Stability of hippocampal place cell activity across the rat estrous cycle

Female rodents in behavioral neuroscience: Narrative review on the methodological pitfalls

Since the NIH 'Sex as biological variable' policy, the percentage of studies including female subjects have increased largely. Nonetheless, many researchers fail to adequate their protocols to include females. In this narrative review, we aim to discuss the methodological pitfalls of the inclusion of female rodents in behavioral neuroscience. We address three points to consider in studies: the manipulations conducted only in female animals (such as estrous cycle monitoring, ovariectomy, and hormone replacement), the consideration of males as the standard, and biases related to interpretation and publication of the results. In addition, we suggest guidelines and perspectives for the inclusion of females in preclinical research.

The estrous cycle modulates hippocampal spine dynamics, dendritic processing, and spatial coding

Histological evidence suggests that the estrous cycle exerts a powerful effect on CA1 neurons in mammalian hippocampus. Decades have passed since this landmark observation, yet how the estrous cycle shapes dendritic spine dynamics and hippocampal spatial coding in vivo remains a mystery. Here, we used a custom hippocampal microperiscope and two-photon calcium imaging to track CA1 pyramidal neurons in female mice over multiple cycles. Estrous cycle stage had a potent effect on spine dynamics, with heightened density during periods of greater estradiol (proestrus). These morphological changes were accompanied by greater somatodendritic coupling and increased infiltration of back-propagating action potentials into the apical dendrite. Finally, tracking CA1 response properties during navigation revealed enhanced place field stability during proestrus, evident at the single-cell and population level. These results establish the estrous cycle as a driver of large-scale structural and functional plasticity in hippocampal circuits essential for learning and memory.

Sexually dimorphic organization of open field behavior following moderate prenatal alcohol exposure

BACKGROUND

Prenatal alcohol exposure (PAE) can produce deficits in a wide range of cognitive functions but is especially detrimental to behaviors requiring accurate spatial information processing. In open field environments, spatial behavior is organized such that animals establish "home bases" marked by long stops focused around one location. Progressions away from the home base are circuitous and slow, while progressions directed toward the home base are non-circuitous and fast. The impact of PAE on the organization of open field behavior has not been experimentally investigated.

METHODS

In the present study, adult female and male rats with moderate PAE or saccharin exposure locomoted a circular high walled open field for 30 minutes under lighted conditions.

RESULTS

The findings indicate that PAE and sex influence the organization of open field behavior. Consistent with previous literature, PAE rats exhibited greater locomotion in the open field. Novel findings from the current study indicate that PAE and sex also impact open field measures specific to spatial orientation. While all rats established a home base on the periphery of the open field, PAE rats, particularly males, exhibited significantly less clustered home base stopping with smaller changes in heading between stops. PAE also impaired progression measures specific to distance estimation, while sex alone impacted progression measures specific to direction estimation.

CONCLUSIONS

These findings support the conclusion that adult male rats have an increased susceptibility to the effects of PAE on the organization of open field behavior.

Spatiotemporally random and diverse grid cell spike patterns contribute to the transformation of grid cell to place cell in a neural network model

The medial entorhinal cortex and the hippocampus are brain regions specialized in spatial information processing. While an animal navigates around an environment, grid cells in the medial entorhinal cortex spike at multiple discrete locations, forming hexagonal grid patterns, and each grid cell is spatiotemporally dynamic with a different grid size, spacing, and orientation. In contrast, place cells in the hippocampus spike when an animal is at one or more specific locations, called a “place field”. While an animal traverses through a place field, the place cell’s spike phases relative to the hippocampal theta-frequency oscillation advance in phase, known as the “spike phase precession” phenomenon and each spike encodes the specific location within the place field. Interestingly, the medial entorhinal cortical grid cells and the hippocampal place cells are only one excitatory synapse apart. However, how the spatiotemporally dynamic multi-peaked grid cell activities are transformed into hippocampal place cell activities with spike phase precession phenomenon is yet unknown. To address this question, we construct an anatomically and physiologically realistic neural network model comprised of 10,000 grid cell models, each with a spatiotemporally dynamic grid patterns and a place cell model connected by excitatory synapses. Using this neural network model, we show that grid cells’ spike activities with spatiotemporally random and diverse grid orientation, spacing, and phases as inputs to place cell are able to generate a place field with spike phase precession. These results indicate that spatiotemporally random and diverse grid cell spike activities are essential for the formation of place cell activity observed in vivo.

