Intrinsic cholinergic excitation in the rat neostriatum: nicotinic and muscarinic receptors

Mechanism of action of the nicotinic acetylcholine receptor

Nicotinic acetylcholine receptors at peripheral synapses mediate rapid and effective excitatory synaptic transmission. The functional properties of peripheral and central nicotinic acetylcholine receptors are similar, yet in the central nervous system nicotinic receptors do not appear to occur postsynaptically at many excitatory synapses. Two properties of nicotinic receptors are that significant Ca2+ influx can occur through the receptor channel and that at low agonist concentrations steady activation of nicotinic receptors can occur. These are discussed in the context of presynaptic and postsynaptic localizations of nicotinic receptors.

Action-potential-independent GABAergic tone mediated by nicotinic stimulation of immature striatal miniature synaptic transmission

Stimulation of presynaptic nicotinic acetylcholine receptors (nAChRs) increases the frequency of miniature excitatory synaptic activity (mEPSCs) to a point where they can promote cell firing in hippocampal CA3 neurons. We have evaluated whether nicotine regulation of miniature synaptic activity can be extended to inhibitory transmission onto striatal medium spiny projection neurons (MSNs) in acute brain slices. Bath application of micromolar nicotine typically induced 12-fold increases in the frequency of miniature inhibitory synaptic currents (mIPSCs). Little effect was observed on the amplitude of mIPSCs or mEPSCs under these conditions. Nicotine stimulation of mIPSCs was dependent on entry of extracellular calcium because removal of calcium from perfusate was able to block its action. To assess the potential physiological significance of the nicotine-stimulated increase in mIPSC frequency, we also examined the nicotine effect on evoked IPSCs (eIPSCs). eIPSCs were markedly attenuated by nicotine. This effect could be attributed to two potential mechanisms: transmitter depletion due to extremely high mIPSC rates and/or a reduction in presynaptic excitability associated with nicotinic depolarization. Treatment with low concentrations of K(+) was able to in part mimic nicotine's stimulatory effect on mIPSCs and inhibitory effect on eIPSCs. Current-clamp recordings confirmed a direct depolarizing action of nicotine that could dampen eIPSC activity leading to a switch to striatal inhibitory synaptic transmission mediated by tonic mIPSCs.

Altered distribution of striatal activity-dependent synaptic plasticity in the 3-nitropropionic acid model of Huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder characterized by involuntary choreiform movements, neuropsychiatric disturbances and cognitive decline. The hyperkinetic phenomenology has commonly been attributed to a disturbance of the basal ganglia function, mainly the neostriatum, but its pathophysiology mechanisms remain unclear. Activity-dependent long-term changes in synaptic efficacy, such as long-term potentiation (LTP) and long-term depression (LTD), are widely considered to be the cellular models for acquisition and storage of information in neuronal networks. Both LTP and LTD have been described at the corticostriatal pathway and they might be probably involved not only in physiological motor behavior processing but also in disease states affecting that pathway. Systemic injection of 3-nitropropionic acid (3-NP) induces excitotoxic striatal lesions and abnormal movements in rodents, resembling those seen in HD. We examined synaptic plasticity in dorsolateral striatum slices prepared from both control and 3-NP-treated rats by recording extracellular field potentials. Our results reinforce the idea that both forms of activity-dependent synaptic plasticity can be recorded at the dorsolateral region of striatum by the same stimulating protocol in control rats and suggest that 3-NP-induced striatal lesions may be associated with suppression of LTD expression in the sensorimotor striatum.

Acetylcholine attenuates synaptic GABA release to supraoptic neurons through presynaptic nicotinic receptors

Both inhibitory GABAergic and excitatory glutamatergic inputs to supraoptic nucleus (SON) neurons can influence the release of vasopressin and oxytocin. Acetylcholine is known to excite SON neurons and to increase vasopressin release. The functional significance of cholinergic receptors, located at the presynaptic nerve terminals, in the regulation of the excitability of SON neurons is not fully known. In this study, we determined the role of presynaptic cholinergic receptors in regulation of the inhibitory GABAergic inputs to the SON neurons. The magnocellular neurons in the rat hypothalamic slice were identified microscopically, and the spontaneous miniature inhibitory postsynaptic currents (mIPSCs) were recorded using the whole-cell voltage-clamp technique. The mIPSCs were abolished by the GABA(A) receptor antagonist, bicuculline (10 microM). Acetylcholine (100 microM) significantly reduced the frequency of mIPSCs of SON neurons from 3.59+/-0.36 to 1.62+/-0.20 Hz (n=37), but did not alter the amplitude and the decay time constant of mIPSCs. Furthermore, the nicotinic receptor antagonist, mecamylamine (10 microM, n=13), eliminated the inhibitory effect of acetylcholine on mIPSCs of SON neurons. The muscarinic receptor antagonist, atropine (100 microM), did not alter significantly the effect of acetylcholine on mIPSCs in most of the 17 SON neurons studied. These results suggest that the excitatory effect of acetylcholine on the SON neurons is mediated, at least in part, by inhibition of presynaptic GABA release. Activation of presynaptic nicotinic receptors located in the GABAergic terminals plays a major role in the cholinergic regulation of the inhibitory GABAergic input to SON neurons.

Potentiation of glutamatergic synaptic input to supraoptic neurons by presynaptic nicotinic receptors

The release of vasopressin and oxytocin from the supraoptic nucleus (SON) neurons is tonically regulated by excitatory glutamatergic and inhibitory GABAergic synaptic inputs. Acetylcholine is known to excite SON neurons and to elicit vasopressin release. Cholinergic receptors are located pre- and postsynaptically in the SON, but their functional significance in the regulation of SON neurons is not fully understood. In this study, we determined the role of presynaptic cholinergic receptors in regulation of the excitatory glutamatergic inputs to the SON neurons. The magnocellular neurons in the rat hypothalamic slices were identified microscopically, and the spontaneous miniature excitatory postsynaptic currents (mEPSCs) were recorded using the whole cell voltage-clamp technique. The mEPSCs were abolished by the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (20 microM). Acetylcholine (100 microM) significantly increased the frequency of mEPSCs of 38 SON neurons from 1.87 +/- 0.36 to 3.42 +/- 0.54 Hz but did not alter the amplitude (from 19.61 +/- 0.90 to 19.34 +/- 0.84 pA) and the decay time constant of mEPSCs. Furthermore, the nicotinic receptor antagonist mecamylamine (10 microM, n = 16), but not the muscarinic receptor antagonist atropine (100 microM, n = 12), abolished the excitatory effect of acetylcholine on the frequency of mEPSCs. These data provide new information that the excitatory effect of acetylcholine on the SON neurons is mediated, at least in part, by its effect on presynaptic glutamate release. Activation of presynaptic nicotinic, but not muscarinic, receptors located in the glutamatergic terminals increases the excitatory synaptic input to the SON neurons of the hypothalamus.

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.

Alterations with aging and ischemia in nicotinic acetylcholine receptor subunits alpha4 and beta2 messenger RNA expression in postmortem human putamen. Implications for susceptibility to parkinsonism

Nicotine activates the dopaminergic system and acts to alleviate hypokinetic disorders (parkinsonism). The frequency of parkinsonism increases with age and is sometimes associated with multiple small infarcts (status lacunaris) in the putamen. To investigate changes with aging in control cases free from neurological disease and changes in cases with multiple small infarcts (status lacunaris) in the putamen, the present study determined nicotinic acetylcholine receptor (nAChR) subunit alpha4 and beta2 messenger RNA (mRNA) expression in the postmortem human putamen using the reverse transcription-polymerase chain reaction (RT-PCR). In controls, alpha4 subunit mRNA expression was unaltered, but beta2 subunit mRNA expression decreased significantly with age. In cases with status lacunaris, both beta2 and alpha4 subunit mRNA expressions were significantly lower than in the control cases. The reduction in beta2 mRNA expression alone, or in both alpha4 and beta2 mRNA expressions, suggests a reduction in functional nAChRs in the putamen, which may in part explain the susceptibility to hypokinetic disorders of the elderly and subjects with ischemic damage in the striatum.

