Hippocampal theta activity and behavioral sequences in a reward-directed approach locomotor task

Current Models and Future Directions for Understanding the Neural Circuitries of Maternal Behaviors in Rodents

Maternal behaviors in rodents include a number of subcomponents, such as nursing, nest building, licking and grooming of pups, pup retrieval, and maternal aggression. Because each behavior involves a unique motor pattern, a unique ensemble neural circuitry must underlie each behavior. To what extent there is overlap in terms of brain regions and specific neurons for each circuit is being actively investigated. This review will first examine overlapping and separate components of pup retrieval and maternal aggression circuitries while examining a central role for medial preoptic area (MPA) in both behaviors. With an emphasis on experimental approaches, the review will then highlight recent findings and propose future directions for understanding maternal behavior regulation. Finally, examples for why studying the neural basis of maternal behaviors can bring insights to other areas of neuroscience, such as feeding, addiction, and anxiety and aggression regulation will be provided.

Hippocampal theta rhythm and drug-related reward-seeking behavior: an analysis of cocaine-induced conditioned place preference in rats

Drug cravings are elicited by environmental stimuli associated with the rewarding effects of drugs. As an animal model of such associative learning, conditioned place preference (CPP) is widely used. Since the hippocampus is closely related to reward memory and the hippocampal local field potential (LFP), and in particular the theta rhythm is known to be associated with bodily movements, the theta rhythm might be one of the key neural substrates. On the basis of this assumption, we recorded the behaviors and hippocampal LFP of eight rats during cocaine-induced acquisition and expression of CPP. The earliest appearance of phase-locked theta activity was observed before the rats entered the cocaine-paired environment after conditioning; after entrance, the theta disappeared. This phase-locked theta was stronger when the rats stayed for a long time in the cocaine-paired environment. Our observation suggested that the phase-locked hippocampal theta rhythm is related to the approaching behavior of the rat caused by reward memory. Thus, the role of the hippocampus in drug craving should be emphasized further.

Behavior-dependent coordination of multiple theta dipoles in the hippocampus

Theta (4-10 Hz) oscillations in the hippocampus are thought to be important for plasticity, temporal coding, learning, and memory. The hippocampal system has been postulated to have two (or more) rhythmic sources of theta oscillations, but little is known about the behavior-dependent interplay of theta oscillations in different subregions and layers of the hippocampus. We tested rats in a hippocampus-dependent delayed spatial alternation task on a modified T-maze while simultaneously recording local field potentials from dendritic and somatic layers of the dentate gyrus, CA3, and CA1 regions using high-density, 96-site silicon probes. We found that while theta oscillations were generally coherent throughout the hippocampus, the power, coherence, and phase of theta oscillations fluctuated in a layer-specific manner, confirming the presence of multiple interdependent dipoles. Layer-dependent changes in the power and coherence of theta oscillations varied with aspects of both the memory and control (non-mnemonic) tasks, but only a small fraction of the variance could be explained by running speed or acceleration. Furthermore, the phase lag between theta oscillations in the CA3 and CA1 pyramidal layers was significantly smaller on the maze arm approaching the T-junction than on other arms of the alternation task or on comparable segments of control tasks. Overall, our findings reveal a consortium of layer-specific theta dipoles (current sinks and sources) generated by the rhythmic flow of ions into and out of hippocampal cells. Moreover, our data suggest that these different theta generators flexibly coordinate hippocampal regions and layers to support behavioral task performance.

Decline in hippocampal theta activity during cessation of locomotor approach sequences: amplitude leads frequency and relates to instrumental behavior

Hippocampal theta frequency and amplitude decrease as locomotor approach slows and the goal is reached. This study compared the declines of these theta parameters and related them to behavioral events. Theta activity was recorded with bipolar electrodes spanning cornu Ammon, sector 1 or cornu Ammon, sectors 2/3 cell layers of the dorsal hippocampus in 12 rats trained to approach and depress a treadle which exposed a milk dipper. Behavioral events were identified using a video capture system (20-ms sampling) synchronized to the hippocampal recording system (10-ms sampling). Peri-event averages of theta activity were made around the initial paw contact with the treadle, the presentation of the dipper, and the first lick at the dipper. Phase relationships between averaged hippocampal slow wave activity and behavioral events occasionally were found but they were inconsistent. In averages of both amplitude and frequency, times of minimum were less variable around paw contact indicating that compared with reward presentation and consummatory behavior, it more closely related to the processes determining the declines. Theta amplitude declined more rapidly than frequency and reached an earlier minimum in averages around initial paw contact and dipper presentation. Mean amplitude minimum occurred after the paw contact at 159 ms but the decline of frequency continued into the licking bout with its minimum occurring at 343 ms. The findings indicate that during the termination of approach locomotion, the amplitude of hippocampal theta activity is closely related to specific expected sensorimotor events.

Amplitude, frequency, and phase analysis of hippocampal theta during sensorimotor processing in a jump avoidance task

In the current study, rats implanted with hippocampal recording electrodes were trained to avoid shock in a jump avoidance task, using three separate heights, 28, 33, and 38 cm. The objectives were to measure the progression of Type 2 (immobility-related) hippocampal theta amplitude and frequency recorded during jump avoidances at each of the levels, and determine if there was a consistent relationship between the phase of the Type 1 (movement related) theta jump wave and the moment of movement initiation at each of the jump heights. The results demonstrated that the immobility period prior to the execution of the jump could be divided into two components: a sensory processing period and a movement preparation period. Comparing these two periods, average amplitudes were higher while frequency remained relatively constant during the sensory processing period. During the movement preparation period there was a negative correlation between amplitude and frequency: amplitude declined rapidly and frequency increased rapidly. During the execution of the jump, theta amplitude (Type 1) and frequency were positively correlated, both reaching peak values. The separate analyses of the individual jump heights provided further support for the precise relationships between theta parameters and the magnitude of the required movements. A significant phase preference was demonstrated for the highest jump height, with movement initiation occurring around the trough of theta recorded from the stratum moleculare of the dentate region. The findings supported the conclusion that the generation of hippocampal theta band oscillation and synchrony was related to sensorimotor processing and integration.