Stimulatory effects of bombesin-like peptides on hypothalamic arcuate neurons in rat brain slices

Effects of central gastrin-releasing peptide on glucose metabolism

Gastrin-releasing peptide (GRP) mediated signals in the central nervous system (CNS) influence many functions associated with energy metabolism. The purpose of the present study was to investigate the central effect of GRP on glucose metabolism in the male rat. Intracerebroventricular (icv) administration of GRP caused an immediate hyperglycaemia which was sustained till the end of the infusion. The rise in plasma glucose levels was accompanied by an increase in endogenous glucose production (EGP), as well as increases in plasma glucagon and insulin concentrations. Furthermore, no differences in plasma corticosterone levels were noted between control and GRP treated rats. These results demonstrate that central GRP increases plasma glucose levels, probably by stimulating pancreatic glucagon release and concomitantly or subsequently endogenous glucose production.

The suprachiasmatic nucleus changes the daily activity of the arcuate nucleus α-MSH neurons in male rats

Timing of metabolic processes is crucial for balanced physiology; many studies have shown the deleterious effects of untimely food intake. The basis for this might be an interaction between the arcuate nucleus (ARC) as the main integration site for metabolic information and the suprachiasmatic nucleus (SCN) as the master clock. Here we show in male rats that the SCN influences ARC daily neuronal activity by imposing a daily rhythm on the α-MSH neurons with a peak in neuronal activity at the end of the dark phase. Bilateral SCN lesions showed a complete disappearance of ARC neuronal rhythms and unilateral SCN lesions showed a decreased activation in the ARC at the lesioned side. Moreover light exposure during the dark phase inhibited ARC and α-MSH neuronal activity. The daily inhibition of ARC neuronal activity occurred in light-dark conditions as well as in dark-dark conditions, demonstrating the inhibitory effect to be mediated by increased SCN (subjective) day neuronal activity. Injections into the SCN with the neuronal tracer cholera toxin B showed that α-MSH neurons receive direct projections from the SCN. The present study demonstrates that the SCN activates and possibly also inhibits depending on the moment of the circadian cycle ARC α-MSH neurons via direct neuronal input. The persistence of these activity patterns in fasted animals demonstrates that this SCN-ARC interaction is not necessarily satiety associated but may support physiological functions associated with changes in the sleep-wake cycle.

Neuromedin B and gastrin-releasing peptide excite arcuate nucleus neuropeptide Y neurons in a novel transgenic mouse expressing strong Renilla green fluorescent protein in NPY neurons

Neuropeptide Y (NPY) is one of the most widespread neuropeptides in the brain. Transgenic mice were generated that expressed bright Renilla green fluorescent protein (GFP) in most or all of the known NPY cells in the brain, which otherwise were not identifiable. GFP expression in NPY cells was confirmed with immunocytochemistry and single-cell reverse transcription-PCR. NPY neurons in the hypothalamic arcuate nucleus play an important role in energy homeostasis and endocrine control. Whole-cell patch clamp recording was used to study identified arcuate NPY cells. Primary agents that regulate energy balance include melanocortin receptor agonists, AgRP, and cannabinoids; none of these substances substantially influenced electrical properties of NPY neurons. In striking contrast, neuropeptides of the bombesin family, including gastrin-releasing peptide and neuromedin B, which are found in axons in the mediobasal hypothalamus and may also be released from the gut to signal the brain, showed strong direct excitatory actions at nanomolar levels on the NPY neurons, stronger than the actions of ghrelin and hypocretin/orexin. Bombesin-related peptides reduced input resistance and depolarized the membrane potential. The depolarization was attenuated by several factors: substitution of choline for sodium, extracellular Ni(2+), inclusion of BAPTA in the pipette, KB-R7943, and SKF96365. Reduced extracellular calcium enhanced the current, which reversed around -20 mV. Together, these data suggest two mechanisms, activation of nonselective cation channels and the sodium/calcium exchanger. Since both NPY and POMC neurons, which we also studied, are similarly directly excited by bombesin-like peptides, the peptides may function to initiate broad activation, rather than the cell-type selective activation or inhibition reported for many other compounds that modulate energy homeostasis.

The distribution of the orphan bombesin receptor subtype-3 in the rat CNS

Bombesin receptor subtype 3 (BRS-3) is an orphan G-protein coupled receptor that shares between 47 and 51% homology with other known bombesin receptors. The natural ligand for BRS-3 is currently unknown and little is known about the mechanisms regulating BRS-3 gene expression. Unlike other mammalian bombesin receptors that have been shown to be predominantly expressed in the CNS and gastrointestinal tract, expression of the BRS-3 receptor in the rat brain has previously not been observed. To gain further understanding of the biology of BRS-3, we have studied the distribution of BRS-3 mRNA and protein in the rat CNS. The mRNA expression pattern was studied using reverse transcription followed by quantitative polymerase chain reaction. Using immunohistological techniques, the distribution of BRS-3 protein in the rat brain was investigated using a rabbit affinity-purified polyclonal antiserum raised against an N-terminal peptide. The BRS-3 receptor was found to be widely expressed in the rat brain at both mRNA and protein levels. Particularly strong immunosignals were observed in the cerebral cortex, hippocampal formation, hypothalamus and thalamus. Other regions of the brain such as the basal ganglia, midbrain and reticular formation were also immunopositive for BRS-3. In conclusion, our neuroanatomical data provide evidence that BRS-3 is as widely expressed in the rat brain as other bombesin-like peptide receptors and suggest that this receptor may also have important roles in the CNS, mediating the functions of a so far unidentified ligand.

Dopamine as a prolactin (PRL) inhibitor

Dopamine is a small and relatively simple molecule that fulfills diverse functions. Within the brain, it acts as a classical neurotransmitter whose attenuation or overactivity can result in disorders such as Parkinson's disease and schizophrenia. Major advances in the cloning and characterization of biosynthetic enzymes, transporters, and receptors have increased our knowledge regarding the metabolism, release, reuptake, and mechanism of action of dopamine. Dopamine reaches the pituitary via hypophysial portal blood from several hypothalamic nerve tracts that are regulated by PRL itself, estrogens, and several neuropeptides and neurotransmitters. Dopamine binds to type-2 dopamine receptors that are functionally linked to membrane channels and G proteins and suppresses the high intrinsic secretory activity of the pituitary lactotrophs. In addition to inhibiting PRL release by controlling calcium fluxes, dopamine activates several interacting intracellular signaling pathways and suppresses PRL gene expression and lactotroph proliferation. Thus, PRL homeostasis should be viewed in the context of a fine balance between the action of dopamine as an inhibitor and the many hypothalamic, systemic, and local factors acting as stimulators, none of which has yet emerged as a primary PRL releasing factor. The generation of transgenic animals with overexpressed or mutated genes expanded our understanding of dopamine-PRL interactions and the physiological consequences of their perturbations. PRL release in humans, which differs in many respects from that in laboratory animals, is affected by several drugs used in clinical practice. Hyperprolactinemia is a major neuroendocrine-related cause of reproductive disturbances in both men and women. The treatment of hyperprolactinemia has greatly benefited from the generation of progressively more effective and selective dopaminergic drugs.