The administration of a relatively high dose of antidepressant drugs produces an increased neuronal firing rate of the lateral septal nucleus (LSN) in the rat and a decreased immobility in rats forced to swim. However, it is unknown whether a minimally effective low-dose 21-day treatment with the selective serotonin reuptake inhibitor, fluoxetine, while reducing immobility in the forced swim test, also increases the neuronal firing rate of the LSN in Wistar rats. The total time of immobility decreased with a daily injection of 0.5, 1.0 or 2.0 mg/kg of fluoxetine (p < 0.001), and the lowest dose increasing the latency to the first immobility period (p < 0.0001) was 1.0 mg/kg. Therefore, the action of the 21-day fluoxetine treatment (1.0 mg/kg) on the firing rate of LSN neurones was tested in another group of rats. A total amount of 78 single-unit extracellular recordings was taken from the LSN of eight control rats (n = 40) and eight fluoxetine treated rats (n = 38). The LSN firing rate in the fluoxetine group was double (18.3 +/- 2.5 spikes per 10 s, p < 0.05) that in the control group (7.0 +/- 0.9 spikes per 10 s), and the first order interval of firing proved to be significantly lower in the fluoxetine group compared to the control group (384.3 +/- 22.3 and 639.7 +/- 27.5 ms, respectively; p < 0.05). In conclusion, the increased neuronal tiring rate of the LSN in the animals treated with a low dose of fluoxetine may be associated with an increased motivation to escape from the stressful situation that the forced swim represents.

Odors from amniotic fluid produce signs of calmness in mammals suggesting some anxiolytic-like properties. Experimental models, such as the defensive burying, elevated plus maze, and open field tests offer well-controlled approaches to the study of putative anxiolytic substances using rats. Using gas chromatography-mass spectrometry, we first identified eight fatty acids (lauric, myristic, palmitic, palmitoleic, stearic, oleic, elaidic, and linoleic acids) as consistently present in human amniotic fluid. We then used the defensive burying and elevated plus maze tests to compare the action of diazepam (2 mg/kg), fresh amniotic fluid, and a mixture of its fatty acids with two vehicles (i.e. propylene glycol and centrifuged amniotic fluid with a low fatty acid content). No significant differences in estradiol or progesterone content were found between fresh amniotic fluid and centrifuged amniotic fluid using the microparticle enzyme immunoassay. Compared with the vehicle, diazepam, fresh amniotic fluid, and the fatty acid mixture increased burying latency, reduced cumulative burying, and increased the time spent in the open arms of the elevated plus maze in both sexes without altering general locomotor activity. We conclude that the fatty acids contained in human amniotic fluid exert anxiolytic-like effects, with minimal or no participation of female gonadal steroids.

No abstract

Since allopregnanolone reduces the total time of immobility in rats submitted to the forced swimming test, we decided to explore whether this neuroactive steroid shares other antidepressant-like actions, such as increasing the neuronal firing rate in the lateral septal nucleus (LSN). In order to discard the influence of the oestrous cycle on immobility and on the firing rate of LSN neurons, all Wistar rats used in the study underwent ovariectomy before treatments. A group of rats received different doses of allopregnanolone (0.5, 1.0, 2.0 and 3.0 mg/kg, i.p.) 1 hour before being forced to swim in order to identify the minimum effective dose diminishing immobility. None of the tested doses of allopregnanolone produced significant changes in motor activity in the open-field test. The minimum dose of allopregnanolone producing a significant reduction in the total time of immobility (p<0.05) against the vehicle was 1.0 mg/kg, while 2.0 mg/kg and above also increased the latency to the first period of immobility (p<0.05). The minimum effective dose of allopregnanolone reducing immobility in the forced swimming test (1.0 mg/kg) significantly (p <0.05) produced a higher (twofold) neuronal firing rate in LSN neurons, but did not produce any change in septofimbrial nucleus neurons, which fired at a rate similar to that of vehicle-treated rats. The pretreatment with the non-competitive GABAA receptor antagonist, picrotoxin (1.0 mg/kg), blocked the aforementioned actions of allopregnanolone on both immobility and LSN firing rate. In conclusion, allopregnanolone produces an antidepressant-like effect in the forced swimming test, associated with an increase in the LSN neuronal firing rate, seemingly mediated by the GABAA receptor.