Postsynaptic receptors for cholecystokinin in the thalamic reticular nucleus: a possible modulatory system for sensory transmission

The effect of isolation rearing on volitional ethanol consumption and central CCK/dopamine systems in Fawn-Hooded rats

Numerous studies have demonstrated that socially isolating rats (from weaning) produces a sustained anxious phenotype and an enhanced response to psychostimulant drugs such as amphetamine and cocaine. In addition, isolation rearing has been shown to induce significant changes in the mesolimbic dopamine system. These data indicate that isolation rearing not only induces an anxiogenic phenotype but also induces neurochemical changes in reward nuclei of the brain, which is correlated with an enhanced response to psychostimulants. For these reasons, the effect of isolation rearing on volitional ethanol consumption was examined in Fawn-Hooded (FH) rats and correlated with neurochemical changes in central dopamine and cholecystokinin systems. Social isolation from weaning produced an anxiogenic phenotype as measured by a decreased time spent on the open arms of an elevated plus-maze. Interestingly, isolation-rearing induced a greater proportion of FH rats to acquire preference for ethanol while having no effect on the amount of ethanol consumed by alcohol-preferring rats. In addition, isolation rearing induced a number of changes in central CCK/dopamine systems.

Comparative analysis of the central CCK system in Fawn Hooded and Wistar Kyoto rats: extended localisation of CCK-A receptors throughout the rat brain using a novel radioligand

The neuropeptide cholecystokinin has been implicated in the actions of a number of central processes including anxiety and reward. For this reason, the aim of the present study was to compare the density of CCK-A and -B receptors and the mRNA encoding preproCCK throughout the brains of an alcohol-preferring (Fawn Hooded) rat strain with that of a non-alcohol-preferring (Wistar Kyoto) strain of rat. Our study revealed significant differences with regard to the central CCK system of the FH compared to the WKY rat, including differences in CCK-A receptor binding throughout the dorsal medulla, and altered CCK-B binding density throughout the cerebral cortex and reticular nucleus of the thalamus. The most striking result, given the altered behavioural phenotype of the FH rat, was the 33% lower density of CCKmRNA measured throughout the ventral tegmental area of the FH rat when compared to the WKY. This study also reports on a protocol to utilise a novel radioligand, [125I]-D-Tyr-Gly-A-71378, for autoradiographic detection of CCK-A receptors throughout the rat brain. As previously reported, CCK-A receptors were located throughout the area postrema, interpeduncular nucleus and nucleus tractus solitarii; however, binding to CCK-A receptors was also visualised throughout the medial pre-optic area, the arcuate nucleus and the circumventricular regions of the ventral hypothalamus, regions known to contain CCK-A receptors but which were previously undetectable using autoradiography in rat brain.

Cholinergic modulation of neuronal responses to cholecystokinin in anesthetized rats

The aim of this study was to investigate whether the effects of the neuropeptide cholecystokinin on neuronal firing can be changed by acetylcholine in various structures of the brain. Single unit activity was extracellularly recorded in rats anesthetized with urethane. The neurons were located in several nuclei of the thalamus, the basal ganglia and the cerebral cortex. Neurons responding to the sulfated octapeptide of cholecystokinin (CCK-8S) were mainly activated by the drug [Wilcoxon test (Wt) p < 0.0001, n=113]. Thalamic neurons could also increase the number of burst discharges (Wt p < 0.005, n=39). Iontophoretically administered acetylcholine could reduce the activating effects of CCK-8S on firing and burst discharges. In its presence, even inhibitory effects of CCK-8S predominated (Wt p < 0.0001, n=113). The suppressive action seemed not to depend on the direction of the effect of acetylcholine itself and concerned neurons of all locations studied. Atropine could diminish or block the suppressive action of acetylcholine. In the presence of both drugs, CCK-8S mainly activated the neurons (Wt p < 0.005, n=43). Atropine itself did not significantly change the responses to CCK-8S (Wt p > 0.05). It can be concluded that cholecystokinin may reduce neuronal firing instead of increasing it during activation of the cholinergic system.

