Published: Jun 25, 2019 by Chloe
One of the many remarkable things about the human brain is its capacity for life-long plasticity. Within brain regions involved in emotional learning, like the hippocampus and the amygdala, brain plasticity allows for the formation and re-formation of emotional associations, including reappraisal of the salience and valence of emotional stimuli. Disorders that involve emotional deregulation, such as major depressive disorder or autism spectrum disorders, likely involve disrupted neural plasticity within emotional circuits that prohibits or interferes with emotional learning. What are the cellular and molecular substrates for plasticity within emotional brain circuitry? An enigmatic population of cells in an equally enigmatic region of the amygdala may be a piece of the puzzle of emotional plasticity.
The paralaminar (PL) nucleus of the amygdala is the enigmatic region in question. It is present in humans, non-human primates, and sheep, and it is questionable whether it exists in rodents or if it is just too small to reliably detect. Within the PL, there exists a somewhat mysterious population of immature neurons. These cells contain protein markers indicative of immaturity, like TUJ1, DCX, and/or PSA-NCAM, and morphologically show leading or trailing processes, indicative of potential migration. These immature cells have been observed in non-human primates, as well as in humans in a recent paper by Sorrells et al.
Immature neurons in the human amygdala
The first question that Sorrells et al. sought to answer was whether immature neurons in the PL are derived from the caudal ganglionic eminence (CGE), a major source of inhibitory interneurons and a proliferative region near where the immature neurons are found. To investigate this question, the authors stained for several transcription factors that are expressed in the CGE, including SP8, COUPTFII, and PROX1. COUPTFII expression was found in the CGE and amygdala: the COUPTFII+ cells in the CGE were positive for SP8 and PROX1. The PL was identified as a layer of COUPTFII+, SP8-, PROX1- cells between the basolateral amygdala (BLA) and CGE. The different transcription factor expression patterns between the PL and the CGE suggest that the PL is distinct from the neighboring CGE. Neurons in the PL did not stain for secretagogin (SCGN), a marker of CGE migratory inhibitory interneurons. Instead, PL neurons expressed the excitatory neuron transcription factor TBR1, further distinguishing the PL from the CGE by its excitatory cell population.
During gestation and at birth, PL neurons also express immature neuron markers like DCX and PSA-NCAM, with few mature NeuN+ cells. Through childhood, adolescence, and adulthood, there were declining numbers of immature cells and increasing numbers of mature cells, suggesting that the excitatory neurons in the PL slowly mature throughout life. This maturation was most pronounced during adolescence, but immature cells were still present in adulthood, up to 77 years of age (the oldest age examined in the study).
The question remains whether the immature cells in the PL that continue to be observed late in life reflect slowly-maturing cells or adult-born neurons. The authors examined the number of Ki-67+ cells, a marker of neural proliferation. At birth there were Ki-67+ cells in the PL, but they were more frequently found between the PL and the remnants of the CGE. In early postnatal life and early adulthood, there were still Ki-67+ cells, but they were observed mostly adjacent to the PL rather than within it. There were a few Ki-67+ cells within the PL but they were not densely found there compared to the the rest of the BLA. Furthermore, the Ki-67+ cells that were present within or around the PL did not overlap with DCX+/PSA-NCAM+ cells. So it is unlikely that the population of immature neurons in the PL derives from neurons born after early postnatal development. In other words, there is not strong evidence for continuing neurogenesis in the PL, though the possibility cannot be completely ruled out.
In summary, the presence of immature cells in the PL throughout the lifespan could be a substrate for ongoing plasticity in this brain region. The amygdala is highly involved in emotional processing, raising the intriguing possibility that immature cells in the human PL are involved in emotional learning and plasticity throughout the lifespan. They may also be involved in emotional maturation during adolescence, given that more mature neurons start appearing in the PL during this developmental time period. It is also possible that a subpopulation of the immature cells in the PL are migrating away or dying, given their decline with age and their migratory morphology. That is another possibility yet to be explored.
The function of these immature PL cells remains unknown. It will be difficult to study their function in rodent models, since the existence of a PL is rodents is unlikely. The piriform cortex may be somewhat analogous, however. Populations of DCX+ and PSA-NCAM+ cells that slowly mature into excitatory neurons, much like the in the PL, have been observed in the rodent piriform cortex.
