Pharmacologically defined M1 and M2 muscarinic cholinergic binding sites in the cat's substantia nigra: development and maturity

Role of pedunculopontine cholinergic neurons in the vulnerability of nigral dopaminergic neurons in Parkinson's disease

Pedunculopontine nucleus (PPN) cholinergic neurons, which exert excitatory nicotinic control over substantia nigra dopaminergic neurons, degenerate in Parkinson's disease (PD). This finding and other studies showing that nicotine, the preferential agonist of nicotinic acetylcholine receptors, is neuroprotective in experimental models of PD suggest that a deficit in PPN excitatory cholinergic inputs might contribute to the death of nigral dopaminergic neurons in PD. To explore this possibility, we used lesion paradigms of dopaminergic and/or cholinergic systems in rats and monkeys. Consistent with our hypothesis, we observed that stereotaxic lesioning of PPN cholinergic neurons with diphtheria toxin coupled to urotensin II resulted in a significant loss of nigral dopaminergic neurons in rats and induced morphological changes in these neurons in macaques. Unexpectedly, a lesion of dopaminergic neurons induced by unilateral striatal injection of 6-hydroxydopamine (6-OHDA) in rats, or by repeated systemic injections of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in macaques, led to a 29% and 7% loss of PPN cholinergic neurons, respectively. Lastly, when the PPN cholinergic lesion was performed in rats in which the dopaminergic lesion induced by 6-OHDA was in progress, loss of cholinergic neurons was more drastic than when each neurotransmitter system was lesioned separately. Thus, our results suggest that strong PPN cholinergic and dopaminergic interactions may be an important mechanism in the pathophysiology of PD.

Intrinsic and integrative properties of substantia nigra pars reticulata neurons

The GABA projection neurons of the substantia nigra pars reticulata (SNr) are output neurons for the basal ganglia and thus critical for movement control. Their most striking neurophysiological feature is sustained, spontaneous high frequency spike firing. A fundamental question is: what are the key ion channels supporting the remarkable firing capability in these neurons? Recent studies indicate that these neurons express tonically active type 3 transient receptor potential (TRPC3) channels that conduct a Na-dependent inward current even at hyperpolarized membrane potentials. When the membrane potential reaches -60 mV, a voltage-gated persistent sodium current (I(NaP)) starts to activate, further depolarizing the membrane potential. At or slightly below -50 mV, the large transient voltage-activated sodium current (I(NaT)) starts to activate and eventually triggers the rapid rising phase of action potentials. SNr GABA neurons have a higher density of I(NaT), contributing to the faster rise and larger amplitude of action potentials, compared with the slow-spiking dopamine neurons. I(NaT) also recovers from inactivation more quickly in SNr GABA neurons than in nigral dopamine neurons. In SNr GABA neurons, the rising phase of the action potential triggers the activation of high-threshold, inactivation-resistant Kv3-like channels that can rapidly repolarize the membrane. These intrinsic ion channels provide SNr GABA neurons with the ability to fire spontaneous and sustained high frequency spikes. Additionally, robust GABA inputs from direct pathway medium spiny neurons in the striatum and GABA neurons in the globus pallidus may inhibit and silence SNr GABA neurons, whereas glutamate synaptic input from the subthalamic nucleus may induce burst firing in SNr GABA neurons. Thus, afferent GABA and glutamate synaptic inputs sculpt the tonic high frequency firing of SNr GABA neurons and the consequent inhibition of their targets into an integrated motor control signal that is further fine-tuned by neuromodulators including dopamine, serotonin, endocannabinoids, and H₂O₂.

