Dextromethorphan affects ventilation differently in male and female rats

Sex differences in breathing

Breathing is a vital behavior that ensures both the adequate supply of oxygen and the elimination of CO₂, and it is influenced by many factors. Despite that most of the studies in respiratory physiology rely heavily on male subjects, there is much evidence to suggest that sex is an important factor in the respiratory control system, including the susceptibility for some diseases. These different respiratory responses in males and females may be related to the actions of sex hormones, especially in adulthood. These hormones affect neuromodulatory systems that influence the central medullary rhythm/pontine pattern generator and integrator, sensory inputs to the integrator and motor output to the respiratory muscles. In this article, we will first review the sex dependence on the prevalence of some respiratory-related diseases. Then, we will discuss the role of sex and gonadal hormones in respiratory control under resting conditions and during respiratory challenges, such as hypoxia and hypercapnia, and whether hormonal fluctuations during the estrous/menstrual cycle affect breathing control. We will then discuss the role of the locus coeruleus, a sexually dimorphic CO₂/pH-chemosensitive nucleus, on breathing regulation in males and females. Next, we will highlight the studies that exist regarding sex differences in respiratory control during development. Finally, the few existing studies regarding the influence of sex on breathing control in non-mammalian vertebrates will be discussed.

Sexual dimorphism of cardiopulmonary regulation in the arcuate nucleus of the hypothalamus

The arcuate nucleus of the hypothalamus (ANH) interacts with other hypothalamic nuclei, forebrain regions, and downstream brain sites to affect autonomic nervous system outflow, energy balance, temperature regulation, sleep, arousal, neuroendocrine function, reproduction, and cardiopulmonary regulation. Compared to studies of other ANH functions, how the ANH regulates cardiopulmonary function is less understood. Importantly, the ANH exhibits structural and functional sexually dimorphic characteristics and contains numerous neuroactive substances and receptors including leptin, neuropeptide Y, glutamate, acetylcholine, endorphins, orexin, kisspeptin, insulin, Agouti-related protein, cocaine and amphetamine-regulated transcript, dopamine, somatostatin, components of renin-angiotensin system and gamma amino butyric acid that modulate physiological functions. Moreover, several clinically relevant disorders are associated with ANH ventilatory control dysfunction. This review highlights how ANH neurotransmitter systems and receptors modulate breathing differently in male and female rodents. Results highlight the significance of the ANH in cardiopulmonary regulation. The paucity of studies in this area that will hopefully spark investigations of sexually dimorphic ANH-modulation of breathing.

Evaluating the safety and efficacy of dextromethorphan/quinidine in the treatment of pseudobulbar affect

Pseudobulbar affect (PBA) is a common manifestation of brain pathology associated with many neurological diseases, including amyotrophic lateral sclerosis, Alzheimer's disease, stroke, multiple sclerosis, Parkinson's disease, and traumatic brain injury. PBA is defined by involuntary and uncontrollable expressed emotion that is exaggerated and inappropriate, and also incongruent with the underlying emotional state. Dextromethorphan/quinidine (DM/Q) is a combination product indicated for the treatment of PBA. The quinidine component of DM/Q inhibits the cytochrome P450 2D6-mediated metabolic conversion of dextromethorphan to its active metabolite dextrorphan, thereby increasing dextromethorphan systemic bioavailability and driving the pharmacology toward that of the parent drug and away from adverse effects of the dextrorphan metabolite. Three published efficacy and safety studies support the use of DM/Q in the treatment of PBA; significant effects were seen on the primary end point, the Center for Neurologic Study-Lability Scale, as well as secondary efficacy end points and quality of life. While concentration-effect relationships appear relatively weak for efficacy parameters, concentrations of DM/Q may have an impact on safety. Some special safety concerns exist with DM/Q, primarily because of the drug interaction and QT prolongation potential of the quinidine component. However, because concentrations of dextrorphan (which is responsible for many of the parent drug's side effects) and quinidine are lower than those observed in clinical practice with these drugs administered alone, some of the perceived safety issues may not be as relevant with this low dose combination product. However, since patients with PBA have a variety of other medical problems and are on numerous other medications, they may not tolerate DM/Q adverse effects, or may be at risk for drug interactions. Some caution is warranted when initiating DM/Q treatment, particularly in patients with underlying risk factors for torsade de pointes and in those receiving medications that may interact with DM/Q.