Connectivity may provide insight into function
The PL receives projections from the lateral nucleus of the amygdala as well as from the hippocampus. deCampo and Fudge describe the implications of the PL’s connectivity patterns, together with the population of immature cells:
The PL, with its immature neuronal component, is located at the crossroads of circuitry involved in contextual conditioning [...] Its main afferent inputs appear relatively specific, carrying information about temporal and spatial information (via hippocampus) that converges with polymodal sensory information occurring in the present (via the ventral lateral nucleus). This arrangement suggests that excitatory signals involved in the process of contextual learning or memory formation may shape the fate of immature neurons.
In monkeys, the PL receives input from the ventral uncal region of the hippocampus (unique to humans and non-human primates; part of CA1), which plays a role in autobiographical memory and contextual learning. Connections between this region of the hippocampus and the amygdala may place the PL within circuitry serving autobiographical memory, emotional salience processing, and contextual fear.
Interestingly, hippocampal lesions increase the number of mature neurons in the monkey PL, according to a study by Chareyron et al. The number of immature neurons in the PL increased or decreased due to neonatal or adult legion, respectively. These findings suggests that lesion of the hippocampus, presumably due to its connectivity with the PL, impacts the differentiation of immature neurons in the PL. More broadly, this suggests that the fate of immature PL neurons is activity- and experience-dependent.
Hippocampal-amygdala connectivity is further necessary for the recall of past emotional experiences, and activity within hippocampal-amygdala pathways are strongly associated with mood. The PL is therefore likely involved in emotional expression and regulation and may be disrupted somehow in mood disorders.
Implications for neuropsychiatric disorders
Contextual fear learning and the emotional salience of memory is resistant to rewiring or erasure. For instance, contextual fear memories are easily recalled and very difficult to erase. Following extinction of the fear memory, reactivation and relapse is easily triggered. According to Fudge et al.: “If immature neurons participate in contextual fear conditioning, their enrichment in PSA-NCAM and DCX implies a special sensitivity to neural plasticity.” PSA-NCAM and DCX are not just markers for immature cells; they participate in increasing the plasticity of these neurons. High PSA-NCAM expression permits a lower threshold for long-term potentiation (LTP), one of the most common forms of neural plasticity. PSA-NCAM also regulates neural differentiation, migration, and neurite outgrowth in young neurons. DCX is responsible for microtubule stabilization, axon extension, and neural migration.
The heightened plasticity of PL neurons could go in either direction: the immature cells may play a role in maintaining a fearful memory (something similar has been observed in the dentate gyrus) or they could serve as a cellular substrate for new emotional learning that can overwrite and truly erase past emotional associations. So, immature neurons in the PL may help stabilize emotional associations or render them malleable. Either way, they could be a therapeutic target for neuropsychiatric disorders like anxiety, depression, and PTSD.
Major depression and suicide: Maheu et al. and Varea et al.
There are currently no studies about the PL and depression; however, this study by Maheu et al. examines DCX and PSA-NCAM expression in the basolateral (BLA; which likely includes the PL) and central (CeA) amygdala of humans. They found that DCX and PSA-NCAM expression was increased in the BLA of depressed subjects that did not die by suicide. By contrast, Varea et al. found that depressed patients showed decreased PSA-NCAM expression in the basolateral and basomedial amygdala; they did not further differentiate subjects with depression.
Regarding the depressed suicide subjects, the authors write: “the absence of any difference in the expression of these markers among depressed suicides may reflect a state of variable neuronal adaptability in the amygdala according to the severity of the depressive phenotype […] it may be the absence of structural remodeling in the amygdala, rather than its presence, that is associated with more severe pathological outcomes.” In other words, the increase in markers of plasticity that the authors observe in depressed non-suicide subjects may be an adaptive response to stress and/or depression. A somewhat similar phenomenon has been observed in sex differences in the response to stress in the prefrontal cortex (PFC). Following chronic stress, males show structural remodeling of pyramidal cells in the PFC while females do not, but females show more susceptibility to subsequent stressors. Therefore, plasticity in the case of stress and/or depression may be important for resilience and recovery. There are some misgivings about this interpretation, however. For one, it is a large assumption that depressed subjects who died by suicide had more severe depression than those who did not. Suicidality does not necessarily correlate with severity of depression, and sometimes not with depression at all. Further complicating matters, antidepressants have been shown to reduce PSA-NCAM in the BLA.
Autism: Avino et al.
Avino et al. examine the amygdala in autism spectrum disorders. This study considered the basal nucleus and the PL of the amygdala as a single unit, and found that volume increased from juvenility to adulthood in both humans and monkeys. In the basal nucleus, including in other amygdalar nuclei (lateral, central), the number of neurons in children with autism spectrum disorders (ASD) is initially greater than age-matched neurotypical children. By adulthood, however, individuals with ASD have fewer neurons in amygdala nuclei.
