[Thesis]. Manchester, UK: The University of Manchester; 2019.
Schizophrenia is a chronic and severe psychiatric disorder that follows a remitting
and relapsing course of action. Impaired cognitive functions are a core feature of
schizophrenia, which persist throughout the patientsĂ˘ life despite life-long treatment
with antipsychotics (APs). While the neurocognitive effects of long-term AP treatment
remain unclear, several lines of evidence point towards its detrimental impact on
cognition, accompanied by structural brain alterations in patients with schizophrenia.
Pre-clinical models provide a platform for systematic investigation into the neurocognitive
effects of long-term AP treatment. However, model results so far lack translational
validity due to methodological limitations. Substantial evidence suggests that disruptions
in the functional interaction between the hippocampal formation (HF) and medial prefrontal
cortex (mPFC) contributes to the cognitive impairments associated with the disease.
In particular, pre-clinical investigations emphasise a key role for the direct pathway
from the ventral hippocampus (vHipp) to the mPFC in mediating higher-order cognitive
functions; these include episodic memory, executive function and goal-directed behaviour,
deficits in which are well documented in patients with schizophrenia. Timely transfer
and accurate processing of information between brain regions is governed by processes
of synaptic plasticity which are thought to be modulated by AP treatments. Modulations
of synaptic plasticity in this pathway in response to long-term treatment with APs
could advance current understanding of APsĂ˘ mechanism of action on cognition and
its neural correlates. Since studying processes of synaptic plasticity are challenging
in humans, their examination in pre-clinical models is essential.
Using the well-validated sub-chronic (sc) phencyclidine (PCP; scPCP) model for cognitive
impairments associated with schizophrenia, this project investigated the neurocognitive
effects of long-term treatment with haloperidol and olanzapine to address some of
the methodological issues associated with pre-clinical research in this field. Performance
in two variations of the novel object recognition (NOR) task formed the primary measure
of cognition in the studies reported here (Chapters 3, 4 and 6). The disrupted NOR
(dNOR; classic one-trial NOR test) test was employed to examine the ability of long-term
AP treatment to rescue scPCP-induced memory deficits. In contrast to dNOR, performance
of scPCP-treated rats is intact in the continuous NOR (cNOR). Therefore, cNOR was
employed to examine potentially negative effects of long-term treatment with APs.
Furthermore, through in vivo electrophysiological recordings under anaesthetised conditions,
this project characterised the synaptic properties (synaptic connectivity, short-
and long-term synaptic plasticity) of the vHipp-mPFC pathway in the scPCP model for
the first time (Chapter 5). This was followed by an investigation into the impact
of long-term haloperidol treatment on synaptic properties of the vHipp-mPFC pathway
in the scPCP model (Chapter 6).
Results presented in Chapter 3 were inconclusive in determining the influence of 22
days of treatment with haloperidol (0.1 mg/kg/day, oral administration; p.o.) and
olanzapine (1.5 mg/kg/day; p.o.) on dNOR performance. Performance was assessed once
weekly on days 1, 8, 15 and 22 of AP treatment and at two other time points during
treatment washout period. In this study, the presence of a robust scPCP-induced dNOR
deficit could not be confirmed. It was reasoned that this could have been caused by
excess handling prior to scPCP dosing and during AP treatment period. Furthermore,
high variability in the dNOR outcome rendered the findings of this study inconclusive.
In Chapter 4, investigations were limited to haloperidol (0.5mg/kg/day), which was
delivered via subcutaneous osmotic minipumps over 28 days. In addition to the dNOR
test, the cNOR test was also introduced as a measure of cognition. Tests were repeated
at 6 time points throughout the study (1 dNOR and cNOR assessment prior to osmotic
minipump implant and 2 other dNOR and cNOR testing sessions post minipump implant).
