The mechanism of obesity development may be simplistically described as energy intake in excess of energy loss. But the energy intake is determined by the interplay of different factors to varying degrees under diverse circumstances modified by the fed or sated state, the mental state, as well as the exposure or availability of bland vs. palatable food in a sociable or unsociable environment. Hence it is difficult to ascribe a specific defect in any one peptide or pathway to be predominantly responsible for obesity development. Further the individual contribution of different candidate culprits under varying circumstances may greatly fluctuate, making generalisations regarding obesity pathogenesis difficult.
As an example, mice and men may eat more in the fasted state than in the fed state. But this feeding can be further modified by offering bland vs. tasty food! or in other words, wanting food vs. liking food. A starved mouse may work harder to reach tasty food than it would to get at bland chow! Subtle behavioural changes like these would greatly modify energy balances in the long term by producing opportunistic over-consumption of food, and hence not only appetite, but also the drive to quench it and the availability of means to quench it would also determine development of obesity. A starving man on death row offered ad libitum food on the penultimate day may eat less than a starving punk let loose at MacDonalds. The contribution of the emotional component to appetite and its mechanisms will be difficult to study using rodents.
(Not necessarily in order)
Identify, Clone and sequence the gene of a candidate peptide
Establish sites of peptide expression (e.g.: demonstrate immunoreactivity for the peptide in neurons or tissues)
Assess the target site of action for the peptide (e.g.: c-fos studies in the brain suggesting peptide induced activity)
Assess for synthesis of the peptide in tissues (mRNA expression in the tissues)
Establish active and inactive forms of the peptide in circulation or tissue
Determine levels of the peptide in serum and tissues in lean and obese animals
Determine change in peptide concentration in blood and CNS with weight loss and weight gain
Identify receptors and receptor subtypes for the peptide
Characterise receptor polymorphisms and their phenotypes
Determine receptor expression and concentration in various sites of the brain/hypothalamic nuclei and periphery (gut and adipose tissue)
Identify co-expression of receptors or peptides in the hypothalamus to determine possible inter-system interactions on appetite control
Demonstrate phylogenetic preservation of the peptide across species
Determine variations of other known peptides in relation to the candidate peptide in the lean and obese animals
Conversely, Determine variations of the candidate peptide in relation to other know peptides
Identify the modifying factors or regulators of the peptide
nutrient composition of meal (fat vs. protein vs. carbohydrate)
calorie value of meals
vagal contribution (cephalic phase vs. gastric phase)
Inject a peptide and demonstrate appetite modulation in rodent or man
Assess effect of varied routes of administration of the peptide (centrally into the ventricle, peripherally into the veins or peritoneum)
Differentiate between the effects elicited by pharmacological and physiological doses of the peptide
Assess for species specificity of the injected peptide
Determine the effect of blockade of actions of the peptide using
Knock-out mice (targeted gene disruption) which do not express the peptide
Receptor knock out animals
Blockade of downstream effector peptides if identified
Determine the effect of over-expression of the peptide (knock-in) using trangenics
Study the effects of tissue specific gene knock in or knock outs
Determine the effects of homozygote and heterozygote states for the gene
Determine the effects of long term administration especially tolerance and development of antibodies
Difficulties with Obesity Studies
The rat is useful as a model for obesity as it has similar omnivorous tendencies and preferences for tasty food as do humans. But No man is a mouse. Rodents have brown fat which is minimal in humans. Rodents show predominantly nocturnal feeding.
While a demonstration of phylogenetic preservation of a peptide and its structure in different species suggests a possible physiological role for the peptide, inter-species differences can produce confusing results in studies involving peptide administration if the appropriate species-specific peptide is not used. Cross reactivity from other homologous peptides has to be ruled out to ensure that measurement are in-fact reflecting the actual peptide under study. Dosing adjustment depending on the site of administration (e.g. injection into hypothalamic nuclei) is also important to avoid confusing results secondary to high levels of the peptide diffusing into surrounding brain areas.
Measurement of serum or tissue levels of a peptide has to distinguish between the inactive, active or total levels of the peptide.
Changes of a peptide with meals has to be distinguished as cause or effect with regard to energy intake. The role of a peptide in energy balance has to be clarified with regard to short and long term energy balance. While a meal related fluctuation may be demonstrable, the long term changes in fat mass consequent to energy intake may influence the levels of the peptide as well.
A molecule injected centrally may produce effects different to its peripheral administration confusing interpretation of its role in energy balance. (e.g.: Pancreatic polypeptide) For example, a peptide which normally does not cross the blood brain barrier may produce appetite modulation when injected centrally, but the action would not probably be effected physiologically. Thus a mere stimulation or suppression of appetite does not necessarily mean that the particular peptide has physiological relevance.
A peptide should not produce decreased food intake due to increased drowsiness (e.g.. cannabinoids) or due to increased nausea or due to production of irritability (e.g. peritonitis in animals on intra-peritoneal injection), as these are not direct actions on the appetite regulation pathway and are unlikely to be acceptably translated into humans.
Developmental transgenic (knock in) or targeted gene deletion (knock out) studies may result in compensation by other systems or peptides resulting in effects that may not be totally attributable to the absence or presence of the peptide under study.
Types of Rodents used in Obesity Research
Types of rodents used in Diabetes Research
Guinea pigs for carbohydrate restriction and lipid studies
Phases of Human Trials:
Phase I study: A new research treatment is given to a small number of patients. Aspects determined during this phase are: the best way to give a new treatment, how much of it can be safely given harmful side effects Once a peptide has been tested in laboratory animals, studies are done in humans in different phases
Phase II :Each newphase of a clinicaltrail depends on and builds on information from an earlier phase with greater numbers of patients. If a treatment has shown to be effective in Phase II, it moves to Phase III.
Phase III: the new treatment is directly compared to the old to compare effectiveness and tolerability.
Phase IV studies, the new research treatment becomes part of standard treatment in patient care and is used together with other effective drugs, or with surgery