Expression of cholecystokinin and somatostatin genes in the human thalamus

Sleep Spindles: Mechanisms and Functions

Sleep spindles are burstlike signals in the electroencephalogram (EEG) of the sleeping mammalian brain and electrical surface correlates of neuronal oscillations in thalamus. As one of the most inheritable sleep EEG signatures, sleep spindles probably reflect the strength and malleability of thalamocortical circuits that underlie individual cognitive profiles. We review the characteristics, organization, regulation, and origins of sleep spindles and their implication in non-rapid-eye-movement sleep (NREMS) and its functions, focusing on human and rodent. Spatially, sleep spindle-related neuronal activity appears on scales ranging from small thalamic circuits to functional cortical areas, and generates a cortical state favoring intracortical plasticity while limiting cortical output. Temporally, sleep spindles are discrete events, part of a continuous power band, and elements grouped on an infraslow time scale over which NREMS alternates between continuity and fragility. We synthesize diverse and seemingly unlinked functions of sleep spindles for sleep architecture, sensory processing, synaptic plasticity, memory formation, and cognitive abilities into a unifying sleep spindle concept, according to which sleep spindles 1) generate neural conditions of large-scale functional connectivity and plasticity that outlast their appearance as discrete EEG events, 2) appear preferentially in thalamic circuits engaged in learning and attention-based experience during wakefulness, and 3) enable a selective reactivation and routing of wake-instated neuronal traces between brain areas such as hippocampus and cortex. Their fine spatiotemporal organization reflects NREMS as a physiological state coordinated over brain and body and may indicate, if not anticipate and ultimately differentiate, pathologies in sleep and neurodevelopmental, -degenerative, and -psychiatric conditions.

The thalamic reticular nucleus: structure, function and concept

On the basis of theoretical, anatomical, psychological and physiological considerations, Francis Crick (1984) proposed that, during selective attention, the thalamic reticular nucleus (TRN) controls the internal attentional searchlight that simultaneously highlights all the neural circuits called on by the object of attention. In other words, he submitted that during either perception, or the preparation and execution of any cognitive and/or motor task, the TRN sets all the corresponding thalamocortical (TC) circuits in motion. Over the last two decades, behavioural, electrophysiological, anatomical and neurochemical findings have been accumulating, supporting the complex nature of the TRN and raising questions about the validity of this speculative hypothesis. Indeed, our knowledge of the actual functioning of the TRN is still sprinkled with unresolved questions. Therefore, the time has come to join forces and discuss some recent cellular and network findings concerning this diencephalic GABAergic structure, which plays important roles during various states of consciousness. On the whole, the present critical survey emphasizes the TRN's complexity, and provides arguments combining anatomy, physiology and cognitive psychology.

Somatostatin inhibits thalamic network oscillations in vitro: actions on the GABAergic neurons of the reticular nucleus

We examined the effects of somatostatin (SST) on neurons in the thalamic reticular nucleus (RT) using whole-cell patch-clamp techniques applied to visualized neurons in rat thalamic slices. SST, acting via sst(5) receptors and pertussis toxin-sensitive G-proteins, activated an inwardly rectifying K(+) (GIRK) current in 20 of 28 recorded cells to increase input conductance 15 +/- 3% above control and inhibited N-type Ca(2+) currents in 17 of 24 neurons via voltage-dependent mechanisms. SST reversibly depressed evoked EPSCs (eEPSCs) to 37 +/- 8% of control without altering their kinetics. SST-mediated inhibition of eEPSCs showed short-term relief from block during 25 Hz stimulus trains. SST also reduced the frequency (33 +/- 8%) but not the amplitude of miniature EPSCs (mEPSCs). These data indicate that SST mediates presynaptic inhibition of glutamate release onto RT neurons. In current-clamp recordings, SST preferentially inhibited burst discharges mediated by near-threshold corticothalamic EPSPs and intracellularly applied depolarizing currents. SST had powerful effects on in vitro intrathalamic rhythms, which included a shortening of the duration and a reduction in spike count within each oscillatory event. Furthermore, there was a paradoxical increase in the synchrony of epileptiform oscillations, likely mediated by a suppression of the responses to weak synaptic inputs in RT. We conclude that SST suppresses discharges in RT neurons, via presynaptic inhibition of glutamate release and postsynaptic activation of GIRK channels, leading to the dampening of both spindle-like and epileptiform thalamic network oscillations. SST may act as an important endogenous regulator of physiological and pathological thalamocortical network activities.

