Basolateral Amygdala Inputs to the Medial Entorhinal Cortex Selectively Modulate the Consolidation of Spatial and Contextual Learning.

Although evidence suggests that the basolateral amygdala (BLA) and dorsal hippocampus (DH) work together to influence the consolidation of spatial/contextual learning, the circuit mechanism by which the BLA selectively modulates spatial/contextual memory consolidation is not clear. The medial entorhinal cortex (mEC) is a critical region in the hippocampus-based system for processing spatial information. As an efferent target of the BLA, the mEC is a candidate by which the BLA influences the consolidation of such learning. To address several questions regarding this issue, male Sprague Dawley rats received optogenetic manipulations of different BLA afferents immediately after training in different learning tasks. Optogenetic stimulation of the BLA-mEC pathway using ChR2(E123A) after spatial and cued-response Barnes maze training enhanced and impaired retention, respectively, whereas optical inhibition of the pathway using eNpHR3.0 produced trends in the opposite direction. Similar stimulation of the BLA-posterior dorsal striatum pathway had no effect. BLA-mEC stimulation also selectively enhanced retention for the contextual, but not foot shock, component of a modified contextual fear-conditioning procedure. In both sets of experiments, only stimulation using bursts of 8 Hz light pulses significantly enhanced retention, suggesting the importance of driving activity in this frequency range. An 8 Hz stimulation of the BLA-mEC pathway increased local field potential power in the same frequency range in the mEC and in the DH. Together, the present findings suggest that the BLA modulates the consolidation of spatial/contextual memory via projections to the mEC and that activity within the 8 Hz range is critical for this modulation.SIGNIFICANCE STATEMENT The mechanism by which the basolateral amygdala (BLA) influences the consolidation of spatial/contextual memory is unknown. Using an optogenetic approach with multiple behavioral procedures, we found that immediate posttraining 8 Hz stimulation of BLA projections to the medial entorhinal cortex (mEC) enhanced retention for spatial/contextual memory, impaired retention for cued-response memory, and had no effect on foot shock learning for contextual fear conditioning. Electrophysiological recordings confirmed that 8 Hz stimulation of this pathway increased activity in the 8 Hz range in the mEC and in the dorsal hippocampus, a region critical for spatial memory consolidation. This suggests that coordinated BLA activity with downstream regions in the 8 Hz activity range immediately after training is important for consolidation of multiple memory forms.

Basolateral amygdala projections to ventral hippocampus modulate the consolidation of footshock, but not contextual, learning in rats.

The basolateral amygdala (BLA) modulates memory consolidation for a variety of types of learning, whereas other brain regions play more selective roles in specific kinds of learning suggesting a role for differential consolidation via distinct BLA pathways. The ventral hippocampus (VH), an efferent target of the BLA, has been suggested to selectively process emotion-related learning, yet whether the BLA → VH pathway modulates memory consolidation, and does so in a learning-specific manner, is unknown. To address this issue, the BLA of male Sprague-Dawley rats was bilaterally transduced to express either ChR2(E123A) or eArchT3.0. Fiber optic probes were implanted in the VH to provide illumination of BLA axons. Rats then underwent a modified contextual fear conditioning task permitting separation of context and footshock learning. On day 1, rats received 3 min of pre-exposure to the apparatus. On day 2, rats were placed into the apparatus, received an immediate footshock, and quickly removed. Retention was tested on day 4. Optical stimulation of the BLA → VH pathway following footshock, but not context, training using trains of 40-Hz light pulses enhanced retention. Continuous optical inhibition of this pathway for 15 min starting 25 min after footshock training impaired retention. These findings indicate that BLA → VH projections influence the consolidation for footshock, but not context, learning of a modified CFC task and provide direct evidence that BLA projections to other brain regions modulate memory consolidation selectively depending on the kind of learning involved.

The rostromedial tegmental nucleus modulates behavioral inhibition following cocaine self-administration in rats.

Recent findings suggest that the mesolimbic dopamine neurons, known to promote cocaine-seeking behavior, are strongly inhibited by a newly characterized region of the midbrain known as the rostromedial tegmental nucleus (RMTg). The RMTg appears to be involved in generating reward-prediction error signals and inhibition of motivated behaviors, suggesting its potential involvement in the extinction of cocaine seeking as well. Therefore, to address this question, male Sprague-Dawley rats underwent surgeries for implantation of catheters and cannulas targeted at the RMTg. After cocaine self-administration, rats underwent modified extinction training. Pre- or post-training intra-RMTg microinjections of the allosteric AMPA receptor potentiator PEPA during the first 5 days of extinction training appeared to enhance the retention of the extinction learning. Following the extinction training, rats underwent cue-induced reinstatement or an 'inactivation-alone' extinction tests. RMTg inactivation before a cue-induced reinstatement session or inactivation alone before a standard extinction session increased overall lever pressing. To determine whether these effects generalized to other motivated behaviors, additional experiments examining food-seeking behavior were also conducted. The results from the food-seeking experiments indicate that PEPA microinjections into the RMTg did not influence the extinction of food seeking and that, at least in rats that had not been given PEPA during the extinction learning experiments, RMTg inactivation had no effect on lever pressing during the cue-induced reinstatement or inactivation-alone tests. These findings suggest that the RMTg provides general behavioral inhibition and is potentially involved in learning to extinguish cocaine-seeking behavior in rats.

Posttraining optogenetic manipulations of basolateral amygdala activity modulate consolidation of inhibitory avoidance memory in rats.

Memory consolidation studies, including those examining the role of the basolateral amygdala (BLA), have traditionally used techniques limited in their temporal and spatial precision. The development of optogenetics provides increased precision in the control of neuronal activity that can be used to address the temporal nature of the modulation of memory consolidation. The present experiments, therefore, investigated whether optogenetically stimulating and inhibiting BLA activity immediately after training on an inhibitory avoidance task enhances and impairs retention, respectively. The BLA of male Sprague-Dawley rats was transduced to express either ChR2(E123A) or archaerhodopsin-3 from the Halorubrum sodomense strain TP009 (ArchT). Immediately after inhibitory avoidance training, rats received optical stimulation or inhibition of the BLA, and 2 d later, rats' retention was tested. Stimulation of ChR2(E123A)-expressing neurons in the BLA using trains of 40-Hz light pulses enhanced retention, consistent with recording studies suggesting the importance of BLA activity at this frequency. Light pulses alone given to control rats had no effect on retention. Inhibition of ArchT-expressing neurons in the BLA for 15 min, but not 1 min, significantly impaired retention. Again, illumination alone given to control rats had no effect on retention, and BLA inhibition 3 h after training had no effect. These findings provide critical evidence of the importance of specific frequency patterns of activity in the BLA during consolidation and indicate that optogenetic manipulations can be used to alter activity after a learning event to investigate the processes underlying memory consolidation.

Optogenetic inhibition of cocaine seeking in rats.

Inhibitory optogenetics was used to examine the roles of the prelimbic cortex (PL), the nucleus accumbens core (NAcore) and the PL projections to the NAcore in the reinstatement of cocaine seeking. Rats were microinjected into the PL or NAcore with an adeno-associated virus containing halorhodopsin or archaerhodopsin. After 12 days of cocaine self-administration, followed by extinction training, animals underwent reinstatement testing along with the presence/absence of optically induced inhibition via laser light. Bilateral optical inhibition of the PL, NAcore or the PL fibers in the NAcore inhibited the reinstatement of cocaine seeking.