Perineuronal nets in cortical processing and plasticity

Perineuronal nets (PNNs) are a specialized form of extracellular matrix in the CNS, embodying parvalbumin expressing inhibitory neurons. The PNN assembles towards the end of the period of heightened plasticity, the critical period, in parallel with maturation of the inhibitory network. Degradation of PNNs with the enzyme Chondroitinase ABC (chABC) reopens for plasticity in adults. Together this suggests that PNNs serves as a break on plasticity by stabilizing synapses and maintaining the inhibitory-excitatory balance in the cortex. How the PNN contributes to cortical processing and how its removal opens for plasticity remain elusive. I investigated how the removal of PNNs in primary visual cortex (V1) influences cortical processing and plasticity in rats, by injecting chABC in V1 and measuring neuronal activity with chronically implanted tetrodes. The PNN was completely degraded after three days, and then reassembled over 60 days. All functional studies were performed within 21 days of enzymatic treatment. Degradation of the PNN caused a non-significant reduction in activity of the inhibitory neurons with more than 50% reduction in mean firing rate compared to controls. Tuning properties, such as orientation selectivity and OD were unaffected. In order to elucidate how removal of the PNN influences plasticity I used monocular deprivation (MD) to induce activity-dependent plasticity. One eyelid was sutured shut and neuronal activity recorded daily; after five days, the suture was removed and OD reassessed. In accordance with previous studies, MD for five days in the chABC-injected animals produced a shift in OD. Already after one day of MD, neurons contralateral to the deprived eye showed a 50% reduction in firing rate and continued to be reduced throughout the MD period. Conversely, neurons ipsilateral to the deprived eye showed more than 90% increase in firing rate after one day of MD, after which the activity stabilized. The reduced activity of inhibitory neurons after PNN degradation supports the hypothesis that the increased adult plasticity may be caused by a shift in the inhibitory-excitatory balance. The increased activity seen ipsilateral to the deprived eye could be an early indication of a functional change. I have also studied the effects of anesthesia on cortical processing. For more than 50 years, anesthetized animals have been used to study the visual system. To what extent general anesthetics affect populations of neurons and response properties of single cells have not been determined. I found that neurons in V1 respond very differently to anesthesia; while some were stable or showed increase in firing rate compared to in the awake state, most neurons showed reduced firing rate. Furthermore, the stability in orientation tuning between the two states was highly variable between cells. Altogether, anesthesia should be used with caution when investigating cortical function

Differential Expression and Cell-Type Specificity of Perineuronal Nets in Hippocampus, Medial Entorhinal Cortex and Visual Cortex Examined in the Rat and Mouse

Perineuronal nets (PNNs) are specialized extracellular matrix (ECM) structures that condense around the soma and proximal dendrites of subpopulations of neurons. Emerging evidence suggests that they are involved in regulating brain plasticity. However, the expression of PNNs varies between and within brain areas. A lack of quantitative studies describing the distribution and cell-specificity of PNNs makes it difficult to reveal the functional roles of PNNs. In the current study, we examine the distribution of PNNs and the identity of PNN-enwrapped neurons in three brain areas with different cognitive functions: the dorsal hippocampus, medial entorhinal cortex (mEC) and primary visual cortex (V1). We compared rats and mice as knowledge from these species are often intermingled. The most abundant expression of PNNs was found in the mEC and V1, while dorsal hippocampus showed strikingly low levels of PNNs, apart from dense expression in the CA2 region. In hippocampus we also found apparent species differences in expression of PNNs. While we confirm that the PNNs enwrap parvalbumin-expressing (PV+) neurons in V1, we found that they mainly colocalize with excitatory CamKII-expressing neurons in CA2. In mEC, we demonstrate that in addition to PV+ cells, the PNNs colocalize with reelin-expressing stellate cells. We also show that the maturation of PNNs in mEC coincides with the formation of grid cell pattern, while PV+ cells, unlike in other cortical areas, are present from early postnatal development. Finally, we demonstrate considerable effects on the number of PSD-95-gephyrin puncta after enzymatic removal of PNNs

An updated suite of viral vectors for in vivo calcium imaging using intracerebral and retro-orbital injections in male mice

Genetically encoded Ca2+ indicators (GECIs) are used to measure neural activity. Here, authors screen GECI constructs for suitability with systemic injections and soma-targeting, and modify a soma-targeting peptide for improved expression rate