Postsynaptic integration of cholinergic and dopaminergic signals on medium-sized GABAergic projection neurons in the neostriatum

The effects of cholinergic drugs and the interaction between cholinergic and dopaminergic compounds were studied on electrically evoked [3H]gamma-aminobutyric acid (GABA) overflow in slices of the rat neostriatum. Slices were prepared and loaded with [3H]GABA in the presence of beta-alanine and then superfused with Krebs-bicarbonate buffer containing aminooxyacetic acid and nipecotic acid to inhibit GABA uptake and metabolism, respectively. The nonselective muscarinic agonist oxotremorine (0.1-10 microM) increased the release of [3H]GABA and the selective M1 receptor agonist McN-A-343 (0.1-10 microM) exerted similar effect. The stimulatory effect of oxotremorine (10 microM) on [3H][GABA overflow was antagonized by the nonselective muscarinic antagonist atropine (1 microM) and the selective M1 receptor antagonist pirenzepine (0.1-1.0 microM). The M2 receptor antagonist methoctramine (1.0 microM) did not alter the stimulatory effect of oxotremorine. Of the muscarinic receptor antagonists atropine, pirenzepine, and methoctramine (1.0 microM) failed to affect [3H]GABA overflow. The M3 receptor antagonist p-F-HHSiD (1 microM) increased [3H]GABA overflow and p-F-HHSiD and oxotremorine were found to be additive in increasing this effect. The D2 dopamine receptor antagonist sulpiride (10 microM) increased the electrical stimulation-induced [3H]GABA overflow, and this stimulation was counteracted by concomitant administration of atropine (1 microM). McN-A-343 and sulpiride also increased the KCl-induced [3H]GABA overflow from superfused neostriatal slices and tetrodotoxin (1 microM) did not affect these stimulations. These data indicate that the release of GABA in the neostriatum is under the control of M1 stimulatory and M3 inhibitory muscarinic receptors. Dopamine, which exerts inhibition on GABA release via D2 receptors, may counteract the M1 facilitation, and M1 and D2 receptors involved in the cholinergic-dopaminergic interaction may be located postsynaptically on medium-sized spiny GABAergic projection neurons.

Effects of cholecystokinin agonists on striatal neurons are reduced by acetylcholine

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

Sensory gating in a computer model of the CA3 neural network of the hippocampus

We have developed a unique computer model of the CA3 region of the hippocampus that simulates the P50 auditory evoked potential response to repeated stimuli in order to study the neuronal circuits involved in a sensory processing deficit associated with schizophrenia. Our computer model of the CA3 hippocampal network includes recurrent activation from within the CA3 region as well as input from the entorhinal cortex and the medial septal nucleus. We used the model to help us determine if the cortical and septal inputs to the CA3 hippocampus alone are responsible for the gating of auditory evoked activity, or if the strong recurrent activity within the CA3 region contributes to this phenomenon. The model suggests that the medial septal input is critical for normal gating; however, to a large extent the activity of the medial septal input can be replaced by simulated stimulation of the hippocampal neurons by a nicotinic agonist. The model is thus consistent with experimental data that show that nicotine restores gating of the N40 evoked potential in fimbria-fornix lesioned rats and of the P50 evoked potential in schizophrenic patients.

Modulatory actions of dopamine on NMDA receptor-mediated responses are reduced in D1A-deficient mutant mice

The role of D1 dopamine (DA) receptors in mediating the ability of DA to modulate responses attributable to activation of NMDA receptors was examined in mice lacking D1A dopamine receptors. Specifically, experiments were designed to test the hypothesis that the ability of DA to potentiate responses mediated by activation of NMDA receptors was attributable to activation of D1 receptors. Based on this hypothesis, we would predict that in the D1A mutant mouse, either DA would not induce enhancement of NMDA-mediated responses, or the enhancement would be severely attenuated. The results provided evidence to support the hypothesis. In mutant mice, DA and D1 receptor agonists did not potentiate responses mediated by activation of NMDA receptors. In contrast, in control mice, both DA and D1 receptor agonists markedly potentiated responses mediated by activation of NMDA receptors. The effects of DA in attenuating responses mediated by activation of non-NMDA receptors also were altered in the mutant, suggesting that this action of DA may require coupling or interactions between D1 and D2 receptors. The present studies also provided an opportunity to assess some of the basic electrophysiological and morphological properties of neostriatal neurons in mice lacking D1A DA receptors. Resting membrane potential, action potential parameters, input resistance, excitability, somatic size, dendritic extent, and estimates of spine density in mutants and controls were similar, suggesting that these basic neurophysiological and structural properties have not been changed by the loss of the D1A DA receptor.

Neocortical damage alters synaptic responses of neostriatal neurons in vitro

The present experiments were designed to investigate the physiological impact of a partial decortication upon neostriatal synaptic responses using intracellular recording techniques in the in vitro brain slice preparation. In the intact rat, the locally evoked neostriatal synaptic response is primarily mediated by excitatory amino acid receptor activation. Following neocortex damage, the contributions of both N-methyl-D-aspartate and non-N-methyl-D-aspartate receptor activation were significantly diminished, although responses remained robust in amplitude and duration. Components of the locally evoked synaptic response mediated by activation of GABAA receptors were relatively unchanged, while presynaptic inhibition mediated by activation of GABAB receptors was markedly reduced. Furthermore, the normally minimal acetylcholine contribution to the synaptic response was significantly increased after neocortical damage. This enhanced cholinergic role in the generation of the synaptic response appeared to be mediated primarily by activation of nicotinic receptors. Thus, neocortical damage leads to novel physiological relationships between intrinsic neostriatal cholinergic interneurons and the GABAergic projection neurons. One possibility is that cholinergic interneurons have the potential for substituting for the loss of excitation created by the absence of neocortical glutamatergic input.

Neuromodulatory actions of dopamine on synaptically-evoked neostriatal responses in slices

The present experiments were designed to further examine the hypothesis that receptor subtype determines the direction of dopamine's (DA) ability to modulate neostriatal neuronal responses. We have reported that DA potentiates responses mediated by activation of N-methyl-d-aspartate (NMDA) receptors, but attenuates responses mediated by activation of non-NMDA receptors in neocortex [Cepeda et al. (1992b) Synapse, 11:330-341] and neostriatum [Cepeda et al. (1993) Proc. Natl. Acad. Sci. U.S.A., 90:9576-9580]. In these studies, responses to excitatory amino acids (EAAs) were evoked by microphoretic application of agonists. The present studies examined whether this differential modulation also applies to components of synaptic responses evoked by electrical stimulation of neostriatal afferents and mediated by activation of specific subtypes of EAA receptors. Using brain slices, the actions of DA and its receptor specific agonists on components of neostriatal synaptic responses that were mediated either by NMDA or non-NMDA receptors were assessed. Responses mediated by NMDA receptors were potentiated by DA while those mediated by non-NMDA receptors were attenuated. These findings provide further support for the hypothesis that the direction of modulatory action of DA is determined by the specific subtype of EAA receptor activated. In addition, the enhancement of NMDA receptor-mediated responses was mimicked by application of SKF 38393, a D1 receptor agonist. Quinpirole, a D2 receptor agonist, consistently attenuated responses mediated by activation of non-NMDA receptors. Thus, the complex modulatory actions of DA are dependent upon combinations of co-activation of specific subtypes of EAA and DA receptors. These findings are of clinical relevance since the actions of DA and EAAs have been implicated in neurological and affective disorders.

