Systemic inhibition of the Na(+)/H (+) exchanger type 3 in intact rats activates brainstem respiratory regions

Opposing effects on the phrenic motor pathway attributed to dopamine-D1 and -D3/D2 receptor activation

Previous in vivo studies revealed that dopamine-D1-agonists elevate excitability of ventral respiratory column (VRC) neurons and increase discharge activity in the phrenic motor output through actions in the brainstem. In this in vivo study performed on pentobarbital-anesthetized cats, we show that D1-agonists (SKF-38393, dihydrexidine) given intravenously enhanced discharge activity in VRC inspiratory neurons and the phrenic nerve in two stages; discharge intensity first increased to a peak and then discharge duration increased. Cross-correlation analysis of VRC inspiratory neuron and phrenic nerve discharges showed that both stages increased strength of coupling between medullary inspiratory neurons and the phrenic motoneuron output. Intracellular recording and microiontophoresis experiments indicated that D1-agonists produced their stimulatory effects indirectly through actions on synaptic inputs to VRC inspiratory neurons. Because other laboratories have provided evidence that dopamine acting on other types of receptors depresses respiratory neuron excitability we tested the effects of piribedil, an agonist that activates receptors of the generally depressant D3/D2-dopamine receptor family, on phrenic nerve activity. Piribedil depressed phrenic nerve inspiratory discharge intensity, prolonged discharge duration, slowed burst frequency and slowed rate of action potential augmentation. The effects of piribedil were partially counteracted by intravenous injection of dihydrexidine. We propose that under normal, steady state conditions, D1-receptor-mediated excitatory modulation of phrenic motor output overrides D3/D2-receptor mediated inhibition.

Interactions between postnatal sustained hypoxia and intermittent hypoxia in the adulthood to alter brainstem structures and respiratory function

Neural plasticity is defined as a persistent change in the morphology and/or function based on prior experiences. Plasticity is well evident when the triggering experience occurs early in life, but in the case of respiratory control plasticity, it also can be triggered in adult life. We have combined a 10 days postnatal hypoxic (PH) (0-10 days of age;11% O(2)) and a 15 days intermittent hypoxia (IH) exposures in the adulthood (90-105 days of age; 5% O(2), 40 s/20% O(2), 80 s; 8 h/day) to test if early PH interacts with IH of the adulthood to generate detrimental plastic changes. After recording of ventilatory parameters, the brains were studied immunocytochemically for localization of the organization pattern of non-phosphorylated subunit of neurofilament H (NFH) and tyrosine hydroxylase (TH) expression in the nucleus tractus solitarius (Sol) and caudal (CVL) and rostral ventrolateral reticular (RVL) nuclei, areas related to central cardio-respiratory regulation. In comparison to control, PH male rats (but not females) at 1 month of age hyperventilated at rest, in response to moderate hypoxia (12% O(2)) and 5% CO(2), the effect being due to increased tidal volume. At 3.5 months sex differences in ventilation disappeared and it was indistinguishable between control and PH. IH tended to decrease ventilation in both control (C) and PH animals. PH augmented PENH values in air and in hypoxic conditions when compared with C group. IH in both groups, tended to decrease the PENH value, being statistically different in PH+IH. Results also show an increment of disorganization of NFH-positive labeled structures at the level of Sol and CVL/RVL nuclei in PH, IH and HP+HI groups. PH rats showed differences in the number of TH-positive neurons at the level of CVL/RVL nuclei, which was increased in the PH and PH+IH groups with respect to C one. In conclusion, PH alters the central morpho-physiological organization and the catecholaminergic components of cardio-respiratory nuclei, whose effects were enhanced after a period of IH in the adulthood.

Switching control of sympathetic activity from forebrain to hindbrain in chronic dehydration

We investigated the mechanisms responsible for increased blood pressure and sympathetic nerve activity (SNA) caused by 2-3 days dehydration (DH) both in vivo and in situ preparations. In euhydrated (EH) rats, systemic application of the AT(1) receptor antagonist Losartan and subsequent pre-collicular transection (to remove the hypothalamus) significantly reduced thoracic (t)SNA. In contrast, in DH rats, Losartan, followed by pre-collicular and pontine transections, failed to reduce tSNA, whereas transection at the medulla-spinal cord junction massively reduced tSNA. In DH but not EH rats, selective inhibition of the commissural nucleus tractus solitarii (cNTS) significantly reduced tSNA. Comparable data were obtained in both in situ and in vivo (anaesthetized/conscious) rats and suggest that following chronic dehydration, the control of tSNA transfers from supra-brainstem structures (e.g. hypothalamus) to the medulla oblongata, particularly the cNTS. As microarray analysis revealed up-regulation of AP1 transcription factor JunD in the dehydrated cNTS, we tested the hypothesis that AP1 transcription factor activity is responsible for dehydration-induced functional plasticity. When AP1 activity was blocked in the cNTS using a viral vector expressing a dominant negative FosB, cNTS inactivation was ineffective. However, tSNA was decreased after pre-collicular transection, a response similar to that seen in EH rats. Thus, the dehydration-induced switch in control of tSNA from hypothalamus to cNTS seems to be mediated via activation of AP1 transcription factors in the cNTS. If AP1 activity is blocked in the cNTS during dehydration, sympathetic activity control reverts back to forebrain regions. This unique reciprocating neural structure-switching plasticity between brain centres emphasizes the multiple mechanisms available for the adaptive response to dehydration.