Sleep variables are unaltered by zolantidine in rats: Are histamine H2-receptors not involved in sleep regulation?

The effects of the H1-receptor antagonist diphenhydramine and the brain-penetrating H2-receptor antagonist zolantidine were studied in rats implanted for chronic sleep recordings. Diphenhydramine (1.0-4.0 mg/kg) significantly increased slow wave sleep and decreased wakefulness. Zolantidine (0.25-8.0 mg/kg) had no significant effects on any of the sleep parameters examined. One possibility is that zolantidine did not enter the brain in sufficient concentration to produce significant changes on sleep and wakefulness. Another possibility is that blockade of H2-receptor involved parts of the brain other than those implicated in the sleep-wake cycle. The feasibility remains that H2-receptors are not involved in sleep regulation. The absence of selective, brain-penetrating H2-receptor agonists precludes a more detailed analysis of the role of this subtype of receptor in the control of sleep and wakefulness.     

Effects of histamine on MK-801-induced memory deficits in radial maze performance in rats

The effects of histamine on the spatial memory deficits induced by MK-801 were investigated using the eight-arm radial maze paradigm in rats. Intracerebroventricular (i.c.v.) injection of histamine or thioperamide, and intraperitoneal (i.p.) injection of histidine improved the spatial memory deficits induced by MK-801. Similar results were obtained with 2-thiazolylethylamine. In contrast, 4-methylhistamine showed no significant effect. Based on these observations, it seems likely that the protective effect of histamine on MK-801-induced spatial memory deficit is mediated by H(1)-receptors.     

Nitric Oxide Donors SIN-1 and SNAP Promote Nonrapid-Eye-Movement Sleep in Rats

We previously showed that inhibition of brain NO production suppresses sleep in rats and rabbits. In the present experiments we studied the effects of stimulation of NO-receptive brain mechanisms on sleep. Male rats were injected intracerebroventricularly with the NO donor S-nitroso-N-acetylpenicillamine (SNAP, 400 μg) or molsidomine (SIN-1, 7 and 70 μg). Seven micrograms of SIN-1 did not affect sleep, but increased the delta wave activity of the electroencephalogram (EEG) during nonrapid-eye-movement sleep (NREMS) and suppressed EEG alpha and beta activities in NREMS and delta, theta, and beta activities during wakefulness. Seventy micrograms of SIN-1 significantly increased NREMS after a latency of ≈9 h. EEG power was suppressed in each frequency band during rapid-eye-movement sleep (REMS) and wakefulness, whereas during NREMS, delta activities were increased after the injection of 7 μg SIN-1, and higher frequencies were suppressed after both doses. On the recovery day sleep remained elevated, but EEG power returned to baseline. The effects of SNAP on NREMS were similar to those of SIN-1, but REMS was decreased and slight increases in brain temperature accompanied the sleep changes. The EEG theta, alpha, and beta activities were suppressed in both wakefulness and REMS. Collectively, these results are consistent with the hypothesis that NO plays a role in the regulation of vigilance.     

Intracerebroventricular injection of erythropoietin enhances sleep in the rat

Systemic injection of erythropoietin (EPO) over several days reduces sleep fragmentation in patients with periodic limb movements in sleep (PLMS). However, there are no studies concerning the effects of EPO on spontaneous sleep. In this study, we determined the effects of intracerebroventricular (i.c.v.) administration of EPO on spontaneous rat sleep. Three doses of EPO (25, 75, and 125 ng) were injected i.c.v. at the onset of the dark period. All doses of EPO increased non-rapid eye movement sleep (NREMS). In addition, high and low doses of EPO (125 and 25 ng) increased rapid eye movement sleep (REMS), but the medium dose of EPO (75 ng) inhibited REMS. Electroencephalogram slow-wave activity during NREMS also increased following the two higher doses of EPO. In contrast, EPO injection during the light period failed to affect sleep. Brain temperature (Tbr) was not affected by any dose of EPO. These results suggest that EPO could be part of the cytokine network involved in sleep regulation.     

