Edward B. Han

The neuromodulator dopamine plays a critical role in orchestrating behavior by signaling actions and rewards. Dopamine signaling in the hippocampus is necessary for the formation of long-term memories but the functional correlates, anatomical organization, and spatiotemporal scale of hippocampal dopamine release remain largely unknown. Using high-resolution, two-photon imaging of optical sensors in the hippocampus of male and female mice, we identified dopamine release in two closely apposed spatial domains. In pavlovian conditioning, there was no significant dopamine release in early training but over days to weeks of experience, dopamine transients appeared at rewards in the “deep” domain of CA1 (basal dendritic layer). Surprisingly these transients did not strongly encode reward. The act of stopping and licking, used to collect reward, was sufficient to produce deep dopamine transients even without the conditioned stimulus or reward. Similarly, hippocampal dopamine transients showed no reward prediction error at surprise rewards. In a spatial goal-directed task, deep dopamine domain transients at rewards persisted but dopamine ramps now appeared, ramping up in the superficial domain (cell body and apical dendrites) during reward approach while ramping down in the deep domain. Our results reveal anatomically segregated hippocampal dopamine release domains with dynamic signaling that is distinct from striatal dopamine. Rather than a unitary volume and function of dopamine, we identify multiple spatial domains and functional signals that likely play distinct and dissociable roles in hippocampal-dependent learning and memory.

The cellular etiology of seizures in CLN2 disease, a childhood-onset neurodegenerative lysosomal storage disorder caused by a deficiency of tripeptidyl peptidase 1 (TPP1), remains elusive. Given that Cln2 ^R207X/R207X mice display fatal spontaneous seizures and an early loss of several cortical GABAergic interneuron populations, we hypothesized that these 2 events might be causally related. To study the cell-autonomous effects of interneuron-specific TPP1 deficiency, we first generated transgenic mice expressing loxP -flanked lysosomal membrane–tethered TPP1 (TPP1LAMP1 mice) on the Cln2 ^R207X/R207X genetic background, and then crossed TPP1LAMP1 mice with Vgat-Cre mice. These Vgat-Cre; TPP1LAMP1 mice accumulated storage material in cortical and striatal interneurons. Vgat-Cre; TPP1LAMP1 mice also died more readily after pentylenetetrazole-induced seizures, indicating that interneuron-specific TPP1 deficiency renders these mice more susceptible to seizure-induced mortality. We also selectively activated interneurons using designer receptors exclusively activated by designer drugs (DREADDs) in Vgat-Cre; Cln2 ^R207X/R207X mice. Electroencephalogram monitoring revealed that DREADD-mediated activation of interneurons markedly accelerated the onset of spontaneous seizures and seizure-associated death in Vgat-Cre; Cln2 ^R207X/R207X mice, suggesting that modulating interneuron activity can exacerbate epileptiform abnormalities. Taken together, these results provide mechanistic insights into the underlying etiology of seizures and premature death that characterize CLN2 disease.

Layer 1 of V1 has been shown to receive locomotion-related signals from the dorsal lateral geniculate (dLGN) and lateral posterior (LP) thalamic nuclei (Roth et al., 2016). Inputs from the dLGN terminate in M2+ patches while inputs from LP target M2− interpatches (D’Souza et al., 2019) suggesting that motion related signals are processed in distinct networks. Here, we investigated by calcium imaging in head-fixed awake mice whether L2/3 neurons underneath L1 M2+ and M2− modules are differentially activated by locomotion, and whether distinct networks of feedback connections from higher cortical areas to L1 may contribute to these differences. We found that strongly locomotion-modulated cell clusters during visual stimulation were aligned with M2− interpatches, while weakly modulated cells clustered under M2+ patches. Unlike M2+ patch cells, pairs of M2− interpatch cells showed increased correlated variability of calcium transients when the sites in the visuotopic map were far apart, suggesting that activity is integrated across large parts of the visual field. Pathway tracing further suggests that strong locomotion modulation in L2/3 M2− interpatch cells of V1 relies on looped, like-to-like networks between apical dendrites of MOs-, PM- and RSP-projecting neurons and feedback input from these areas to L1. M2− interpatches receive strong inputs from SST neurons, suggesting that during locomotion these interneurons influence the firing of specific subnetworks by controlling the excitability of apical dendrites in M2− interpatches.

