some lesion loci are, in sharp contrast to current hypotheses about functional localization in the brain, not associated with impairments, but with significant improvements of a specific behavior. For such lesion loci the correlation of lesion size and behavioral performance may yield significant positive relationships (that is, increasing behavioral improvement with increasing lesion size); these relationships are contrasted by the significant negative relationships obtained for lesions of brain regions thought to be primarily involved in a given behavior. Thus, the lesion size may be a good predictor of the behavioral performance, depending on the lesion location and on the behavior under measurement. The behaviors analysed in this study were discrimination or delayed reaction or delayed matching-to-sample. The former two behaviors involve habit-like learning and are thought to be mediated by corticostriate functional pathways in the brain and the latter behavior implies the learning of single events, being thought to be mediated by corticolimbic functional pathways in the brain. Improved performances were observed for habit-like behaviors after lesions of brain regions (lateral frontal, premotor/motor, parietal, inferotemporal cortex, amygdala and fornix) being not primarily involved in a given behavior but possibly being able to inhibit the corticostriate pathways. Interestingly, lesions of subareas of the neostriatum were found to cause impairments in habit-like behaviors presumably being processed via these subareas (e.g. head of the caudate nucleus and delayed reaction), but to cause significant improvements in other behaviors (e.g. head of the caudate nucleus and visual discrimination). Thus, it may be concluded that diverse systems of functionally interconnected brain regions may maintain reciprocal inhibitions, with the result that a lesion within one system not only leads to a loss of one behavior, but in addition leads to a modification, may be a facilitation, of another behavior.(ABSTRACT TRUNCATED AT 400 WORDS)

Sulfhydryl proteins and the engram

A scheme, which is selectional not instructional, is described for the molecular basis of the engram. It uses as reactants intermediates formed during the catabolism of neuronal membrane sulfhydryl proteins. It is proposed that these compounds undergoing intermolecular association generate a large number of diverse structures, some of which are selected for further complex formation. The basis of selection is the binding of a particular compound to membrane lipid, which enhances its reaction with another bound compound. Because the composition and configuration of membrane lipids are influenced by environmental factors, the scheme implies that incoming sensory signals are matched with endogenously generated chemical structures.

The deoxyglucose method in the ferret brain. II. Glucose utilization images and normal values

To measure cerebral glucose utilization with the autoradiographic deoxyglucose method, the tracer transfer rate constants and lumped constants must be known. 2-Deoxyglucose (2-DG) and fluorodeoxyglucose (FDG) constants were determined in 18 gray and white matter brain structures of the anesthetized ferret. The ferret is a domestic carnivore particularly suitable for deoxyglucose studies because of its small brain size and low body weight. The average gray matter rate constants for tracer transfer across the blood-brain barrier are similar for 2-DG and FDG in the ferret brain (K*1 = 0.21 ml/g/min and k*2 = 0.39 min-1). The rate constant for the rate-limiting step of tracer phosphorylation, k*3, is 1.6 times higher for FDG than for 2-DG (0.21 vs. 0.13 min-1). Loss of metabolized tracer is about 1-1.5%/min throughout the ferret brain for both tracers as estimated for a 180 min experimental period. Taking into account this loss, the lumped constant is 0.92 for FDG and 0.68 for 2-DG. Glucose utilization values in the brain of the anesthesized ferret range from 33 mumol/100 g/min in the corpus callosum to 104 mumol/100 g/min in the caudate nucleus. Representative glucose utilization images of coronal sections of the ferret brain are shown. Brain structures are identified on the same slices counterstained with thionin.

Diencephalic amnesia: a reorientation towards tracts?

The principal thalamic and hypothalamic structures implicated in mnemonic information processing are the mediodorsal nucleus of the thalamus, the pulvinar, anterior thalamus, and laterodorsal nucleus, the mamillary body, and the mamillothalamic tract and internal medullary lamina. Determining the contribution of an individual region in memory is quite difficult as it is nearly impossible to find a circumscribed damage of only one region. On the contrary, some illnesses affecting primarily the diencephalon, such as Korsakoff's disease, tend to involve several structures together. Furthermore, even when cases with similar circumscribed diencephalic damage can be found, these will not necessarily demonstrate the same outcome on the behavioral level. Therefore, the role or contribution of individual memory-related diencephalic structures has to be inferred by comparing a number of cases and by then extracting distinct features common to a given group. Such an approach revealed that the contributions of the two fiber systems mentioned above, mamillothalamic tract and internal medullary lamina, might be more important in processing information long-term than had been acknowledged previously and might be more important than that of the nuclear masses mentioned, especially of the mediodorsal thalamus. This outcome underlines the view that emphasizing interactions between brain regions rather than single static masses will provide a more realistic picture of how the nervous system acts in information processing.

