We first determined whether repeated exposure to MK-801 during the periadolescent transition period (0.1 mg/kg/day × 5 days from PD35 to PD40; see Methods and Materials for details) is associated with a hyperactive PFC state in adulthood, by means of local field potential recordings in vivo. All recordings were conducted from the medial PFC (infralimbic and prelimbic regions) and changes in prefrontal local field potentials to ventral hippocampal stimulation were assessed in adulthood within the PD65-85 age period (). We observed that saline and MK-801-treated animals exhibited similar prefrontal response curves to increasing current intensities of hippocampal stimulation (), indicating that the basal network state in the PFC was not affected by periadolescent MK-801 exposure.
Figure 1 (a) Diagram depicting the recording arrangement used to study the impact of MK-801 exposure on ventral hippocampal-induced synaptic responses in the medial PFC (infralimbic and prelimbic regions) in vivo. Traces are examples of hippocampal stimulation-induced (more ...)
We next assessed the impact of periadolescent MK-801 treatment on hippocampal train stimulation-induced facilitation and depression of local field potentials in the PFC. Hippocampal train stimulation at 10Hz (10 pulses at 100ms interval/15s) elicited a distinct pattern of sustained facilitation of the evoked field potential response in the PFC that was undistinguishable among saline and MK-801-treated animals (). This was not the case when the hippocampal train stimulation was delivered at 20Hz (10 pulses at 50ms interval/15s). While a transient attenuation of the prefrontal response was observed in the saline group, a shift to sustained facilitation of the field potential response emerges in the PFC of MK-801-treated animals (). Similarly, MK-801 exposure during periadolescence also disrupted the prefrontal response to ventral hippocampal drive at 40 Hz. A pattern of profound suppression of the evoked field potential response was observed in the PFC of saline and MK-801-treated animals (). However, the magnitude of this inhibition was markedly reduced in the PFC of animals with a history of periadolescent MK-801 treatment (). Together, these results indicate that periadolescent MK-801 exposure selectively diminished the normal inhibitory control of prefrontal processing of ventral hippocampal inputs in a frequency-dependent manner.
Figure 2 Disruption of ventral hippocampal-induced frequency dependent prefrontal LFP inhibition by periadolescent MK-801 treatment. (a) Saline (n=12) and MK-801-treated (n=10) rats exhibited similar degrees of LFP facilitation in the PFC in response to hippocampal (more ...)
To determine if the frequency-dependent disruption of prefrontal inhibition observed in the periadolescent-treated group is age dependent, the impact of MK-801 exposure during adulthood (i.e., PD75-80) was assessed following the same experimental design described above. As for the periadolescent cohort, all recordings were conducted within 3-5 weeks (i.e., PD105-125 range) from the last saline or MK-801 injection (). Similar to the periadolescent-treated group, the characteristic hippocampal 10Hz-induced facilitation of the prefrontal response was indistinguishable among saline and MK-801-treated animals (). Interestingly, MK-801 treatment in adulthood failed to alter the pattern of 20 and 40Hz-induced field potential inhibition in the PFC (). Together, these results indicate that MK-801-induced disruption of the ventral hippocampal-induced frequency-dependent prefrontal inhibition is age dependent. Notably, both adolescent- and adult-exposed saline groups exhibited similar pattern of prefrontal responses at 20Hz and 40Hz despite the differences in testing ages (PD65-85 vs. PD105-125; main effect of group, p=0.9, two-way ANOVA).
Figure 3 Summary of the effects of MK-801 treatment administered during adulthood. (a) Diagram illustrating the experimental design. Five non-contingent injections of saline (n=8) or MK-801 (n=8) were performed in adult rats and changes in prefrontal LFP response (more ...)