Basal ganglia control of substantia nigra dopaminergic neurons

Although substantia nigra dopaminergic neurons are spontaneously active both in vivo and in vitro, this activity does not depend on afferent input as these neurons express an endogenous calcium-dependent oscillatory mechanism sufficient to drive action potential generation. However, afferents to these neurons, a large proportion of them GABAergic and arising from other nuclei in the basal ganglia, play a crucial role in modulating the activity of dopaminergic neurons. In the absence of afferent activity or when in brain slices, dopaminergic neurons fire in a very regular, pacemaker-like mode. Phasic activity in GABAergic, glutamatergic, and cholinergic inputs modulates the pacemaker activity into two other modes. The most common is a random firing pattern in which interspike intervals assume a Poisson-like distribution, and a less common pattern, often in response to a conditioned stimulus or a reward in which the neurons fire bursts of 2-8 spikes time-locked to the stimulus. Typically in vivo, all three firing patterns are observed, intermixed, in single nigrostriatal neurons varying over time. Although the precise mechanism(s) underlying the burst are currently the focus of intensive study, it is obvious that bursting must be triggered by afferent inputs. Most of the afferents to substantia nigra pars compacta dopaminergic neurons comprise monosynaptic inputs from GABAergic projection neurons in the ipsilateral neostriatum, the globus pallidus, and the substantia nigra pars reticulata. A smaller fraction of the basal ganglia inputs, something less than 30%, are glutamatergic and arise principally from the ipsilateral subthalamic nucleus and pedunculopontine nucleus. The pedunculopontine nucleus also sends a cholinergic input to nigral dopaminergic neurons. The GABAergic pars reticulata projection neurons also receive inputs from all of these sources, in some cases relaying them disynaptically to the dopaminergic neurons, thereby playing a particularly significant role in setting and/or modulating the firing pattern of the nigrostriatal neurons.

Inhibitory avoidance memory deficit induced by scopolamine: Interaction of cholinergic and glutamatergic systems in the ventral tegmental area

Interaction of cholinergic and glutamatergic inputs in the ventral tegmental area (VTA) influencing a learned behavior is a topic of great interest. In the present study the effect of intra-VTA administration of a nonselective muscarinic acetylcholine antagonist, scopolamine, and N-methyl-d-aspartate (NMDA) receptor agents by themselves as well as their interactions on consolidation and retrieval of inhibitory avoidance (IA) memory have been investigated. A step-through inhibitory avoidance task was used for memory assessment in male Wistar rats. The results showed that intra-VTA administration of scopolamine (1 and 2microg/rat) and NMDA receptor antagonist, MK-801 (0.75 and 1microg/rat) immediately after training, impaired consolidation of IA memory. Interestingly, co-administration of an ineffective dose of MK-801 (0.5microg/rat) with ineffective doses of scopolamine (0.25 and 0.5microg/rat) significantly decreased the consolidation process. Post-training intra-VTA injections of NMDA (0.001 and 0.01microg/rat) had no effects by itself, whereas its co-administration with scopolamine (2microg/rat) prevented the effect of the later drug. The results also showed that pre-test intra-VTA administration of scopolamine (3 and 4microg/rat) and MK-801 (1 and 2microg/rat) impaired retrieval of the IA memory. Moreover, co-administration of an ineffective dose of MK-801 (0.5microg/rat) with ineffective doses of scopolamine (1 and 2microg/rat) increasingly reduced the retrieval of the IA memory. On the contrary to its post-training treatment, pre-test administration of NMDA either alone or in combination with scopolamine caused no significant effect on retrieval of IA memory. It can be concluded that muscarinic acetylcholine and NMDA glutamate receptors in the VTA are involved in the mechanism(s) underlying consolidation and retrieval of the IA memory.

Developmental aspects of the cholinergic system

Beyond its importance in sustaining or modulating different aspects of the activity of the central nervous system (CNS), the cholinergic system plays important roles during development. In the current review, we focus on the developmental aspects associated with major components of the cholinergic system: Acetylcholine, choline acetyltransferase, vesicular acetylcholine transporter, high-affinity choline transporter, acetylcholinesterase, nicotinic and muscarinic receptors. We describe when and where each one of these components is first identified in the CNS and the changes in their levels that occur during the course of prenatal and postnatal development. We also describe how these components are relevant to many events that occur during the development of the CNS, including progenitor cells proliferation and differentiation, neurogenesis, gliogenesis, neuronal maturation and plasticity, axonal pathfinding, regulation of gene expression and cell survival. It will be noticed that evidence regarding the developmental aspects of the cholinergic system comes mostly from studies that used agonists, such as nicotine, and antagonists, such as hemicholinium-3. Studies using immunohistochemistry and genetically altered mice also provided valuable information.