Prolactin, vasoactive intestinal peptide, and peptide histidine methionine elicit selective increases in REM sleep in rabbits

The purpose of these experiments was to determine whether (1) vasoactive intestinal peptide (VIP) produces effects on rabbit sleep similar to those reported for rats and cats; (2) peptide histidine methionine (PHM), a peptide closely related to VIP, mimics the sleep effects of VIP; and (3) pituitary prolactin (PRL), a pituitary hormone that has a sleep-related secretory pattern and for which VIP and PHM act as releasing factors, has similar effects on sleep. VIP or PHM (0.01, 0.1 and 10. nmol/kg) was intracerebroventricularly (i.c.v.) injected; PRL (ovine PRL, 45 and 200 IU/kg) was subcutaneously (s.c.) administered. Sleep-wake activity and brain temperature were recorded for 6 h. For controls, rabbits received artificial cerebrospinal fluid i.c.v. or PRL-vehicle s.c. VIP and PHM promoted rapid eye movement sleep (REMS), although these effects were not dose-dependent. In addition, the high dose of VIP and PHM transiently increased wakefulness. Increases in REMS occurred only during hours 2–6 after i.c.v. injection of VIP and peptide histidine leucine (PHI). After s.c. injection of PRL, REMS started to increase in postinjection hour 3. The effect of the high dose was significantly more pronounced than that of the small dose. Each substance enhanced the frequency of REMS episodes, and the high dose of PRL also increased the duration of REMS bouts. These results are consistent with the hypothesis that VIP is involved in physiological regulation of REMS, and that the VIP- and PHM-induced increases in REMS may be mediated via release of PRL.     

Insulin-like growth factor-1 (IGF-1)-induced inhibition of growth hormone secretion is associated with sleep suppression

The hypothalamic growth hormone (GH)-releasing hormone (GHRH) promotes non-rapid eye movement sleep (NREMS). Insulin-like growth factor-1 (IGF-1) acts as a negative feedback in the somatotropic axis inhibiting GHRH and stimulating somatostatin. To determine whether this feedback alters sleep, rats and rabbits were injected intracerebroventricularly (i.c.v.) with IGF-1 (5.0 and 0.25 microgram, respectively) and the sleep-wake activity was studied. Compared to baseline (i.c.v. injection of physiological saline), IGF-1 elicited prompt suppressions in both NREMS and rapid eye movement sleep (REMS) in postinjection hour 1 in rats and rabbits. The intensity of NREMS (characterized by the slow wave activity of the EEG by means of fast-Fourier analysis) was significantly enhanced 7 to 11 h postinjection in rats. Plasma GH concentrations were measured in 30-min samples after i.c.v. IGF-1 injection in rats and a significant suppression of GH secretion was observed 30 min postinjection. The simultaneous inhibition of the somatotropic axis and sleep raises the possibility that the sleep alterations also result from an IGF-1-induced suppression of GHRH. The late increases in NREMS intensity are attributed to metabolic actions of IGF-1 or to a release of GHRH from the IGF-1-induced inhibition.     

The baroreflex contribution to spontaneous heart rhythm assessed with a mathematical model in rats

