Cholecystokinin-induced activity changes of dorsal lateral geniculate neurons in rats.

The effects of iontophoretically applied cholecystokinin fragments and cholecystokinin antagonists on neurons of the dorsal lateral geniculate nucleus were investigated with extracellular recordings in rats anesthetized with urethane. The peptide cholecystokinin-8S, which has affinity for both cholecystokinin-A and -B receptors, altered the baseline firing as well as the responses to visual stimuli of about one half of the investigated neurons (90 out of 190). Excitatory effects predominated (P < 0.01, Wilcoxon test), although inhibitory effects were also observed. The effects of cholecystokinin-8S were dose-dependent. Neurons sensitive to cholecystokinin-8S could be found in all regions of the dorsal lateral geniculate nucleus, but they differed in their susceptibility to cholecystokinin in relation to their location. The B-agonist, BOC-cholecystokinin-4, also changed the baseline firing as well as the light-evoked activity of dorsal lateral geniculate nucleus neurons. The effects were either excitatory or inhibitory. Changes induced by cholecystokinin-8S could be effectively blocked by the cholecystokinin-B antagonist, CAM 1028 (19 out of 22 cholecystokinin-sensitive neurons tested). The cholecystokinin-A antagonist, Ge 410, blocked cholecystokinin-induced effects in 10 out of 16 neurons. These results indicate that the modulation of geniculate cell firing by cholecystokinin is mediated by both A-and B-receptor types.

Decreased inhibition by leptin of hypothalamic arcuate neurons in neonatally overfed young rats.

Rats neonatally overnourished due to a reduced litter size develop persisting overweight throughout life. A presumed mechanism leading to this 'malprogramming' consists of an acquired change of the responsiveness to leptin of the neuronal system regulating feeding behavior. The study reports significant differences in the effects of leptin on single unit activity of the arcuate nucleus in brain slices of normal compared with early postnatally overfed juvenile rats. The firing rate of arcuate neurons in normal rats was inhibited by leptin (Wilcoxon test p < 0.0001, n = 42), whereas it was not changed in obese rats (Wt p = 0.24, n = 35). The reduced inhibition by leptin of arcuate neurons in neonatally overfed rats might indicate an acquired hypothalamic leptin resistance contributing to persistent hyperphagia and overweight.

Inhibition by insulin of hypothalamic VMN neurons in rats overweight due to postnatal overfeeding.

Single unit activity was studied in brain slices of normal and overweight adolescent rats, the latter grown up until weaning in small litters of three pups per mother (SL). Significantly fewer neurons of the ventromedial hypothalamic nucleus (VMN) were activated by insulin in overweight SL rats than in normal (NL) rats (chi2 p < 0.01). Although there is no significant difference between NL and SL rats in the number of VMN neurons responsive to insulin, the neurons differ in the type of reaction. In overweight SL rats neurons were mainly inhibited by insulin (Wilcoxon test p < 0.0001, n = 45). This altered response to the satiety signal insulin in postnatally overnourished rats might contribute to their persisting hyperphagia and overweight.

Increased response to NPY of hypothalamic VMN neurons in postnatally overfed juvenile rats.

Rats postnatally overnourished due to a reduced litter size become persistently overweight. A presumed pathophysiological mechanism consists of a change in the activity and responsiveness to neuropeptides of the neuronal system regulating feeding behavior. This study aimed to find differences in the action of neuropeptide Y, orexin-A and cholecystokinin on single unit activity of the ventromedial hypothalamic nucleus in brain slices of normal and postnatally overfed juvenile rats. NPY inhibited significantly more neurons (15 of 23) of obese than of normal rats (6 of 27; p < 0.01, chi2). Orexin-A and CCK-8S mainly activated the neurons without significant differences between the groups. In conclusion, the stronger inhibition by NPY of VMN neurons which signal satiety might contribute to increased feeding behavior in postnatally overfed rats.

Effects of cholecystokinin agonists on striatal neurons are reduced by acetylcholine.

The present study investigated whether acetylcholine, a transmitter of striatal interneurons, modulates responses of neostriatal neurons to agonists of the neuropeptide cholecystokinin (CCK). Single unit activity was recorded in rats anesthetized with urethane. Acetylcholine and CCK agonists (the CCKA receptor agonists A-71378 and A-71623; the CCKB receptor agonist Suc-CCK-4) were iontophoretically administered alone and in combination. The CCK agonists excited about one third of the neurons. The excitatory effects of both the CCKB and the CCKA receptor agonists were mainly reduced or changed to suppression of activity by acetylcholine (Wilcoxon test p < 0.001). Atropine did not significantly change the neuronal responses to the CCK agonists. The suppressive action of acetylcholine could be diminished by additional administration of atropine. The results suggest that the modulatory action of cholecystokinin does not only depend on the actual state of excitability in striatal neurons, but could be changed by acetylcholine released from interneurons.

