Temporal covariance of pre- and postsynaptic activity regulates functional connectivity in the visual cortex

1. It has been suggested from mathematical models and in vivo experiments in the visual cortex that periods of temporal covariance of pre- and postsynaptic activity can lead to a potentiation or depression of synaptic efficacy. We directly tested this hypothesis in vitro in the guinea pig and cat visual cortex. 2. Intracellular recordings were made in brain slices from 63 neurons in layers 2-4 in bicuculline-free artificial cerebrospinal fluid. Twenty-nine cells (n = 25 from pigmented guinea pigs and 4 from cats) were taken through a complete series of control and test protocols to evaluate the covariance hypothesis. Some (n = 7) cells that were taken through the complete experimental protocols were also filled intracellularly with biocytin. Compound postsynaptic potentials (PSPs) were evoked by low-frequency (0.2-1.0 Hz), weak (20% of threshold intensity) stimulation of the cortical white matter and/or intracortical sites in layers 2-3. 3. In one series of experiments we paired PSPs with imposed coincident depolarizing (S+) or hyperpolarizing (S-) pulses (mean +/- 2.8 nA for 50-80 ms) of the postsynaptic neuron (n = 54 PSPs; > 1 pairing protocol was often run on an individual cell). Controls consisted of analyzing the same number of S+ or S- pairings but with long temporal delays [called fixed delay pairings (FDPs)] between the test pathway stimulation and the onset of the intracellular current pulse (120 ms) and pseudopairings (PP) consisting of evoked PSPs and delivery of intracellular current injection pulses in a phase-independent manner. Twenty-one of 54 PSPs subjected to pairing were significantly modified by the protocol. The S+ protocol significantly (P < 0.05, Kolmogorov-Smirnov test) increased the peak amplitudes of 8 of 22 PSPs (+20 to +55%); the S- protocol significantly decreased the peak amplitudes of 13 of 32 PSPs (-15 to -88%), whereas the FDP and PP protocols generally did not cause significant changes in the PSPs (0% and 4%, respectively). Significant changes in PSPs persisted in most cases for 10-20 min. 4. Another series of experiments consisted of evaluating for the same cell the effects of evoking a PSP from one stimulation site without concomitant postsynaptic activation and alternately evoking a PSP from the other stimulation site with S+ or S- pairing (n = 25 PSPs). Only the paired pathway showed the predicted effects on the PSP (S+ pairing causing an increase in peak PSP amplitude and S- pairing causing a decrease in peak PSP amplitude).(ABSTRACT TRUNCATED AT 400 WORDS)