Dorsal Raphe Serotonin Neurons in Mice: Immature Hyperexcitability Transitions to Adult State during First Three Postnatal Weeks Suggesting Sensitive Period for Environmental Perturbation

Abstract

Trauma during early life is a major risk factor for the development of anxiety disorders and suggests that the developing brain may be particularly sensitive to perturbation. Increased vulnerability most likely involves altering neural circuits involved in emotional regulation. The role of serotonin in emotional regulation is well established, but little is known about the postnatal development of the raphe where serotonin is made. Using whole-cell patch-clamp recording and immunohistochemistry, we tested whether serotonin circuitry in the dorsal and median raphe was functionally mature during the first 3 postnatal weeks in mice. Serotonin neurons at postnatal day 4 (P4) were hyperexcitable. The increased excitability was due to depolarized resting membrane potential, increased resistance, increased firing rate, lack of 5-HT 1A autoreceptor response, and lack of GABA synaptic activity. Over the next 2 weeks, membrane resistance decreased and resting membrane potential hyperpolarized due in part to potassium current activation. The 5-HT 1A autoreceptor-mediated inhibition did not develop until P21. The frequency of spontaneous inhibitory and excitatory events increased as neurons extended and refined their dendritic arbor. Serotonin colocalized with vGlut3 at P4 as in adulthood, suggesting enhanced release of glutamate alongside enhanced serotonin release. Because serotonin affects circuit development in other brain regions, altering the developmental trajectory of serotonin neuron excitability and release could have many downstream consequences. We conclude that serotonin neuron structure and function change substantially during the first 3 weeks of life during which external stressors could potentially alter circuit formation.

Activity of Raphé Serotonergic Neurons Controls Emotional Behaviors

“…First, between P2 and P21, DR but not MR 5-HTergic neurons undergo a molecular, morphological and physiological transition which appears crucial for its proper maturation, including the establishment of 5-HT1a-dependent auto-inhibition (Donaldson et al, 2014; Gross et al, 2002; Rood et al, 2014). Hence, a postnatal increase in extracellular 5-HT levels might preferentially affect DR 5-HTergic neurons maturation.…”

“…Hence, a postnatal increase in extracellular 5-HT levels might preferentially affect DR 5-HTergic neurons maturation. Second, developmental SSRI exposure may differentially alter the establishment of specific inputs onto DR and MR 5-HT neurons (Dorocic et al, 2014; Lechin et al, 2006), including those from local GABAergic interneurons (Challis et al, 2013; Rood et al, 2014). Finally, MR and DR send reciprocal projections to each other (Bang et al, 2012; Muzerelle et al, 2014; Vertes et al, 1999), and developmental interference with the establishment of interconnectivity can lead to permanently altered relative 5-HTergic activity.…”

Identification of Serotonergic Neuronal Modules that Affect Aggressive Behavior

“…While Drd1a/Pet1 neuron labeling/silencing commenced embryonically, that for Drd2/Pet1 neurons began at adolescence. Adolescence in mice is associated with functional changes in DR serotonin neurons and has been identified as a sensitive period for 5-HT-mediated modulation of emotional behavior in mice (Gross et al, 2002, Yu et al, 2014, Liu et al, 2010, Rood et al, 2014). From our present data, we can deduce that embryonic or early post-natal silencing of the Drd2/Pet1 neurons is not necessary to induce behavioral phenotypes, but rather that subtype-specific neuronal silencing during later post-natal periods is sufficient.…”

Chronic social isolation reduces 5-HT neuronal activity via upregulated SK3 calcium-activated potassium channels

“…The slices were then recovered for a minimum of 2 hr in ACSF solution containing: 128 mM NaCl, 10 mM D-glucose, 26 mM NaHCO 3 , 2 mM CaCl 2 , 2 mM MgSO 4 , 3 mM KCl, 1.25 mM NaH 2 PO 4 , pH 7.4 and saturated with 95% O 2 /5% CO 2 at 31–33ºC. L-tryptophan (2.5 µM) was included during the recovery period to maintain 5-HT synthesis (Rood et al, 2014). Recording was performed in ACSF oxygenated with 95% O 2 /5% CO 2 at 31–33ºC flowing at a rate of 3–4 ml/min.…”