ANIMALS: Male Sprague-Dawley rats weighting 320±70 g were used. In the day of the experiment, rats were anesthesized under cloral hidrate anesthesia (450 mg/kg, i.p.) and received suplementary dosis of anesthesic (50 mg/kg, i.p.) for each hour.

MICRODIALYSIS AND TREATMENTS: The rats were implanted with microdialysis probes of a membrane length of 3mm and 2mm located, repectively, in the ventral hippocampus and the median raphe nucleus by using a stereotaxic instrument. Artificial cerebrospinal fluid (CSF) containing 3 micromolar paroxetine was perfused through the probe located in the hippocampus at a flow rate of 1 microliter/min. After three hours of stabilization, rats were injected with saline, paroxetine (10 mg/kg, i.p.) or a combined treatment with systemic paroxetine plus (-)-pindolol perfused continuosly through the probe located in the raphe nuclei at doses of 10 and 100 micromolar. The effect of treatments was monitorized for 4 hours following the application and dialysates were collected each 30 min.

MEASURES:The content of 5-HT in the dialysates was measured by HPLC with coulochemical detection.

Systemic paroxetine administration caused a maximum of 56% inhibition of the increase in extracellular 5-HT induced with the local aplication of paroxetine 3 microM (Table 1). When the percentage of total 5-HT released during the experiment was considered, the paroxetine-treated group showed a 43% lower 5-HT than animals treated with saline.

TABLE 1. Mean percentage (with respect to pretreatment levels) of 5-HT released from the hippocampus of the rat treated with paroxetine (10 mg/kg, i.p.) or paroxetine plus (-)-pindolol (10 and 100 micromolar) continuosly perfused in the median raphe. Dialysates were obtained in presence of 3 microM paroxetine perfused through the probe.

Saline     Paroxetine       Paroxetine                 Paroxetine
                         +pindolol 10 microM       +Pindolol 100 microM
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108.3±11.7   61.2±5.9         77.7±7.2                91.8±11.4
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The treatment with (-)-pindolol (10 and 100 micromolar) applied locally in the raphe nucleus reversed dose-dependently the paroxetine-induced inhibition of 5-HT release in ventral hippocampus. In fact, release of 5-HT in the hippocampus of rats receiving paroxetine and pindolol 10 micromolar was a 28% less than that of animals receiving saline; while the group of rats treated with pindolol 100 micromolar showed only a 15% of reduction in the percentage 5-HT release as compared to saline-treated rats.

The major finding of the present study is that local administration of (-)-pindolol 10 to 100 micromolar in the median raphe nucleus, reduced dose-dependently the decrease in extracellular 5-HT levels induced by the systemic treatment of paroxetine. These data agree with other works showing that pindolol coadministered by continuos s.c. infusion with an acute dose of SSRIs increased 5-HT levels over those obtained from rats treated with SSRIs alone (4). Similar responses were also reported following treatments that combinated systemic aplication of SSRIs and 5-HT_1A antagonists more selective that (-)-pindolol (3, 5).

Since samples were collected in presence of 3 micromolar paroxetine applied continuosly through the probe located in the hippocampus, it seems to be likely that the effect of (-)-pindolol on hippocampal extracellular 5-HT was due to its antagonistic properties on the somatodendritic 5-HT_1A receptors, thus restoring neuronal activity and enhancing 5-HT release in the projection areas.