Behavioral anxiolysis without reduction of hippocampal theta frequency after histamine application in the lateral septum of rats

Hippocampal theta activity is linked to various processes, including locomotion, learning and memory, and defense and affect (i.e., fear and anxiety). Interestingly, all classes of clinically effective anxiolytics, as well as experimental compounds that decrease anxiety in pre‐clinical animal models of anxiety, reduce the frequency of hippocampal theta activity elicited by stimulation of the reticular formation in freely behaving or anesthetized animals. In the present experiments, we found that bilateral histamine infusions (0.5 µg/hemisphere) into the lateral septum (LS) of rats decreased anxiety‐like responses in two models of anxiety, the elevated plus maze and novelty‐induced suppression of feeding test. Surprisingly, these same infusions significantly increased hippocampal theta frequency elicited by reticular stimulation in urethane‐anesthetized rats. In contrast to these findings, additional experiments showed that the clinically effective anxiolytic buspirone (40 mg/kg, i.p.) reduced theta frequency, confirming previous observations. Taken together, the dissociation of behavioral anxiolysis and theta frequency reduction noted here suggest that hippocampal theta frequency is not a direct index of anxiety levels in rodents. Further, the mechanisms underlying the behavioral and physiological effects elicited by histamine in the LS require further study. © 2014 Wiley Periodicals, Inc.     

Hippocampal theta rhythm is reduced by suppression of the H-current in septohippocampal GABAergic neurons

Hippocampal learning and memory tasks are tightly coupled to the hippocampal theta rhythm, which is critically dependent on the medial septum/diagonal band of Broca (MSDB) although the underlying mechanisms remain unclear. The MSDB sends both cholinergic and GABAergic projections to the hippocampus. Here we show that: (i) septo-hippocampal GABAergic but not cholinergic neurons have a pacemaking current, the H-current, and that its selective blockade by ZD7288 reduces their spontaneous firing in rat brain slices; and (ii), local infusions of ZD7288 into the MSDB reduce exploration and sensory evoked hippocampal theta bursts in behaving rats. Thus, the H-current in septohippocampal GABAergic neurons modulates the hippocampal theta rhythm.     

Hippocampal theta‐driving cells revealed by Granger causality

The two‐dipole model of theta generation in hippocampal CA1 suggests that the inhibitory perisomatic theta dipole is generated by local GABAergic interneurons. Various CA1 interneurons fire preferentially at different theta phases, raising the question of how these theta‐locked interneurons contribute to the generation of theta oscillations. We here recorded interneurons in the hippocampal CA1 area of freely behaving mice, and identified a unique subset of theta‐locked interneurons by using the Granger causality approach. These cells fired in an extremely reliable theta‐burst pattern at high firing rates (～90 Hz) during exploration and always locked to ascending phases of the theta waves. Among theta‐locked interneurons we recorded, only these cells generated strong Granger causal influences on local field potential (LFP) signals within the theta band (4–12 Hz), and the influences were persistent across behavioral states. Our results suggest that this unique type of theta‐locked interneurons serve as the local inhibitory theta dipole control cells in shaping hippocampal theta oscillations. © 2012 Wiley Periodicals, Inc.     

Distinct contributions of human hippocampal theta to spatial cognition and anxiety

Current views of the hippocampus assign this structure, and its prominent theta rhythms, a key role in both cognition and affect. We studied this duality of function in humans, where no direct evidence exists. Whole‐head magnetoencephalographic (MEG) data were recorded to measure theta activity while healthy participants (N = 25) navigated two virtual Morris water mazes, one in which they risked receiving aversive shocks without warning to induce anxiety and one in which they were safe from shocks. Results showed that threat of shock elevated anxiety level and enhanced navigation performance as compared to the safe condition. MEG source analyses revealed that improved navigation performance during threat was preferentially associated with increased left septal (posterior) hippocampal theta (specifically 4–8 Hz activity), replicating previous research that emphasizes a predominant role of the septal third of the hippocampus in spatial cognition. Moreover, increased self‐reported anxiety during threat was preferentially associated with increased left temporal (anterior) hippocampal theta (specifically 2–6 Hz activity), consistent with this region's involvement in mediating conditioned and innate fear. Supporting contemporary theory, these findings highlight simultaneous involvement of the human hippocampus in spatial cognition and anxiety, and clarify their distinct correlates. © 2012 Wiley Periodicals, Inc.     

