Distinct muscarinic receptors inhibit release of gamma-aminobutyric acid and excitatory amino acids in mammalian brain

Ontogeny of altered dendritic morphology in the rat prefrontal cortex, hippocampus, and nucleus accumbens following Cesarean delivery and birth anoxia

We used a delayed Cesarean birth model and the Golgi-Cox staining method to investigate the effects of perinatal anoxia on prefrontal cortex (PFC) and hippocampal (CA1) pyramidal neurons as well as nucleus accumbens (NAcc) medium spiny neurons. Dendritic morphology in these regions was studied on postnatal days (P) 2, 7, 14, 21, 35, and 70 in male Sprague-Dawley rats born either vaginally (VAG) or by Cesarean section either with (C + anoxia) or without (C-only) anoxia. The most striking birth group differences seen were at the level of dendritic spine densities on P35. During this postnatal period the dendritic spine density of PFC neurons was significantly lower in C + anoxia and C-only animals than in VAG controls; however, by P70 PFC spine densities in all birth groups were comparable. In contrast, hippocampal spine densities on P35 were comparably greater in C + anoxia animals than in VAG controls, whereas in C-only animals spine densities were lower than controls; here again, by P70 all groups had comparable hippocampal spine densities. In NAcc greater spine densities were seen on medium spiny neurons of C + anoxia animals on P35. These findings provide evidence that perinatal insult in the form of Cesarean birth with or without anoxia alters the dendritic development of PFC and hippocampal pyramidal neurons and to some extent also of NAcc medium spiny neurons. They also suggest that perinatal anoxia can alter the neuronal development of key structures thought to be affected in such late-onset dopamine-related disorders as schizophrenia and Attention Deficit Hyperactivity Disorder (ADHD).

The role of acetylcholine in cocaine addiction

Central nervous system cholinergic neurons arise from several discrete sources, project to multiple brain regions, and exert specific effects on reward, learning, and memory. These processes are critical for the development and persistence of addictive disorders. Although other neurotransmitters, including dopamine, glutamate, and serotonin, have been the primary focus of drug research to date, a growing preclinical literature reveals a critical role of acetylcholine (ACh) in the experience and progression of drug use. This review will present and integrate the findings regarding the role of ACh in drug dependence, with a primary focus on cocaine and the muscarinic ACh system. Mesostriatal ACh appears to mediate reinforcement through its effect on reward, satiation, and aversion, and chronic cocaine administration produces neuroadaptive changes in the striatum. ACh is further involved in the acquisition of conditional associations that underlie cocaine self-administration and context-dependent sensitization, the acquisition of associations in conditioned learning, and drug procurement through its effects on arousal and attention. Long-term cocaine use may induce neuronal alterations in the brain that affect the ACh system and impair executive function, possibly contributing to the disruptions in decision making that characterize this population. These primarily preclinical studies suggest that ACh exerts a myriad of effects on the addictive process and that persistent changes to the ACh system following chronic drug use may exacerbate the risk of relapse during recovery. Ultimately, ACh modulation may be a potential target for pharmacological treatment interventions in cocaine-addicted subjects. However, the complicated neurocircuitry of the cholinergic system, the multiple ACh receptor subtypes, the confluence of excitatory and inhibitory ACh inputs, and the unique properties of the striatal cholinergic interneurons suggest that a precise target of cholinergic manipulation will be required to impact substance use in the clinical population.

Effects of Striatal GABAA-Receptor Blockade on Striatal and Cortical Activity in Monkeys

To elucidate the role of ambient striatal γ-aminobutyric acid (GABA) in the regulation of neuronal activity in the basal ganglia–thalamocortical circuits, we studied the effects of blocking striatal GABAA receptors on the electrical activities of single striatal neurons, on local field potentials (LFPs) in the striatum, and on motor cortical electroencephalograms (EEGs) in two monkeys. Striatal LFPs were recorded with a device that allowed us to simultaneously record field potentials and apply drugs by reverse microdialysis at the same site. Administration of the GABAA-receptor antagonist gabazine (SR95531, 10 and 500 μM) induced large-amplitude LFP fluctuations at the infusion site, occurring every 2–5 s for about 2 h after the start of the 20-min drug administration. These events were prevented by cotreatment with a GABAA-receptor agonist (muscimol, 100 μM) or a combination of ionotropic glutamate receptor antagonists (CNQX and MK-801, each given at 100 μM). Gabazine (10 μM) also increased the firing of single neurons recorded close to the injection site, but in most cases there was no correlation between single-neuron activity and the concomitantly recorded LFP signals from the same striatal region. In contrast, intrastriatal application of gabazine increased the correlation between striatal LFPs and EEG, and resulted in the appearance of recurrent EEG events that were temporally related to the striatal LFP events. These data provide evidence that a GABAergic “tone” in the monkey striatum controls the spontaneous activity of striatal neurons, as well as the level of striatal and cortical synchrony.

5-HT6 receptor antagonist reversal of emotional learning and prepulse inhibition deficits induced by apomorphine or scopolamine

5-HT6 receptors have been implicated in consolidation of visuospatial and reward-based learning tasks. Since 5-HT6 receptors may be important in modulation of sensory gating which is often affected in schizophrenic patients, we tested whether Ro 4368554, a 5-HT6 selective antagonist at a dose of 10 mg/kg, could reverse the loss of prepulse inhibition from apomorphine or scopolamine. In addition, we also tested whether Ro 4368554 altered fear conditioning using fear potentiated startle, a model for emotional learning. Prepulse inhibition of startle was disrupted by apomorphine (0.5 mg/kg) when prepulse emissions were 5 dB above background but not above 15 dB, while scopolamine (0.5 mg/kg) caused disruption at both prepulse levels. Scopolamine-mediated disruption was not reversed by Ro 4368854 but apomorphine-mediated disruption was significantly ameliorated by 5-HT6 inhibition. For fear potentiated startle, scopolamine and/or Ro 4368554 were administered before two daily fear conditioning sessions; rats were tested on the following day. Rats that received scopolamine displayed no fear potentiated startle but Ro 4368554 reversed this scopolamine deficit. Additionally, we mapped Fos induction in rats treated with scopolamine and/or Ro 4368554; scopolamine increased Fos expression in the central nucleus of the amygdala and this was attenuated by Ro 4368554. In summary, we have demonstrated the efficacy of 5-HT6 antagonists in modulating sensory gating and fear conditioning, and thus may be of therapeutic use for schizophrenia-related disorders.

Involvement of the sigma1 (sigma1) receptor in the anti-amnesic, but not antidepressant-like, effects of the aminotetrahydrofuran derivative ANAVEX1-41

BACKGROUND AND PURPOSE

Tetrahydro-N, N-dimethyl-5, 5-diphenyl-3-furanmethanamine hydrochloride (ANAVEX1-41) is a potent muscarinic and sigma(1) (sigma (1)) receptor ligand. The sigma (1) receptor modulates glutamatergic and cholinergic responses in the forebrain and selective agonists are potent anti-amnesic and antidepressant DRUGS. WE HAVE HERE ANALYSED THE SIGMA (1) COMPONENT IN THE BEHAVIOURAL EFFECTS OF ANAVEX1-41.

EXPERIMENTAL APPROACH

Binding of ANAVEX1-41 to muscarinic and sigma (1) receptors were measured using cell membranes. Behavioural effects of ANAVEX1-41 were tested in mice using memory (spontaneous alternation, passive avoidance, water-maze) and antidepressant-like activity (forced swimming) procedures.

KEY RESULTS

In vitro, ANAVEX1-41 was a potent muscarinic (M(1)>M(3), M(4)>M(2) with K(i) ranging from 18 to 114 nM) and selective sigma (1) ligand (sigma (1), K(i)=44 nM; sigma (2), K(i)=4 microM). In mice, ANAVEX1-41 failed to affect learning when injected alone (0.03-1 mg kg(-1)), but attenuated scopolamine-induced amnesia with a bell-shaped dose response (maximum at 0.1 mg kg(-1)). The sigma (1) antagonist BD1047 blocked the anti-amnesic effect of ANAVEX1-41 on both short- and long-term memories. Pretreatment with a sigma (1) receptor-directed antisense oligodeoxynucleotide prevented effects of ANAVEX1-41 only in the passive avoidance procedure, measuring long-term memory. ANAVEX1-41 reduced behavioural despair at 30 and 60 mg kg(-1), without involving the sigma (1) receptor, as it was not blocked by BD1047 or the antisense oligodeoxynucleotide.

CONCLUSIONS AND IMPLICATIONS

ANAVEX1-41 is a potent anti-amnesic drug, acting through muscarinic and sigma (1) receptors. The latter component may be involved in the enhancing effects of the drug on long-term memory processes.

Towards a muscarinic hypothesis of schizophrenia

Although the neurotransmitter dopamine plays a prominent role in the pathogenesis and treatment of schizophrenia, the dopamine hypothesis of schizophrenia fails to explain all aspects of this disorder. It is increasingly evident that the pathology of schizophrenia also involves other neurotransmitter systems. Data from many streams of research including pre-clinical and clinical pharmacology, treatment studies, post-mortem studies and neuroimaging suggest an important role for the muscarinic cholinergic system in the pathophysiology of schizophrenia. This review will focus on evidence that supports the hypothesis that the muscarinic system is involved in the pathogenesis of schizophrenia and that muscarinic receptors may represent promising novel targets for the treatment of this disorder.