Developmental pattern of M1 and M2 muscarinic gene expression and receptor levels in cat carotid body, petrosal and superior cervical ganglion

Using real-time reverse transcriptase polymerase chain reaction, Northern blot, and Western blot analyses, we evaluated the developmental pattern of mRNA and protein expression level of muscarinic M1 and M2 receptors in the carotid body, petrosal ganglion and superior cervical ganglion of 1-day, 15-day, 2-month-old and adult cats. mRNA expression and protein levels of tyrosine hydroxylase, the rate limiting enzyme for dopamine synthesis, were also assessed. Carotid body M1 receptor mRNA, increased significantly by approximately 100% and 300% in 2-month and adult vs. 1- and 15-day-old cats, but protein level decreased gradually being approximately 50% lower compared with 1-day-old cats. In the petrosal ganglion, muscarinic M1 receptor mRNA level was higher in 15-day-old cats vs. 1-day-old, 2-month-old and adult cats and protein levels were about 30% lower than in 1- and 15-day-old cats. In the superior cervical ganglion, muscarinic M1 receptor mRNA was approximately 50% and 80% higher in 2-month-old and adult cats than 1- and 15-day-old, but no changes in the protein level except in 15-day-old cats which was approximately 40% higher than 1-day-old. There was no change of muscarinic M2 receptor mRNA or protein level in the carotid body or petrosal ganglion. However, in the superior cervical ganglion, the significant increase of mRNA of 30% and 50% in 2-month-olds and adults, respectively was not associated with an increase in receptor protein. Tyrosine hydroxylase mRNA and protein level decreased significantly with age in the carotid body and petrosal ganglion. In the superior cervical ganglion, the age dependent increase in tyrosine hydroxylase mRNA was not associated with any changes in the protein level. These results show that the expression of muscarinic M1 and M2 receptors are age and organ-dependent in cats. Consequently, these changes may modulate chemosensory activity during development since muscarinic M1 receptor is predominantly involved in postsynaptic chemosensory activity, while muscarinic M2 receptor modulates acetylcholine and dopamine release from chemosensitive cells.

Diet restriction in mice causes a decrease in hippocampal choline uptake and muscarinic receptors that is restored by administration of tyrosine: interaction between cholinergic and adrenergic receptors influencing cognitive function

We have studied the effects of diet restriction (DR) to 60% and 40% of daily requirements, and tyrosine administration on cognitive function in mice, to define the nutritional-neurochemical interactions on autonomic tone involved in behavior and energy regulation. Cognitive function in the Morris Water maze was significantly impaired after 40% DR compared to both control and 60% DR. It was restored after tyrosine in association with increased M1 cholinergic and beta-adrenergic receptor function, and decreased alpha-adrenergic function. DR to 40% significantly decreased choline uptake (p <.05) and M1 receptor number (Bmax) (p <.05), without changes in affinity (Kd), choline acetyl transferase (ChAT) or acetyl cholinesterase (AChE) activity. Tyrosine administration significantly increased choline uptake (Bmax) (p <.05) and M1 density in the 40% DR (p <.01) without changes in affinity. ChAT activity was decreased after tyrosine--significantly after 40% DR (p <.05) while AChE was not affected. Both M1 mRNA and protein were not influenced by DR or tyrosine administration. Tyrosine hydroxylase mRNA was decreased significantly by 40% DR (p <.01). The effect of DR and tyrosine appeared to be both pre- and post-synaptic, indicating modulation of cholinergic activity by adrenergic tone. Nutritional effect on behavior and autonomic tone may have implications for the treatment of mood changes associated with weight loss and semi-starvation.

