Project Summary/Abstract. Fragile X syndrome (FX) is a widespread type of inherited intellectual disability.
Effective treatments that target mechanisms underlying FX are currently lacking. FX is the foremost monogenic
cause of autism spectrum disorders, and thus many individuals with FX exhibit abnormal social behaviors.
Individuals with FX also often engage in aberrant spatial behaviors such as “elopement”, wandering off and
getting lost. The hippocampus is a brain structure that is particularly vulnerable to FX. Much evidence suggests
that hippocampal areas CA2 and CA1 are important for social behaviors and spatial memory, respectively. Yet,
few studies have investigated whether disturbances in neurophysiological mechanisms in CA2 and CA1 could
underlie impaired social and spatial cognitive functioning in FX. This project’s goal is to address this gap in
knowledge by investigating the extent to which subcellular, cellular, circuit, and neuronal population mechanisms
of social and spatial memory operations in the hippocampus are impaired in rodent models of FX. The studies
will employ state-of-the-art in vivo and in vitro electrophysiological techniques. In vivo approaches will be used
to assess whether impairments in cellular responses in CA2 and coordinated neuronal population activity in CA1
could explain deficits in social and spatial cognition in FX. In vitro experiments will be conducted to uncover
cellular mechanisms underlying altered intrinsic properties of and plasticity in CA2 neurons and aberrant
inhibitory circuits in CA1. Models of FX in two species, specifically Fmr1 knockout (KO) rats and mice, will be
used, allowing comparison of FX pathophysiology across species. Specific Aim 1 will assess whether correlated
neuronal spiking activity between CA2 and one of its major inputs, CA3, is weaker in Fmr1 KO rats than wildtype
rats during exploration of social stimuli. Specific Aim 2 will employ whole cell and patch clamp recordings,
including recordings directly from dendrites, in hippocampal slices to test whether CA2 neurons in Fmr1 KO rats
and mice show impaired synaptic plasticity and deficient responses to the social neuropeptide, oxytocin. Specific
Aim 3 will test whether reactivation, or “replay”, of spike sequences from populations of CA1 neurons that code
for previously learned spatial trajectories is disrupted in Fmr1 KO rats. Replay is critical for spatial memory
operations, and thus disrupted replay could contribute to impaired spatial cognition and behavior in FX. Replay
of CA1 neuronal spike sequences is temporally coordinated by properly timed activation of specific CA1 inhibitory
interneurons. Thus, disrupted replay of CA1 spike sequences in FX may reflect disturbances in CA1 inhibitory
circuits. Specific Aim 4 will employ whole cell recordings from CA1 pyramidal neurons, specific classes of CA1
interneurons, and connected CA1 interneuron-pyramidal cell pairs to test the hypothesis that inhibitory circuits
are disrupted in FX. Successful completion of these Aims will provide novel insights about specific mechanisms
underlying aberrant social and spatial cognition and behaviors in FX. Gaining a deeper understanding of FX
mechanisms is expected to suggest novel targets for intervention in FX.
