Noradrenergic Profile of Hippocampal Formation Theta Rhythm in Anaesthetized Rats.

The present study was undertaken to identify the noradrenergic receptors underlying the production of hippocampal formation (HPC) type 2 theta rhythm. The experiments were performed on urethanized rats wherein type 2 theta is the only rhythm present. In three independent stages of experiments, the effects of noradrenaline (NE) and selective noradrenergic α and ß agonists and antagonists were tested. We indicate that the selective activation of three HPC noradrenergic receptors, α1, α2 and ß1, induced a similar effect (i.e., inhibition) on type 2 theta rhythm. The remaining HPC ß2 and ß3 noradrenergic receptors do not seem to be directly involved in the pharmacological mechanism responsible for the suppression of theta rhythm in anaesthetized rats. Obtained results provide evidence for the suppressant effect of exogenous NE on HPC type 2 theta rhythm and show the crucial role of α1, α2 and ß1 noradrenergic receptors in the modulation of HPC mechanisms of oscillations and synchrony. This finding is in contrast to the effects of endogenous NE produced by electrical stimulation of the locus coeruleus (LC) and procaine injection into the LC (Broncel et al., 2020).

Effects of locus coeruleus activation and inactivation on hippocampal formation theta rhythm in anesthetized rats.

Previously obtained data suggests that noradrenaline (NE) released from the efferent locus coeruleus (LC) endings in hippocampal formation (HPC) may serve as an important modulating signal involved in the pharmacological mechanisms responsible for the production of type 2 theta rhythm in rats. Hence, two distinct hypotheses were tested in the present study: 1/ if the decrease in HPC level of NE is correlated with the desynchronization of HPC field potential, then the inhibition of LC would be expected to abolish HPC type 2 theta rhythm; 2/ if the increase in HPC NE level is correlated with synchronization of HPC field potential, then the stimulation of LC would be expected to produce type 2 theta. The experiments were performed using an experimental model of HPC type 2 theta rhythm recorded in urethanized rats. It was demonstrated that electrical stimulation of LC produced type 2 theta rhythm whereas procaine injection into LC, in contrast, reversibly abolished type 2 theta. The possible relation of type 2 theta rhythm with some disturbances of Alzheimer disease are addressed.

Vagal nerve stimulation as a promising tool in the improvement of cognitive disorders.

Vagal nerve stimulation (VNS) is known as an effective method of treatment in a number of neurological disorders. The low risk of side effects also makes it useful in clinical trials in other diseases. Branches of the vagal nerve innervate the anatomical structures known to be involved in memory processing. That is why it seems justified that several studies emphasize the impact of VNS on the cognitive and memory function in both healthy volunteers and patients with epilepsy and Alzheimer's disease. Results have shown that VNS can modulate different types of memory depending the protocol of stimulation in non-demented patients after both short term and chronic VNS application. Transcutaneous vagal nerve stimulation (tVNS), which is a non-invasive method of VNS, opens up new perspectives for different clinical applications.