Interactive effects of cholecystokinin-8S and serotonin on spontaneously active neurons in ventromedial hypothalamic slices.

The influence of cholecystokinin (CCK-8S) and serotonin (5-HT) on the discharge rate of spontaneously active ventromedial hypothalamic (VMH) neurons was investigated in brain slices. Drugs were drop-applied individually and concomitantly into the slice chamber. CCK-8S (0.1-2.5 microM) produced a dose-dependent increase in firing rate mainly mediated by the CCK(B) receptor subtype, because Suc-CCK-4 (a CCK(B) receptor agonist) acted like CCK-8S and A-71378 (a CCK(A) receptor agonist) rarely induced excitatory effects. The main response to serotonin application (2-20 microM) was an inhibition that could be mimicked by 8-OH-DPAT (a 5-HT1A receptor agonist). S-UH-301 (a 5-HT1A receptor antagonist) reversibly diminished or blocked this effect. Other 5-HT agonists like DOI and 2-Methyl-5-HT did not evoke relevant responses. Co-administration of CCK-8S and 5-HT induced counteracting effects at which CCK-8S significantly reduced the prevailing suppressive effect of serotonin. It is concluded that both substances, CCK and 5-HT, have a reciprocal influence on the regulation of neuronal activity within the VMH, a structure, which is involved in the mediation of signals for the state of satiety.

Neostriatal neurons of rats can be influenced by cholecystokinin-A receptor agonists.

The present study investigated the effects of agonists of the neuropeptide cholecystokinin (CCK) on neostriatal neurons in order to confirm the existence of CCK-A receptors in the rat caudate-putamen. Single unit activity was recorded in rats anaesthetized with urethane. The CCK-A receptor agonists A-71378 (desamino-Tyr(SO3H)-Nle-Gly-Trp-Nle-(N-methyl)Asp-Phe-NH2), and A-71623 (Boc-Trp-Lys(epsilon-N-2-methylphenylamino-carbonyl)-Asp-(N-methyl )Phe-NH2, as well as the CCK-B receptor agonist Suc-CCK-4 (Suc-Trp-(N-methyl)Nle-Asp-Phe-NH2) were iontophoretically administered with multibarrel capillaries. About one-third of the neurons responded to the CCK-A receptor agonists. Excitatory effects predominated. The CCK-B receptor agonist also induced mainly increases in the neuronal firing. Several neurons were affected by both types of agonists. The results suggest that in the rat neostriatum not only CCK-B receptors, but also CCK-A receptors seem to mediate the effects of cholecystokinin.

Interactive effects of cholecystokinin-8S and various serotonin receptor agonists on the firing activity of neostriatal neuronesin rats.

In rats anaesthetized with urethane single unit activity was extracellularly recorded in the neostriatum, and several drugs were microiontophoretically ejected. Separate administration of the sulfated octapeptide cholecystokinin (CCK-8S), serotonin (5-HT) or 8-OH-DPAT (a 5-HT(1A/7) receptor agonist) predominantly induced increases in the neuronal discharge rates (Wilcoxon test significant P<0.05), whereas the 5-HT(2A/2C) receptor agonist DOI affected only a few neurones and mainly reduced firing. After coadministration of CCK-8S and serotonin, activating effects also predominated (Wt P<0.05), but the neuronal responsiveness was significantly reduced (Chi2P<0.01). Similarly, concomitant application of CCK-8S and 8-OH-DPAT led to significant activation accompanied with a reduction of inhibitory effects. The block of serotonin- or 8-OH-DPAT-effects through specific 5-HT(1A) receptor antagonists implies the involvement of this receptor subtype within the striatum. In conclusion, concomitant action of CCK-8S and serotonin induces a mean level of neuronal activation that might promote normal function.

Modulation of visually evoked responses in units of the ventral lateral geniculate nucleus of the rat by somatic stimuli.

Single unit activity was recorded from the ventral part of the lateral geniculate nucleus (vLGN) in rats anaesthetized with urethane. Most of the cells located laterally in the nucleus were excited by light. The studied vLGN neurones did not respond to electrical stimulation of the tail, but about half of them changed their response to light significantly when the light flash was paired with the electrical stimulation. When the tail stimulus preceded the light, the changes consisted in a pronounced facilitation of flash-evoked activity. When the electrical stimulus was applied after the flash in a forward conditioning paradigm, facilitations were less pronounced and responses of some neurones were suppressed. These results are in contrast to those of similar experiments on the dorsal LGN, neurones of which were mainly facilitated by the conditioning paradigm. Thus, light-evoked activity of ventral geniculate cells can be enhanced by arousal-related processes.