Oral administration of dextromethorphan does not produce neuronal vacuolation in the rat brain

Dextromethorphan is a widely used antitussive agent, also showing increased recreational abuse. Dextromethorphan and its metabolite dextrorphan are non-competitive antagonists at the N-methyl-d-aspartate (NMDA) receptor ion channel. Single doses of some NMDA receptor antagonists produce neuropathologic changes in neurons of the retrosplenial/posterior cingulate cortices (RS/PC), characterized by vacuolation or neurodegeneration. To determine whether dextromethorphan produces these characteristic lesions, dextromethorphan was administered orally either as a single dose of 120mg/kg to female rats, or daily for 30 days at doses of 5-400 mg/(kg day) to male rats and 5-120mg/(kg day) to female rats. Brains were examined microscopically for evidence of neuronal vacuolation (4-6h postdose) and neurodegeneration ( approximately 24 or 48h postdose). Administration of dextromethorphan at 120mg/(kg day) in females, and at > or =150mg/(kg day) in males produced marked behavioral changes, indicative of neurologic effects. Mortality occurred at the highest doses administered. There were no detectable neuropathologic changes following single or repeated oral administration of dextromethorphan at any dose. Administration of MK-801 (9mg/kg) produced both cytoplasmic vacuolation and neuronal degeneration in neurons of the RS/PC cortex. Thus characteristic neuropathologic changes found with more potent NMDA receptor antagonists do not occur following single or repeated oral administration of dextromethorphan.

Dextromethorphan protects male but not female mice with brain ischemia

The non-competitive N-methyl-D-aspartate receptor antagonist dextromethorphan is protective against some types of brain injury. Unilateral carotid ligation in postnatal day 12 CD1 mice produces ischemic brain injury. To evaluate the neuroprotective potential of dextromethorphan against ischemic injury in the immature brain, seven litters of postnatal day 12 CD1 mice received either dextromethorphan or vehicle after a unilateral carotid ligation. Only the male pups were protected, and brain injury was unchanged in the female pups treated with dextromethorphan. These results suggest that dextromethorphan neuroprotection against ischemic injury in the immature brain is sex-dependent.

Sex-specific densities of estrogen receptors alpha and beta in the subnuclei of the nucleus tractus solitarius, hypoglossal nucleus and dorsal vagal motor nucleus weanling rats

In rats ventilatory responses to N-methyl-d-aspartate (NMDA) receptor modulation are sexually dimorphic and may be altered by manipulating brain levels of estrogen receptors. Here we used image analysis and immunohistochemistry in weanling male and female rats to quantitate areas and densities of ER alpha and ER beta-positive neurons within medullary regions associated with cardiopulmonary regulation including the hypoglossal nucleus, subnuclei of the nucleus of the solitary tract (NTS), and the dorsal motor nucleus of the vagus. Weanling rats were selected because ventilation, metabolic rate, and body and brain weights are comparable at this age and there are no large fluctuations in plasma hormone levels. Females, relative to males, had smaller areas in the A2 region and parts of the NTS. Counts and densities for ER alpha were greater in females than males in almost all regions studied. In contrast sex differences in ER beta were found in fewer nuclei, but in those higher counts and densities were noted in females. In general, ER beta-positive neurons in the brainstem regions examined were less prevalent than ER alpha neurons. Thus, in weanling rats sex affected ER alpha and ER beta neuronal densities in brainstem regions associated with cardiopulmonary regulation that may be responsible for sex differences in control of breathing.

