D1 and D2 dopamine receptor-regulated gene expression of striatonigral and striatopallidal neurons

The striatum, which is the major component of the basal ganglia in the brain, is regulated in part by dopaminergic input from the substantia nigra. Severe movement disorders result from the loss of striatal dopamine in patients with Parkinson's disease. Rats with lesions of the nigrostriatal dopamine pathway caused by 6-hydroxydopamine (6-OHDA) serve as a model for Parkinson's disease and show alterations in gene expression in the two major output systems of the striatum to the globus pallidus and substantia nigra. Striatopallidal neurons show a 6-OHDA-induced elevation in their specific expression of messenger RNAs (mRNAs) encoding the D2 dopamine receptor and enkephalin, which is reversed by subsequent continuous treatment with the D2 agonist quinpirole. Conversely, striatonigral neurons show a 6-OHDA-induced reduction in their specific expression of mRNAs encoding the D1 dopamine receptor and substance P, which is reversed by subsequent daily injections of the D1 agonist SKF-38393. This treatment also increases dynorphin mRNA in striatonigral neurons. Thus, the differential effects of dopamine on striatonigral and striatopallidal neurons are mediated by their specific expression of D1 and D2 dopamine receptor subtypes, respectively.

Dopamine acts on D2 receptors to increase potassium conductance in neurones of the rat substantia nigra zona compacta

1. Intracellular recordings were made from neurones in the substantia nigra zona compacta in slices of rat mesencephalon in vitro. The majority of neurones fired action potentials spontaneously at 0.2-5.6 Hz. Dopamine, applied either by superfusion or from the tip of a pressurized pipette, prevented spontaneous action potential firing and hyperpolarized the membrane. 2. When the membrane potential was held negative to the threshold for action potential firing, the hyperpolarization evoked by dopamine was accompanied by a fall in input resistance. Under voltage clamp, dopamine produced an outward membrane current associated with an increase in membrane conductance. The effects of superfused dopamine on firing rate, membrane potential and membrane current were concentration dependent in the range 1-100 microM. 3. The reversal potential for the hyperpolarizations and the outward currents produced by dopamine was -109.7 +/- 1.7 mV (n = 12) when the potassium concentration was 2.5 mM and -74.0 +/- 5.0 mV (n = 4) when the potassium concentration was 10.5 mM. The change in reversal potentials in these and intermediate potassium concentrations was described by the Nernst equation. 4. The outward current induced by dopamine was reversibly reduced by barium (100-300 microM) and by high concentrations of tetraethylammonium (greater than or equal to 10 mM). Calcium-free solutions with cobalt (0.5-2 mM) did not reduce the current in response to dopamine during the first 5 min of their application. Currents and hyperpolarizations caused by dopamine were unaffected by tetrodotoxin (1 microM). 5. The hyperpolarization produced by dopamine was mimicked by the D2 receptor agonist quinpirole (LY 171555, 0.1-3 microM) and was blocked by the D2 receptor agonists domperidone and (-)-sulpiride. Agonists and antagonists at D1 receptors had no effect. 6. (-)-Sulpiride (30 nM-30 microM) produced a progressive shift to the right in the concentration-response curve to either dopamine or quinpirole. Schild analysis of the antagonism between (-)-sulpiride and quinpirole suggested competitive antagonism with a dissociation equilibrium constant for (-)-sulpiride of about 13 nM. 7. It is concluded that dopamine acts on D2 receptors on neurones of the rat substantia nigra pars compacta to increase the membrane potassium conductance.

