Neuroinflammation is considered a highly preserved physiological defensive process, triggered by various insults that can reach the brain, aimed at restoring homeostasis. It allows the elimination of the underlying cause, repair injured tissue, and restore normal functions. However, under some circumstances, neuroinflammation can also have detrimental consequences.
Over the last decades, life expectancy is getting longer, and the traditional 65-year chronological age threshold for entering old age has recently been questioned in favor of the fixed remaining life expectancy as a better indicator . Similarly, the life expectancy of dogs and cats has been increasingly expanding during the last years, mainly thanks to the tremendous improvement of veterinary nutrition and medicine . Currently, dogs are considered senior when they enter the last 25% of their predicted life expectancy, while feline pets are viewed as “old” between 10 and 14 years, with geriatric age beginning thereafter .
In human medicine, successful aging is a multidimensional term to describe seniors with minimal physical and health impairment, autonomous and satisfactory social interactive life, as well as prompt cognitive function . In veterinary medicine, “successfully aging” pets are considered those who do not show important impairments in daily routine  while usually manifesting slow deterioration in general activity and playing behavior . Failure to respond to commands, decreased interest toward novelty, increased time spent sleeping, and frequency of phobias, as well as reduced ability to cope with mild social challenges are further features of normal aging in pets . Learning and memory are not negatively affected in most cases, although cognitive abilities tend to slow down over time , especially if comorbidities are present, due to the recently acknowledged “frailty syndrome” of the elderly .
Age-related cognitive decline in pets is a hot topic in veterinary medicine, mainly due to its striking similarities with human Alzheimer’s disease (AD). Cognitive dysfunction syndrome (CDS) refers to unsuccessful brain aging with AD-like cognitive decline in elderly pets. CDS encompasses behavioral (i.e., neuropsychological)  and physical signs . The neuropsychological signs of canine and feline CDS are well described and classically summarized with the acronym D.I.S.H.A. that stands for Disorientation, altered Interactions, Sleep-wake cycle changes, breaking in the House soiling and altered Activity levels with newly onset anxiety . Disorientation, or confusional mental status, is a common finding (over half of the cases), with affected dogs mainly showing aimless stereotypic wandering or stopping in front of the wrong edge of the front door. Vocalization and staring at the empty space as well as getting “stuck” in corners or narrow places are further frequent signs (Figure 1).
Altered interaction with owners is reported in about 50% of CDS dogs, and increased aggressive behavior can also be observed, albeit less frequently . Sleep-wake cycle disturbance is one of the commonest clinical features and is characterized by an extremely altered REM cycle with frequent awakening during the night and increased daytime sleep . Lost housetraining has been reported in about one-third of CDS patients, and in 16%, active soiling could be observed. Altered emotions with increased novelty anxiety are also frequently detected . The main neurobehavioral signs of canine CDS are summarized in Table 1.
|Mental status and spatial orientation (confusional status)
Get lost in a known environment
Awaiting the door opening on the wrong side
Inability to circumnavigate unknown objects
Less interested in environmental stimuli
|Relationships (social interaction)
Less interested in being touched
Ignoring the return of the owner
Social behavior is disrupted
Increased need for physical contact (is “needy”)
Starring at objects or empty space, fly biting
Increased licking behavior (on the owner or objects)
Apathetic, less interested in exploring
Seems to not be interested anymore in known stimuli
Eats more than usual
Eats less than usual
Reduced time spent cleaning itself
Often irritable or anxious
Shows signs of separation anxiety that has never had before
Short period of sleep interrupted by frequent abrupt awakenings
Sleeps more than usual during daytime
|Learning and memory
Loss of housetraining, urinating or defecating in front of the owner
Does not request to go out anymore
Despite regular daily activity eliminates only when back home
Eliminates where it sleepsIt is incontinent
|Learned behavior and commands
Struggle in performing a previously learned task
Struggle to recognize a member of the family or other known people/animals
Struggle to respond to commands
Struggle to learn new commands or tasks
A very interesting clinical parallelism with AD in human beings is the recognition of an intermediate phase between clinical normal aging subjects and clinically demented pets. This intermediate step has also been named mild cognitive impairment (MCI), borrowing it from human medicine . MCI has been defined as reduced cognitive ability without significant interference with daily life . A recent study showed that dogs with MCI exhibit reduced social interaction and novelty fear without signs of breaking of the housetraining nor sleep–awake cycle disturbance .
On the physical side of CDS, a large web-based survey recently performed in dogs aged 10 years or older found that vision impairment, smell disturbance, tremor, swaying or falling, and head ptosis were the main physical disturbances related to CDS . Interestingly, similar signs are also prevalent in demented human patients  and extrapyramidal signs within an MCI state are predictive of faster progression to full dementia . In this context, it is noteworthy that dogs with CDS were found to be twice as likely to show neurologic deficits compared to “successfully aging” dogs .
Accumulated evidence suggests that AD pathogenesis is not limited to the neuronal compartment but comprises important interactions with immunological mechanisms in the central nervous system (CNS). It is now well accepted that neuroinflammation contributes to AD pathology. Neuroinflammation is driven by the activation of different brain cells, mainly microglia, astrocytes, and CNS mast cells. If the activity of these cells is under control, neuroinflammation operates correctly and then is turned off. On the contrary, if cell responses escape the physiological control systems, neuroinflammation remains “on” fostering pathological conditions, such as cognitive dysfunction and chronic pain (Figure 2).
