Role of isotocin in regulation of the hypophysiotropic dopamine neurons in the preoptic area of the catfish, Clarias batrachus

Abstract

Dopamine (DA) has emerged as the potent inhibitory neuromodulator of LH secretion and reproduction in teleosts. The DA neurons located in the anterior subdivision of nucleus preopticus periventricularis (NPPa) in the preoptic area (POA) innervate the pituitary gland and regulate the LH cells. Although reduction in the inhibitory DAergic tone is crucial for stimulatory action of GnRH on LH cells, the role of other hypothalamic factors is suggested but not fully understood. Nonapeptide, isotocin (IST) has emerged as a likely candidate that may also influence the LH cell function. IST neurons reside in the nucleus preopticus and innervate LH cells. While IST treatment dramatically elevated LH secretion, the IST levels in brain peaked during spawning. In a pilot study on the catfish, Clarias batrachus, we observed a dense network of IST-immunoreactive (IST-ir) fibers in NPPa, the region known to harbor hypophysiotropic DA neurons. Application of double immunofluorescence method showed dense IST-ir fiber network around the tyrosine hydroxylase (TH)-ir neurons in NPPa region. A great majority of the TH-ir neurons in NPPa were contacted by IST-ir fibers during spawning phase. The NPPa therefore seems to be a site for intense interaction of DA and IST. The IST-ir fiber innervation in NPPa showed reproduction phase-dependent changes. The percent fluorescent area of IST-ir fibers showed gradual increase from the resting through prespawning phases (Resting: 7.5 ± 1.04; Preparatory: 8.6 ± 0.8; Prespawning: 15.5 ± 1.4) reaching a peak in spawning phase (28 ± 2.3; P<0.001). Compared to spawning phase, a drastic reduction in IST-ir fibers in the NPPa was observed during the postspawning phase (8.4 ± 0.9; P<0.001). Superfused slices of the POA of C. batrachus treated with IST peptide resulted in a significant reduction in TH-immunoreactivity in the NPPa [Control: 45.3 ± 4.2; IST peptide (5 μM): 29.4 ± 4.7; P<0.05]. We suggest that the intense interaction between IST and DA in the NPPa, most probably of the inhibitory nature, may be critical to the regulation of LH cells and reproduction in teleosts.

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