In Spontaneously Hypertensive (SHR) and Wistar-Kyoto (WKY) normotensive rats, we quantified the extent to which spontaneous fluctuations of heart period (HP) may be determined from arterial pressure based on linear baroreflex properties. We analyzed time series (30-s length) of low-frequency (<0.8 Hz) fluctuations of HP and mean arterial pressure obtained during quiet wakefulness, rapid-eye-movement sleep (REMS) and non-rapid-eye-movement sleep (NREMS) as well as a control set of surrogate isospectral data with random phase. HP was modeled as the summed output of two parallel linear transfer functions with arterial pressure as input. The mean square difference between modeled and recorded HP was minimized by varying model parameters. The percentage of time series, in which such difference was lower than half the measured HP variance was significantly lower in REMS (6+/-1%, SHR; 5+/-1%, WKY) than either in quiet wakefulness (25+/-2%, SHR; 35+/-3%, WKY) or NREMS (33+/-3%, SHR; 27+/-3%, WKY), and in quiet wakefulness, it was significantly lower in SHR than in WKY. In surrogate data, these percentages were significantly lower than in recorded data during quiet wakefulness and NREMS, but not during REMS. The extent to which linear baroreflex properties explain spontaneous heart rhythm thus depends on the interaction between the behavioral state and the hypertensive disease, and in REMS, may be accounted for by chance couplings between HP and arterial pressure.     

The effects of intracerebroventricular application of 8-Br-cGMP and LY-83,583, a guanylyl cyclase inhibitor, on sleep-wake activity in rats

Cyclic GMP is the second messenger that mediates most of the neuronal effects of nitric oxide (NO). Several lines of evidence suggest that NO-ergic mechanisms play an integral role in the regulation of vigilance. In the present study, we tested the effects of the activation of cGMP-receptive mechanisms and the inhibitor of guanylyl cyclase (GC), LY-83,583, on sleep in rats. Rats were injected intracerebroventricularly (icv) with 0.16, 4, 100, and 500 microg or 2.5 mg 8-Br-cGMP, a membrane-permeable analogue of cGMP, or 1 and 100 microg LY-83,583. Administration of 4 microg-2.5 mg 8-Br-cGMP increased wakefulness and suppressed rapid-eye-movement sleep (REMS) and non-REMS (NREMS) in rats when given before dark onset but not when given before the light period. The GC inhibitor LY-83,583 strongly promoted NREMS and suppressed REMS during the light period of the day. Furthermore, LY-83,583 induced striking increases in the delta-wave activity of the electroencephalogram (EEG) during NREMS, whereas EEG activity above the 4.5 Hz wave range was suppressed in all vigilance states. Our finding that cGMP has an arousal-promoting activity is in line with the hypothesis that NO/cGMP signaling pathway is involved in the regulation of vigilance.     

Involvement of histaminergic neurons in arousal mechanisms demonstrated with H3-receptor ligands in the cat

The effects of histamine H3-receptor ligands on sleep-waking parameters were studied in freely moving cats. Oral administration of (R)alpha-methylhistamine (alpha MHA), a H3-agonist, caused a significant increase in deep slow wave sleep while that of thioperamide, a H3-antagonist, enhanced wakefulness in a marked and dose-dependent manner. The arousal effects of thioperamide were prevented by pretreatment with alpha MHA or mepyramine, a H1-receptor antagonist. The findings support the hypothesis that the histaminergic neurons are critically involved in arousal mechanisms and suggest that H3-receptors play an active part in these mechanisms by regulating histamine transmission.     

Anandamide-induced sleep is blocked by SR141716A, a CB1 receptor antagonist and by U73122, a phospholipase C inhibitor.

Anandamide (ANA) alters sleep by increasing the amount of time spent in slow wave sleep 2 (SWS2) and rapid eye movement sleep (REMS) at the expense of wakefulness (W) in rats. In this report, we describe a similar effect of ANA when injected itracerebroventricularly (i.c.v.) or into the peduriculopontine tegmental nucleus (PPTg) and the lack of an effect when ANA is administered into the medial preoptic area (MPOA). Furthermore, the i.c.v. or PPTg administration of SR141716A, a CB1 antagonist, or U73122, a PLC inhibitor, 15 min prior to ANA, readily prevents the ANA induced changes in sleep. The present results suggest that a cannabinoid system in the PPTg may be involved in sleep regulation and that the cannabinoid effect is mediated by the CB1 receptor coupled to a PLC second messenger system.     

