Luciane H. Gargaglioni

Involvement of serotoninergic receptors in the anteroventral preoptic region on hypoxia-induced hypothermia

Hypoxia causes a regulated decrease in body temperature (Tb). There is circumstantial evidence that the neurotransmitter serotonin (5-HT) in the anteroventral preoptic region (AVPO) mediates this response. However, which 5-HT receptor(s) is (are) involved in this response has not been assessed. Thus, we investigated the participation of the 5-HT receptors (5-HT1, 5-HT2, and 5-HT7) in the AVPO in hypoxic hypothermia. To this end, Tb of conscious Wistar rats was monitored by biotelemetry before and after intra-AVPO microinjection of methysergide (a 5-HT1 and 5-HT2 receptor antagonist, 0.2 and 2 microg/100 nL), WAY-100635 (a 5-HT(1A) receptor antagonist, 0.3 and 3 microg/100 nL), and SB-269970 (a 5-HT7 receptor antagonist, 0.4 and 4 micro/100 nL), followed by 60 min of hypoxia exposure (7% O2). During the experiments, the mean chamber temperature was 24.6 +/- 0.7 degrees C (mean +/- SE) and the mean room temperature was 23.5 +/- 0.8 degrees C (mean +/- SE). Intra-AVPO microinjection of vehicle or 5-HT antagonists did not change Tb during normoxic conditions. Exposure of rats to 7% of inspired oxygen evoked typical hypoxia-induced hypothermia after vehicle microinjection, which was not affected by both doses of methysergide. However, WAY-100635 and SB-269970 treatment attenuated the drop in Tb in response to hypoxia. The effect was more pronounced with the 5-HT7 antagonist since both doses (0.4 and 4 microg/0.1 microL) were capable of attenuating the hypothermic response. As to the 5-HT(1A) antagonist, the attenuation of hypoxia-induced hypothermia was only observed at the higher dose. Therefore, the present results are consistent with the notion that 5-HT acts on both 5-HT(1A) and 5-HT7 receptors in the AVPO to induce hypothermia, during hypoxia.

The nucleus raphe magnus modulates hypoxia-induced hyperventilation but not anapyrexia in rats

The nucleus raphe magnus (NRM) is one of the brainstem cell groups involved in physiological responses to hypoxia. Thus, we tested the hypothesis that the NRM modulates hypoxia-induced hyperventilation and anapyrexia. To this end, we assessed the participation of NRM in the respiratory and thermoregulatory responses to hypoxia using ibotenic acid lesions produced in the NRM of rats. Our results demonstrated that, under resting breathing, NRM plays no role in ventilation or body temperature. Hypoxia caused hyperventilation and anapyrexia in all groups. NMR lesions elicited an increased ventilatory response to hypoxia due to a higher tidal volume (V(T)) but did not affect hypoxia-induced anapyrexia. Therefore, we conclude that NRM exerts an inhibitory modulation of breathing during hypoxia, acting on V(T), but plays no role in the hypoxia-induced anapyrexia.

Role of nucleus isthmi in the ventilatory response to hypoxia of Bufo paracnemis.

Nucleus isthmi (NI) is a mesencephalic structure of the amphibian brain that has recently been reported to participate in CO2-ventilatory response. The present study was designed to test the hypothesis that NI is also involved in hypoxia-induced hyperventilation and in the breathing pattern of the toad Bufo paracnemis. Pulmonary ventilation was directly measured by pneumotachography method in control, sham-operated and NI-lesioned toads exposed to normoxia and hypoxia (7 and 5% inspired O2). Under normoxic conditions, NI lesion caused no significant change in the ventilatory pattern or in the pulmonary ventilation. Hypoxia caused a significant (P < 0.05) increase in ventilation in control and sham-operated animals mainly due to an elevated VT. The hypoxia-induced hyperventilation was greater (P < 0.05) in the NI-lesioned toads, due to increases in both fR and VT. Such increased fR under hypoxia was due to a higher number of breaths per burst. The data indicate that NI plays no role under normoxic conditions but is involved in the ventilatory response to hypoxia, exercising an inhibitory modulation on pulmonary ventilation.

