Effects of different routes of nicotine administration on gastric morphology and hormonal secretion in rats

Neurobiological mechanisms of nicotine's effects on feeding and body weight

Nicotine, a neuroactive substance in tobacco products, has been widely studied for its effects on feeding and body weight, mostly focusing on the involvement of nervous system, metabolism, hormones, and gut microbiota. To elucidate the action mechanism of nicotine on feeding and body weight, especially the underlying neurobiological mechanisms, we reviewed the studies on nicotine's effects on feeding and body weight by the regulation of various nerve systems, energy expenditure, peripheral hormones, gut microbiota, etc. The role of neuronal signaling molecules such as AMP-activated protein kinase (AMPK) and kappa opioid receptor (κOR) were specialized in the nicotine-regulating energy expenditure. The energy homeostasis-related neurons, pro-opiomelanocortin (POMC), agouti-related peptide (AgRP), prolactin-releasing hormone (Prlh), etc, were discussed about the responsibility for nicotine's effects on feeding. Nicotine's actions on hypothalamus and its related neural circuits were described in view of peripheral nervous system, reward system, adipose browning, hormone secretion, and gut-brain axis. Elucidation of neurobiological mechanism of nicotine's actions on feeding and body weight will be of immense value to the therapeutic strategies of smoking, and advance the medicine research for the therapy of obesity.

Exposure to nicotine regulates prostaglandin E2 secretion and autophagy of granulosa cells to retard follicular maturation in mammals

Exposure to nicotine by cigarette smoking have shown strongly defectives on the physiological function of ovaries, which in turn leads to disorders of fertility in women. However, the potential molecular mechanisms remain to be elucidated. In this study, we notably found that nicotine was likely to specifically raise the expression of histone deacetylase 3 (HDAC3) to promote the apoptosis and autophagy of granulosa cells (GCs) and block follicular maturation. Moreover, prostaglandin E2 (PGE2) inhibited the apoptosis of GCs and facilitated follicular maturation, and nicotine appeared to inhibit PGE2 secretion by freezing the expression of cyclooxygenase 1 (COX1), which was the rate-limiting and essential enzyme for PGE2 synthesis. Epigenetically, the nicotine was observed to diminish the histone H3 lysine 9 acetylation (H3K9ac) level and compact the chromatin accessibility in -1776/-1499 bp region of COX1 by evoking the expression of HDAC3, with the deactivated Cas9-HDAC3/sgRNA system. Mechanistically, the COX1 protein was found to pick up and degrade the autophagy related protein beclin 1 (BECN1) to control the autophagy of GCs. These results provided a potential new molecular therapy to recover the damage of female fertility induced by nicotine from cigarette smoking.

Acute nicotine intake increases feeding behavior through decreasing glucagon signaling in dependent male and female rats

Chronic use of nicotine is known to dysregulate metabolic signaling through altering circulating levels of feeding-related hormones, contributing to the onset of disorders like type 2 diabetes. However, little is known about the acute effects of nicotine on hormonal signaling. We previously identified an acute increase in food intake following acute nicotine, and we sought to determine whether this behavior was due to a change in hormone levels. We first identified that acute nicotine injection produces an increase in feeding behavior in dependent rats, but not nondependent rats. We confirmed that chronic nicotine use increases circulating levels of insulin, leptin, and ghrelin, and these correlate with rats' body weight and food intake. Acute nicotine injection in dependent animals decreased circulating GLP-1 and glucagon levels, and administration of glucagon prior to acute nicotine injection prevented the acute increase in feeding behavior. Thus, acute nicotine injection increases feeding behavior in dependent rats by decreasing glucagon signaling.

