Unilateral blockade of the dorsal ascending noradrenergic bundle and septal elicitation of hippocampal theta rhythm

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 β agonists and antagonists were tested. We indicate that the selective activation of three HPC noradrenergic receptors, α1, α2 and β1, induced a similar effect (i.e., inhibition) on type 2 theta rhythm. The remaining HPC β2 and β3 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 β1 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).

Medial septal cholinergic mediation of hippocampal theta rhythm induced by vagal nerve stimulation

BACKGROUND

Electrical vagal nerve stimulation (VNS) has been used for years to treat patients with drug-resistant epilepsy. This technique also remains under investigation as a specific treatment of patients with Alzheimer's disease. Recently we discovered that VNS induced hippocampal formation (HPC) type II theta rhythm, which is involved in memory consolidation. In the present study, we have extended our previous observation and addressed the neuronal substrate and pharmacological profile of HPC type II theta rhythm induced by VNS in anesthetized rats.

METHODS

Male Wistar rats were implanted with a VNS cuff electrode around the left vagus nerve, a tungsten microelectrode for recording the HPC field activity, and a medial septal (MS) cannula for the injection of a local anesthetic, procaine, and muscarinic agents. A direct, brief effect of VNS on the HPC field potential was evaluated before and after medial-septal drug injection.

RESULTS

Medial septal injection of local anesthetic, procaine, reversibly abolished VNS-induced HPC theta rhythm. With the use of cholinergic muscarinic agonist and antagonists, we demonstrated that medial septal M1 receptors are involved in the mediation of the VNS effect on HPC theta field potential.

CONCLUSION

The MS cholinergic M1 receptor mechanism integrates not only central inputs from the brainstem synchronizing pathway, which underlies the production of HPC type II theta rhythm, but also the input from the vagal afferents in the brain stem.

Development of a theoretically-derived human anxiety syndrome biomarker

“Anxiety disorders” are extremely common; and are a major source of health costs and lost work days. Their diagnosis is currently based on clinical symptom check lists and there are no biological markers to diagnose specific syndromal causes. This paper describes: 1) a detailed theory of the brain systems controlling anxiolytic-insensitive threat-avoidance and anxiolytic-sensitive threat-approach — where, in specific brain structures, activity generates specific normal behaviours, hyperactivity generates abnormal behaviours, and hyper-reactivity (hypersensitivity to input) generates specific clinical syndromes; 2) a rodent model of systemic anxiolytic action (rhythmical slow activity), linked to the theory, that over a period of 40 years has shown predictive validity with no false positives or false negatives — and which is likely to assay the sensitivity of endogenous systems that control anxiety; and, 3) derivation from this rodent-based theory of a specific non-invasive biomarker (goal-conflict-specific rhythmicity) for the threat-approach system in humans. This new biomarker should allow division of untreated “anxiety” patients, with superficially similar clusters of symptoms, into distinct high scoring (syndromal) and low scoring groups with different treatment-responses. This would be the first theoretically-derived biomarker for any mental disorder and should: 1) predict treatment efficacy better than current symptom-based diagnoses; 2) provide a human single dose test of novel anxiolytics; 3) provide a starting point for developing biomarkers for other “anxiety” syndromes; and so, 4) greatly improve treatment outcomes and cost-effectiveness.

Noradrenergic control of cortico-striato-thalamic and mesolimbic cross-structural synchrony