ZD7288, a selective hyperpolarization-activated cyclic nucleotide-gated channel blocker,inhibits hippocampal synaptic plasticity

The selective hyperpolarization-activated cyclic nucleotide-gated(HCN) channel blocker 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyrimidinium chloride(ZD7288) blocks the induction of long-term potentiation in the perforant path–CA3 region in rat hippocampus in vivo. To explore the mechanisms underlying the action of ZD7288, we recorded excitatory postsynaptic potentials in perforant path–CA3 synapses in male Sprague-Dawley rats. We measured glutamate content in the hippocampus and in cultured hippocampal neurons using high performance liquid chromatography, and determined intracellular Ca~(2+) concentration([Ca~(2+)]i) using Fura-2. ZD7288 inhibited the induction and maintenance of long-term potentiation, and these effects were mirrored by the nonspecific HCN channel blocker cesium. ZD7288 also decreased glutamate release in hippocampal tissue and in cultured hippocampal neurons. Furthermore, ZD7288 attenuated glutamate-induced rises in [Ca~(2+)]i in a concentration-dependent manner and reversed 8-Br-c AMP-mediated facilitation of these glutamate-induced [Ca~(2+)]i rises. Our results suggest that ZD7288 inhibits hippocampal synaptic plasticity both glutamate release and resultant [Ca~(2+)]i increases in rat hippocampal neurons.     

Bidirectional pattern-specific plasticity of the slow afterhyperpolarization in rats: role for high-voltage activated Ca2+ channels and I h

A burst of action potentials in hippocampal neurons is followed by a slow afterhyperpolarization (sAHP) that serves to limit subsequent firing. A reduction in the sAHP accompanies acquisition of several types of learning, whereas increases in the sAHP are correlated with cognitive impairment. The present study demonstrates in vitro that activity-dependent bidirectional plasticity of the sAHP does not require synaptic activation, and depends on the pattern of action potential firing. Whole-cell current-clamp recordings from CA1 pyramidal neurons in hippocampal slices from young rats (postnatal days 14-24) were performed in blockers of synaptic transmission. The sAHP was evoked by action potential firing at gamma-related (50 Hz, gamma-AHP) or theta frequencies (5 Hz, theta-AHP), two firing frequencies implicated in attention and memory. Interestingly, when the gamma-AHP and theta-AHP were evoked in the same cell, a gradual potentiation of the gamma-AHP (186 ± 31%) was observed that was blocked using Ca(2+) channel blockers nimodipine (10 μm) or ω-conotoxin MVIIC (1 μm). In experiments that exclusively evoked the sAHP with 50 Hz firing, the gamma-AHP was similarly potentiated (198 ± 44%). However, theta-burst firing pattern alone resulted in a decrease (65 ± 19%) of the sAHP. In these experiments, application of the h-channel blocker ZD7288 (25 μm) selectively prevented enhancement of the gamma-AHP. These data demonstrate that induction requirements for bidirectional AHP plasticity depend on the pattern of action potential firing, and result from distinct mechanisms. The identification of novel mechanisms underlying AHP plasticity in vitro provides additional insight into the dynamic processes that may regulate neuronal excitability during learning in vivo.     

ZD7288 inhibits postsynaptic glutamate receptor-mediated responses at hippocampal perforant path-granule cell synapses

Hyperpolarization-activated channels (Ih) are widely expressed in the nervous system and believed to play an important role in the regulation of membrane excitability and rhythmic activity. Recent evidence suggests that Ih may be involved in long-term potentiation (LTP) in the hippocampus; however, the results are controversial. To explore the possible causes of these differing results, the effects of Ih blockers on synaptic activity were evaluated in mouse hippocampal slices. ZD7288 (20 micro m), a selective Ih blocker, apparently prevented the induction of LTP, while Cs+ (1 mm), a commonly used Ih blocker, had no effect on LTP at hippocampal perforant path-dentate granule cell synapses. In addition, ZD7288 but not Cs+ abolished basal synaptic transmission. Results from voltage-clamp experiments showed that ZD7288 produced a very little inhibition on hyperpolarization-activated currents, indicating a weak expression of the Ih in granule neurons. Outside-out patch recordings revealed that ZD7288 inhibited glutamate receptor-mediated responses, while Cs+ had no effect on them. Meanwhile, ZD7288 reduced both alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate and N-methyl-d-aspartate receptor-mediated excitatory postsynaptic currents. The results suggest that ZD7288-induced reduction of synaptic transmission may result from its inhibition of the postsynaptic glutamate receptors on dentate granule neurons.     