Cholinergic interneurons control the excitatory input to the striatum

How the extent and time course of presynaptic inhibition depend on the action potentials of the neuron controlling the terminals is unknown. We investigated this issue in the striatum using paired recordings from cholinergic interneurons and projection neurons. Glutamatergic EPSCs were evoked in projection neurons and cholinergic interneurons by stimulation of afferent fibers in the cortex and the striatum, respectively. A single spike in a cholinergic interneuron caused significant depression of the evoked glutamatergic EPSC in 34% of projection neurons located within 100 microm and 41% of cholinergic interneurons located within 200 microm. The time course of these effects was similar in the two cases, with EPSC inhibition peaking 20-30 ms after the spike and disappearing after 40-80 ms. Maximal depression of EPSC amplitude was up to 27% in projection neurons and to 19% in cholinergic interneurons. These effects were reversibly blocked by muscarinic receptor antagonists (atropine or methoctramine), which also significantly increased baseline EPSC (evoked without a preceding spike in the cholinergic interneuron), suggesting that some tonic cholinergic presynaptic inhibition was present. This was confirmed by the fact that lowering extracellular potassium, which silenced spontaneously active cholinergic interneurons, also increased baseline EPSC amplitude, and these effects were occluded by previous application of muscarinic receptor antagonists. Collectively, these results show that a single spike in a cholinergic interneuron exerts a fast and powerful inhibitory control over the glutamatergic input to striatal neurons.

Acetyl-l-carnitine protects striatal neurons against in vitro ischemia: The role of endogenous acetylcholine

The neuronal death after ischemia is closely linked to the essential role of mitochondrial metabolism. Inhibition of mitochondrial respiratory chain reduces ATP generation leading to a dysregulation of ion metabolism. Acetyl-L-carnitine (ALC) influences the maintenance of key mitochondrial proteins for maximum energy production and it may play a neuroprotective role in some pathological conditions. In this study we have analyzed ALC-mediated neuroprotection on an in vitro model of brain ischemia. Field potential recordings were obtained from a rat corticostriatal slice preparation. In vitro ischemia (oxygen and glucose deprivation) was delivered by switching to a solution in which glucose was omitted and oxygen was replaced with N2. Ten minutes of in vitro ischemia caused an irreversible loss of the field potential amplitude. Pretreatment with ALC produced a progressive and dose-dependent recovery of the field potential amplitude following in vitro ischemia. The neuroprotective effect of ALC was stereospecific since the pretreatment with two different carnitine-related compounds did not cause neuroprotection. The choline transporter inhibitor hemicholinium-3 blocked the neuroprotective effect of ALC. ALC-mediated neuroprotection was also prevented either by the non-selective muscarinic antagonist scopolamine, or by the putative M2-like receptor antagonist methoctramine. Conversely, the effect of ALC was not altered by the M1-like receptor antagonist pirenzepine. These findings show that ALC exert a neuroprotective action against in vitro ischemia. This neuroprotective effect requires the activity of choline uptake system and the activation of M2 muscarinic receptors.

Endocannabinoid-mediated short-term suppression of excitatory synaptic transmission to medium spiny neurons in the striatum

Medium spiny neurons in the dorsal striatum receive glutamatergic excitatory synaptic inputs from the cerebral cortex. These synapses undergo long-term depression that requires release of endocannabinoids from medium spiny neurons and activation of cannabinoid CB1 receptors. However, it remains unclear how cortico-striatal synapses exhibit endocannabinoid-mediated short-term suppression, which has been found in various brain regions including the hippocampus and cerebellum. Endocannabinoids are released from postsynaptic neurons by strong depolarization and resultant Ca2+ elevation or activation of postsynaptic Gq/11-coupled receptors such as group I metabotropic glutamate receptors (mGluRs) and M1/M3 muscarinic acetylcholine receptors. Moreover, endocannabioids are effectively released when weak depolarization is combined with Gq/11-coupled receptor activation. We found that muscarinic activation induced transient suppression of excitatory synaptic transmission to medium spiny neurons, which was independent of retrograde endocannabinoid signaling but was mediated directly by presynaptic muscarinic receptors. Neither postsynaptic depolarization alone nor depolarization and muscarinic activation caused suppression of cortico-striatal synapses. In contrast, activation of group I mGluRs readily suppressed cortico-striatal excitatory synaptic transmission. Furthermore, postsynaptic depolarization induced clear suppression when combined with group I mGluR activation. These results indicate that group I mGluRs but not muscarinic receptors contribute to endocannabinoid-mediated short-term suppression of cortico-striatal excitatory synaptic transmission.

The neurochemical and behavioral effects of the novel cholinesterase-monoamine oxidase inhibitor, ladostigil, in response to L-dopa and L-tryptophan, in rats

The novel drugs, ladostigil (TV3326) and TV3279, are R and S isomers, respectively, derived from a combination of the carbamate cholinesterase (ChE) inhibitor, rivastigmine, and the pharmacophore of the monoamine oxidase (MAO) B inhibitor, rasagiline. They were developed for the treatment of comorbidity of dementia with Parkinsonism. In the present study, we determined the effects of these drugs on both aminergic neurotransmitter levels and motor behavioral activity in naïve and in L-dopa- or L-tryptophan-induced rats. Chronic treatment of rats with ladostigil (52 mg kg(-1) for 21 days) inhibited hippocampal and striatal MAO A and B activities by >90%, increased striatal levels of dopamine and serotonin, and inhibited striatal ChE activity by approximately 50%. Chronic TV3279 (26 mg kg(-1) for 21 days) similarly inhibited approximately 50% of striatal ChE activity, but did not affect MAO activity or amine levels. In sharp contrast to the inductive effect of the MAO A/B inhibitor, tranylcypromine (TCP), on stereotyped hyperactivity in response to L-dopa (50 mg kg(-1)) or L-tryptophan (100 mg kg(-1)), ladostigil completely inhibited these behavioral hyperactivity syndromes. Accordingly, acute rivastigmine (2 mg kg(-1)) and chronic TV3279 abolished the ability of TCP to initiate L-dopa-induced hyperactivity, while scopolamine (0.5 mg kg(-1)) reversed the inhibitory effect of chronic ladostigil on L-dopa-induced hyperactivity, suggesting that ladostigil may attenuate successive locomotion by activating central cholinergic muscarinic receptors.Finally, while chronic ladostigil administration to naïve rats resulted in preserved spontaneous motor behavior, acute treatment with ladostigil decreased motor performance, compared to control animals. In contrast, chronic as well as acute treatments with TV3279 reduced spontaneous motor activity. Thus, the aminergic potentiation by ladostigil may counteract its cholinergic inhibitory effect on spontaneous motor behavior. Our results suggest that potentiation of both aminergic and cholinergic transmission systems by ladostigil contributes equally to motor behavior performance, which is substantially impaired in comorbidity of dementia with Parkinsonism including dementia with Lewy bodies (DLB).

Striatal glutamate release evoked in vivo by NMDA is dependent upon ongoing neuronal activity in the substantia nigra, endogenous striatal substance P and dopamine

The aim of the present microdialysis study was to investigate whether the increase in striatal glutamate levels induced by intrastriatal perfusion with NMDA was dependent on the activation of extrastriatal loops and/or endogenous striatal substance P and dopamine. The NMDA-evoked striatal glutamate release was mediated by selective activation of the NMDA receptor-channel complex and action potential propagation, as it was prevented by local perfusion with dizocilpine and tetrodotoxin, respectively. Tetrodotoxin and bicuculline, perfused distally in the substantia nigra reticulata, prevented the NMDA-evoked striatal glutamate release, suggesting its dependence on ongoing neuronal activity and GABA(A) receptor activation, respectively, in the substantia nigra. The NMDA-evoked glutamate release was also dependent on striatal substance P and dopamine, as it was antagonized by intrastriatal perfusion with selective NK(1) (SR140333), D(1)-like (SCH23390) and D(2)-like (raclopride) receptor antagonists, as well as by striatal dopamine depletion. Furthermore, impairment of dopaminergic transmission unmasked a glutamatergic stimulation by submicromolar NMDA concentrations. We conclude that in vivo the NMDA-evoked striatal glutamate release is mediated by activation of striatofugal GABAergic neurons and requires activation of striatal NK(1) and dopamine receptors. Endogenous striatal dopamine inhibits or potentiates the NMDA action depending on the strength of the excitatory stimulus (i.e. the NMDA concentration).

Cholinergic Control of Firing Pattern and Neurotransmission in Rat Neostriatal Projection Neurons: Role of CaV2.1 and CaV2.2 Ca2+ Channels

Besides a reduction of L-type Ca2+-currents (CaV1), muscarine and the peptidic M1-selective agonist, MT-1, reduced currents through CaV2.1 (P/Q) and CaV2.2 (N) Ca2+ channel types. This modulation was strongly blocked by the peptide MT-7, a specific muscarinic M1-type receptor antagonist but not significantly reduced by the peptide MT-3, a specific muscarinic M4-type receptor antagonist. Accordingly, MT-7, but not MT-3, blocked a muscarinic reduction of the afterhyperpolarizing potential (AHP) and decreased the GABAergic inhibitory postsynaptic currents (IPSCs) produced by axon collaterals that interconnect spiny neurons. Both these functions are known to be dependent on P/Q and N types Ca2+ channels. The action on the AHP had an important effect in increasing firing frequency. The action on the IPSCs was shown to be caused presynaptically as it coursed with an increase in the paired-pulse ratio. These results show: first, that muscarinic M1-type receptor activation is the main cholinergic mechanism that modulates Ca2+ entry through voltage-dependent Ca2+ channels in spiny neurons. Second, this muscarinic modulation produces a postsynaptic facilitation of discharge together with a presynaptic inhibition of the GABAergic control mediated by axon collaterals. Together, both effects would tend to recruit more spiny neurons for the same task.

