Development of the baroreflex in the young rat

Deletion of neurturin impairs development of cholinergic nerves and heart rate control in postnatal mouse hearts

The neurotrophic factor neurturin is required for normal cholinergic innervation of adult mouse heart and bradycardic responses to vagal stimulation. Our goals were to determine effects of neurturin deletion on development of cardiac chronotropic and dromotropic functions, vagal baroreflex response, and cholinergic nerve density in nodal regions of postnatal mice. Experiments were performed on postnatal C57BL/6 wild-type (WT) and neurturin knockout (KO) mice. Serial electrocardiograms were recorded noninvasively from conscious pups using an ECGenie apparatus. Mice were treated with atenolol to evaluate and block sympathetic effects on heart rate (HR) and phenylephrine (PE) to stimulate the baroreflex. Immunohistochemistry was used to label cholinergic nerves in paraffin sections. WT and KO mice showed similar age-dependent increases in HR and decreases in PR interval between postnatal days (P) 2.5 and 21. Treatment with atenolol reduced HR significantly in WT and KO pups at P7.5. PE caused a reflex bradycardia that was significantly smaller in KO pups. Cholinergic nerve density was significantly less in nodal regions of P7.5 KO mice. We conclude that cholinergic nerves have minimal influence on developmental changes in HR and PR, QRS, and QTc intervals in mouse pups. However, cholinergic nerves mediate reflex bradycardia by 1 week postnatally. Deletion of neurturin impairs cholinergic innervation of the heart and the vagal efferent component of the baroreflex early during postnatal development.

Perinatal sulfur dioxide exposure alters brainstem parasympathetic control of heart rate

AIMS

Sulfur dioxide (SO₂) is an air pollutant that impedes neonatal development and induces adverse cardiorespiratory health effects, including tachycardia. Here, an animal model was developed that enabled characterization of (i) in vivo alterations in heart rate and (ii) altered activity in brainstem neurons that control heart rate after perinatal SO₂ exposure.

METHODS AND RESULTS

Pregnant Sprague-Dawley dams and their pups were exposed to 5 parts per million SO₂ for 1 h daily throughout gestation and 6 days postnatal. Electrocardiograms were recorded from pups at 5 days postnatal to examine changes in basal and diving reflex-evoked changes in heart rate following perinatal SO₂ exposure. In vitro studies employed whole-cell patch-clamp electrophysiology to examine changes in neurotransmission to cardiac vagal neurons within the nucleus ambiguus upon SO₂ exposure using a preparation that maintains fictive inspiratory activity recorded from the hypoglossal rootlet. Perinatal SO₂ exposure increased heart rate and blunted the parasympathetic-mediated diving reflex-evoked changes in heart rate. Neither spontaneous nor inspiratory-related inhibitory GABAergic or glycinergic neurotransmission to cardiac vagal neurons was altered by SO₂ exposure. However, excitatory glutamatergic neurotransmission was decreased by 51.2% upon SO₂ exposure. This diminished excitatory neurotransmission was tetrodotoxin-sensitive, indicating SO₂ exposure impaired the activity of preceding glutamatergic neurons that synapse upon cardiac vagal neurons.

CONCLUSIONS

Diminished glutamatergic, but unaltered inhibitory neurotransmission to cardiac vagal neurons provides a mechanism for the observed SO₂-induced elevated heart rate via an impairment of brainstem cardioinhibitory parasympathetic activity to the heart.

Multifocal haemorrhagic brain damage following hypoxia and blood pressure lability: case report and rat model

AIMS

Haemorrhagic brain damage is frequently encountered as a complication of premature birth. Much less frequently, multifocal petechial haemorrhage is identified in asphyxiated term newborns. Our goal was to develop an experimental rat model to reproduce this pattern of brain damage.

METHODS

Neonatal rat pups were exposed to a 24-h period of 10% or 8% hypoxia followed by a single dose of phenylephrine. Acute and subacute changes, as well as long-term outcomes, were investigated by histology, brain magnetic resonance imaging and behavioural assessment. Immunostaining for vascular endothelial growth factor and caveolin-1 was performed in the rat brains as well as in a 17-day human case.

RESULTS

Small foci of haemorrhage were identified in almost all regions of the rat brain subjected to hypoxia plus phenylephrine, but not hypoxia alone. Exposure to 8% hypoxia was associated with more haemorrhagic foci than 10% hypoxia. With rare exceptions, the blood deposits were too small to be detected by magnetic resonance imaging. Altered immunohistochemical detection of vascular endothelial growth factor and caveolin-1 in the child and the rat model suggests a role for blood-brain barrier compromise. There were no clear behavioural changes and no residual morphological abnormalities in the 78-day follow-up of the rats.

CONCLUSIONS

We conclude that transient hypoxia, in a dose-dependent manner, can weaken the vasculature and predispose to brain haemorrhage in the situation of labile blood pressure. Persistent hypoxia is likely to be important in the genesis of permanent severe brain damage.

