Uptake 2 Transporter Blockade Can Ameliorate Sociability Deficits

Background:  Impaired social behavior is a persistent core symptom of autism that is generally treatment-resistant. Clinical findings and basic studies in rodents demonstrate that serotonin (5-HT) transmission is often disrupted in the socially-impaired brain. While selective 5-HT reuptake inhibitors (SSRIs) such as Prozac (fluoxetine) enhance sociability in limited subpopulations of patients, their efficacy is diminished when 5-HT transporter (SERT) function is compromised.  Aside from SERT, auxilliary transporters of 5-HT in the brain include organic cation transporters (OCTs) and the plasma membrane monoamine transporter (PMAT) among others collectively known as “uptake 2”.  Uptake 2 transporters typically have lower affinity for 5-HT, but remove it from extracellular fluid with greater capacity than SERT. Our hypothesis is that if uptake 2 is blocked, impaired social behavior may improve.

Objectives:  Our goal was to characterize the acute and sub-chronic effects of blocking uptake 2 transporters on the social behavior of two mouse lines with impaired sociability: BTBR T^+Itpr3tf/J and SERT -/- mice, in three-chamber sociability tests. Only male mice were examined in these studies, since autism incidence is higher in males than females.

Methods:  We utilized the pseudoisocyanine decynium-22 (D-22) to block uptake 2 transporters, and administered it to mice via acute interperitoneal injection or sub-chronic (2 weeks) delivery of D-22 via surgically-implanted osmotic minipumps.  Subsequently behavioral performance in three chamber sociability tests was assessed.  In vitro uptake of [³H] 5-HT, [³H] citalopram binding and their competition with D-22 was performed in cortical and hippocampal synaptosomes or membranes to compare relative blockade capacity and affinity to the SSRI fluoxetine. Pharmacokinetics of D-22 in serum and brain were assessed by i.p. injection and HPLC.

Results:  Acute injections of D-22 (at 0.1 – 0.01 mg/kg) enhanced social interaction preference in both BTBR and SERT -/- mice, in terms of sniffing behavior and chamber dwelling time.  Inherent social novelty preference was not diminished by D-22 treatment in BTBR mice. Furthermore, sub-chronic administration of D-22 improved social interaction preference in both mouse lines, with no apparent adverse effects. D-22 blocks 5-HT uptake in mouse brain in vitro with Km = 90 ± 10 nM, and has little affinity for SERT (Ki > 3000 nM).  D-22 also appears to cross the blood-brain barrier. 

Conclusions:  Blockade of uptake 2 transporters appears to be an effective short or long-term treatment strategy for impaired social behavior in mice. However, clinical effects of uptake 2 blockade remain to examined, and a search for more selective uptake 2 blockers is warranted.