Aging reduces neostriatal responsiveness to N-methyl-D-aspartate and dopamine: an in vitro electrophysiological study

Excitatory amino acids and dopamine interact to control information flow in the neostriatum. The present study was designed to examine some of the age-induced alterations in the interaction of these two neurotransmitter systems. First, responsiveness of neostriatal neurons to glutamate and N-methyl-D-aspartate was compared in neurons from young and in aged animals. N-Methyl-D-aspartate function was chosen for emphasis because declines in cognitive processes during aging are thought to involve changes in this excitatory amino acid receptor. Second, the age-related changes in dopamine's ability to modulate responses mediated by excitatory amino acid receptors was examined. Specifically, the ability of dopamine to differentially modulate responses induced by N-methyl-D-aspartate and glutamate was assessed. There is considerable evidence for alterations in dopamine receptors and behavioral responses to dopamine in aged animals. It thus becomes important to determine how these alterations are reflected at an electrophysiological level. The responses to application of excitatory amino acid agonists and dopamine as well as changes in synaptic responses mediated by activation of N-methyl-D-aspartate receptors were assessed in 69 neurons obtained from young Fischer 344 rats (3-5 months) and young cats (3-4 years) and 69 neurons obtained from aged Fischer 344 rats (24-26 months) and aged cats (10-16 years) using an in vitro slice preparation. The results indicated that populations of aged neurons from both rats and cats displayed qualitative and quantitative alterations in responses to iontophoretic application of excitatory amino acid receptor agonists. These alterations included lack of response, unusual responses consisting of depolarizations without action potentials or combinations of prepotentials and full amplitude action potentials. Threshold currents for induction of responses were also significantly elevated in neurons from aged animals. Synaptic response components mediated by activation of N-methyl-D-aspartate receptors in aged rats were reduced as well. Exposure to Mg(2+)-free artificial cerebrospinal fluid resulted in marked increases in the size of responses evoked by local stimulation in young neurons from rats. These increases, which are mediated by activation of N-methyl-D-aspartate receptors, were significantly attenuated in aged neurons. The ability of dopamine to modulate responses mediated by activation of excitatory amino acid receptors was reduced in cells from both aged rats and cats. Subpopulations of cells were either unresponsive to dopamine or required higher iontophoretic current intensities to modulate excitatory amino acid-induced responses. The present findings further document age-induced changes in neostriatal electrophysiology indicating that interactions between excitatory amino acids and dopamine appear to be compromised during aging. They emphasize alterations in N-methyl-D-aspartate receptor function and suggest further than the ability of neostriatal neurons to integrate information is altered during aging. The present findings are supported by data from the literature indicating decreases in N-methyl-D-aspartate receptor function during aging. Furthermore, the decreases in excitatory amino acid function during aging suggest that therapeutic interventions designed to prevent or retard the deleterious effects of age in the neostriatum might be directed toward enhancing excitatory amino acid receptor function.

Neural dynamics in cortex-striatum co-cultures--I. anatomy and electrophysiology of neuronal cell types

An in vitro system was established to analyse corticostriatal processing. Cortical and striatal slices taken at postnatal days 0-2 were co-cultured for three to six weeks. The anatomy of the organotypic co-cultures was determined using immunohistochemistry. In the cortex parvalbumin-positive and calbindin-positive cells, which resembled those seen in vivo, had laminar distributions. In the striatum, strongly stained parvalbumin-positive cells resembling striatal GABAergic interneurons and cholinergic interneurons were scattered throughout the tissue. The soma area of these interneuron classes was larger than the average striatal soma area, thus enabling visual selection of cells by class before recording. Cortical neurons with projections to the striatum showed similar morphological features to corticostriatal projection neurons in vivo. No projections from the striatum to the cortex were found. Intracellular recordings were obtained from 94 neurons. These were first classified on the basis of electrophysiological characteristics and the morphologies of cells in each class were reconstructed. Two types of striatal secondary neurons with unique electrophysiological dynamics were identified: GABAergic interneurons (n = 17) and large aspiny, probably cholinergic, interneurons (n = 15). The electrophysiological and morphological characteristics of cortical pyramidal cells (n = 27), cortical interneurons (n = 1), as well as striatal principal neurons (n = 34), were identical to those reported for similar ages in vivo. Organotypic cortex-striatum co-cultures are therefore suitable as an in vitro system in which to analyse corticostriatal processing. The network dynamics, which developed spontaneously in that system, are examined in the companion paper.

Glutamatergic and GABAergic postsynaptic responses of striatal spiny neurons to intrastriatal and cortical stimulation recorded in slice preparations

Glutamatergic and GABAergic responses of the neostriatal spiny neurons to intrastriatal and cortical stimulation were characterized by intracellular recording in brain slice preparations. This study also demonstrated the role of each response in the spike activity of the spiny neuron. Single neostriatal stimulation induced postsynaptic potentials consisting of multiple components. The early part of the postsynaptic potential, which was isolated by the GABAA antagonist bicuculline methiodide and the N-methyl-D-aspartate antagonist 3-(2-carboxypiperzin-4-yl)-propyl-1-phosphonic acid (CPP), was mainly an alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate receptor-mediated response. Perfusion of magnesium-free medium containing bicuculline methiodide and the AMPA/kainate antagonist 3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline (NBQX) disclosed a large, slow N-methyl-D-aspartate receptor-mediated response. The N-methyl-D-aspartate response in magnesium-containing perfusing medium was small in neurons at the resting membrane potential, but became a significant component when the neurons were depolarized to subthreshold membrane potential. The duration of the N-methyl-D-aspartate response was over 300 ms. The nicotinic antagonists dihydro-beta-erythroidine hydrobromide and mecamylamine failed to change responses to single stimulation. Repetitive intrastriatal stimulation induced a large, long-duration depolarization with action potentials in the spiny neurons. This stimulation-induced response resembles that of the depolarization stage observed in anesthetized animals. Bicuculline methiodide increased the response amplitude. In contrast, CPP reduced the amplitude of the response to the below the spike generation threshold. The CPP-sensitive N-methyl-D-aspartate response was large and lasted several hundred milliseconds after the termination of repetitive stimulation. Responses of the neostriatal neurons to cortical stimulation were similar to those induced after intrastriatal stimulation. CPP greatly reduced both the response amplitude and the number of spikes triggered from the response. Bicuculline methiodide, on the other hand, greatly increased the response amplitude and the number of spikes. The AMPA/kainate response alone, which was isolated by application of bicuculline methiodide and CPP, did not induce sustained depolarization in spiny neurons to repetitive cortical stimulation. Application of NBQX diminished GABAA response to cortical stimulation. This observation indicates that, for neostriatal spiny neurons to respond with GABAA response after cortical stimulation, the AMPA/kainate response must be induced in the GABAergic secondary neurons in the neostriatum. This study indicates that the main synaptic driving forces of neostriatal spiny neurons include AMPA/kainate, N-methyl-D-aspartate and GABAA responses. Although AMPA/kainate response is the main synaptic input, the generation of the action potentials in neostriatal neurons is greatly influenced by both GABAA and N-methyl-D-aspartate responses.

