Manuel J. Rojas

Local functional state differences between rat cortical columns.

Surface evoked potentials (SEPs) during auditory clicks and whisker twitches are usually larger during quiet sleep (QS) over waking and REM sleep. However, SEP amplitudes from single trials fluctuate periodically between high and low values regardless of sleep-wake cycle. To test the hypothesis that state-independent fluctuations represent local functional sleep-like states of individual cortical columns, we examined single trial SEP amplitudes from multiple cortical locations across sleep-wake cycles. Bilateral stimuli produced SEP amplitude fluctuations in each hemisphere that usually covaried (r = 0.4), but with frequent hemispheric differences. Two neighboring whiskers, twitched simultaneously on the same side, produced highly correlated SEPs in neighboring cortical columns (r = 0.9) with frequent divergences. We found 50% more disparity during QS over waking, indicating that the differences did not result from recording noise or stimulus inconsistency. Local SEP fluctuations also followed local differences in the delta wave signal during QS (r = 0.4), suggesting that similar mechanisms may modulate the SEP. The duration of the localized sleep-like (high SEP amplitude) state was dependent on the duration of prior wake-like (low SEP amplitude) state (r = 0.5), suggesting a use dependence of prior functional state period. Since SEP indicators fluctuated independently from whole animal sleep state, and were frequently different between hemispheres and nearby cortical columns, these data support the theory that sleep-like functional states may be localized to brain regions at least as small as cortical columns.

TUMOR NECROSIS FACTOR α: ACTIVITY DEPENDENT EXPRESSION AND PROMOTION OF CORTICAL COLUMN SLEEP IN RATS

Cortical surface evoked potentials (SEPs) are larger during sleep and characterize a sleep-like state in cortical columns. Since tumor necrosis factor alpha (TNF) may be involved in sleep regulation and is produced as a consequence of waking activity, we tested the hypothesis that direct application of TNF to the cortex will induce a sleep-like state within cortical columns and enhance SEP amplitudes. We found that microinjection of TNF onto the surface of the rat somatosensory cortex enhanced whisker stimulation-induced SEP amplitude relative to a control heat-inactivated TNF microinjection. We also determined if whisker stimulation enhanced endogenous TNF expression. TNF immunoreactivity (IR) was visualized after 2 h of deflection of a single whisker on each side. The number of TNF-IR cells increased in layers II-IV of the activated somatosensory barrel column. In two separate studies, unilateral deflection of multiple whiskers for 2 h increased the number of TNF-IR cells in layers II-V in columns that also exhibited enhanced cellular ongogene (Fos-IR). TNF-IR also colocalized with NeuN-IR suggesting that TNF expression was in neurons. Collectively these data are consistent with the hypotheses that TNF is produced in response to neural activity and in turn enhances the probability of a local sleep-like state as determined by increases in SEP amplitudes.

Mechanisms underlying state dependent surface-evoked response patterns

Cortical evoked response potentials (ERPs) display a rich set of waveforms that are both context and state dependent. However, the mechanisms that underlie state dependent ERP patterns are unclear. Determining those mechanisms through analysis of single trial ERP waveform signatures may provide insight into the regulation of cortical column state and the roles that sleep plays in cortical function. We implanted rats with electroencephalogram (EEG) and electromyogram (EMG) electrodes to record ERPs and to assess sleep/wake states continuously during 1-2 s random auditory clicks. Individual cortical auditory ERPs were sorted into one of eight behavioral states, and fell into three categories based on amplitude and latency characteristics. ERPs within waking and rapid eye movement (REM) sleep were predominantly low amplitude and short latency. Approximately 50% of ERPs during light quiet sleep (quiet sleep 1 and quiet sleep 2) exhibited low amplitude, short latency responses, and the remaining ERPs had high amplitude, long latency responses. This distribution was characteristic of EEG fluctuations during low frequency delta waves. Significantly more individual ERPs showed very low amplitudes during deep quiet sleep (quiet sleep 3 and quiet sleep 4), resulting in a lower average ERP. These results support the hypothesis that evoked response amplitudes and waveform patterns follow specific EEG patterns. Since evoked response characteristics distribute differently across states, they could aid our understanding of sleep mechanisms through state-related and local neural signaling.

