We investigated a role for cAMP/protein kinase A (PKA) in light/ glutamate (GLU)-stimulated state changes of the mammalian circadian clock in the suprachiasmatic nucleus (SCN). Nocturnal GLU treatment elevated [cAMP]; however, agonists of cAMP/ PKA did not mimic the effects of light/GLU. Coincident activation of cAMP/PKA enhanced GLU-stimulated state changes in early night but blocked light/GLU-induced state changes in the late night, whereas inhibition of cAMP/PKA reversed these effects. These responses are distinct from those mediated by mitogenactivated protein kinase (MAPK). MAPK inhibitors attenuated both GLU-induced state changes. Although GLU induced mPer1 mRNA in both early and late night, inhibition of PKA blocked this event only in early night, suggesting that cellular mechanisms regulating mPer1 are gated by the suprachiasmatic circadian clock. These data support a diametric gating role for cAMP/PKA in light/GLU-induced SCN state changes: cAMP/PKA promotes the effects of light/GLU in early night, but opposes them in late night.

We have examined the effect on respiration of the steroid hormone progesterone, administered either intravenously or directly into the medulla oblongata in anesthetized and paralyzed male and female cats. The carotid sinus and vagus nerves were cut, and end-tidal Pco2 and temperature were kept constant with servo-controllers. Phrenic nerve activity was used to quantitate central respiratory activity.Repeated doses of progesterone (from 0.1 to 2.0 ,ug/kg, cumulative) caused a sustained (>45 min) facilitation of phrenic nerve activity in female and male cats; however, the response was much more variable in females. Progesterone injected into the region of nucleus tractus solitarii, a respiratory-related area in the medulla oblongata, also caused a prolonged stimulation of respiration. Progesterone administration at high concentration by both routes also caused a substantial hypotension. Identical i.v. doses of other classes of steroid hormones (17f8-estradiol, testosterone, and cortisol) did not elicit the same respiratory effect. Pretreatment with RU 486, a progesterone-receptor antagonist, blocked the facilitatory effect of progesterone. We conclude that progesterone acts centrally through a steroid receptor-mediated mechanism to facilitate respiration.It has been known since the turn of the century that women hyperventilate during pregnancy (1, 2) and during the luteal phase of the menstrual cycle (3). Because the level of circulating progesterone correlates positively with increased ventilation, progesterone has been proposed as the ventilatory stimulant in these conditions (4)-a contention further supported by studies in which exogenously administered progesterone caused hyperventilation in normal male subjects (5, 6) and in patients with breathing disorders (7-12).Even though many animal species hyperventilate during pregnancy when endogenous progesterone levels are elevated (13-15), most fail to respond in a like manner to exogenously administered progesterone under experimental conditions (5,(15)(16)(17). Such data has been interpreted to mean that progesterone is not the respiratory stimulant during pregnancy and the luteal phase of the menstrual cycle. However, a reasonable explanation exists for this apparent discrepancy. Estrogen levels increase during pregnancy, and estrogen causes induction, or upregulation, of progesterone receptors within certain regions of the brain (18,19 Respiratory feedback from the arterial chemoreceptors and the pulmonary mechanoreceptors was blocked by cutting the carotid sinus and vagus nerves. A dorsal laminectomy was performed in one cat, and the spinal cord was transected at C7/T1 to eliminate input from visceral or somatic receptors. One phrenic nerve root (C5) was exposed, cut distally, desheathed, and placed in a bipolar platinum recording electrode that was built into a small acrylic platform placed in a tissue well adjacent to the nerve. Because the electrode had no fixed external attachments (the electrical wires were flexible and moved freely), it was poss...

SUMMARY1. Central respiratory response to acute (10 min) hypoxia, as measured by phrenic nerve activity, was determined in peripheral chemo-denervated cats.2. Hypoxia was induced by ventilating cats for 10 min at reduced inspired oxygen levels (inspired 02 fraction, FI°2 = 0-06-0-15). The degree of hypoxaemia was determined from an arterial blood sample and ranged from 'severe' (arterial 02 pressure, Pa,°< 26 Torr) to 'mild' (Pa, 02 > 35 Torr). The respiratory response was monitored for 1 h following return to ventilation with 100 % oxygen.3. The results confirmed the finding of prolonged ( > 60 min) inhibition of respiration upon return to hyperoxic conditions following severe hypoxia, as reported previously (Millhorn, Eldridge, Kiley & Waldrop, 1984). A new finding was a long-lasting ( > 60 min) facilitation of respiration following exposure to less severe (Pa, 02 > 35 Torr) hypoxia.4. Medullary extracellular fluid pH was measured in six cats. Changes in pH could not explain either the prolonged inhibition following severe hypoxia or the longlasting facilitation observed following mild hypoxia.5. Ablation studies were performed in order to determine the locations of the neuronal substrates for the inhibitory and facilitatory mechanisms. The results of this series of experiments indicate that the mesencephalon is necessary for activation of the inhibitory mechanism, while the facilitatory mechanism requires the presence of higher brain structures, notably the diencephalon.6. Following removal of the diencephalon, the inhibitory response was seen following even mild hypoxic insults, i.e. those shown to produce facilitation in animals with intact brains. In the absence of the mesencephalon, neither prolonged inhibition nor prolonged facilitation could be produced following hypoxia.7. It is proposed that there are two centrally mediated long-lasting responses to acute hypoxia. Facilitation is seen following mild hypoxia. Inhibition is more likely following severe hypoxia. However, both mechanisms appear to be triggered simultaneously and the output of the central respiratory controller reflects the influence of each.

No abstract

SUMMARY1. Previous work from this laboratory has indicated that the mesencephalon is the anatomical substrate for a mechanism capable of inhibiting central respiratory drive in glomectomized cats for periods of up to 1 h or more following brief exposure to systemic hypoxia; phrenic nerve activity was used as an index of central respiratory drive.2. The present study was undertaken to further localize the region responsible for the observed post-hypoxic inhibition of respiratory drive. We studied the phrenic nerve response to stimulations of the mesencephalon in anaesthetized, paralysed peripherally chemo-denervated cats with end-expired Pco2 and body temperature servo-controlled.3. Stimulations of two types were employed. Electrical stimulation allowed rapid determination of sites from which phrenic inhibition could be elicited. Microinjections of excitatory amino acids were used subsequently in order to confine excitation to neuronal cell bodies and not axons of passage.4. Stimulation of discrete regions of the ventromedial aspect of the mesencephalon in the vicinity of the red nucleus produced substantial inhibition of phrenic activity which lasted up to 45 min. Stimulation of other areas of the mesencephalon either produced no phrenic inhibition or resulted in a slight stimulation of phrenic activity.5. The results are discussed in the context of the central respiratory response to hypoxia.