While numerous studies have shown an association between reduced hippocampal neurogenesis and increased risk for depressive-like symptoms Warner-Schmidt and Duman ; Eisch and Petrik , fewer studies have tested the relationship between MDD and OB neurogenesis Siopi et al.
The reduction in levels of neurogenesis and OB volume in the UCMS rat model indicate that the same stress that contributes to the expression of depressive-like behavior can impact OB neurogenesis.
While there is debate about the existence of adult OB neurogenesis in humans Curtis et al. Thus, it is possible that stress could contribute to impairments in olfactory perception in MDD patients, however, these linkages must be formally tested. While specific olfactory impairments have not yet been highlighted in animal models of depression, it would be relevant to test specific parameters, including olfactory acuity and sensitivity as well as discrimination ability in animal models of depression in which reduced OB neurogenesis has been observed.
Importantly, studies showing decreased neurogenesis in animal model of depression have been able to restore levels of neurogenesis in the hippocampus Banasr and Duman ; Rochet et al. To better understand the brain regions involved in olfactory deficits in models of depressive-like behavior, Croy and Hummel posit that olfactory receptor turnover rate in the olfactory epithelium is decreased in depression.
This is a potentially interesting potential mechanism for olfactory disturbance in MDD and could explain impaired olfactory threshold, identification or discrimination, in patient populations Croy and Hummel Other studies have shown that the habenula involved in the transfer of olfactory information to other brain areas might also be affected in depression.
Models of bilateral bulbectomy in rodents revealed a higher level of apoptosis in the habenula during depression, which could contribute to its role in olfactory disturbance in depression Brand and Schaal Numerous studies using the LH rat model of depressive-like behavior have shown important metabolic changes in regions of frontal cortex and hippocampus as well as a reduction in BDNF in the medial prefrontal cortex and dentate gyrus, disturbance in lipid metabolism, glutamatergic metabolism, and neurotransmission Shirayama et al.
LH was also observed to have significant impacts on the habenula, amygdala, insular and cingulate cortex Shumake and Gonzalez-Lima ; Shumake et al. Interestingly, these regions are known to be involved in attention, emotional, and cognitive processes and also in olfactory processing Kesner et al. In this context, alterations of these regions in LH rats may significantly diminish olfactory sensory function, however, olfactory performance has not yet been thoroughly tested in this model.
Possible effects of disturbance in the function of these regions could include diminished olfactory sensitivity or altered hedonic perception of odorants, which have not yet been evaluated in LH rats.
Numerous studies have also analyzed the consequences of repeated exposure to social defeat in mice on neural activity in the prefrontal cortex, cingulate cortex, hippocampal formation, amygdala, and hypothalamic nuclei Sheline et al. As described above, these brain regions form important limbic structures implicated in a wide variety of emotional, cognitive, and behavioral control processes Kumari et al. These regions have also been implicated in the processing of olfactory information.
As one example, social defeat is associated with increased activity in the amygdala which could impact the processing of olfactory signals, influencing the perceived hedonic value, the discriminability, or emotional significance of olfactory stimuli. Based on that work, it seems relevant to test the effects of social defeat on hedonic perception and response to odors. A recent study by Czarnabay et al. Further, MS pups took more time to identify odorants in an olfactory learning task.
Thus, stress during a key period in early development appears to have negative and long-lasting effects on olfactory function in the offspring. While the primary results indicate deficits in olfactory memory of rodents raised in MS conditions, further experiments are needed to test for broader effects of MS on olfactory sensory function and possible mechanisms linking delayed development of major olfactory structures with deficit in olfactory memory.
In summary, studies using animal models of depressive-like behavior have commonly shown a reduced volume of crucial olfactory and limbic structures, including the OB and hippocampus, along with diminished neurogenesis.
Numerous studies have also highlighted dysfunction of major neurotransmitter systems associated with metabolic changes including impacts on the serotonergic and dopaminergic systems in regions that are implicated in the sensing and perception of olfactory signals. While some studies have found links between depressive-like behavior and alterations in olfactory identification, few studies have tested the effects of models of depressive-like behavior on other parameters of olfactory perception.
While depression has been linked with modified hedonic perception of odorants in humans, the linkage between depressive-like phenotypes and hedonic perception of odorants in rodent models remains largely unstudied. Studies focusing on olfactory parameters like odorant sensitivity, discrimination, and hedonic perception of odorants in animal models of depression are required to further our understanding of the links between olfaction and depression and to connect results observed in animal models with results found in depressed patients.
In aggregate, these results illustrate the need for additional work and more thorough analysis of olfactory perception in particular, hedonic processing of odorants in the different animal models of depressive-like behavior. Specifically, there is a lack of important information with regard to results from animal models of depression and olfactory perceptual capacities, including effects on olfactory threshold, discrimination, identification, or hedonic perception that will be needed to draw clearer conclusions regarding the impact of depression on olfaction and its neural basis.
