Edyta Jurkowlaniec

Microinjection of procaine into the pedunculopontine tegmental nucleus suppresses hippocampal theta rhythm in urethane-anesthetized rats

It was found that the cholinergic component of the pedunculopontine tegmental nucleus (PPN) is involved in the generation of theta rhythm in the hippocampus. However, it is still not known how important PPN is in the brainstem theta-generating system, where the nucleus reticularis pontis oralis is regarded as a primary generator. In the present experiment, performed on urethane-anesthetized rats, we studied the effect on the tail pinch-elicited hippocampal theta of unilateral inactivation of PPN by means of direct procaine microinjection. Procaine induced ipsilateral suppression of theta rhythm, manifested as desynchronization of hippocampal EEG, a shift of the fast Fourier transformation (FFT) power peak toward lower frequencies, and a reduction of FFT peak magnitude at theta band. Hippocampal field activity returned to normal (both its FFT peak frequency and magnitude) within 30 min after the injection. The results obtained indicate that PPN is critical for hippocampal theta generation but it may not be involved in encoding theta frequency.

Microinjection of procaine and electrolytic lesion in the ventral tegmental area suppresses hippocampal theta rhythm in urethane-anesthetized rats

The midbrain ventral tegmental area (VTA), a key structure of the mesocorticolimbic system is anatomically connected with the hippocampal formation. In addition mesocortical dopamine was found to influence hippocampus-related memory and hippocampal synaptic plasticity, both being linked to the theta rhythm. Therefore, the aim of the present study was to evaluate the possible role of the VTA in the regulation of the hippocampal theta activity. The study was performed on urethane-anesthetized male Wistar rats in which theta rhythm was evoked by tail pinch. It was found that unilateral, temporal inactivation of the VTA by means of direct procaine injection resulted in bilateral suppression of the hippocampal theta which manifested as a loss of synchronization of hippocampal EEG and respective reduction of the power and also the frequency of the 3-6 Hz theta band. Depression of the power of the 3-6 Hz component of the EEG signal was also seen in spontaneous hippocampal EEG after procaine. The permanent destruction of the VTA by means of unilateral electrocoagulation evoked a long-lasting, mainly ipsilateral depression of the power of the theta with some influence on its frequency. Simultaneously, there was a substantial increase of the power in higher frequency bands indicating decrease of a synchrony of the hippocampal EEG activity. On the basis of these results indicating impairment of synchronization of the hippocampal activity the VTA may be considered as another part of the brainstem theta synchroning system.

Effects of amphetamine on NK-related cytotoxicity in rats differing in locomotor reactivity and social position

The effect of i.p. administration of 1mg/kg of amphetamine (AMPH) on natural killer cells cytotoxicity (NKCC) and number of large granular lymphocytes (LGL-NK) together with plasma corticosterone (CORT) level and WBC was evaluated in male Wistar rats differing in two behavioral features: locomotor reactivity to novelty (high, HR and low, LR responders) and social position (dominants, D and subordinates, S). In the majority of animals AMPH evoked (30 min after administration) an increase in NKCC and LGL (NK) number accompanied by lymphopenia, neutrocytosis, monocytosis, and an increase in CORT level. Changes in NKCC (LU20) showed substantial individual variability: in HR group approximately 513Delta%, p <0.01 (relative to the control); LR group approximately 56Delta%, p >.05; D group approximately 441Delta%, p >0.001; S group approximately 216Delta%, p >0.05; HR/D group approximately 643Delta%, p <.001; HR/S group approximately 414Delta%, p <.001; LR/D group approximately 191Delta%, p >.05; and LR/S group approximately -19Delta%, p .05. The increase in CORT level, lymphopenia, and neutrocytosis indicated a stress-like reaction to AMPH. No significant correlation between NKCC and CORT level was found. The results obtained indicate that AMPH can evoke an increase in NK-related cytotoxic activity quantitatively related to high behavioral reactivity to novelty and social dominance, however NKCC is not related to the AMPH-induced CORT changes.

