Abstracts

The intrinsic optical signal (IOS) is a change in light reflectance from neural tissue that correlates with the underlying electrophysiology. The IOS has recently been applied to the study of epileptiform events in vivo, and holds great promise as a high-resolution ([sim]50 [mu]m) tool for seizure imaging. It has been shown that different information predominates in the IOS at different wavelengths (blood flow for green light, the oxygenation state of hemoglobin at orange, and cell volume effects for red), but the optimal wavelength for imaging epileptiform events has not yet been determined. We investigate the IOS response to 4-aminopyridine (4AP)-induced focal neocortical seizures in the rat at various wavelengths of illuminating light.
Adult (275-250 gram) urethane-anesthetized rats were placed in a stereotactic frame. The skull was thinned using a dental drill. A small area of skull and dura were opened opened and two glass microelectrodes, tips [lt]1 mm apart, were placed [sim]500 [mu]m deep in the somatosensory cortex. One electrode was used to record local field potential; the second was attached to a nanoliter-volume injector through which 4AP was injected (250 nl of a 50 mM solution in 1% NaCl). A 10 bit CCD camera was mounted on a tandem configuration of two 50 mm lenses, and focused 500 mm below the cortical surface. The thinned skull was covered with 1.5% agar (in saline) and with a glass cover slip, and then illuminated with light of various wavelengths (546 nm, 605 nm, 630 nm and 700 nm). Images taken every 600 msec during the seizure were divided by a baseline image prior to seizure onset. Percent change in light reflectance (-[Delta]R/R) was calculated over the time-course of each seizure.
In a 1 mm[sup2] region of interest surrounding the injection site, the largest -[Delta]R/R was obtained at 546 nm (18.4% [plusmn] 5.7%, N=9 seizures). At higher wavelengths, the maximal -[Delta]R/R was 4.82% [plusmn] 0.5% (605 nm, N=3), 6.81% [plusmn] 1.34% (630 nm, N=3) and 0.47% [plusmn] 0.32% (700 nm, N=5). The spatial extent of the seizures was also greatest at 546 nm (41.9 [plusmn] 4.5 mm [sup2], N=8) and was 18.03 [plusmn] 2.9 mm[sup2] (N=3), 29.6 [plusmn] 5.9 mm[sup2] (N=3) and 11.8 [plusmn] 7.7 mm[sup2] (N=4) at 605, 630 and 700 nm, respectively. For all recorded seizures, an inverted optical signal was observed in the region surrounding or adjacent to the seizure focus. Minimal -[Delta]R/R was -4.0% [plusmn] 4.1% (N=9), -10.9 [plusmn] 1.8% (N=3), -5.6% [plusmn] 3.5% (N=3) and -6.6% [plusmn] 3.1% (N=5) at 546, 605, 630 and 700 nm, respectively.
The intrinsic optical signal can be used to obtain high-resolution maps of focal neocortical seizures through the thinned skull in an in vivo rat model. We find that the largest amplitude signal occurs for incident light of 546 nm. Whether this corresponds better with the population of epileptic neurons than does the signal at other wavelengths remains to be determined. An inverted optical signal within the surrounding brain may represent an inhibitory surround and/or shunting of blood to the epileptic focus
[Supported by: The NIH (NINDS).]