With regard to the existing bidirectional relationship between olfaction and depression, it seems that the quality of life of depressed patients is more affected when alterations of the olfactory processing are observed Kohli et al. Quality of life is a complex concept, influenced by the physical health of the subject, their psychological state, their level of independence, their social relationships, and their perception of their environment Rochet et al.
However, since odors provide critical information for survival e. Brand and Schaal ; Croy and Hummel , the impact of depression on olfactory function has the potential to impact or worsen the quality of life in the context of broader pathology. Studying the relationship between depression and olfactory sensory function has the potential to provide unique insights into the possible neurobiological basis of disease, symptom expression, and possible novel targets for treatments and interventions.
However, in executing such studies, key variables must be taken into account when choosing the sample population, model system, and measures to assess these critical questions. In the current review, we have discussed a number of topics related to our current understanding of the relationship between depression and olfactory sensory function in both human clinical populations and animals models of this debilitating disease.
Based on our assessment of the literature, a number of issues remain to be tackled in this field. In clinical populations, there is a great deal of variability in both methods being used to test olfactory sensory function, dimensions of olfactory function being tested sensitivity, discrimination, hedonic valuation , and heterogeneity in selection criteria for inclusion of individuals with varied forms of depression.
In future work, it will be important to take care to assess multiple aspects of olfactory sensory function as well as control for the composition of the population e. We have also highlighted the potential importance of using animal models to further probe these links.
Specifically, more work is needed to assess sensory function at the neural and behavioral level in models of genetic and environmental risk for behavioral profiles associated with depressive-like behavior. The use of animal models may provide the level of control that is often difficult to attain in human samples to further assess the directionality of the relationship between depression and sensory disturbance.
Further, such models provide researchers with the ability to directly probe the neural underpinnings of these effects to better understand the reason for the overlap between sensory disturbance and pathology.
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Mitra R , Sapolsky RM. As well as a sensitivity to pain, depression can leave us feeling sensitive to touch. This means that we might find things like hugs unpleasant or even painful. Certain textures might put us on edge, so we avoid them. Different types of touch and texture can feel intolerable to us. This can make intimacy difficult and might leave us finding it difficult to relax or wind down.
Click To Tweet. Depression can dull our sense of smell. The part of our brain that is responsible for our sense of smell, our olfactory bulbs, can be smaller in those of us with depression than the same part of the brain in those without depression.
The more strongly we experience depression, the smaller our olfactory bulb is likely to be. Our taste can be dampened by depression. When we feel low, or emotionally exhausted , many of us experience noise sensitivity. This can cause us to find everyday sounds hard to cope with which can leave us feeling incredibly irritable and anxious.
Some of us will struggle with loud noises, others might find specific noises particularly annoying, and some of us might find ordinary sounds quite painful. Many of us will experience sensory overload. Sensory overload happens when we have more input from our senses than our brains can cope with.
This can leave us feeling confused, anxious, and sometimes quite distressed. Once we understand how each of our senses affects us, we can begin to manage them and use them to help us. Thinking about touch, if we find certain textures to be particularly abrasive, then we can try and make our environment as soft as possible. We could put soft blankets on our bed, sofa, or even in our car.
If we struggle with the texture of our clothes, then we could add fabric softener to our washes, or we could try wearing a thin cotton top under our clothes so that we have a less abrasive fabric touching our skin.
Sometimes being under something heavy can help us to feel calm, so a heavy jumper or weighted blanket can help to lower our anxiety. Though our sense of smell can be dulled, if there are particular smells that we find helpful then we can make add them to the world around us — for example, we could use aromatherapy oils , or put one or two diffusers around our house.
A major life event or stress can trigger symptoms. This article was contributed by: familydoctor. This information provides a general overview and may not apply to everyone.
Talk to your family doctor to find out if this information applies to you and to get more information on this subject. You may hear conflicting reports from different sources.
The U. Visit The Symptom Checker. Read More. Food Poisoning. Acute Bronchitis. Eustachian Tube Dysfunction. Bursitis of the Hip. Abnormal Uterine Bleeding. High Blood Pressure. Table of Contents. What is sensory processing disorder? Symptoms of sensory processing disorder. Children may be oversensitive if they: Think clothing feels too scratchy or itchy. Think lights seem too bright. Think sounds seem too loud. Think soft touches feel too hard. Experience food textures make them gag. Have poor balance or seem clumsy.
Are afraid to play on the swings. React poorly to sudden movements, touches, loud noises, or bright lights. Have behavior problems. Can spin without getting dizzy. Chew on things including their hands and clothing. Seek visual stimulation like electronics. Have problems sleeping. What causes sensory processing disorder? How is sensory processing disorder diagnosed?
Can sensory processing disorder be prevented or avoided? Sensory processing disorder treatment. There are different types of therapy: Sensory integration therapy SI. A sensory diet at school might include: A time every hour when your child could go for a minute walk. A time twice a day when your child could swing for 10 minutes. Access to in-class headphones so your child can listen to music while working. Access to fidget toys. Access to a desk chair bungee cord.
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