• Balance Between Inhibition and Excitation in Rat Prefrontal Cortex: Modulation by Dopamine, As a Study of Age and Cortical Layer

      Toreson, Kathy Lynn; O'Donnell, Patricio (2010)
      Prefrontal cortical (PFC) pyramidal neurons and fast spiking interneurons receive dopaminergic (DA) and non-DAergic inputs from the ventral tegmental area (VTA). In vivo juxtacellular recordings of neurons in the medial PFC were performed to assess the effect of mesocortical activation. Recorded cells were identified as pyramidal neurons or fast spiking interneurons based on electrophysiological characteristics combined with Neurobiotin staining and parvalbumin immunohistochemistry. Chemical activation of the mesocortical pathway (intra-VTA NMDA microinjection) resulted in an increase in firing in fast spiking interneurons and a decrease in firing in pyramidal neurons. These responses had a similar temporal course, suggesting that mesocortical activation of fast spiking interneurons may contribute to the silencing of pyramidal neurons under these conditions. The PFC is organized into layers, which have distinct compositions of cell types, receptor expression, and afferent and efferent projections. Importantly, the density and distribution of dopamine receptors and mesocortical inputs across cortical layers continue to change as the brain matures and does not reach their final arrangements until after puberty. The functional significance of these variations in cortical layer composition and juvenile and versus developmentally mature PFC remain to be determined. To investigate the effect of mesocortical projection activation on pyramidal neurons of the medial prefrontal cortex as a function of cortical layer and late postnatal development, we performed in vivo juxtacellular recordings in juvenile (P28-35) and young adult (>P60) chloral hydrate anesthetized rats. Activation of the VTA by an electrical stimulation that mimics dopamine cell burst firing (5 pulse 20 Hz train) produced either a simple excitatory or inhibitory response, or a complex multiphasic response, in pyramidal neurons. The most common initial (train) response to electrical stimulation of the VTA in both the juvenile and the young adult groups was inhibition. In both age groups, the delayed (post stimulation train) response was approximately equal in the proportions of cells exhibiting excitation or inhibition. When taking into account cortical layer, there was no difference in the proportion of cells exhibiting excitation or inhibition in either age group. These data demonstrate that although there are variations in the composition of cortical layers, as well as anatomical differences between juvenile and adult PFC, these disparities do not result in major differences in the way pyramidal neurons respond to mesocortical activation.
    • Basolateral Amygdala Stimulation Evokes Feed-Forward Inhibition in the Medial Prefrontal Cortex

      Dilgen, Jonathan E.; O'Donnell, Patricio (2011)
      The amygdala sends a strong projection to the prefrontal cortex (PFC) and both brain regions are critical for high order cognitive processes. This pathway may convey emotional and incentive-value related information to the PFC where it could be used to guide behaviors such as decision making and working memory. Behavioral studies have shown that both the amygdala and PFC are important for emotional processing and learning, but the underlying neural correlates remain unclear. Anatomy studies show that the basolateral amygdala (BLA) sends an excitatory projection to the medial PFC (mPFC). Paradoxically, electrophysiologic evidence indicates that most mPFC neurons are inhibited by BLA stimulation. This apparent incongruence could be explained if mPFC interneurons are considered mediators of BLA to mPFC neurotransmission. In this thesis, I tested the hypothesis that the BLA-mPFC projection includes a feed-forward inhibition mechanism with several experiments using electrophysiological approaches.
    • Dominant-Negative Disrupted in Schizophrenia 1 Alters the Dopaminergic Modulation of Excitatory-Inhibitory Balance in the Prefrontal Cortex

