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Somatostatin is a 14-28 amino acid peptide that is located not only in the gastrointestinal system but also in multiple sites of the human brain. The inhibitory effect of somatostatin on the growth hormone (GH) secretion of the pituitary gland is a well-established phenomenon. There is a general consensus that somatostatin is released into the hypophysial portal blood and modulates GH secretion by hormonal action. In the present study, we explored the possibility that in addition to the hormonal route, somatostatin may also influence GH secretion via influencing the growth hormone-releasing hormone (GHRH) secretion by direct contacts that may be functional synapses. Since the verification of these putative synapses by electron microscopy is virtually impossible in humans due to the long post mortem time, in order to reveal the putative somatostatinergic-GHRH juxtapositions, light microscopic double-label immunohistochemistry was utilized. By examining the slides with high magnification, we observed that the vast majority of the GHRH perikarya received contacting somatostatinergic axonal varicosities in the arcuate nucleus. In contrast, GHRH axonal varicosities rarely contacted somatostatinergic perikarya. The morphology and the abundance of somatostatin to GHRH juxtapositions indicate that these associations are functional synapses, and they represent, at least partially, the morphological substrate of the somatostatin-influenced GHRH secretion. Thus, in addition to influencing the GH secretion directly via the hypophysial portal system, somatostatin may also modulate GH release from the anterior pituitary by regulating the hypothalamic GHRH secretion via direct contacts. The rare GHRH to somatostatin juxtapositions indicate that the negative feedback effect of GH targets the somatostatinergic system directly and not via the GHRH system.

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Although it is a general consensus that opioids modulate growth, the mechanism of this phenomenon is largely unknown. Since endogenous opiates use the same receptor family as morphine, these peptides may be one of the key regulators of growth in humans by impacting growth hormone (GH) secretion, either directly, or indirectly, via growth hormone-releasing hormone (GHRH) release. However, the exact mechanism of this regulation has not been elucidated yet. In the present study we identified close juxtapositions between the enkephalinergic/endorphinergic/dynorphinergic axonal varicosities and GHRH-immunoreactive (IR) perikarya in the human hypothalamus. Due to the long post mortem period electron microscopy could not be utilized to detect the presence of synapses between the enkephalinergic/endorphinergic/dynorphinergic and GHRH neurons. Therefore, we used light microscopic double-label immunocytochemistry to identify putative juxtapositions between these systems. Our findings revealed that the majority of the GHRH-IR perikarya formed intimate associations with enkephalinergic axonal varicosities in the infundibular nucleus/median eminence, while endorphinergic-GHRH juxtapositions were much less frequent. In contrast, no significant dynorphinergic-GHRH associations were detected. The density of the abutting enkephalinergic fibers on the surface of the GHRH perikarya suggests that these juxtapositions may be functional synapses and may represent the morphological substrate of the impact of enkephalin on growth. The small number of GHRH neurons innervated by the endorphin and dynorphin systems indicates significant differences between the regulatory roles of endogenous opiates on growth in humans.

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Galanin released into the hypophysial portal circulation in the hypothalamus may function as a hypophysiotropic factor regulating the anterior pituitary function or it may function as a neurotransmitter/neuromodulator acting at synaptic sites regulating neuronal activity of many neurons in the brain. Catecholamines (adrenaline, noradrenaline, and dopamine) primarily regulate anterior pituitary functions indirectly via innervating hypophysiotropic neurons. The aim of the present studies was to explore with double-label immunocytochemistry if, as in rodents, catecholamines interact with galanin in the human diencephalon. Due to the long post-mortem period and subsequent lack of optimal preservation of the cell membranes in the brain, electron microscopy could not be employed to show the presence of catecholaminergic-immunoreactive synapses on galanin-immunoreactive neurons. Therefore, we used light microscopic immunocytochemistry and high-magnification microscopy with oil immersion to identify putative juxtapositions between catecholamines and galanin-utilizing antisera against key enzymes of catecholamine synthesis (tyrosine hydroxylase (TH), representing all three catecholamines; dopamine-beta-hydroxylase (DBH), representing noradrenaline; and phenylethanolamine-N-methyltransferase (PNMT), representing adrenaline) and galanin. Our studies show that among the three catecholamines, dopamine is the most abundant and the vast majority of catecholaminergic contacts on galanin-immunoreactive neurons is dopaminergic. The number of DBH-immunoreactive contacts is less and the number of PNMT-immunopositive contacts is negligible. Among the hypothalamic regions, the periventricular region above the infundibulum (infundibular or arcuate nucleus) contained the largest number of contacts. These en passant-type intimate associations between catecholamine- and galanin-immunoreactive neuronal elements may be functional synapses and may provide the morphological basis for the catecholamine-mediated galanin release.

