Materials
Drugs
The drugs used in the present study were lithium chloride (Merck, Germany), muscimol (Tocris, UK), bicuculline (Sigma, St. Louis, CA, USA). All drugs were dissolved in sterile 0.9% saline, just before the experiment, except for bicuculline that was dissolved in one drop of glacial acetic acid and made up to a volume of 2 mL with sterile 0.9% saline and then diluted to the required volume. Muscimol and bicuculline were injected into the CA1 regions of the dorsal hippocampi (intra-CA1) and lithium was injected intraperitoneally (IP). Control animals received either saline and/or vehicle.
Methods
Subjects
Male albino NMRI mice (Pasteur Institute; Tehran, Iran) weighing 20-30 g were used. The animals were housed 10 per a Plexiglas cage and maintained in a room with controlled light/dark cycle (12/12 h with light beginning at 7:00 a.m.) and temperature (22 ± 2 °C), with free access to food and water. They were allowed to adapt to the laboratory conditions for at least one week before surgery. The experiments were carried out during the light phase of the photoperiod. Ten animals were used in each group of experiments and each animal was used once only. In this research, 25 groups were examined. The research adhered to the Principles of Laboratory Animal Care (NIH publication #85−23, revised in 1985)
Surgery and cannulae implantation
Mice were anesthetized with intraperitoneal injection of ketamine/xylazine mixture (100 and 10 mg/kg, respectively), and placed in a stereotaxic frame (Stoelting Co. USA). The skin was incised and the skull was cleaned. Two 23-gauge guide cannulae were placed (bilaterally) 1 mm above the intended site of injection according to the atlas of Paxinos and Franklin (2001). Stereotaxic coordinates for the CA1 regions of the dorsal hippocampus were AP: -2 mm from bregma, L: ±1.6 from the midline, and V: -1.5 mm from the skull surface. The guide cannulae were anchored to the skull by a jeweler’s screw and dental cement, and then two stainless steel stylets (30-gauge) were inserted into the guide cannulae to keep them patent prior to injections. All animals were allowed one week to recover from the surgery.
Intra-hippocampal microinjections
For drug infusions, the animals were gently restrained by a hand; the stylets were removed from the guide cannulae and a 30-gauge injection needle was inserted (1 mm beyond the tip of the guide cannula). The injection solutions were administered in a total volume of 1 μL/mouse (0.5 μL/each side) over a 60 sec period followed by an additional 60 sec to facilitate the diffusion of the drugs from the tip of the guide cannula.
Apparatus
The inhibitory avoidance apparatus consisted of a wooden box (30×30×40 cm height). The floor of which was consisted of stainless steel rods (29 parallel rods, 0.3 cm in diameter, set 1 cm apart). A wooden platform (4×4×4 cm) was set in the center of the grid floor. Intermittent electric shocks (1 Hz, 0.5 sec, 40 V DC) were delivered to the grid floor by an isolated stimulator (Grass S44, USA).
Inhibitory avoidance task
A single-trial step-down inhibitory avoidance task was used. Each mouse was gently placed on the wooden platform. When the mouse stepped down from the platform and placed all its four paws on the grid floor, the electric shock was delivered continuously for 15 sec. This training procedure was carried out between 9:00 and 13:00 h. Each mouse was placed on the platform again at 24 h after training and the step-down latency was recorded with a stopwatch as inhibitory avoidance memory. An upper cut-off time of 300 sec was set. The retention test was also carried out between 9:00 and 13:00 h.
Drug treatment
Ten animals were used in each experimental group. In the experiments where animals received two injections, the control groups also received two saline or vehicle injections. The drug doses for lithium and GABA
A agents were selected according to pilot experiments and our previous studies (
32-
37).
Experiment 1
In this experiment effects of post-training administration of lithium on inhibitory avoidance memory, and then effects of pre-test administration of lithium on memory impairment induced by lithium (10 mg/kg) given after training were examined. Four groups of Animals were used in this experiment. One group of animals received post-training and pre-test saline administration. The other groups received immediate post-training lithium (10 mg/kg), and on the test day they received saline or different doses of lithium (5 or 10 mg/kg, IP) 45 min prior to the test.
Experiment 2
This experiment examined effect of intra-CA1 administration of GABAA receptor antagonist, bicuculline, on memory retrieval by the lower dose of lithium. Nine groups of animals were used in this experiment. One group received injections of saline (10 mL/kg) both post-training and pre-test. The other eight groups of animals received lithium (10 mg/kg) after training and divided in two sets with four groups. On the test day, the animals of the first set received saline (10 mL/kg) 45 min before testing plus intra-CA1 injections of vehicle or bicuculline (0.062, 0.125 or 0.25 μg/mouse) five min before testing. The animals of the second set received LiCl (5 mg/kg) 45 min before testing plus intra-CA1 injections of vehicle or bicuculline (0.062, 0.125 or 0.25 μg/mouse) five min prior to the test.
Experiment 3
This experiment examined effect of intra-CA1 administration of GABAA receptor agonist, muscimol, on memory retrieval by the lithium. Twelve groups of animals were used in this experiment. One group received injections of saline (10 mL/kg) both post-training and pre-test. Three groups of the animals received post-training saline, and on the test day they received muscimol (0.01, 0.03 or 0.06 μg/mouse, intra-CA1) five min prior to testing. The other eight groups of animals were received post-training lithium (10 mg/kg) and divided in two sets with four groups. On the test day, the animals of the first set received saline (10 mL/kg) 45 min before testing plus intra-CA1 injections of saline or muscimol (0.01, 0.03 or 0.06 μg/mouse) five min prior to testing. The animals of the second set received LiCl (10 mg/kg) 45 min before testing plus intra-CA1 injections of saline or muscimol (0.01, 0.03 or 0.06 μg/mouse) five min before to the test.
Data analysis
Because of the individual variations, the data were analyzed by one-way analysis of variance (ANOVA) for non-parametrical data (Kruskal-Wallis) followed by a two-tailed Mann-Whitney’s U-test. Then, Holmes Sequential Bonferroni correction was done for paired comparisons. The step-down latencies (as an index of inhibitory avoidance memory) for each experimental group were expressed as the median ± quartiles. In all statistical evaluations, p < 0.05 was used as the criterion for statistical significance.