Role of NMDA- and non-NMDA Subtypes of Glutamate Receptors in A5 Neuronal Structures in the Regulation of Respiration and Circulation during Thermal Nociceptive Stimulation in Rats

Integration of the Proprioceptive and Central Inspiratory Inhibitory Afferent Inputs by Pontine Noradrenergic A5 Neurons in Rats

Inhibitory afferent inputs to pontine A5 noradrenergic neurons (A5 NN) are not known, except partial baroreceptor input. In spontaneously breathing pentobarbital-anesthetized rats, we registered 35 A5 NN that were activated by hypoxia (100% N₂, 10 sec) by more than 5 times in comparison with the background. Cooling of retrotrapezoid nucleus (15°C, 6 sec) completely blocked the motor inspiratory output and A5 NN discharge frequency increased (23/23) by more than 7 times in comparison with the background values. The beginning of A5 NN activation coincided with cessation of inspiratory activity. Short-term passive stretching of the shin muscles (1 sec, 100 g) caused BP drop and complete inhibition of A5 NN (12/12) activated by hypoxia. Inhibitory afferent inputs from proprioceptors and central inspiratory neurons that can limit A5 NN activity were demonstrated.

Role of forward-motility-stimulating factor as an extracellular activator of soluble adenylyl cyclase

Forward-motility-stimulating factor (FMSF) is a protein, originally purified from bubaline serum, that promotes progressive motility of mature spermatozoa. FMSF binds to sperm surface receptors and activates transmembrane adenylyl cyclase (tmAC), causing a rise in intracellular cyclic AMP level ([cAMP]i) and subsequent activation of a protein kinase A/tyrosine kinase-mediated pathway that enhances forward motility. This article further evaluates how FMSF works in the caprine system, particularly identifying the stimulatory effect of this glycoprotein on soluble adenylyl cyclase (sAC). Elevated [cAMP]i, initially resulting from FMSF-dependent activation of tmAC, was associated with the release of Ca(2+) from an intracellular calcium store in the sperm head, likely via an inositol triphosphate-sensitive calcium ion channel. This peak Ca(2+) concentration of ∼125-175 nM was capable of stimulating sAC in vitro in a calmodulin-independent manner, thereby triggering more cAMP production. Our model proposes that a positive-feedback loop mediated by cAMP and Ca(2+) is established in FMSF-stimulated sperm, with cAMP playing the role of a chemical messenger at multiple steps, resulting in the observed progressive motility. Thus, FSMF stimulates a novel signaling cascade that synergistically activate both tmAC and sAC to achieve forward sperm motility.

NMDA Receptors of A5 Area in the Regulation of Blood Pressure and Respiratory Activity during Hypoxia in Rats

In narcotized (40 mg/kg sodium ethaminal intraperitoneally) spontaneously breathing albino rats, experimental short-term hypoxia induced BP drop and increased phrenic nerve firing rate. Unilateral microinjection of a selective NMDA receptor blocker ketamine hydrochloride (50 nl; 4 mM) into A5 area did not affect BP and phrenic nerve fi ring rate. However, against the background of preliminary NMDA receptor blockage, hypoxia more markedly stimulated rhythmic activity of the respiratory center and hypotensive response. Unilateral microinjection of a selective blocker of non-NMDA receptors GAMS (50 nl; 4 mM) into A5 area did not change initial BP and phrenic nerve fi ring rate. The studied parameters in rats exposed to hypoxia after blockage of non-NMDA receptors in A5 area did not differ from the control levels. Thus, activity of the respiratory center and BP in rats during hypoxia is regulated by the structures in A5 area and NMDA glutamate receptors.