Figure 2.
CB1 deficiency does not affect the formation of associative fear memories. A, Experimental procedure of the discriminatory fear-conditioning task. I, General timetable of the experiments. CB1−/− (n = 5) and CB1+/+ (n = 6) were chronically equipped with a recording electrode aimed at the CA1 region of the dorsal hippocampus. After a recovery period of 14 d, the optimal tone intensity for auditory-evoked potential recordings (∼40% of maximal response) was defined per animal and tone frequency (6 and 12 kHz), followed by measurement of the stability of the responses (test recording). II, Schematic representation of the procedures used in the experimental period. Conditioning was performed after 2 d of baseline recordings. At the conditioning day, a series of 20 50 ms tones of either 6 or 12 kHz coterminated with an electric footshock (flash) in the conditioning chamber. The conditioning procedure was repeated two times. Tone series of the frequency used for the conditioning procedures were termed paired tones, and those of the other frequency, which was not presented at the conditioning day, were termed neutral tones. Tone frequencies for the conditioning procedure were counterbalanced among the animals of a given genotype. During the two preconditioning and the two postconditioning days, paired and neutral tones were presented in the neutral test context. III, Time schedule for a recording day. Animals were slightly anesthetized with isoflurane to connect the electrode assembly to the recording hardware. After a 27 min recovery period, the video recorder was switched on. Three minutes later, two 90 tone series, one of 6 kHz and the other of 12 kHz, were presented at a 3 min interval. B, Schematic brain section according to Franklin and Paxinos (1997) showing the correct placement of the electrodes within the CA1 region of the dorsal hippocampus in CB1−/− (•) and CB1+/+ (○) C, D, Both amplitude (C) and slope (D) of the most-negative-going component of the auditory-evoked potentials were potentiated for the paired tone (•, ○) but not for the neutral tone (▪, □) after conditioning. Data (mean ± SEM) were normalized individually to the averaged preconditioning responses (dotted line, 100%). ▪ and •, CB1−/−; □ and ○, CB1+/+. Co, Conditioning day; CS+, paired tones; CSn, neutral tones; fEPSP, field EPSP.