Induction of hippocampal theta rhythm by electrical stimulation of the ventral tegmental area and its loss after septum inactivation

The ventral tegmental area (VTA), which may be one of the structures involved in regulation of hippocampal theta rhythm, sends direct projections to the hippocampus and also to the forebrain septum, the key centres involved in theta generation. In the present study we aimed at assessing which projections from the VTA (direct or through the septum) participate in regulation of hippocampal electric activity. Experiments were conducted on 3 groups of urethanised male Wistar rats. In the first group (n=6) electrical stimulation of the VTA was used to evoke theta rhythm episodes in hippocampus. Stimulation was repeatedly applied in control conditions and after procainic blockade of the septum. The second group (n=6), subjected to unilateral electrical stimulation of the VTA (30-s stimulation at 10-min intervals during 2h) and to subsequent detection of Fos expression, served to measure neuronal activation of the target mesolimbic structures. Activation levels of selected structures were compared to data from analogous stimulation of the zona incerta (ZI, the third group, n=6). Stimulation of the VTA immediately generated regular theta rhythm in both hippocampi. Inactivation of the septum with procaine temporarily abolished this effect. VTA stimulation increased the density of Fos in the ipsilateral nucleus accumbens. Stimulation of the ZI never generated theta but evoked significant induction of Fos expression in the hippocampus. Our data suggest that the projection through which the VTA enhances theta rhythm is not direct but is incorporated into the main route of theta generation, which involves septum as the main relay node.

Involvement of GABAergic transmission in the midbrain ventral tegmental area in the regulation of hippocampal theta rhythm.

Previously we indicated that the ventral tegmental area (VTA) may belong to the system regulating hippocampal theta rhythm. In the present study, we aimed at assessing the role of the GABAergic system of the VTA in regulation of hippocampal electric activity. Male Wistar rats received unilateral intra-VTA microinjection of either bicuculline (50ng/0.5μl, n=9), muscimol (100ng/0.5μl, n=10) or phaclofen (500ng/0.5μl, n=9). 1-min tail pinch stimulations were applied at 10-min intervals to evoke theta rhythm episodes in hippocampus. We analysed peak power (P(max)) and corresponding frequency (F(max)) of EEG signal at delta and theta bands. Bicuculline induced theta rhythm in both hippocampi with 0 latency, continuous for ca. 33min. Phaclofen also induced theta but in this group it appeared with latency (17.45±3.16min on average), lasted for ca. 33.6min and during this time was interrupted by periods of irregular activity of variable length. Tail pinch was not applied in these groups. Muscimol induced an opposite effect: depression of theta P(max) with simultaneous increase in delta P(max) and a decrease in F(max) delta during episodes of tail pinch-evoked theta. This effect had variable latency and no return to the control EEG could be observed. We propose that GABA activity in the VTA is of tonic character, so that abolition of this mechanism produces immediate effect, i.e. theta induction (strong by GABA(A) and weak by GABA(B) receptors blockade), whereas enhancing the already present GABAergic inhibition causes delayed, prolonged changes expressed as gradual loss of theta synchronisation.

Hippocampal theta rhythm after serotonergic activation of the pedunculopontine tegmental nucleus in anesthetized rats.

The pedunculopontine tegmental nucleus (PPN), as a part of reticular formation activating system, is thought to be involved in the sleep/wake cycle regulation, and plays an important role in the generation and regulation of hippocampal rhythmical slow activity. The activity of PPN can be modulated by serotonergic system, mainly through multiple projections from raphe nuclei, which can influence PPN neurons through different classes of 5-HT receptors. In the present study, the effect of intra-PPN injection of two serotonin agonists: 8-OH-DPAT and 5-CT, on hippocampal formation EEG activity was examined in urethane-anesthetized rats. The study found that the microinjections induced prolonged spontaneous theta rhythm in both hippocampi with a short latency. The results obtained suggest that local inhibition of presumably cholinergic neurons in the PPN acts as a trigger for hippocampal theta activity.

Brainstem system of hippocampal theta induction: The role of the ventral tegmental area.

This article summarizes the results of studies concerning the influence of the ventral tegmental area (VTA) on the hippocampal theta rhythm. Temporary VTA inactivation resulted in transient loss of the hippocampal theta. Permanent destruction of the VTA caused a long‐lasting depression of the power of the theta and it also had some influence on the frequency of the rhythm. Activation of glutamate (GLU) receptors or decrease of GABAergic tonus in the VTA led to enhancement of dopamine release and increased hippocampal theta power. High time and frequency cross‐correlation was detected for the theta band between the VTA and hippocampus during paradoxical sleep and active waking. Thus, the VTA may belong to the broad network involved in theta rhythm regulation. This article also presents a model of brainstem–VTA–hippocampal interactions in the induction of the hippocampal theta rhythm. The projections from the VTA which enhance theta rhythm are incorporated into the main theta generation pathway, in which the septum acts as the central node. The neuronal activity that may be responsible for the ability of the VTA to regulate theta probably derives from the structures associated with rapid eye movement (sleep) (REM) sleep or with sensorimotor activity (i.e., mainly from the pedunculopontine and laterodorsal tegmental nuclei and also from the raphe). Synapse 69:553–575, 2015.

Dopaminergic transmission in the midbrain ventral tegmental area in the induction of hippocampal theta rhythm.