Median raphe stimulation‐induced motor inhibition concurrent with suppression of type 1 and type 2 hippocampal theta

This study investigated behavioral, anatomical and electrophysiological effects produced by electrical stimulation of posterior hypothalamic (PH) or median raphe (MR) nuclei, independently and during combined stimulation of both PH and MR. These three stimulation conditions were applied during spontaneous behavior in an open field and during PH stimulation‐induced wheel running, while simultaneously recording hippocampal (HPC) field activity. An additional objective was to determine the effects of MR stimulation on Type 1 movement related theta and Type 2 sensory processing related theta. To achieve the latter, when behavioral studies were completed we studied the same rats under urethane anesthesia and then during urethane anesthesia with the addition of atropine sulfate (ATSO4). Here we demonstrated that electrical stimulation of a localized region of the MR nucleus resulted in a profound inhibition of both spontaneously occurring theta related motor behaviors and the theta related motor behaviors induced by electrical stimulation of the PH nucleus. Furthermore, this motor inhibition occurred concurrently with strong suppression of hippocampal theta field oscillations in the freely moving rat, a condition where the theta recorded is Type 2 sensory processing theta occurring coincidently with Type 1 movement related theta (Bland, 1986). Our results indicate that motor inhibition resulted from stimulation of neurons located in the mid central region of the MR, while stimulation in adjacent regions produced variable responses, including movements and theta activity. The present study provided evidence that the pharmacological basis of the suppression of Type 2 sensory processing HPC theta was cholinergic. However, MR inhibition of PH‐induced wheel running was not affected by cholinergic blockade, which blocks Type 2 theta, indicating that MR stimulation‐induced motor inhibition also requires the suppression of Type 1 theta. © 2015 Wiley Periodicals, Inc.     

The functional role of human right hippocampal/parahippocampal theta rhythm in environmental encoding during virtual spatial navigation

Low frequency theta band oscillations (4–8 Hz) are thought to provide a timing mechanism for hippocampal place cell firing and to mediate the formation of spatial memory. In rodents, hippocampal theta has been shown to play an important role in encoding a new environment during spatial navigation, but a similar functional role of hippocampal theta in humans has not been firmly established. To investigate this question, we recorded healthy participants’ brain responses with a 160‐channel whole‐head MEG system as they performed two training sets of a virtual Morris water maze task. Environment layouts (except for platform locations) of the two sets were kept constant to measure theta activity during spatial learning in new and familiar environments. In line with previous findings, left hippocampal/parahippocampal theta showed more activation navigating to a hidden platform relative to random swimming. Consistent with our hypothesis, right hippocampal/parahippocampal theta was stronger during the first training set compared to the second one. Notably, theta in this region during the first training set correlated with spatial navigation performance across individuals in both training sets. These results strongly argue for the functional importance of right hippocampal theta in initial encoding of configural properties of an environment during spatial navigation. Our findings provide important evidence that right hippocampal/parahippocampal theta activity is associated with environmental encoding in the human brain. Hum Brain Mapp 38:1347–1361, 2017. © 2016 Wiley Periodicals, Inc.     

The interrelationship between movement and cognition: Theta rhythm and the P300 event‐related potential

The relationship among brain electrophysiological activity, motor activity, and cognition has been a matter of great interest. For example, it has been discussed whether hippocampal theta rhythm reflects motor activity or cognitive activity, whereas it is widely accepted that the P300 event‐related potential (ERP) reflects cognitive processes such as updating working memory. Here, we investigated the interrelationships among motor activity, hippocampal theta rhythm, and hippocampal P300 ERP using electrophysiological and behavioral data recorded from rats performing an auditory discrimination task (i.e., the auditory oddball paradigm) in a chamber with and without a running‐wheel. We found that the hippocampal theta rhythm generated during locomotion codes information about self‐motion, and event‐related increases in hippocampal theta rhythm observed when rats performed the auditory discrimination cognitive task reflect a change in motor behavior after learning the cognitive task. Interestingly, the hippocampal P300 ERP occurred coincidently with increases in the power and frequency of hippocampal theta rhythm. In addition, we found that changes in theta rhythm observed during spontaneous wheel running without performing a cognitive task as well as when performing the cognitive task are associated with changes in delta‐ and gamma‐band EEG activities. These major findings are discussed with respect to current hypotheses regarding P300 ERP and theta‐, delta‐, and gamma‐band EEG activities in brain functions. © 2010 Wiley‐Liss, Inc.     

