The effect of partial noradrenergic denervation on corticosterone secretion in the rat

Systemic administration of leptin decreases plasma corticosterone levels: role of hypothalamic norepinephrine

Leptin, an adipocyte-derived hormone, is known to regulate a variety of neuroendocrine functions. It inhibits the hypothalamo-pituitary-adrenal axis (HPA) in several animal models, however, the exact mechanism by which it does so is not known. Since norepinephrine (NE) is a key regulator of the HPA axis, we hypothesized that leptin could suppress HPA activity by decreasing NE levels. To study this, we implanted adult male Sprague-Dawley rats with both a push-pull cannula in the paraventricular nucleus (PVN) and a catheter in the jugular vein. Animals were treated with either 0 or 100 microg or 500 microg of recombinant rat leptin (Lep). Push-pull perfusion was performed from 1000-1600 h. Perfusate samples were collected every 30 min and analyzed for NE levels using HPLC-EC. Blood samples were collected every 60 min and analyzed for corticosterone (CS) levels. To further understand the role of NE in this phenomenon animals were treated with either an alpha1-adrenergic agonist, phenylephrine (PHE; 0.5 mg/kg BW), an alpha2-adrenergic agonist, clonidine (CLON; 0.6 mg/kg BW), or a beta-adrenergic agonist, isoproterenol (ISO; 0.2 mg/kg BW) alone or in combination with 500 microg of Lep. Pre-treatment and hourly post-treatment blood samples were collected, plasma was separated and analyzed for CS levels. Leptin administration decreased NE release in the PVN significantly by 30 min (p<0.05). It also significantly reduced plasma CS levels at 240 and 300 min (p<0.05). Administration of either PHE or CLON in combination with leptin prevented the leptin-induced decrease in CS. In contrast, administration of ISO along with leptin did not prevent the leptin-induced decrease in CS. These results indicate that leptin decreases hypothalamic NE and plasma CS and that this effect is most probably mediated through alpha-adrenergic receptors.

Serotonin modulates the suppressive effects of corticosterone on proliferating progenitor cells in the dentate gyrus of the hippocampus in the adult rat

This series of experiments explores the interaction between corticosterone and serotonin (5-HT) in the regulation of cell proliferation in the dentate gyrus of the adult rat. Intracerebroventricular 5,7-DHT (5,7-dihydroxytryptamine) (either 200 or 300 microg) resulted in highly significant depletion of 5-HT as measured by high performance liquid chromatography in the frontal cortex but had no effect on the number of proliferating cells in the dentate gyrus by measuring 5-bromo-2'-deoxyuridine (BrdU) and Ki-67 cytochemistry. Treatment with PCPA (p-chlorophenylalanine: a tryptophan hydroxylase inhibitor: 300 mg/kg initially followed by 100 mg/kg/day) resulted in reduced proliferation as measured by Ki-67 after 3 days treatment, but not by BrdU uptake, and not after 14 days treatment by either method. In addition, injection of corticosterone (10-40 mg/kg/day) for 8 days significantly reduced proliferation in the dentate gyrus, as expected, measured by both BrdU uptake and Ki-67 immunostaining. Adrenalectomized (ADX) rats with a replacement subcutaneous pellet of corticosterone showed reduced proliferation when given additional corticosterone (10 mg/kg/day for 8 days), but this was prevented by 5-HT depletion (i.c.v. 5,7-DHT). Finally, a dose-response study showed that progressive doses of corticosterone (0-40 mg/kg/day) in ADX rats resulted in diminished suppression of proliferation in 5-HT-depleted compared with 5-HT-intact rats. These results strongly suggest that 5-HT regulates the sensitivity of proliferating cells in the dentate gyrus to corticosterone.

Decreased hippocampal noradrenaline does not affect corticosterone release following electrical stimulation of CA1 pyramidal cells

Bipolar electrodes were implanted into the CA1 pyramidal cells of the dorsal hippocampus and the effect of electrical stimulation of these cells on corticosterone secretion was investigated in freely moving rats. Histology showed that the electrodes were positioned in close proximity to the CA1 pyramidal cells. Rats that were subjected to high intensity electrical stimulation (1, 10, and 100 microA) behaved differently when compared to their sham stimulated controls. They were more active and displayed wet dog shakes. Plasma corticosterone levels increased dose-dependently in rats subjected to different electrical stimulation intensities. Although prior treatment (24 hours) of rats with DSP4 (60 mg/kg, i.p.) significantly reduced hippocampal noradrenaline content by 46%, it did not bring about any behavioural changes. DSP4 treatment also had no effect on electrically stimulated corticosterone release. These data suggested that stimulation of CA1 pyramidal cells may lead to increased corticosterone release and that a decrease in hippocampal noradrenaline concentration was unable to alter this corticosterone response.

Alpha 2- and beta-adrenergic stimulation of corticosterone secretion in rats

Bilateral injection of 6-hydroxydopamine into the medial forebrain bundle (MFB) significantly decreased monoamine concentrations in the hypothalamus. The noradrenaline and serotonin content of the paraventricular nucleus (PVN) was also significantly reduced. These drastic decreases in neurotransmitter concentration did not alter basal secretion of corticosterone. Isoproterenol, a beta-adrenoceptor agonist (1 mg/kg, i.p.), significantly stimulated corticosterone release in saline and MFB lesioned rats. This stimulation did not differ significantly between the two groups. Clonidine, an alpha 2-adrenoceptor agonist, injected either intraperitoneally or intracerebrally just dorsal to the PVN, caused a dose-dependent increase in corticosterone secretion. The stimulation of corticosterone release by clonidine (250 micrograms/kg, i.p.) was antagonised by the selective alpha 2-adrenoceptor antagonist, yohimbine (1 mg/kg, i.p.) and significantly reduced by the MFB lesion. These results suggest that corticosterone secretion is stimulated by activation of alpha 2-adrenoceptors which occur on noradrenergic nerve terminals in the PVN.

Oxaprotiline enantiomers stimulate ACTH and corticosterone secretion in the rat

The effect of oxaprotiline (OXA) enantiomers--of which (+)-OXA inhibits noradrenaline (NA) uptake, whereas (-)-OXA does not--on the secretion of adrenocorticotropin hormone (ACTH) and corticosterone was studied in rats. Both enantiomers dose-dependently and with a similar potency increased the plasma level of ACTH and corticosterone, the effect of (-)-OXA on corticosterone being of a longer duration. The stimulation of ACTH secretion and the inability of (+)- and (-)-OXA to increase the plasma corticosterone concentration in animals pretreated with dexamethasone indicate that secretion of the latter hormone results from the action of the enantiomers at a level superior to the adrenal cortex, i.e. the hypothalamus/pituitary. The corticosterone response to (+)- or (-)-OXA was not modified in rats with a selective lesion of NA nerve endings induced by the neurotoxin DSP-4, nor was it affected by the selective alpha 1-antagonist prazosin, the selective alpha 2-antagonist yohimbine, the mixed alpha 1/alpha 2-antagonist phentolamine, the selective dopamine (D2) receptor antagonist sulpiride and the non-selective 5-hydroxytryptamine (5-HT) receptor antagonist metergoline. These results indicate that neither the NA system nor D2 and 5-HT receptors are involved in the hormonal response to the OXA enantiomers. Although the (+)- and (-)-OXA-induced stimulation of corticosterone secretion was not antagonized by diazepam, ipsapirone, naloxone, or propranolol, it cannot be excluded that both these enantiomers act as non-specific stressors.