Synaptic transmission and modulation in the neostriatum

The neostriatum is the entryway into the basal ganglia and is the site of many of the neurological defects involving basal ganglia function. Thus, it is important to understand the regulation of synaptic transmission at afferent synapses innervating the neostriatum. Cortical glutamatergic and nigral dopaminergic afferent input impinge on neurons in the neostriatum, providing the most significant afferent inputs to this structure. Our understanding of the mechanisms involved in transmission and modulation of transmission at these synapses has greatly increased. It is now apparent that the corticostriatal glutamatergic inputs produce rapid depolarization of striatal neurons via activation of ionotropic AMPA-type glutamate receptors. In addition, transmission is modulated by a number of presynaptic, G-protein-coupled receptors but, surprisingly, relatively little evidence of postsynaptic modulation has been observed. Corticostriatal synapses also express certain forms of plasticity, most notably short- and long- term synaptic depression (STI) and LTD, respectively). It appears that LTD may involve convergent actions of glutamate and dopamine. Striatal LTD may have important roles in information storage and motor set selection in the striatum. However, some aspects of synaptic transmission in the striatum remain unclear. In particular, the exact physiological roles of dopaminergic nigrostriatal input and the role of NMDA-type glutamate receptors are not well understood. In addition, intrastriatal synaptic connections have received relatively little attention as compared with extrinsic input to the neostriatum. Future studies will need to focus on elucidating these aspects of neostriatal function.

Striatal interneurones: chemical, physiological and morphological characterization

The neostriatum is the largest component of the basal ganglia, and the main recipient of afferents to the basal ganglia from the cerebral cortex and thalamus. Studies of the cellular organization of the neostriatum have focused upon the spiny projection neurones, which represent the vast majority of neurones, but the identity and functions of interneurones in this structure have remained enigmatic despite decades of study. Recently, the discovery of cytochemical markers that are specific for each of the major classes of striatal interneurones, and the combination of this with intracellular recording and staining, has revealed the identities of interneurones and some of their functional characteristics in a way that could not have been imagined by the classical morphologists. These methods also suggest some possible modes of action of interneurones in the neostriatal circuitry.

Nicotinic cholinoceptor-mediated excitatory postsynaptic potentials in rat nucleus ambiguus

In rat brainstem slice preparations, intracellular recording from neurons (n = 39) in the compact formation of the nucleus ambiguus (AMBc) revealed spontaneous and miniature excitatory postsynaptic potentials (EPSPs; n = 11) that, along with acetylcholine-induced depolarization, were enhanced by physostigmine (10 microM; n = 2) and blocked by dihydro-beta-erythroidine 1-5 pmol (n = 4). Retrograde neuronal tracing combined with choline acetyltransferase immunocytochemistry demonstrated that the AMBc receives a projection from a subpopulation of cholinergic neurons in the zona intermedialis reticularis parvicellularis. Electrical stimulation of this region in slices evoked fast EPSPs in AMBc neurons (n = 23) that were inhibited by dihydro-beta-erythroidine 2-5 pmol (n = 8), but not by methscopolamine 1 pmol (n = 2). The present findings strongly support the existence of a cholinergic nicotinic synapse mediating fast transmission in brainstem vagal motoneurons.

Normalization by nicotine of deficient auditory sensory gating in the relatives of schizophrenics

Diminished gating of the P50 auditory evoked response to repeated stimuli is a psychophysiological feature of schizophrenia, that is also present in many relatives of patients. Animal models of auditory sensory gating indicate that nicotinic cholinergic neurotransmission is a critical neuronal substrate. The aim of this experiment was to determine if the deficit in sensory gating could be reversed by nicotine administration. Nonsmoking relatives of schizophrenics with abnormal sensory gating were selected as subjects for this initial double-blind trial, to avoid effects of psychotropic medications that might complicate trials in schizophrenic patients themselves. Nicotine-containing gum increased P50 sensory gating to near normal levels within 30 min of administration. The effect was transient; the gating of P50 returned to baseline levels within 1 hr. There was no change observed after placebo administration. In one of the subjects, the anticholinesterase inhibitor physostigmine similarly normalized P50 gating. The results are consistent with the hypothesis that nicotinic cholinergic neurotransmission may mediate a familial psychophysiological deficit in schizophrenia.

Intrastriatal dopaminergic agents, muscarinic stimulation, and GABA antagonism compared for rotation responses in rats

We tested the common hypothesis that rotation on systemic injection of dopaminergic agents in rats with unilateral 6-hydroxydopamine lesions of the substantia nigra is attributable to unequal stimulation of dopamine receptors between the two striata. No rotation occurred when dopamine, apomorphine or amphetamine were injected into dorsal striatum or nucleus accumbens of intact, unanesthetized rats. Intrastriatal haloperidol elicited ipsiversive postural deviation only in conjunction with hypermotility induced by systemic amphetamine. In unilateral 6-hydroxydopamine-lesioned rats, intrastriatal apomorphine elicited rotation directed away from the side of its injection, whether intact or lesioned. Carbachol elicited short-latency rotation, contraversive to injection in dorsal striatum or nucleus accumbens, in both intact and 6-hydroxydopamine-lesioned rats. The rotation response to carbachol was suppressed by atropine administered systemically or into the site of intrastriatal carbachol. Picrotoxin or bicuculline produced contraversive rotation or contralateral myoclonic jerks on injection into the striatum in intact rats. The results show that asymmetric stimulation of striatal dopamine receptors is not sufficient to cause rotation, unless the receptors have been denervated. On the other hand, asymmetric stimulation muscarinic receptors is in itself enough to produce the imbalance of gamma-aminobutyric acid (GABA)ergic striatal outputs responsible for rotation.

Dendritic activity on neostriatal neurons as inferred from somatic intracellular recordings

Orthodromic responses after local field stimulation were studied in a neostriatal slice preparation. Single suprathreshold stimuli evoked slow plateau-like orthodromic responses with repetitive firing and duration proportional to the stimulus intensity. Its persistence and apparent threshold upon hyperpolarization, as well as the actions of tetraethylammonium, QX-314 and Sr2+, suggest that this orthodromic response and associated phenomena such as fast prepotentials or Ca-dependent action potentials, may be generated on dendrites.

Acute tolerance to the locomotor stimulant effects of nicotine in the rat

Studies of peripheral nicotinic receptors have revealed that, after an initial agonist action, the receptors remain inhibited either through continued depolarization blockade due to continued presence of the agonist or through a brief inactivation of the receptor following its activation. If a similar phenomenon occurs at central nervous system nicotinic receptors, then behavioral responses to nicotine should exhibit an acute tolerance (tachyphylaxis). Groups of rats were given either saline or 0.2 mg/kg nicotine injections at 20-min intervals in photocell activity cages. A progressive decline in the locomotor responsiveness to nicotine was observed. The time course of this acute tolerance was observed in other rats given initial 0.2 mg/kg nicotine injections followed at differing time intervals by second 0.2 mg/kg nicotine test injections. The secondary antagonism to nicotine's locomotor stimulant effects was maximal at 45-60 min and recovered by 90-120 min. The locomotor response to 0.2 mg/kg nicotine test injections was observed in other rats following exposure to 1.8 mg/kg nicotine, and the behavioral response was attenuated for more than 5 h.

