Sensory Neuron Diversity in the Inner Ear Is Shaped by Activity.

In the auditory system, type I spiral ganglion neurons (SGNs) convey complex acoustic information from inner hair cells (IHCs) to the brainstem. Although SGNs exhibit variation in physiological and anatomical properties, it is unclear which features are endogenous and which reflect input from synaptic partners. Using single-cell RNA sequencing, we derived a molecular classification of mouse type I SGNs comprising three subtypes that express unique combinations of Ca2+ binding proteins, ion channel regulators, guidance molecules, and transcription factors. Based on connectivity and susceptibility to age-related loss, these subtypes correspond to those defined physiologically. Additional intrinsic differences among subtypes and across the tonotopic axis highlight an unexpectedly active role for SGNs in auditory processing. SGN identities emerge postnatally and are disrupted in a mouse model of deafness that lacks IHC-driven activity. These results elucidate the range, nature, and origins of SGN diversity, with implications for treatment of congenital deafness.

Hippocampal Arc (Arg3.1) expression is induced by memory recall and required for memory reconsolidation in trace fear conditioning.

Mounting evidence suggests that long-lasting, protein synthesis-dependent changes in synaptic strength accompany both the initial acquisition and subsequent recall of specific memories. Within brain areas thought to be important for learning and memory, including the hippocampus, learning-related plasticity is likely mediated in part by NMDA receptor activation and experience-dependent changes in gene expression. In the present study, we examined the role of activity-regulated cytoskeletal-associated protein (Arc/Arg3.1) expression in the acquisition, recall, and reconsolidation of memory in a trace fear conditioning paradigm. First, we show that the expression of Arc protein in ventral hippocampus (VH) is dramatically enhanced by memory recall 24h after the acquisition of trace fear conditioning, and that both memory recall and the associated recall-induced enhancement of Arc expression are blocked by pre-training administration of 2-amino-5-phosphonovaleric acid (APV). Next, we show that while infusion of Arc antisense oligodeoxynucleotides (ODNs) into VH prior to testing had little effect on memory recall, it significantly reduced both Arc protein expression and freezing behavior during subsequent testing sessions. Collectively, these results suggest that Arc/Arg3.1 protein plays an important functional role in both the initial acquisition of hippocampal-dependent memory and the reconsolidation of these memories after recall.

The importance of having Arc: expression of the immediate-early gene Arc is required for hippocampus-dependent fear conditioning and blocked by NMDA receptor antagonism.

Long-lasting, experience-dependent changes in synaptic strength are widely thought to underlie the formation of memories. Many forms of learning-related plasticity are likely mediated by NMDA receptor activation and plasticity-related gene expression in brain areas thought to be important for learning and memory, including the hippocampus. Here, we examined the putative role of activity-regulated cytoskeletal-associated protein (Arc), an immediate-early gene (IEG) whose expression is tightly linked to the induction and maintenance of some forms of neuronal plasticity, in hippocampus-dependent and hippocampus-independent forms of learning. The extent to which learning-induced Arc expression may depend on NMDA receptor activation was also assessed. First, we observed an increase in Arc gene and protein products in both dorsal hippocampus (DH) and ventral hippocampus (VH) of male Sprague Dawley rats after hippocampus-dependent trace and contextual fear conditioning, but not after hippocampus-independent delay fear conditioning. Specific knockdown of Arc using antisense oligodeoxynucleotides (ODNs) in DH or VH attenuated the learning-related expression of Arc protein, and resulted in a dramatic impairment in trace and contextual, but not delay, fear conditioning. Finally, pretraining infusions of the NMDA receptor antagonist APV into the DH or VH blocked the learning-induced enhancement of Arc in a regionally selective manner, suggesting that NMDA receptor activation and Arc translation are functionally coupled to support hippocampus-dependent memory for fear conditioning. Collectively these results provide the first evidence suggesting that NMDA receptor-dependent expression of the IEG Arc in both DH and VH likely underlies the consolidation of a variety of forms of hippocampus-dependent learning.

Dorsal versus ventral hippocampal contributions to trace and contextual conditioning: differential effects of regionally selective NMDA receptor antagonism on acquisition and expression.

The dorsal and ventral subregions of the hippocampus likely play dissociable roles in some forms of learning. For example, we have previously demonstrated that temporary inactivation of ventral, but not dorsal, hippocampus dramatically impaired the acquisition of trace fear conditioning, while temporary inactivation of dorsal, but not ventral, hippocampus impaired spatially guided reinforced alternation (Czerniawski et al. (2009) Hippocampus 19:20-32). Importantly, emerging data suggest that lesions, temporary inactivation, and NMDA receptor antagonism within these subregions can produce quite different patterns of behavioral effects when administered into the same region. Specifically, while neither lesions nor temporary inactivation of dorsal hippocampus impair the acquisition of trace fear conditioning, learning in this paradigm is severely impaired by pre-training administration of the NMDA receptor antagonist dl-2-phosphonovaleric acid (APV) in dorsal hippocampus; the effect of NMDA receptor antagonism within ventral hippocampus on the acquisition and expression of trace conditioning, or on learning in general, has not yet been systematically explored. The present study extends our previous work examining the differential effect of lesions or inactivation of the dorsal and ventral hippocampal subregions by systematically examining the effect of regionally selective pre-training or pre-testing administration of APV on the acquisition and expression of trace and contextual fear conditioning. The results of these studies demonstrate that while pre-training NMDA receptor antagonism within either the dorsal or ventral subregion of the hippocampus impaired the acquisition of both trace and contextual conditioning, pre-testing NMDA receptor antagonism within ventral, but not dorsal, hippocampus impaired the expression of previously-acquired trace and contextual fear conditioning. These data suggest that selectively manipulating the integrity of individual subregions may result in compensatory mechanisms that can support learning, and that NMDA-dependent plasticity within both dorsal and ventral hippocampus is normally required for the acquisition and maintenance of memory in trace and contextual fear conditioning.