Neuromodulatory actions of dopamine in the neostriatum are dependent upon the excitatory amino acid receptor subtypes activated

In the mammalian neostriatum, dopamine modulates neuronal responses mediated by activation of excitatory amino acid receptors. The direction of this modulation varies with the specific subtype of excitatory amino acid receptor activated. Responses evoked by iontophoretic application of glutamate (Glu) and the non-N-methyl-D-aspartate (NMDA) agonists quisqualate and alpha-amino-3-hydroxy-5-methyl-4- isoxazolepropionic acid were significantly attenuated when dopamine was applied. In contrast, responses evoked by NMDA were markedly potentiated. The enhancement of NMDA-evoked excitations was mimicked by bath application of SKF 38393, a D1 receptor agonist. The D1 receptor antagonist SCH 23390 blocked the dopamine enhancement of NMDA-induced excitations. Quinpirole, a D2 receptor agonist, attenuated responses evoked by both NMDA and non-NMDA receptor agonists. These results indicate that the complex modulatory actions of dopamine in the neostriatum are a function of the excitatory amino acid receptor as well as the specific dopamine receptor subtype activated. These findings are of clinical relevance since the actions of dopamine and excitatory amino acids have been implicated in neurological and affective disorders.

Dopaminergic and ligand-independent activation of steroid hormone receptors

The current view of how steroid hormone receptors affect gene transcription is that these receptors, on binding ligand, change to a state in which they can interact with chromatin and regulate transcription of target genes. Receptor activation is believed to be dependent only on this ligand-binding event. Selected steroid hormone receptors can be activated in a ligand-independent manner by a membrane receptor agonist, the neurotransmitter dopamine. In vitro, dopamine faithfully mimicked the effect of progesterone by causing a translocation of chicken progesterone receptor (cPR) from cytoplasm to nucleus. Dual activation by progesterone and dopamine was dissociable, and a serine residue in the cPR was identified that is not necessary for progesterone-dependent activation of cPR, but is essential for dopamine activation of this receptor.

D1 dopamine receptor activation required for postsynaptic expression of D2 agonist effects

D1 and D2 dopamine receptors exert synergistic effects on the firing rates of basal ganglia neurons and on the expression of stereotyped behavior in rats. Moreover, the ability of D2 agonists to induce changes in basal ganglia single unit activity and spontaneous motor activity is dependent upon the presence of endogenous dopamine to stimulate D1 receptors; in rats treated with alpha-methyl-rho-tyrosine to reduce endogenous dopamine levels, the neurophysiological and behavioral effects of the D2 agonist quinpirole are significantly attenuated, while the effects of nonselective agonists like apomorphine, which stimulate both D1 and D2 receptors, or combinations of a D2 agonist and a D1 agonist are not attenuated. Thus, the previously held view that D2 receptors alone are responsible for evoking the changes in behavior and basal ganglia output induced by nonselective dopamine agonists and endogenous dopamine is not supported by these results, which indicate that these phenomena require concurrent stimulation of both dopamine receptor subtypes.

Opposite modulation of cocaine-seeking behavior by D1- and D2-like dopamine receptor agonists

Activation of the mesolimbic dopamine system is known to trigger relapse in animal models of cocaine-seeking behavior. We found that this "priming" effect was selectively induced by D2-like, and not by D1-like, dopamine receptor agonists in rats. Moreover, D1-like receptor agonists prevented cocaine-seeking behavior induced by cocaine itself, whereas D2-like receptor agonists enhanced this behavior. These results demonstrate an important dissociation between D1- and D2-like receptor processes in cocaine-seeking behavior and support further evaluation of D1-like receptor agonists as a possible pharmacotherapy for cocaine addiction.

Modulation of cocaine self-administration in the rat through D-3 dopamine receptors

The reinforcing properties of cocaine are probably mediated by the mesocorticolimbic dopamine pathways in the central nervous system, but not all of the dopamine receptor subtypes involved in cocaine's reinforcing actions have been clearly identified. Recently, the D-3 receptor has been cloned, and its distribution in the brain has been found to be relatively restricted to limbic projections of the midbrain dopamine system. The D-3-selective compounds 7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OHDPAT) and quinpirole potently decreased cocaine self-administration in the rat at doses that were not by themselves reinforcing. Moreover, three dopamine receptor agonists had affinities for binding to the D-3 receptor that correlated highly with their relative potencies in decreasing cocaine self-administration. The D-3 receptor may be involved in the reinforcing effects of cocaine and may be a useful target for the development of new pharmacotherapies for cocaine abuse.

