A neural plasticity hypothesis of schizophrenia

The Imbalanced Plasticity Hypothesis of Schizophrenia-Related Psychosis: A Predictive Perspective

A considerable number of studies have attempted to account for the psychotic aspects of schizophrenia in terms of the influential predictive coding (PC) hypothesis. We argue that the prediction-oriented perspective on schizophrenia-related psychosis may benefit from a mechanistic model that: 1) gives due weight to the extent to which alterations in short- and long-term synaptic plasticity determine the degree and the direction of the functional disruption that occurs in psychosis; and 2) addresses the distinction between the two central syndromes of psychosis in schizophrenia: disorganization and reality-distortion. To accomplish these goals, we propose the Imbalanced Plasticity Hypothesis - IPH, and demonstrate that it: 1) accounts for commonalities and differences between disorganization and reality distortion in terms of excessive (hyper) or insufficient (hypo) neuroplasticity, respectively; 2) provides distinct predictions in the cognitive and electrophysiological domains; and 3) is able to reconcile conflicting PC-oriented accounts of psychosis.

Visual cortical plasticity and the risk for psychosis: An interim analysis of the North American Prodrome Longitudinal Study

BACKGROUND

Adolescence/early adulthood coincides with accelerated pruning of cortical synapses and the onset of schizophrenia. Cortical gray matter reduction and dysconnectivity in schizophrenia are hypothesized to result from impaired synaptic plasticity mechanisms, including long-term potentiation (LTP), since deficient LTP may result in too many weak synapses that are then subject to over-pruning. Deficient plasticity has already been observed in schizophrenia. Here, we assessed whether such deficits are present in the psychosis risk syndrome (PRS), particularly those who subsequently convert to full psychosis.

METHODS

An interim analysis was performed on a sub-sample from the NAPLS-3 study, including 46 healthy controls (HC) and 246 PRS participants. All participants performed an LTP-like visual cortical plasticity paradigm involving assessment of visual evoked potentials (VEPs) elicited by vertical and horizontal line gratings before and after high frequency ("tetanizing") visual stimulation with one of the gratings to induce "input-specific" neuroplasticity (i.e., VEP changes specific to the tetanized stimulus). Non-parametric, cluster-based permutation testing was used to identify electrodes and timepoints that demonstrated input-specific plasticity effects.

RESULTS

Input-specific pre-post VEP changes (i.e., increased negative voltage) were found in a single spatio-temporal cluster covering multiple occipital electrodes in a 126-223 ms time window. This plasticity effect was deficient in PRS individuals who subsequently converted to psychosis, relative to PRS non-converters and HC.

CONCLUSIONS

Input-specific LTP-like visual plasticity can be measured from VEPs in adolescents and young adults. Interim analyses suggest that deficient visual cortical plasticity is evident in those PRS individuals at greatest risk for transition to psychosis.

A psychobiological theory of attachment

This article describes a neurobiological basis for the “first attachment” of the primate infant to its caretaker. The infant normally internalizes a neurobiological “image” of the behavioral and emotional characteristics of its caregiver that later regulates important features of its brain function. Current models of sensorimotor analysis and its relation to emotion suggest that sensorimotor systems are also habit and memory systems, their functional status and lability regulated in part by biogenic amine systems. The intertwined development of neural and social functions can sometimes go awry. If the attachment process fails or the caregiver is incompetent, the infant may become socially dysfunctional. This helps explain the developmental psychopathology and later vulnerability to adult psychopathology that result from disruptions of social attachment.

Causes of changes in brain noradrenaline systems and later effects on responses to social stressors in rhesus monkeys: the cascade hypothesis

Disruption of social attachments in social primates produces a protest-despair response. In rhesus monkeys, the response is probably adaptive in the feral environment, although the despair stage resembles human depression in many respects. The severity of the response varies between individuals and is affected by deprivation of certain classes of social stimuli during development. Social deprivation is associated with differences in the concentrations of noradrenaline (NA) in cerebrospinal fluid and in responses to agents that affect catecholamine systems. Thus, early rearing conditions and pre-existing genetic or perinatal differences between monkeys can have long-term effects on the response to social separation, and NA system release and/or receptor mechanisms are involved. NA systems appear to mediate adaptation to the environment from the level of perception to reorganization of neural tissue. Adaptation to the social environment may involve a cascade of changes that begins with behavioural coping attempts and terminates in structural reorganization of regions of the cerebral cortex. Processes at each level occur within environmentally appropriate but neurobiologically constrained time-frames. The cerebral NA system may be an adaptive mechanism that can fail or be damaged. Behavioural changes caused by such damage or failure would be manifested by inappropriate responses to environmental contingencies and inability to change behaviour to adapt to the prevailing environment. These features of NA system disorder could be common to depression and several other forms of human psychopathology.

