L655,708 a Negative Allosteric Modulator at of Gabaa Receptor Increased Dopamine Neuronal Activity Up to 1 Week After a Single Injection

Background Negative allosteric modulators of gamma-aminobutyric acid A receptors (GABAA-NAMs) bind with high affinity at the α5 subunit located primarily on glutamatergic pyramidal neurons in the prefrontal cortex (PFC) and hippocamps, thereby directly increasing AMPA throughput [2]. Their specificity limits off-target effects, reducing the possibility of adverse side effects. The α5-preferring GABA-NAM L655,708 has been shown to exert rapid and sustained antidepressant-like action in rats subjected to mild stress. Aims & Objectives It is hypothesized that the monoamine system in conjunction with the glutamatergic system, may play a role in the antidepressant effects of L655,708. Method Experiments were performed in male Sprague-Dawley rats anesthetized using chloral hydrate. in vivo electrophysiological recordings of pyramidal and dopaminergic (DA) neurons were carried out in the PFC and ventral tegmental area (VTA), 24 hours after a single administration of vehicle solution or L655,708 (3 mg/kg; i.p.) and 1 week after for the VTA. Neuronal firing was analyzed for changes in spike frequency and burst activity as well as the number of spontaneously active neurons per track in the VTA (population activity). Results A single administration of L655,708 1-day before electrophysiological recordings did not significantly alter pyramidal neuronal activity in the PFC; firing rate (Vehicle: 1.0 ± 0.3 Hz; L655,708 0.9 ± 0.1 Hz), burst rate (Vehicle: 67 ± 4.8; L655,708 62 ± 4.1). However in the VTA, 24 hours following injection of L655,708, there was a robust enhancement in the number of spontaneously active DA neurons compared to vehicle administration, (Vehicle: 1.3 ± 0.1 Hz; L655,708 3.3 ± 0.1 Hz; two-tailed t test t(5)=8, p<0.001). This increase in population activity observed in the VTA 1-day post-treatment was diminished after the administration of flumazenil, a competitive GABAA receptor antagonist, and the administration of NBQX, an AMPA receptor antagonist. Interestingly, 1 week following the injection of L655,708, there was a significant 2-fold enhancement in the number of spontaneously active DA neurons in the VTA compared to vehicle administration (Vehicle: 1.0 ± 0.1 Hz; L655,708 2.4 ± 0.2 Hz; two-tailed t test t(5) =6, p=0.002). Discussion & Conclusions While no changes were observed in the activity of pyramidal neurons in the PFC 24 hours after injection, L655,708 caused a significant increase in the population activity of DA neurons in the VTA 1 day and 1 week after injection. These results align with data showing that this drug produced an antidepressant-like effect in the forced swim test at 1 day and 1-week post-treatment [1]. The reversal of these effects with the administration of flumazenil and NBQX indicates that L-655-708 indeed binds to the benzodiazepine site on GABAA receptors and that the observed response is AMPA receptor-dependent. The increase of DA neuronal activity by L655,708 is unlikely due to an impact of the drug on PFC pyramidal neurons. Concurrently, the increase in DA population activity without the alteration of firing and burst activity may be attributed to an L-655,708 induced increase in ventral hippocampal activation [3].