Differential modulation by carbachol of four separate excitatory afferent systems to the rat subiculum in vitro

The subiculum is a limbic cortical region that receives inputs from hippocampus and other parahippocampal regions. We used horizontal brain slices to study the modulatory effects of muscarinic receptor activation on excitatory afferent systems of the subiculum. Multiple inputs are preserved in these slices. Carbachol (CCh, applied to the bath) induced a decrease in the field responses (40-50% at 50 microM; 60% at 100 microM) to CA1, presubicular (PreS), and medial entorhinal (MEC) stimulation. Subicular responses to lateral entorhinal (LEC) stimuli were not depressed. The M1 receptor antagonist pirenzepine at 1 microM was sufficient to reverse most of the CCh-induced depression of afferent excitation, but 10 microM concentrations were required to eliminate the CCh-induced firing in the isolated subiculum. A partial reversal of the CCh-induced depression of afferent excitation was achieved by the M2 receptor antagonist methoctramine (1 or 10 microM), but these concentrations did not prevent CCh-induced firing. When CA1 afferents were repetitively activated with submaximal stimuli in the presence of CCh, population excitatory postsynaptic potentials (EPSPs) showed modest summation, but every response was smaller than a corresponding events in normal media. Population spikes, particularly late spikes in a train, showed pronounced facilitation during CCh exposure. The NMDA receptor antagonist CPP (10 microM) prevented facilitation of responses to repetitive stimulation in the presence of carbachol. We conclude that CA1, PreS, and MEC afferents to the subiculum exhibit CCh sensitivity similar to that established for area CA3 afferents to CA1, and LEC afferents to subiculum exhibit CCh resistance. Our data suggest that much of the hippocampal formation circuitry is modulated by CCh and the properties of this modulation can explain some specific firing characteristics of hippocampal formation neurons in "cholinergic" versus "noncholinergic" brain states.

Effects of muscarinic acetylcholine receptor activation on membrane currents and intracellular messengers in medium spiny neurones of the rat striatum

Acetylcholine, acting through muscarinic receptors, modulates the excitability of striatal medium spiny neurones. However, the underlying membrane conductances and intracellular signalling pathways have not been fully determined. Our aim was to characterize excitatory effects mediated by M1 muscarinic acetylcholine receptors in these neurones using whole-cell patch-clamp recordings in brain slices of postnatal rats. Under voltage-clamp, muscarine evoked an inward current associated with an increase in cell membrane resistance. The current, which reversed at -85 mV, was sensitive to the M1 receptor antagonist pirenzepine. Blocking the potassium conductance attenuated the response and the residual current was further reduced by ruthenium red (50 microm) and reversed at +15 mV. Simultaneous recordings from cholinergic interneurones and medium spiny neurones in conjunction with spike-triggered averaging revealed small unitary excitatory postsynaptic currents in four of 39 cell pairs tested. The muscarine-induced inward current was attenuated by a phospholipase C (PLC) inhibitor, U73122, but not by a protein kinase C inhibitor, chelerythrine, or by the intracellular calcium chelator 1,2-bis(2-aminophenoxy) ethane-N,N,N',N'-tetra-acetic acid, suggesting that the current was associated with PLC in a protein kinase C- and Ca2+ -independent manner. The phosphatidylinositol 4-kinase inhibitor wortmannin (10 microm) reduced the recovery of the inward current, indicating that the recovery process was dependent on the removal of diacylglycerol and/or inositol 1,4,5 triphosphate or resynthesis of phospholipid phosphatidylinositol 4,5-bisphophate. Ratiometric measurement of intracellular calcium after cell loading with fura-2 demonstrated a muscarine-induced increase in calcium signal that originated mainly from intracellular stores. Thus, the cholinergic excitatory effect in striatal medium spiny neurones, which is important in motor disorders associated with altered cholinergic transmission in the striatum such as Parkinson's disease, is mediated through M1 receptors and the PLC-dependent pathway.

Presynaptic muscarinic acetylcholine receptors suppress GABAergic synaptic transmission in the intermediate grey layer of mouse superior colliculus

The intermediate grey layer (the stratum griseum intermediale; SGI) of the superior colliculus (SC) receives cholinergic inputs from the parabrachial region of the brainstem. It has been shown that cholinergic inputs activate nicotinic acetylcholine (nACh) receptors on projection neurons in the SGI. Therefore, it has been suggested that they facilitate the initiation of orienting behaviours. In this study, we investigated the effect of muscarinic acetylcholine (mACh) receptor activation on GABAergic synaptic transmission to SGI neurons using the whole-cell patch-clamp recording technique in slice preparations from mice. The GABAA receptor-mediated inhibitory postsynaptic currents (IPSCs) evoked in SGI neurons by focal electrical stimulation were suppressed by bath application of 10 microm muscarine chloride. During muscarine application, both the paired-pulse facilitation index and the coefficient of variation of IPSCs increased; however, the current responses induced by a transient pressure application of 1 mm GABA were not affected by muscarine. Muscarine reduced frequencies of miniature IPSCs (mIPSCs) while the amplitudes of mIPSCs remained unchanged. These results suggested that mAChR-mediated inhibition of IPSCs was of presynaptic origin. The suppressant effect of muscarine was antagonized by an M1 receptor antagonist, pirenzepine dihydrochloride (1 microM), and a relatively specific M3 receptor antagonist, 4-DAMP methiodide (50 nM). By contrast, an M2 receptor antagonist, methoctramine tetrahydrochloride (10 microM), was ineffective. These results suggest that the cholinergic inputs suppress GABAergic synaptic transmission to the SGI neurons at the presynaptic site via activation of M1 and, possibly, M3 receptors. This may be an additional mechanism by which cholinergic inputs can facilitate tectofugal command generation.

A potential cholinergic mechanism of procaine's limbic activation

The local anesthetic procaine, when administered to humans intravenously (i.v.), yields brief intense emotional and sensory experiences, and concomitant increases in anterior paralimbic cerebral blood flow, as measured by positron emission tomography (PET). Procaine's high muscarinic affinity, together with the distribution of muscarinic receptors that overlaps with brain regions activated by procaine, suggests a muscarinic contribution to procaine's emotional and sensory effects. This study evaluates the effects of procaine on cerebral muscarinic cholinergic receptors in the anesthetized rhesus monkey. Whole brain and regional muscarinic receptor binding was measured before and after procaine administration on the same day in three anesthetized rhesus monkeys with PET and the radiotracer 3-(3-(3[18F]fluoropropylthio)-1,2,5-thiadiazol-4-yl)-1,2,5,6-tetrahydro-1-methylpyridine ([18F]FP-TZTP), a cholinergic ligand that has preferential binding to muscarinic (M(2)) receptors. On separate days each animal received six different doses of i.v. procaine in a randomized fashion. Procaine blocked up to approximately 90% of [18F]FP-TZTP specific binding globally in a dose-related manner. There were no regional differences in procaine's inhibitory concentration for 50% blockade (IC50) for [18F]FP-TZTP. Tracer delivery, which was highly correlated to cerebral blood flow in previous monkey studies, was significantly increased at all doses of procaine with the greatest increases occurring near procaine's IC50 for average cortex. Furthermore, anterior limbic regions showed greater increases in tracer delivery than nonlimbic regions. Procaine has high affinity to muscarinic M2 receptors in vivo in the rhesus monkey. This, as well as a preferential increase of tracer delivery to paralimbic regions, suggests that action at these receptors could contribute to i.v. procaine's emotional and sensory effects in man. These findings are consistent with other evidence of cholinergic modulation of mood and emotion.

Two distinct classes of muscarinic action on hippocampal inhibitory synapses: M2-mediated direct suppression and M1/M3-mediated indirect suppression through endocannabinoid signalling

The cholinergic system in the CNS plays important roles in higher brain functions, primarily through muscarinic acetylcholine receptors. At cellular levels, muscarinic activation produces various effects including modulation of synaptic transmission. Here we report that muscarinic activation suppresses hippocampal inhibitory transmission through two distinct mechanisms, namely a cannabinoid-dependent and cannabinoid-independent mechanism. We made paired whole-cell recordings from cultured hippocampal neurons of rats and mice, and monitored inhibitory postsynaptic currents (IPSCs). When cannabinoid receptor type 1 (CB1) was blocked, oxotremorine M (oxo-M), a muscarinic agonist, suppressed IPSCs in a subset of neuron pairs. This suppression was associated with an increase in paired-pulse ratio, blocked by the M(2)-preferring antagonist gallamine, and was totally absent in neuron pairs from M(2)-knockout mice. When CB1 receptors were not blocked, oxo-M suppressed IPSCs in a gallamine-resistant manner in cannabinoid-sensitive pairs. This suppression was associated with an increase in paired-pulse ratio, blocked by the CB1 antagonist AM281, and was completely eliminated in neuron pairs from M(1)/M(3)-compound-knockout mice. Our immunohistochemical examination showed that M(2) and CB1 receptors were present at inhibitory presynaptic terminals of mostly different origins. These results indicate that two distinct mechanisms mediate the muscarinic suppression. In a subset of synapses, activation of M(2) receptors at presynaptic terminals suppresses GABA release directly. In contrast, in a different subset of synapses, activation of M(1)/M(3) receptors causes endocannabinoid production and subsequent suppression of GABA release by activating presynaptic CB1 receptors. Thus, the muscarinic system can influence hippocampal functions by controlling different subsets of inhibitory synapses through the two distinct mechanisms.