Serotoninergic receptors in the anteroventral preoptic region modulate the hypoxic ventilatory response

Hypothalamus is a site of integration of the hypoxic and thermal stimuli on breathing and there is evidence that serotonin (5-HT) receptors in the anteroventral preoptic region (AVPO) mediate hypoxic hypothermia. Once 5-HT is involved in the hypoxic ventilatory response (HVR), we investigated the participation of the 5-HT receptors (5-HT1, 5-HT2 and 5-HT7) in the AVPO in the HVR. To this end, pulmonary ventilation (V(E)) of rats was measured before and after intra-AVPO microinjection of methysergide (a 5-HT1 and 5-HT2 receptor antagonist), WAY-100635 (a 5-HT1A receptor antagonist) and SB-269970 (a 5-HT7 receptor antagonist), followed by 60 min of hypoxia exposure (7% O2). Intra-AVPO microinjection of vehicles or 5-HT antagonists did not change V(E) during normoxic conditions. Exposure of rats to 7% O2 evoked typical hypoxia-induced hyperpnea after vehicle microinjection, which was not affected by methysergide. WAY-100635 and SB-269970 treatment caused an increased HVR, due to a higher tidal volume. Therefore, the current data provide the evidence that 5-HT acting on 5-HT1A and 5-HT7 receptors in the AVPO exert an inhibitory modulation on the HVR.

Basal midbrain modulation of tonic immobility in the toad Bufo paracnemis.

Tonic immobility (TI) is considered to be a final stage in a sequence of defensive responses occurring in the prey/predator encounter. It is known that the basal midbrain of toads is involved in the organization of defensive behavior and analgesia. This study investigated the effect of electrolytic or neurotoxic lesions of two mesencephalic regions [tegmentum (TEG) and interpeduncular nucleus (IPN)] on the latency and duration of TI (induced by postural inversion and by movement restriction) and on the latency of the motor response to a nociceptive stimulus (hot plate) in toads. Electrolytic lesions of TEG and IPN promoted an increase in the duration of TI episodes. Neurotoxic lesion of these two regions also caused an increase in the duration of TI episodes. The effect was more intense in the animals with electrolytic lesion, possibly due to more extensive damage associated with this procedure or to damage of passage fibers. The results suggest that lesions of the midbrain TEG liberate basic circuits placed caudally and are involved in the organization of the TI response. It remains to be determined if the IPN exerts its effect directly on the caudal levels or by acting via the mesencephalic TEG. Lesions do not interfere with the latency of the motor response to a thermal noxious stimulus, indicating that the lesioned regions do not affect the reflexive response and are not essential for the perception of the noxious stimulus.

Brain monoaminergic neurons and ventilatory control in vertebrates.

Monoamines (noradrenaline (NA), adrenaline (AD), dopamine (DA) and serotonin (5-HT) are key neurotransmitters that are implicated in multiple physiological and pathological brain mechanisms, including control of respiration. The monoaminergic system is known to be widely distributed in the animal kingdom, which indicates a considerable degree of phylogenetic conservation of this system amongst vertebrates. Substantial progress has been made in uncovering the participation of the brain monoamines in the breathing regulation of mammals, since they are involved in the maturation of the respiratory network as well as in the modulation of its intrinsic and synaptic properties. On the other hand, for the non-mammalian vertebrates, most of the knowledge of central monoaminergic modulation in respiratory control, which is actually very little, has emerged from studies using anuran amphibians. This article reviews the available data on the role of brain monoaminergic systems in the control of ventilation in terrestrial vertebrates. Emphasis is given to the comparative aspects of the brain noradrenergic, adrenergic, dopaminergic and serotonergic neuronal groups in breathing regulation, after first briefly considering the distribution of monoaminergic neurons in the vertebrate brain.

Role of glutamate in the nucleus isthmi on the hypoxia- and hypercarbia-induced hyperventilation of toads.

The nucleus isthmi (NI) is a mesencephalic structure of the amphibian brain that has been reported to participate in CO(2) chemoreception and in the ventilatory response to hypoxia. In the present study, we assessed the role of glutamatergic transmission inside the NI on the hypoxic and hypercarbic drive to breathing. We compared the respiratory responses to 7 and 5% inspired O(2) and 3% inspired CO(2) after microinjecting 10 nmol/100 nl of kynurenic acid (an antagonist of L-glutamate receptors) into the NI of toads (Bufo paracnemis). Kynurenic acid had no effect under resting conditions. Both hypoxia and hypercarbia elicited an increase in ventilation in all groups, with hypoxia acting on tidal volume (V(T)) and hypercarbia on frequency (f). The microinjection of kynurenic acid into the NI caused an increased ventilatory response to hypoxia and hypercarbia due to a higher V(T). We conclude that glutamatergic transmission in the NI has an inhibitory effect when the respiratory drive is high, acting on V(T).

