Bartosz Caban

The posterior hypothalamic area (PHa), including the supramammillary nucleus (SuM) and posterior hypothalamic nuclei, forms a crucial part of the ascending brainstem hippocampal synchronizing pathway, that is involved in the frequency programming and modulation of rhythmic theta activity generated in limbic structures. Recent investigations show that in addition to being a modulator of limbic theta activity, the PHa is capable of producing well-synchronized local theta field potentials by itself. The purpose of this study was to examine the ability of the PHa to generate theta field potentials and accompanying cell discharges in response to glutamatergic stimulation under both in vitro and in vivo conditions. The second objective was to examine the electrophysiological properties of neurons located in the SuM and posterior hypothalamic nuclei. Extracellular in vivo and in vitro as well as intracellular in vitro experiments revealed that glutamatergic stimulation of PHa with kainic acid induces well-synchronized local theta field oscillations in both the supramammillary and posterior hypothalamic nuclei. Furthermore, the glutamatergic PHa theta rhythm recorded extracellularly was accompanied by the activity of specific subtypes of theta-related neurons. We identify, for the first time, a subpopulation of supramammillary and posterior hypothalamic neurons that express clear subthreshold membrane potential oscillations in the theta frequency range.

Theta oscillations generated in hippocampal (HPC) and cortical neuronal networks are involved in various aspects of brain function, including sensorimotor integration, movement planning, memory formation and attention. Disruptions of theta rhythms are present in individuals with brain disorders, including epilepsy and Alzheimer’s disease. Theta rhythm generation involves a specific interplay between cellular (ion channel) and network (synaptic) mechanisms. HCN channels are theta modulators, and several medications are known to enhance their activity. We investigated how different doses of lamotrigine (LTG), an HCN channel modulator, and antiepileptic and neuroprotective agent, would affect HPC theta rhythms in acute HPC slices (in vitro) and anaesthetized rats (in vivo). Whole-cell patch clamp recordings revealed that LTG decreased GABA_A-fast transmission in CA3 cells, in vitro. In addition, LTG directly depressed CA3 and CA1 pyramidal neuron excitability. These effects were partially blocked by ZD 7288, a selective HCN blocker, and are consistent with decreased excitability associated with antiepileptic actions. Lamotrigine depressed HPC theta oscillations in vitro, also consistent with its neuronal depressant effects. In contrast, it exerted an opposite, enhancing effect, on theta recorded in vivo. The contradictory in vivo and in vitro results indicate that LTG increases ascending theta activating medial septum/entorhinal synaptic inputs that over-power the depressant effects seen in HPC neurons. These results provide new insights into LTG actions and indicate an opportunity to develop more precise therapeutics for the treatment of dementias, memory disorders and epilepsy.

In the originally published version of this manuscript, there was an error in Figure 2. In panel D of the figure, one of the asterisks that shows significance was placed in the wrong spot. Figure 2 has been updated as follows online:

Information coding in the hippocampus relies on the interplay between various neuronal ensembles. We discovered that the application of a cholinergic agonist, carbachol (Cch), which triggers oscillatory activity in the gamma range, induces the activity of matrix metalloproteinase 9 (MMP-9)—an enzyme necessary for the maintenance of synaptic plasticity. Using electrophysiological recordings in hippocampal organotypic slices, we show that Cch potentiates the frequency of miniature inhibitory and excitatory postsynaptic currents (mIPSCs and mEPSCs, respectively) in CA1 neurons and this effect is MMP-9 dependent. Interestingly, though MMP-9 inhibition prevents the potentiation of inhibitory events, it further boosts the frequency of excitatory mEPSCs. Such enhancement of the frequency of excitatory events is a result of increased synaptogenesis onto CA1 neurons. Thus, the function of MMP-9 in cholinergically induced plasticity in the hippocampus is to maintain the fine-tuned balance between the excitatory and the inhibitory synaptic transmission.

Theta rhythms have been recorded from rat brain slices of the posterior hypothalamic area (PHa), including the supramammillary and posterior hypothalamic nuclei. Additionally, in numerous studies theta‐related neurons were identified in the PHa according to the classification of Bland and Colom (Progress in Neurobiology, 41, 157–208, 1993). It is currently widely accepted that the PHa contributes to the process of HPC theta frequency programming at least in certain behavioral states. The postnatal development of the HPC and its ability to generate theta has also been a subject of studies. Specifically, it was found that theta oscillations are present in the HPC of 8–10 days old rat pups and turn into a well‐synchronized and high‐amplitude activity in the following few days. In our current study, we therefore focused on the postnatal development of cholinergically‐induced theta rhythm and theta‐related neuronal activity in PHa slices obtained from 8 to 24 days old rat pups. Theta activity was observed in the PHa preparations at the age of 8–10 days and then progressively increased its probability of occurrence, amplitude and synchrony up to the age of 22–24 days when it reached a plateau phase. A steady increase in the number of recorded neurons correlated with local theta oscillations was also observed.

Data obtained in in vitro experiments and urethane anaesthetized animals have revealed that the mechanisms responsible for the generation of hippocampal cholinergic theta rhythm are specifically affected by the administration of broad spectrum gap junctions (GJs) blocker - carbenoxolone (CBX). The aim of this study was to examine the effect of GJs modulation on the production of posterior hypothalamic theta. Specifically, we were interested in evaluating whether CBX could attenuate the theta rhythm recorded from the supramammillary nucleus and posterior hypothalamic nuclei, in both in vitro and in vivo preparations. The data we obtained from in vitro and in vivo preparations demonstrated that the administration of CBX did not suppress cholinergically induced theta in posterior hypothalamic area (PHa) slices nor the theta rhythm observed in the PHa of urethane anaesthetized rats. Moreover, the application of trimethylamine, while very effective in the enhancement of hippocampal theta rhythm, did not produce any changes in theta oscillations observed in either in vitro or in vivo posterior hypothalamic area preparations. These data show that electrical coupling via GJs is not involved in theta rhythm generation in the PHa. Surprisingly, we observed a significant enhancement of theta activity in response to the carbenoxolone administration in both in vitro and in vivo PHa preparations. The theta rhythm enhancement detected in those experiments was attenuated by the application of spironolactone (mineralocorticoid receptors antagonist). We suggest that the observed excitatory effects of CBX on posterior hypothalamic oscillatory activity in the theta band could be mediated by mineralocorticoid receptors.

Kowalczyk et al. (Hippocampus 2014; 24:7‐20) were probably the first to conduct a systemic study of posterior hypothalamic area (PHa) theta rhythm in anesthetized rats. They demonstrated that local PHa theta field potentials were tail‐pinch resistant and could be generated in urethane‐anesthetized rats independently of ongoing hippocampal formation theta rhythm. These in vivo data were also confirmed in PHa slice preparations perfused with cholinergic agonist, carbachol. In the current experiments we extend our earlier observations concerning PHa theta rhythm. Specifically, PHa field potentials were analyzed in relation to the ongoing local cell firing repertoire. Single‐unit discharge patterns of cells localized in the posterior hypothalamic and supramammillary nuclei were characterized according to the criteria that was developed previously to classify theta‐related cells in the hippocampal formation. The present study demonstrated that in addition to the earlier described theta‐related cells (theta‐on, theta‐off and gating cells) the PHa also contains cells discharging in a very regular manner, which were labelled “timing cells”. This type of neuron has not been previously documented. We suggest that “timing cells” form a part of the ascending brainstem synchronizing pathway, provideing a regular rhythmic signal which facilitates the transduction of tonic discharges of cells localized in the brain stem into theta‐frequency rhythmic discharges.