Muscarinic antagonists in substantia nigra influence the decarboxylation of L-dopa in striatum

This study was designed to investigate whether anticholinergic drugs acting at the level of the substantia nigra can affect basal extracellular dopamine concentrations and the levodopa (L-dopa)-induced increases in dopamine levels in the striatum. Dual probe in vivo microdialysis in freely moving rats was used. One microdialysis probe was implanted in the substantia nigra and the other in the ipsilateral striatum. Muscarinic receptor antagonists were perfused into the substantia nigra and changes in neurotransmitter levels in the substantia nigra and at the axon terminals in the striatum were monitored simultaneously. Nigral perfusion of the non-selective muscarinic receptor antagonist trihexyphenidyl (1 mM) produced an increase in extracellular dopamine and gamma-aminobutyric acid (GABA) levels in the substantia nigra. Perfusion with the muscarinic M(1) receptor antagonist telenzepine (0.1 microM) produced a significant decrease in nigral dopamine and GABA levels in the substantia nigra. The muscarinic M(2) receptor antagonist methoctramine (75 microM) produced an increase in dopamine levels in the substantia nigra. No significant changes in nigral extracellular GABA levels were observed. The L-dopa-induced increases in extracellular dopamine levels in the striatum were clearly attenuated under nigral perfusion of these drugs. This in vivo study demonstrates that anticholinergic drugs perfused at the level of the substantia nigra can modulate dopamine and GABA levels and attenuate the L-dopa decarboxylation in the striatum, possibly via modulation of the nigrostriatal dopaminergic system. We add further evidence that the substantia nigra is an important site of action of antimuscarinic drugs. The attenuation of L-dopa-induced dopamine release in the striatum exerted by nigral perfusion of these antimuscarinic drugs is probably mediated via different mechanisms. This attenuation is regarded as a beneficial effect of the muscarinic antagonists as adjuncts to L-dopa in Parkinson's disease treatment. We postulate that drugs that enhance dopamine release, after L-dopa administration, in a less extreme way than L-dopa administered on its own could prevent further neurodegeneration and dyskinesias thought to result from extremely high extracellular dopamine levels following L-dopa treatment.

Ultrastructural localization of the alpha4-subunit of the neuronal acetylcholine nicotinic receptor in the rat substantia nigra

The distribution of the alpha4-subunit of the neuronal nicotinic acetylcholine receptor (nAChR) in the rat brain was examined at light and electron microscopy levels using immunohistochemical staining. In the present study we demonstrate the specificity, in both tissue homogenates and brain sections, of a polyclonal antibody raised against the rat nAChR alpha4-subunit. The characterization of this antibody involved: (1) Western blot analysis of rat brain homogenates and membrane extracts from cells previously transfected with diverse combinations of neuronal nAChR subunits, and (2) immunohistochemistry using transfected cells and rat brain tissue. At the light microscope level, the alpha4-subunit-like-immunoreactivity (LI) was widely distributed in the rat brain and matched the distribution of the alpha4-subunit transcripts observed previously by in situ hybridization. Strong immunohistochemical labeling was detected in the mesencephalic dopaminergic nuclei. The nAChRs in this region are thought to be responsible for the modulation of dopaminergic transmission. The neurotransmitter identity of alpha4-immunolabeled neurons in the substantia nigra pars compacta (SNpc) and the ventral tegmental area was thus assessed by investigating the possible colocalization of the nAChR alpha4-subunit with tyrosine hydroxylase using confocal microscopy. The double labeling experiments unambiguously indicated that the alpha4-subunit-LI is present in dopaminergic neurons. At the electron microscope level, the neurons in the SNpc exhibited alpha4-subunit-LI in association with a minority of postsynaptic densities, suggesting that the alpha4-subunit may be a component of functional nAChRs mediating synaptic transmission between midbrain cholinergic neurons and mesencephalic dopaminergic neurons.

Interaction of benztropine and haloperidol actions on rat substantia nigra dopamine cell electrophysiological activity in vivo

Despite the known efficacy of antimuscarinic agents in treating dopamine-related movement disorders, their effects on dopamine cell activity have not been well studied. We investigated the effects of systemically administered benztropine, an antimuscarinic agent that also inhibits the dopamine transporter, on substantia nigra dopamine neuron electrophysiological activity. Benztropine caused a dose-dependent inhibition of the firing rate of dopamine neurons in control rats but exerted baseline-dependent changes in burst firing. In rats pre-treated with haloperidol, which prevents the effects of dopamine transporter inhibitors on dopamine cell activity, benztropine either increased or decreased firing rate; however, it consistently increased burst firing. Thus, the antimuscarinic and psychostimulant properties of benztropine have differential effects on dopamine neuron firing rate and burst firing. The increase in burst firing seen in the presence of haloperidol may mediate some of the therapeutic effects of benztropine in the treatment of antipsychotic drug-induced movement disorders.