Locally evoked potentials in slices of the rat nucleus accumbens: NMDA and non-NMDA receptor mediated components and modulation by GABA

In a slice preparation of the rat nucleus accumbens (Acb), local electrical stimulation elicited a field potential composed of two negative peaks, followed by a positive wave. The early negative peak was identified as a non-synaptic compound action potential, the late negative peak as a monosynaptic population spike (PS) and the positive wave as a mixture of an excitatory and an inhibitory postsynaptic potential (PSP). Both the PS and the PSP exhibited a marked degree of paired-pulse facilitation. The quisqualate/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 2 microM) and the broadly acting glutamate receptor antagonist kynurenic acid (300 microM) reversibly abolished or reduced both the PS and PSP. In contrast, nicotinic, muscarinic and N-methyl-D-aspartate (NMDA) receptor antagonists had no suppressive action. Washout of Mg2+ from the superfusion medium reversibly enhanced and prolonged the PSP and this effect was blocked by the NMDA receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (D-AP-5). The gamma-aminobutyric acid antagonist picrotoxin (60 microM) enhanced the PS and induced secondary spikes which were superimposed on a prolonged PSP. Most of this prolongation was abolished by D-AP-5. It is concluded that locally evoked synaptic responses in the Acb are mediated by glutamate or aspartate, and that NMDA receptor mediated activity evoked by low frequency stimulation is substantial in Mg2(+)-free medium or during reduced GABAA receptor activity, but not under normal conditions.

Synaptic potentials in the rat neostriatum in dissociated embryonic cell culture

Neostriatal cells of embryonic days 19-21 were grown in dissociated cell culture. To test whether the cultures contained predominantly neostriatal cells, a glyoxylic acid staining procedure was used which, after dopamine loading, stained neostriatal cells but not neurons of embryonic neocortical tissue. Whole cell current clamp recording was performed in the neurons after 1-2 weeks in cell culture. Although cells could be driven to discharge by direct depolarization, spontaneous activity was low. All cells responded to gamma-aminobutyric acid (GABA) (0.1-0.5 mM), and the majority of them responded to glutamate (Glu) (0.1 mM). Only about 50% were depolarized by acetylcholine (ACh) (0.1-0.5 mM). Atropine (1-10 microM) did not block this depolarization. Barrages of postsynaptic potentials (PSPs) were induced by applications of Glu or ACh, even if the neuron under observation was not depolarized. All PSPs were depressed by bicuculline (50 microM), indicating their mediation by GABAergic receptors. Exclusively GABAergic PSPs were also observed in cultures raised in the presence of nerve growth factor. The study indicates that neostriatal cells form GABAergic, but not excitatory cholinergic synapses when cultured at this embryonic age under our conditions, resembling the pattern of development observed in slices obtained from neonatal rats.

Pharmacology of nicotine

In recent years progress in basic neuropsychopharmacology and clinical addiction research have allowed the conclusion that tobacco smoking essentially represents an addiction to nicotine. Parallel to this work, experimental research in biochemistry, physiology and pharmacology has provided detailed descriptions of the structure and function of the nicotinic receptor, the biologic mediator of the many actions of nicotine. This article reviews current knowledge of nicotinic mechanisms in the peripheral and central nervous systems as well as some implications for the notion of smoking as an addiction to nicotine. In particular this review will focus on the effects of nicotine on brain dopamine and noradrenaline systems since these neuronal systems appear to be crucially involved in the rewarding and stimulant effects of addictive drugs.

Kynurenic acid antagonizes the excitatory postsynaptic potential elicited in neostriatal neurons in the in vitro slice of the rat

Stimulation of corpus callosum in rat brain slices evoked an excitatory postsynaptic potential (EPSP) in neostriatal neurons. This EPSP is greatly reduced by exposing the slice to kynurenic acid (KY). The action of KY is reversed when the preparation is bathed in normal Ringer's solution. KY reduces the EPSP amplitude in a dose-dependent fashion, with 1 mM KY attenuating the potential by 86%. The effect is not due to a conductance change, since KY treatment does not alter the neuron's input resistance.

Muscarinic slow EPSPs in neostriatal and hippocampal neurons in vitro

Cholinergic slow excitatory postsynaptic potentials (slow EPSPs) can be elicited by presynaptic tetanic stimulation in brain slices obtained from rat neostriatum or guinea pig hippocampus. Slow EPSPs are generated by a reduction of a K-leak-conductance. In hippocampal neurons slow EPSPs are amplified by the reduction of an outward current termed IAHP through the activation of a second muscarinic receptor subtype. While hippocampal slow EPSPs might be involved in information processing across hippocampal pathways, muscarinic modulation in the neostriatum consists of a presynaptic tuning of nicotinic fast synaptic transmission.

Pharmacological profile of nicotinic acetylcholine receptors in the rat prefrontal cortex: an electrophysiological study in a slice preparation

The specificity of nicotinic receptors in the neocortex has been questioned previously because: (i) electrophysiological responses to nicotine could not be blocked by nicotinic antagonists, and (ii) the effect of nicotine was not mimicked by acetylcholine. In the present study, the presence of functional nicotinic receptors in rat neocortex has been assessed in a slice preparation of prefrontal cortex, using evoked field potential and unit recordings. Nicotine and the nicotinic agonists, dimethylphenylpiperazinium, cytisine, acetylthiocholine, applied by iontophoresis, produced an increase in the negative wave of field potentials, reflecting an increased excitability of cortical neurons. This effect was blocked by the selective probe for neuronal nicotinic receptors Toxin F (1.4 microM in the perfusion medium) and by dihydro-beta-erythroidine (100 microM). Alpha-bungarotoxin, the blocker of skeletal muscle acetylcholine receptor had no effect. Iontophoretically applied acetylcholine, muscarine and pilocarpine, on the other hand, produced a decrease in the field potential amplitude, which was blocked by atropine and scopolamine (1-10 microM). In the presence of eserine (10 microM), the muscarinic effect of acetylcholine was dramatically altered, leading to the development of a nicotinic response sensitive to Toxin F. Thus, the physiological activation of nicotinic receptors in rat prefrontal cortex appears to require higher concentrations of acetylcholine than do muscarinic receptors. Our results show that: (i) the rat prefrontal cortex possesses functional nicotinic receptors with a pharmacological profile clearly distinct from muscle receptors, and (ii) a nicotinic effect of acetylcholine can be revealed when its degradation by acetylcholinesterase is inhibited.

Excitatory amino acids in synaptic excitation of rat striatal neurones in vitro

1. Intracellular recordings were made from rat striatal neurones in vitro. The cells had resting membrane potentials greater than -60 mV and action potentials greater than 70 mV with spike overshoot of 10-30 mV. 2. In the presence of bicuculline intrastriatal stimulation evoked an excitatory postsynaptic potential (EPSP). The relationship between EPSP amplitude and membrane potential was not linear. The EPSP decreased in amplitude and duration for values of membrane potential more negative than -80 mV and increased in amplitude and duration for values of membrane potential more positive than -50 mV. 3. The mean reversal potential for the EPSP recorded with electrodes filled with potassium methyl-sulphate was -9.2 +/- 1.7 mV (mean +/- S.E.M.) in presence of bicuculline (30 microM). A similar reversal potential was obtained with CsCl-filled electrodes. 4. The endogenous broad-spectrum excitatory amino acid antagonist, kynurenic acid (100-500 microM), reduced the EPSP in a dose-dependent way, maximally by 80% at 500 microM, but a residual depolarization remained even at high antagonist concentrations. This effect was associated sometimes with a membrane depolarization and an increase in input resistance. 5. In normal artificial cerebro-spinal fluid solution and at resting membrane potential the specific N-methyl-D-aspartate (NMDA) antagonist, (D,L)-2-amino-7-phosphonoheptanoic acid (([D,L)-AP7), did not affect the EPSP amplitude. However, this antagonist partially reduced the EPSP amplitude when the membrane was depolarized beyond -50 mV by intracellular current injection. 6. The nicotinic cholinergic antagonist mecamylamine (10 microM) caused a partial (24 +/- 3%) reduction of EPSP amplitude at resting potential in normal medium. However, in the cells where a reduction of EPSP amplitude was observed it was always accompanied by membrane depolarization (7.1 +/- 2.1 mV). (+)-Tubocurarine and hexamethonium were without effect at 10 microM. 7. When Mg2+ was removed from the bathing solution, the EPSP increased in amplitude (89 +/- 9.5%) and duration. In Mg2+-free medium at resting membrane potential (D,L)-AP7 (30 microM) partially reduced EPSP amplitudes (59 +/- 2.5%). 8. It is proposed that a major component of the EPSP evoked by intrastriatal stimulation is mediated by excitatory amino acids. At resting membrane potential and in normal medium only non-NMDA receptors seem to contribute to the synaptic depolarization, but at depolarized potentials and in Mg2+-free medium an NMDA receptor-mediated component of the EPSP can be demonstrated.

