Fast network oscillations in the hippocampus that range from approximately 150 to 250 Hz are termed ripples (O’Keefe & Nadel 1978; Buszáki et al. 1992). These ripples are thought to be due to simultaneous excitation of pyramidal cells and interneuronal networks and represent IPSPs on the somata of the pyramidal cells due to feedback from basket cells (Buszáki et al. 1992; Chrobak & Buszáki, 1996). However, Draguhn et al. (1998) hypothesized that gap junctions located between axons of principal neurones mediate ripples observed in vitro. High frequency oscillations have been recorded in the entorhinal cortex (EC) of both epileptic humans (Bragin et al. 1999) as well as epileptic and non-epileptic rodents (Chrobak & Buszáki, 1996; Bragin et al. 2002). Using slices from non-epileptic rats, Dhillon & Jones (2000) have demonstrated that neurones in layer III of the medial EC (mEC) exhibit electrical coupling, most likely mediated by gap junctions.
Combined entorhinal-hippocampal slices (~450 µm) were taken from adult (200-250g) Wistar rats after terminal anaesthesia using ketamine-xylazine (administered intramuscularly), and intracardial perfusion with artificial cerebrospinal fluid (ACSF) in which NaCl was replaced with sucrose. Slices were maintained at an interface of oxygenated ACSF and humidified gas (95 % O2 and 5 % CO2) at 36 °C
Using conventional extracellular recording techniques, we are now able to demonstrate ripples only in layer III of the mEC. The ripples had a mean (± S.E.M.) frequency of 192.5 ± 7.2 Hz and were approximately 100-200 µV in amplitude (n = 3). These ripples were observed in the absence of synaptic transmission (ACSF [Ca2+]o), they had a mean frequency of 215.5 ± 11.8 Hz (n = 5) and also displayed similar amplitude range to those observed in physiological ACSF. There was no significant difference in the frequency in either condition (P > 0.05, Student’s paired t test). In the presence of gap junction blockers such as octanol (500 µM) and carbenoxolone (100-200 µM), spontaneous ripples were significantly suppressed (P < 0.05, Student’s paired t test).
Thus, we can demonstrate high frequency activity in an in vitro slice preparation of the mEC, and in agreement with previous work these ripples are located in the only lamina of the mEC that exhibits electrical coupling, supporting the notion that gap junctions are crucial in the generation of these ripples.
This work was supported by the MRC and GSK plc.