Parallel processing of short-term memory for sensitization in Aplysia

How is the short-term memory for a single form of learning distributed among the various elements of a neuronal circuit? To answer this question, we examined the short-term memory for sensitization, using the siphon component of the defensive gill- and siphon-withdrawal reflex. We found that the memory for short-term sensitization is represented by at least four sites of circuit modification, each involving a different type of plasticity. These include (1) presynaptic facilitation of the sensory neuron connections onto both interneurons and motorneurons; (2) presynaptic inhibition at the connections of the L30 inhibitory neurons onto the excitatory interneuron L29; (3) posttetanic potentiation of the excitatory connections made by L29 onto a specific subclass of siphon motorneurons, the LFS cells; and (4) an increase in the tonic firing rate of the LFS siphon motor neurons, resulting in neuromuscular facilitation. Each of the heterosynaptic changes seems to involve a common modulatory transmitter and to utilize a common second messenger system. Moreover, each of these sites seems capable of encoding a different component of the short-term memory. Facilitation of the connections of sensory neurons should contribute to the increase in amplitude of the response; the disinhibition of the L29 interneurons and the posttetanic potentiation at L29 synapses should contribute to an increase in the duration of the response; and the increase in tonic firing of the LFS subclass of siphon motor neurons seems capable of contributing both to an increase in response amplitude and to changes in response topography.

Cerebral glucose utilization after aversive conditioning and during conditioned fear in the rat

Regional cerebral glucose utilization (rCMRglu) was studied in rats with and without previous aversive conditioning. Four groups of rats were studied. Two groups of rats were aversely conditioned by placing them in a shock chamber (conditioned stimulus) where they received random footshocks. The two remaining groups were placed in the shock chamber but not conditioned. Regional CMRglu and systemic parameters (heart rate, mean arterial blood pressure (MABP), blood gases and pH, plasma catecholamines, and plasma glucose) were measured in unconditioned and conditioned rats in the presence and in the absence of the conditioned stimulus. The changes in rCMRglu described below appeared to be global and not limited to specific regions. Results are as follows: (1) transferring unconditioned rats to the shock chamber had no significant effect on rCMRglu even though the systemic parameters indicated a stress response. It appears that stress capable of inducing changes in heart rate, MABP, and plasma catecholamines is not necessarily accompanied by increases in cerebral glucose utilization. (2) Conditioned rats not exposed to the shock chamber at the time rCMRglu was measured had decreased rates of rCMRglu compared to rats that were not conditioned. Except for plasma epinephrine, which increased after conditioning, systemic parameters were not affected. (3) Conditioned fear, elicited by transferring conditioned rats to the shock chamber, increased rCMRglu when compared to a control group that was conditioned to footshock using the same paradigm but not exposed to the shock chamber at the time rCMRglu was measured. The systemic parameters indicated a stress response in conditioned rats transferred to the shock chamber.(ABSTRACT TRUNCATED AT 250 WORDS)

Double-label autoradiographic deoxyglucose method for sequential measurement of regional cerebral glucose utilization

A new double-label autoradiographic glucose analog method for the sequential measurement of altered regional cerebral metabolic rates for glucose in the same animal is presented. This method is based on the sequential injection of two boluses of glucose tracer labeled with two different isotopes (short-lived 18F and long-lived 3H, respectively). An operational equation is derived which allows the determination of glucose utilization for the time period before the injection of the second tracer; this equation corrects for accumulation and loss of the first tracer from the metabolic pool occurring after the injection of the second tracer. An error analysis of this operational equation is performed. The double-label deoxyglucose method is validated in the primary somatosensory ("barrel") cortex of the anesthetized rat. Two different rows of whiskers were stimulated sequentially in each rat; the two periods of stimulation were each preceded by an injection of glucose tracer. After decapitation, dried brain slices were first exposed, in direct contact, to standard X-ray film and then to uncoated, "tritium-sensitive" film. Results show that the double-label deoxyglucose method proposed in this paper allows the quantification and complete separation of glucose utilization patterns elicited by two different stimulations sequentially applied in the same animal. The double-label deoxyglucose is of potential usefulness in sensory physiology since it makes possible the separate mapping of regional cerebral glucose utilization patterns elicited by two sequentially applied sensory stimulations in the same animal. The method allows the quantification of a step-like change in regional cerebral glucose utilization in the same animal. It could be used to study the cerebral metabolic effects induced by neuropharmacological agents or surgical interventions applied during the experiment. Using each animal as its own control eliminates intersubject variability. Thus experimental cost and effort can be saved, and the reliability of the results obtained can be increased.