Involvement of CNS cholinergic systems in alcohol drinking of P rats

Experiments were undertaken to determine if CNS muscarinic- and nicotinic-cholinergic receptors are involved in regulating alcohol drinking of rats from the selectively-bred alcohol-preferring P line. Intracerebroventricular (i.c.v.) drug infusions were administered into the lateral ventricle of female P rats 15 minutes before ethanol access. The muscarinic antagonists pirenzepine and scopolamine were tested on limited access (4 hours/day) to a 10% (v/v) ethanol solution. Food and water were available ad libitum. Nicotine and the nicotinic antagonist mecamylamine were tested on limited access (4 hours/day) to 10% (v/v) ethanol and 0.0125% saccharin solutions. Food was available ad libitum and water was available during the remaining 20 hours. The baseline ethanol intakes ranged between an average of 3.0 ± 0.3 g/kg/4 hours and 3.4 ± 0.3 g/kg/4 hours. Administration of 40-100 m g pirenzepine (M1-selective antagonist) had no effect on ethanol, food or water consumption. However, 20-80 m g scopolamine, a non-selective muscarinic antagonist, dosedependently decreased ethanol intake as much as 60% (p < 0.05) without altering food or water consumption. The nicotinic antagonist mecamylamine (20-120 m g) did not alter ethanol intake, but nicotine (40-80 m g) dose-dependently decreased ethanol drinking as much as 60% within the first 30 minutes (p < 0.05) without an effect on saccharin intake. The results suggest that: (a), muscarinic receptors, with the possible exception of the M1 subtype, are involved in regulating alcohol drinking and (b), activation of nicotinic receptors can reduce alcohol drinking of the P line of rats.

Increased striatal dopamine efflux follows scopolamine administered systemically or to the tegmental pedunculopontine nucleus

The cholinergic cells of the tegmental pedunculopontine nucleus monosynaptically excite dopaminergic neurons of the substantia nigra. In vivo electrochemical methods were used to monitor dorsal striatal dopamine efflux in awake rats following intraperitoneal scopolamine injections and following the direct application of scopolamine to the tegmental pedunculopontine nucleus. Systemic injections of scopolamine (1.0, 3.0 or 10.0 mg/kg) resulted in dose-related increases in peak striatal dopamine oxidation currents of between 1.1 and 2.0 nA. Increases began within 10-20 min after injection and peaked after 40-90 min. Unilateral microinjections of scopolamine into the tegmental pedunculopontine nucleus (10, 50 or 100 micrograms/0.5 microliter) resulted in dose-related increases in dopamine oxidation currents that peaked 60-90 min postinjection (2.9-5.0 nA). Carbachol (4.0 micrograms/0.5 microliter) injected unilaterally into the tegmental pedunculopontine nucleus 20 min before 100 micrograms tegmental pedunculopontine nucleus scopolamine, or injected bilaterally 20 min before 3.0 mg/kg systemic scopolamine, attenuated the increases produced by scopolamine alone. The carbachol preinjection tests suggest that the effects of both systemic and tegmental pedunculopontine nucleus scopolamine treatments are mediated largely by muscarinic receptors near the tegmental pedunculopontine nucleus. These findings are consistent with the proposal that enhanced activation of substantia nigra dopamine cells results from scopolamine-induced disinhibition of the tegemental pedunculopontine nucleus cholinergic cell group via blockade of their inhibitory autoreceptors.

Comparison of the behavioural effects of infusion of carbachol and acetylcholinesterase into the rat substantia nigra

It has been postulated for many years that acetylcholinesterase (AChE) may play a nonclassical role in the substantia nigra, unrelated to its ability to hydrolyse acetylcholine. In this study the behavioural effects of unilateral infusion of AChE and a cholinergic agonist, carbachol, were compared. Carbachol induced ipsiversive circling over a very short time scale (minutes), whereas AChE induced contraversive circling, but over a longer time course-10 days. Both agents showed selectivity of response within the substantia nigra: acetylcholinesterase was only effective when infused into the most rostral region of the substantia nigra and its effects were limited to the pars compacta. In contrast, carbachol had effects in both the pars compacta and reticulata, with a graded sensitivity to carbachol in the rostral/caudal plane; infusions into rostral regions induced high rates of circling compared to more caudal areas, suggesting that the cholinergic input to the substantia nigra is not homogenous, but greater in rostral regions. This disparity between the effects of carbachol and AChE would, therefore, suggest that AChE is not exerting its long-term behavioural actions via a cholinergic mechanism, both in terms of time course of the response and the areas within the substantia nigra sensitive to these agents.