Emerging role of mitochondrial DAMPs, aberrant mitochondrial dynamics and anomalous mitophagy in gut mucosal pathogenesis

Gastroduodenal inflammation and ulcerative injuries are increasing due to expanding socio-economic stress, unhealthy food habits-lifestyle, smoking, alcoholism and usage of medicines like non-steroidal anti-inflammatory drugs. In fact, gastrointestinal (GI) complications, associated with the prevailing COVID-19 pandemic, further, poses a challenge to global healthcare towards safeguarding the GI tract. Emerging evidences have discretely identified mitochondrial dysfunctions as common etiological denominators in diseases. However, it is worth realizing that mitochondrial dysfunctions are not just consequences of diseases. Rather, damaged mitochondria severely aggravate the pathogenesis thereby qualifying as perpetrable factors worth of prophylactic and therapeutic targeting. Oxidative and nitrosative stress due to endogenous and exogenous stimuli triggers mitochondrial injury causing production of mitochondrial damage associated molecular patterns (mtDAMPs), which, in a feed-forward loop, inflicts inflammatory tissue damage. Mitochondrial structural dynamics and mitophagy are crucial quality control parameters determining the extent of mitopathology and disease outcomes. Interestingly, apart from endogenous factors, mitochondria also crosstalk and in turn get detrimentally affected by gut pathobionts colonized during luminal dysbiosis. Although mitopathology is documented in various pre-clinical/clinical studies, a comprehensive account appreciating the mitochondrial basis of GI mucosal pathologies is largely lacking. Here we critically discuss the molecular events impinging on mitochondria along with the interplay of mitochondria-derived factors in fueling mucosal pathogenesis. We specifically emphasize on the potential role of aberrant mitochondrial dynamics, anomalous mitophagy, mitochondrial lipoxidation and ferroptosis as emerging regulators of GI mucosal pathogenesis. We finally discuss about the prospect of mitochondrial targeting for next-generation drug discovery against GI disorders.

The Role of Ghrelin/GHS-R1A Signaling in Nonalcohol Drug Addictions

Drug addiction causes constant serious health, social, and economic burden within the human society. The current drug dependence pharmacotherapies, particularly relapse prevention, remain limited, unsatisfactory, unreliable for opioids and tobacco, and even symptomatic for stimulants and cannabinoids, thus, new more effective treatment strategies are researched. The antagonism of the growth hormone secretagogue receptor type A (GHS-R1A) has been recently proposed as a novel alcohol addiction treatment strategy, and it has been intensively studied in experimental models of other addictive drugs, such as nicotine, stimulants, opioids and cannabinoids. The role of ghrelin signaling in these drugs effects has also been investigated. The present review aims to provide a comprehensive overview of preclinical and clinical studies focused on ghrelin's/GHS-R1A possible involvement in these nonalcohol addictive drugs reinforcing effects and addiction. Although the investigation is still in its early stage, majority of the existing reviewed experimental results from rodents with the addition of few human studies, that searched correlations between the genetic variations of the ghrelin signaling or the ghrelin blood content with the addictive drugs effects, have indicated the importance of the ghrelin's/GHS-R1As involvement in the nonalcohol abused drugs pro-addictive effects. Further research is necessary to elucidate the exact involved mechanisms and to verify the future potential utilization and safety of the GHS-R1A antagonism use for these drug addiction therapies, particularly for reducing the risk of relapse.

A population-based investigation of the association between alcohol intake and serum total ghrelin concentrations among cigarette-smoking, non-alcohol-dependent male individuals

BACKGROUND

Ghrelin plays significant roles in regulating appetite, food intake, and metabolism. Furthermore, the ghrelin system is increasingly being studied in relation to alcohol seeking behaviors. To this end, it is important to understand the possible effects of alcohol intake on the ghrelin system. The aim of the present study was to investigate the association between alcohol drinking and circulating ghrelin levels in a large sample of cigarette-smoking, non-alcohol-dependent male individuals.

METHODS

We utilized data from two nested case-control studies (study A, n = 807; study B, n = 976) based within the Alpha-Tocopherol, Beta-Carotene Cancer Prevention (ATBC) trial. Data on alcohol consumption (grams of pure alcohol consumed per day) were obtained via a food frequency questionnaire. Blood samples were also collected (after 12 h of fasting), and serum concentrations of total ghrelin were measured by radioimmunoassay.

RESULTS

Dichotomous comparison between alcohol drinkers (>0 g/day of alcohol intake) and non-drinkers (0 g/day of alcohol intake) found higher total ghrelin levels among individuals who drank alcohol than those who did not, with statistically significant results in study A [F (1, 798) = 4.32, P = 0.03] and less robust results in study B [F (1, 966) = 2.62, P = 0.10], controlling for a list of factors that may influence ghrelin levels and/or differ between drinkers and non-drinkers. Bivariate correlational analysis among drinkers found no association between the quantity of daily alcohol intake and blood total ghrelin concentrations.