      Cardarelli, Ross Andrew; O'Donnell, Patricio (2013)
      Dysfunction of inhibitory cells, especially of fast-spiking interneurons within the prefrontal cortex, is a convergent phenotype existing in patients with schizophrenia and within animal models of the disorder. Despite a large amount of research into the molecular and behavioral consequences of different manipulations, the electrophysiological phenotype of cells within relevant brain areas including the prefrontal cortex remains underexplored. In this dissertation, I utilized whole cell recordings to first differentiate between classes of neurons in the normal-developing prefrontal cortex and then discerned the pattern of their modulation by dopamine receptor D2 agonists. I then contrasted this normal, late-maturing phenotype with that in mice carrying a truncated human disrupted in schizophrenia 1 (DISC1) allele. I will show that fast-spiking interneurons in mice expressing mutated DISC1 lose their normal electrophysiological phenotype including a failure to attain a normal adult firing profile and dopamine response, and conclude that early-life gene disruption can result in electrophysiological dysfunction of GABAergic interneurons.
    • Neurophysiological Effects of NMDA Receptor Antagonism in Adolescence vs. Adulthood: EEG as a Translational Tool to Study Schizophrenia

      Sullivan, Elyse; O'Donnell, Patricio (2014)
      Electroencephalogram (EEG) stands out as a highly translational tool for psychiatric research, yet rodent and human EEG are not typically obtained in the same way. The first goal of this study was to develop a tool to record skull EEG in awake behaving rats in a similar manner to how human EEG are obtained. Using this novel rodent EEG technique, I tested whether acute NMDA receptor (NMDAR) antagonism replicates the effects of acute NMDAR-antagonism in humans, and found a strikingly similar shift in EEG oscillatory power from low to high frequencies in rodents, mimicking the effects reported in humans. After validating the translational potential of this approach, I paired EEG with local field recordings to investigate if a disruption to NMDAR signaling during a critical period of development caused long-lasting alterations in neural oscillations in adulthood. I found that rats exposed to an NMDAR-antagonist during adolescence, a critical period for the development of local and long-range synchrony in the brain, showed decreased gamma (30-50 Hz) synchrony between the medial prefrontal cortex (mPFC) and ventral hippocampus (HP) that was evident after NMDAR-antagonist re-exposure during adulthood. The same treatment given to adult rats did not produce this disruption to long-range synchrony, suggesting that adolescent disruption of NMDA signaling may have more robust long-term effects than a similar adult disruption. Interestingly, we did not find alterations in local oscillatory activity in these regions, suggesting that long-range synchrony may be more sensitive than local synchrony to blockade of NMDAR signaling during adolescence. Given that deficits in long-range coherence are seen in patients with schizophrenia, this manipulation in rodents may prove useful in understanding mechanisms behind this phenomenon. Overall, the experiments presented in this Dissertation provide support for the use of EEG as a cross-species translational tool to study schizophrenia across acute and developmental animal models.
    • The Nucleus Accumbens as a Switchboard: Heterosynaptic Suppression in the Ventral Striatum

      Calhoon, Gwendolyn Gabrielle; O'Donnell, Patricio (2013)
      Many brain circuits control behavior through the integration of information provided by separate inputs onto a common target neuron. Medium spiny neurons (MSNs) in the ventral striatum (VS) receive converging excitatory afferents from the prefrontal cortex (PFC), hippocampus (HP), and thalamus, among others, and the integration of these inputs is critical for shaping goal-directed behaviors. Although under baseline conditions the membrane activity of MSNs is controlled largely by the HP, the PFC can elicit up states in MSNs during periods of high frequency activity, such as that which occurs during decision making epochs. Moreover, during epochs of high PFC activity, the VS loses synchrony with the HP. It is therefore possible that PFC inputs locally attenuate responses to other glutamatergic inputs to the VS. We investigated whether strong, transient PFC activation can disengage the VS from the HP by measuring the effect of high frequency PFC stimulation on MSN responses to stimulation of other synaptic inputs. Using in vivo intracellular recordings, we found that delivering trains of stimuli to the PFC suppresses HP- and thalamus-evoked synaptic responses in the VS, partially through a GABAergic mediator. These findings indicate that high frequency PFC activity overrides HP control of MSN up state transitions, and provide evidence of heterosynaptic inhibition in this system in vivo. This interaction may enable the PFC to exert influence on basal ganglia loops during decision-making instances with minimal disturbance from ongoing contextual inputs.
    • Prefrontal Cortical Fast-Spiking Interneurons in Healthy Animals and in a Developmental Model of Schizophrenia