Theta-gamma cascades and running speed

Oscillations in the hippocampal local field potential at theta and gamma frequencies are prominent during awake behavior and have demonstrated several behavioral correlates. Both oscillations have been observed to increase in amplitude and frequency as a function of running speed. Previous investigations, however, have examined the relationship between speed and each of these oscillation bands separately. Based on energy cascade models where "…perturbations of slow frequencies cause a cascade of energy dissipation at all frequency scales" (Buzsaki G. Rhythms of the Brain, 2006), we hypothesized that cross-frequency interactions between theta and gamma should increase as a function of speed. We examined these relationships across multiple layers of the CA1 subregion, which correspond to synaptic zones receiving different afferents. Across layers, we found a reliable correlation between the power of theta and the power of gamma, indicative of an amplitude-amplitude relationship. Moreover, there was an increase in the coherence between the power of gamma and the phase of theta, demonstrating increased phase-amplitude coupling with speed. Finally, at higher velocities, phase entrainment between theta and gamma increases. These results have important implications and provide new insights regarding how theta and gamma are integrated for neuronal circuit dynamics, with coupling strength determined by the excitatory drive within the hippocampus. Specifically, rather than arguing that different frequencies can be attributed to different psychological processes, we contend that cognitive processes occur across multiple frequency bands simultaneously with organization occurring as a function of the amount of energy iteratively propagated through the brain. NEW & NOTEWORTHY Often, the theta and gamma oscillations in the hippocampus have been believed to be a consequence of two marginally overlapping phenomena. This perspective, however, runs counter to an alternative hypothesis in which a slow-frequency, high-amplitude oscillation provides energy that cascades into higher frequency, lower amplitude oscillations. We found that as running speed increases, all measures of cross-frequency theta-gamma coupling intensify, providing evidence in favor of the energy cascade hypothesis.

Sex Differences in Long-Term Potentiation at Temporoammonic-CA1 Synapses: Potential Implications for Memory Consolidation

Sex differences in spatial memory have long been observed in humans, non-human primates and rodents, but the underlying cellular and molecular mechanisms responsible for these differences remain obscure. In the present study we found that adolescent male rats outperformed female rats in 7 d and 28 d retention probes, but not in learning trials and immediate probes, in the Morris water maze task. Male rats also had larger long-term potentiation (LTP) at hippocampal temproammonic-CA1 (TA-CA1) synapses, which have been implicated to play a key role in place field and memory consolidation, when protocols designed to elicit late-stage LTP (LLTP) were used. Interestingly, the ratio of evoked AMPA/NMDA currents was found to be smaller at TA-CA1 synapses in male rats compared to female rats. Protein biotinylation experiments showed that male rats expressed more surface GluN1 receptors in hippocampal CA1 stratum lacunosum-moleculare (SLM) than female rats, although GluA1 expression was also slightly higher in male rats. Taken together, our results suggest that differences in the expression of AMPA and NMDA receptors may affect LTP expression at TA-CA1 synapses in adolescent male and female rats, and thus possibly contribute to the observed sex difference in spatial memory.

Instability of spatial encoding by CA1 hippocampal place cells after peripheral nerve injury

Several authors have shown that the hippocampus responds to painful stimulation and suggested that prolonged painful conditions could lead to abnormal hippocampal functioning. The aim of the present study was to evaluate whether the induction of persistent peripheral neuropathic pain would affect basic hippocampal processing such as the spatial encoding performed by CA1 place cells. These place cells fire preferentially in a certain spatial position in the environment, and this spatial mapping remains stable across multiple experimental sessions even when the animal is removed from the testing environment. To address the effect of prolonged pain on the stability of place cell encoding, we chronically implanted arrays of electrodes in the CA1 hippocampal region of adult rats and recorded the multichannel neuronal activity during a simple food-reinforced alternation task in a U-shaped runway. The activity of place cells was followed over a 3-week period before and after the establishment of an animal model of neuropathy, spared nerve injury. Our results show that the nerve injury increased the number of place fields encoded per cell and the mapping size of the place fields. In addition, there was an increase in in-field coherence while the amount of spatial information content that a single spike conveyed about the animal location decreased over time. Other measures of spatial tuning (in-field firing rate, firing peak and number of spikes) were unchanged between the experimental groups. These results demonstrate that the functioning of spatial place cells is altered during neuropathic pain conditions.

