Pulmonary and systemic vascular effects of serotonin in conscious newborn lambs

Does serotonin deficiency lead to anosmia, ageusia, dysfunctional chemesthesis and increased severity of illness in COVID-19?

Different mechanisms forwarded to understand anosmia and ageusia in coronavirus patients are not adequate to explain reversible anosmia and ageusia, which are resolved quickly. In addition, the reason behind the impaired chemesthetic sensations in some coronavirus patients remains unknown. In the present paper it is proposed that SARS-CoV-2 patients suffer from depletion of tryptophan, as ACE2, a key element in the process of absorption of tryptophan from the food, is significantly reduced in the patients as coronavirus uses ACE2 as the receptor to enter the host cells. The tryptophan depletion leads to a deficit of serotonin (5-HT) in SARS-COV-2 patients because tryptophan is the precursor in the synthesis of 5-HT. Such 5-HT deficiency can explain anosmia, ageusia and dysfunctional chemesthesis in COVID-19, given the fact that 5-HT is an important neuromodulator in the olfactory neurons, taste receptor cells and transient receptor potential channels (TRP channels) involved in chemesthesis. In addition, 5-HT deficiency worsens silent hypoxemia and depresses hypoxic pulmonary vasoconstriction leading to increased severity of the disease. Also, the levels of anti-inflammatory melatonin (synthesized from 5-HT) and nicotinamide adenine dinucleotide (NAD⁺, produced from niacin whose precursor is the tryptophan) might decrease in coronavirus patients resulting in the aggravation of the disease. Interestingly, selective serotonin reuptake inhibitors (SSRIs) may not be of much help in correcting the 5-HT deficiency in COVID-19 patients, as their efficacy goes down significantly when there is depletion of tryptophan in the system. Hence, tryptophan supplementation may herald a radical change in the treatment of COVID-19 and accordingly, clinical trials (therapeutic / prophylactic) should be conducted on coronavirus patients to find out how tryptophan supplementation (oral or parenteral, the latter in severe cases where there is hardly any absorption of tryptophan from the food) helps in curing, relieving or preventing the olfactory, gustatory and chemesthetic dysfunctions and in lessening the severity of the disease.

Intermittent severe hypoxia induces plasticity within serotonergic and catecholaminergic neurons in the neonatal rat ventrolateral medulla

5-HT neurons contribute to autoresuscitation and survival during intermittent severe hypoxia (IsH). In adults, catecholaminergic neurons in the ventrolateral medulla (VLM) contribute to the autonomic response to hypoxia. We hypothesized that 1) catecholaminergic neurons in the neonatal VLM are activated following IsH, 2) this activation is compromised following an acute loss of brain stem 5-HT, and 3) IsH induces cellular and/or transcriptomic plasticity within catecholaminergic and serotonergic neurons that are within or project to the VLM, respectively. To test these hypotheses, we treated rat pups with 6-fluorotryptophan, a tryptophan hydroxylase (TPH) inhibitor, and then exposed treated and vehicle controls to IsH or air. Along with immunohistochemistry to detect tyrosine hydroxylase (TH)- or Fos-positive neurons, we used RNA sequencing to resolve the effects of IsH and 5-HT deficiency on the expression of serotonergic and catecholaminergic system genes in the VLM. 5-HT deficiency compromised autoresuscitation and survival. IsH significantly increased the number of identifiable TH-positive VLM neurons, an effect enhanced by 5-HT deficiency (P = 0.003). Contrary to our hypothesis, 5-HT-deficient pups had significantly more Fos-positive neurons following IsH (P = 0.008) and more activated TH-positive neurons following IsH or air (P = 0.04). In both groups the expression of the 5-HT transporter and TPH2 was increased following IsH. In 5-HT-deficient pups, the expression of the inhibitory 5-HT1A receptor was decreased following IsH, while the expression of DOPA decarboxylase was increased. These data show that the serotonergic and catecholaminergic systems in the VLM of the neonatal rat are dynamically upregulated by IsH, potentially adapting cardiorespiratory responses to severe hypoxia.

Pulmonary arterioles from rats with congenital diaphragmatic hernias are hypoplastic but not hyperresponsive

BACKGROUND

Infants born with congenital diaphragmatic hernias (CDH) frequently die as a result of pulmonary hypertension and persistent fetal circulation. The pulmonary vessels of infants with CDH have decreased total cross-sectional area, increased muscle content, and muscularization of intra-acinar arterioles that are normally not muscularized. These structural alterations are believed to result in exaggerated responses to normal vasoconstrictor stimuli.

METHODS

The authors used the nitrofen-induced CDH model in rats to determine whether the vasoconstrictor responses of pulmonary arterioles are exaggerated in this animal model of CDH. The authors compared the responses of isolated third-generation pulmonary arterioles from normal rats and from rats with nitrofen-induced CDH to K+-induced depolarization, phenylephrine, angiotensin II, serotonin, and the thromboxane A2 agonist, U46619.

RESULTS

It was found that the intraluminal diameter of third-generation pulmonary arterioles from CDH rats was significantly less than in controls (129 +/- 5 micron v 152 +/- 9 micron, respectively). In addition, the ratio of wall thickness to vessel internal diameter was increased in the third-generation pulmonary arterioles of rats with nitrofen-induced CDH (0.62 +/- 0.4 v 0.50 +/- 0.5 for controls). Responses to K+-induced depolarization, phenylephrine, angiotensin II, serotonin, and U46619, however, were not different for pulmonary arterioles from control and CDH rats.

CONCLUSION

These data suggest that the structural alterations of the pulmonary vasculature observed in infants with CDH may not cause exaggerated vasoconstrictor responses to normal vasoconstrictor stimuli.