Role of nitric oxide in sleep regulation: effects of L-NAME, an inhibitor of nitric oxide synthase, on sleep in rats

The effect of N(G)-nitro-L-arginine methyl ester (L-NAME), a competitive inhibitor of enzyme nitric oxide synthase (NOS), on spontaneous sleep during the light period, was studied in adult rats implanted for chronic sleep recordings. L-NAME was injected by subcutaneous (s.c.) or intracerebroventricular (i.c.v.) routes or was infused directly into the dorsal raphe nuclei (DRN). Subcutaneous (1.25-5.0 mg/kg) or i.c.v. (0.25-1.0 mg) administration of L-NAME increased waking (W) and reduced slow wave sleep (SWS) and rapid-eye-movement sleep (REMS) during the first 3 h of recording. On the other hand, direct application of L-NAME into the DRN (50.0-150.0 microg) induced an increment of W and a reduction of SWS without suppressing REMS. Values of W and SWS were significantly different compared with those of controls during the 6-h recording period. The effects of L-NAME observed after s.c. or i.c.v. administration confirm previous studies in rabbits and rats, in which the NOS inhibitor reduced sleep and increased W in a dose-dependent manner. It is possible that REMS suppression after L-NAME could be related to a reduction of acetylcholine release in areas critical for REMS promotion. A decrease in gamma-aminobutyric acid (GABA) release after nitric oxide synthesis inhibition could play a role in the reduction of SWS.     

Angiotensin AT1 receptors in the preoptic area negatively modulate the cardiovascular and ACTH responses induced in rats by intrapreoptic injection of prostaglandin E2

We previously reported that brain angiotensin II type 2 (AT₂) receptors contribute to the hyperthermia induced by intrahypothalamic (intrapreoptic (i.p.o.)) administration of prostaglandin E₂ (PGE₂) in rats. The present study was carried out to investigate the role of angiotensin II (ANG II) receptors in the cardiovascular and adrenocorticotropic hormone (ACTH) responses induced in rats by i.p.o. injection of PGE₂. PGE₂ (100 ng) produced marked increases in blood pressure, heart rate, and plasma ACTH concentration. These changes were significantly enhanced by i.p.o. treatment with an AT₁-receptor antagonist, losartan, while an AT₂-receptor antagonist, CGP 42112A, had no effect. In contrast, losartan, but not CGP 42112A, reduced the pressor and ACTH responses to i.p.o. injection of a large dose of “exogenous” ANG II (25 ng). These results suggest that while “endogenous” ANG II exerts inhibitory effects on both the cardiovascular and the ACTH responses to i.p.o. PGE₂ by way of preoptic AT₁-receptors, a large dose of exogenous ANG II produces effects opposite to those induced by the endogenous ANG II that is released locally and in small amounts by i.p.o. PGE₂.     

Increase of waking and reduction of NREM and REM sleep after nitric oxide synthase inhibition: prevention with GABA(A) or adenosine A(1) receptor agonists

The effect of N(G)-nitro-L-arginine methyl ester (L-NAME), a competitive inhibitor of enzyme nitric oxide synthase (NOS), on spontaneous sleep during the light period, was studied in adult rats implanted for chronic sleep recordings. L-NAME was injected by subcutaneous (s.c.) route or was infused directly into the dorsal raphe nucleus (DRN). Subcutaneous (46.0--185.0 micromol/kg) administration of L-NAME increased waking (W), slow wave sleep (SWS) and rapid-eye-movement sleep (REMS) latency, whereas SWS, REMS and the number of REM periods were reduced. Direct application of L-NAME into the DRN (0.37--1.1 micromol) induced an increment of W and a reduction of SWS and REMS. Values corresponding to SWS and REMS latency, and the number of REM periods remained within control levels. Subcutaneous administration of the GABA(A) receptor agonist muscimol (1.7--3.5 micromol/kg) or the adenosine A(1) receptor agonist L-PIA [L(-)N(6)-(2-phenylisopropyl)adenosine] (0.1--0.3 micromol/kg) induced slight but inconsistent changes of W, light sleep (LS), SWS and REMS that did not attain significance. Pretreatment with muscimol (1.7--3.5 micromol/kg, s.c.) or L-PIA (0.1--0.3 micromol/kg, s.c.) antagonized the increase of W and reduction of SWS and REMS induced by s.c. (92.0 micromol/kg) or intra-DRN (0.74 micromol) administration of L-NAME. However, neither muscimol nor L-PIA prevented the increase of REMS latency induced by L-NAME 92.0 micromol/kg, s.c. Our findings tend to indicate that the change of behavioral state observed after systemic or intra-DRN administration of L-NAME is partly related to the reduction of GABA and adenosine at critical sites in the CNS.     