Although normal dopaminergic tone has been shown to be essential for the induction of cortico-striatal and mesolimbic theta oscillatory activity, the influence of norepinephrine on these brain networks remains relatively unknown. To address this question, we simultaneously recorded local field potentials and single-neuron activity across 10 interconnected brain areas (ventral striatum, frontal association cortex, hippocampus, primary motor cortex, orbital frontal cortex, prelimbic cortex, dorsal lateral striatum, medial dorsal nucleus of thalamus, substantia nigra pars reticularis, and ventral tegmental area) in a combined genetically and pharmacologically induced mouse model of hyponoradrenergia. Our results show that norepinephrine (NE) depletion induces a novel state in male mice characterized by a profound disruption of coherence across multiple cortico-striatal circuits and an increase in mesolimbic cross-structural coherence. Moreover, this brain state is accompanied by a complex behavioral phenotype consisting of transient hyperactivity, stereotypic behaviors, and an acute 12-fold increase in grooming. Notably, treatment with a norepinephrine precursors (l-3,4-dihydroxyphenylalanine at 100 mg/kg or l-threo-dihydroxyphenylserine at 5 mg/kg) or a selective serotonin reuptake inhibitor (fluoxetine at 20 mg/kg) attenuates the abnormal behaviors and selectively reverses the circuit changes observed in NE-depleted mice. Together, our results demonstrate that norepinephrine modulates the dynamic tuning of coherence across cortico-striato-thalamic circuits, and they suggest that changes in coherence across these circuits mediate the abnormal generation of hyperactivity and repetitive behaviors.

Effects of thyrotropin-releasing hormone on the sleep EEG and nocturnal hormone secretion in male volunteers

Various peptides including corticotropin-releasing hormone (CRH) exert selective effects on sleep structure and noctural secretions of cortisol and growth hormone (GH). In animal studies analeptic effects and sleep disturbances after thyrotropin-releasing hormone (TRH) administration have been observed; studies of endocrine function in depressed patients suggest a pathological activity of CRH and TRH as compared with that in healthy volunteers. As the role of TRH in the regulation of the sleep endocrine pattern in humans has not yet been clarified, we performed a study to examine the effects of pulsatile administration of TRH on the sleep EEG pattern and the nocturnal secretions of cortisol and GH in 7 healthy male subjects. The sleep EEG was recorded from 23.00 to 07.00 h, and blood samples were collected every 20 min from 20.00 to 07.00 h for the analysis of GH and cortisol concentrations during intravenous administration of placebo or 4 x 50 microgram TRH at 22.00, 23.00, 24. 00, and 01.00 h. In contrast to the well-known effects of CRH on the sleep endocrine pattern, TRH exerts only a weak effect on the sleep EEG which is reflected in a slight decrease in sleep efficiency associated with a trend to wakefulness during the night. Furthermore, after TRH administration, the cortisol rise appeared earlier, and a nonsignificant tendency to an increased secretion of cortisol during the first half of the night was found. The GH secretion did not differ significantly after application of TRH or placebo. The activating, albeit weak, effect of TRH on the sleep EEG and nocturnal cortisol secretion in healthy volunteers confirms and adds to the results previously observed in animals. On the basis of these findings, we surmise that TRH may contribute to the disturbed sleep continuity seen in depressed patients, probably acting as a cofactor of CRH in a synergistic manner.

State-dependent effects of some neuropeptides and neurotransmitters on neuronal activity of the medial septal area in brain slices of the ground squirrel, Citellus undulatus

Neuronal activity of the medial septal area was recorded extracellularly in brain slices taken from hibernating (winter) and waking (summer) ground squirrels. The effects of neuropeptides identified in the brain tissue of hibernators (Thr-Ser-Lys-Tyr, Thr-Ser-Lys-Tyr-Arg and Asp-Tyr) on the background activity and responses to electrical stimulation of the median forebrain bundle were analysed. For comparison, the effects of bath application of noradrenaline and serotonin were also tested. Spontaneous activity in half of all neurons (47-56%) was changed under the influence of neuropeptides in hibernating ground squirrels, while in waking ground squirrels the proportion of responsive neurons was significantly lower (25-30%). The tendency for higher efficacy in hibernating ground squirrels was observed for serotonin; only noradrenaline was equally effective in both groups of animals. Electrically evoked responses of the medial septal nucleus-nucleus of the diagonal band neurons were also strongly modulated by neuropeptides; their changes could occur in the absence of shifts in the level and pattern of spontaneous activity. All three neuropeptides had differential action on the level of spontaneous activity, as well as on inhibitory and excitatory components of electrically evoked responses. Thus, the character and distribution of the effects were state dependent and differed greatly in hibernating and waking ground squirrels. The experiments confirmed that medial septal nucleus-nucleus of the diagonal band neurons have higher excitability and responsiveness to some neuropeptides and neurotransmitters in hibernating ground squirrels.The data obtained suggest an increased latent excitability and responsiveness of septal neurons during hibernation and their possible active participation in urgent arousal under the influence of sensory signals.