We next asked the question of whether the prefrontal disruptions observed following periadolescent MK-801 treatment could be attributed to a developmental impairment of the normal frequency-dependent regulation of synaptic transmission in the PFC by the ventral hippocampus. Towards this goal, the pattern of prefrontal response to ventral hippocampal stimulation at 10, 20 and 40Hz was compared across 3 developmentally distinct age groups of naïve rats: PD30-40, PD45-55 and PD60-80. We observed no differences in the 10Hz-induced prefrontal facilitation among the three age groups (). However, the characteristic 20Hz-induced transient attenuation of the evoked field potential observed in adult animals became apparent only in the PD45-55 age group (). In the PD30-40 PFC, a 20Hz-induced sustained facilitation of the evoked response was found instead (), resembling that observed in the adult PFC of periadolescent MK-801-exposed rats (see for comparison). Similarly, the degree of prefrontal field potential suppression observed at 40Hz in the PD30-40 animals was significantly less pronounced when compared to the response observed in the PD45-55 and PD60-80 age groups (). Once again, such attenuated 40Hz-induced prefrontal inhibition in the PD30-40 group resembles that recorded in the PFC of adult rats with a periadolescent history of MK-801 treatment. Together, these results indicate that the periadolescent MK-801-induced prefrontal disinhibition could be due to a developmental disruption of the normal acquisition of frequency-dependent inhibitory mechanism that occurs after PD40 ().
Figure 4 Developmental regulation of ventral hippocampal-induced facilitation and suppression of LFP transmission in the PFC. (a) The pattern of prefrontal LFP facilitation to ventral hippocampal stimulation at 10 Hz were indistinguishable among rats from PD30-40 (more ...)
It is well known that GABAergic interneurons play a crucial role in mediating the enhanced prefrontal inhibitory response observed during the normal periadolescent transition period (Tseng and O'Donnell, 2007a
). To test this hypothesis, we first examined if the effects of periadolescent MK-801 treatment could be acutely reproduced through a disruption of local prefrontal GABAergic transmission via infusion of the GABA-A receptor antagonist picrotoxin into the PFC of adult naïve rats (PD65-85). We found that while the pattern of hippocampal 10 Hz-induced prefrontal facilitation remained unaffected (
), a shift from the distinct 20 Hz-induced transient attenuation to a sustained facilitation of the evoked field potential response was observed in the PFC following local infusion of picrotoxin (
). Similarly, the characteristic hippocampal 40 Hz-induced suppression in the prefrontal field potential response was markedly attenuated by picrotoxin (
). In sum, these results indicate that the distinctive hippocampal-induced frequency-dependent field potential inhibition observed in the normal adult PFC is mediated by local prefrontal GABAergic transmission. Furthermore, a functional impairment of this latter is sufficient to elicit a PFC state resembling that induced by periadolescent MK-801 exposure (
Figure 5 Prefrontal disinhibition induced by local administration of the GABA-A antagonist picrotoxin. (a) The magnitude of ventral hippocampal-induced prefrontal LFP facilitation at 10 Hz was not affected by local microinjection of picrotoxin (50 μM) (more ...)
The above findings led us to further hypothesize that periadolescent MK-801 treatment could exert a developmental dysregulation of local prefrontal GABAergic transmission. In order to test this, we examined the impact of the GABA-A α1 receptor positive allosteric modulator Indiplon in reversing the abnormal disinhibited state observed in the PFC of animals treated with MK-801 during periadolescence. We found that local prefrontal infusion of Indiplon did not alter the pattern of prefrontal facilitation induced by hippocampal 10 Hz stimulation (). However, the abnormal 20 Hz-induced facilitation of the evoked field potential response observed in the adult PFC of periadolescent MK-801-treated rats was not longer apparent following local administration of Indiplon (). Instead, a shift to the typical 20 Hz-induced transient attenuation of the evoked response was obtained, which resembles the pattern observed in saline controls (see for comparison). Similarly, local prefrontal application of Indiplon rescued the abnormally attenuated 40 Hz-induced suppression of the evoked field potential response observed in MK-801-exposed rats to saline control levels (). Together, these results indicate that an upregulation of local prefrontal GABAA receptor-mediated transmission is sufficient to normalize the enduring PFC disinhibitory state induced by periadolescent MK-801exposure ().
Figure 6 Reversal actions of the GABA-A α1 positive allosteric modulator Indiplon on MK-801-induced prefrontal LFP disinhibition. (a) At 10 Hz, local prefrontal infusion of Indiplon did not change the pattern of ventral hippocampal stimulation-induced (more ...)