Medial prefrontal cortex cells show dynamic modulation with the hippocampal theta rhythm dependent on behavior

Both the hippocampus and the medial prefrontal cortex are essential for successful performance in learning- and memory-related tasks. Within the hippocampus the theta rhythm plays an integral role in the timing of action potentials of hippocampal neurons responding to elements of any given task. Medial prefrontal cortex (mPFC) neurons display firing rate changes to specific facets of behavioral tasks (Jung et al., 1998. Cereb Cortex 8:437--450). We recorded units in the mPFC and field potentials in the hippocampus to determine whether behaviorally correlated mPFC cells fired with phase relationships to the hippocampal theta rhythm. In two different behavioral tasks (running a linear track and foraging in two distinct environments) we found mPFC cells that alternated between theta entrained firing and nonphasic firing depending on the ongoing behavior, while other cells were modulated during all conditions in both tasks. The majority of the mPFC cells with a significant correlation of firing rate changes with behavior were entrained to hippocampal theta. Cells that fired to specific events during only one direction of running were predisposed to theta modulation only in that direction. mPFC neurons have the capability to respond to behaviorally relevant elements by dynamically alternating between hippocampal theta entrained and nonphasic firing.     

The pharmacology of hippocampal theta cells: Evidence that the sensory processing correlate is cholinergic

The firing repertoires of theta cells in the CA1 and dentate layers of the hippocampal formation of the freely moving rabbit were analyzed during 3 behavioral conditions: (1) voluntary motor patterns, termed type 1 theta behaviors; (2) automatic motor patterns, termed type 2LIA behaviors; (3) alert immobility with presentation of sensory stimuli, termed type 2 theta behavior. Cholinergic manipulations were shown to effect the firing repertoires of theta cells during the type 2 theta behavior condition (sensory processing) and not the other two behavioral conditions. A hypothesis of a sensorimotor processing function of the hippocampal formation is presented and discussed.     

In vivo contribution of h-channels in the septal pacemaker to theta rhythm generation

One of the most intriguing network-level inferences made on the basis of in vitro and modelling data regarding the role of Ih current was that they participate in rhythmogenesis in different parts of the brain. The nature of Ih contribution to various neuronal oscillations is far from uniform however, and the proper evaluation of the role of Ih in each particular structure requires in situ investigations in the intact brain. In this study we tested the effect of Ih blockade in the medial septum on hippocampal theta rhythm in anaesthetized and freely behaving rats. We could not confirm the recent report of elimination of theta by septal injection of ZD7288 [C. Xu et al. (2004) Eur. J. Neurosci., 19, 2299-2309]; the observed effects were more subtle and more specific. We found that Ih blockade in the medial septum substantially decreased the frequency of hippocampal oscillations without changing the context in which theta occurred, i.e. specific behaviours in freely moving rats and spontaneous switching and brainstem stimulation under anaesthesia. Septal injection of ZD7288 eliminated atropine-resistant theta elicited by high intensity electrical stimulation of the reticular formation in anaesthetized rats but was ineffective in combination with the muscarinic agonist, carbachol. Thus, functional Ih was necessary for the septum to generate or transmit high frequency theta rhythm elicited by strong ascending activation, whereas low frequency theta persisted after Ih blockade. These results suggest that Ih plays a specific role in septal theta generation by promoting fast oscillations during exploratory behaviour and rapid eye movement sleep.     

Prestimulus theta in the human hippocampus predicts subsequent recognition but not recall

Human theta (4?8 Hz) activity in the medial temporal lobe correlates with memory formation; however, the precise role that theta plays in the memory system remains elusive (Hanslmayr and Staudigl, 2014 ). Recently, prestimulus theta activity has been associated with successful memory formation, although its specific cognitive role remains unknown (e.g., Fell et al., 2011). In this report, we demonstrate that prestimulus theta in the hippocampus indexes encoding that supports old‐new recognition memory but not recall. These findings suggest that human hippocampal prestimulus theta may preferentially participate in the encoding of item information, as opposed to associative information. © 2014 Wiley Periodicals, Inc.     