CONCLUSION

These results indicate elevated ghrelin levels among alcohol drinkers and provide additional/relevant information on the complex interaction between alcohol use and the ghrelin system.

Nicotine and energy balance: A review examining the effect of nicotine on hormonal appetite regulation and energy expenditure

Nicotine has been shown to decrease appetite, food intake (FI) and body weight, but the mechanisms are unclear. The purpose of this review was to examine research on the effects of nicotine on energy balance by exploring physiological mechanisms and hormone regulation related to FI, subjective appetite and energy expenditure (EE). We searched PubMed and MEDLINE, and included articles investigating the effects of nicotine on central appetite regulation, FI, leptin, peptide-YY (PYY), ghrelin, glucagon-like peptide-1 (GLP-1), adiponectin, cholecystokinin (CCK), orexin, and EE. A total of 65 studies were included in the qualitative synthesis and review. Our findings suggest that the decrease in appetite and FI may be attributed to nicotinic alterations of neuropeptide Y (NPY) and pro-opiomelanocortin (POMC) but the effect of nicotine on FI remains unclear. Furthermore, nicotine increases resting EE (REE) and physical activity EE (PAEE) in both smokers and non-smokers; and these increases may be a result of the catecholaminergic effect of nicotine. Decreases in body weight and appetite experienced by nicotine users results from increased EE and changes in the central hypothalamic regulation of appetite. There is not enough evidence to implicate a relationship between peripheral hormones and changes in appetite or FI after nicotine use. Although nicotine increases REE and PAEE, the effect of nicotine on other components of EE warrants further research. We conclude that further research evaluating the effect of nicotine on appetite hormones, FI and EE in humans is warranted.

THE INTRIGUING LIGAND-DEPENDENT AND LIGAND-INDEPENDENT ACTIONS OF THE GROWTH HORMONE SECRETAGOGUE RECEPTOR ON REWARD-RELATED BEHAVIORS

The growth hormone secretagogue receptor (GHSR) is a G-protein-coupled receptor (GPCR) highly expressed in the brain, and also in some peripheral tissues. GHSR activity is evoked by the stomach-derived peptide hormone ghrelin and abrogated by the intestine-derived liver-expressed antimicrobial peptide 2 (LEAP2). In vitro, GHSR displays ligand-independent actions, including a high constitutive activity and an allosteric modulation of other GPCRs. Beyond its neuroendocrine and metabolic effects, cumulative evidence shows that GHSR regulates the activity of the mesocorticolimbic pathway and modulates complex reward-related behaviors towards different stimuli. Here, we review current evidence indicating that ligand-dependent and ligand-independent actions of GHSR enhance reward-related behaviors towards appetitive stimuli and drugs of abuse. We discuss putative neuronal networks and molecular mechanisms that GHSR would engage to modulate such reward-related behaviors. Finally, we briefly discuss imaging studies showing that ghrelin would also regulate reward processing in humans. Overall, we conclude that GHSR is a key regulator of the mesocorticolimbic pathway that influences its activity and, consequently, modulates reward-related behaviors via ligand-dependent and ligand-independent actions.

Metabolic Effects of Testosterone Hormone Therapy in Normal and Orchiectomized Male Rats: From Indirect Calorimetry to Lipolytic Enzymes

BACKGROUND AND AIM

Changes in total energy expenditure (TEE) and substrate metabolism may help explain the metabolic actions of testosterone (T). This study measured respiratory quotient (RQ), TEE, ghrelin, insulin, and key lipolysis enzyme concentrations in relation to body weight (wt) and food intake (FI) in both normal and bilaterally orchiectomized rats with/without T treatment.

