Dendritic spines are small protrusions that receive 90% of excitatory cortical synapses and are critically important to neural function. Each dendritic spine is supported by a dynamic actin cytoskeleton that responds to internal and external cues to allow spine development, elongation, retraction and movement. Multiple proteins have roles in spinogenesis, but until now, a regulatory role for actin itself has not been established. Here, we show that, in the acute slice preparation, actin expression increases during a period of rapid spinogenesis. Furthermore, actin overexpression in organotypic hippocampal cultures leads to a significant increase in spine density on CA1 pyramidal cells. Specifically, the number of filopodia (long, thin protrusions without heads) increases by 38% on secondary apical dendrites and 88% on basal dendrites and the number of elongated spines with heads increases by 162% on secondary apical dendrites and 113% on basal dendrites. Synapsin-I immunostaining demonstrated that the majority of filopodia and elongated spines are apposed by axon terminals. Additionally, we show that overexpressed actin enters both new and established spines within 24 h. These data demonstrate that neurons undertaking spinogenesis upregulate actin expression, that actin overexpression per se increases spine density, and that both new and established spines incorporate exogenous actin.

Affective disorders represent one of the greatest global burdens of disease. Work in patients with affective disorders demonstrates that serotonin (5-HT) signaling within the prefrontal cortex, particularly at the level of the 5-HT receptors, plays an integral role in both the pathology and treatment of these diseases. Surprisingly, the characterization of the prefrontal 5-HT receptors under both healthy and pathological conditions remains incomplete. The technique of whole cell electrophysiological recording provides an unparalleled tool for investigating the functional effects of these 5-HT receptors on neurons in acute prefrontal cortical slices. The objectives of my thesis were to delve deeper into the 5-HT receptor subtypes that modulate the prefrontal cortex in the healthy control rodents and to examine how this modulation was disrupted in a rodent model of affective disorders. In work from healthy control rodents, I examined two prefrontal 5-HT receptor-mediated currents. I show for the first time the presence of the 5-HT1A receptor during the early postnatal period, a critical developmental window during which this receptor programs adult anxiety behaviors. In adulthood, I characterized an inhibitory current mediated by the 5-ht5A receptor; findings that will permit the classification of this receptor within the 5-HT receptor family. Collectively, this investigation of functional early 5-HT1A receptors and adult 5-ht5A receptors offers a novel conceptual framework for understanding 5-HT receptor modulation of the healthy prefrontal cortex. To model vulnerability to affective disorder in the rodent, I used the early stress of maternal separation. In early stress rodents, I observed a marked increase in 5-HT1A receptor currents during the early postnatal period, the critical time window for the programming of anxiety. By comparison, in adulthood I found that rodents exposed to early stress displayed increased 5-HT2A receptor currents. These findings provide novel insight into the developmental and long-lasting pathology underlying early stress, indicating that the early prefrontal 5-HT1A receptor and adult prefrontal 5-HT2A receptors as a potential therapeutic target in treatment of affective disorders At a fundamental level, the findings provided herein offer critical insight into the cellular mechanisms underlying affective disorders, one of the most debilitating and costly diseases worldwide.