The impact of oxytocin and GlyT1 inhibitors on social behaviour

Schizophrenia is a complex mental disorder characterised by various symptoms which fall into three categories of positive, negative and cognitive. In particular, negative symptoms are poorly treated by current medications although several adjunctive therapies are under investigation including Glycine Transporter (GlyT1) inhibitors and neuropeptides such as oxytocin. Despite accumulating preclinical and clinical evidence that such compounds can influence social behaviour and improve negative symptoms in patients, there is little information as to the precise mechanisms by which they work. Therefore, the aims of this thesis were to ultimately determine some of the key regions and potential signalling pathways activated following administration of these compounds in Lister-hooded rats. Firstly, a functional map of GlyT1 inhibitor RO4993850, an analogue to Bitopertin, identified the selective activation of neurons within the rostral and caudal prefrontal cortex (PFC), suggesting potential NMDA receptor activation in brain areas involved in motivation and goal-directed behaviour. This was further assessed in a novel ‘dual-hit’ neonatal-PCP isolation-rearing rodent model for schizophrenia which was shown herein to induce locomotor hyperactivity and social deficits including reduced social interaction (an index for negative symptoms) and increased communication (as assessed by ultrasonic vocalisations (USVs)). Interestingly neonatal-PCP isolation-reared rats emitted more pro-social 50 kHz USVs which were also longer in duration and had a greater change in call bandwidth compared to controls. Neonatal-PCP isolation-rearing was also shown to selectively decrease parvalbumin expression (a calcium binding protein present in GABAergic interneurons) in the hippocampus but not in the rostral PFC sub-regions assessed, producing similar changes to other rodent models. Microdialysis studies however revealed no change to basal PFC and striatal dopamine levels in these rats. Chronic treatment with the GlyT1 inhibitor RO4993850 improved social deficits in the social interaction test and altered both USV emissions and call characteristics but showed no effect on locomotor hyperactivity, parvalbumin expression in either the PFC or hippocampus, nor dopamine overflow in the PFC or striatum. Finally, an established dose of the neuropeptide oxytocin which did not influence core body temperature, was shown to attenuate PCP-induced hyperactivity, increase pro-social behaviour and selectively enhance dopamine release in the nucleus accumbens (NAc) in group-housed Lister-hooded rats. Thereby providing supporting evidence for regionally-specific oxytocin-dopaminergic interactions within the mesocorticolimbic circuits responsible for regulating associative and rewarding behaviour. There are therefore several potential mechanisms by which both GlyT1 inhibitors and oxytocin can influence social behaviour, most likely via activation of key brain loci involved in motivation. Although further work is required, results herein indicate the potential of GlyT1 inhibitors and oxytocin as adjunctive therapies to treat predominant negative symptoms in schizophrenia

Oxytocin attenuates phencyclidine hyperactivity and increases social interaction and nucleus accumben dopamine release in rats

The pituitary neuropeptide oxytocin promotes social behavior, and is a potential adjunct therapy for social deficits in schizophrenia and autism. Oxytocin may mediate pro-social effects by modulating monoamine release in limbic and cortical areas, which was investigated herein using in vivo microdialysis, after establishing a dose that did not produce accompanying sedative or thermoregulatory effects that could concomitantly influence behavior. The effects of oxytocin (0.03–0.3 mg/kg subcutaneous) on locomotor activity, core body temperature, and social behavior (social interaction and ultrasonic vocalizations) were examined in adult male Lister-hooded rats, using selective antagonists to determine the role of oxytocin and vasopressin V1a receptors. Dopamine and serotonin efflux in the prefrontal cortex and nucleus accumbens of conscious rats were assessed using microdialysis. 0.3 mg/kg oxytocin modestly reduced activity and caused hypothermia but only the latter was attenuated by the V1a receptor antagonist, SR49059 (1 mg/kg intraperitoneal). Oxytocin at 0.1 mg/kg, which did not alter activity and had little effect on temperature, significantly attenuated phencyclidine-induced hyperactivity and increased social interaction between unfamiliar rats without altering the number or pattern of ultrasonic vocalizations. In the same rats, oxytocin (0.1 mg/kg) selectively elevated dopamine overflow in the nucleus accumbens, but not prefrontal cortex, without influencing serotonin efflux. Systemic oxytocin administration attenuated phencyclidine-induced hyperactivity and increased pro-social behavior without decreasing core body temperature and selectively enhanced nucleus accumbens dopamine release, consistent with activation of mesocorticolimbic circuits regulating associative/reward behavior being involved. This highlights the therapeutic potential of oxytocin to treat social behavioral deficits seen in psychiatric disorders such as schizophrenia

No evidence from complementary data sources of a direct glutamatergic projection from the mouse anterior cingulate area to the hippocampal formation