Muscarinic receptor stimulation increases the spontaneous [3H]GABA release in the rat substantia nigra through muscarinic receptors localized on striatonigral terminals

The effect of muscarinic agonists on the spontaneous release of [3H]GABA was investigated in vitro on rat substantia nigra slices. Acetylcholine (5 x 10(-5) M) in the presence of eserine (5 x 10(-5) M) induced a 12.3% increase of the spontaneous release of [3H]GABA. Similarly, carbachol (5 x 10(-4) M) enhanced by 9% the release of [3H]GABA. This effect was Ca(2+)-dependent, it was abolished in the presence of 0.4 mM Ca2+ and enhanced from 9 to 17% when Ca(2+)-concentration of the superfusion medium was increased from 1.3 to 2.4 mM. The carbachol effect was mediated by muscarinic receptors since it was abolished by atropine (2 x 10(-6) M). The pharmacologically M2 muscarinic receptor subtypes seems to be involved as the carbachol-induced effect was abolished by AF-DX384MS (10(-6) M), an M2 antagonist and was only partially reversed by pirenzepine (10(-5) and 10(-4) M), an M1 antagonist which at these doses also block the M2 receptors. The absence of effect of SCH23390 (10(-6) M) a D1 antagonist as well as the lack of effect of CNQX (10(-5) M) and dizocilpine maleate (10(-6) M), two glutamate antagonists, on the carbachol-induced effect indicated that neither dopamine (through D1 receptors) nor glutamate (through ionotropic receptors) were involved in the response. In addition, the persistence of the carbachol-induced effect in the presence of tetrodotoxin (2 x 10(-7) M) suggests a direct muscarinic-mediated modulation of [3H]GABA. The localization of muscarinic receptors on striatonigral fibres was confirmed by autoradiographic studies showing a decrease of [3H]pirenzepine binding in the substantia nigra after a unilateral striatal lesion induced by kainic acid injection. This latter result provides evidence of the presence of M1 receptors on striatonigral terminals as the concentration of [3H]pirenzepine used (10 nM) is M1-selective. These results indicate a cholinergic modulation of GABA release in the rat substantia nigra mediated by muscarinic receptors localized on striatonigral terminals. The involvement of the m4 muscarinic receptor subtype that have a M1/M2 pharmacology is discussed.

Pedunculopontine nucleus in the squirrel monkey: cholinergic and glutamatergic projections to the substantia nigra

The distribution and chemospecificity of the pedunculonigral neurons have been studied in squirrel monkeys (Saimiri sciureus) with cholera toxin subunit B (CTb) and fluorogold (FG) as retrograde tracers combined with immunohistochemistry for choline acetyltransferase (ChAT), glutamate, and the calcium binding protein calbindin D-28k. The injection of either CTb or FG into the substantia nigra produces prominent retrograde cell labeling in the mesopontine tegmentum. Labeled neurons are particularly numerous at the level of the decussation of the superior cerebellar peduncle, where they abound principally in the pars dissipata of the pedunculopontine nucleus (PPN). A significant proportion of retrogradely labeled neurons in the PPN display ChAT immunoreactivity. Within the entire PPN, approximately 25% of the retrogradely labeled neurons express ChAT immunoreactivity, but proportions of doubly labeled neurons are about 35%, 25%, and 15% in the rostral, middle, and caudal thirds of the PPN, respectively. These doubly labeled neurons are scattered among numerous retrogradely labeled neurons that are ChAT-negative and whose number increases along the rostrocaudal extent of the PPN. Several retrogradely labeled neurons in the PPN display glutamate immunoreactivity, but very few express calbindin. This study provides the first direct evidence for the involvement of cholinergic and glutamatergic neurons in the pedunculonigral projection in primates. Furthermore, the fact that some neurons of the PPN display both ChAT and glutamate immunoreactivity indicates that single neurons in the mesopontine tegmentum may exert a two-fold effect upon dopaminergic neurons of the substantia nigra. This dual cholinergic and glutamatergic pedunculonigral projection may play a crucial role in the functional organization of primate basal ganglia.

