The Role of Electrode Size on the Incidence of Spreading Depression and on Cortical Cerebral Blood Flow as Measured by H₂ Clearance

Abstract

Summary: Cerebral blood flow was measured by the Hz clearance method 30 and 60 min after the implantation of 300, 250, 125, or 50 !--l m diameter platinum-iridium elec trodes 2 mm deep into the right parietal cortex of normo thermic, normocarbic halothane-anesthetized rats. An other group of animals had 50 !--l m electrodes inserted I mm. In all animals, the presence or absence of a wave of spreading depression (SD) was noted at the time of im plantation, with recordings made with glass micropi pettes. H2 flow values were compared with those mea sured in gray matter from the same anatomical region (but from different rats), using [ 3 H]nicotine. The incidence of SD ranged from 60% following insertion of 300 !--l m elec trodes to 0% with 50 !--l m electrodes. H2 clearance flows also varied with electrode size, from 77 ± 21 ml 100 g� 1 min � 1 (mean ± standard deviation) with 300 !--l m elec trodes to 110 ± 31 and 111 ± 16 ml 100 g� 1 min� 1 with 125 and 50 !--l m electrodes, respectively (insertion depth of 2 mm). A CBF value of 155 ± 60 ml 100 g� 1 min � 1 wasThe hydrogen clearance method for the measure ment of CBF offers several advantages over other techniques. It permits the simultaneous determina tions of local CBF in multiple sites. These measure ments can be repeated many times over periods ranging from hours to days. The method is also technically simple to perform and requires rela tively inexpensive equipment. However, in spite of the widespread use of this technique, questions re- garding its validity have been recently raised. It has been known for many years that in spite of good correlations between CBF values determined by H2 clearance and other flow measurement methods, the absolute values obtained with H2 clearance are often lower than those obtained with other tech niques. For example, Furlow measured CBF of 80 ml 100 g � 1 min � 1 in the rostral neocortex of awake (but paralyzed) rats using implanted 200 /-lm diam eter platinum electrodes (Furlow, 1982). In con trast, using e 4 C]iodoantipyrine autoradiography, a flow of 139 ml 100 g -1 min -1 was obtained for the same area (in a separate group of animals). More importantly, Tomida et al. measured regional CBF in gerbils by H2 clearance (using a 250 /-lm diameter electrode) and by either eH]nicotine or

Cerebral autoregulation during moderate hypothermia in rats.

“…The observed CBFLDF values are normally distributed with a coefficient of variability similar to that seen with other flow methods. 26 There are also clear differences between group 1 and group 2, the magnitudes of which are similar to those reported by others.10 Our measurements were also made through the intact dura. This was done to avoid changing the cortical microenvironment and to limit the potential for injury.…”

Cortical Spreading Depression Protects against Subsequent Focal Cerebral Ischemia in Rats

“…This electrode may not be ideal, but the waveforms recorded were con sistent with previously reported characteristics of CSD (McLachlan, 1992;Somjen et al, 1992). CSD may be generated by insertion of the electrode itself (Verhaegen et al , 1992) and by the microdialysis probe (Tomida et al, 1989), but this would have affected both groups equally. In fact, no CSDs were recorded during the microdialysis probe implanta tion in any animals.…”

Calcitonin gene-related peptide reduces brain injury in a rat model of focal cerebral ischemia.

“…The spreading depression injury from electrode implantation can lower CBF by 30% to 70% in acute experiments, 22 but our use of small electrodes (0.125 mm in diameter) placed to an intradural depth of 1 mm minimizes this effect. 23 The technique only provides CBF data from a small volume of tissue around the electrode and does not provide a complete picture of MCA blood flow; however, the standardization of electrode placement allows repeatable comparisons of CBF within groups of experiments.…”