The paradigm of complexity in clinical neurocognitive science

Neurocognitive science represents the modern approach to integrating the subdisciplines aimed at a scientific study of the brain-mind system. This relatively new discipline recognizes, implicitly or explicitly, that this is a complex system whose states and processes are determined by multiple bio-psycho-social variables and order parameters. In a generic perspective, all neurocognitive science is complex, as it is multidisciplinary, but in some studies, complexity has become a more defined scientific paradigm using its own specific empirical and theoretical tools. Some neuroscientists consider complexity science as a specific and formalized paradigm. Between their contributions, the author will try to highlight some current promising paths and new frontiers for neuroscience. In this perspective, he will mostly focus on those contributions directly related to clinical perspectives. This is the reason why some seminal contributions more focused on physiological functioning might not be mentioned.

Cingulate cortex synaptic terminal proteins and neural cell adhesion molecule in schizophrenia

The neuronal organization and patterns of afferent innervation are abnormal in the cingulate cortex in schizophrenia, and associated changes in synaptic terminals could be present. A panel of monoclonal antibodies was defined with biochemical and fusion protein studies as detecting syntaxin (antibody SP6), synaptophysin (antibody SP4) and synaptosomal-associated protein-25 (antibody SP12). These antibodies and a polyclonal antibody reactive with neural cell adhesion molecule were used to investigate the cingulate cortex in schizophrenia. Immunocytochemistry indicated that syntaxin immunoreactivity had a considerably wider distribution than synaptophysin. Overall, multivariate analysis indicated increased synaptic terminal protein immunoreactivity in schizophrenia compared to controls (P=0.004). Controlled for age and post mortem interval, syntaxin immunoreactivity was significantly elevated in schizophrenia (P=0.004), and neural cell adhesion molecule immunoreactivity was also elevated (P=0.05). The neural cell adhesion molecule to synaptophysin ratio was increased (P=0.005), possibly indicating the presence of less mature synapses in schizophrenia. Elevated syntaxin immunoreactivity is consistent with increased glutamatergic afferents to the cingulate cortex in schizophrenia, and combined with the neural cell adhesion molecule to synaptophysin ratio results suggests that synaptic function in this region in schizophrenia may be abnormal.

The concept of affect logic: an integrative psycho-socio-biological approach to understanding and treatment of schizophrenia

In this article, the psycho-socio-biologically integrative concept of affect-logic, and its relevance for a comprehensive understanding and therapy of schizophrenia, is briefly presented. This concept has been developed by the author over the past 20 years, on the basis of the literature, of clinical experience and his own research into long-term evolution, rehabilitation, effects of milieu-therapy, and nonlinear evolutionary dynamics of the illness. It postulates, basically, that fundamental affective states (or emotions, feelings, moods) are continuously and inseparably linked to all cognitive functioning (or "thinking" and "logic" in a broad sense), and that affects have essential organizing and integrating effects on cognition. Schizophrenia is understood as an altered mode of affective-cognitive interaction based, possibly, on disturbed (loosened) affective-cognitive connections. This hypothesis leads to: 1) an integrative psycho-socio-biological model of long-term evolution of the illness; 2) a new understanding of psychopathological core phenomena such as ambivalence, incoherence, and emotional flattening; 3) an innovative therapeutic approach based on an emotion-relaxing milieu and style of care; and 4) the hypothesis that schizophrenia could basically be an affective (and not a cognitive) disease, of another kind than mania or melancholia, however.

