No abstract

1 Unlike long-term potentiation, long-term depression (LTD) 4 Adenosine decreased the spike size in a concentration-dependent manner, but failed to induce LTD. 5 Alphaxalone and 5a-pregnan-3a-ol-20-one at concentrations that did not have any effect themselves on the population spike (0.5 and 1 gM), potentiated the inhibitory effect of muscimol on the population spike size, including concentrations which were not effective by themselves. Both steroids were able to potentiate the ability of muscimol to induce LTD. 6 Bicuculline, 5 gM, reversed the LTD induced by muscimol, 10 gM. 7 The NMDA receptor antagonist (±)-2-amino-5-phosphonopentanoic acid (2-AP5), the NMDA/ metabotropic antagonist 2-AP3 and selective metabotropic antagonist L-(+)-2-amino-3-phosphonopropionic acid (L(+)-AP3) failed to modify the LTD. Similarly, quisqualic acid and (IS, 3R)-aminocyclopentane dicarboxylic acid (ACPD) a selective agonist at metabotropic receptors did not induce LTD or short-term depression, whereas kynurenic acid prevented the reversal of the LTD obtained at 1 Hz. 8 It is concluded that LTD can be induced by the selective activation of GABAA receptors. The lack of involvement of glutamate receptors in our protocol confirms the unique nature of the LTD described here. The phenomenon of GABA-induced LTD and its reversal by 1 Hz stimulation may represent a novel type of long-lasting depression by which inhibitory interneurones can modulate pyramidal cell excitability in a frequency-dependent manner.

Four experiments were conducted to study the following: i) the influence of different concentrations of sucrose (0.15, 0.3 and 0.5 M with osmolality of 308, 500 and 760 mOsm/kg, respectively) in diluents and control diluent (370 mOsm/kg) on intensity of motility and progressive motility of goat sperm without rehydration and freezing step in four incubation periods (0, 0.5, 2 and 4 h after dilution); ii) the influence of gradual dilution (in 3 steps) on improvements in ascertained results of the first experiment; iii) cryoprotective effects of different concentrations of sucrose (0.15, 0.22, 0.29 and 0.37 M with osmolality of 450, 560, 740 and 920 mOsm/kg, respectively) plus 7% glycerol and 20% egg yolk in basic diluent (Tris-Citric acid-Fructose) and iv) the effect of two concentrations of sucrose (0.15 and 0.22 M) with and without glycerol (7%). In experiment 1, we obtained better results for control diluent, 0.15 and 0.3 M sucrose supplemented diluents with 0 and 0.5 h incubation periods. In experiment 2, apart from a slight improvement, similar tendencies to experiment 1 were observed. In experiment 3, we obtained the best result for diluent with 0.22 M sucrose with regard to intensity of motility, progressive motility, live sperm and normal acrosomes (40±4%, 3.1±0.2, 37±4% and 37±4%, repectively). In experiment 4, we obtained the best result for diluent with 0.22 M sucrose plus 7% glycerol in regard to intensity of motility, progressive motility and live sperm (39±3%, 3.6±0.4 and 41±4%, respectively). The characteristic normal acrosomes in diluents without glycerol, i.e. diluents with 0.15 and 0.22 M sucrose showed better results (39±8 and 42±6% respectively). With regard to the release of hyaluronidase enzyme there were no significant differences between diluents (p>0.05). The results of the diluents with 0.15 and 0.22 M sucrose without glycerol were slightly lower than those with glycerol (69±11 and 70±11 vs. 72±11 and 70±11 ×120×10 6 units ml -1 , respectively). In conclusion, the use of concentrated sucrose solutions showed that goat sperm can tolerate osmolality up to 560 mOsm (0.22 M) in the freezing period. In addition, glycerol proved to be a necessary cryoprotective agent in the cryopreservation of goat sperm, particularly for intensity of motility, progressive motility and live sperm.