Adenosine 3',5'-cyclic monophosphate stimulates dopamine biosynthesis in the median eminence of rat hypothalamic slices

Salsolinol induces a decrease in cyclic AMP at the median eminence and an increase at the adenohypophysis in lactating rats

Investigating the cellular events in the pituitary gland, the intracellular cyclic AMP (cAMP) of the median eminence (ME), neuro-intermediate lobe (NIL) and the anterior lobe (AL) have been measured following 15-min of intravenous injection of salsolinol (SAL). Parallel to the elevation of plasma prolactin (PRL), SAL induced a significant decrease of cAMP concentration in the ME. In contrast, SAL injection resulted in a significant increase of cAMP at the level of the AL. Changes in cAMP of the NIL as well as in the plasma level of vasopressin (VP) could not be detected. The observed changes in the level of cAMP following the acute treatment of SAL in the ME and the AL seems to be related to interacting neuroendocrine signals delivered from the ME to the AL through the long portal vessels to release PRL.

Presynaptic H3 autoreceptors modulate histamine synthesis through cAMP pathway

Histamine H3 receptors modulate histamine synthesis, although little is known about the transduction mechanisms involved. To investigate this issue, we have used a preparation of rat brain cortical miniprisms in which histamine synthesis can be modulated by depolarization and by H3 receptor ligands. When the miniprisms were incubated in presence of forskolin, dibutyryl-cAMP, or 3-isobutyl-1-methylxanthine (IBMX), histamine synthesis was stimulated in 34, 29, and 47%, respectively. These stimulations could be prevented by the selective cAMP protein kinase blocker Rp-adenosine 3',5'-cyclic monophosphothioate triethylamine (Rp-cAMPs). Preincubation with the H3 receptor agonist imetit prevented IBMX- (100% blockade) and forskolin- (70% blockade) induced stimulation of histamine synthesis. The H3 inverse agonist thioperamide enhanced histamine synthesis in the presence of 1 mM IBMX or 30 mM potassium (+47 and +45%, respectively). Similarly, the H3 antagonist clobenpropit enhanced histamine synthesis in the presence of 30 mM potassium (+ 59%). The cAMP-dependent protein kinase blockers Rp-cAMPs and PKI14-22 could impair the effects of thioperamide and clobenpropit, respectively. These results indicate that the adenylate cyclase-protein kinase A pathway is involved in the modulation of histamine synthesis by H3 autoreceptors present in histaminergic nerve terminals.

Activation of cultured rat hypothalamic dopaminergic neurons by long-term but not short-term treatment with prolactin

Pituitary prolactin (PRL) secretion is inhibited by hypothalamic GABAergic and dopaminergic (DAergic) systems. PRL, in turn, appears to be capable of activating these neurons, thus, providing for a negative feedback regulation. We have recently shown that cultured hypothalamic GABAergic- but not DAergic neurons respond to PRL with a rapid increase in intracellular free calcium. Here, we demonstrate that cultured hypothalamic DAergic neurons can be activated in terms of synthesis of dihydroxyphenylalanine (DOPA) by long-term PRL treatment. Short-term PRL treatment was ineffective. It is concluded that hypothalamic DAergic neurons are indeed capable of responding to PRL. However, their response differs from that of GABAergic neurons with respect to time scale and signal transduction. We suggest that the two types of hypothalamic cells are involved in separate feedback loops that provide for tonic and rapid regulation of pituitary PRL secretion, respectively.

Effect of aging on in vitro dopamine biosynthesis in the median eminence of rat hypothalamic slices

The rates of basal and cyclic AMP-dependent DOPA accumulation in the median eminence of hypothalamic slices were not different between ovariectomized young and aged rats. However, the rate of Ca2+-dependent, depolarization-induced DOPA accumulation was smaller in aged rats, suggesting that the Ca2+ system in the regulation of dopamine biosynthesis in tuberoinfundibular dopaminergic neurons is altered by aging.