Many pharmacotherapies, already in use for human patients with psychosis, are also effective at treating ‘schizophrenia-related’ phenotypes in MAM-treated rats. Specifically, D2R antagonists such as haloperidol and sertindole can acutely reduce dopaminergic neuron activity in the VTA of MAM-treated rats but not controls, reflective of the fast actions of antipsychotics in schizophrenia patients (Valenti et al., 2011). Behaviourally, acute treatment with haloperidol, risperidone, and clozapine, have been shown to attenuate spontaneous, and MK- 801-induced, hyperactivity in MAM-treated rats (Le Pen et al., 2011).
A primary purpose of a preclinical animal manipulation is to test potential treatments for the disorder the manipulation recapitulates, many studies have been done with MAM-treated rats to explore putative therapies. To this end, the MAM-manipulation has generated some interesting discoveries. Diazepam, a GABAA receptor PAM, has been found to enhance GABAergic neurotransmission in MAM-treated rats after sub-chronic administration during the puberty stage of development (PND 31 - 40), resulting in normal VTA dopamine neuron activity,
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and amphetamine induced hyperlocomotion, when assayed in adulthood (PND 60 - 80; Du and Grace, 2013)). Recently, this approach was extended to MAM-treated rat amygdala hyperactivity with a similarly positive result of attenuated neuronal activity and reduced measures of anxiety in the elevated plus maze (Du and Grace, 2016). A common adjunct treatment for schizophrenia, valproic acid, has also been shown to be effective at reducing ‘schizophrenia-related’ phenotypes (PPI and histone deacetylase 2 expression) in MAM-treated rats when administered in early adolescence (PND 23 – 29) or in adulthood (Bator et al., 2015). Interestingly, a study utilizing a combination of forward and reverse translation, found that a history of chronic D2R antagonists treatment in MAM-treated rats resulted in a blunted response to selective enhancement of GABAAα5 receptors (Gill et al., 2014), a finding with powerful implications for treatment approaches in schizophrenia patients.
MAM-based studies on enhancement of GABAergic signalling have also found that acute treatment with an α5-GABAR PAM also reduces the hyperactivity of dopaminergic neurons and amphetamine induced hyperlocomotion (Gill et al., 2011). The authors of the study were also able to show that increasing inhibitory activity within the ventral hippocampus via local administration of the PAM had the same effect, indicating ventral hippocampus hyperactivity as a primary target for therapeutics. Additional studies with local administration of tetrodotoxin (Lodge and Grace, 2007), GABAergic interneuron cell grafts (Perez and Lodge, 2013), and deep brain stimulation (Perez et al., 2013), targeted to the ventral hippocampus, also proved therapeutic for hyperdopaminergic activity in MAM- treated rats.
5.2 Rationale
The empirical evidence supporting the MAM-manipulation is compelling. While, D2R antagonists have proven effective at treating the positive symptoms associated with the disorder, novel pharmacotherapies are required to reduce the cognitive deficits (Conn et al., 2009). A 2007 clinical trial found that patients with
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schizophrenia had reduced positive and negative symptoms in response to a glutamatergic pharmacotherapy (Patil et al., 2007). Indeed, Gastambide et al. (2012) found that enhancement of glutamatergic signalling via an mGluR5 PAM normalised the ASST reversal learning performance in MAM-treated rats. Unfortunately, the compound used in the study was unfit for long-term human consumption. However, the potential for glutamatergic enhancement of ASST performance in MAM-treated rats (and hopefully eventually patients) drove the studies in this experimental chapter.
The NMDAR PAM ORG49209 is a synthetic steroid which our lab has preliminary evidence supporting it as putative cognitive normaliser (Chase dissertation 2013). With the discovery of novel steroid binding sites and steroid- mediated regulation of NMDARs (Park-Chung et al., 1997; Paul et al., 2013), interest has emerged in directly targeting NMDARs via synthetic derivatives of these compounds. The cholesterol metabolite 24 (S)-hydroxycholesterol is an endogenous neuroactive steroid which has its own allosteric NMDAR binding site. 24 (S)-hydroxycholesterol can potently enhance NMDAR-mediated Ca2+ influx as measured by increased NMDA-stimulated excitatory postsynaptic currents (EPSCs) in cultured mouse hippocampal neurons (Paul et al.). Derived from 24 (S)- hydroxycholesterol, the synthetic neurosteroid ORG49209 (SGE201) also enhances NMDAR activity as measured by increased EPSCs, in addition to enhanced high frequency stimulation-induced long-term potentiation in rat hippocampal slices (Paul et al.). Experiments from our lab found that 15 days of treatment with 10 mg/kg ORG49209 improved reversal learning performance in aged rats (12 - 18 months old; Chase, 2013). Since ORG49209 directly targets the NMDAR, it is of interest to discover if the compound may be effective at ameliorating ‘schizophrenia-related’ behaviours. The aims of this chapter were to: 1) replicate previous findings of reversal learning and set-shifting deficits in MAM- treated rats, and 2) to explore the potential for ORG49209 to normalise any deficits found.
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5.3 Methods and materials
5.3.1 Animals
Male Sprague Dawley rats exposed to MAM (22 mg/kg/ml) or saline vehicle on gestational day 17 were obtained from (Lilly, Wokingham, UK). A total of 28 male rats were obtained (control = 14, MAM = 14). Rats were housed in standard housing conditions (two to three per cage, 0700 - 1900 hr light phase, controlled temperature and humidity, and ad libitum water). Animals were at least 5 months old at the beginning of behavioural testing. One month prior to behavioural testing rats were placed on a food control regimen of 15 - 20 g per day, to increase their motivation to work for food reward. All animals continued to gain weight at a normal rate and were handled during weighing and general husbandry procedures.