Dissociation of hippocampus and caudate nucleus memory systems by posttraining intracerebral injection of dopamine agonists

The effect of posttraining intracerebral injections of the indirect dopamine (DA) agonist d-amphetamine, the direct D2 agonist LY 171555, and the direct D1 agonist SKF-38393 on the acquisition of two 8-arm radial maze tasks were examined. On a win-stay task, a light cue signaled the location of food in 4 randomly selected maze arms on each trial, and animals were required to visit each of the lit arms twice within a trial. Posttraining intracaudate injection of d-amphetamine (10.0 and 15.0 micrograms), LY 171555 (2.0 micrograms), and SKF-38393 (5.0 micrograms) all improved win-stay acquisition in relation to saline-injected controls. In contrast, posttraining intrahippocampal injection of DA agonists had no effect on win-stay acquisition. On a win-shift task, rats were allowed to obtain food from 4 randomly selected maze arms, followed by a delay period in which they were removed from the maze. They were returned to the maze for a retention test in which only those arms that had not been visited before the delay contained food. Posttraining intrahippocampal (but not intracaudate) injection of d-amphetamine (5.0 micrograms), LY 171555 (2.0 micrograms), and SKF-38393 (5.0 micrograms) all improved win-shift retention in relation to saline-injected controls. The results demonstrate a double dissociation of hippocampus and caudate nucleus memory functions and show that posttraining injection of both D1 and D2 agonists modulate the memory processes subserved by both hippocampus and caudate nucleus.

Dopamine receptor subtypes colocalize in rat striatonigral neurons

Dopaminergic neurons of the substantia nigra provide one of the major neuromodulatory inputs to the neostriatum. Recent in situ hybridization experiments have suggested that postsynaptic dopamine receptors are segregated in striatonigral and striatopallidal neurons. We have tested this hypothesis in acutely isolated, retrogradely labeled striatonigral neurons by examining the neuromodulatory effects of selective dopaminergic agonists on Na currents and by probing single-cell antisense RNA populations with dopamine receptor cDNAs. In most of the neurons examined (20/31), the application of the D1 dopamine receptor agonist SKF 38393 reduced evoked whole-cell Na+ current. The D2 agonists quinpirole and bromocriptine had mixed effects; in most neurons (23/42), whole-cell Na+ currents were reduced, but in others (8/42), currents were increased. In cell-attached patch recordings, bath application of SKF 38393 decreased currents as in whole-cell recordings, whereas quinpirole consistently (6/10) enhanced currents--suggesting that D2-like receptors could act through membrane delimited and non-delimited pathways. Changes in evoked current were produced by modulation of peak conductance and modest shifts in the voltage dependence of steady-state inactivation. Antisense RNA probes of dopamine receptor cDNA Southern blots consistently (5/5) revealed the presence of D1, D2, and D3 receptor mRNA in single striatonigral neurons. These findings argue that, contrary to a strict receptor segregation hypothesis, many striatonigral neurons colocalize functional D1, D2, and D3 receptors.

Obligatory D-1/D-2 receptor interaction in the generation of dopamine agonist related behaviors

Administered independently, the selective D-1 and D-2 agonists SKF 38393 and LY 171555 elicited dose dependent increases in complex motor behaviors such as locomotion or grooming. Typical stereotypic behaviors were observed only when these agents were combined and only at the higher doses of the D-1 agonist. AMPT pretreatment blocked the effects of the agonists administered independently, but all classes of behavior could be induced when they were administered in combination. Thus, expression of dopamine agonist induced behaviors requires concurrent activation of D-1 and D-2 receptors; the nature of the behavior appears to depend upon the ratio of D-1 to D-2 receptor activation.