Microglia modifications have also been described in humans, where the presence of microglial activation and proliferation in several brain regions of AD patients have been observed [134–137]. Very recently, a study in family-owned domestic dogs with CDS has shown a statistically significant increase of microglial numbers in CDS dogs compared to age-match controls . Moreover, signs of microglia hypertrophy and activation were also observed . Studies over the last twenty years have progressively brought to light the involvement of glial reactivity and inflammation in AD, even if the underpinning molecular mechanisms have not yet been fully clarified. For a long while, it was believed that glial cells acquire a reactive phenotype and foster inflammation upon β-amyloid deposition or neurodegeneration. Nowadays, accumulated evidence indicates that reactive gliosis and inflammation occur in the very early phase of the disease before histopathological modifications . On the contrary, other studies on rodent models of AD indicate that the early astroglia response is represented by cell atrophy that may have important consequences for synaptic connectivity, thereby contributing to cognitive deficits . These signs of atrophy appear first in the entorhinal cortex and affect astrocytes located afar from senile plaques in the later stages of AD.
Some authors have hypothesized that mast cells are one of the first brain cells that sense amyloid peptides, thus exerting an important role in AD onset and (possibly) progression . In agreement, post-mortem studies have shown mast cells surround amyloid plaques in AD patients in higher numbers in comparison to age-matched control patients . The evidence available so far, although limited, supports a possible role for mast cells in AD pathology  and opens exciting new fields of research for neuroscientists.
Overall, mast cells, as well as microglia and astrocytes, are now considered critical effectors during several neuroinflammatory disorders, including AD . The modulation of the crosstalk between mast cells and glial cells is emerging as a valuable approach to treat these brain pathologies .
Recently, endocannabinoids and their lipid congeners are emerging as pro-resolving agents due to their ability to stimulate resolving programs during neuroinflammation . Among these compounds, great attention had been focused on palmitoylethanolamide (PEA), an anandamide congener. PEA is an endogenous lipid compound, an amide of ethanolamide and palmitic acid, firstly isolated from soy lecithin . Interestingly, PEA is able to modulate the activity of both glial and mast cells, thus representing a very promising therapeutic tool to treat neurological disorders . These effects are likely due to the ability of PEA to interact with the endocannabinoid system  and partially involve the CB2 receptor . Under some circumstances, PEA may potentiate the actions of the canonical endocannabinoids, AEA and 2-AG, by increasing their levels . In addition to these mechanisms, experiments using selective antagonists and murine models where PPAR-α was genetically ablated indicate that the anti-inflammatory and neuroprotective properties of PEA involve the activations of PPAR-α [180,210,227–229].
Through PPAR-α, PEA attenuated in vitro and in vivo β-amyloid-induced glial over-reactivity and the release of pro-inflammatory mediators, including inducible nitric oxide synthase (iNOS), cyclooxygenase (COX)-2, interleukin (IL)-1β, and tumor necrosis factor (TNF)-α . PEA also promoted neuronal viability in primary astrocytes derived from the prefrontal cortex of 3xTg-AD mice, a transgenic murine model of AD . All these data concur to demonstrate the key role of PPAR-α in controlling neuroprotective and anti-inflammatory pathways . Moreover, studies showed that the protective effects exerted by PEA also include the activation of GPR55  and TRPV1 . Figure 3 summarizes the proposed mechanisms of action of PEA.
The reduction of endogenous levels of PEA during conditions characterized by non-resolving neuroinflammation, such as unsuccessful brain aging, suggests the opportunity to restore PEA endogenous reserves, thus fostering and re-activating physiological pro-resolving responses.
In order to overcome the low bioavailability of PEA following oral administration, its particle size has been reduced through micronization techniques giving rise to the so-called ultramicronized (um)-PEA. The pharmacological potential of um-PEA in controlling neuroinflammation — either singly or co-ultramicronized with the antioxidant luteolin (i.e., co-ultra PEA) — has been proven by numerous investigations in various models of neurodegenerative conditions and senile dementia, both in vitro and in vivo . In rat hippocampal slices and neuroblastoma cells challenged with β-amyloid 1–42, PEA in the ultramicronized or co-ultramicronized formulation exhibited anti-inflammatory and anti-apoptotic effects, as well as the ability to decrease the expression of markers of oxidative stress and astroglial injuries, such as iNOS and GFAP . In rats that received an intrahippocampal infusion of β-amyloid 1–42, chronic treatment for 14 days with co-ultra PEA prevented astrocyte hypertrophy as well as the production of pro-inflammatory cytokines and enzymes, compared to vehicle-treated animals . Moreover, in this AD model, co-ultra PEA also prevented the decrease in gene expression of glial-derived and brain-derived neurotrophins . In 3xTg-AD mice, i.e., an animal model of AD exhibiting age-dependent β-amyloid and tau pathologies, chronic administration of um-PEA reduced brain levels of several pro-inflammatory mediators and showed neuroprotective effects . Importantly, um-PEA also prevented the impaired performance in cognitive tasks as well as reduced the AD-like pathology in these animals, as shown by the decrease of β-amyloid formation and tau protein phosphorylation in the hippocampus .