Hippocampal and striatal dependent navigation: sex differences are limited to acquisition

Estrogen has been demonstrated to enhance the use of hippocampal-based place learning while reducing the use of striatal-based motor-response strategy (Korol, D.L., Malin, E.L., Borden, K.A., Busby, R.A., & Couper-Leo, J. (2004). Shifts in preferred learning strategy across the estrous cycle in female rats. Horm. Behav. 45, 330-338). Previous research has focused on task acquisition and the switch from a place to motor-response navigation with training. The current paradigm allowed an examination of the interplay between these two systems by having well-trained animals switch strategies "on demand." Female and male Sprague-Dawley rats were taught a motor-response task on a plus maze. The rats were then introduced to a place task and taught to switch, by cue, from the motor-response to place strategy. Finally, the rats were trained to continuously alternate between place and motor-responses strategies. The maze configuration allowed for an analysis of cooperative choices (both strategies result in the same goal arm), competitive choices (both strategies result in different goal arms), and single strategy choices (can only use the motor-response strategy). The results indicate that sex and estrogen-related effects on navigation strategy are limited to the initial stages of learning a task. The role of sex and estrogen is diminished once the task is well learned, and presumably, the relative involvement of the hippocampal and striatal systems is established.

Changes in hippocampal function of ovariectomized rats after sequential low doses of estradiol to simulate the preovulatory estrogen surge

In adult female rats, robust hippocampal changes occur when estradiol rises on the morning of proestrus. Whether estradiol mediates these changes, however, remains unknown. To address this issue, we used sequential injections of estradiol to simulate two key components of the preovulatory surge: the rapid rise in estradiol on proestrous morning, and the slower rise during the preceding day, diestrus 2. Animals were examined mid-morning of simulated proestrus, and compared to vehicle-treated or intact rats. In both simulated and intact rats, CA1-evoked responses were potentiated in hippocampal slices, and presynaptic mechanisms appeared to contribute. In CA3, multiple population spikes were evoked in response to mossy fiber stimuli, and expression of brain-derived neurotrophic factor was increased. Simulation of proestrous morning also improved performance on object and place recognition tests, in comparison to vehicle treatment. Surprisingly, effects on CA1-evoked responses showed a dependence on estradiol during simulated diestrus 2, as well as a dependence on proestrous morning. Increasing estradiol above the physiological range on proestrous morning paradoxically decreased evoked responses in CA1. However, CA3 pyramidal cell activity increased further, and became synchronized. Together, the results confirm that physiological estradiol levels are sufficient to profoundly affect hippocampal function. In addition: (i) changes on proestrous morning appear to depend on slow increases in estradiol during the preceding day; (ii) effects are extremely sensitive to the peak serum level on proestrous morning; and (iii) there are striking subfield differences within the hippocampus.

Differential behavioral state-dependence in the burst properties of CA3 and CA1 neurons

Brief bursts of fast high-frequency action potentials are a signature characteristic of CA3 and CA1 pyramidal neurons. Understanding the factors determining burst and single spiking is potentially significant for sensory representation, synaptic plasticity and epileptogenesis. A variety of models suggest distinct functional roles for burst discharge, and for specific characteristics of the burst in neural coding. However, little in vivo data demonstrate how often and under what conditions CA3 and CA1 actually exhibit burst and single spike discharges. The present study examined burst discharge and single spiking of CA3 and CA1 neurons across distinct behavioral states (awake-immobility and maze-running) in rats. In both CA3 and CA1 spike bursts accounted for less than 20% of all spike events. CA3 neurons exhibited more spikes per burst, greater spike frequency, larger amplitude spikes and more spike amplitude attenuation than CA1 neurons. A major finding of the present study is that the propensity of CA1 neurons to burst was affected by behavioral state, while the propensity of CA3 to burst was not. CA1 neurons exhibited fewer bursts during maze running compared with awake-immobility. In contrast, there were no differences in burst discharge of CA3 neurons. Neurons in both subregions exhibited smaller spike amplitude, fewer spikes per burst, longer inter-spike intervals and greater spike amplitude attenuation within a burst during awake-immobility compared with maze running. These findings demonstrate that the CA1 network is under greater behavioral state-dependent regulation than CA3. The present findings should inform both theoretic and computational models of CA3 and CA1 function.