Adi Mizrahi

High-resolution in vivo imaging of hippocampal dendrites and spines

Structural changes in hippocampal dendrites and dendritic spines are thought to be a consequence of a wide range of experience- and activity-dependent manipulations. We explored the dynamics of hippocampal dendritic spines in vivo by developing a surgical preparation of the adult mouse brain that enabled two-photon imaging of fluorescently labeled CA1 pyramidal neurons. Dendritic trees and spines were repeatedly visualized over many hours in exquisite detail. We tested spine stability under both control conditions and during prolonged epileptic seizures. Remarkably, spines remained structurally stable after 30 min of experimental induction of epileptic seizures. Spines began to disappear only several hours after induction of epileptic activity. We thus demonstrate that this technique provides a methodology for direct in vivo optical studies of the intact mammalian hippocampus.

Sensory Input Enhances Synaptogenesis of Adult-Born Neurons

The adult mammalian brain maintains a prominent stem cell niche in the subventricular zone supplying new neurons to the olfactory bulb. We examined the dynamics of synaptogenesis by imaging the formation and elimination of clusters of a postsynaptic marker (PSD95), genetically targeted to adult-born neurons. We imaged in vivo adult-born periglomerular neurons (PGNs) during two phases of development, immaturity and maturity. Immature PGNs showed high levels of PSD95 puncta dynamics during 12–72 h intervals. Mature PGNs were more stable compared with immature PGNs but still remained dynamic, suggesting that synaptogenesis persists long after these neurons integrated into the network. By combining intrinsic signal and two photon imaging we followed PSD95 puncta in sensory enriched glomeruli. Sensory input upregulated the development of adult-born PGNs only in enriched glomeruli. Our data provide evidence for an activity-based mechanism that enhances synaptogenesis of adult-born PGNs during their initial phases of development.

Experience-dependent plasticity of mature adult-born neurons

The adult olfactory bulb and hippocampus are continuously supplied with newborn neurons that are thought to possess a capacity for plasticity only at a young neuronal age, mainly during the early stages of integration into the network. We find that the two main types of adult-born neurons in the mouse olfactory bulb undergo experience-dependent plasticity long after maturation and integration, as evidenced by stabilization of synaptic turnover rates. Thus, the potential time window for plasticity of adult-born neurons extends well into maturity.

Long-Term Imaging Reveals Dynamic Changes in the Neuronal Composition of the Glomerular Layer

The mammalian olfactory bulb (OB) contains a rich and highly heterogeneous network of local interneurons (INs). These INs undergo continuous turnover in the adult OB in a process known as “adult neurogenesis.” Although the overall magnitude of adult neurogenesis has been estimated, the detailed dynamics of the different subpopulations remains largely unknown. Here we present a novel preparation that enables long-term in vivo time-lapse imaging in the mouse OB through a chronic cranial window in a virtually unlimited number of sessions. Using this preparation, we followed the turnover of a specific neuronal population in the OB, the dopaminergic (DA) neurons, for as long as 9 months. By following the same population over long periods of time, we found clear addition and loss of DA neurons in the glomerular layer. Both cell addition and loss increased over time. The numbers of new DA cells were consistently and significantly higher than lost DA cells, suggesting a net increase in the size of this particular population with age. Over a 9 month period of adult life, the net addition of DA neurons reached ∼13%. Our data argue that the fine composition of the bulbar IN network changes throughout adulthood rather than simply being replenished.

Circuit formation and maintenance—perspectives from the mammalian olfactory bulb

The rodent olfactory bulb (OB) is becoming a model system for studying how neuronal circuits develop and maintain. The OB has typical components of a sensory circuit such as ordered sensory inputs, diverse populations of interneurons, substantial neuromodulatory innervation, and projection neurons that transfer information to higher brain centers. Additionally, the OB is unique because its sensory afferents and a subset of its interneurons are continuously replaced throughout adulthood. Here, we review some recent findings on the development and maintenance of the mammalian OB circuitry. We review some of the known developmental strategies of the major OB components and discuss the ways in which the OB circuitry preserves stability in the face of ongoing changes.

Enhanced Synaptic Integration of Adult-Born Neurons in the Olfactory Bulb of Lactating Mothers

One of the most dramatic events during the life of adult mammals is the transition into motherhood. This transition is accompanied by specific maternal behaviors, displayed by the mother, that ensure the survival and the well-being of her offspring. The execution of these behaviors is most likely accompanied by plastic changes in specific neuronal circuits, but these are still poorly defined. In this work, we studied the mammalian olfactory bulb (OB), which has been shown to be an essential brain region for maternal behaviors in mice. In the OB, we focused on adult-born neurons, which are continuously incorporated into the circuit during adulthood, thus providing a potential substrate for heightened plasticity after parturition. We analyzed the dynamics and morphological characteristics of adult-born granule cells (abGCs), innervating the OB of primiparous lactating mothers, shortly after parturition as well as in naive females. In vivo time-lapse imaging of abGCs revealed that dendritic spines were significantly more stable in lactating mothers compared with naive virgins. In contrast, spine stability of resident GCs remained unchanged after parturition. In addition, while spine size distribution of abGCs was approximately similar between mothers and naive virgins, the spine density of abGCs was lower in lactating mothers and the density of their presynaptic components was higher. These structural features are indicative of enhanced integration of adult-born neurons into the bulbar circuitry of lactating mothers. This enhanced integration may serve as a cellular mechanism, supporting changes in olfactory coding of new mothers during their first days following parturition.

