Somatostatin-like immunoreactivity in the region of the prepacemaker nucleus in weakly electric knifefish,...

The diencephalic prepacemaker nucleus (PPn) is a bilateral cluster of neurons that controls frequency modulations of the otherwise very regular electric organ discharge cycle in weakly electric knifefish, Eigenmannia. Ultrastructural evidence had suggested that the action of excitatory and inhibitory synapses contacting PPn neurons is modulated by neuropeptides. In this investigation, we examined the distribution of somatostatin-like immunoreactive (S-IR) structures in the region of the PPn. In transverse sections, we found 5 bilateral S-IR structures in the region of the PPn: a 'dorsohorizontal stripe'; a 'lateral cell group'; a 'diagonal stripe'; a 'tubercular stripe'; and a 'hypothalamic stripe'. The dorsohorizontal stripe, consisting of S-IR fibers, terminals, and roughly 200 cell bodies unilaterally, stretches from the edge of the third ventricle at the thalamic dorsal-posterior nucleus and the central posterior nucleus approximately 400 microns laterally to the PPn. S-IR cell bodies show a distinct pattern of distribution within this stripe. Almost half of the somata are located within 50 microns of the ventricle. These ventricular cells are small and often densely clustered. Laterally, towards the PPn, the number of labelled cells decreases, whereas their size gradually increases. The lateral cell group consists of roughly 20 somata in the medial region of the subelectrosensorius nucleus. Fibers of the diagonal stripe travel from the hypothalamus dorsalis to the PPn. Fibers of the tubercular stripe originate from S-IR cell bodies in the medial zone of the periventricular nucleus of the posterior tuberculum and merge with the diagonal stripe. Fibers of the hypothalamic stripe connect the hypothalamus lateralis and the diagonal stripe. The density of immunolabelling in the hypothalamic stripe is significantly higher in mature than in immature females.