Neonatal sex steroids affect ventilatory responses to aspartic acid and NMDA receptor subunit 1 in rats

We hypothesized that administration of estradiol benzoate to males and testosterone propionate to female neonatal rat pups alters sex-specific ventilatory responses to aspartic acid with correspondent changes in N-methyl-D-aspartate receptor subunit 1 (NR1) expression determined by Western blot in specific brain regions. One-day-old rat pups received estradiol benzoate, testosterone propionate, or vehicle and were studied at weanling and adulthood. Different groups had distinct patterns of changes in tidal volume and frequency of breathing after aspartic acid administration. NR1 expression in hypothalamus was altered by age, sex, and treatment. Medullary and pontine NR1 expression correlated with baseline ventilation and magnitude of the ventilatory response to aspartic acid in some groups. Thus 1) tidal volume and breathing frequency patterns in response to aspartic acid are gender, age, and treatment dependent; 2) sex, age, and exogenous steroid hormones affect NR1 expression primarily in the hypothalamus; and 3) there is correlation between NR1 expression in pons and medulla with ventilatory parameters.

NMDA receptor-mediated modulation of ventilation in obese Zucker rats

BACKGROUND

Ventilation in response to hypoxia is reduced in some obese humans and is believed to represent part of the pathogenesis of obesity hypoventilation syndrome (OHS). Ventilation in response to hypoxic exposure is closely related to the release of excitatory neurotransmitters, in particular glutamate, acting specifically on N-methyl-D-aspartate (NMDA) receptors.

OBJECTIVES

The aim of the present study was to investigate whether NMDA receptor-mediated mechanisms are responsible for the altered ventilatory response to sustained hypoxia observed in obese Zucker (Z) rats.

SUBJECTS

Seven lean and seven 15-week-old obese male Z rats were studied.

MEASUREMENTS

Ventilation ([V](E)) at rest and during 30 min sustained hypoxic (10% O(2)) exposure was measured by the barometric method. [V](E) was assessed following the blinded-random administration of equal volumes of either saline (vehicle) or dextromethorphan (DM, 10 mg/kg), a non-competitive glutamate NMDA receptor antagonist.

RESULTS

DM had no effects on resting [V(E) in both lean and obese rats during room air breathing. Lean rats treated with DM exhibited a significant (P<0.05) depression in [V](E), V(T), and V(T)/T(I) during either the early (5 min) or the late phase (30 min) of ventilatory response to sustained hypoxia. In contrast, DM administration in obese rats did not change [V(E), V(T), or V(T)/T(I) during the early phase of ventilatory response to hypoxia. During the late phase of ventilatory response to hypoxia. obese rats treated with DM exhibited a similar depression in [V](E) and V(T) as observed in lean rats, but had no significant change in V(T)/T(I) during the 30 min hypoxic exposure.

CONCLUSION

Our findings indicate that altered glutamatergic mechanisms acting on NMDA receptors are partially responsible for a blunted early phase of ventilatory response to hypoxia noted in obese rats and also contribute to their reduced neural respiratory drive.

Gender-specific effects of CNQX administered into the arcuate nucleus on ventilatory patterns in rats

This study evaluated the effect of microinjection of the non-N-methyl-D-aspartate (NMDA) receptor antagonist, cyano, 3-dihydro-7-nitrogluinoxaline-2, 3-dione (CNQX), into the arcuate nucleus of the hypothalamus on ventilation in male and female rats. Conscious rats received saline or 50, 100, or 200 pmol concentrations of CNQX on separate days. Significant interactions between dose and gender were observed on frequency, inspiratory (TI) and expiratory (TE) time, and tidal volume. CNQX depressed frequency, but increased tidal volume in female rats. Effects of CNQX in males on these ventilatory parameters were considerably less. In CNQX-treated females the decrease in frequency of breathing was primarily due to an increase in TI. Exposure of CNQX-treated female rats to hypercapnia, but not to hypoxia transiently decreased TI. No effect of CNQX was noted on oxygen consumption or body temperature. Thus, non-NMDA receptors in the arcuate nucleus are involved in modulating ventilatory patterns in a gender-specific manner independent of its effects on oxygen consumption or body temperature.