The effects of alpha 2 agonists and antagonists on hippocampal theta activity in the freely moving rat

Staying awake--a genetic region that hinders α2 adrenergic receptor agonist-induced sleep

How external stimuli prevent the onset of sleep has been little studied. This is usually considered to be a non-specific type of phenomenon. However, the hypnotic drug dexmedetomidine, an agonist at α₂ adrenergic receptors, has unusual properties that make it useful for investigating this question. Dexmedetomidine is considered to produce an ‘arousable’ sleep-like state, so that patients or animals given dexmedetomidine become alert following modest stimulation. We hypothesized that it might be more difficult to make mice unconscious with dexmedetomidine if there was a sufficient external stimulus. Employing a motorized rotating cylinder, which provided a continuous and controlled arousal stimulus, we quantitatively measured the ability of such a stimulus to prevent dexmedetomidine loss of righting reflex in two inbred strains of mice (C57BL/6 and 129X1). We found that whereas the C57BL/6 strain required a strong stimulus to prevent dexmedetomidine-induced hypnosis, the 129X1 strain stayed awake even with minimal stimuli. Remarkably, this could be calibrated as a simple threshold trait, i.e. a binary ‘yes–no’ response, which after crossing the two mouse strains behaved as a dominant-like trait. We carried out a genome-wide linkage analysis on the F₂ progeny to determine if the ability of a stimulus to prevent dexmedetomidine hypnosis could be mapped to one or more chromosomal regions. We identified a locus on chromosome 4 with an associated Logarithm of Odds score exceeding the pre-established threshold level. These results show that complex traits, such as the ability of a stimulus to reverse drug-induced hypnosis, may have precise genetic determinants.

Ventral subicular lesion alters rhythmical slow wave activity (theta) of CA1 area of hippocampus and entorhinal cortex

The present study demonstrates the effect of ibotenic acid lesioning of ventral subiculum on the theta activity of CA1 area of hippocampus and the entorhinal cortex during REM sleep. Ibotenic acid lesioning of ventral subiculum, has increased the absolute power with no noticeable change in the relative power of theta of the CA1 area. In contrast, it has decreased both the absolute and relative power of entorhinal cortical theta. Subicular output may serve to modulate the synchronous neuronal activity of entorhinal cortex and CA1 pyramidal cells during REM sleep.

Hippocampal theta activity after systemic administration of a non-peptide delta-opioid agonist in freely-moving rats: relationship to D1 dopamine receptors

Hippocampal theta activity was acquired and processed off-line from digitized EEG recordings after subcutaneous (s.c.) administration of the non-opioid delta agonist BW 373U86 (0.5-2.5 mg/kg) in freely-moving rats. Relative theta power spectral analysis, implemented by a signal processing software, showed that BW 373U86 induced a dose-dependent increase in the slow component of theta band (Type 2 theta), while movement-related fast theta band (Type 1 theta) failed to show significant changes. Moreover, the increase in relative Type 2 theta power showed a maximal change at 1 mg/kg of BW 373U86, while higher doses, although effective in increasing relative Type 2 theta, induced locomotion and irregularly increased Type 1 hippocampal theta activity. The administration of 10.0 mg/kg of the delta antagonist Naltrindole (NLI) 30 min before BW 373U86, abolished hippocampal Type 2 theta increase. The rise of relative Type 2 theta power induced by BW 373U86 (1-2.5 mg/kg) was greatly attenuated by 0.1 mg/kg of the selective dopamine (DA) D1 antagonist SCH 23390. Administration of 0.1 mg/kg of SCH 23390 alone did not modify hippocampal Type 2 theta. These results indicate that delta receptors modulate the expression of hippocampal Type 2 theta and dopamine, through D1 receptors, exerts a permissive role on this influence.

Neonatal vs. adult unilateral hippocampal lesions: differential alterations in contralateral hippocampal theta rhythm

Subcortical damage often has more severe consequences in neonates than in adults. For example, unilateral hippocampal lesions in adult rats typically lead to transient memory deficits, whereas neonatal lesions cause lasting learning impairment. We hypothesized that the defects triggered by unilateral damage may include synaptic dysfunction in the contralateral hippocampus. Consequently, we examined the hippocampal theta rhythm, an EEG pattern thought to be associated with learning. Initial comparisons between intact and lesioned rats revealed no obvious differences in basal theta rhythm properties. However, manipulations of ascending brainstem projections to hippocampus with drugs specific for serotonergic, noradrenergic and cholinergic receptors uncovered differences. Antagonism of 5-HT3 receptors known to promote learning significantly increased theta frequency in controls and adult lesioned rats, but not after neonatal damage. In contrast, blockade of noradrenergic-alpha2 receptors had no effect. Antagonism of cholinergic receptors which typically impairs learning disrupted theta and caused irregular, high-amplitude activity that was significantly more pronounced in the lesioned groups. A final approach involved pharmacological facilitation of AMPA receptor-mediated currents, using a drug which enhances memory. This treatment significantly enhanced theta frequency in controls and animals lesioned as adults. In contrast, it failed to do so in rats lesioned at birth. These observations suggest that latent dysfunction in contralateral hippocampal physiology may contribute to the lasting memory deficits seen after unilateral hippocampal lesion in neonates.

Alpha 2 modulation of type 1 and type 2 hippocampal theta in the freely moving rat

In order to determine the brain sites involved in the noradrenergic modulation of hippocampal theta, the alpha 2 agonist detomidine was infused into one of three brain sites in the freely moving animal: hippocampus, median raphe nucleus (MRN), or locus coeruleus (LC). Bilateral hippocampal recording electrodes were implanted in 45 animals. Infusions of detomidine into the hippocampus produced an attenuation of type 1 (movement) theta. Detomidine infused into the MRN produced a release of type 2 (immobility) theta, although having no effect on type 1 theta. No significant changes were noted in the hippocampal EEG activity following infusions of detomidine into the LC. Infusions of Xylocaine into each of the above brain sites were ineffective in eliciting changes in hippocampal EEG activity. No behavioral effects were noted following infusions of detomidine or Xylocaine. The present results suggest that the attenuation of type 1 theta occurs at the hippocampus, while the release of type 2 theta occurs at the MRN.