Hippocampal rhythmic slow activity (RSA, theta) is regulated by many brainstem structures, including the midbrain ventral tegmental area (VTA). This work aimed at assessing the role of the dopaminergic (DA) transmission of the VTA in this regulation. Male Wistar rats (n=35) in urethane anaesthesia received an intra-VTA microinjection of either flupenthixol (FLU; doses of 5.0, 2.5, 1.25 and 0.625 μg) or amphetamine (AMPH; 2.5 and 5.0 μg) following control solvent microinjection. Peak power (Pmax) and corresponding peak frequency (Fmax) for delta and theta bands were extracted from EEG recording. Flupenthixol at a dose of 1.25 μg evoked long-lasting theta, continuing for 32.0 min on average, with a mean latency of 7.1 min. Other doses of FLU caused an increase of Pmax theta and reduction of Pmax delta without generating visually recognizable, regular theta rhythm. 5 μg of AMPH evoked theta continuing for 24.4 min on average, with a mean latency of 9.7 min. The lower dose was much less effective, with its outcome resembling the one after the less active FLU doses. During pharmacologically induced theta rhythm, both after FLU and AMPH, brief episodes of asynchronous activity appeared periodically, and they were more frequent and longer in AMPH groups. AMPH may act locally on multiple sites, inhibiting DA cells in somatodendritic region but also increasing dopamine release in target structures, and this, depending on AMPH dose, can lead to induction of theta rhythm. Locally administered DA antagonist on the other hand, when used at a proper dose, can produce theta most likely by the mechanism of inhibiting autoreceptors.

Theta activity in local field potential of the ventral tegmental area in sleeping and waking rats

Hippocampal theta rhythm appears in two vigilance states: active waking and paradoxical sleep. The ventral tegmental area (VTA) is active in sleep and waking and is connected to the hippocampus. We assessed the relationship between local field potential (LFP) of the VTA and sleep-waking stages in freely moving rats. Electrical activity of the VTA was divided into: quiet waking (W), waking with theta (WT), slow wave sleep (SWS) and paradoxical sleep (PS), depending on the hippocampal signal and the animals behavior. We analyzed total power in the VTA signal and we also extracted peak power (Pmax) and corresponding frequency (Fmax) in theta and delta bands from both the VTA and hippocampal recording. In the VTA the 6-9 Hz band had the highest power during PS, and the ratio of the 6-9 to 3-6 Hz power was highest during both PS and WT, which accentuated Pmax of this particular theta sub-band. During W, a very slight increase (or plateau) in signal power was seen in theta range. Pmax and Fmax of theta were higher in PS than in both WT and W, and these parameters did not differ between W and WT. During WT and PS, Fmax in the 6-9 Hz band was greatly correlated between the VTA and hippocampus signal. We also detected high cross-correlation in power spectra between the hippocampus and the VTA (for delta and theta, during WT and PS). The results suggest that the VTA may belong to the broad network involved in theta rhythm induction.

NMDA-glutamatergic activation of the ventral tegmental area induces hippocampal theta rhythm in anesthetized rats.

Glutamate afferents reaching the ventral tegmental area (VTA) affect dopamine (DA) cells in this structure probably mainly via NMDA receptors. VTA appears to be one of the structures involved in regulation of hippocampal theta rhythm, and this work aimed at assessing the role of glutamatergic activation of the VTA in the theta regulation. Male Wistar rats (n=17) were divided into groups, each receiving intra-VTA microinjection (0.5 μl) of either solvent (water), glutamatergic NMDA agonist (0.2 μg) or antagonist (MK-801, 3.0 μg). Changes in local field potential were assessed on the basis of peak power (Pmax) and corresponding peak frequency (Fmax) for the delta (0.5-3 Hz) and theta (3-6 Hz) bands. NMDA microinjection evoked long-lasting hippocampal theta. The rhythm appeared with a latency of ca. 12 min post-injection and lasted for over 30 min; Pmax in this band was significantly increased for 50 min, while simultaneously Pmax in the delta band remained lower than in control conditions. Theta Fmax and delta Fmax were increased in almost entire post-injection period (by 0.3-0.5 Hz and 0.3-0.7 Hz, respectively). MK-801 depressed the sensory-evoked theta: tail pinch could not induce theta for 30 min after the injection; Pmax significantly decreased in the theta band and at the same time it increased in the delta band. Theta Fmax decreased 10 and 20 min post injection (by 0.4-0.5 Hz) and delta Fmax decreased in almost entire post injection period (by 0.3-0.7 Hz). NMDA injection generates theta rhythm probably through stimulation of dopaminergic activity within the VTA.