Abstracts

Rationale: Exposure to a clinically relevant triple cocktail of general anesthetics (GAs), such as isoflurane, nitrous oxide, and midazolam during synaptogenesis alters brain structures including the thalamus. This study has shown that a single exposure to GAs at P7 leads to alterations in the action potential firing properties of the nucleus reticularis thalami (nRT). The nRT is a major mediator of burst and tonic AP firing within thalamocortical networks. Thus, it was decided to investigate how GAs affect thalamocortical circuits in vivo by using electroencephalogram (EEG) in combination with Gamma Butyrolactone (GBL), a well characterized absence seizure inducing agent (Snead.,1991). The objective of this study was to understand changes in thalamocortical EEG as a result of exposure to GAs during synaptogenesis. Methods: P7 Sprague Dawley (SD) rats were first given an intraperitoneal (i.p.) injection of 9 mg/Kg midazolam, and then subjected to a mixture of 80 % N2O and 0.75 % isoflurane for 6 hours. Sham rats were treated to a mock i.p. injection consisting of 0.1% DMSO/H2O, and separated from their mother for 6 hours. Hyperpolarization induced AP firing patterns were characterized using live brain slices from P9-18 rats. EEG recordings were obtained from the somatosensory cortex with bilateral stainless steel electrodes accompanied by an operational amplifier (Texas Instruments TL2274x). Reference electrodes were placed within the cerebellum (Zaman et al., 2011). Operational amplifiers were surgically implanted utilizing isoflurane anesthesia during P19. Electrode placements utilized the following coordinates: from bregma (mm) -2.2 rostral caudal, 5.5 medial lateral, 1.5 dorsal ventral. EEG recordings coupled with video were acquired from p19 to p25. Recordings were obtained utilizing tethered wires coupled to a freely moving commutator. EEG was analyzed utilizing Harmonie Stellate software. GBL (70mg/Kg) was administered i.p. 4 days post implant.Results: Hyperpolarization induced rebound AP frequencies were found to be significantly elevated in GAs versus sham nRT neurons (figure 1). In terms of EEG, the average GBL induced EEG event duration was found to be increased for GAs 4.813 0.08 sec (N=196 events) versus that of Sham 3.955 0.1213 sec (N=111 events p< 0.01 Student s t-test). The average total GBL event duration was also significantly increased for GAs 1267 143 sec (N=6) as opposed to Sham 733 86 sec. (N=6 p< 0.0001 student s T-test). Conclusions: These findings suggest that a single exposure to clinically relevant GAs leads to a lasting increase in intrinsic cellular excitability of nRT neurons. This in turn causes alterations in GBL mediated EEG of thalamocortical neurons. Changes in thalamocortical neuron function may lead to altered cognition and/or seizure. Future experiments will be performed in order to understand alterations in ion channel properties that underlie nRT rebound AP firing such as T-type Calcium channels.