Early life experience shapes the functional organization of stress-responsive visceral circuits

Emotions are closely tied to changes in autonomic (i.e., visceral motor) function, and interoceptive sensory feedback from body to brain exerts powerful modulatory control over motivation, affect, and stress responsiveness. This manuscript reviews evidence that early life experience can shape the structure and function of central visceral circuits that underlie behavioral and physiological responses to emotive and stressful events. The review begins with a general discussion of descending autonomic and ascending visceral sensory pathways within the brain, and then summarizes what is known about the postnatal development of these central visceral circuits in rats. Evidence is then presented to support the view that early life experience, particularly maternal care, can modify the developmental assembly and structure of these circuits in a way that impacts later stress responsiveness and emotional behavior. The review concludes by presenting a working hypothesis that endogenous cholecystokinin signaling and subsequent recruitment of gastric vagal sensory inputs to the caudal brainstem may be an important mechanism by which maternal care influences visceral circuit development in rat pups. Early life experience may contribute to meaningful individual differences in emotionality and stress responsiveness by shaping the postnatal developmental trajectory of central visceral circuits.

Maternal dietary tryptophan deficiency alters cardiorespiratory control in rat pups

Malnutrition during pregnancy adversely affects postnatal forebrain development; its effect upon brain stem development is less certain. To evaluate the role of tryptophan [critical for serotonin (5-HT) synthesis] on brain stem 5-HT and the development of cardiorespiratory function, we fed dams a diet ∼45% deficient in tryptophan during gestation and early postnatal life and studied cardiorespiratory variables in the developing pups. Deficient pups were of normal weight at postnatal day (P)5 but weighed less than control pups at P15 and P25 (P < 0.001) and had lower body temperatures at P15 (P < 0.001) and P25 (P < 0.05; females only). Oxygen consumption (Vo(2)) was unaffected. At P15, deficient pups had an altered breathing pattern and slower heart rates. At P25, they had significantly lower ventilation (Ve) and Ve-to-Vo(2) ratios in both air and 7% CO(2). The ventilatory response to CO(2) (% increase in Ve/Vo(2)) was significantly increased at P5 (males) and reduced at P15 and P25 (males and females). Deficient pups had 41-56% less medullary 5-HT (P < 0.01) compared with control pups, without a difference in 5-HT neuronal number. These data indicate important interactions between nutrition, brain stem physiology, and age that are potentially relevant to understanding 5-HT deficiency in the sudden infant death syndrome.

Quantitative evaluation of ontogenetic change in heart rate and its autonomic regulation in newborn mice with the use of a noninvasive piezoelectric sensor

A reliable basal heart rate (HR) measurement in freely moving newborn mice was accomplished for the first time by using a novel noninvasive piezoelectric transducer (PZT) sensor. The basal HR was approximately 320 beats/min at postnatal day (P)0 and increased with age to approximately 690 beats/min at P14. Contribution of autonomic control to HR was then assessed. Sympathetic blockade with metoprolol significantly reduced basal HR at both P6 (-236 +/- 23 beats/min; mean +/- SE) and P12 (-105 +/- 8 beats/min), but atropine was without effect, indicating the predominant tonic adrenergic stimulation and absence of vagal control for basal HR in newborn mice. In contrast to stable basal HR during 5-min recording, HR measured by ECG (ECG-HR) was markedly decreased because of the restraint stress of attaching ECG electrodes, with accompanying freezing behavior. ECG-HR lowered and further decreased gradually during 5 min (slow cardiodeceleration) at P0-P3 and rapidly decreased and gradually recovered within 5 min (transient bradycardia) at P9-P14. The response was not uniform in P4-P8 mice: they showed either of these two patterns or sustained bradycardia (9-29%), and the number of mice that showed transient bradycardia increased with age (30-100%) during the period. Studies with autonomic blockade suggest that the slow cardiodeceleration and transient bradycardia are mediated mainly by withdrawal of adrenergic stimulation and phasic vagal activation, respectively, and the autonomic control of HR response to restraint stress is likely to change from the withdrawal of adrenergic stimulation to the phasic vagal activation at different stages during P4-P8 in individual mice. The PZT sensor may offer excellent opportunities to monitor basal HR of small animals noninvasively.

Beta1-adrenoceptor or alpha1-adrenoceptor activation initiates early odor preference learning in rat pups: support for the mitral cell/cAMP model of odor preference learning

We proposed that mitral cell beta1-adrenoceptor activation mediates rat pup odor preference learning. Here we evaluate beta1-, beta2-, alpha1-, and alpha2-adrenoceptor agonists in such learning. The beta1-adrenoceptor agonist, dobutamine, and the alpha1-adrenoceptor agonist, phenylephrine, induced learning, and both exhibited an inverted U-curve dose-response relationship to odor preference learning. Phenylephrine-induced learning occurred in the presence of propranolol to prevent indirect activation of beta-adrenoceptors. Alpha1-adrenoceptor mediation may represent a novel mechanism inducing learning or may increase cAMP in mitral cells via indirect activation of GABA(B) receptors. Neither the beta2-adrenoceptor agonist, salbutamol, nor the alpha2-adrenoceptor agonist, clonidine, induced learning.