The connectivity and interplay between the prefrontal cortex and hippocampus underpin various key cognitive processes, with changes in these interactions being implicated in both neurodevelopmental and neurodegenerative conditions. Understanding the precise cellular connections through which this circuit is organised is, therefore, vital for understanding these same processes. Overturning earlier findings, a recent study described a novel excitatory projection from anterior cingulate area to dorsal hippocampus. We sought to validate this unexpected finding using multiple, complementary methods: anterograde and retrograde anatomical tracing, using anterograde and retrograde adeno-associated viral vectors, monosynaptic rabies tracing, and the Fast Blue classical tracer. Additionally, an extensive data search of the Allen Projection Brain Atlas database was conducted to find the stated projection within any of the deposited anatomical studies as an independent verification of our own results. However, we failed to find any evidence of a direct, monosynaptic glutamatergic projection from mouse anterior cingulate cortex to the hippocampus proper

Oxytocin attenuates phencyclidine hyperactivity and increases social interaction and nucleus accumben dopamine release in rats

The pituitary neuropeptide oxytocin promotes social behavior, and is a potential adjunct therapy for social deficits in schizophrenia and autism. Oxytocin may mediate pro-social effects by modulating monoamine release in limbic and cortical areas, which was investigated herein using in vivo microdialysis, after establishing a dose that did not produce accompanying sedative or thermoregulatory effects that could concomitantly influence behavior. The effects of oxytocin (0.03-0.3mg/kg s.c.) on locomotor activity, core body temperature and social behavior (social interaction and ultrasonic vocalisations) were examined in adult male Lister-hooded rats, using selective antagonists to determine the role of oxytocin and vasopressin V1A receptors. Dopamine and serotonin (5-HT) efflux in the prefrontal cortex (PFC) and nucleus accumbens (NAc) of conscious rats were assessed using microdialysis. 0.3mg/kg oxytocin modestly reduced activity and caused hypothermia but only the latter was attenuated by the V1A receptor antagonist, SR49059 (1mg/kg i.p.). Oxytocin at 0.1mg/kg, which did not alter activity or temperature, significantly attenuated PCP-induced hyperactivity and increased social interaction between unfamiliar rats without altering the number or pattern of ultrasonic vocalisations. In the same rats, oxytocin (0.1 mg/kg) selectively elevated dopamine overflow in the NAc (F(1, 12)=7.983, P=0.0153), but not PFC, without influencing 5-HT efflux. Systemic oxytocin administration attenuated PCP-induced hyperactivity and increased pro-social behavior without decreasing core body temperature and selectively enhanced NAc dopamine release, consistent with activation of mesocorticolimbic circuits regulating associative/reward behavior being involved. This highlights the therapeutic potential of oxytocin to treat social behavioral deficits seen in psychiatric disorders such as schizophrenia and autism

The impact of oxytocin and GlyT1 inhibitors on social behaviour

Schizophrenia is a complex mental disorder characterised by various symptoms which fall into three categories of positive, negative and cognitive. In particular, negative symptoms are poorly treated by current medications although several adjunctive therapies are under investigation including Glycine Transporter (GlyT1) inhibitors and neuropeptides such as oxytocin. Despite accumulating preclinical and clinical evidence that such compounds can influence social behaviour and improve negative symptoms in patients, there is little information as to the precise mechanisms by which they work. Therefore, the aims of this thesis were to ultimately determine some of the key regions and potential signalling pathways activated following administration of these compounds in Lister-hooded rats. Firstly, a functional map of GlyT1 inhibitor RO4993850, an analogue to Bitopertin, identified the selective activation of neurons within the rostral and caudal prefrontal cortex (PFC), suggesting potential NMDA receptor activation in brain areas involved in motivation and goal-directed behaviour. This was further assessed in a novel ‘dual-hit’ neonatal-PCP isolation-rearing rodent model for schizophrenia which was shown herein to induce locomotor hyperactivity and social deficits including reduced social interaction (an index for negative symptoms) and increased communication (as assessed by ultrasonic vocalisations (USVs)). Interestingly neonatal-PCP isolation-reared rats emitted more pro-social 50 kHz USVs which were also longer in duration and had a greater change in call bandwidth compared to controls. Neonatal-PCP isolation-rearing was also shown to selectively decrease parvalbumin expression (a calcium binding protein present in GABAergic interneurons) in the hippocampus but not in the rostral PFC sub-regions assessed, producing similar changes to other rodent models. Microdialysis studies however revealed no change to basal PFC and striatal dopamine levels in these rats. Chronic treatment with the GlyT1 inhibitor RO4993850 improved social deficits in the social interaction test and altered both USV emissions and call characteristics but showed no effect on locomotor hyperactivity, parvalbumin expression in either the PFC or hippocampus, nor dopamine overflow in the PFC or striatum. Finally, an established dose of the neuropeptide oxytocin which did not influence core body temperature, was shown to attenuate PCP-induced hyperactivity, increase pro-social behaviour and selectively enhance dopamine release in the nucleus accumbens (NAc) in group-housed Lister-hooded rats. Thereby providing supporting evidence for regionally-specific oxytocin-dopaminergic interactions within the mesocorticolimbic circuits responsible for regulating associative and rewarding behaviour. There are therefore several potential mechanisms by which both GlyT1 inhibitors and oxytocin can influence social behaviour, most likely via activation of key brain loci involved in motivation. Although further work is required, results herein indicate the potential of GlyT1 inhibitors and oxytocin as adjunctive therapies to treat predominant negative symptoms in schizophrenia