Response of supraoptic magnocellular neurons to stimulation of forebrain alpha-adrenoceptors

Acute increases in arterial blood pressure do not reduce plasma vasopressin levels stimulated by angiotensin II or hyperosmolality in rats

The present study sought to determine whether an acute increase in arterial blood pressure (ABP) reduces plasma vasopressin (VP) levels stimulated by ANG II or hyperosmolality. During an intravenous infusion of ANG II (100 ng·kg−1·min−1), attenuation of the ANG II-evoked increase in ABP with diazoxide or minoxidil did not further enhance plasma VP levels in rats. When VP secretion was stimulated by an infusion of hypertonic saline, coinfusion of the α-adrenergic agonist phenylephrine (PE) significantly increased ABP but did not reduce plasma VP levels. In fact, plasma VP levels were enhanced. The enhancement of plasma VP levels cannot be explained by a direct stimulatory action of PE, as plasma VP levels of isosmotic rats did not change during a similar infusion of PE. An infusion of endothelin-1 in hyperosmotic rats significantly raised ABP but did not reduce plasma VP levels; rather, VP levels increased as observed with PE. In α-chloralose-anesthetized rats infused with hypertonic saline, inflation of an aortic cuff to increase ABP and stimulate arterial baroreceptors did not reduce plasma VP levels. In each experiment, plasma oxytocin levels paralleled plasma VP levels. Collectively, the present findings suggest that an acute increase in ABP does not inhibit VP secretion.

Presynaptic noradrenergic regulation of glutamate inputs to hypothalamic magnocellular neurones

Glutamate and norepinephrine transmitter systems play critical roles in the synaptic control of hypothalamic magnocellular neurones. We recently reported on a norepinephrine-sensitive glutamate circuit within the paraventricular nucleus (PVN) that projects to magnocellular neurones. Here, we present evidence for norepinephrine regulation of glutamate release in the PVN and supraoptic nucleus (SON) via actions on presynaptic terminals. Whole-cell synaptic currents were recorded in magnocellular neurones of the SON and PVN in an acute slice preparation. Bath application of norepinephrine (100 microm) caused a robust, reversible increase in the frequency of spontaneous glutamatergic excitatory postsynaptic currents in 100% of SON neurones (246%) and in 88% of PVN magnocellular neurones (259%). The norepinephrine-induced increase in glutamate release was mediated by activation of both presynaptic alpha1 receptors and alpha2 receptors, but the alpha1-receptor component was the predominant component of the response. The presynaptic actions of norepinephrine were predominantly, although not completely, resistant to blockade of Na-dependent spikes, implicating a presynaptic terminal locus of action. Interestingly, the spike-dependent component of the response was greater in PVN than in SON magnocellular neurones. This robust presynaptic facilitation of glutamate release by norepinephrine, combined with the known excitatory postsynaptic actions of norepinephrine, activational effects on local glutamate circuits, and inhibitory effects on gamma-aminobutyric acid release, indicate a strong excitatory role of norepinephrine in the regulation of oxytocin and vasopressin release during physiological stimulation.

Inhibition of supraoptic vasopressin neurones following systemic clonidine

Experiments were undertaken in urethane-anaesthetized rats to investigate the effects of systemic clonidine on the firing of supraoptic vasopressin (VP) neurones, which were identified by their characteristic phasic activity pattern. Injection of clonidine (50 micrograms/kg i.v.) reduced the firing of all 16 VP neurones tested, and their overall mean activity decreased from 5.09 +/- 1.01 to 1.63 +/- 0.64 spikes/second (P < 0.02). In VP cells which were already firing phasically before clonidine, the inhibition resulted in complete quiescence. In VP cells which were originally continuously active, the inhibition resulted in a switch to phasic activity. This inhibitory effect, which was prevented by prior injection of the alpha-2 antagonist idazoxan (0.5 mg/kg), had a mean duration of 11.8 +/- 1.8 min. Subsequent experiments revealed that i.v. clonidine (50 micrograms/kg) caused a transient rise in blood pressure, but this had a shorter time-course and was unlikely to account for the prolonged neuronal inhibition. It was concluded that systemic clonidine acts centrally to suppress the activity of hypothalamic VP neurones, thereby explaining the fall in plasma VP levels found in previous studies.