Postsynaptic M1 and M3 receptors are responsible for the muscarinic enhancement of retrograde endocannabinoid signalling in the hippocampus

The cholinergic system is crucial for higher brain functions including learning and memory. These functions are mediated primarily by muscarinic acetylcholine receptors (mAChRs) that consist of five subtypes (M(1)-M(5)). A recent study suggested a novel role of acetylcholine as a potent enhancer of endocannabinoid signalling that acts retrogradely from postsynaptic to presynaptic neurons. In the present study, we further investigated the mechanisms of this cholinergic effect on endocannabinoid signalling. We made paired whole-cell recordings from cultured hippocampal neurons, and monitored inhibitory postsynaptic currents (IPSCs). The postsynaptic depolarization induced a transient suppression of IPSCs (DSI), a phenomenon known to involve retrograde signalling by endocannabinoids. The cholinergic agonist carbachol (CCh) markedly enhanced DSI at 0.01-0.3 microM without changing the presynaptic cannabinoid sensitivity. The facilitating effect of CCh on DSI was mimicked by the muscarinic agonist oxotremorine-M, whereas it was eliminated by the muscarinic antagonist atropine. It was also blocked by a non-hydrolizable analogue of GDP (GDP-beta-S) that was applied intracellularly to postsynaptic neurons. The muscarinic enhancement of DSI persisted to a substantial degree in the neurons prepared from M1-knockout and M3-knockout mice, but was virtually eliminated in the neurons from M1/M3-compound-knockout mice. CCh still enhanced DSI significantly under the blockade of postsynatpic K(+) conductance, and did not significantly influence the depolarization-induced Ca(2+) transients. These results indicate that the activation of postsynaptic M1 and M3 receptors facilitates the depolarization-induced release of endocannabinoids.

Cholinergic modulation of memory in the basolateral amygdala involves activation of both m1 and m2 receptors

Muscarinic cholinergic activation is a critical component of basolateral amygdala (BLA)-mediated modulation of memory consolidation. The receptor(s) mediating this activation during consolidation have not been elucidated. This study investigated the roles of muscarinic subtype 1 (m1) and subtype 2 (m2) receptors in memory enhancement, by post-training intra-BLA infusions of the non-selective muscarinic agonist oxotremorine. Rats received intra-BLA infusions of either oxotremorine alone (10 microg in 0.2 microl per side), oxotremorine together with the selective m1 antagonist telenzipine (1.7, 5.0, 17 or 50 nmol/side), oxotremorine with the selective m2 antagonist methoctramine (1.7, 5.0, 17 or 50 nmol/side), oxotremorine with a combination of the above doses of telenzipine and methoctramine, or only vehicle, immediately after inhibitory avoidance training. Performance on a 48-hour retention test was significantly enhanced in oxotremorine-treated rats relative to vehicle-infused controls. Intra-BLA co-infusion of oxotremorine with either telenzipine (5, 17 or 50 nmol/side) or methoctramine (17 or 50 nmol/side) blocked the oxotremorine-induced enhancement. Combinations of these antagonists did not act additively to block memory enhancement by oxotremorine. These findings indicate that modulation of memory consolidation induced by cholinergic influences within the BLA requires activation of both m1 and m2 receptor synapses. Plausible mechanisms for m1- and m2-mediated influences on BLA circuitry are discussed.

Muscarinic and nicotinic cholinergic mechanisms in the mesostriatal dopamine systems

The striatum and its dense dopaminergic innervation originating in the midbrain, primarily from the substantia nigra pars compacta and the ventral tegmental area, compose the mesostriatal dopamine (DA) systems. The nigrostriatal system is involved mainly in motor coordination and in disorders such as Tourette's syndrome, Huntington's disease, and Parkinson's disease. The dopaminergic projections from the ventral tegmental area to the striatum participate more in the processes that shape behaviors leading to reward, and addictive drugs act upon this mesolimbic system. The midbrain DA areas receive cholinergic innervation from the pedunculopontine tegmentum and the laterodorsal pontine tegmentum, whereas the striatum receives dense cholinergic innervation from local interneurons. The various neurons of the mesostriatal systems express multiple types of muscarinic and nicotinic acetylcholine receptors as well as DA receptors. Especially in the striatum, the dense mingling of dopaminergic and cholinergic constituents enables potent interactions. Evidence indicates that cholinergic and dopaminergic systems work together to produce the coordinated functioning of the striatum. Loss of that cooperative activity contributes to the dysfunction underlying Parkinson's disease.

Modulatory action of acetylcholine on striatal neurons: microiontophoretic study in awake, unrestrained rats

Cholinergic interneurons innervate virtually all medium spiny striatal cells, but the relevance of this input in regulating the activity and afferent responsiveness of these cells remains unclear. Studies in anaesthetized animals and slice preparations have shown that iontophoretic acetylcholine (ACh) either weakly excites or inhibits striatal neurons. These differential responses may reflect cholinergic receptor heterogeneity but may be also related to the different activity states of recorded units and different afferent inputs specific in each preparation. Single-unit recording was combined with iontophoresis in awake, unrestrained rats to examine the effects of ACh and selective muscarinic (oxotremorine M or Oxo-M) and nicotinic agonists (nicotine or NIC) on dorsal and ventral striatal neurons. These effects were tested on naturally silent, spontaneously active and glutamate-stimulated units. We found that iontophoretic ACh primarily inhibited spontaneously active and glutamate-stimulated units; the direction of the ACh response, however, was dependent on the firing rate. The effects of ACh were generally mimicked by Oxo-M and, surprisingly, by NIC, which is known to excite units in most central structures, including striatal neurons in anaesthetized preparation. Given that NIC receptors are absent on striatal cells but located primarily on dopamine terminals, we assessed the effects of NIC after complete blockade of dopamine receptors induced by systemic administration of a mixture of D1 and D2 antagonists. During dopamine receptor blockade the number of NIC-induced inhibitions dramatically decreased and NIC had mainly excitatory effects on striatal neurons. Thus, our data suggest that under physiologically relevant conditions ACh acts as a state-dependent neuromodulator, and its action involves not only postsynaptic but also presynaptic cholinoreceptors located on dopamine- and glutamate-containing terminals.

Role of muscarinic receptors in the activation of the ventral subiculum and the consequences for dopamine release in the nucleus accumbens

The nucleus accumbens receives limbic inputs from a number of brain regions, including the ventral subiculum. In rats, activation of the ventral subiculum following microinjection of N-methyl-D-aspartate (NMDA) or carbachol increases locomotor activity, whilst ventral subiculum application of NMDA also increases dopamine efflux in the ipsilateral nucleus accumbens. Microdialysis experiments were therefore conducted to ascertain the consequences for dopamine release in the nucleus accumbens following ventral subiculum administration of carbachol, and to explore the acetylcholine receptor subtype(s) that might be involved. We report that, in anaesthetised rats, ventral subiculum administration of carbachol increased dopamine levels in the nucleus accumbens. The response was attenuated by co-administration with atropine, whilst administration of nicotine and the alpha-7 nicotinic acetylcholine receptor agonist AR-R17779 (spiro[1-azabicyclo[2,2,2]octane-3,5'-oxazolidine]-2'-one monohydrochloride) failed to evoke a response. Oxotremorine-M produced a dose-dependent increase in dopamine efflux confirming sensitivity to muscarinic receptor stimulation. However, the ventral subiculum was insensitive to xanomeline and pilocarpine, muscarinic M(1) receptor-preferring agonists, but sensitive to BuTAC ([5R-[exo]-6-[butylthio]-1,2,5-thiadiazol-3-yl]-1-azabicyclo[3.2.1])octane), a muscarinic M(2)/M(4) receptor agonist. The dopamine response to oxotremorine-M was significantly attenuated, although not abolished by co-administration with the M(2)/M(4) receptor antagonist methoctramine, and studies combining oxotremorine-M with (-)-bicuculline, indicated a dual action in the ventral subiculum that was dependent and independent of reduced GABA neurotransmission. The data presented indicates that activation of the ventral subiculum by carbachol increases dopamine efflux in the nucleus accumbens by stimulation of muscarinic receptors, and that the ventral subiculum-nucleus accumbens projection system is sensitive to muscarinic M(2)/M(4) receptor stimulation.