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Previous studies have demonstrated that catecholaminergic, tyrosine hydroxylase (TH)-immunoreactive (IR) perikarya and fibers are widely distributed in the human hypothalamus. Since TH is the key and rate-limiting enzyme for catecholaminergic synthesis, these IR neurons may represent dopaminergic, noradrenergic or adrenergic neural elements. However, the distribution and morphology of these neurotransmitter systems in the human hypothalamus is not entirely known. Since the different catecholaminergic systems can be detected by identifying the neurons containing the specific key enzymes of catecholaminergic synthesis, in the present study we mapped the catecholaminergic elements in the human hypothalamus using immunohistochemistry against the catecholaminergic enzymes, TH, dopamine beta-hydroxylase (DBH) and phenylethanolamine-N-methyltransferase (PNMT). Only a few, PNMT-IR, adrenergic neuronal elements were found mainly in the infundibulum and the periventricular zone. DBH-IR structures were more widely distributed in the human hypothalamus occupying chiefly the infundibulum/infundibular nucleus, periventricular area, supraoptic and paraventricular nuclei. Dopaminergic elements were detected by utilizing double label immunohistochemistry. First, the DBH-IR elements were visualized; then the TH-IR structures, that lack DBH, were detected with a different chromogen. In our study, we conclude that all of the catecholaminergic perikarya and the majority of the catecholaminergic fibers represent dopaminergic neurons in the human hypothalamus. Due to the extremely small number of PNMT-IR, adrenergic structures in the human hypothalamus, the DBH-IR fibers represent almost exclusively noradrenergic neuronal processes. These findings suggest that the juxtapositions between the TH-IR and numerous peptidergic systems revealed by previous reports indicate mostly dopaminergic synapses.

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Galanin and neuropeptide Y (NPY) are among the most abundant neuropeptides in the hypothalamus. The role of NPY and galanin in the regulation of the secretory activity of the anterior pituitary has been well established. In addition, the two peptides interact with a number of neurons synthesizing the releasing and inhibiting hormones and a large number of other neuropeptides. The aim of the present studies was to explore if, as in rodents, NPY innervates galanin-immunoreactive (IR) neurons in the human diencephalon. Due to the long post mortem period and subsequent lack of optimal preservation of the cell membranes in the brain, electron microscopy could not be employed to show the presence of NPY-IR synapses on galanin-IR neurons. Therefore, we used light microscopic double label immunocytochemistry and high magnification microscopy with oil immersion to identify putative juxtapositions between NPY and galanin. Our studies show that similarly to rats, numerous NPY-IR nerve terminals surrounded galanin-IR neurons in the human hypothalamus. Among the hypothalamic regions, the infundibulum (infundibular or arcuate nucleus) contained the largest number of galanin-IR neurons heavily surrounded with NPY-IR nerve terminals. These en passant-type intimate associations between NPY-IR and galanin-IR neuronal elements may be functional synapses and may provide the morphological basis for the NPY-mediated galanin release. Consequently, NPY-galanin communication may mediate effects of NPY on neuronal systems innervated by galanin, and therefore may play a pivotal role in the regulation of reproduction, growth, energy and metabolism.

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