Peptidergic modulation of intrathalamic circuit activity in vitro: actions of cholecystokinin

Cholecystokinin (CCK)-mediated actions on intrathalamic rhythmic activities were examined in an in vitro rat thalamic slice preparation. Single electrical stimuli in the thalamic reticular nucleus (nRt) evoked rhythmic activity (1-15 sec duration) in nRt and the adjacent ventrobasal nucleus (VB). Low CCK concentrations (20-50 nM) suppressed rhythmic oscillations in 43% of experiments but prolonged such activities in the remaining slices. Higher CCK concentrations (100-400 nM) had a predominantly antioscillatory effect. Suppression of oscillations was associated with a relatively large membrane depolarization of nRt neurons that changed their firing mode from phasic (burst) to tonic (single-spike) output. This decreased burst discharge of nRt neurons during CCK application reduced inhibitory drive onto VB neurons from multiple peaked inhibitory postsynaptic currents (IPSCs) to single peaked inhibitory events. We hypothesize that suppression of inhibitory drive onto VB neurons decreases their probability of burst output, which, together with a reduction of nRt burst output, dampens the oscillatory activity. Low CCK concentrations, which produced little or no depolarization of nRt neurons, did not alter the firing mode of the nRt neurons. However, the probability of burst output from nRt neurons in response to subthreshold stimuli was increased in low CCK concentrations, presumably leading to an increase in the number of nRt neurons participating in the rhythmic activity. Our findings suggest that the neuropeptide CCK, by altering the firing characteristics of nRt neurons, has powerful modulatory effects on intrathalamic rhythms; the ultimate action was dependent on CCK concentration and resting state of these cells.

Ontogeny of somatostatin gene expression in rat forebrain

With hybridization histochemistry, somatostatin (SRIF) mRNA was detected in several neuronal populations of the basal diencephalon (anterior and posterior) and basal telencephalon (lateral) for the first time on the 14th day of gestation (E14). On E16, a large increase of the extent of expression was found in these populations. In addition, cells in the medial telencephalon and a few cells in the future allocortex also contained SRIF mRNA for the first time. In the prenatal period, the expression in the above populations continued to mature and individual nuclei with SRIF mRNA began to be recognizable. At birth, the overall pattern of SRIF gene expression was established but the ventral portions (hypothalamus, amygdala, allocortical areas) had higher levels of expression than the more dorsal ones (striatum and neocortex). Over the first 2 wk of life, this difference decreased and an adult-like pattern was found at postnatal day 21. We demonstrate that most of SRIF gene expression development takes place before birth. This description may serve as a basis for studies on the putative functions of SRIF during brain ontogeny.

Expression of soluble guanylyl cyclase gene in adult rat brain

The synthesis of the intracellular messenger, cyclic GMP, is catalysed by particular or soluble guanylyl cyclase (sGCY). sGCY is activated by nitric oxide, a compound with putative neurotransmitter functions, especially in long-term potentiation. Hybridization histochemistry with a probe complementary to the rat lung large (alpha 1) subunit was used to assess the exact localization of sGCY mRNA in the rat brain. Many cells in the olfactory bulb contained sGCY mRNA. In the whole cerebral cortex, sGCY mRNA was found in all layers, with a predominance in layers II-III. A similar pattern was found in the olfactory tuberculum, in continuation with the piriform cortex and the cortical amygdaloid nucleus. All parts of the striatum expressed sGCY mRNA. sGCY mRNA was also found in the habenula medialis, in the pinealis in some diencephalic nuclei, and in the granule cell layers of the cerebellum. This study provides a description of the normal anatomy of sGCY gene expression in the rat forebrain as a basis for the study of the modulation of expression after physiological and pharmacological manipulations.