The time course of sigma activity and slow-wave activity during NREMS in cortical and thalamic EEG of the cat during baseline and after 12 hours of wakefulness.

The extrapolation from recent neurophysiological findings concerning the dependency of spindle and slow-wave oscillations of thalamocortical neurons on membrane potential to macroscopic EEG events, predicts a reciprocal relation between spindle activity and slow-wave activity (SWA) in thalamic and cortical EEG during non-rapid-eye-movement sleep (NREMS). To test this hypothesis, the EEG recorded in 8 cats, from the nucleus centralis lateralis of the thalamus and from the skull during a 12-h baseline dark period and during a 12-h recovery dark period, following a 12-h sleep deprivation, were analyzed. Per 12-s epoch, sleep-wake behaviour was determined and spectral power density was computed in the slow-wave frequency range (0.5-4.0 Hz) and in the spindle frequency region (sigma activity: 11.0-14.5 Hz). To analyze the development of EEG power densities in the course of NREMS and during the transition from NREMS to REMS, the last epoch of wakefulness and the first 15 epochs of NREMS, as well as the last epochs of NREMS and the first epoch of REMS were selected from the NREM-REM cycles. For each animal the values were averaged over 4-h intervals. In the cortical EEG, SWA was minimal at NREMS onset and increased progressively in the course of NREMS. SWA declined sharply prior to REMS. sigma Activity increased gradually towards a uniform level after NREMS onset. During the transition to REMS, sigma activity initially increased and then decreased rapidly. In the thalamic EEG, the time course of SWA paralleled that of the cortex. However, the development of sigma activity during the first part of NREMS differed: in the thalamic EEG, sigma activity was maximal during the beginning of NREMS and slightly decreased thereafter. After sleep deprivation, SWA within NREMS was markedly enhanced in both the cortical and the thalamic EEG. Sigma activity was attenuated in the thalamic EEG, whereas in the cortical EEG it was temporarily elevated. The present data show that, in the thalamic EEG, an inverse relation exists between spindle and slow-wave activity during baseline NREMS. This relation is preserved after sleep deprivation. In the cortical EEG, a reciprocal relation between spindling and SWA is less evident.     

Sleep-wake behavior and responses to sleep deprivation of mice lacking both interleukin-1 beta receptor 1 and tumor necrosis factor-alpha receptor 1

