Disruption of mesolimbic regulation of prefrontal cholinergic transmission in an animal model of schizophrenia and normalization by chronic clozapine treatment

Abstract

Abnormal mesolimbic control of cortical cholinergic activity has been hypothesized to contribute to the cognitive symptoms of schizophrenia. Stimulation of NMDA receptors in nucleus accumbens (NAC) increases acetylcholine (ACh) release in prefrontal cortex (PFC), an activation thought to contribute to attentional processing. Thus, the effects of intra-NAC perfusion of NMDA (250-400 microM) on ACh release in PFC were determined in rats receiving lesions of the ventral hippocampus (VH) as neonates (nVHLX), a neurodevelopmental model of schizophrenia, or as adults (aVHLX). NMDA elevated ACh release (100-150% above baseline) in adults sham-lesioned as neonates or in aVHLX rats. Adult nVHLX were unresponsive to NAC NMDA receptor stimulation. The inability of nVHLX to respond to NMDA emerged over development as a separate experiment demonstrated that evoked ACh release was normal before puberty (100-150% increase) yet, in these same nVHLX animals, absent after puberty. Amphetamine-evoked ACh release was assessed in nVHLX animals to exclude potential limitations in release capacity. Amphetamine produced greater increases in ACh release than in shams, indicating that nVHLX does not impair the capacity of cholinergic neurons to release ACh. Finally, the ability of 13 days of pretreatment with clozapine (1.25 mg/kg/day) to reinstate NMDA-evoked cortical ACh efflux was determined. Clozapine treatment normalized NMDA-evoked ACh release in nVHLX animals. These experiments show that mesolimbic regulation of cortical ACh release is disrupted in postpubertal nVHLX rats and normalized by low-dose treatment of clozapine; supporting the usefulness of nVHLX animals for research on the neuronal mechanisms underlying the cognitive symptoms of schizophrenia.

Figure 1

Representative coronal sections of the ventral hippocampal region of animals that received sham-lesions as adults (a) or neonates (c), or infusions of ibotenic acid as adults (b) or neonates (d; scales: 200 μm; all 4 sections are from Bregma AP levels −4.5 – −4.8 mm). At both ages, the lesions removed the ventral CA3 and CA1 portions of the hippocampus and extended posteriorly to include the ventral dentate gyrus and subiculum. Damage to the adjacent piriform (Pir) and enthorinal cortex remained generally very limited. Additional landmark structure indicated on the sections from sham-lesioned brains are the dorsal lateral geniculate nucleus (DLG), the ventral part of the medial geniculate nucleus (MGV) as well as the substantia nigra (SN).

Figure 2

Mean (± S.E.M.) ACh efflux in the mPFC of sham-lesioned controls (n = 9) and nVHLX rats (n = 6) receiving, in pseudo-randomized order, perfusions of vehicle (aCSF) or NMDA (400 μM) into the NAC shell during two separate dialysis sessions as adults. Following baseline collections (1–4), aCSF or NMDA was perfused for 1 hr (5–8). Following drug perfusion, aCSF was re-perfused for 45 min (9–11) until the conclusion of the dialysis session. In sham animals, perfusion of NMDA produced a robust increase in cortical ACh efflux above that observed during aCSF perfusion. In nVHLX animals, however, perfusion of NMDA failed to increase cortical ACh efflux above baseline levels.

Figure 3

Mean (± S.E.M.) ACh efflux in the mPFC of sham-lesioned controls (n = 6) and aVHLX rats (n = 6) receiving, in pseudo-randomized order, perfusions of vehicle (aCSF) or NMDA (250 μM) into the NAC shell during two separate dialysis sessions. Following baseline collections (1–4), aCSF or NMDA was perfused for 1 hr (5–8). Following drug perfusion, aCSF was re-perfused for 45 min (9–11) until the conclusion of the dialysis session. Perfusion of NMDA resulted in a sustained increase in cortical ACh efflux, above that observed during aCSF perfusion, in both sham and aVHLX animals. There was no effect of lesion on the NMDA-stimulated release.

Figure 4

Mean (± S.E.M.) ACh efflux in the mPFC of sham-lesioned controls (n = 6) and nVHLX rats (n = 6) tested at pre-pubertal (panel a; Days 30–38) and again at post-pubertal ages (panel b; Days 56–64) for the effects of intra-NAC perfusions of aCSF or NMDA (250 μM). When rats were tested pre-puberty, both sham and nVHLX exhibited an NMDA-stimulated release of ACh (collections 5–8). However, when these same rats were retested post-puberty, the responsivity of nVHLX rats to NMDA was lost and only sham-lesioned rats exhibited a drug-induced stimulation of ACh.

Figure 5

Mean (± S.E.M.) ACh efflux in the mPFC of shams (n = 6) or nVHLX (n = 6) rats receiving systemic injections of amphetamine (AMPH; 2 mg/kg, i.p.) as adults. Following baseline collections (1–4), AMPH was injected. In contrast to the effects of NMDA (Figs. 2 and 4b), administration of amphetamine resulted in robust increases in cortical ACh efflux in both sham and nVHLX groups, with lesioned animals exhibiting a potentiated release relative to that seen in control rats.

Figure 6

Mean (± S.E.M.) ACh efflux in the mPFC of sham (n = 8) or nVHLX (n = 8) rats following pretreatment (daily injections for 13 days) with either saline (0.9%) or clozapine (1.25 mg/kg, i.p.) prior to microdialysis testing as adults. Each animal received, in pseudo-randomized order, perfusions of vehicle (aCSF) and NMDA (250 μM) into the NAC shell during two dialysis sessions. The period for baseline infusions of aCSF and NMDA was the same as those reported in previous experiments (Figs. 2–4). In the control condition of saline-pretreatment (Panel a), the results of Experiment 1A (Fig. 2) were replicated. Namely, NMDA perfusion increased cortical ACh efflux in sham control animals whereas this effect was absent in nVHLX animals. Following pre-treatment with clozapine (Panel b), nVHLX animals exhibited a sham-control-like stimulation of ACh release following intra-NAC perfusion of NMDA.