Layer 1 of V1 has been shown to receive locomotion-related signals from the dorsal lateral geniculate (dLGN) and lateral posterior (LP) thalamic nuclei ( Roth et al., 2016 ). Inputs from the dLGN terminate in M2+ patches while inputs from LP target M2- interpatches ( D’Souza et al., 2019 ) suggesting that motion related signals are processed in distinct networks. Here, we investigated by calcium imaging in head-fixed awake mice whether L2/3 neurons underneath L1 M2+ and M2- modules are differentially activated by locomotion, and whether distinct networks of feedback connections from higher cortical areas to L1 may contribute to these differences. We found that strongly locomotion-modulated cell clusters during visual stimulation were aligned with M2- interpatches, while weakly modulated cells clustered under M2+ patches. Unlike M2+ patch cells, pairs of M2- interpatch cells showed increased correlated variability of calcium transients when the sites in the visuotopic map were far apart, suggesting that activity is integrated across large parts of the visual field. Pathway tracing further suggests that strong locomotion modulation in L2/3 M2- interpatch cells of V1 relies on looped, like-to-like networks between apical dendrites of MOs-, PM- and RSP-projecting neurons and feedback input from these areas to L1. M2- interpatches receive strong inputs from SST neurons, suggesting that during locomotion these interneurons influence the firing of specific subnetworks by controlling the excitability of apical dendrites in M2- interpatches.

Summary: Numerous studies have identified dopamine signaling in the hippocampus as necessary for certain types of learning and memory. Since dopamine in the striatum is strongly tied to rewards, dopamine in the hippocampus is thought to reinforce reward learning. Despite the critical influence of dopamine on hippocampal function, little is known about dopamine release in the hippocampus or the specific ways dopamine can influence hippocampal function. Based on the functional complexity of hippocampal circuitry, we hypothesized the existence of multiple dopamine signaling domains. Using optical dopamine sensors, two-photon imaging, and head-fixed behaviors, we identified two functionally and spatially distinct dopamine domains in the hippocampus. The “superficial” domain (cell somata and apical dendrites) showed reward-related dopamine transients early in Pavlovian conditioning but were replaced by “deep” domain transients (basal dendritic layer) with experience. These two domains also play distinct roles in a hippocampal-dependent, goal-directed virtual reality task where mice use exploratory licks to discover the location of a hidden reward zone. Here, positive dopamine ramps appeared in the superficial domain as mice approached the reward zone, similar to those seen in the striatum. At the same time, the deep domain showed strong reward-related transients. These results reveal small-scale, anatomically segregated, dopamine domains in the hippocampus. Furthermore dopamine domain activity had temporal-specificity for different phases of behavior. Finally, the subcellular scale of dopamine domains suggests specialized postsynaptic pathways for processing and integrating functionally distinct dopaminergic influences.

Inhibition plays a powerful role in regulating network excitation and plasticity; however, the activity of defined interneuron types during spatial exploration remain poorly understood. Using two-photon calcium imaging, we recorded hippocampal CA1 somatostatin- and parvalbumin-expressing interneurons as mice performed a goal-directed spatial navigation task in new visual virtual reality (VR) contexts. Activity in both interneuron classes was strongly suppressed but recovered as animals learned to adapt the previously learned task to the new spatial context. Surprisingly, although there was a range of activity suppression across the population, individual somatostatin-expressing interneurons showed consistent levels of activity modulation across exposure to multiple novel environments, suggesting context-independent, stable network roles during spatial exploration. This work reveals population-level temporally dynamic interneuron activity in new environments, within which each interneuron shows stable and consistent activity modulation.