Muscarinic and nicotinic presynaptic modulation of EPSCs in the nucleus accumbens during postnatal development

We have studied the modulatory effects of cholinergic agonists on excitatory postsynaptic currents (EPSCs) in nucleus accumbens (nAcb) neurons during postnatal development. Recordings were obtained in slices from postnatal day 1 (P1) to P27 rats using the whole cell patch-clamp technique. EPSCs were evoked by local electrical stimulation, and all experiments were conducted in the presence of bicuculline methchloride in the bathing medium and with QX-314 in the recording pipette. Under these conditions, postsynaptic currents consisted of glutamatergic EPSCs typically consisting of two components mediated by AMPA/kainate (KA) and N-methyl-D-aspartate (NMDA) receptors. The addition of acetylcholine (ACh) or carbachol (CCh) to the superfusing medium resulted in a decrease of 30-60% of both AMPA/KA- and NMDA-mediated EPSCs. In contrast, ACh produced an increase ( approximately 35%) in both AMPA/KA and NMDA receptor-mediated EPSCs when administered in the presence of the muscarinic antagonist atropine. These excitatory effects were mimicked by the nicotinic receptor agonist 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP) and blocked by the nicotinic receptor antagonist mecamylamine, showing the presence of a cholinergic modulation mediated by nicotinic receptors in the nAcb. The antagonistic effects of atropine were mimicked by pirenzepine, suggesting that the muscarinic depression of the EPSCs was mediated by M(1)/M(4) receptors. In addition, the inhibitory effects of ACh on NMDA but not on AMPA/KA receptor-mediated EPSC significantly increased during the first two postnatal weeks. We found that, under our experimental conditions, cholinergic agonists produced no changes on membrane holding currents, on the decay time of the AMPA/KA EPSC, or on responses evoked by exogenous application of glutamate in the presence of tetrodotoxin, but they produced significant changes in paired pulse ratio, suggesting that their action was mediated by presynaptic mechanisms. In contrast, CCh produced consistent changes in the membrane and firing properties of medium spiny (MS) neurons when QX-314 was omitted from the recording pipette solution, suggesting that this substance actually blocked postsynaptic cholinergic modulation. Together, these results suggest that ACh can decrease or increase glutamatergic neurotransmission in the nAcb by, respectively, acting on muscarinic and nicotinic receptors located on excitatory terminals. The cholinergic modulation of AMPA/KA and NMDA receptor-mediated neurotransmission in the nAcb during postnatal development could play an important role in activity-dependent developmental processes in refining the excitatory drive on MS neurons by gating specific inputs.

Multiple muscarinic acetylcholine receptor subtypes modulate striatal dopamine release, as studied with M1-M5 muscarinic receptor knock-out mice

A proper balance between striatal muscarinic cholinergic and dopaminergic neurotransmission is required for coordinated locomotor control. Activation of striatal muscarinic acetylcholine receptors (mAChRs) is known to modulate striatal dopamine release. To identify the mAChR subtype(s) involved in this activity, we used genetically altered mice that lacked functional M1-M5 mAChRs [knock-out (KO) mice]. In superfused striatal slices from wild-type mice, the non-subtype-selective muscarinic agonist oxotremorine led to concentration-dependent increases in potassium-stimulated [3H]dopamine release (by up to 60%). The lack of M1 or M2 receptors had no significant effect on the magnitude of these responses. Strikingly, oxotremorine-mediated potentiation of stimulated striatal [3H]dopamine release was abolished in M4 receptor KO mice, significantly increased in M3 receptor-deficient mice, and significantly reduced (but not abolished) in M5 receptor KO mice. Additional release studies performed in the presence of tetrodotoxin suggested that the dopamine release-stimulating M4 receptors are probably located on neuronal cell bodies, but that the release-facilitating M5 and the release-inhibiting M3 receptors are likely to be located on nerve terminals. Studies with the GABA(A) receptor blocker bicuculline methochloride suggested that M3 and M4 receptors mediate their dopamine release-modulatory effects via facilitation or inhibition, respectively, of striatal GABA release. These results provide unambiguous evidence that multiple mAChR subtypes are involved in the regulation of striatal dopamine release. These findings should contribute to a better understanding of the important functional roles that the muscarinic cholinergic system plays in striatal function.

Intrinsic membrane properties and synaptic inputs regulating the firing activity of the dopamine neurons

Dopamine (DA) neurones of the ventral mesencephalon are involved in the control of reward related behaviour, cognitive functions and motor performances, and provide a critical site of action for major categories of neuropsychiatric drugs, such as antipsychotic agents, dependence producing drugs and anti-Parkinson medication. The midbrain DA neurones are mainly located in the substantia nigra pars compacta (SNPC) and the ventral tegmental area (VTA). Intrinsic membrane properties regulate the activity of these neurones. In fact, they possess several conductances that allow them to fire in a slow pacemaker-like mode. The internal set of membrane currents interact with afferent synaptic inputs which, especially in in vivo conditions, contribute to accelerate or decelerate the firing activity of the cells in accordance with the necessity to optimise the release of dopamine in the terminal fields. In particular, discrete excitatory and inhibitory inputs transform the firing from a low regular into a bursting pattern. The bursting activity promotes dopamine release being very important in cognition and motor performances. In the present paper we review electrophysiological data regarding the role of glutamatergic and cholinergic and GABAergic afferent inputs in regulating the midbrain DAergic neuronal activity.

Dual cholinergic control of fast-spiking interneurons in the neostriatum

GABAergic interneurons appear to play a fundamental role in the functioning of the neostriatum by modulating the spiking of striatal projection neurons with great efficacy. The powerful and strongly divergent output of the GABAergic interneurons neurons suggests that modulation of their activity may be particularly effective at controlling the functioning of the entire neostriatal circuitry. Acetylcholine is one of the main modulators of striatal functioning. The effects of acetylcholine on fast-spiking (FS) GABAergic interneurons were studied with whole-cell recording in an in vitro slice preparation. Acetylcholine exerted two distinct effects on fast-spiking interneurons. Acetylcholine directly depolarized FS interneurons by acting on nondesensitizing soma-dendritic nicotinic receptors. In addition, acetylcholine attenuated the GABAergic inhibition of projection neurons by fast-spiking interneurons through activation of presynaptic muscarinic receptors. It is suggested that the nicotinic excitation of FS interneurons may play an important role in translating the effect of the brief behaviorally contingent cessation of firing of the tonically active cholinergic interneurons to the output neurons of the neostriatum. In contrast, the muscarinic presynaptic inhibitory mechanism may be engaged primarily during longer-lasting elevations of extracellular acetylcholine levels.

Dual cholinergic control of fast-spiking interneurons in the neostriatum

GABAergic interneurons appear to play a fundamental role in the functioning of the neostriatum by modulating the spiking of striatal projection neurons with great efficacy. The powerful and strongly divergent output of the GABAergic interneurons neurons suggests that modulation of their activity may be particularly effective at controlling the functioning of the entire neostriatal circuitry. Acetylcholine is one of the main modulators of striatal functioning. The effects of acetylcholine on fast-spiking (FS) GABAergic interneurons were studied with whole-cell recording in an in vitro slice preparation. Acetylcholine exerted two distinct effects on fast-spiking interneurons. Acetylcholine directly depolarized FS interneurons by acting on nondesensitizing soma-dendritic nicotinic receptors. In addition, acetylcholine attenuated the GABAergic inhibition of projection neurons by fast-spiking interneurons through activation of presynaptic muscarinic receptors. It is suggested that the nicotinic excitation of FS interneurons may play an important role in translating the effect of the brief behaviorally contingent cessation of firing of the tonically active cholinergic interneurons to the output neurons of the neostriatum. In contrast, the muscarinic presynaptic inhibitory mechanism may be engaged primarily during longer-lasting elevations of extracellular acetylcholine levels.

Protein kinase C is involved in M1-muscarinic receptor-mediated facilitation of L-type Ca2+ channels in neurons of the major pelvic ganglion of the adult male rat

We used patch clamp recording techniques to determine if muscarinic signaling mechanisms are present in dissociated autonomic neurons obtained from the major pelvic ganglion, which provides the cholinergic innervation of the urinary bladder and other pelvic organs. The M1 specific agonist, McN-A-343 (2-30 microM) enhanced Ca2+ currents in approximately 37% of neurons (by 50-80%). This enhancement was reduced by atropine (5-10 microM) or a PKC inhibitor (bisindolylmaleimide, 50-200 nM). In responsive neurons Ca2+ currents were also enhanced by the phorbol ester, phorbol-12,13-dibutyrate (50-300 nM) and the dihydropyridine agonist Bay K 8644 (5 microM) and had kinetics of activation and inactivation as expected for L-type Ca2+ channels. We conclude that in a subpopulation of MPG neurons, M1-mediated activation of PKC phosphorylates and enhances L-type Ca2+ channel activities. This muscarinic facilitatory mechanism in MPG neurons may be the same as the M1-mediated facilitation of transmitter release reported previously at the nerve terminals in the urinary bladder.

Muscarinic activation of inwardly rectifying K(+) conductance reduces EPSPs in rat hippocampal CA1 pyramidal cells

1. To determine how acetylcholine (ACh) modulates the somatodendritic processing of EPSPs, we performed whole-cell recordings from CA1 pyramidal cells of hippocampal slices and examined the effect of the cholinergic agonist, carbachol (CCh), on alpha-amino-3-hydroxy-5-methyl isoxazole-4-propionate (AMPA) EPSPs, miniature EPSPs, and EPSP-like waveforms evoked by brief dendritic glutamate pulses (glutamate-evoked postsynaptic potentials, GPSPs). 2. Although CCh is known to enhance the intrinsic excitability of the neuron in several ways, activation of atropine-sensitive (muscarinic) receptors on the apical dendrite or the soma of CA1 pyramidal cells consistently reduced the amplitude of EPSPs and GPSPs. 3. Cholinergic inhibition of evoked and simulated EPSP waveforms displayed considerable voltage dependence, with the amplitude of the postsynaptic potentials progressively declining with membrane hyperpolarization indicating the involvement of an inwardly rectifying current. 4. Extracellular Ba(2+) (200 microM) and tertiapin (30 nM), a novel and selective blocker of G protein-activated, inwardly rectifying K(+) (GIRK) channels, completely blocked the effect of CCh on GPSP amplitude. 5. Muscarinic reduction of GPSPs was not sensitive to the M1 receptor-preferring antagonist, pirenzepine, but was suppressed by the M2 receptor-preferring antagonist, methoctramine, and by the allosteric M2 receptor antagonist, gallamine. 6. In voltage-clamp recordings, CCh induced an ion current displaying inward rectification in the hyperpolarizing direction, which was identified as a GIRK current based on its sensitivity to low Ba(2+) and tertiapin. Its pharmacological profile paralleled that of the cholinergic GPSP reduction. 7. We link the observed reduction of postsynaptic potentials to the cholinergic activation of a GIRK conductance, which serves to partially shunt excitatory synaptic input.