Despite these differences, Avino et al. observed a similar age-related decline in the immature neuron marker bcl-2 in the PL between ASD and neurotypical individuals. This suggests that maturational processes may be intact in ASD. However, it is still possible that the migratory capacity of immature cells is impaired somehow in ASD. The authors propose that immature neurons from the PL migrate to the dorsal amygdala. If migration from the PL is impaired in ASD, that could explain in part the lack of an age-related increase in neuron number in other amygdala regions. It is also still possible that axonogenesis, dendritogenesis, and other indicators of cell morphology are altered in some ways not observed here, potentially indicative of altered development.
Early life stress: deCampo et al.
In the PL, early life stress was found to downregulate expression of tbr1, a transcription factor necessary for directing neuroblasts to a glutamatergic phenotype. At this time, there is not much to conclude about this study, other than it does indicate that the immature population of cells in the PL is susceptible to stress and therefore likely bears relevance to stress-related neuropsychiatric conditions like anxiety and depression. It is worth noting, however, stress exposure during early life or other periods of development can have very different mechanistic effects than stress exposure during adulthood, even if there are similar behavioral outcomes. The authors point out as well that their findings “may point to either regression or acceleration of young neurons along their developmental trajectory.” More research, as always, is needed.
Some thoughts on brain region-specific plasticity
Immature neurons in the PL seem well-poised to play a role in various neuropsychiatric disorders as well as in the stress response. They likely confer plasticity to emotional circuits, though the functional outcomes of this plasticity are unknown.
Brain regions are engaged in a balancing act between stability and plasticity. In the PFC, for instance, it appears that in cases of chronic stress and anxiety/depression, neuronal plasticity is reduced. Antidepressant treatments (SSRIs, ketamine, talk therapy, and transcranial magnetic stimulation) are thought to re-engage mechanisms of plasticity in the PFC to allow for therapeutic rewiring of neuronal circuits. In the logic of this phenomenon, more immature cells in the PL, incurring plasticity to emotional circuits, would likely be therapeutic for emotional disorders - it would allow for emotional updating and rewiring, rather than being captured by a maladaptive circuit. Fluoxetine treatment increases expression of PSA-NCAM in the PFC, as well as inducing other molecular and cellular changes indicative of increased plasticity. Maybe something similar is happening in the amygdala. However, the amygdala is such a heterogeneous brain region, with many subdivisions with differing functions and connectivity. Increased plasticity may be therapeutic in one area and detrimental in another. In the central amygdala, PSA-NCAM expression is reduced by chronic restraint stress and increased by chronic antidepressant treatment. But as previously mentioned, antidepressant treatment has been reported to reduce PSA-NCAM expression in the BLA. Interestingly, the BLA responds to stress in a way opposite that of the PFC (excitatory neurons in the PFC undergo dendritic atrophy while they experience hypertrophy in the BLA). It is possible that therapies for affective disorders favor plasticity in some brain regions (the PFC) and stability in others (the BLA). How the PL fits into the stability-plasticity puzzle of emotional function and dysfunction remains to be explored.
References
- Avino et al. Neuron numbers increase in the human amygdala from birth to adulthood, but not in autism. PNAS (2018).
- Chareyron et al. Selective lesion of the hippocampus increases the differentiation of immature neurons in the monkey amygdala. PNAS (2016).
- deCampo et al. Maternal deprivation alters expression of neural maturation gene tbr1 in the amygdala paralaminar nucleus in infant female macaques. Developmental Psychobiology (2015).
- deCampo and Fudge. Where and what is the paralaminar nucleus? A review on a unique and frequently overlooked area of the primate amygdala. Neuroscience and Biobehavioral Reviews (2011).
- Fudge et al. Revisiting the hippocampal-amygdala pathway in primates: association with immature-appearing neurons. Neuroscience (2012).
- Maheu et al. Amygdalar expression of proteins associated with neuroplasticity in major depression and suicide. Journal of Psychiatric Research. (2013).
- Rotheneichner et al. Cellular plasticity in the adult murine piriform cortex: Continuous maturation of dormant precursors into excitatory neurons. Cerebral Cortex (2018).
- Sorrells et al. Immature excitatory neurons develop during adolescence in the human amygdala. Nature Communications (2019).
- Varea et al. Chronic fluoxetine treatment increases the expression of PSA-NCAM in the medial prefrontal cortex. Neuropsychopharmacology (2007).
- Varea et al. Expression of PSA-NCAM and synaptic proteins in the amygdala of psychiatric disorder patients. Journal of Psychiatric Research (2012).