Data indicate that interleukin (IL)-1β and tumor necrosis factor-α (TNFα) are involved in the regulation of non-rapid eye movement sleep (NREMS). Previous studies demonstrate that mice lacking the IL-1β type 1 receptor spend less time in NREMS during the light period, whereas mice lacking the p55 (type 1) receptor for TNFα spend less time in NREMS during the dark period. To further investigate roles for IL-1β and TNFα in sleep regulation we phenotyped sleep and responses to sleep deprivation of mice lacking both the IL-1β receptor 1 and TNFα receptor 1 (IL-1R1/TNFR1 KO). Male adult mice (IL-1R1/TNFR1 KO, n = 14; B6129SF2/J, n = 14) were surgically instrumented with EEG electrodes and with a thermistor to measure brain temperature. After recovery and adaptation to the recording apparatus, 48 h of undisturbed baseline recordings were obtained. Mice were then subjected to 6 h sleep deprivation at light onset by gentle handling. IL-1R1/TNFR1 KO mice spent less time in NREMS during the last 6 h of the dark period and less time in rapid eye movement sleep (REMS) during the light period. There were no differences between strains in the diurnal timing of delta power during NREMS. However, there were strain differences in the relative power spectra of the NREMS EEG during both the light period and the dark period. In addition, during the light period relative power in the theta frequency band of the REMS EEG differed between strains. After sleep deprivation, control mice exhibited prolonged increases in NREMS and REMS, whereas the duration of the NREMS increase was shorter and there was no increase in REMS of IL-1R1/TNFR1 KO mice. Delta power during NREMS increased in both strains after sleep deprivation, but the increase in delta power during NREMS of IL-1R1/TNFR1 KO mice was of greater magnitude and of longer duration than that observed in control mice. These results provide additional evidence that the IL-1β and TNFα cytokine systems play a role in sleep regulation and in the alterations in sleep that follow prolonged wakefulness.     

Role of nitric oxide in sleep regulation: effects of -NAME, an inhibitor of nitric oxide synthase, on sleep in rats

The effect of N^G-nitro- -arginine methyl ester ( -NAME), a competitive inhibitor of enzyme nitric oxide synthase (NOS), on spontaneous sleep during the light period, was studied in adult rats implanted for chronic sleep recordings. -NAME was injected by subcutaneous (s.c.) or intracerebroventricular (i.c.v.) routes or was infused directly into the dorsal raphe nuclei (DRN). Subcutaneous (1.25–5.0 mg/kg) or i.c.v. (0.25–1.0 mg) administration of -NAME increased waking (W) and reduced slow wave sleep (SWS) and rapid-eye-movement sleep (REMS) during the first 3 h of recording. On the other hand, direct application of -NAME into the DRN (50.0–150.0 μg) induced an increment of W and a reduction of SWS without suppressing REMS. Values of W and SWS were significantly different compared with those of controls during the 6-h recording period. The effects of -NAME observed after s.c. or i.c.v. administration confirm previous studies in rabbits and rats, in which the NOS inhibitor reduced sleep and increased W in a dose-dependent manner. It is possible that REMS suppression after -NAME could be related to a reduction of acetylcholine release in areas critical for REMS promotion. A decrease in γ-aminobutyric acid (GABA) release after nitric oxide synthesis inhibition could play a role in the reduction of SWS.     

The effects of histamine H3-receptor antagonists on amygdaloid kindled seizures in rats

The effects of histamine H₃-receptor antagonists, thioperamide, and clobenpropit on amygdaloid kindled seizures were investigated in rats. Both intracerebroventricular (i.c.v.) and intraperitoneal (i.p.) injections of H₃-antagonists resulted in a dose-related inhibition of amygdaloid kindled seizures. An inhibition induced by thioperamide was antagonized by an H₃-agonist [(R)-α-methylhistamine] and H₁-antagonists (diphenhydramine and chlorpheniramine). On the other hand, an H₂-antagonist (cimetidine and ranitidine) caused no antagonistic effect. Metoprine, an inhibitor of N-methyltransferase was also effective in inhibiting amygdaloid kindled seizure, and this effect was augmented by thioperamide treatment.     