Cholinergic synaptic input to different parts of spiny striatonigral neurons in the rat

The postsynaptic targets of cholinergic boutons in the rat neostriatum were assessed by examination in the electron microscope of boutons that were immunoreactive for choline acetyltransferase, the synthetic enzyme for acetylcholine. These boutons formed symmetrical synaptic specializations with neostriatal neurons. Of 209 immunoreactive synaptic boutons observed in random searches of the neostriatum, 45% made contact with dendritic shafts, 34% with dendritic spines, and 20% with neuronal perikarya. Many of the postsynaptic structures had ultrastructural characteristics of the most common type of striatal neuron, the medium-size densely spiny neuron. This was confirmed by the examination in the electron microscope of Golgi-impregnated medium-size spiny neurons from sections that had also been immunostained for choline acetyltransferase. Immunoreactive boutons formed symmetrical synaptic specializations with all parts of the neurons examined, i.e., perikarya, proximal and distal dendritic shafts, and dendritic spines. Two of the Golgi-impregnated medium-size spiny neurons that received input from the cholinergic boutons were also retrogradely labelled with horseradish peroxidase that had been injected into the substantia nigra, they were thus further characterized as striatonigral neurons. Similarly, seven retrogradely labelled perikarya of striatonigral neurons were found to receive input from the cholinergic boutons. It is concluded that cholinergic boutons in the neostriatum form synaptic specializations and that one of their major targets is the medium-size densely spiny neuron that projects to the substantia nigra. The topography of the cholinergic afferents of these cells is distinctly different from that of other boutons derived from local neurons and from boutons that form asymmetrical synaptic specializations, but it is similar to that of the dopaminergic boutons originating from neurons in the substantia nigra.

Spontaneous synaptic potentials in dopamine-denervated neostriatal neurons

Intracellular spontaneous activity was recorded in neostriatal slices from rats with 6-hydroxydopamine-induced lesion of the left nigrostriatal dopaminergic system. Recordings were made at different times after denervation. Dopaminergic denervation caused the appearance of spontaneous synaptic potentials, which were present even after 8 months. The results suggest a tonic inhibitory influence of the dopaminergic innervation on the synaptic input of neostriatal neurons.

Choline acetyltransferase-immunoreactive neurons and terminals in the rat septal complex: a combined light and electron microscopic study

A monoclonal antibody against choline acetyltransferase (ChAT), the acetylcholine synthesizing enzyme, was used to determine the morphological characteristics of cholinergic neurons and axon terminals within the rat septum. Light microscopy revealed numerous large fusiform or multipolar ChAT-immunoreactive neurons in the medial septal nucleus/diagonal band complex (MSDB). In contrast, virtually no immunostained cells were found in the lateral septum (Nc. septalis dorsalis and Nc. septalis lateralis). Fine immunostained fibers were most abundant close to the midline in the MSDB mainly following an ascending course. A few thin ChAT-immunoreactive fibers and terminallike pericellular punctate structures were observed in the inner part of the dorsal septal nucleus. Electron microscopy of ChAT-immunoreactive neurons revealed large cell bodies rich in cytoplasmic organelles. The cell nuclei regularly exhibited multiple invaginations of the nuclear membrane. Only rarely were terminals found that established synaptic contacts on the cell bodies of immunostained neurons. In contrast, numerous terminals formed synaptic contacts on immunoreactive dendrites. ChAT-immunopositive terminals were studied in thin sections from the MSDB and from the dorsal septal nucleus. In both regions they appeared as heavily immunostained vesicle-filled boutons that established symmetric and asymmetric synaptic contacts. In the dorsal septal nucleus immunostained terminals often showed a basketlike arrangement around immunonegative cell bodies. Our fine structural study provides evidence that cholinergic neurons in the MSDB are similar to cholinergic neurons in the basal nucleus and neostriatum, which have been described by other investigators. The presence of cholinergic synapses in the septal complex indicates that this region not only contains cholinergic projection neurons, but receives a cholinergic input itself.

Kynurenic acid blocks suprachiasmatic nucleus responses to optic nerve stimulation

An in vitro slice preparation of the mouse hypothalamus was used to determine the effects of pharmacological agents on the field potentials that are evoked in the suprachiasmatic nucleus (SCN) by stimulation of the optic nerve. Postsynaptic components of these responses were identified by lowering the concentration of calcium in the superfusate. Bath application of kynurenate, an antagonist of excitatory amino acid neurotransmission, reversibly blocked postsynaptic responses in the SCN. The evoked responses in the SCN were not affected by the acetylcholinergic agents (+)-tubocurarine, scopolamine, physostigmine, or carbachol. These results suggest that excitatory amino acid receptors mediate responses of SCN neurons to retinal input, but do not support a role for acetylcholine.

Intracellular studies on the dopamine-induced firing inhibition of neostriatal neurons in vitro: evidence for D1 receptor involvement

Intracellular recordings were obtained from rat neostriatal slices. Bath-applied dopamine (1-10 microM) produced a reversible inhibition of the action potentials evoked by direct stimulation and a decrease in the amplitude of the intrastriatally evoked depolarizing postsynaptic potentials. No change in membrane potential was detected during the application of 1-10 microM dopamine. Dopamine application also produced a decrease in anomalous rectification in the depolarizing direction. This subthreshold inward rectification was abolished by tetrodotoxin, but not by calcium-free and cadmium (0.1-1 mM)-containing solutions. The dopamine-induced decrease in excitatory postsynaptic potential amplitude was evident at resting membrane potential or at more positive levels, but was absent at hyperpolarized values of the membrane potential. Addition of bicuculline (50-500 microM) to the medium did not affect the inhibitory action of dopamine. The inhibitory action of dopamine also persisted in calcium-free and cadmium-containing solutions. The adenosine 3',5'-cyclic monophosphate analogue, 8-bromo-adenosine 3',5'-cyclic monophosphate (0.1-1 mM), mimicked the effects produced by D1 receptor activation. Bath application of 2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine (SKF 38393) (1-10 microM), a selective D1 dopaminergic agonist, mimicked the effects of micromolar concentrations of dopamine. The D2 dopaminergic agonists, 4,4a,5,6,7,8,8a,9-octahydro-5-n-propyl-2H-pyrazolo-3,4-g-quinoline (LY 171555) and bromocriptine (both at 10 nM-10 microM), had no effects on neostriatal cells. The inhibition induced by micromolar doses of dopamine or SKF 38393 was antagonized by bath applications of R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin++ +-7-ol (SCH 23390; 0.1-10 microM), a D1-selective antagonist, but not by sulpiride (10 nM-10 microM), a D2 antagonist. We conclude that the inhibitory effect of dopamine on rat striatal neurons is postsynaptically mediated by the activation of D1 dopaminergic receptors via the reduction of a voltage-dependent tetrodotoxin-sensitive inward conductance.