      Lewis, Eastman; O'Donnell, Patricio (2014)
      Schizophrenia is a serious mental illness from which approximately 1% of the world's population suffers. Current treatments are effective against positive symptoms such as hallucinations and delusions, but do little to ameliorate the cognitive deficits which are a devastating component of the disease. A current hypothesis is that hypofunction of cortical fast spiking interneurons (FSI) is an important substrate of these deficits. However, the mechanisms producing this hypofunction are unclear. Based on the observation that NMDA receptor antagonists like ketamine are psychotomimetic and produce a disinhibited cortex, it has been suggested that hypofunctional NMDA receptors may underlie the hypothesized deficit in FSI function. Recently, this has been called into question based on the observation that NMDA receptors contribute minimally to synaptic responses in adult FSI. Here we tested whether adult FSI express functional NMDA receptors in the adult rat cortex. We found that while only a subset of FSI express NMDA receptors at synaptic locations, all FSI express functional NMDA receptors at some location. In light of this, the hypothesis that hypofunctional NMDA receptors on FSI contribute to cortical disinhibition and cognitive symptoms in schizophrenia should not be disregarded. As schizophrenia is a developmental disorder with late adolescent or early adult onset, it is important to understand possible links between early-life abnormalities and adult-onset symptoms. In a second set of experiments, we used the neonatal ventral hippocampal lesion (NVHL) model of schizophrenia to test the possibility that developmental oxidative stress could provide a link between neonatal insults and cortical dysfunction in adulthood. Indeed, we found evidence that developmental oxidative stress causes electrophysiological deficits in the adult NVHL cortex. This implicates oxidative stress as a link between early-life insults and adult-onset psychiatric disorders. Furthermore, this indicates that developmental antioxidant treatment may be beneficial for individuals at high risk for developing schizophrenia.
    • Prefrontal kappa-opioid regulation of local neurotransmission

      Tejeda, Hugo Alejandro; O'Donnell, Patricio (2013)
      Kappa-opioid receptors (KORs) are mediators of motivational processes, mood/emotion, and stress reactivity. KORs are enriched in brain regions that mediate such behaviors, including the medial prefrontal cortex (mPFC). The mPFC guides behavior through its connections with limbic brain regions such as the amygdala, hippocampus, and ventral tegmental area. A critical knowledge gap exists in our understanding of the role of KOR systems in modulating mPFC afferents. Thus, we designed a series of studies to understand the role of KORs in regulating dopamine (DA) and limbic glutamatergic afferents to the mPFC. First, we determined the role of KOR systems in regulating neurotransmission in the mesocortical DA pathway by utilizing a combination of neurochemical techniques in rats and in transgenic mice. We found that mPFC KOR activation and antagonism decreased and increased DA output, respectively. Genetic ablation of KOR in DA neurons abolished the inhibitory effect of mPFC KOR signaling on local DA output. These findings suggest that mPFC KORs negatively modulate the mesocortical DA pathway by directly acting on DA varicosities in the mPFC. Second, we examined the role of mPFC KOR systems in regulating mPFC extracellular glutamate and glutamatergic limbic inputs utilizing neurochemical and electrophysiological techniques in rats. We found that extracellular glutamate and glutamate-driven GABA levels were inhibited by mPFC KORs. mPFC KOR activation inhibited synaptic transmission in the BLA to mPFC pathway in a manner not overcome by BLA burst stimulation. KORs do not inhibit the hippocampus to mPFC pathway. These findings show that mPFC KORs inhibit glutamatergic afferents in a pathway-specific manner. Lastly, we characterized heterosynaptic interactions between the BLA and hippocampus in the mPFC, and determined the role of KORs in these interactions. BLA train stimulation inhibits hippocampal inputs to the mPFC in an activity-dependent manner, while hippocampus stimulation is without effect on BLA-evoked responses. KOR antagonism does not modify BLA-evoked heterosynaptic suppression, suggesting that BLA-evoked heterosynaptic suppression is KOR-independent. This series of studies demonstrate that mPFC KORs regulate mesocortical DA and limbic pathways, but may not be involved in limbic interactions. These findings provide a biological framework whereby mPFC KOR signaling alters mPFC-dependent behaviors.