Modeling disturbances of cholinergic systems : possible relevance for schizophrenia

Abstract

Schizophrenia is a severe, chronic mental disorder. The etiology remains unknown but alterations in cholinergic neurotransmission is one, among other, disturbances in the brain that have been implicated in schizophrenia. The main aim of the thesis projects has been to evaluate if disturbances in central cholinergic function can contribute to schizophrenic symptoms. In all studies an immunotoxin, 192 IgG-saporin (192-sap), was used to selectively kill cholinergic neurons in the basal forebrain.

Paper I examined the effect of central cholinergic denervation on dopamine (DA)-mediated functions. Because schizophrenia is often considered to be a developmental disorder, we also examined the effect of neonatal cholinergic denervation. Adult and neonatal rats received intracerebroventricular (icv) injections of 192-sap to selectively destroy the cholinergic neurons in the basal forebrain projecting to hippocampus and neocortex. We found that adult lesioned rats showed increased spontaneous horizontal activity and a remarkable increase in locomotor response to amphetamine as evidenced by increased horizontal and vertical activity. The behavioral response to the DA receptor agonist apomorphine was not changed by denervation. There were no marked changes of spontaneous or drug-induced locomotor activity in adult rats that had been treated with 192-sap as neonates.

Paper II evaluated the relative contribution of the loss of septohippocampal versus basalocortical cholinergic projections for the amphetamine hyper-response seen in icv 192-sap injected rats. Since icv delivery of 192-sap also destroys a population of p75 receptor expressing Purkinje neurons, this cell loss needed to be taken into consideration as well. Cortex cerebri and hippocampus were selectively cholinergically denervated by intraparenchymal injections of 192-sap into nucleus basalis magnocellularis (NBM) and the medial septum/diagonal band of Broca, respectively. Selective loss of Purkinje cells in cerebellum was achieved by icv delivery of OX7 saporin. Selective cholinergic denervation of cortex cerebri, but not denervation of hippocampus or damage to cerebellum elicited DArgic hyper-reactivity similar to that seen in previous icv 192-sap experiments.

Paper III: Disturbances in glutamatergic functions, and especially hypofunction of the NMDA receptor, is considered as a potential contributing factor in schizophrenia. Since cholinergic afferents from NBM are known to modulate glutamate transmission in neocortex, we hypothesized that cortical cholinergic denervation might result in changes in glutamatergic activity. Therefore, we lesioned the cholinergic corticopetal projections by local infusion of 192-sap into NBM of rats. Possible effects of this lesion on glutamatergic systems were examined by phencyclidine (PCP) (an NMDA receptor antagonist)-induced locomotor activity, and also by NMDA-receptor binding. Cholinergic lesioning of neocortex lead to enhanced sensitivity to PCP in the form of a dramatic increase in horizontal activity. Further, NMDA-receptor binding was unaffected in denervated rats.

Paper IV investigated if the increased sensitivity to amphetamine seen in rats with cortical cholinergic denervation was paralleled by an increased amphetamine-induced release of DA in nucleus accumbens (NAC) and/or striatum. The corticopetal cholinergic projections were lesioned by local infusion of 192-sap into NBM in adult rats. Amphetamine-induced DA release in NAC and striatum were monitored by in vivo microdialysis two to three weeks after lesioning. Amphetamine caused a greater release of DA in NAC but not in striatum of rats with cortical cholinergic denervation compared to sham lesioned controls. The duration of the amphetamine-effect was also significantly longer in the lesioned group.

Paper V: Structural changes in schizophrenic brains such as enlarged lateral ventricles and reduced volume of cortical areas have been reported. Therefore we measured possible changes in volume or cortical thickness after cortical cholinergic denervation. Rats received unilateral cholinergic denervation produced by local infusion of the immunotoxin 192-sap into the left NBM while the right side was sham lesioned. MRI scans were made, 3 and 8 weeks postlesion. Age matched naïve rats served as controls. We did not find a significant difference in cortical thickness or hippocampal size between left and right hemispheres in the denervated animals. However, when comparing differences in grayscale values between the left and right side in T2-weighted images 8 weeks after the lesion the left, denervated side was significantly darker in saporin treated rats. This could indicate reduced cerebral blood flow in the cholinergically denervated cortical mantle.

Paper VI: Schizophrenic patients often have cognitive dysfunctions such as impaired working memory, which might be linked to cholinergic alterations. Thus we investigated if cortical cholinergic deficits affected learning and memory. The corticopetal cholinergic system was lesioned with local infusion of the immunotoxin 192-sap into. Working memory was investigated in a delayed matching-to-place task in the water maze. We found that control animals readily learned the task, while saporin-treated rats clearly had impaired learning and memory abilities.

Conclusions: Cholinergic denervation of cortex cerebri in adult rats leads to markedly increased behavioral responsiveness to DArgic stimulation by d-amphetamine. This is paralleled by increased release of DA in NAC. The behavioral responses to phencyclidine, a drug used to model aspects of schizophrenia, are also strongly potentiated. Animals with selective cortical cholinergic denervations also display impaired memory functions. A further observation was that effects of cholinergic denervation of cortex cerebri can be detected by standard MR imaging. This suggests that the denervation may alter cortical blood flow and/or overall neuronal activity in cortex cerebri. These observations are compatible with a possible role of cholinergic deficits in schizophrenia, and suggests ways in which cholinergic, glutamatergic and DArgic hypotheses of schizophrenia may be linked.