Post-pubertal disruption of medial prefrontal cortical dopamine-glutamate interactions in a developmental animal model of schizophrenia

Abstract

Background: A neonatal ventral hippocampal lesion (NVHL) induces behavioral and physiological anomalies mimicking pathophysiological changes of schizophrenia. Because prefrontal cortical (PFC) pyramidal neurons recorded from adult NVHL rats exhibit abnormal responses to activation of the mesocortical dopaminergic (DA) system, we explored whether these changes are due to an altered DA modulation of pyramidal neurons.

Methods: Whole-cell recordings were used to examine the effects of DA and glutamate agonists on cell excitability in brain slices obtained from pre- (postnatal day [PD] 28-35) and post-pubertal (PD > 61) sham and NVHL animals.

Results: N-methyl d-aspartate (NMDA), alpha-amino-3-hydroxy-5-methylisoxazole propionate (AMPA), and the D(1) agonist SKF38393 increased excitability of deep layer pyramidal neurons in a concentration-dependent manner. The opposite effect was observed with the D(2) agonist quinpirole. The effects of NMDA (but not AMPA) and SKF38393 on cell excitability were significantly higher in slices from NVHL animals, whereas quinpirole decrease of cell excitability was reduced. These differences were not observed in slices from pre-pubertal rats, suggesting that PFC DA and glutamatergic systems become altered after puberty in NVHL rats.

Conclusions: A disruption of PFC dopamine-glutamate interactions might emerge after puberty in brains with an early postnatal deficit in hippocampal inputs, and this disruption could contribute to the manifestation of schizophrenia-like symptoms.

Figure 1

Neonatal ventral hippocampal lesion (NVHL). (A) Coronal sections showing the ventral hippocampus of a sham rat (top) and a typical NVHL (bottom), characterized by cell loss (thick arrows), and enlarged ventricles (thin arrows). (B) Drawings illustrating the extension of the neonatal lesion observed in adulthood. Gray and dark areas indicate maximal and minimal extents of damage, respectively.

Figure 2

Whole-cell patch clamp recording of medial prefrontal cortical (PFC) pyramidal neurons from adult animals. (A) Typical responses to depolarizing and hyperpolarizing somatic current pulses (300 msec, from −300 to +100 pA) of a deep layer pyramidal neuron recorded from an adult rat. (B) Image of a typical deep-layer pyramidal neuron recorded from the medial PFC and labeled with Neurobiotin. Black arrowheads indicate the apical dendrite and the white arrowhead points to the cell body. (C) Current-voltage plot obtained from the traces shown in (A). Currents larger than −100 pA yielded an evident inward rectification (arrowhead) in the hyperpolarizing direction.

Figure 3

The inhibitory effect of quinpirole on prefrontal cortical (PFC) pyramidal cell excitability is reduced in neonatal ventral hippocampal lesion (NVHL) animals. (A) Line graphs summarizing the concentration-dependent effect of quinpirole (n = 5– 6 cells/dose) on the excitability of PFC pyramidal neurons recorded from adult (postnatal day [PD] 61–78) NVHL and sham-operated animals (all data are mean ± SD; ***p < .0005, **p < .005, *p < .01, Tukey post hoc test after significant two-way analysis of variance). Only higher doses (2 and 4 μmol/L) yielded the inhibitory effect in NVHL animals (^#p < .0002 compared with baseline, Tukey post hoc test). (B) Representative traces illustrating the effect of 1 μmol/L quinpirole on PFC pyramidal neuron excitability. Quinpirole (1 μmol/L) reduced the number of evoked spikes from 4 to 2 spikes in the PFC of sham animals, whereas no apparent effect was observed in the lesioned group.

Figure 4

The excitatory effect of SKF38393 on prefrontal cortical (PFC) pyramidal cell excitability is enhanced in neonatal ventral hippocampal lesion (NVHL) animals. (A) Line graphs summarizing the concentration-dependent effect of SKF38393 (n = 5– 6 cells/dose) on the excitability of PFC pyramidal neurons recorded from adult (postnatal day [PD] 61–78) sham-operated and NVHL animals (***p < .0005, **p < .005, Tukey post-hoc test after significant two-way analysis of variance). (B) Traces of responses of two representative pyramidal neurons recorded from a sham and an NVHL rat during baseline and after bath application of 2 μmol/L SKF38393. The number of evoked spikes was not affected by SKF38393 in the neuron recorded from the sham rat. In contrast, bath application of 2 μmol/L SKF38393 increased pyramidal cell excitability from 2 to 3 spikes in the lesioned rat.

Figure 5

The excitatory effect of N-methyl d-aspartate (NMDA) on PFC pyramidal cell excitability is enhanced in NVHL animals. (A) Line graphs summarizing the concentration-dependent effect of NMDA (n = 5–7 cells/dose) on the excitability of pyramidal neurons recorded in the PFC of adult (PD 61–78) sham-operated and NVHL animals (***p < .0002, **p < .001, Tukey post hoc test after significant two-way analysis of variance). (B) Representative traces of two PFC pyramidal neurons recorded from a sham and a lesioned animal illustrating their response to 2 μmol/L NMDA. After 5 min of NMDA, the number of evoked spikes increased from 2 t o 3 spikes in the sham group, whereas a more pronounced increase (from 2 t o 7 spikes) was observed in the lesioned animal. Abbreviations as in Figure 4.

Figure 6

The effect of α-amino-3-hydroxy-5-methylisoxazole propionate (AMPA) on PFC pyramidal cell excitability is unchanged in NVHL animals. The PFC pyramidal neuron excitability increases in a concentration-dependent manner after bath application of AMPA (top: number of evoked spikes; bottom: first spike latency, n = 5– 6 cells/dose). The PFC pyramidal neurons recorded from sham and NVHL animals (PD 61–78) exhibited similar increase in the number of evoked spikes and decrease of first spike latency. Abbreviations as in Figure 4.

Figure 7

Concentration-dependent effect of quinpirole (A), SKF38393 (B), and NMDA (C) on PFC pyramidal cell excitability (number of spikes evoked by somatic current pulses) recorded from pre-pubertal (PD 28 –36) animals. No differences were observed between sham-operated animals and those with an NVHL (4 –5 cells/dose). Abbreviations as in Figure 5. (*P <.05, **P <.01, ***P <.001.)

Figure 8

Disruption of D₂–NMDA interactions in the PFC of NVHL animals. Bar graphs summarize the effect of quinpirole on NMDA responses observed in the PFC of (A) sham and (B) NVHL animals (PD > 61). Bath application of quinpirole (1 μmol/L) significantly decreased the excitatory effect of NMDA (4 μmol/L) on PFC pyramidal neuron excitability (**p < .001, Tukey post hoc test after significant two-way analysis of variance). In contrast, the increase in the number of evoked spikes and decrease in first spike latency induced by NMDA were not affected by quinpirole in NVHL animals. Abbreviations as in Figure 5.

Figure 9

Disruption of D₂-AMPA interactions in the PFC of NVHL animals. Bar graphs summarize the effect of quinpirole on AMPA responses recorded in the PFC of (A) sham and (B) NVHL animals (PD > 61). The excitatory effects of .2 μmol/L AMPA on the number of evoked spikes and first spike latency were significantly attenuated with 1 μmol/L quinpirole in the PFC of sham animals (**p < .001, Tukey post hoc test after significant two-way analysis of variance). In contrast, these inhibitory actions of quinpirole on AMPA responses were not observed in PFC pyramidal neurons from NVHL rats. Abbreviations as in Figure 6.