State Dependent Auditory Evoked Hemodynamic Responses Recorded Optically with Indwelling Photodiodes

Implantable optical technologies provide measurements of cerebral hemodynamic activity from freely behaving animals without movement constraint or anesthesia. In order to study state-dependent neural evoked responses and the consequential hemodynamic response, we simultaneously measured EEG and scattered light changes in chronically implanted rats. Recordings took place under freely behaving conditions, allowing us to compare the evoked responses across wake, sleep, and anesthetized states. The largest evoked electrical and optical responses occurred during quiet sleep compared to wake and REM sleep, while isoflurane anesthesia showed a large, late burst of electrical activity synchronized to the stimulus but an earlier optical response.

In vitro and in vivo noise analysis for optical neural recording

Laser diodes (LD) are commonly used for optical neural recordings in chronically recorded animals and humans, primarily due to their brightness and small size. However, noise introduced by LDs may counteract the benefits of brightness when compared to low-noise light-emitting diodes (LEDs). To understand noise sources in optical recordings, we systematically compared instrument and physiological noise profiles in two recording paradigms. A better understanding of noise sources can help improve optical recordings and make them more practical with fewer averages. We stimulated lobster nerves and a rat cortex, then compared the root mean square (RMS) noise and signal-to-noise ratios (SNRs) of data obtained with LED, superluminescent diode (SLD), and LD illumination for different numbers of averages. The LED data exhibited significantly higher SNRs in fewer averages than LD data in all recordings. In the absence of tissue, LED noise increased linearly with intensity, while LD noise increased sharply in the transition to lasing and settled to noise levels significantly higher than the LEDs, suggesting that speckle noise contributed to the LDs higher noise and lower SNRs. Our data recommend low coherence and portable light sources for in vivo chronic neural recording applications.

Electroencephalographic spectrum power of sheep's brain after stunning

The electroencephalographic spectrum power (ESP) shows the frequency band components of the brain electrical activity. The electroencephalogram (EEG) has a characteristic ESP during different behavioural states such as waking or sleeping and also during pathological states as epilepsy or brain death. Methods of slaughtering the farm animals should prevent needless suffering; all the animals are expected to be unconscious and insensible to pain before being hoisted, but there are no systematic studies of the animals brain activity after stunning. This study evaluated the brain activity of sheep after stunning by means of percussion or electrical shock. Brain activity after electrical shock showed an epileptiform EEG, with decreasing delta power and increasing theta and gamma band power. After percussion stunning, the EEG showed a slightly decrease of high frequency power.

Running wheel training does not change neurogenesis levels or alter working memory tasks in adult rats

Background Exercise can change cellular structure and connectivity (neurogenesis or synaptogenesis), causing alterations in both behavior and working memory. The aim of this study was to evaluate the effect of exercise on working memory and hippocampal neurogenesis in adult male Wistar rats using a T-maze test. Methods An experimental design with two groups was developed: the experimental group (n = 12) was subject to a forced exercise program for five days, whereas the control group (n = 9) stayed in the home cage. Six to eight weeks after training, the rats’ working memory was evaluated in a T-maze test and four choice days were analyzed, taking into account alternation as a working memory indicator. Hippocampal neurogenesis was evaluated by means of immunohistochemistry of BrdU positive cells. Results No differences between groups were found in the behavioral variables (alternation, preference index, time of response, time of trial or feeding), or in the levels of BrdU positive cells. Discussion Results suggest that although exercise may have effects on brain structure, a construct such as working memory may require more complex changes in networks or connections to demonstrate a change at behavioral level.

Evoked Optical Response under Wake, Sleep and Anesthetized States

Concurrent electrical and optical measurements of auditory cortex responses exhibits state dependent hemodynamic activity. When compared to wake, quiet sleep elicits large, late optical signals, REM signals are large, while Isoflurane signals are phase shifted.