Histamine reduces firing and bursting of anterior and intralaminar thalamic neurons and activates striatal cells in anesthetized rats.

Histamine is known to play a role in the regulation of waking behavior as well as in processes of memory and reinforcement. The striatum and thalamic nuclei as the intralaminar complex and the anterior group can be involved in these functions. Little is known about the action of histamine on neurons of these brain structures. Single unit activity was extracellularly recorded in rats anesthetized with urethane. Firing of anterior and intralaminar thalamic neurons that responded to iontophoretically administered histamine was predominantly reduced (Wilcoxon test (Wt), P<0.05, n=49 and 63, respectively), whereas striatal neurons were mainly activated by the drug (Wt, P<0.05, n=29). Thalamic neurons also significantly reduced the number of burst discharges and the proportion of spikes involved in bursts. The histaminergic effects could be blocked by H1 or H2 receptor antagonists. In conclusion, histamine may control waking behavior also via nonspecific thalamic nuclei and basal ganglia circuits. Through modulation of the transmission in the anterior thalamus it may exert an influence on learning and emotional processes.

Influence of atropine microinjection into nucleus reticularis thalami on activity of lateral geniculate nucleus neurones in freely moving rats.

The influence of the microinjection of atropine into the visual part of the nucleus reticularis thalami (TR) on flash-evoked unit responses of the dorsal lateral geniculate nucleus (dLGN) was investigated in freely moving rats. Atropine induced a significant prolongation and accentuation of the postexcitatory inhibitory phases in water-deprived trained rats, to which light flashes acted as conditioned stimuli and initiated a drinking behaviour. In satiated relaxed rats atropine did not change the unit activity significantly. Acetylcholine injected into TR caused a facilitation of the dLGN transmission. Since the decrease of flash-evoked excitatory response phases in thirsty, trained rats as compared to satiated ones could not be blocked by atropine, not all behaviourally dependent changes are mediated by a disinhibitory action of cholinergic fibres on TR.

Action of cholecystokinin and serotonin on lateral hypothalamic neurons of rats.

Discharges of spontaneously active lateral hypothalamic neurons were extracellularly recorded during iontophoretic administration of cholecystokinin (CCK-8S) or/and serotonin (5-HT) in anesthetized rats. The main results are the following. (1) The proportion of neurons responsive to CCK-8S was 62% (61/99) and that responsive to 5-HT 42% (33/78). (2) Out of the neuronal sample, 36% were influenced by both transmitters, allowing an interaction between the two systems. (3) Co-ejection of CCK and 5-HT elicited a response in 40% of the tested neurons, which was a significantly smaller responsiveness than with separate ejection of CCK-8S. The effect resulted from a reduced number of excited neurons whereas the number of inhibitions did not change. The results show that effects of 5-HT and CCK can converge on the same neuron within the lateral hypothalamus. This might be of relevance in the regulation of feeding behavior.

Different responses of ventromedial hypothalamic neurons to leptin in normal and early postnatally overfed rats.

Leptin is crucially involved in the central nervous regulation of body weight. Neurons of the ventromedial hypothalamic nucleus (VMH) express leptin receptors and signal satiety with increase in their firing. Normally, leptin mainly activates VMH neurons. Rats grown up in small litters (SL) develop persistent hyperphagia and obesity throughout life. We studied single unit activity in hypothalamic brain slices of juvenile SL rats overweight due to early postnatal overfeeding (Mann-Whitney U-test, P < 0.001). VMH neurons of normal rats were mainly activated by leptin (Wilcoxon test, P < 0.05, n = 39), whereas neurons of overweight SL rats were mainly inhibited (Wt, P < 0.001, n = 33). This clearly altered response to leptin in neonatally overnourished rats might contribute to their persistent overweight throughout life.

Action of urethane on dorsal lateral geniculate neurons.

Light evoked neuronal responses of urethane anaesthetized rats were compared with those recorded in freely moving animals. The results demonstrate that urethane reduces the firing frequency, the response probability as well as the number of excitatory light evoked response phases. These changes are combined with a prolongation of the postexcitatory inhibitory phases. The latencies to diffuse flashes are increased in geniculate neuronal responses as well as in evoked potentials recorded from the visual cortex. A similar relation exists between diminution of late geniculate excitations and suppression of cortical photic afterdischarges.