Individual motor activity--relationships to dopaminergic responses

Owing to motor activity mice were divided into two groups in a running-wheel test: low-active mice (LAM) and high-active mice (HAM). Locomotor activities in the running wheel and in glass boxes are compared. The HAM showed a more intensive explorative behavior than LAM and were also more responsive in terms of exogenous factors than LAM. In contrast, LAM showed higher locomotor activity than HAM after habituation. Analyzing the response of LAM and HAM to dopaminergic agonists such as apomorphine, bromocriptine, and amphetamine, the role of specific dopaminergic mechanisms for the two types is discussed. Although apomorphine mainly stimulated the climbing activity in HAM, bromocriptine (climbing activity) and amphetamine (locomotion) had stronger effects in LAM. Differences may be assumed between LAM and HAM concerning the nigrostriatal and/or mesolimbic dopaminergic mechanisms. On the one hand, climbing activity following apomorphine application accompanied by stereotypes may suggest a stronger activation of striatal dopaminergic mechanisms in HAM. On the other hand, climbing activity following bromocriptine accompanied by jumping behavior, as well as the stimulation of locomotion after amphetamine, suggests a more effective activation of mesolimbic dopaminergic structures in LAM.

The major hallucinogens and the central cytoskeleton: an association beyond coincidence? Towards sub-cellular mechanisms in schizophrenia

There appears to be a remarkably consistent structural and functional relationship between the phenylethylamine hallucinogens and the microtubule inhibitor colchicine. Such a relationship is not sustained in simple form through to the indoleamine hallucinogens and the indole based Vinca alkaloids. However, LSD and the more potent hallucinogens retain the full potential to disrupt the structure of the brain's cytoskeleton indirectly via serotonin and the raphe system. Serotonin appears to have a direct role in regulating and maintaining microtubules and microfilaments. It appears that a second receptor mediated action is required for full hallucinogenic activity. It is deduced that cytoskeletal restraints may have a role in governing central information processing. A theory for the cellular mechanisms of thought disorder and drug induced hallucinations is proposed. Schizophrenia may reflect a subtle disorder of central cytoskeletal function.

Non-competitive regulation of phencyclidine/sigma-receptors by the N-methyl-D-aspartate receptor antagonist D-(-)-2-amino-5-phosphonovaleric acid

D-(-)-2-Amino-5-phosphonovaleric acid (D(-)AP5), a selective, potent competitive antagonist of N-methyl-D-aspartate (NMDA)-type excitatory amino acid receptors was used to investigate the relationship between NMDA receptors and phencyclidine (PCP) binding. Incubation of rat brain membranes with D(-)AP5 decreased the apparent number of PCP/sigma-receptors in dose-dependent fashion without affecting their affinity for [2-thienyl-3H]cyclohexylpiperidine (TCP). These data, taken together with electrophysiological evidence that PCP non-competitively antagonizes NMDA receptor-mediated transmission, are consistent with the hypothesis that the PCP/sigma-receptor may be situated in or near a channel regulated by an NMDA receptor complex.

Increased plasma phospholipase-A2 activity in schizophrenic patients: reduction after neuroleptic therapy

Phospholipase-A2 (PLA2) is a key enzyme in the metabolism of phospholipids, and it may play an important role in neuronal function and neuronal plasticity. We determined the activity of PLA2 in the plasma of 20 drug-free schizophrenic patients, 6 nonschizophrenic psychiatric patients, and 21 healthy controls. Schizophrenic patients showed significantly higher plasma PLA2 activity than controls, and higher than our nonschizophrenic patients. Seventy percent of the schizophrenics had enzyme activity higher than the highest value from the control group. The increased plasma PLA2 activity in schizophrenics was reduced to the level of the controls after 3 weeks of neuroleptic treatment. These findings warrant further study for possible implications of this increased PLA2 activity in the etiopathology of schizophrenia.

Cross-sensitization between foot shock stress and enkephalin-induced motor activity

Injection of the enkephalin analog D-Ala2-Met5-enkephalinamide (DALA) into the ventral tegmental area (VTA) of rats has been shown to activate dopamine (DA) neurons projecting to the nucleus accumbens, thereby increasing spontaneous motor activity. Furthermore, daily DALA injection into the VTA results in a progressive enhancement in the motor stimulant effect of subsequent administration of either DALA, intra-VTA, or amphetamine, intraperitoneally. A similar behavioral sensitization occurs after daily amphetamine administration, and cross-sensitization between stress and amphetamine has been demonstrated. Considering that the stimulant effect of both DALA and amphetamine is mediated, at least in part, via enhanced DA release, the present study determined whether or not cross-sensitization could be produced between intra-VTA injection with DALA and mild foot shock stress. It was found that rats receiving a daily injection with DALA (1.0 micrograms/side) into the VTA for 5 days demonstrated a significantly greater increase in DA metabolism in the nucleus accumbens and septum in response to foot shock (0.14 mA over 20 min) than did control rats. Conversely, rats receiving daily foot shock for 5 days had a significantly greater motor stimulant response to intra-VTA injection with DALA than sham-shocked rats. Furthermore, the foot shock-induced behavioral sensitization to DALA was persistent for at least 10 days. As enkephalin is found endogenously in the VTA, it is possible that hypersecretion of neuronal enkephalin could sensitize an individual to subsequent environmental stress. These data are discussed in terms of the DA hypothesis of schizophrenia.