Orexinergic theta rhythm in the rat hippocampal formation: In vitro and in vivo findings

Previous in vivo data suggested that orexin neuropeptides (ORX_A and ORX_B) synthetized in hypothalamic neurons were involved in the mechanism of generation of the hippocampal formation theta rhythm. Surprisingly, this suggestion has never been directly proved by experiments using intraseptal or intrahippocampal administration of orexins. In this study, involving the use of in vitro hippocampal formation slices and in vivo model of anesthetized rat, we provide the first convergent electropharmacological evidence that in the presence of both ORX_A and ORX_B the hippocampal formation neuronal network is capable of producing oscillations in the theta band. This effect of orexin peptides was antagonized by selective blockers of orexin receptors (OX₁R and OX₂R), SB 334867 and TCS OX2 29, respectively. These results provide evidence for a novel, orexinergic mechanism responsible for the production of theta rhythm in the hippocampal formation neuronal network. © 2015 Wiley Periodicals, Inc.     

The theta‐related firing activity of parvalbumin‐positive neurons in the medial septum‐diagonal band of broca complex and their response to 5‐HT1A Receptor stimulation in a rat model of Parkinson's disease

The parvalbumin (PV)‐positive neurons in the medial septum‐diagonal band of Broca complex (MS‐DB) play an important role in the generation of hippocampal theta rhythm involved in cognitive functions. These neurons in this region express a high density of 5‐HT_1A receptors which regulate the neuronal activity and consequently affect the theta rhythm. In this study, we examined changes in the theta‐related firing activity of PV‐positive neurons in the MS‐DB, their response to 5‐HT_1A receptor stimulation and the corresponding hippocampal theta rhythm, and the density of PV‐positive neurons and their co‐localization with 5‐HT_1A receptors in rats with 6‐hydroxydopamine lesions of the substantia nigra pars compacta (SNc). The lesion of the SNc decreased the rhythmically bursting activity of PV‐positive neurons and the peak frequency of hippocampal theta rhythm. Systemic administration of 5‐HT_1A receptor agonist 8‐OH‐DPAT (0.5–128 µg/kg, i.v.) inhibited the firing rate of PV‐positive neurons and disrupted rhythmically bursting activity of the neurons and the theta rhythm in sham‐operated and the lesioned rats, respectively. The cumulative doses producing inhibition and disruption in the lesioned rats were higher than that of sham‐operated rats. Furthermore, local application of 8‐OH‐DPAT (0.005 μg) in the MS‐DB also inhibited the firing rate of PV‐positive neurons and disrupted their rhythmically bursting activity in sham‐operated rats, while having no effect on PV‐positive neurons in the lesioned rats. The lesion of the SNc decreased the density of PV‐positive neurons in the MS‐DB, and percentage of PV‐positive neurons expressing 5‐HT_1A receptors. These results indicate that the lesion of the SNc leads to suppression of PV‐positive neurons in the MS‐DB and hippocampal theta rhythm. Furthermore, the lesion decreases the response of these neurons to 5‐HT_1A receptor stimulation, which attributes to dysfunction and/or down‐regulation of 5‐HT_1A receptor expression on these neurons. These changes may be involved in cognitive impairments of Parkinson's disease. © 2013 Wiley Periodicals, Inc.     

Septohippocampal GABAergic neurons mediate the altered behaviors induced by n‐methyl‐D‐aspartate receptor antagonists

