Ultraweak signals can cause synaptic depression and adaptation

Steady-state PERG adaptation: a conspicuous component of response variability with clinical significance

PURPOSE

To investigate within-test variability of the steady-state PERG (SS-PERG).

METHODS

SS-PERGs were recorded in response to black-white horizontal gratings (1.6 cycles/deg, 98% contrast, 15.63 reversals/s, LED display, 25 deg square field, 800 cd/sqm mean luminance) using skin electrodes. PERG and noise (± reference) signals were averaged over 1024 epochs (~ 2.2 min) and Fourier analyzed to retrieve SS-PERG amplitude and phase. SS-PERGs were split into 16 partial averages (samples) of 64 epochs each, and corresponding amplitudes and phases combined in polar coordinates to assess their dispersion (within-test variability). To assess time-dependent variability, samples were clustered in four successive time segments of ~ 33 s each. Amplitude adaptation was defined as amplitude difference between initial and final clusters, and PERG phase adaptation as the corresponding phase difference. To determine the dynamic range of SS-PERG adaptation, recording was performed in normal controls of different age (n = 32) and patients with different severity of optic nerve dysfunction (early manifest glaucoma, EMG, n = 7; non-arteritic ischemic optic neuropathy, NAION, n = 5).

RESULTS

Amplitude adaptation was largest in younger controls (amplitude adaptation ÷ noise, SNR = 9.5, 95% CI 13.1, 5.9) and progressively decreased with increasing age (older subjects, SNR = 5.5, 95% CI 9.2, 1.8) and presence of disease (EMG: SNR = 2.4, 95% CI 3.5, 1.4; NAION: SNR = 1.9, 95% CI 6.5,-2.2). In 11 young subjects, amplitude adaptation was repeatable (test-retest in two sessions a week apart; intraclass correlation coefficient = 0.59). Phase adaptation was not significantly different from zero in all groups.

CONCLUSIONS

SS-PERG adaptation accounts for a sizeable portion of the within-test variability. As it has robust SNR, sufficient test-retest variability, and is altered in disease, it may have physiological and clinical significance. This study suggests that SS-PERG protocols should include adaptation in addition to SS-PERG amplitude and phase/latency.

Single Ca2+ channels and exocytosis at sensory synapses

Hair cell synapses in the ear and photoreceptor synapses in the eye are the first synapses in the auditory and visual system. These specialized synapses transmit a large amount of sensory information in a fast and efficient manner. Moreover, both small and large signals with widely variable kinetics must be quickly encoded and reliably transmitted to allow an animal to rapidly monitor and react to its environment. Here we briefly review some aspects of these primary synapses, which are characterized by a synaptic ribbon in their active zones of transmitter release. We propose that these synapses are themselves highly specialized for the task at hand. Photoreceptor and bipolar cell ribbon synapses in the retina appear to have versatile properties that permit both tonic and phasic transmitter release. This allows them to transmit changes of both luminance and contrast within a visual field at different ambient light levels. By contrast, hair cell ribbon synapses are specialized for a highly synchronous form of multivesicular release that may be critical for phase locking to low-frequency sound-evoked signals at both low and high sound intensities. The microarchitecture of a hair cell synapse may be such that the opening of a single Ca(2+) channel evokes the simultaneous exocytosis of multiple synaptic vesicles. Thus, the differing demands of sensory encoding in the eye and ear generate diverse designs and capabilities for their ribbon synapses.

Nonlinear interactions between excitatory and inhibitory retinal synapses control visual output

The visual system is highly sensitive to dynamic features in the visual scene. However, it is not known how or where this enhanced sensitivity first occurs. We investigated this phenomenon by studying interactions between excitatory and inhibitory synapses in the second synaptic layer of the mouse retina. We found that these interactions showed activity-dependent changes that enhanced signaling of dynamic stimuli. Excitatory signaling from cone bipolar cells to ganglion cells exhibited strong synaptic depression, attributable to reduced glutamate release from bipolar cells. This depression was relieved by amacrine cell inhibitory feedback that activated presynaptic GABA(C) receptors. We found that the balance between excitation and feedback inhibition depended on stimulus frequency; at short interstimulus intervals, excitation was enhanced, attributable to reduced inhibitory feedback. This dynamic interplay may enrich visual processing by enhancing retinal responses to closely spaced temporal events, representing rapid changes in the visual environment.