Carbachol-induced phosphoinositide metabolism in slices of rat substantia nigra pars reticulata

Carbachol produced an approximately 2-fold stimulation, with respect to basal, of [3H]inositol monophosphate ([3H]IP1) accumulation in microdissected slices of rat substantia nigra pars reticulata (SNr), measured as a percentage of [3H]inositol incorporation. The response to carbachol was inhibited by 75 +/- 7% by 1 microM pirenzepine, consistent with a muscarinic M1 receptor-mediated response. Carbachol-induced [3H]IP1 accumulation was not reduced by pretreatment of the SNr slices with NMDA, in contrast to the 85% inhibition observed in striatal slices. The results suggest that measurement of carbachol-induced phosphoinositide hydrolysis may be a feasible approach for studying muscarinic M1 receptor function in slices of SNr.

Neurotransmitter regulation of dopamine neurons in the ventral tegmental area

Over the last 10 years there has been important progress towards understanding how neurotransmitters regulate dopaminergic output. Reasonable estimates can be made of the synaptic arrangement of afferents to dopamine and non-dopamine cells in the ventral tegmental area (VTA). These models are derived from correlative findings using a variety of techniques. In addition to improved lesioning and pathway-tracing techniques, the capacity to measure mRNA in situ allows the localization of transmitters and receptors to neurons and/or axon terminals in the VTA. The application of intracellular electrophysiology to VTA tissue slices has permitted great strides towards understanding the influence of transmitters on dopamine cell function, as well as towards elucidating relative synaptic organization. Finally, the advent of in vivo dialysis has verified the effects of transmitters on dopamine and gamma-aminobutyric acid transmission in the VTA. Although reasonable estimates can be made of a single transmitter's actions under largely pharmacological conditions, our knowledge of how transmitters work in concert in the VTA to regulate the functional state of dopamine cells is only just emerging. The fact that individual transmitters can have seemingly opposite effects on dopaminergic function demonstrates that the actions of neurotransmitters in the VTA are, to some extent, state-dependent. Thus, different transmitters perform similar functions or the same transmitter may perform opposing functions when environmental circumstances are altered. Understanding the dynamic range of a transmitter's action and how this couples in concert with other transmitters to modulate dopamine neurons in the VTA is essential to defining the role of dopamine cells in the etiology and maintenance of neuropsychiatric disorders. Further, it will permit a more rational exploration of drugs possessing utility in treating disorders involving dopamine transmission.

A comparison of behaviour following stimulation of the anterior substantia nigra by direct cholinergic agonists and anticholinesterases

Microinjections of carbachol, a muscarinic cholinergic receptor agonist, into the anterior substantia nigra increase feeding, drinking and sexual behaviour if there is a pre-existing tendency to respond and a low baseline rate of behaviour. The present experiment was undertaken to compare the effects of carbachol with other cholinergic stimulants. Groups of 6-12 satiated rats received 0.5 microliter microinjections into the anterior substantia nigra of 0.1-5.0 micrograms carbachol, 0.1-5.0 micrograms nicotine, 2.5-10.0 micrograms eserine, and 1.25-5.0 micrograms or 0.1-1.0 microgram neostigmine (each dissolved in sterile saline) and the effects on feeding, drinking, locomotion, grooming, rearing and sniffing were examined. Carbachol, nicotine and low doses of neostigmine stimulated eating in a dose-dependent manner. The increased feeding following neostigmine was over a shorter time-period than following carbachol or nicotine. Neither carbachol nor nicotine had any significant effect on behaviour other than eating. The higher doses of neostigmine increased the frequency of sniffing and rearing, but not eating, and no dose of eserine had a clear effect on behaviour. These data are discussed in terms of their relationship to the cholinergic input to substantia nigra which excites pars compacta dopamine-containing neurones.