Localization of CCK receptors in thalamic reticular neurons: a modeling study

In an earlier experimental study, intracellular recording suggested that cholecystokinin (CCK) suppresses a K+ conductance in thalamic reticular (RE) neurons, yet the reversal potential of the CCK response, revealed using voltage clamp, was hyperpolarized significantly relative to the K+ equilibrium potential. Here, biophysical models of RE neurons were developed and used to test whether suppression of the K+ conductance, gK, can account for the CCK response observed in vitro and also to determine the likely site of CCK receptors on RE neurons. Suppression of gK in model RE neurons can reproduce the relatively hyperpolarized reversal potential of CCK responses found using voltage clamp if the voltage clamp becomes less effective at hyperpolarized potentials. Three factors would reduce voltage-clamp effectiveness in this model: the nonnegligible series resistance of the voltage-clamp electrode, a hyperpolarization-activated mixed cation current (Ih) in RE neurons, and the dendritic location of CCK-sensitive K+ channels. Although suppression of gK in the dendritic compartments of model RE neurons simulates both the magnitude and reversal potential of the CCK response, suppression of gK in just the somatic compartment of model RE neurons fails to do so. Thus the model predicts that CCK should effectively suppress K+ conductance RE neuron dendrites and thereby regulate burst firing in RE neurons. This may explain the potent effects of CCK on intrathalamic oscillations in vitro.

Modulation of spindle oscillations by acetylcholine, cholecystokinin and 1S,3R-ACPD in the ferret lateral geniculate and perigeniculate nuclei in vitro

The transition from sleep to waking is associated with the abolition of spindle waves and the appearance of tonic activity in thalamocortical neurons and thalamic reticular/perigeniculate GABAergic cells. We tested the possibility that changes such as these may arise through modulation of the leak potassium current, IKL, by examining the effects of neurotransmitters known to modulate this current on spindle wave generation in the ferret geniculate slice maintained in vitro. Local application of agents that reduce IKL in thalamocortical neurons, including acetylcholine, DL-muscarine chloride and the glutamate metabotropic receptor agonist 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD), to spontaneously spindling thalamocortical neurons resulted in a 5-10 mV membrane depolarization and the abolition of spindle waves. Local application of 1S,3R-ACPD and cholecystokinin-8-sulfate, both of which reduce IKL, to GABAergic neurons of the perigeniculate nucleus resulted in a 10-20 mV membrane depolarization, appearance of tonic discharge and the abolition of spindle wave generation. Local application of 1S,3R-ACPD and cholecystokinin to the perigeniculate nucleus while recording from thalamocortical neurons resulted in the abolition of spindle wave-associated inhibitory postsynaptic potentials and the occurrence of a continuous barrage of smaller amplitude inhibitory postsynaptic potentials, presumably in response to depolarization and tonic discharge of perigeniculate neurons. These results indicate that modulation of IKL in thalamocortical neurons and perigeniculate neurons is capable of abolishing the generation of spindle waves in thalamic networks. Through the modulation of IKL, ascending and descending activating systems may control the state of the thalamus such that the transition from slow wave sleep to waking is associated with the abolition of slow, synchronized rhythms and the facilitation of a state that is conducive to sensory receptor field analysis, arousal and perception.

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.

Distribution of cholecystokinin-like-immunoreactive neurons in the guinea pig forebrain