Mitral cell presynaptic Ca(2+) influx and synaptic transmission in frog amygdala

Dextran-conjugated Ca(2+) indicators were injected into the accessory olfactory bulb of frogs in vivo to selectively fill presynaptic terminals of mitral cells at their termination in the ipsilateral amygdala. After one to three days of uptake and transport, the forebrain hemisphere anterior to the tectum was removed and maintained in vitro for simultaneous electrophysiological and optical measurements. Ca(2+) influx into these terminals was compared to synaptic transmission between mitral cells and amygdala neurons under conditions of reduced Ca(2+) influx resulting from reduced extracellular [Ca(2+)], blockade of N- and P/Q-type channels, and application of the cholinergic agonist carbachol. Reducing extracellular [Ca(2+)] had a non-linear effect on release; release was proportional to Ca(2+) influx raised to the power of approximately 3.6, as observed at numerous other synapses. The N-type Ca(2+) channel blocker, omega-conotoxin-GVIA (1 microM), blocked 77% of Ca(2+) influx and 88% of the postsynaptic field potential. The P/Q-type Ca(2+) channel blocker, omega-agatoxin-IVA (200 nM), blocked 19% of Ca(2+) influx and 25% of the postsynaptic field, while the two toxins combined to block 92% of Ca(2+) influx and 97% of the postsynaptic field. The relationship between toxin blockade of Ca(2+) influx and synaptic transmission was therefore only slightly non-linear; release was proportional to Ca(2+) influx raised to the power approximately 1.4. Carbachol (100 microM) acting via muscarinic receptors had no effect on the afferent volley, but rapidly and reversibly reduced Ca(2+) influx through both N- and P/Q-type channels by 51% and postsynaptic responses by 78%, i.e. release was proportional to Ca(2+) raised to the power approximately 2.5. The weak dependence of release on changes in Ca(2+) when channel toxins block channels suggests little overlap between Ca(2+) microdomains from channels supporting release or substantial segregation of channel subtypes between terminals. The proportionately greater reduction of transmission by muscarinic receptors compared to Ca(2+) channel toxins suggests that they directly affect the release machinery in addition to reducing Ca(2+) influx.

Coordinated expression of muscarinic receptor messenger RNAs in striatal medium spiny neurons

The postsynaptic effects of acetylcholine in the striatum are largely mediated by muscarinic receptors. Two of the five cloned muscarinic receptors (M1 and M4) are expressed at high levels by the medium spiny neurons-the principal projection neurons of the striatum. Previous studies have suggested that M4 muscarinic receptors are found primarily in medium spiny neurons that express substance P and participate in the "direct" striatonigral pathway. This view is difficult to reconcile with electrophysiological studies suggesting that nearly all medium spiny neurons exhibit responses characteristic of M4 receptors. To explore this apparent discrepancy, the coordinated expression of M1-M5 receptor messenger RNAs in identified medium spiny neurons was assayed using single-cell reverse transcription-polymerase chain reaction techniques. Nearly all medium spiny neurons had detectable levels of M1 receptor messenger RNA. Although M4 receptor messenger RNA was detected more frequently in substance P-expressing neurons (70%), it was readily seen in a substantial population of enkephalin-expressing neurons (50%). To provide a quantitative estimate of transcript abundance, quantitative reverse transcription-polymerase chain reaction experiments were performed. These studies revealed that M4 messenger RNA was expressed by both substance P and enkephalin neurons, but was roughly five-fold higher in abundance in substance P-expressing neurons. This quantitative difference provides a means of reconciling previous estimates of M4 receptor distribution and function.

Short-Term Plasticity at Inhibitory Synapses in Rat Striatum and Its Effects on Striatal Output

Two forms of short-term plasticity at inhibitory synapses were investigated in adult rat striatal brain slices using intracellular recordings. Intrastriatal stimulation in the presence of the ionotropic glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (20 μM) andd,l-2-amino-5-phosphonovaleric acid (50 μM) produced an inhibitory postsynaptic potential (IPSP) that reversed polarity at −76 ± 1 (SE) mV and was sensitive to bicuculline (30 μM). The IPSP rectified at hyperpolarized membrane potentials due in part to activation of K+ channels. The IPSP exhibited two forms of short-term plasticity, paired-pulse depression (PPD) and synaptic augmentation. PPD lasted for several seconds and was greatest at interstimulus intervals (ISIs) of several hundred milliseconds, reducing the IPSP to 80 ± 2% of its control amplitude at an ISI of 200 ms. Augmentation of the IPSP, elicited by a conditioning train of 15 stimuli applied at 20 Hz, was 119 ± 1% of control when sampled 2 s after the conditioning train. Augmentation decayed with a time constant of 10 s. We tested if PPD and augmentation modify the ability of the IPSP to prevent the generation of action potentials. A train of action potentials triggered by a depolarizing current injection of constant amplitude could be interrupted by stimulation of an IPSP. If this IPSP was the second in a pair of IPSPs, it was less effective in blocking spikes due to PPD. By contrast, augmented IPSPs were more effective in blocking spikes. The same results were achieved when action potentials were triggered by a depolarizing current injection of varying amplitude, a manipulation that produces nearly identical spike times from trial to trial and approximates the in vivo behavior of these neurons. These results demonstrate that short-term plasticity of inhibition can modify the output of the striatum and thus may be an important component of information processing during behaviors that involve the striatum.

A predicted cortical serotonergic/cholinergic/GABAergic interface as a site of pathology in schizophrenia

1. The pathological process that precipitates schizophrenia has yet to be identified. However, many lines of evidence suggest that a change in the functioning of the frontal cortex is an important abnormality that underlies schizophrenia. 2. Studies in Brodmann's area 9, obtained post-mortem, have shown changes in 5-hydroxytryptamine 5-HT2A, muscarinic M1 and GABA(A) receptors in tissue from subjects with schizophrenia. 3. Animal studies suggest a site in the cortex where there would be an interaction between serotonergic and cholinergic innervation and that this interaction would involve the 5-HT2A and the M1 receptor. This site, in turn, would be a potent modulator of GABA activity and, hence, levels of GABA(A) receptors. 4. From combining these data, a theoretical site is proposed that, if proven to exist in human cortex, is likely to be central to the pathology of that illness.

Muscarinic agonist carbachol depresses excitatory synaptic transmission in the rat basolateral amygdala in vitro

Intracellular recordings in slice preparations of the basolateral amygdala were used to test which excitatory amino acid receptors mediate the excitatory postsynaptic potentials due to stimulation of the external capsule. These recordings were also used to examine the action of muscarinic agonists on the evoked excitatory potentials. Intracellular recordings from amygdaloid pyramidal neurons revealed that carbachol (2-20 microM) suppressed, in a dose-dependent manner, excitatory postsynaptic responses evoked by stimulation of the external capsule (EC). This effect was blocked by atropine. The estimated effective concentration to produce half-maximal response (EC(50)) was 6.2 microM. Synaptic suppression was observed with no changes in the input resistance of the recorded cells, suggesting a presynaptic mechanism. In addition, the results obtained using the paired-pulse protocol provided additional support for a presynaptic action of carbachol. To identify which subtype of cholinergic receptors were involved in the suppression of the EPSP, four partially selective muscarinic receptor antagonists were used at different concentrations: pirenzepine, a compound with a similar high affinity for muscarinic M1 and M4 receptors; gallamine, a noncompetitive antagonist for M2; methoctramine, an antagonist for M2 and M4; and 4-diphenylacetoxy-N-methylpiperidine, a compound with similar high affinity for muscarinic receptors M1 and M3. None of them independently antagonized the suppressive effect of carbachol on the evoked EPSP completely, suggesting that more than one muscarinic receptor subtype is involved in the effect. These experiments provide evidence that in the amygdala muscarinic agonists block the excitatory synaptic response, mediated by glutamic acid, by acting on several types of presynaptic receptors.

Cholinergic modulation of cortical function: a hypothetical role in shifting the dynamics in cortical network

Wide innervation of cholinergic projections throughout the cortex implies that acetylcholine (ACh) plays an essential role in information processing, but how it works is still enigmatic. Experimental as well as theoretical work in the olfactory cortex and hippocampus suggests that ACh, via the muscarinic receptors, serves to shift the dynamics of the cortical networks into a state where afferent influence predominates over intracortical influence. Recent experiments in the visual and somatosensory cortex suggested that this hypothesis could be extended to neocortex. In addition, participation of the nicotinic receptors in regulating the synaptic response in the somatosensory cortex further substantiates this hypothesis. This hypothesis, derived mainly from in vitro work, also seemed to account for results from in vivo experiments without any obvious inconsistencies.