Lactate as a modulator of hypoxia-induced hyperventilation

In the present study, we tested the hypothesis that lactate, which is a classic companion of hypoxic stress in mammals, is a modulator of hypoxia-induced hyperventilation. To this end, pulmonary ventilation (V(E)) of male Wistar rats was measured by whole body plethysmograph, and dichloroacetate (DCA, 100 mg/kg) was used to inhibit lactate production. Plasma lactate levels, arterial pH (pHa), arterial carbon dioxide partial pressure (PaCO(2)), arterial oxygen partial pressure (PaO(2)), plasma bicarbonate (HCO3(-)) and oxygen consumption (VO(2)) were determined as well. In normoxia, intraperitoneal DCA elicited a decrease only in plasma lactate levels. Hypoxia caused an increase in V(E), pHa and plasma lactate levels and parallel to decreases in PaCO(2), PaO(2) and VO(2) in the control group. DCA administration markedly reduced the ventilatory response to hypoxia by acting on tidal volume (V(T)). This reduced ventilatory response caused by DCA was independent of VO(2). In conclusion, the present study indicates that lactate contributes to the initiation and maintenance of hypoxia-induced hyperventilation in rats, modulating the adjustments in V(T).

Nitric oxide pathway in the nucleus raphe magnus modulates hypoxic ventilatory response but not anapyrexia in rats.

Nucleus raphe magnus (NRM) is one of the cellular groups of the brainstem that is involved in the physiologic responses to hypoxia and contains nitric oxide (NO) synthase. In the present study, we assessed the role of NO pathway in the NRM on the hypoxic ventilatory response (HVR) and anapyrexia (a regulated decrease in body temperature). To this end, pulmonary ventilation (VE) and body temperature (Tb) of male Wistar rats were measured before and after microinjection of N-monomethyl-L-arginine (L-NMMA, a nonselective nitric oxide synthase inhibitor, 12.5 microg/0.1 microl) into the NRM, followed by hypoxia. Control rats received microinjection of saline. Under resting conditions, L-NMMA treatment did not affect pulmonary VE or Tb. Typical hypoxia-induced hyperventilation and anapyrexia were observed after saline treatment. L-NMMA into the NRM reduced the HVR but did not affect hypoxia-induced anapyrexia. In conclusion, the present study indicates that NO in the NRM is involved in HVR, exerts an inhibitory modulation on the NRM neurons but does not mediate hypoxia-induced anapyrexia.

Is lactate a mediator of hypoxia-induced anapyrexia?

Abstract.Hypoxia causes a regulated decrease in body core temperature (Tc), termed anapyrexia, which seems to be a very effective way of preventing hypoxia-associated cell damage. Since during several pathological states the supply of O2 is a limiting factor, the clinical importance of anapyrexia is evident. However, the mechanisms involved in this response remain unclear. We tested the hypothesis that lactate, a classic companion of hypoxia, is a mediator of hypoxia-induced anapyrexia, using the inhibitor of acid lactic production dichloroacetate (DCA). Each of 28 rats was placed in a chamber ventilated with humidified air at an ambient temperature of 24–26xa0°C. After a control period of 30xa0min the animals were given saline or 100xa0mg/kg DCA i.p. Then, 30xa0min later, the chamber was flushed with a 7% O2 gas mixture for 2xa0h. At the end of the experiment, the animals were decapitated and blood samples collected for measurements of plasma lactate. Tc was measured by biotelemetry. DCA did not affect the Tc or basal lactate levels of normoxic rats. Hypoxia elicited a significant decrease in Tc and an increase in plasma lactate levels. Although DCA decreased plasma lactate levels during hypoxia, it caused no change in the course of hypoxia-induced anapyrexia. Correspondingly, no correlation was found between the drop in Tc and the rise in plasma lactate during hypoxic conditions. These results do not support the hypothesis that lactate is a mediator of hypoxia-induced anapyrexia in rats.