METHODS

In total, thirty-two male Wistar rats (300-400 g) were divided into four groups (n = 8/group), including (a) sham-operated and vehicle-injected group (Sham), (b) T-treated sham group (T-Sham) for which sham-operated rats were injected with IM testosterone undecanoate (100 mg/kg, for one week), (c) orchiectomy and vehicle-injected group (Orch), and (d) T-replaced orchiectomy group (T-Orch). After one week, FI and wt were automatically recorded, indirect calorimetry parameters were measured, and blood samples were collected to measure T, ghrelin, insulin, growth hormone (GH), glucose, hormone-sensitive lipase (HSL), adipocyte triglyceride lipase (ATGL), free fatty acids (FFA), and lipid profiles.

RESULTS

Orchiectomy decreased ghrelin, GH, and insulin levels, increased TEE and RQ, and lowered FI and wt. The T-Orch group exhibited increased levels of ghrelin (3-fold), insulin, GH, blood levels of lipolysis products, TEE, and FI in addition to reduced glucose levels (P < 0.05). This group demonstrated no significant changes in wt. In the T-Sham group, T increased ghrelin and insulin levels (P < 0.05) with strong positive correlations (r = 0.663 and 0.644, respectively, P < 0.05), increased ATGL levels, RQ toward carbohydrate utilization ranges, and TEE, and reduced HSL levels (P < 0.05) with insignificant changes in FI or wt.

CONCLUSIONS

T administration in orchiectomized rats significantly increased orexigenic mediators such as ghrelin and insulin without inducing any significant changes in wt. The mechanism for this finding might be the increased TEE and the stimulation of lipolysis through the ATGL enzyme. The associated rise of GH might help in interference with accumulation of lipid in adipose tissue. Apart from the effect on GH, T-Sham showed similar effects of T supplementation.

Effect of nicotine on placental ischemia-induced complement activation and hypertension in the rat

Preeclampsia is a pregnancy-specific condition manifested by new-onset maternal hypertension with systemic inflammation, including increased innate immune system complement activation. While exact pathophysiology is unknown, evidence suggests that inadequate spiral artery invasion and resulting utero-placental insufficiency is the initiating event. Cigarette smoking during pregnancy decreases the risk of preeclampsia. Nicotine, a major component of cigarettes, stimulates the efferent cholinergic anti-inflammatory pathway through peripherally expressed nicotinic acetylcholine receptors (nAChR) and is known to attenuate ischemia-reperfusion injury in kidney and liver. Prior studies indicated that complement activation was critical for placental ischemia-induced hypertension in a rat model. Thus, it was hypothesized here that nicotine was responsible for the protective effect of cigarette smoking in preeclampsia and would attenuate placental ischemia-induced systemic complement activation and hypertension. The Reduced Utero-placental Perfusion Pressure (RUPP) model in the pregnant rat was employed to induce placental ischemia, resulting in complement activation, fetal resorptions, and hypertension. On gestation day (GD)14, nicotine (1 mg/kg) or saline was administered via subcutaneous injection prior to RUPP surgery and daily through GD18. On GD19, placental ischemia significantly increased mean arterial pressure (MAP) in saline injected animals. However, the placental ischemia-induced increase in blood pressure was not evident in nicotine-treated animals and nicotine treatment significantly increased MAP variability. Circulating C3a was measured as an indicator of complement activation and increased C3a in RUPP compared to Sham persisted with nicotine treatment, as did fetal resorptions. These data suggested to us that nicotine may contribute to the decreased risk of preeclampsia with cigarette smoking, but this protective effect was confounded by additional effects of nicotine on the cardiovascular system.

The Role of the Ghrelin System in Drug Addiction

In the past years, a significant volume of research has implicated the appetitive hormone ghrelin in the mechanisms underlying drug use and addiction. From a neuroscientific standpoint, ghrelin modulates both reward and stress pathways, two key drivers of substance use behaviors. Previous investigations support a connection between the ghrelin system and alcohol, stimulants, and tobacco use in both animals and humans, while the research on opioids and cannabis is scarce. In general, upregulation of the ghrelin system seems to enhance craving for drugs as well as substances use. On the other hand, acute and chronic exposure to drugs of abuse influences the ghrelin system at different levels. This chapter summarizes the literature on the relationship between the ghrelin system and substance-related behaviors. We also review recent work investigating the ghrelin system as a potential pharmacological target for treating substance use disorders and discuss the need for additional research.