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.

Conditioning-related changes of unit activity in the dorsal lateral geniculate nucleus of urethane-anaesthetized rats.

Changes in geniculate unit activity of urethane-anaesthetized and freely moving rats were investigated during conditioning. The conditioned stimulus (CS) was a flash which was paired with an electrical stimulation of the tail as unconditioned stimulus (US). The discharge rates evoked by the CS during forward conditioning were significantly higher in responding units than those evoked by reversal of CS and US (backward conditioning) or by pseudoconditioning. Tail stimulation alone did not cause significant changes in the firing rate of most of the neurons. In 25% of the investigated neurons the facilitation of activity evoked by forward conditioning persisted during an extinction period of more than 15 min. The effect of conditioning on neuronal activity appeared to be comparable in urethane-anaesthetized rats and in freely moving ones which responded to the US with a slight freezing behavior.

Cholecystokinin affects the neuronal discharge mode in the rat lateral geniculate body.

Neurons of the dorsal lateral geniculate body (dLGB) discharge either continuously with single spikes or in bursts, that is, groups of action potentials with an interspike interval < or = 4 ms. The influence of the sulphated octapeptide of cholecystokinin (CCK-8S) on the discharge mode of dLGB neurons was studied in rats anesthetized with urethane. Unit activity was recorded during repeated stimulation with diffuse light. The occurrence of bursts was determined during control conditions and during iontophoretic administration of CCK-8S and its antagonists. CCK-8S frequently increased the number of bursts. The percentage of spikes involved in bursts was mainly augmented in neurons that responded after short latency to light or were excited by light-off. It was predominantly reduced in units responding to light after long latency with a primary excitation. CCKA receptors seem to contribute to burst induction, since CCK-8S reduced bursting in the presence of the CCKA receptor antagonist Ge 410 and increased bursting when applied with the CCKB receptor antagonist PD 135,158. These bursts were frequently related to light stimuli.

SLOW and FAST lateral geniculate neurons are differently influenced by acetylcholine.

In rats anesthetized with urethane, potentials of 108 neurons were recorded extracellularly in the dorsal part of the lateral geniculate body (dLGB). Neuronal responses to diffuse light stimuli were studied before and during the iontophoretic application of acetylcholine (ACh). Although individual cells of all groups of functionally different neuron types could be influenced by ACh, responses to flashes were most pronounced and uniformly enhanced in the groups of SLOW ON-like cells located in the dorsolateral and caudal parts of the dLGB. The activity in primary response phases to light flashes increased also in caudally located SLOW OFF-like cells. In the group of ventromedially located FAST OFF-like cells the postinhibitory offdischarge in the response to flash was significantly augmented. Only few cells of FAST ON-like groups were affected and some of them inhibited by ACh. off

Interaction of cholecystokinin and glutamate agonists within the dLGN, the dentate gyrus, and the hippocampus.

The interaction of sulfated cholecystokinin (CCK-8S) with excitatory amino acids (EAA) was studied on single units of the dorsal lateral geniculate nucleus (dLGN), the dentate gyrus, and the hippocampal CA3 region in rats anaesthetized with urethane. lontophoretic co-administration of small, individually ineffective currents of CCK-8S and kainic acid or N-methyl-D-aspartate repeatedly elicited an increase of the discharge rate in nearly all geniculate and half of the dentate neurons but not in those of the CA3 region. The effect could be reduced by the CCKB receptor antagonist PD 135,158 more often than by the CCKA antagonist KL 1001. The increased firing due to co-administration of CCK and kainate could also be suppressed by the non-NMDA antagonist CNQX but not by the NMDA antagonists CPP or AP-5, which were otherwise able to prevent the neuron from responding to co-administration of CCK and NMDA. It is suggested that in distinct brain regions the effectivity of the "low level" EAA transmission may be enhanced by small amounts of CCK-8S. This is thought to be mediated by a coactivation of CCK and EEA receptors.

Lateral geniculate unit activity in freely moving rats. II. Relation to conditioning and motivation.

Unit activity of the lateral geniculate body was recorded on successive days using implanted semimicroelectrodes. Unit responses to light stimuli recorded in water deprived and satiated animals were compared. They differed in dependence on whether the light stimulus is a conditioned one (leading to a behavioral response) or is acting merely as a physical event without relevance to the animal’s adaptive behavior. This was confirmed using the following experimental procedures: (i) recording from one cell population on several successive days under equal conditions, (ii) elaborating a conditioned reflex during recording, and (iii) comparing unit responses to light stimuli in relation to the rat’s behavioral responses (correct, not correct) during the first training days.