A comparison of the acute effects of a tricyclic and a MAOI antidepressant on septal driving of hippocampal rhythmical slow activity

In free-moving male rats, the function relating frequency to the threshold current required to drive hippocampal rhythmical slow activity (RSA) with septal stimulation has a minimum at 130 ms. Both classical anxiolytics (e.g. benzodiazepines) and the novel anxiolytic buspirone show similar effects on septal driving of RSA. The tricyclic antidepressant imipramine may be as effective as anxiolytic drugs in treatment of generalized anxiety disorder. The antidepressant monoamine oxidase inhibitor phenelzine has also been reported to be effective in treating anxiety, but this may reflect an action on "atypical depression" rather than "anxiety". The present study therefore compared the effects of acute administration of imipramine and phenelzine on septal driving of RSA to determine whether either would mimic anxiolytics in this test. Rats were chronically implanted with septal stimulating electrodes and subicular recording electrodes. Three groups of rats received IP injection of either imipramine (5.9-13.3 mg/kg or 13.3-30 mg/kg) or phenelzine (0.2-5.4 mg/kg). The effects produced by imipramine were very similar to the effects produced by anxiolytic drugs. In contrast, the effects produced by phenelzine were essentially opposite to those of both anxiolytic drugs and imipramine. The present experiment suggests that imipramine may act as a true anxiolytic, in addition to its conventional antidepressant properties. In contrast, phenelzine may be effective in cases where the etiology is essentially that of depression even when the symptomatology appears to be that of anxiety.

Effect of DSP-4, pCPA, and haloperidol on hippocampal electrical activity and behavior in rabbits

An experimental model, including novel and fearful stimuli, has been used to study the effect of noradrenaline, serotonin, and dopamine depletion on hippocampal electrical activity and behavior in freely moving rabbits. DSP-4 (N-(2-chloroethyl)-N-ethyl-2- bromobenzylamine hydrochloride, 40 mg/kg ip), a selective hippocampal noradrenaline depletor, decreased the overall exploratory activity and significantly increased RSA (rhythmic slow activity) percentage. The high frequencies of the hippocampal RSA were significantly reduced. When this noradrenaline depletion was coupled with a serotonin depression by p-chlorophenylalanine (pCPA), the above described effect was potentiated. In particular, the frequency distribution of RSA was characterized by a further reduction of the high values with a concomitant increase of the low frequency band. Moreover, a more evident decrease of the exploratory activity and a similar increase of RSA percentage was observed. These results show that the hippocampal electrical activity is modulated by noradrenaline and serotonin by an inhibitory effect on RSA occurrence and a frequency selection. The block of dopamine receptors by chronic haloperidol administration (5 mg/kg ip/day) did not seem to exert any effect on RSA parameters. Results are discussed in the light of attentional and emotional theories.

Effects of early undernutrition on hippocampal development and function

The possibility of a "hippocampal syndrome' was investigated in mature animals whose mothers had received a period of undernutrition during pregnancy and throughout lactation. Spontaneous alternation behaviour was abolished, and performance was found to be at chance level. Significant depletions in cell numbers were found throughout the hippocampal region; however, hippocampal theta rhythm remained and threshold levels for theta driving appeared normal. It was concluded that the behavioural differences observed did no arise from some simple deficit in the circuitry controlling hippocampal theta rhythm and that it was more likely due to an impairment at the synaptic level. The similarity of these, and other recent behavioural observations, to those of animals with hippocampal damage may provide a reasonable basis for investigating a number of learning deficits induced by early undernutrition.