Altered hypothalamic monoamine metabolism and pituitary prolactin regulation in female spontaneously hypertensive rats

Differential Hypothalamic-pituitary-adrenal Response to Stress among Rat Strains: Methodological Considerations and Relevance for Neuropsychiatric Research

The hormones of the hypothalamic-pituitary-adrenal (HPA) axis, particularly glucocorticoids (GCs), play a critical role in the behavioral and physiological consequences of exposure to stress. For this reason, numerous studies have described differences in HPA function between different rodent strains/lines obtained by genetic selection of certain characteristics not directly related to the HPA axis. These studies have demonstrated a complex and poorly understood relationship between HPA function and certain relevant behavioral characteristics. The present review first remarks important methodological considerations regarding the evaluation and interpretation of resting and stress levels of HPA hormones. Then, it presents works in which differences in HPA function between Lewis and Fischer rats were explored as a model for how to approach other strain comparisons. After that, differences in the HPA axis between classical strain pairs (e.g. High and Low anxiety rats, Roman high- and low-avoidance, Wistar Kyoto versus Spontaneously Hypertensive or other strains, Flinder Sensitive and Flinder Resistant lines) are described. Finally, after discussing the relationship between HPA differences and relevant behavioral traits (anxiety-like and depression-like behavior and coping style), an example for main methodological and interpretative concerns and how to test strain differences is offered.

The activity of catecholamine synthesis in the hypothalamus of female normotensive Wistar Kyoto and spontaneously hypertensive rats

The activity of catecholamine synthesis in the hypothalamus, as determined by the rate of 3,4-dihydroxyphenylalanine (DOPA) accumulation after the administration of a DOPA decarboxylase inhibitor, was compared among Wistar, spontaneously hypertensive (SH), and genetically matched normotensive Wistar Kyoto (WKY) rats. DOPA accumulation in the median eminence, an index of the activity of tuberoinfundibular dopaminergic neurons, was greater in SH rats than Wistar and WKY rats while DOPA accumulation in the medial preoptic area was smaller in Wistar rats than SH and WKY rats. No strain difference was found in DOPA accumulation in the corpus striatum, which represents the activity of nigrostriatal dopaminergic neurons. These results suggest that there are differences in catecholamine synthesis in the hypothalamus not only between SH and WKY rats but also between WKY and Wistar rats.

Yohimbine-induced alterations of monoamine metabolism in the spontaneously hypertensive rat of the Okamoto strain (SHR). II. The central nervous system (CNS)

Steady state levels of monoamine neurotransmitters were examined in SHR, a genetic model of hypertension and compared to its normotensive control (WKY). SHR and WKY were also challenged with alpha 2-adrenergic antagonists, (yohimbine, YOH, idazoxan) or an alpha 1-antagonist (prazosin) and alterations in CNS monoamine metabolism evaluated. SHR were found to have elevated levels of NE and 5-HT in a number of brain regions involved in cardiovascular control when compared to WKY. DA levels and metabolism were also altered in the SHR. Blockade of alpha 2-adrenoceptors and other direct and indirect actions of YOH exacerbated the abnormalities in central monoaminergic neurotransmission in SHR. Significant decreases in NE content were produced by YOH or idazoxan treatment in both SHR and WKY, presumably the result of the inhibition of alpha 2-adrenoceptor medicated presynaptic control of NE release. YOH treatment abolished the differences in steady state levels of NE between SHR and WKY, however, idazoxan did not. YOH administration resulted in significant increases in DA and 5-HT in a number of brain regions of both SHR and WKY. Idazoxan or prazosin produced few changes in DA and 5-HT metabolism except for increases in DA content in the spinal cord and brainstem of SHR given idazoxan. The YOH-induced increases in DA and 5-HT content of SHR were of a greater magnitude than the WKY in several brain regions. DOPAC levels were significantly elevated by YOH in both WKY and SHR, reflecting the antidopaminergic properties of YOH. 5-HIAA content was significantly reduced by YOH in a number of brain regions in both SHR and WKY, however, this effect was attenuated in several brain regions in SHR. The results of the present study demonstrate the multifarious nature of the alterations in CNS monoamine metabolism in SHR.

Paracrine interactions in the anterior pituitary

The topographical affinity between certain cell types in rat anterior pituitary as well as the presence of biogenic amines, neuropeptides, growth and tissue factors in specific cell types suggest participation of paracrine control mechanisms in the regulation of anterior pituitary hormone secretion. Due to the recent advances in the separation of pituitary cell types and the development of three-dimensional cell cultures, direct experimental evidence for control by intercellular messengers has become available. The stimulation of PRL release from superfused pituitary cell aggregates by LHRH has been shown to be mediated by gonadotrophs. Gonadotrophs appear to secrete a factor with PRL-releasing activity. Gonadotrophs also modulate the stimulation of PRL release by angiotensin II. Interaction of somatotrophs with an unknown small-sized cell type strongly amplifies the GH response to adrenaline, GRF and VIP. The latter phenomenon requires the permissive action of glucocorticoids. Some of these in vitro observations can be correlated with recently reported in vivo actions of LHRH, PRL and angiotensin II and with pathophysiological changes in the pituitary.