In addition to dNOR, the effectiveness of scPCP treatment in this study was also examined
by assessment of its locomotor activity response to an acute dose of amphetamine (1mg/kg)
administered intraperitoneally (i.p.). While results of the dNOR test (prior to implant)
could not confidently confirm the effectiveness of scPCP treatment, findings of the
locomotor activity in response to amphetamine suggested that the scPCP treatment had
been effective. Overall, the behavioural findings of this study were also inconclusive
due to performance variability. By limiting the number of NOR testing sessions, Chapter
6, following the same treatment plan as Chapter 4, showed that 28 days of treatment
with haloperidol did not impair cognitive performance in the cNOR task in scPCP and
control rats. Investigations into the synaptic properties of the vHipp-mPFC pathway
(Chapter 5), showed a significant reduction in strength of glutamatergic synaptic
connectivity from vHipp to the mPFC in scPCP treated rats, although the vHipp-mPFC
pathway was still able to support synaptic facilitation and long-term potentiation
in scPCP treated rats. The general pattern of the results points towards compromised
inhibitory mechanisms manifested as hyper excitability/plasticity in this pathway
in the scPCP model, which is consistent with studies in other animal models of the
disease. Investigations in this chapter further suggested a significant reduction
in the excitability threshold in the scPCP treated rats, an effect which might also
involve disturbances in ĂŽÂ˛-adrenoceptor-mediated effects. As presented in Chapter
6, 28 days of treatment with haloperidol appeared to have reduced the strength of
synaptic connectivity in the sub-chronic Vehicle (0.9% saline; scVeh) treated rats.
This effect did not reach statistical significance in comparison to the scVeh-control
treatment group. This trend was absent in the haloperidol treated scPCP rats in comparison
to its control. The amplitude of synaptic connectivity was, however, appeared to have
been reduced (also not statistically significant) to the same extent in both scVeh
and scPCP treatment groups in response to long-term haloperidol treatment. This may
point towards the effect of haloperidol in increasing the activity of a subset of
inhibitory mechanisms, which may be involved in regulating response strength and size.
Interestingly, investigations into short- and long-term synaptic plasticity showed
that long-term haloperidol treatment induced a state of hyper excitability/plasticity
in the vHipp-mPFC pathway in both scVeh and scPCP rats, which was significantly more
robust in the scPCP treatment groups. These results suggest that the observed hyper-excitability
may be due to disruptions in GABAB-D2-NMDA receptor interaction. Collectively, these
results suggest that different inhibitory mechanisms may be involved in regulating
the vHipp-mPFC responses, which may be differentially affected by haloperidol.
In conclusion, the behavioural studies presented in this thesis highlighted that the
scPCP model is susceptible to the effects of handling, which can interfere with study
outcome. In addition, these studies suggested, that repeated administration of dNOR
and cNOR tests, results in pronounced performance variability, which leads to ambiguous
findings. In spite of these challenges, the behavioural studies presented in this
thesis were able to demonstrate that, for the duration studied, haloperidol did not
impair performance on the cNOR task in the scPCP and scVeh treatment groups. Through
the use of electrophysiological techniques, the studies presented in this thesis were
able to investigate previously unexplored aspects of the scPCP model, which contributes
to its validity with relevance to cognitive impairments associated with schizophrenia.
Results of these studies demonstrated deficits in synaptic connectivity and highlighted
a general reduction in inhibitory tone, manifested as hyper excitability/plasticity
in the vHipp-mPFC pathway in scPCP-treated rats. These disturbances, which may be
responsible for cognitive impairments in schizophrenia, were further exacerbated by
long-term haloperidol treatment. Functional consequences of these disturbances were
not reflected in the behavioural paradigms employed, as these tests do not depend
on vHipp-mPFC interaction for successful performance. Further studies are required
to determine the behavioural and cognitive impact of these synaptic alterations using
more complex and sensitive behavioural paradigms, which engage the vHipp-mPFC pathway.
In addition to its role in mediating cognitive processes, disruptions in the activity
of the vHipp-mPFC pathway, specifically the hyperactivity of the vHipp, are also thought
to be involved in psychosis. The hyper-excitability in the vHipp-mPFC pathway following
long-term treatment with haloperidol may be indicative of the processes of dopamine
super-sensitivity and AP-induced psychosis, instances of which are observed in the
clinic. Therefore, investigations into synaptic properties of the vHipp-mPFC pathway
may provide a platform for better understanding of disease processes and contribute
to advancements in novel drug development with improved efficacy in treating positive
symptoms and cognitive impairments associated with schizophrenia.