Lindsey L. Glickfeld

Supralinear increase of recurrent inhibition during sparse activity in the somatosensory cortex

The balance between excitation and inhibition in the cortex is crucial in determining sensory processing. Because the amount of excitation varies, maintaining this balance is a dynamic process; yet the underlying mechanisms are poorly understood. We show here that the activity of even a single layer 2/3 pyramidal cell in the somatosensory cortex of the rat generates widespread inhibition that increases disproportionately with the number of active pyramidal neurons. This supralinear increase of inhibition results from the incremental recruitment of somatostatin-expressing inhibitory interneurons located in layers 2/3 and 5. The recruitment of these interneurons increases tenfold when they are excited by two pyramidal cells. A simple model demonstrates that the distribution of excitatory input amplitudes onto inhibitory neurons influences the sensitivity and dynamic range of the recurrent circuit. These data show that through a highly sensitive recurrent inhibitory circuit, cortical excitability can be modulated by one pyramidal cell.

Interneurons hyperpolarize pyramidal cells along their entire somatodendritic axis.

Although GABAergic interneurons are the main source of synaptic inhibition in the cortex, activation of GABAA receptors has been shown to depolarize specific neuronal compartments, resulting in excitation. By using a noninvasive approach to monitor the effect of individual interneurons on the pyramidal cell population, we found that rat hippocampal interneurons hyperpolarized pyramidal cells irrespective of the location of their synapses along the somato-dendritic axis.

Cortico-cortical projections in mouse visual cortex are functionally target specific

Neurons in primary sensory cortex have diverse response properties, whereas higher cortical areas are specialized. Specific connectivity may be important for areal specialization, particularly in the mouse, where neighboring neurons are functionally diverse. To examine whether higher visual areas receive functionally specific input from primary visual cortex (V1), we used two-photon calcium imaging to measure responses of axons from V1 arborizing in three areas with distinct spatial and temporal frequency preferences. We found that visual preferences of presynaptic boutons in each area were distinct and matched the average preferences of recipient neurons. This specificity could not be explained by organization within V1 and instead was due to both a greater density and greater response amplitude of functionally matched boutons. Projections from a single layer (layer 5) and from secondary visual cortex were also matched to their target areas. Thus, transmission of specific information to downstream targets may be a general feature of cortico-cortical communication.

Sensory Neuron Signaling to the Brain: Properties of Transmitter Release from Olfactory Nerve Terminals

Olfactory receptor neurons (ORNs) convey sensory information directly to the CNS via conventional glutamatergic synaptic contacts in olfactory bulb glomeruli. To better understand the process by which information contained in the odorant-evoked firing of ORNs is transmitted to the brain, we examined the properties of glutamate release from olfactory nerve (ON) terminals in slices of the rat olfactory bulb. We show that marked paired pulse depression is the same in simultaneously recorded periglomerular and tufted neurons, and that this form of short-term plasticity is attributable to a reduction of glutamate release from ON terminals. We used the progressive blockade of NMDA receptor (NMDAR) EPSCs by MK-801 [(5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5-10-imine hydrogen maleate] and stationary fluctuation analysis of AMPA receptor (AMPAR) EPSCs to determine the probability of release (Pr) of ON terminals; both approaches indicated that Pr is unusually high (≥0.8). The low-affinity glutamate receptor antagonists γ-d-glutamylglycine and l-amino-5-phosphonovaleric acid blocked ON-evoked AMPAR- and NMDAR-mediated EPSCs, respectively, to the same extent under conditions of low and high Pr, suggesting that multivesicular release is not a feature of ON terminals. Although release from most synapses exhibits a highly nonlinear dependence on extracellular Ca2+, we find that the relationship between glutamate release and extracellular Ca2+ at ON terminals is nearly linear. Our results suggest that ON terminals have specialized features that may contribute to the reliable transmission of sensory information from nose to brain.

Removable cranial windows for long-term imaging in awake mice

Cranial window implants in head-fixed rodents are becoming a preparation of choice for stable optical access to large areas of the cortex over extended periods of time. Here we provide a highly detailed and reliable surgical protocol for a cranial window implantation procedure for chronic wide-field and cellular imaging in awake, head-fixed mice, which enables subsequent window removal and replacement in the weeks and months after the initial craniotomy. This protocol has facilitated awake, chronic imaging in adolescent and adult mice over several months from a large number of cortical brain regions; targeted virus and tracer injections from data obtained using prior awake functional mapping; and functionally targeted two-photon imaging across all cortical layers in awake mice using a microprism attachment to the cranial window. Collectively, these procedures extend the reach of chronic imaging of cortical function and dysfunction in behaving animals.

