Neuron Mar;6(3): Control of postsynaptic Ca2+ influx in developing neocortex by excitatory and inhibitory neurotransmitters. Yuste R, Katz LC. Laboratory of Neurobiology, Rockefeller University, New York, New York We assessed the pathways by which excitatory and inhibitory neurotransmitters elicit postsynaptic changes in [Ca2+]i in brain slices of developing rat and cat neocortex, using fura 2. Glutamate, NMDA, and quisqualate transiently elevated [Ca2%]i in all neurons. While the quisqualate response relied exclusively on voltage-gated Ca2+ channels, almost all of the NMDA-induced Ca2+ influx was via the NMDA ionophore itself, rather than through voltage-gated Ca2+ channels. Glutamate itself altered [Ca2+]i almost exclusively via the NMDA receptor. Furthermore, synaptically induced Ca2+ entry relied almost completely on NMDA receptor activation, even with low-frequency stimulation. The inhibitory neurotransmitter GABA also increased [Ca2+]i, probably via voltage-sensitive Ca2+ channels, whereas the neuromodulator acetylcholine caused Ca2+ release from intracellular stores via a muscarinic receptor. Low concentrations of these agonists produced nonperiodic [Ca2+]i oscillations, which were temporally correlated in neighbouring cells. Optical recording with Ca2(+)-sensitive indicators may thus permit the visualization of functional networks in developing cortical circuits.

Calcium imaging of cortical microcircuits

Single-cell resolution imaging of Ca 2+ influx due to action potentials L5 pyramid loaded with 50µM fura imaged by photodiode array at 1.6 kHz (0.6ms/frame)

Whole-cell filled AM filled

Trains of action potentials

50 Hz 40 Hz

Cortical circuits in vitro are spontaneously active: spontaneous activity as a tool, let the circuit speak

V IV II/III Automatic identification of cells

Detection of calcium transients

Cell number a Spontaneous synchronizations of a small % of neurons Low temporal resolution- 1sec/frame

Spontaneous coactivations have specific spatial patterns

9 mV 5 s -70 mV, 0 pA 500 ms 9 mV 1.3 s 9 mV 500 ms Synchronizations correspond to UP states

UP states can last several seconds

Stereotyped dynamics of circuit coactivations

Cortical motifs and songs: repeated sequences of activity Intermediate temporal resolution- 50 msec/frame

Shuffling tests

Photodiode array: 0.6 msec/frame

Local synchronizations

Sequential activations of cells

Pacemakers

Pacemakers are more regular

Repeated network activity measured in a single cell 10 KHz resolution i iii iv

10 pA 200 ms Repeated motifs of spontaneous activity in slices

Millisecond precision

Correlation between intracellular and optical repetitions

Repetitions in vivo Ilan Lampl/David Ferster

What is role of thalamic stimulation on cortical dynamics? L4 L2/3 L5 Adapted from Brecht et al 2003

Thalamic Stimulation 4-8 stimuli 40 Hz 200  s 50 – 100  A Thalamus “Barrel” Cortex Stimulation Electrode Imaging Layer 4 response to thalamic stimulation

20 mV 1 s Thalamic stimulation generates cortical UP states UP states Prolonged depolarizations ~ 10 mV depolarized from rest Preferential state for action potential generation Coincident with multiple nearby neurons Vm -70 mV

500 ms 20 mV Spontaneous Spontaneous activity also generates cortical UP states

Triggered Triggered Core Spontaneous X 5X 4 Spontaneous Core Overlap Overlap Core Spontaneous activity and thalamic stimulation engage the same neurons !!!

Triggered Spontaneous Overlap 5mV 20mV 500 ms 1 s Amplitude Duration No. APs Similar Spontaneous and Evoked Intracellular UP states # of APs Amplitude Duration Correlation of UPstates within cells

TriggeredSpontaneous Core Frame Number Identical Network Dynamics during Spontaneous and Evoked Network Events- 100 msec/frame Time

10 mV 500 ms 5 mV 100 ms Millisecond Precision in the Repetition of Synaptic inputs during spontaneous and thalamic UP states

Novel types of spontaneous network dynamics Data: Reverberating activity is prevalent at all temporal scales Spatiotemporal patterns are real: statistics, two techniques, spatial profile, UP states, they can be triggered Sparse dynamics: small number of cells Single neurons can participate in many patterns Repetitions never exact Thalamic stimulation triggers internal states Speculation: Spatially organized ensembles: related to circuit features? Preferred states: attractors or metastable states? Precisely repeated dynamics: Abeles’ synfire chains? Cortex as a giant CPG?

Spinal Central Pattern Generator Cortical Microcircuit Cortex as a giant CPG

Buqing Mao-postdoc Rosa Cossart-postdoc Dimitry Aronov-undergraduate student Yuji Ikegaya-visiting professor Gloster Aaron-postdoc Jason McLean-postdoc Brendon Watson-MD PhD student National Eye Institute- HHMI

Synfire chains hypothesis- Moshe Abeles Synchronous firing Nonlinear gain paradoxically reduces jitter Faithful propagation Faithful repetition Precise Firing Sequences

Two theories of brain function: Feed forward: Sherrington Hubel &Wiesel Receptive fields Speed of processing Feedback: Brown Lorente/Hebb Llinás Recurrent connectivity Spontaneous activity

Pyramidal neurons in layer 5

1 2 3 Frame Number Triggered Naive Core An Already Existing Network Mediates the Observed Dynamics Time

10 mV 500 ms An Already Existing Network Mediates the Observed Dynamics

40 % 30 % <10 % Thalamus Even in L4, the vast majority of excitatory synapses arise locally within cortex (20 % long corticocortical excitatory connections)

Circuit attractors Attractors Input Inputs Adapted from Wilson, 1999 Memories

Example of an emergent computation

Synfire chains

Evidence for synfire chains Abeles PFS Spatial navigation in hippocampus Birdsong sequences CPGs Arguments against Statistics Nonlinear null hypotheses Mechanism unknown

-52 mV -72 mV -52 mV UP states promote precise firing patterns in response to thalamic input 50 mV 25 ms Train of Stimuli During DOWN state Single Thalamic Stimulation During DOWN state Train of Stimuli During UP state 1 st Spike <2 ms jitter 2nd Spike <5 ms jitter 1 st Spike 40 ms jitter

Searching for repeats of activity in a single neuronal recording Examine the covariance, h(  ), between segments: (AxB), (AxC),...(BxC), (BxD),......

Two competing world views: How is perception shaped? Feed Forward Feedback EmpiricismNativism

Synchronizations correspond to maximum organization