Plasticity during motherhood: changes in excitatory and inhibitory layer 2/3 neurons in auditory cortex.

Maternal behavior can be triggered by auditory and olfactory cues originating from the newborn. Here we report how the transition to motherhood affects excitatory and inhibitory neurons in layer 2/3 (L2/3) of the mouse primary auditory cortex. We used in vivo two-photon targeted cell-attached recording to compare the response properties of parvalbumin-expressing neurons (PVNs) and pyramidal glutamatergic neurons (PyrNs). The transition to motherhood shifts the average best frequency of PVNs to higher frequency by a full octave, with no significant effect on average best frequency of PyrNs. The presence of pup odors significantly reduced the spontaneous and evoked activity of PVN. This reduction of feedforward inhibition coincides with a complimentary increase in spontaneous and evoked activity of PyrNs. The selective shift of PVN frequency tuning should render pup odor-induced disinhibition more effective for high-frequency stimuli, such as ultrasonic vocalizations. Indeed, pup odors increased neuronal responses of PyrNs to pup ultrasonic vocalizations. We conclude that plasticity in the mothers is mediated, at least in part, via modulation of the feedforward inhibition circuitry in the auditory cortex.

Elevated Correlations in Neuronal Ensembles of Mouse Auditory Cortex Following Parturition

The auditory cortex is malleable by experience. Previous studies of auditory plasticity have described experience-dependent changes in response profiles of single neurons or changes in global tonotopic organization. However, experience-dependent changes in the dynamics of local neural populations have remained unexplored. In this study, we examined the influence of a dramatic yet natural experience in the life of female mice, giving birth and becoming a mother on single neurons and neuronal ensembles in the primary auditory cortex (A1). Using in vivo two-photon calcium imaging and electrophysiological recordings from layer 2/3 in A1 of mothers and age-matched virgin mice, we monitored changes in the responses to a set of artificial and natural sounds. Population dynamics underwent large changes as measured by pairwise and higher-order correlations, with noise correlations increasing as much as twofold in lactating mothers. Concomitantly, changes in response properties of single neurons were modest and selective. Remarkably, despite the large changes in correlations, information about stimulus identity remained essentially the same in the two groups. Our results demonstrate changes in the correlation structure of neuronal activity as a result of a natural life event.

Single neuron and population coding of natural sounds in auditory cortex.

The auditory system drives behavior using information extracted from sounds. Early in the auditory hierarchy, circuits are highly specialized for detecting basic sound features. However, already at the level of the auditory cortex the functional organization of the circuits and the underlying coding principles become different. Here, we review some recent progress in our understanding of single neuron and population coding in primary auditory cortex, focusing on natural sounds. We discuss possible mechanisms explaining why single neuron responses to simple sounds cannot predict responses to natural stimuli. We describe recent work suggesting that structural features like local subnetworks rather than smoothly mapped tonotopy are essential components of population coding. Finally, we suggest a synthesis of how single neurons and subnetworks may be involved in coding natural sounds.

Long‐term changes in the morphology and synaptic distributions of adult‐born neurons

The adult mammalian olfactory bulb (OB) is continuously supplied with adult‐born neurons. While some new neurons die shortly after arrival into the OB, others persist throughout the life of the animal. Here we followed the long‐term morphological changes in adult‐born periglomerular neurons and granule cells from the mouse OB well after they mature. We present a dataset of dendritic morphology and synaptic distributions from >100 adult‐born neurons as imaged in vivo and reconstructed in 3D. The dataset currently includes a substantial range of neuronal ages (0.5–11 months old). Using this dataset, we show that the morphological steady‐state which adult‐born periglomerular neurons reach soon after maturation is not maintained in older neurons. Rather, total dendritic length decreases after 6 months of age. We find that this morphological decrease in “old” periglomerular neurons is regulated by the age of the animal, and is independent of neuronal age. This suggests that morphological development of adult‐born neurons is regulated extrinsically. Our dendritic morphology dataset of 3D reconstructions is made available to the scientific community so it may serve as a useful resource for comparative morphological studies of the OB, and in particular of adult neurogenesis. J. Comp. Neurol. 519:2212–2224, 2011.