Complementary Modulation of Somatic Inhibition by Opioids and Cannabinoids

Somatic inhibition, which is critical for determining the spike output of principal cells, is mediated by two physiologically distinct classes of GABAergic interneurons called basket cells. In the hippocampus, despite both targeting the somatic membrane of CA1 pyramidal cells, these two classes of basket cells are active at different times. Differential modulation of these two types of basket cells could hence be important for regulating the activity patterns of CA1 pyramidal cells at very specific periods during ongoing activity. Indeed, cannabinoids selectively suppress the output of one class of basket cell. Whether opioids, another major modulator of inhibition in the hippocampus, also selectively suppress somatic inhibition is not known. Here, we show that basket cells are selectively modulated by either opioids or cannabinoids, but not both. We also find that basket cells are integrated into specific inhibitory subnetworks that are themselves under differential control of opioids and cannabinoids. Furthermore, because the two interneuron types are activated at different times, opioids and cannabinoids suppress different epochs of inhibition. This cell-type specific sensitivity to neuromodulators allows for a fine control of the temporal structure of hippocampal activity.

Mouse Primary Visual Cortex Is Used to Detect Both Orientation and Contrast Changes

In mammals, the lateral geniculate nucleus (LGN) and the superior colliculus (SC) are the major targets of visual inputs from the retina. The LGN projects mainly to primary visual cortex (V1) while the SC targets the thalamus and brainstem, providing two potential pathways for processing visual inputs. Indeed, cortical lesion experiments in rodents have yielded mixed results, leading to the hypothesis that performance of simple visual behaviors may involve computations performed entirely by this subcortical pathway through the SC. However, these previous experiments have been limited by both their assays of behavioral performance and their use of lesions to change cortical activity. To determine the contribution of V1 to these tasks, we trained mice to perform threshold detection tasks in which they reported changes in either the contrast or orientation of visual stimuli. We then reversibly inhibited V1 by optogenetically activating parvalbumin-expressing inhibitory neurons with channelrhodopsin-2. We found that suppressing activity in V1 substantially impaired performance in visual detection tasks. The behavioral deficit depended on the retinotopic position of the visual stimulus, confirming that the effect was due to the specific suppression of the visually driven V1 neurons. Behavioral effects were seen with only moderate changes in neuronal activity, as inactivation that raised neuronal contrast thresholds by a median of only 14% was associated with a doubling of behavioral contrast detection threshold. Thus, detection of changes in either orientation or contrast is dependent on, and highly sensitive to, the activity of neurons in V1.

A mouse model of higher visual cortical function.

During sensory experience, the retina transmits a diverse array of signals to the brain, which must be parsed to generate meaningful percepts that can guide decisions and actions. Decades of anatomical and physiological studies in primates and carnivores have revealed a complex parallel and hierarchical organization by which distinct visual features are distributed to, and processed by, different brain regions. However, these studies have been limited in their ability to dissect the circuit mechanisms involved in the transformation of sensory inputs into complex cortical representations and action patterns. Multiple groups have therefore pushed to explore the organization and function of higher visual areas in the mouse. Here we review the anatomical and physiological findings of these recent explorations in mouse visual cortex. These studies find that sensory input is processed in a diverse set of higher areas that are each interconnected with specific limbic and motor systems. This hierarchical and parallel organization is consistent with the multiple streams that have been found in the higher visual areas of primates. We therefore propose that the mouse visual system is a useful model to explore the circuits underlying the transformation of sensory inputs into goal-directed perceptions and actions.

Self-administering cannabinoids

Endocannabinoids, which are typically released by principal cells in response to prolonged depolarization, act as retrograde messengers to inhibit synaptic transmission. A recent study shows that in a specific subtype of cortical interneuron, endocannabinoids released under similar circumstances can also act cell-autonomously. Here, endocannabinoids endow these neurons with a memory of their own activity in the form of a long-term change in excitability.

High-fidelity optical excitation of cortico-cortical projections at physiological frequencies.

Optogenetic activation of axons is a powerful approach for determining the synaptic properties and impact of long-range projections both in vivo and in vitro. However, because of the difficulty of measuring activity in axons, our knowledge of the reliability of optogenetic axonal stimulation has relied on data from somatic recordings. Yet, there are many reasons why activation of axons may not be comparable to cell bodies. Thus we have developed an approach to more directly assess the fidelity of optogenetic activation of axonal projections. We expressed opsins (ChR2, Chronos, or oChIEF) in the mouse primary visual cortex (V1) and recorded extracellular, pharmacologically isolated presynaptic action potentials in response to axonal activation in the higher visual areas. Repetitive stimulation of axons with ChR2 resulted in a 70% reduction in the fiber volley amplitude and a 60% increase in the latency at all frequencies tested (10-40 Hz). Thus ChR2 cannot reliably recruit axons during repetitive stimulation, even at frequencies that are reliable for somatic stimulation, likely due to pronounced channel inactivation at the high light powers required to evoke action potentials. By comparison, oChIEF and Chronos evoked photocurrents that inactivated minimally and could produce reliable axon stimulation at frequencies up to 60 Hz. Our approach provides a more direct and accurate evaluation of the efficacy of new optogenetic tools and has identified Chronos and oChIEF as viable tools to interrogate the synaptic and circuit function of long-range projections.