synaptic plasticity: angelman’s/autism and psychosis
There is a recent article in Nature Neuroscience by Philpot et al regarding how experience-dependent synaptic plasticity is downregulated in Angelmans’ syndrome and perhaps in Autism too, as the Ube3a gene involved is implicated in both disorders.
First a little history about Angelman– it is a disorder caused by deletion/lack of a maternally imprinted UBE3a gene in chromosomal region 15q11-q13 . It is typically contrasted with Prader-Willi syndrome which is caused by a paternally imprinted gene malfunction in the same chromosomal region. Christopher Badcock has used this to contrast Autism (related to Angelman) and Psychosis (more common in PWS) to argue that Autism and Psychosis are due to a genomic imprinting tug of war between fathers and mothers genes.
I have written about Badcock’s and Crespi’s thesis before and how it fits in with my views on Autism and Psychosis; suffice it to say that I am seeing the new study primarily from this prism of Autism and Psychosis dichotomy.
First , let us see what the study tells us:
It uses mouse model that contains silenced maternal Ube3a genes (Ube3a m-p+ mouse), thus trying to make a mouse model of Angelman.
What it found was:
1) Ube3a expression was markedly reduced in Ube3am-/p+ mice compared with wild-type mice in all three brain regions (visual neocortex, hippocampus,cerebellam). Consistent with previous observations, this attenuation was brain specific, as Ube3a was highly expressed in the liver of both Ube3am+/p- and Ube3am-/p+ mice.
2) To determine the physiological consequences of Ube3a loss on neocortical development, we examined the developmental acquisition of spontaneous excitatory synaptic transmission by recording miniature excitatory postsynaptic currents (mEPSCs) in layer 2/3 pyramidal neurons of visual cortex (see Supplementary Table 1 online for intrinsic membrane properties of recorded neurons). Consistent with previous findings24, 25, mEPSC amplitudes decreased and frequency increased during development in wild-type mice . Just before eye opening (postnatal day 10, P10), mEPSC frequency and amplitude were indistinguishable between wild-type and Ube3am-/p+ mice . Thereafter, mEPSC frequency failed to develop normally in Ube3am-/p+ mice
3)Although dark rearing had no measurable effect on mEPSC amplitude in wild-type mice at P25 , sensory deprivation strongly attenuated the normal developmental increase in mEPSC frequency in wild-type mice . In contrast, dark rearing did not affect mEPSC amplitude or frequency in Ube3am-/p+ mice. Consequently, mEPSC frequency in normally reared Ube3am-/p+ mice was not significantly different from that of dark-reared wild-type mice . These findings demonstrate that, although Ube3a is not necessary for the initial sensory-independent establishment of synaptic connectivity, it is selectively required for experience-dependent maturation of excitatory circuits.
4)We therefore compared the properties of neocortical long-term depression (LTD) and LTP at layer 2/3 synapses in visual cortex of wild-type and Ube3am-/p+ mice at both young (P25) and adult (P100) ages. Because layer 2/3 pyramidal neurons receive major inputs from layer 4 pyramidal neurons, layer 2/3 field potentials were evoked by layer 4 stimulation . We began by measuring LTD in young mice using a standard stimulation protocol (1 Hz for 15 min). Although LTD was reliably induced in young wild-type mice, it was absent in young Ube3am-/p+ mice . We also observed deficits in LTP induction. A relatively weak induction protocol (three 1-s trains of 40-Hz stimulation) elicited LTP in young wild-type mice, but failed to reliably induce LTP in young Ube3am-/p+ mice . To test whether the neocortex of Ube3am-/p+ mice was capable of expressing LTP, we also applied a strong LTP stimulation protocol (two 1-s trains of 100-Hz stimulation). This protocol consistently induced LTP in both Ube3am-/p+ and wild-type mice. Thus, as with LTP deficits in hippocampus8, 9, the LTP induction machinery is impaired in the visual cortex of Ube3am-/p+ mice and this deficit in LTP can be overcome with strong stimulation.
5)To determine whether the plasticity deficits in Angelman syndrome mice persisted into adulthood, we tested LTD and LTP in adults (P100). In adult wild-type mice, LTD induced by 1-Hz stimulation was absent, as expected27, whereas LTP could be induced with strong stimulation. In adult Ube3am-/p+ mice, however, neither of these protocols were effective at modifying synaptic strength. These results indicate that wild-type mice show attenuated neocortical plasticity as they mature and that this attenuation of plasticity is more severe in the absence of Ube3a . Furthermore, these data indicate that plasticity defects in Angelman syndrome mice persist into adulthood.
..and so on (go read the full paper)
In a nutshell, what they found was that in presence of visual stimuli, the plasticity (measured by LTP/LTD ) of visual cortex was adversely affected. As sensory stimulus would normally be available while developing, this would adversely affect the plasticity in adolescence/ critical periods and also continue into adulthood.
Thus, Autism/ Angelman are charechterised by less synaptic plasticity in adulthood and during critical development periods. Paradoxically, this loss of synaptic plasticity is concomitant on their it being experience-dependent or having sensory stimuli. If the organism is sensory deprived, it may still retain the normal synaptic plasticity exhibited by similar sensory deprived normal people.