Paraventricular nucleus injections of noradrenaline: cardiovascular effects in conscious Long-Evans and Brattleboro rats

The cardiovascular effects of noradrenaline injected into the hypothalamic paraventricular nucleus (PVN) were investigated in conscious Long-Evans (control) rats and homozygous vasopressin (AVP)-deficient Brattleboro rats. Unilateral microinjection of noradrenaline (3-30 nmol) into the PVN produced dose-dependent increases in systolic and diastolic blood pressure of Long-Evans rats, and a concomitant decrease in heart rate. Only the highest dose of noradrenaline tested (30 nmol) caused a significant pressor response in Brattleboro rats (9 +/- 4/9 +/- 4 mm Hg, systolic/diastolic, n = 7) which was significantly smaller than the response produced by the same dose of noradrenaline in Long-Evans rats (32 +/- 8/27 +/- 6 mm Hg, n = 7). Intravenous pretreatment of Long-Evans rats with the V1-receptor antagonist, d(CH2)5Tyr[Et]DAVP, almost completely abolished the pressor effect of noradrenaline (10 nmol) without significantly attenuating the bradycardia. The alpha 2-adrenoceptor antagonist, idazoxan (4 nmol), injected into the PVN abolished the pressor response produced by noradrenaline (10 nmol) in Long-Evans rats but had no significant effect on the bradycardia. Pretreatment with the alpha 1-adrenoceptor antagonist, prazosin (0.7 nmol), significantly attenuated both the pressor and bradycardic effects of noradrenaline in Long-Evans rats. These results suggest that the pressor response produced by microinjection of noradrenaline into the hypothalamic PVN of conscious Long-Evans rats is mediated largely through stimulation of alpha 2-adrenoceptors and is dependent, in part, on release of AVP into the circulation. A component of the bradycardia seen with this intervention may be mediated through stimulation of alpha 1-adrenoceptors in the PVN.

Activation of supraoptic magnocellular neurons by gamma 2-melanocyte stimulating hormone (gamma 2-MSH)

The effects of intracarotid infusions of the peptide gamma 2-melanocyte stimulating hormone (gamma 2-MSH) on electrophysiologically and immunohistochemically identified supraoptic nucleus (SON) units were investigated. Over a wide dose range this agent always excited SON units, while control infusions of vehicle had no effect. Because neural responses invariably preceded blood pressure elevation, it appears that gamma 2-MSH excitation of the magnocellular system was due to a direct effect on the central nervous system and was not a result of systemic cardiovascular responses. These results suggest a forebrain gamma 2-MSH sensitive site in the activation of SON magnocellular neurons.

Neuronal plasticity in the hedgehog supraoptic nucleus during hibernation

The purpose of the present study was to identify processes of plasticity in the receptive field of neurosecretory neurons of the supraoptic nucleus during hibernation in the hedgehog, in order to correlate them with the increased neurosecretory activity observed in this nucleus during this annual period. Using the Rapid Golgi method, a quantitative study was conducted in the receptive field of bipolar and multipolar neurons (the main components of the nucleus). Results indicate a generalized increase in the following characteristics: (1) number of dendritic spines per millimeter along the dendritic shafts; (2) degree of branching in the dendritic field; and (3) dendritic density around the neuronal soma. These data demonstrate modification of the dendritic field in the supraoptic nucleus during hibernation, a change undoubtedly related to functional conditions. Since the observed changes affect structures such as dendritic spines which are directly related to the arrival of neural afferences, the discussion is centered on the types of stimuli which may be responsible for the observed processes.