Alterations in preoptic unit activity on stimulation of caudal brain stem EEG-synchronizing structures

A mathematical model towards understanding the mechanism of neuronal regulation of wake-NREMS-REMS states

In this study we have constructed a mathematical model of a recently proposed functional model known to be responsible for inducing waking, NREMS and REMS. Simulation studies using this model reproduced sleep-wake patterns as reported in normal animals. The model helps to explain neural mechanism(s) that underlie the transitions between wake, NREMS and REMS as well as how both the homeostatic sleep-drive and the circadian rhythm shape the duration of each of these episodes. In particular, this mathematical model demonstrates and confirms that an underlying mechanism for REMS generation is pre-synaptic inhibition from substantia nigra onto the REM-off terminals that project on REM-on neurons, as has been recently proposed. The importance of orexinergic neurons in stabilizing the wake-sleep cycle is demonstrated by showing how even small changes in inputs to or from those neurons can have a large impact on the ensuing dynamics. The results from this model allow us to make predictions of the neural mechanisms of regulation and patho-physiology of REMS.

Mesolimbic component of the ascending cholinergic pathways: electrophysiological-pharmacological study

The cholinergic input from the pontomesencephalic cholinergic neurons to the diencephalic and basal forebrain structures has been implicated in a number of limbically controlled overt behaviors. The cellular mechanism by which the cholinergic terminals initiate behavioral manifestations is not clear. The objective of this study was to investigate the effects of the ascending cholinergic projection from the laterodorsal tegmental nucleus (LDT) on neuronal firing in the anterior hypothalamic-medial preoptic region (AHMP), known to be involved in agonistic behavior. Experiments were performed on urethan-anesthetized rats. Iontophoretic application of carbachol (CCh) into the vicinity of single cells in the AHMP caused a dose-dependent decrease in the mean firing rate of 83% of units and an increase in 10% of units. The inhibitory effect of CCh, but not the excitatory effect, was reversed by iontophoretic pretreatment with scopolamine. The inhibition of the firing rate was repeatable for the same dose of CCh and dose dependent. Electrical stimulation of neurons in the LDT caused a comparable, current-dependent decrease in the mean firing rate of AHMP neurons that also was reversed by pretreatment of neurons in the AHMP with scopolamine. The antagonizing effects of scopolamine were reversible with time. The same units in the AHMP that inhibited their firing to stimulation of the LDT also responded with a similar inhibition to local iontophoretic CCh. Finally, the fluorescent carbocyanine dye, 4-(4-(dihexadecylamino)styryl)-N-methylpyridinium iodide, (DiA), has been used as a retrograde axonal tracer and was injected into the recording sites immediately after the electrophysiological recordings. After 1 wk, DiA dye was found in numerous neurons in the LDT as shown by the fluorescence confocal microscopy. Results of the study suggest that LDT cholinergic neurons project and terminate in the AHMP and that their activation causes a decrease in the mean firing rate of the AHMP neurons. It is postulated that this inhibitory effect is implicated in the initiation of some of the behavioral patterns like defensive or alarm vocalization and behavioral inhibition.

Comparison of rostro-caudal brain stem influence on preoptic neurons and cortical EEG

The changes in activity of preoptic area (POA) neurons, and cortical EEG, upon stimulation of the caudal brain stem reticular formation (CBS) and the rostral brain stem reticular formation (RBS) are compared in this study. Low frequency (LF) stimulation of the CBS (which induced EEG synchronization) and the RBS (which generally did not affect the EEG) had an excitatory influence on a majority of the affected neurons of the POA. In contrast, high frequency (HF) stimulation of the CBS (which produced EEG desynchronization in many instances) and the RBS (which induced EEG desynchronization in all instances) resulted in inhibition of a majority of the affected POA neurons. A larger number of neurons responded to HF stimulation of both brain stem regions, as compared to LF stimulation. The changes induced in the POA neurons, upon stimulation of the two brain stem reticular structures, were not dependent on simultaneous changes in the cortical EEG, except during some cases of stimulation-induced EEG desynchronization.