We hypothesize that selective lesion of the septohippocampal GABAergic neurons suppresses the altered behaviors induced by an N‐methyl‐D ‐aspartate (NMDA) receptor antagonist, ketamine or MK‐801. In addition, we hypothesize that septohippocampal GABAergic neurons generate an atropine‐resistant theta rhythm that coexists with an atropine‐sensitive theta rhythm in the hippocampus. Infusion of orexin‐saporin (ore‐SAP) into the medial septal area decreased parvalbumin‐immunoreactive (GABAergic) neurons by ～80%, without significantly affecting choline‐acetyltransferase‐immunoreactive (cholinergic) neurons. The theta rhythm during walking, or the immobility‐associated theta induced by pilocarpine, was not different between ore‐SAP and sham‐lesion rats. Walking theta was, however, more disrupted by atropine sulfate in ore‐SAP than in sham‐lesion rats. MK‐801 (0.5 mg/kg i.p.) induced hyperlocomotion associated with an increase in frequency, but not power, of the hippocampal theta in both ore‐SAP and sham‐lesion rats. However, MK‐801 induced an increase in 71–100 Hz gamma waves in sham‐lesion but not ore‐SAP lesion rats. In sham‐lesion rats, MK‐801 induced an increase in locomotion and an impairment of prepulse inhibition (PPI), and ketamine (3 mg/kg s.c.) induced a loss of gating of hippocampal auditory evoked potentials. MK‐801‐induced behavioral hyperlocomotion and PPI impairment, and ketamine‐induced auditory gating deficit were reduced in ore‐SAP rats as compared to sham‐lesion rats. During baseline without drugs, locomotion and auditory gating were not different between ore‐SAP and sham‐lesion rats, and PPI was slightly but significantly increased in ore‐SAP as compared with sham lesion rats. It is concluded that septohippocampal GABAergic neurons are important for the expression of hyperactive and psychotic symptoms an enhanced hippocampal gamma activity induced by ketamine and MK‐801, and for generating an atropine‐resistant theta. Selective suppression of septohippocampal GABAergic activity is suggested to be an effective treatment of some symptoms of schizophrenia. © 2012 Wiley Periodicals, Inc.     

The CAN‐In network: A biologically inspired model for self‐sustained theta oscillations and memory maintenance in the hippocampus

During working memory tasks, the hippocampus exhibits synchronous theta‐band activity, which is thought to be correlated with the short‐term memory maintenance of salient stimuli. Recent studies indicate that the hippocampus contains the necessary circuitry allowing it to generate and sustain theta oscillations without the need of extrinsic drive. However, the cellular and network mechanisms supporting synchronous rhythmic activity are far from being fully understood. Based on electrophysiological recordings from hippocampal pyramidal CA1 cells, we present a possible mechanism for the maintenance of such rhythmic theta‐band activity in the isolated hippocampus. Our model network, based on the Hodgkin‐Huxley formalism, comprising pyramidal neurons equipped with calcium‐activated nonspecific cationic (CAN) ion channels, is able to generate and sustain synchronized theta oscillations (4–12 Hz), following a transient stimulation. The synchronous network activity is maintained by an intrinsic CAN current (I_CAN), in the absence of constant external input. When connecting the pyramidal‐CAN network to fast‐spiking inhibitory interneurons, the dynamics of the model reveal that feedback inhibition improves the robustness of fast theta oscillations, by tightening the synchronization of the pyramidal CAN neurons. The frequency and power of the theta oscillations are both modulated by the intensity of the I_CAN, which allows for a wide range of oscillation rates within the theta band. This biologically plausible mechanism for the maintenance of synchronous theta oscillations in the hippocampus aims at extending the traditional models of septum‐driven hippocampal rhythmic activity. © 2017 Wiley Periodicals, Inc.     

Obituary: Cornelius H. Vanderwolf

C.H. Vanderwolf described motor correlates of hippocampal theta oscillations and uncovered two broad classes: atropine‐sensitive and atropine‐resistant rhythm with likely different behavioral and cognitive significance. © 2015 Wiley Periodicals, Inc.     

Hippocampal theta sequences

The activity of individual hippocampal principal neurons is spatially localized such that each neuron is active only when the animal occupies an area of the environment known as the cell's place field. Additionally, the activity of hippocampal neurons exhibits a particular temporal relationship to the hippocampal EEG, such that spikes fired by the neuron occur at progressively earlier phases of the co-occurring theta rhythm in the EEG as the animal traverses the place field. This relationship is known as theta precession. A long-standing prediction following the observation of theta precession has been that among a collection of hippocampal neurons recorded simultaneously, the neurons will fire in sequences reflecting the behavioral order of the place fields. Here we examine this prediction. We show that clear, ordered sequences occur during theta, which we name theta sequences, in which a portion of the animal's spatial experience is played out in forwards order. We further investigate the relationship of theta sequences to phase precession by shuffling spike phases in such a way as to preserve the relationship between spike phase and position. This jitter significantly reduces the prevalence of theta sequences while leaving theta phase precession intact, suggesting that the presence of theta phase precession is not trivially predictive of theta sequences. Finally, we discuss the relationship between theta sequences and individual place fields, and the possible functional role of theta sequences in navigational learning.