Removal of perineuronal nets disrupts recall of a remote fear memory

Throughout life animals learn to recognize cues that signal danger and instantaneously initiate an adequate threat response. Memories of such associations may last a lifetime and far outlast the intracellular molecules currently found to be important for memory processing. The memory engram may be supported by other more stable molecular components, such as the extracellular matrix structure of perineuronal nets (PNNs). Here, we show that recall of remote, but not recent, visual fear memories in rats depend on intact PNNs in the secondary visual cortex (V2L). Supporting our behavioral findings, increased synchronized theta oscillations between V2L and basolateral amygdala, a physiological correlate of successful recall, was absent in rats with degraded PNNs in V2L. Together, our findings suggest a role for PNNs in remote memory processing by stabilizing the neural network of the engram

Optogenetic pacing of medial septum parvalbumin-positive cells disrupts temporal but not spatial firing in grid cells

Grid cells in the medial entorhinal cortex (MEC) exhibit remarkable spatial activity patterns with spikes coordinated by theta oscillations driven by the medial septal area (MSA). Spikes from grid cells progress relative to the theta phase in a phenomenon called phase precession, which is suggested as essential to create the spatial periodicity of grid cells. Here, we show that optogenetic activation of parvalbumin-positive (PV+) cells in the MSA enabled selective pacing of local field potential (LFP) oscillations in MEC. During optogenetic stimulation, the grid cells were locked to the imposed pacing frequency but kept their spatial patterns. Phase precession was abolished, and speed information was no longer reflected in the LFP oscillations but was still carried by rate coding of individual MEC neurons. Together, these results support that theta oscillations are not critical to the spatial pattern of grid cells and do not carry a crucial velocity signal

Optogenetic pacing of medial septum parvalbumin-positive cells disrupts temporal but not spatial firing in grid cells

Grid cells in the medial entorhinal cortex (MEC) exhibit remarkable spatial activity patterns with spikes coordinated by theta oscillations driven by the medial septal area (MSA). Spikes from grid cells progress relative to the theta phase in a phenomenon called phase precession, which is suggested as essential to create the spatial periodicity of grid cells. Here, we show that optogenetic activation of parvalbumin-positive (PV+) cells in the MSA enabled selective pacing of local field potential (LFP) oscillations in MEC. During optogenetic stimulation, the grid cells were locked to the imposed pacing frequency but kept their spatial patterns. Phase precession was abolished, and speed information was no longer reflected in the LFP oscillations but was still carried by rate coding of individual MEC neurons. Together, these results support that theta oscillations are not critical to the spatial pattern of grid cells and do not carry a crucial velocity signal

Perineuronal nets stabilize the grid cell network

Grid cells are part of a widespread network which supports navigation and spatial memory. Stable grid patterns appear late in development, in concert with extracellular matrix aggregates termed perineuronal nets (PNNs) that condense around inhibitory neurons. It has been suggested that PNNs stabilize synaptic connections and long-term memories, but their role in the grid cell network remains elusive. We show that removal of PNNs leads to lower inhibitory spiking activity, and reduces grid cells’ ability to create stable representations of a novel environment. Furthermore, in animals with disrupted PNNs, exposure to a novel arena corrupted the spatiotemporal relationships within grid cell modules, and the stored representations of a familiar arena. Finally, we show that PNN removal in entorhinal cortex distorted spatial representations in downstream hippocampal neurons. Together this work suggests that PNNs provide a key stabilizing element for the grid cell network

Perineuronal nets stabilize the grid cell network

Grid cells are part of a widespread network which supports navigation and spatial memory. Stable grid patterns appear late in development, in concert with extracellular matrix aggregates termed perineuronal nets (PNNs) that condense around inhibitory neurons. It has been suggested that PNNs stabilize synaptic connections and long-term memories, but their role in the grid cell network remains elusive. We show that removal of PNNs leads to lower inhibitory spiking activity, and reduces grid cells’ ability to create stable representations of a novel environment. Furthermore, in animals with disrupted PNNs, exposure to a novel arena corrupted the spatiotemporal relationships within grid cell modules, and the stored representations of a familiar arena. Finally, we show that PNN removal in entorhinal cortex distorted spatial representations in downstream hippocampal neurons. Together this work suggests that PNNs provide a key stabilizing element for the grid cell network