Intrinsic membrane properties of neostriatal neurons can account for their low level of spontaneous activity

Electroresponsiveness of neostriatal neurons was studied by intracellular recording in a rat brain slice preparation maintained in standard solution or in solution containing K-channel blockers. In standard solution, the neurons fired repetitively at increasing frequencies with increasing amplitude of direct depolarization. The firing pattern was independent of the membrane potential from which firing was induced. In the presence of tetraethylammonium (20 mM), long-lasting (300-500 ms) plateau potentials could be elicited by the injection of short (5-10 ms) current pulses. Plateau potentials persisted in Na-free solution, in the presence of tetrodotoxin (1-3 microM) and if Ca in the perifusate was replaced by Ba. The plateau was blocked by Cd (500 microM). The plateaux were followed by depolarizing after-potentials. When the plateau potential failed due to fatigue, a small slow depolarization of short duration (10-30 ms) was elicited in Na-free or tetrodotoxin-containing solution, which increased in amplitude with membrane hyperpolarization. This slow depolarization was blocked by Cd, indicating that it was also mediated by Ca. By intrastriatal stimulation in the presence of 4-aminopyridine a long-lasting, voltage-dependent depolarization was triggered from the enhanced postsynaptic potential. In contrast, in the presence of tetraethylammonium, postsynaptic potentials were only slightly increased if they were compared at sizes subthreshold for the plateau potentials. It is concluded that neostriatal neurons, although being characterized as "silent" and "non-bursting", possess slow conductances for inward currents which they share with other mammalian central neurons. However, in contrast, to other central neurons, their Ca-spikes are suppressed by their K-conductances and, in contrast to oscillating neurons, low-threshold Ca-potentials are not prominent.

Flight induced by microinjection of D-tubocurarine or alpha-bungarotoxin into medial hypothalamus or periaqueductal gray matter: cholinergic or GABAergic mediation?

Microinjections of various doses (50-300 ng) of the nicotinic antagonist D-tubocurarine (TUBO) into the rat's medial hypothalamus (MH) or dorsal periaqueductal gray (PAG) produced flight reactions characterized by jumps. Two different types of flight reactions were produced depending on whether the drug was injected into the MH or into the PAG. MH injections provoked an increase in both locomotor activity and rearing together with well-oriented jumps. PAG injections provoked either freezing reactions or running with explosive jumps, but no increase in rearing. In addition, the rat exhibited an asymmetry in responsiveness to tactile stimulation. These reactions also differed depending on whether the drug was injected into the dorsal or ventral PAG. Behavioral reactions similar to those produced by TUBO were also produced by microinjection of the GABA receptor antagonist bicuculline into the same brain sites. Among the 4 putative cholinergic antagonists tested under the same conditions only alpha-bungarotoxin produced effects that were qualitatively similar to those induced by D-tubocurarine or bicuculline. Gallamine and hemicholinium produced tremor when injected into sites located near the ventricular system at either the MH or the PAG level, while vocalizations were only produced by PAG injections. Hexamethonium produced no marked effect. The hypothesis that flight reactions induced by D-tubocurarine or alpha-bungarotoxin do not result from their antinicotinic action but rather from a direct effect on GABAergic transmission is discussed.

Muscarinic modulation of calcium dependent plateau potentials in rat neostriatal neurons

Intracellular recording from neostriatal neurons in rat brain slices revealed effects of the acetylcholine (ACh) agonist carbachol (Cch, 1-10 mumol/l), of the anticholinesterase physostigmine (10 mumol/l) and of the muscarinic antagonist atropine (10 mumol/l) on plateau potentials elicited in the presence of K-blockers. Plateau potentials elicited in the presence of K-blockers were Ca-dependent, since they persisted in Na-free solution, were resistant to tetrodotoxin (TTX, 3 mumol/l) and blocked by Cd (0.1-0.5 mmol/l). Cch reduced the duration of the plateau potentials and made them more susceptible to fatigue. These effects were antagonized by atropine (1-10 mumol/l), but not by Ba (100-200 mumol/l) or 4-aminopyridine (4-AP, 0.5 mmol/l). Physostigmine (10 mumol/l) had the same atropine-sensitive effects as Cch on the plateau potential. Atropine (10 mumol/l), by itself, prolonged the duration of the plateau potential. High concentrations (100 mumol/l) of Cch did not further reduce the duration of the plateau potential, instead, the duration re-increased with prolonged exposure. The re-increase of the plateau-spike duration was later masked by bursting activity. The opposing effects of low and high concentrations of Cch on the plateau potential duration corresponded to effects of this drug on intrastriatally evoked EPSPs in that low concentrations of Cch reduced the EPSP amplitude, but high concentrations re-increased it after a transient decrease. It is concluded that the muscarinic effect of ACh in the neostriatum is to modulate Ca-influx and that this effect is exerted in a tonic manner.

Muscarinic slow excitation and muscarinic inhibition of synaptic transmission in the rat neostriatum

Intracellular recording from neurones in rat neostriatal slices was used to compare the muscarinic effects of endogenous acetylcholine (ACh) released from cholinergic neostriatal synapses with the action of exogenously applied muscarinic agonists. Repetitive electrical stimulation in the neostriatum evoked a series of fast excitatory post-synaptic potentials (e.p.s.p.s) followed by a short, variable period of input resistance decrease. In the presence of the acetylcholinesterase (AChE) inhibitor, physostigmine, these potentials were followed by a slow e.p.s.p. which lasted about 60 s. Higher stimulus intensities were needed to elicit the slow e.p.s.p. than the fast e.p.s.p. The slow e.p.s.p. could not be observed after a single stimulus. Its amplitude was graded and increased with stimulus strength. The slow e.p.s.p. was blocked by the muscarinic antagonist atropine (10 microM) and by Ba2+ (100 microM). Input resistance increased during the slow e.p.s.p. Depolarization of the cell increased the size of the slow e.p.s.p. and hyperpolarization decreased it. Simultaneously, resting input resistance increased with membrane depolarization and decreased with membrane hyperpolarization. Repetitive intrastriatal stimulation was followed by a hyperpolarization instead of the depolarization at membrane potentials negative to -75 mV. Input resistance increased during this hyperpolarization as it did during the slow e.p.s.p. The slow e.p.s.p. persisted at membrane potentials of -70 to -80 mV if K+ concentration in the saline was reduced from 5 to 2 mM. In 10 mM-K+, the repetitive stimulation was followed by a hyperpolarization even at membrane potentials as low as -60 to -50 mV. Bath perfusion of high concentrations (100 microM) of muscarine or carbachol induced a sustained increase in the input resistance. The muscarinic agonists also reduced the amplitude of intrastriatally evoked fast e.p.s.p.s; however, this effect was transient and compensated by the increase in input resistance. The effects of the muscarinic agonists on input resistance and e.p.s.p. amplitude were antagonized by atropine (10 microM). Sustained decreases of e.p.s.p. amplitude were induced by the bath application of low doses (0.5-10 microM) of muscarine or carbachol. Input resistance was not altered. Atropine (1-10 microM) antagonized this effect. A sustained reduction of fast e.p.s.p. amplitude resulted also from inhibition of AChE by application of physostigmine (1-100 microM). Input resistance and neuronal excitability were not affected by AChE blockade.(ABSTRACT TRUNCATED AT 400 WORDS)

Non-cholinergic synaptic excitation in neostriatum: pharmacological evidence for mediation by a glutamate-like transmitter

We studied the synaptic pharmacology of an excitatory pathway in the neostriatum using electrophysiological techniques in tissue slices from rats. In response to single electrical stimuli, two negative, extracellular potentials (N-1 and N-2) were recorded through micropipette electrodes within 150-450 micron of the stimulating cathode. N-2 was reversibly reduced or abolished by reducing the concentration of calcium in the bathing medium, while N-1 was unaffected. Both N-1 and N-2 were reversibly abolished by the local anaesthetic procaine. Single-unit, extracellular action potentials were, at times, associated with either N-1 or N-2. Intracellular recordings showed action potentials at N-2 latency arising from graded, monophasic, depolarizing potentials. Bath-applied cholinoceptor and dopamine receptor antagonists failed to reduce N-2. By contrast, antagonists of excitatory amino acid transmitters reversibly reduced or abolished N-2. gamma-D-Glutamylglycine (GG), (+/-)-cis-2,3-piperidine dicarboxylic acid (PDA) and DL-2-amino-4-phosphonobutyric acid (APB) blocked N-2 with ED50S of 0.79 mM, 1.0 mM and 1.1 mM, respectively. (-)-Baclofen reversibly blocked N-2 with an ED50 of 0.79 microM; (+)-baclofen was 330 times less potent. The results suggest that N-1 results from direct activation of fibre tracts or cell bodies, while N-2 is a population spike mediated by excitatory synapses whose natural transmitter pharmacologically resembles glutamate.