Hippocampal network activity is tightly regulated by local inhibitory interneurons. Suppression of inhibition has been proposed to accelerate learning by enhancing network activity and plasticity; however, the activity dynamics of hippocampal interneurons during learning remain poorly understood. Furthermore, it is unknown if individual interneurons are stochastically suppressed across different learning episodes, mirroring the random remapping of place cells, or if instead they exhibit consistent patterns of activity suppression. These critical properties define how inhibition shapes and controls learning at a network level. To uncover the functional circuit dynamics of inhibition during novelty-induced learning, we recorded calcium activity from hippocampal CA1 interneurons using two-photon imaging as mice learned a virtual reality (VR) goal-directed spatial navigation task in new visual contexts. Here we focused on dendrite-targeting somatostatin-expressing interneurons (SOM-ints), which powerfully control burst firing and synaptic plasticity in excitatory neurons. We found robust activity suppression in SOM-ints upon exposure to novel virtual environments; activity then recovered over repeated exposures to the novel environment as the animal learned goal locations. At a population level, we found a continuum of activity suppression, from interneurons strongly suppressed to moderately activated during learning. Surprisingly, each interneuron exhibited a stable level of activity modulation: when animals were switched into a second novel environment, the magnitude of activity suppression was strongly correlated across remapping sessions. This work reveals dynamic inhibition suppression triggered by novel environments and the gradual return of inhibition with learning. Furthermore, unlike the stochastic remapping of place cells, inhibitory networks display a stable activity structure across learning episodes. This functional inhibitory circuit architecture suggests that individual interneurons play specialized and stereotyped roles during learning, perhaps by differentially regulating pyramidal subnetworks specialized for plasticity and stability.

Exposure to environmental contexts associated with drug use can induce cravings that promote continued use and/or relapse. Opioid abuse is marked by high relapse rates, suggesting that contextual memories formed during opioid use may be particularly strong. While it is known that reward-seeking behavior is controlled by the mesolimbic reward circuit, little is understood about how contextual memories are altered by drug use. The dorsal hippocampus (dHPC) is necessary for multiple types of contextual learning and the place-specific activity of CA1 place cells map out space in a given environment. Here we examined the neuronal representation of context as animals developed morphine-paired environmental associations using a conditioned place preference (CPP) paradigm. To investigate changes in the hippocampal encoding before, during, and after drug-pairing, we developed a virtual reality (VR) morphine CPP (Mor-CPP) paradigm and used in vivo two-photon calcium imaging to record the activity of CA1 pyramidal neurons. We found increased activity in rewarded contexts following real-time operant conditioning with water rewards, but not after Mor-CPP training, suggesting different neural encoding mechanisms for natural reinforcers and morphine.

Network activity is strongly tied to animal movement; however, hippocampal circuits selectively engaged during locomotion or immobility remain poorly characterized. Here we examined whether distinct locomotor states are encoded differentially in genetically defined classes of hippocampal interneurons. To characterize the relationship between interneuron activity and movement, we usedin vivo, two-photon calcium imaging in CA1 of male and female mice, as animals performed a virtual-reality (VR) track running task. We found that activity in most somatostatin-expressing and parvalbumin-expressing interneurons positively correlated with locomotion. Surprisingly, nearly one in five somatostatin or one in seven parvalbumin interneurons were inhibited during locomotion and activated during periods of immobility. Anatomically, the somata of somatostatin immobility-activated neurons were smaller than those of movement-activated neurons. Furthermore, immobility-activated interneurons were distributed across cell layers, with somatostatin-expressing cells predominantly in stratum oriens and parvalbumin-expressing cells mostly in stratum pyramidale. Importantly, each cell's correlation between activity and movement was stable both over time and across VR environments. Our findings suggest that hippocampal interneuronal microcircuits are preferentially active during either movement or immobility periods. These inhibitory networks may regulate information flow in “labeled lines” within the hippocampus to process information during distinct behavioral states.SIGNIFICANCE STATEMENTThe hippocampus is required for learning and memory. Movement controls network activity in the hippocampus but it's unclear how hippocampal neurons encode movement state. We investigated neural circuits active during locomotion and immobility and found interneurons were selectively active during movement or stopped periods, but not both. Each cell's response to locomotion was consistent across time and environments, suggesting there are separate dedicated circuits for processing information during locomotion and immobility. Understanding how the hippocampus switches between different network configurations may lead to therapeutic approaches to hippocampal-dependent dysfunctions, such as Alzheimer's disease or cognitive decline.