How does this relate to Psychosis? If my thesis is correct that autism and Psychosis are opposites, then I would predict that in either prader-willi or in Psychosis (scheziphrenia etc) there should be excessive experience-dependent plasticity. I was glad to learn that I am not the first one to make that proposition, but someone back in 1995 has argued for Hippocampal synaptic plasticity as an endophenotyoe for Episodic Psychosis. I now quote heavily form that article.
Here is the abstract:
Structural change in the hippocampal formation has become popular as a proposed neurobiological substrate for schizophrenic disorders. It is postulated that behavioral plasticity in the form of long-term potentiation of hippocampal synaptic transmission is an attractive putative mechanism for the mediation of transient psychosis. Moreover, the disturbed hippocampal neuroarchitecture found in schizophrenic brain may be susceptible to potentiation and dysfunctional to the degree that delusions and hallucinations develop. Partial and selective blockade of the receptors mediating potentiation may prove to be an efficient means of preventing psychotic episodes and avoiding further damage to the involved network. Basic research, utilizing experimental models such as intraventricular kainic acid injection, may help to clarify the anatomical and physiological substrate of psychosis.
The Main thesis of the paper is:
1. Anatomical, physiological, pharmacological, and behavioral findings are most consistent with the view that neuropathological changes within the limbic system, specifically within the hippocampal formation, may represent a biological substrate of schizophrenia.
2. The biological mechanism underlying transient psychosis may be long-term potentiation (LTP) of synaptic transmission within the hippocampal formation.
3. The effects of dopamine manipulation on these behaviors may be mediated by direct actions on the compromised limbic system of the psychotic patient.
Associative plasticity within hippocampus occurs in the form of long-term potentiation (LTP), an experience-dependent increase in synaptic efficacy. Experimentally, LTP is produced by tetanic stimulation of afferent systems (Bliss and Lomo 1973) and has been shown to facilitate simple associative learning (Berger 1984) but disrupt more complex forms of associative plasticity (Robinson et al 1989). Hippocampal LTP has been observed to occur as a consequence of stimulus pairings in classical conditioning (Weisz et al 1984) and appears to be mediated by N-methyl-Daspartate (NMDA) receptors (Harris et al 1984). Pharmacological blockade of NMDA receptors has been shown to disrupt learning and memory in a variety of forms, including simple associations (Stillwell and Robinson 1990), spatial learning (Morris et al 1986; Heale and Harley 1990; Shapiro and Caramanos 1990), conditioned fear (Miserendino et al 1990; Kim et al 1991), olfactory memory (Staubli et al 1989) and gustatory memory (Welzl et al 1990). Some evidence, however, suggests that deficits involve motor impairment as well as disrupted learning (Keith and Rudy 1990)
Hippocampal function is particularly sensitive to neurochemical modulation, and the expression of monoamine receptors in the temporal lobe is altered in schizophrenics (Joyce 1993). Antipsychotics that reduce endogenous dopamine levels (Losonczy et al 1987) exert significant effects on the hippocampus and LTP. Trifluoperazine inhibits induction of LTP in hippocampus (Finn et al 1980), whereas the dopamine antagonist domperidone has been shown to prevent the maintenance of LTP (Frey et al 1990). Long-term effects of antipsychotic drugs include functional supersensitivity of hippocampal pyramidal neurons (Bijak and Smialowski 1989). Thus, individuals with deranged hippocampal neuroarchitecture would be prone to cognitive dysfunction (including, perhaps, perceptual distortion and other schizophrenic symptoms), differentially susceptible to stress, and responsive to amelioration of symptoms via dopamine antagonism. It may be more than coincidence that the time lag between administration of antipsychotic medication (which results in near immediate decrement in dopamine levels) and the attenuation of psychotic symptoms weeks later (Kane 1987) is remarkably consistent with the time parameters of LTP decay (Douglas and Goddard 1975). Also, the selective disruption of “weak” associative responses by antipsychotic drugs (van der Heyden and Bradford 1988) is consistent with interactions between NMDA-receptor blockade and stimulation intensity on induction of LTP (Reed and Robinson 1991).
From the above, at least to me, it is clear that anti-psychotics may work by decreasing LTP/LTD that is enhanced in episodic psychosis. A propensity towards increased experience-dependent enhancement of synaptic palsticty may be at work here and paradoxically the same approach of sensory deprivation, as in Angelman/ Autism may work here too.
Here is the summary:
In summary, potentiation of hippocampal synaptic transmission may be the neurophysiological basis of episodic psychosis. (Post  has proposed a similar process in the amygdala as a useful model in understanding the progression of recurrent affective disorders.) More selective blockade of the NMDA receptor, which mediates LTP, may prove an effective means of attenuating positive symptoms and preventing further accrual of cellular damage in hippocampus.
In my own summation, I am convinced that we would find more synaptic plasticity in Psychotic people and that hyper-plasticity to hypo-plasticity is another dimension on which the autistics and psychotics differ and this again is a result of the genomic imprinting mediated tug-pf-war between the maternal and paternal genomes.
PORT, R., & SEYBOLD, K. (1995). Hippocampal synaptic plasticity as a biological substrate underlying episodic psychosis Biological Psychiatry, 37 (5), 318-324 DOI: 10.1016/0006-3223(94)00128-P
Koji Yashiro, Thorfinn T Riday, Kathryn H Condon, Adam C Roberts, Danilo R Bernardo, Rohit Prakash, Richard J Weinberg, Michael D Ehlers & Benjamin D Philpot (2009). Ube3a is required for experience-dependent maturation of the neocortex Nature Neuroscience
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