Behavioral and neurochemical effect of daily injection with neurotensin into the ventral tegmental area

Many lines of evidence indicate an excitatory role by neurotensin (NT) on mesolimbic dopamine neurons in the ventral tegmental area (VTA). In support of this postulate, NT microinjection into the VTA of rats produces a dopamine-dependent increase in spontaneous motor activity that is associated with an increase in dopamine metabolism in the nucleus accumbens. In this study it was found that after daily intra-VTA injection with NT, both the motor hyperactivity and increase in dopamine metabolism were significantly enhanced. Further, the increased motor response to NT was present after 7 days without daily administration. While the augmented motor response could be produced with the carboxy-terminal fragment NT8-13, the NH2-terminal fragment, NT1-8, was ineffective. The enhancement of motor activity was only produced by NT injection into the A10 dopamine region and not adjacent nuclei. These results suggest that daily administration with NT into the VTA will potentiate the responsiveness of mesolimbic dopamine neurons to subsequent injection with NT.

Interactions between neuropeptides and dopamine neurons in the ventromedial mesencephalon

Cholecystokinin (CCK), enkephalin, neurotensin (NT), substance P (SP) and substance K (SK) are five neuropeptides that exist in neuronal perikarya or fibers in the vicinity of the A10 dopamine neurons in the ventromedial mesencephalon. Based upon this anatomical proximity, many investigations have been evaluating the possibility that these peptides may influence the function of the A10 dopamine neurons. A variety of experimental techniques have been employed in this regard, including anatomical, electrophysiological, neurochemical and behavioral methodologies. Measurement of immunoreactive peptide levels with radioimmunoassay, and visualization of peptidergic neurons and fibers with immunocytochemistry has demonstrated not only that peptides exist in the vicinity of A10 dopamine neurons, but using double labeling techniques NT and CCK have been found to coexist with dopamine in the same neuron. Further, by combining retrograde tracing technique with immunocytochemistry, the origin of some peptidergic afferents to the ventromedial mesencephalon has been determined. With the exception of CCK-8, microinjection into the ventromedial mesencephalon of rats with all the peptides or potent analogues produces a dose-related increase in spontaneous motor activity. For SP, NT and enkephalin the motor response has been blocked by dopamine antagonists. Further, an increase in dopamine metabolism in mesolimbic dopamine terminal fields is produced concurrent with the behavioral hyperactivity. These data indicate that SP, SK, enkephalin and NT can activate dopamine neurons in the ventromedial mesencephalon. This postulate is supported by electrophysiological studies showing an excitatory action by iontophoretic administration of peptide onto dopamine neurons. However, in some studies, excitatory electrophysiological effects were not observed. While some observations are contradictory, sufficient data has accumulated that tentative postulates and conclusions can be made about how these peptides may influence the A10 dopamine neurons. Further, speculations are offered as to the role this modulatory action may play in the many behaviors and pathologies thought to involve these dopamine neurons.

Mesolimbic dopamine lesions produce an augmented behavioral response to enkephalin

The mesolimbic dopamine (DA) system was lesioned by the injection of 6-hydroxydopamine into the nucleus accumbens or ventral tegmental area of rats. The rats were then examined in a photocell apparatus for their response to injection with the enkephalin analogue, D-Ala2-Met5-enkephalinamide (DALA), into the nucleus accumbens. Rats with DA lesions showed a significantly greater increase in photocell counts in response to DALA than did sham-lesioned animals. The potentiated motor response was dose-related, and rats having the greatest depletion of DA in the nucleus accumbens showed the greatest behavioral response to DALA. It was concluded that chronic impairment of dopaminergic innervation to the nucleus accumbens results in an enhanced response to DALA injected directly into the nucleus accumbens.