Local GABAergic modulation of acetylcholine release from the cortex of freely moving rats

Cortical perfusion with GABA agonists and antagonists modulates the spontaneous release of cortical acetylcholine and GABA in freely moving rats. Twenty-four hours after implantation of a dialysis fibre, cerebral cortex spontaneously released acetylcholine (3.8 +/- 0.2 pmol/10 min) and GABA (6.6 +/- 0.4 pmol/10 min) at a stable rate. Local administration of GABA (1 or 5 mM) or the GABAA agonist muscimol (25 or 50 microM) had no effect on the spontaneous release of acetylcholine. However, bicuculline (1-25 microM), a GABAA antagonist, added to the dialysis perfusate, elicited a concentration-dependent increase of acetylcholine release to approximately double that of control. This effect of bicuculline (25 microM) was completely prevented by coperfusion with muscimol (50 microM). Local administration of the GABAB receptor agonist baclofen (10 or 50 microM) elicited a concentration-dependent increase in spontaneous acetylcholine release with a maximal increase of about 60%. Intracortical administration of baclofen also decreased the spontaneous release of GABA. The GABAB receptor antagonist CGP 35348 (1 mM), administered alone for 20 min through the dialysis fibre, was without effect on spontaneous acetylcholine release; however, it completely blocked both the baclofen-induced increase in acetylcholine release and the decrease in GABA release. These results suggest that cortically released GABA exerts a tonic influence on cholinergic activity.

Muscarinic receptors depress GABAergic synaptic transmission in rat midbrain dopamine neurons

The effects of muscarine and nicotine on evoked and spontaneous release of GABA were studied using intracellular and whole-cell patch-clamp recordings from rat midbrain dopamine neurons in an in vitro slice preparation. Muscarine (30 microM) reversibly depressed the pharmacologically isolated inhibitory postsynaptic potential evoked by local electrical stimulation. The maximal inhibition of the inhibitory postsynaptic potential amplitude was 39.6+/-5%. This depressant effect of muscarine was blocked by the M3/M1 receptor antagonist 4-diphenylacetoxy-N-methylpiperidine methiodide (100 nM), but was slightly affected by the M1/M3 receptor antagonist pirenzepine (1 microM). In addition, muscarine decreased the frequency of the miniature synaptic currents without any effect on their amplitude. Moreover, muscarine did not change the GABA-induced hyperpolarization, indicating that its effect on the inhibitory postsynaptic potential is mediated by presynaptic receptors. On the contrary, the cholinergic agonist nicotine did not change the frequency or the amplitude of the spontaneous glutamatergic and GABAergic synaptic currents. Our data indicate that a prevalent activation of presynaptic M3 muscarinic receptors inhibits the GABA-mediated synaptic events, while the activation of nicotinic receptors does not affect the release of glutamate and GABA on midbrain dopamine neurons.

Dendritic and axonal targeting of the vesicular acetylcholine transporter to membranous cytoplasmic organelles in laterodorsal and pedunculopontine tegmental nuclei

Autoregulation of cholinergic neurons in the laterodorsal tegmental (LDT) and pedunculopontine (PPT) nuclei has been implicated in many functions, most importantly in drug reinforcement and in the pathophysiology of schizophrenia. This autoregulation is attributed to the release of acetylcholine, but neither the storage or release sites are known. To determine these sites, we used electron microscopy for the immunocytochemical localization of antipeptide antiserum raised against the vesicular acetylcholine transporter (VAchT) that is responsible for the uptake of acetylcholine into storage vesicles. The cellular and subcellular distribution of VAchT was remarkably similar in the two regions by by using each of two methods, immunogold and immunoperoxidase. In both PPT and LDT nuclei, VAchT labeling was seen mainly on membranous organelles including the trans-Golgi network in many somata. VAchT-immunoreactive tubulovesicles resembling saccules of smooth endoplasmic reticulum were often seen near the plasma membrane in dendrites. The VAchT-containing dendrites comprised almost 50% of the labeled profiles (1027/2129) in PPT and LDT nuclei. The remaining VAchT-immunoreactive profiles were primarily small unmyelinated axons and axon terminals. In axon terminals, VAchT was densely localized to membranes of small synaptic vesicles. The VAchT-immunoreactive axon terminals formed either symmetric or asymmetric synapses. The postsynaptic targets of these axon terminals included dendrites that were with (36/110) or without (74/110) VAchT immunoreactivity. Our results suggest that dendrites, as well as axon terminals, have the potential for storage and release of acetylcholine in the LDT and PPT nuclei. The released acetylcholine is likely to play a major role in autoregulation of mesopontine cholinergic neurons.

Acetylcholine-mediated modulation of striatal function

Striatal spiny neurones serve as a major anatomical locus for the relay of cortical information flow through the basal ganglia. these projection neurones also represent the main synaptic target of cholinergic interneurones, whose physiological role in striatal activity still remains largely enigmatic. The striatal cholinergic system has been implicated in the pathophysiology of movement disorders such as Parkinson's disease, but the cellular mechanisms underlying cholinergic-neurone function are still unknown. On the basis of in vitro electrophysiological evidence, obtained from a rat corticostriatal-slice preparation, we propose that endogenous ACh exerts a complex modulation of striatal synaptic transmission, which produces both short-term and long-term effects. ACh-mediated mechanisms might be of crucial importance in processing the cortical inputs to the striatum.

Acetylcholine receptor activation enhances NMDA-mediated responses in the rat neostriatum

The influence of acetylcholine (ACh) upon N-methyl-D-aspartate (NMDA) receptor activation of neostriatal neurons is unknown. In the present study, we used both in vitro intracellular and in vivo electroencephalographic recordings in rats to examine this question. In vitro, iontophoretic application of carbachol, a cholinergic receptor agonist, significantly increased the NMDA-mediated response of neostriatal projection neurons. Carbachol alone had mild excitatory effects. In vivo, intrastriatal NMDA produced focal epileptiform activity restricted to the neostriatum. NMDA applied in conjunction with carbachol produced significantly greater epileptiform activity which propagated to the neocortex. These results suggest that ACh and NMDA receptor co-activation leads to potentiation of the neuronal responses both at the site of the interaction and at the endpoint of the cortico-striato-cortical circuit.

Muscarinic control of intracortical connections to layer II in rat entorhinal cortex slice

The cholinergic system is critically involved in oscillatory network activity and synaptic plasticity in the entorhinal cortex (EC) hippocampal formation. Here we demonstrate robust inhibition of field potentials in layer II of the medial EC evoked by stimulation in the deep EC or in the lateral layer II by carbachol (CCh, 0.1-100 microM, K(D) approximately 1 microM). This effect appears not to be mediated by suppression of presynaptic Ca(2+)-signals since paired pulse facilitation was increased by CCh. Blockade of the effect by the muscarinic antagonists atropine and pirenzepine demonstrates mediation by muscarinic receptors, most likely of the M1 subtype. The effect is characterized by absence of desensitization and should be important for laminar shaping of oscillatory activity and synaptic plasticity during acetylcholine-dependent theta-rhythmic activity.

Muscarinic acetylcholine receptors in the hippocampus, neocortex and amygdala: a review of immunocytochemical localization in relation to learning and memory

Immunocytochemical mapping studies employing the extensively used monoclonal anti-muscarinic acetylcholine receptor (mAChR) antibody M35 are reviewed. We focus on three neuronal muscarinic cholinoceptive substrates, which are target regions of the cholinergic basal forebrain system intimately involved in cognitive functions: the hippocampus; neocortex; and amygdala. The distribution and neurochemistry of mAChR-immunoreactive cells as well as behaviorally induced alterations in mAChR-immunoreactivity (ir) are described in detail. M35+ neurons are viewed as cells actively engaged in neuronal functions in which the cholinergic system is typically involved. Phosphorylation and subsequent internalization of muscarinic receptors determine the immunocytochemical outcome, and hence M35 as a tool to visualize muscarinic receptors is less suitable for detection of the entire pool of mAChRs in the central nervous system (CNS). Instead, M35 is sensitive to and capable of detecting alterations in the physiological condition of muscarinic receptors. Therefore, M35 is an excellent tool to localize alterations in cellular cholinoceptivity in the CNS. M35-ir is not only determined by acetylcholine (ACh), but by any substance that changes the phosphorylation/internalization state of the mAChR. An important consequence of this proposition is that other neurotransmitters than ACh (especially glutamate) can regulate M35-ir and the cholinoceptive state of a neuron, and hence the functional properties of a neuron. One of the primary objectives of this review is to provide a synthesis of our data and literature data on mAChR-ir. We propose a hypothesis for the role of muscarinic receptors in learning and memory in terms of modulation between learning and recall states of brain areas at the postsynaptic level as studied by way of immunocytochemistry employing the monoclonal antibody M35.

Epileptiform activity generated by endogenous acetylcholine during blockade of GABAergic inhibition in immature and adult rat hippocampus

We tested the effects of the acetylcholinesterase inhibitor eserine (10 microM), an indicator of the activity of endogenous ACh, on the generation of epileptiform discharges during blockade of inhibitory GABA(A)-mediated potentials by bicuculline (10 microM), in the CA3 area of hippocampal slices from postnatal days 4-20 (P4-P20) immature and adult rats. Eserine provoked or significantly increased the frequency of spontaneous synchronous epileptiform discharges, in 6/22 (27%) P4-P10 slices, 34/35 P11-P20 slices and 18/18 adult slices, an epileptogenic effect. In immature slices, spontaneous discharges showed a stable frequency throughout perfusion with eserine, while in 5/11 adult slices an initial fast frequency was followed by a slower steady-state one. The cholinergic agonist carbachol (CCh, 25 microM) provoked only transient or no spontaneous synchronous discharges in adult slices (n=8), thus suggesting that massive activation of cholinergic receptors may lead to suppression of epileptiform activity in adult brain. Stimulus-induced excitatory CA3 responses, were depressed by eserine in approximately half of 20 P4-P10, 45 P11-P20 and 11 adult slices. The depression consisted of a decrease in the amplitude, duration, and number of population spikes of the field potentials by about 30%, a minor neuroprotective effect, which did not change with maturation. The different developmental profiles of the epileptogenic and neuroprotective effects of endogenous ACh suggest that they are mediated by different mechanisms. These experiments demonstrate that, endogenous ACh is sufficient to induce epileptogenesis during a decrease or failure of GABAergic inhibition, in both >/=P10 immature and in adult hippocampus. We therefore suggest that clinical or behavioral conditions which raise the concentration of endogenous ACh may lower the threshold to seizures.