Late development of intrinsic excitation in the rat neostriatum: an in vitro study

Functional maturation of intrinsic circuitry in the neostriatum was studied by intracellular recording and intracellular staining with Lucifer yellow in slices obtained from rat pups at postnatal days (P)1-20 and from adult rats. The most striking observation was that intrastriatal stimulation elicited predominantly inhibitory responses in slices obtained from animals of P1-6. In contrast, intrastriatally evoked responses in slices obtained after P10 were predominantly excitatory. The inhibitory postsynaptic potentials (IPSPs) recorded in slices obtained from pups were blocked by bicuculline (50 microM) and exhibited a reversal potential at about -60 mV which shifted in depolarizing direction when intracellular Cl- activity increased. Thus, these IPSPs correspond to IPSPs observed in adult animals. It is concluded that maturation of excitatory synapses is the main change during postnatal development. The changes of postsynaptic potentials were paralleled by the appearance of spines on dendrites around P7 as revealed by intracellular staining. The apparent input resistances and time constants of young neurons were very high and responses to large current injections were often distorted by humps which could not be observed in adult neurons. Only young neurons responded with bursts to synaptic activation in the presence of bicuculline (50 microM). It appears that dendritic conductances have a stronger influence on somatic discharge in the electrotonically compact young neurons than in adult neurons.

Effects of 4-aminopyridine (4-AP) on rat neostriatal neurons in an in vitro slice preparation

Effects of 4-aminopyridine (4-AP) on the rat neostriatal neuron were studied using the in vitro slice preparation. The intracellularly recorded neurons had resting membrane potentials of more than 50 mV and were capable of generating action potentials with the amplitude greater than 60 mV. Application of 4-AP in the superfusing media depolarizes the cell membrane and increases its input resistance. Local electrical stimulation induces excitatory postsynaptic potentials (EPSPs) overlapping with inhibitory postsynaptic potentials (IPSPs) in these neurons. 4-AP application enhances the amplitude and duration of the postsynaptic potentials. With application of higher concentration of 4-AP, local stimulation induces a second EPSP and a bicuculline sensitive long duration depolarization. These results indicate that 4-AP clearly has effects on local stimulation-induced postsynaptic responses of neostriatal neurons. Possible mechanisms underlying the 4-AP actions on neurotransmission in the neostriatal slice are discussed.

Active membrane properties of rat neostriatal neurons in an in vitro slice preparation

The active membrane properties of rat neostriatal neurons have been studied in an in vitro slice preparation. All the neurons examined had resting membrane potentials of more than 50 mV and generated action potentials with amplitudes exceeding 70 mV. The morphological characteristics of the neurons identified by intracellular labeling with HRP indicated that they were medium spiny neurons. Depolarizing current injection through the recording microelectrode generated slow depolarizing potentials and repetitive action potentials with frequencies ranging from less than 10 Hz to over 300 Hz. Adaptation of action potentials was observed when long duration depolarizing current was injected. Depolarizing current injections revealed that the membrane of the striatal neuron had an anomalous rectification when the membrane potential was depolarized to the resting potential. A possible bases for the anomalous rectification might involve inactivation of K-conductance and slow inward Ca- and/or Na-currents. Local electrical stimulation evoked depolarizing postsynaptic potentials (DPSPs) followed by long-lasting small depolarizations. In a double stimulation test, a potentiation of the test DPSP was observed at interstimulus time interval of up to 80 ms. Post-tetanic potentiation of DPSPs was also seen in these neurons. Tests utilizing depolarizing current injection, intracellular Cl- injection, and Cl-conductance blocking drugs indicated that the DPSPs were composed of EPSPs and overlapping IPSPs. The nature of the long-lasting small depolarization succeeding the DPSPs could not be conclusively determined. However, available data suggest that the slow inward Ca-current may be responsible for this response.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo regulation of [3H]acetylcholine recognition sites in brain by nicotinic cholinergic drugs

The in vivo regulation of [3H]acetylcholine [( 3H]ACh) recognition sites on nicotinic receptors in rat brain was examined by administering drugs that increase stimulation of nicotinic cholinergic receptors, either directly or indirectly. After 10 days of treatment with the cholinesterase inhibitor diisopropyl fluorophosphate, [3H]ACh binding in the cortex, thalamus, striatum, and hypothalamus was decreased. Scatchard analyses indicated that the decrease in binding in the cortex was due to a reduction in the apparent density of [3H]ACh recognition sites. In contrast, after repeated administration of nicotine (5-21 days), the number of [3H]ACh recognition sites was increased in the cortex, thalamus, striatum, and hypothalamus. Similar effects were observed in the cortex and thalamus following repeated administration of the nicotinic agonist cytisin. The nicotinic antagonists mecamylamine and dihydro-beta-erythroidine did not alter [3H]ACh binding following 10-14 days of administration. Further, concurrent treatment with these antagonists and nicotine did not prevent the nicotine-induced increase in these binding sites. The data indicate that [3H]ACh recognition sites on nicotinic receptors are subject to up- and down-regulation, and that repeated administration of nicotine results in a signal for up-regulation, probably through protracted desensitization at the recognition site.

Patterns of muscarinic cholinergic binding in the striatum and their relation to dopamine islands and striosomes

The distribution of muscarinic cholinergic binding sites in the striatum was studied in relation to the locations of other neurochemical markers in the developing rat, cat, ferret, and human. In addition, patterns of striatal muscarinic binding were studied in the adult cat. Receptor binding autoradiography was carried out with tritiated propylbenzilylcholine mustard [( 3H]-PrBCM), an irreversible muscarinic antagonist, and subsequent serial section analyses involved comparisons among patterns of muscarinic binding, catecholamine histofluorescence, acetylcholinesterase (AChE) staining, Nissl staining, and cell labeling with [3H]-thymidine. Muscarinic binding in the immature striatum was characterized by local patchiness as well as regional density gradients in all species, with the most complex patterns appearing in the human. Patches of dense muscarinic binding were shown to lie in register with fluorescent dopamine islands (rat, cat, ferret), with AChE-positive patches (all species), and with clusters of neurons pulse-labeled by exposure to [3H]-thymidine on embryonic day 27 (ferret). At the developmental stages examined, the [3H]-PrBCM-positive patches were roughly aligned with regions of weak Nissl staining (cat, human). Striatal [3H]-PrBCM binding in the adult cat was dense, and though it usually appeared nearly homogeneous, in some sections patches of elevated binding were present. These had as counterparts, in neighboring sections, AChE-poor striosomes. We conclude that during development muscarinic cholinergic function is compartmentalized in the striatum in association with dopamine-containing afferents, and that this compartmentalization may persist to some degree in the adult.