Histamine H1-receptors mediate phosphoinositide hydrolysis in astrocyte-enriched primary cultures

Astrocyte-enriched primary cultures of newborn rat brain hemispheres, prelabeled with [³H]inositol, accumulated [³H]inositol phosphate but not [³H]inositol bis-and tris-phosphate, after exposure to histamine for 60 min in the presence of 10 mM LiCl. The response to histamine was not a function of contaminating meningeal fibroblasts since no accumulation of [³H]inositol phosphate was elicited by histamine in meningeal cultures. The stimulation of phosphoinositide hydrolysis by histamine in astrocytes was dose-dependent (EC₅₀ = 1.7 μM, maximal effect = 345% over basal levels) and was mimicked by several H₁-receptor agonists. The use of selectiver receptor antagonists confirmed that the histamine response was the result of activation of H₁-receptors. The histamine-induced [³H]inositol phosphate accumulation was completely abolished by omission of Ca²⁺ from the incubation medium. Astrocyte membranes specifically bound the radiolabeled H₁-antagonist, [³H]mepyramine with an affinity (K_d = 5.9 nM) and a density of binding sites (B_max = 113 fmol/mg protein) similar to rat brain. These results demonstrate the presence of functional histamine H₁-receptors in rat brain astrocytes and suggest a role for histamine as a neuromodulator of astrocyte function.     

An inverse agonist of the histamine H(3) receptor improves wakefulness in narcolepsy: studies in orexin-/- mice and patients

Narcolepsy is characterized by excessive daytime sleepiness (EDS), cataplexy, direct onsets of rapid eye movement (REM) sleep from wakefulness (DREMs) and deficiency of orexins, neuropeptides that promote wakefulness largely via activation of histamine (HA) pathways. The hypothesis that the orexin defect can be circumvented by enhancing HA release was explored in narcoleptic mice and patients using tiprolisant, an inverse H(3)-receptor agonist. In narcoleptic orexin(-/-) mice, tiprolisant enhanced HA and noradrenaline neuronal activity, promoted wakefulness and decreased abnormal DREMs, all effects being amplified by co-administration of modafinil, a currently-prescribed wake-promoting drug. In a pilot single-blind trial on 22 patients receiving a placebo followed by tiprolisant, both for 1 week, the Epworth Sleepiness Scale (ESS) score was reduced from a baseline value of 17.6 by 1.0 with the placebo (p>0.05) and 5.9 with tiprolisant (p<0.001). Excessive daytime sleep, unaffected under placebo, was nearly suppressed on the last days of tiprolisant dosing. H(3)-receptor inverse agonists could constitute a novel effective treatment of EDS, particularly when associated with modafinil.     

Functional mapping of transsynaptic effects of local manipulation of inhibition in gerbil auditory cortex

Cortical networks are under the tonic influence of inhibition which is mainly mediated by GABA. The state of inhibition of small neuronal populations in the auditory cortex (AC) field AI of gerbils was altered by local microinjection of GABA, of the GABA(A)-receptor agonist 4-piperidine-sulfonic acid (P4S) and the GABA(A)-receptor antagonists bicuculline methiodide (BMI) and SR-95531. In order to elucidate direct and transsynaptic effects of the alterations of inhibition produced by these substances we used the 2-fluoro-2-deoxy-D-[(14)C(U)] glucose (FDG) mapping method. The injection of GABA (10 mM) caused no significant changes in FDG labeling but P4S caused a marked decrease of local FDG uptake in a small region surrounding the injection site but in no other region. The injection of the GABA(A)-receptor antagonists caused massive increases of FDG uptake within the entire ipsilateral AC, whereas the contralateral AC was not significantly affected in spite of prominent callosal connections. However, disinhibited excitatory output from the ipsilateral AC is suggested by a strong increase in FDG labeling of the corticothalamic fiber tract and ipsilateral structures like medial geniculate nucleus, caudal striatum, and lateral amygdaloid nucleus and a structure at the caudoventral margin of the thalamic reticular nucleus, presumably the subgeniculate nucleus, a structure with hitherto unknown connections and function. No alteration of FDG uptake could be detected in the inferior colliculus, another main descending target structure of the AC. In summary, the effects resulting from microinjection of GABA(A)-receptor antagonists reflect a differential influence of the AC on its anatomically connected target regions. The findings demonstrate the potential of the method of focal application of neuroactive substances in combination with the FDG technique for mapping their transsynaptic influences which are hard to derive from anatomical tracing studies alone.