Muscarinic presynaptic inhibition of neostriatal glutamatergic afferents is mediated by Q-type Ca2+ channels

Cholinergic presynaptic inhibition was investigated on neostriatal glutamatergic transmission. Paired pulse facilitation (PPF) of orthodromic population spikes (PS) were used to construct a concentration-response relationship for muscarine on presynaptic inhibition. Muscarine had an effect proportional to its extracellular concentration with an EC50 (mean +/- standard estimation error) of: 2.5 +/- 1.5 nM, and a maximal effect (saturation) of 245 +/- 16%. Several peptidic toxins against some voltage-gated Ca2+-channels increased PPF indicating that the Ca2+-channels they block participate in transmitter release. However, neither 1 microM omega-conotoxin GVIA, a specific blocker of N-type Ca2+-channels, nor 10-30 nM omega-agatoxinTK, a selective blocker of P-type Ca2+-channels, were able to occlude muscarine's effect on presynaptic inhibition. Nevertheless, 100-400 nM omega-agatoxinTK occluded muscarine's action on PPF in a dose-dependent manner. These results are consistent with Q-type Ca2+-channels mediating muscarinic presynaptic inhibition of neostriatal afferents.

Effects of medial prefrontal cortical injections of GABA receptor agonists and antagonists on the local and nucleus accumbens dopamine responses to stress

Stress stimulates dopamine (DA) release in nucleus accumbens (NAcc) but will do so more strongly in medial prefrontal cortex (PFC). Evidence indicates, however, that the cortical DA response to stress acts to dampen the concurrent increase in NAcc DA release. In the present study, we used voltammetry to investigate the role of PFC GABA in regulating the NAcc DA response to stress. The results of Experiment 1 show that the NAcc stress response is inhibited following bilateral cortical microinjections of baclofen (GABAB receptor agonist). While phaclofen (GABAB receptor antagonist) blocked the effect of baclofen, it had no significant effect of its own. Intra-PFC injections of muscimol (GABAA receptor agonist) and bicuculline (GABAA receptor antagonist) had no effect on the DA stress response in NAcc. In Experiment 2, we explored the possibility that GABA influences the NAcc DA stress response indirectly by modulating stress-induced DA release in PFC. None of the drugs tested had an effect on the PFC stress response at a dose (1 nmol) that produced reliable effects on the NAcc stress response. At an order of magnitude higher dose, however, locally applied phaclofen and muscimol enhanced and attenuated, respectively, the DA stress response in PFC. These results were validated in Experiment 3 by showing that intra-PFC injections of GBR-12395 (DA uptake blocker) and quinpirole (D2/D3 receptor agonist) dose-dependently enhanced and inhibited, respectively, the local DA stress response. Together, these findings indicate that increased GABA transmission in PFC exerts an inhibitory influence on the NAcc DA response to stress, and that this action is mediated primarily but not exclusively by GABAB receptors which may be located both on cortical output neurons and on DA terminals.

Presynaptic muscarinic (M3) receptors reduce excitatory transmission in dopamine neurons of the rat mesencephalon

The effects of carbachol (0.01-30 microM) and muscarine (10-30 microM) on the excitatory synaptic potentials were studied using conventional intracellular recordings from dopaminergic neurons in rat mesencephalic slices. Both muscarinic agonists reversibly reduced the excitatory synaptic potentials, evoked by local electrical stimulation. The EC50 for carbachol was determined to be 4.5 microM. The maximal degree of the excitatory synaptic potentials suppression caused by carbachol and muscarine was around 40% of control. This suppression was completely blocked by the non-specific muscarinic antagonist atropine (1 microM) and the selective M3 antagonist 4-diphenylacetoxy-N-methylpiperidine methiodide (1 microM). Other antagonists, preferentially acting at M1, M2 and M4 receptors, were not effective. Furthermore, the acetylcholinesterase inhibitor, physostigmine (50 microM), decreased the amplitude of the excitatory synaptic potentials, indicating that ambient acetylcholine can depress this potential. Direct depolarizing responses to glutamate were not changed by muscarine. In addition, muscarine facilitated the second excitatory synaptic potentials during a paired-pulse protocol. Thus, the effect of the muscarinic agonists is attributable to a presynaptic locus of action. The action of muscarine was not mediated by an N-ethylmaleimide-sensitive G-protein since it was not modified by a treatment of the slices with this agent. The calcium channels blockers, omega-conotoxin GIVA, omega-agatoxin IVA and omega-conotoxin MVIIC did not affect the action of muscarine on the excitatory synaptic potentials. When the potassium currents were reduced by extracellular barium and 4-aminopyridine, the muscarinic agonists still depressed the excitatory synaptic potentials. Our data indicate that presynaptically located M3 receptors modulate the excitatory transmission to midbrain dopaminergic neurons via a N-ethylmaleimide-insensitive G-protein which activates mechanisms neither linked to N-, P-, Q-type calcium channels nor to barium- and 4-aminopyridine-sensitive potassium channels.

Activation of muscarinic M3-like receptors and beta-adrenoceptors, but not M2-like muscarinic receptors or alpha-adrenoceptors, directly modulates corticostriatal neurotransmission in vitro

The aim of this study was to characterize the modulation of synaptic transmission in the glutamatergic corticostriatal pathway by cholinergic and adrenergic receptors. In coronal slices of mouse brain, negative-going field potentials were recorded in the dorsal striatum in response to stimulation of the overlying white matter, and their susceptibility to various pharmacological manipulations was studied. The responses were mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors, since they were augmented by aniracetam (0.5-1.5 mM), a positive modulator of AMPA-type glutamate receptors, and blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (> or = 10 microM), a selective antagonist of AMPA receptors. Carbachol (10 microM), a muscarinic agonist, reduced the size of responses and abolished paired-pulse depression; these effects being consistent with previous studies indicating that muscarinic activation inhibits release of glutamate in the corticostriatal pathway. Muscarinic antagonists could block the effect of carbachol. Their rank order was: 10 microM scopolamine (a non-selective muscarinic antagonist) > or = 1 microM 4-diphenylacetoxy-N-methyl-piperidine (M3/M1 antagonist)>1 microM pirenzepine (M1 antagonist)>10 microM methoctramine (M2 antagonist). McN-A-343 (1-10 microM), an M1 muscarinic agonist, was ineffective in this preparation. In contrast, isoproterenol (10-30 microM), a beta-adrenergic agonist, slightly increased the synaptic responses, but it did not affect paired-pulse depression. None of alpha-adrenergic agents (30 nM-1.0 microM dexmedetomidine, an alpha2-adrenergic agonist, 0.3 microM atipamezole, an alpha2-adrenergic antagonist or 30 microM phenylephrine, an alpha1-adrenergic agonist) influenced the size of the responses; neither did these drugs alter paired-pulse depression. These results indicate that the activation of striatal M3-like muscarinic receptors and beta-adrenoceptors, but not M2-like muscarinic receptors and alpha-adrenoceptors, modulates directly corticostriatal glutamatergic neurotransmission.

Antinociception after both peripheral and intrathecal injection of oxotremorine is modulated by spinal nitric oxide

The present study investigated the role of spinal nitric oxide (NO) in the antinociception induced by intraperitoneal (i.p.) and intrathecal (i.th.) injection of oxotremorine. The experiments were carried out on male Wistar rats, which had cannulas chronically implanted in the lumbar enlargement of the spinal cord. Antinociceptive effects were evaluated using a tail-flick and a paw pressure test. To raise the spinal NO level, the rats received the NO donor, 3-morpholino-sydnonimine (SIN-1, 10 and 100 microg/5 microl); to lower the NO level, the inhibitor of NO synthase, N-nitro-L-arginine methyl ester (L-NAME, 50 and 400 microg/5 microl), was administered. Both those substances were injected i.th. Systemic injections of oxotremorine (0.02 and 0.1 mg/kg) produced a significant increase in the thermal nociceptive threshold, while the mechanical threshold was affected only by the higher dose (0.1 mg/kg) of the muscarinic agonist. I.th. injections of oxotremorine (0.1 ng, 1 ng, 1 microg/5 microl) produced significant antinociception in both those tests. I.th. administration of SIN-1 in doses which themselves did not affect the nociceptive threshold antagonized both the peripheral and central oxotremorine antinociception. I.th. administration of L-NAME (50 and 400 microg/5 microl) did not change the nociceptive threshold, but dose-dependently potentiated the effects of oxotremorine injected i.p. in both tests; however, the effect of i.th. administration of oxotremorine was potentiated only in the tail-flick test. Our results demonstrate that irrespective of the way of its injection, the antinociceptive effect of oxotremorine is modulated by activity of the spinal NO. Moreover, our results further support the hypothesis that NO present in the spinal cord exerts pronociceptive effects.