The distribution of cholecystokinin (CCK)-immunoreactive nerve fibers and cell bodies was studied in the forebrain of control and colchicine-treated guinea pigs by using an antiserum directed against the carboxyterminus of CCK octapeptide (CCK-8) in the indirect immunoperoxidase technique. Virtually all forebrain areas examined contained immunoreactive nerve fibers. A dense innervation was visualized in; neocortical layers II-III, piriform cortex, the medial amygdala, the medial preoptic area, a circumventricular organ-like structure located at the top of the third ventricle in the preoptic area, the subfornical organ, the posterior bed nucleus of the stria terminalis, the posterior globus pallidus (containing labeled woolly fiber-like profiles), the ventromedial hypothalamus, the median eminence, and the premammillary nucleus. A moderately dense innervation was visualized elsewhere excepted in the septum and thalamus where labeled axons were comparatively few. Immunoreactive perikarya were abundant in: neocortex (especially layers II-III), piriform cortex, amygdala, the median preoptic nucleus, the bed nucleus of the stria terminalis, the hypothalamic paraventricular (parvicellular part), arcuate, and dorsomedial (pars compacta) nuclei, the dorsal and perifornical hypothalamic areas, and throughout the thalamus. Areas also containing a moderate number of labeled cell bodies were the medial preoptic area, the globus pallidus, the caudate-putamen, and the periventromedial area in the hypothalamus. Immunostained perikarya were absent or only occasionally observed in the septum, the suprachiasmatic nucleus, the magnocellular hypothalamoneurohypophyseal nuclei, and the ventral mesencephalon. In the adenohypophysis, corticomelanotrophs were labeled in both males and females, and thyrotrophs were labeled in females only. This distribution pattern of CCK-8 immunoreactivity is compared to those previously recorded in other mammals. This shows that very few features are peculiar to the the guinea pig. It is discussed whether some interspecific differences in immunostaining are real rather than methodological.

Physiological differentiation within the auditory part of the thalamic reticular nucleus of the cat

Spike trains of 153 single units were recorded in the caudoventral part of the thalamic reticular nucleus (RE) of 7 nitrous oxide anaesthetized cats. Functional properties defined by spontaneous activity pattern, studied by mean of auto renewal density histograms, were used to subdivide the units into 4 groups. Types I (18%), II (56%) and III (15%) were defined by an increasing bursting activity and Type IV (11%) by firing no bursts spontaneously. The responses to auditory stimuli confirmed that the caudoventral part of RE is tightly related to central auditory pathways. Responses to white noise bursts (200 ms duration) significantly let appear that Type I units responded in a high proportion (greater than 70%) until 80 ms after the stimulus onset, Type II units where mostly affected during the entire stimulus duration, and Type III units showed preferentially late responses. The units responsive to high frequencies (greater than 8 kHz) were mostly located in the dorsal and the units responsive to low frequencies (less than 2 kHz) in the anteroventral sector of auditory RE. However, only a loosely tonotopy is supported by this study. The neuronal circuitry within RE was shown to be stable when white noise bursts were delivered. Cross-correlograms indicated a large proportion of interconnected units (64%) and signs of mutual inhibition between neighboring RE units (11%). The hypothesis is discussed that the auditory RE exerts a fine control on the time-dependent analysis of the incoming auditory input to the cerebral cortex. The complex intranuclear connectivity suggests that the cell types correspond to distinct patterns of functional connections.

Postnatal development of cholecystokinin (CCK) binding sites in the rat forebrain and midbrain: an autoradiographic study

The postnatal development of cholecystokinin (CCK) binding sites in the rat forebrain and midbrain was studied by in vitro receptor autoradiography. In the majority of structures, the densities of sites were low over the first week after birth, increased until the third week, and decreased over the fourth week to reach adult levels. However, both the rate of increase and the extent of the decrease varied in large proportions among structures. For instance, labeling in the neocortex underwent its largest increase from postnatal day 10, while this increase was already begun at day 7 in the paleocortex. On the other hand, over the fourth postnatal week, the densities could either remain roughly constant (cingulate cortex), slightly decrease (thalamic reticular nucleus), or even return to background levels (pyramidal layer of hippocampus). These different timetables may depend mostly on the differential growth of cells expressing the CCK receptor gene within the developing CNS. The absence of CCK binding sites in most of the regions during the early postnatal period precludes a major role of this peptide in the embryonic development of the rat brain. However, in some regions as the ventromedial hypothalamic nucleus, the endopyriform cortex or the medial nucleus of amygdala, 30-50% of the adult levels were already present at birth. Whether this observation reflects an earlier functional maturation of these structures or a direct participation of the corresponding CCK systems in their development remains to be established.