Dopamine D-1 receptor agonists combined with the selective D-2 agonist quinpirole facilitate the expression of oral stereotyped behaviour in rats

The behaviour of rats was studied after combined treatment with the selective DA D-2 agonist quinpirole and three selective D-1 agonists (SK & F 38393, SK & F 75670 and Lu 24-040). The effects on behaviour were compared with those on receptor binding and adenylate cyclase (AC). While the D-1 agonists alone did not induce stereotyped behaviour, quinpirole induced dose-dependent hyperactivity (locomotion, sniffing, head movements and rearing), whereas licking/biting was absent or seen only occasionally. Combined treatment with quinpirole and a D-1 agonist was followed by dose-dependent licking and occasional biting behaviour. The D-1 agonists had similar efficacies, but SK & F 75670 and Lu 24-040 were more potent than SK & F 38393. The maximal effects of SK & F 38393 plus quinpirole were effectively blocked by either a D-1 antagonist (SCH 23390) or a D-2 antagonist (YM 09151-2) confirming the close relation between D-1 and D-2 receptor sites in the brain. Good correspondence was found between affinities to D-1 receptors [( 3H]SCH 23390 binding) in vitro and the EC50 values for stimulation of AC activity. However, the maximal effects on DA-sensitive AC activity were less for SK & F 75670 and Lu 24-040 than for SK & F 38393. Thus, the results indicate that efficacies in the adenylate cyclase assay are dissociated from those on behaviour. Furthermore, the data indicate that in normal rats D-1 receptors are functionally relevant since D-1 agonists facilitate the expression of oral stereotyped behaviour after combination with a D-2 agonist.

Dopamine activation of the arachidonic acid cascade as a basis for D1/D2 receptor synergism

Understanding the actions of the neurotransmitter dopamine in the brain is important in view of its roles in neuropsychiatric illnesses. Dopamine D1 receptors, which stimulate both adenylyl cyclase and phospholipase C, and D2 receptors, which inhibit them, can nevertheless act synergistically to produce many electrophysiological and behavioral responses. Because this functional synergism can occur at the level of single neurons, another, as yet unidentified, signalling pathway activated by dopamine has been hypothesized. We report here that in Chinese hamster ovary (CHO) cells transfected with the D2 receptor complementary DNA, D2 agonists potently enhanced arachidonic acid release, provided that such release has been initiated by stimulating constitutive purinergic receptors or by increasing intracellular Ca2+. In CHO cells expressed D1 receptors, D1 agonists exert no such effect. When D1 and D2 receptors are coexpressed, however, activation of both subtypes results in a marked synergistic potentiation of arachidonic acid release. The numerous actions of arachidonic acid and its metabolites in neuronal signal transduction suggest that facilitation of its release may be implicated in dopaminergic responses, such as feedback inhibition mediated by D2 autoreceptors, and may constitute a molecular basis for D1/D2 receptor synergism.

Biphasic effect of D-2 agonist quinpirole on locomotion and movements

The effects of the D-2 agonist quinpirole on forward progression, and on vertical and lateral movements, were measured for 2 h in rats injected with either saline, 0.03, 0.125, 0.5 or 8 mg/kg of the drug. Results showed that the drug had a biphasic effect: the lowest dose decreased and the high doses increased the amount of locomotion and of movement. The decrease in activity produced by the low dose had its onset within minutes after administration of the drug; persistent and marked hyperactivity was observed from about 60-80 min after injection of 0.5-8 mg/kg of quinpirole. Moreover, in animals injected with intermediate doses, the excitation was preceded by a brief period of reduced locomotion. It is suggested that the biphasic effect may reflect two independent actions of the drug, possibly on activity in the nucleus accumbens.

Apomorphine-induced penile erection and yawning: site of action in brain

Microinjection of the dopamine (DA) agonist apomorphine into the paraventricular nucleus of the hypothalamus (PVN) induced penile erection and yawning in rats. A significant effect was elicited by a dose of apomorphine as low as 5 ng. The symptomatology usually began within 5 min after the microinjection, lasted for 30-50 min, and was identical to that induced by the systemic administration of the drug. Stereotypy and hypermotility were never observed after apomorphine microinjection into the PVN, even at the highest dose tested (1 microgram). Microinjections of the same doses of apomorphine into the hypothalamic ventromedial and dorsomedial nucleus, preoptic area, caudate nucleus, nucleus accumbens and substantia nigra, were ineffective. LY 171555, a specific D2 Da receptor agonist, and (+)-3-PPP, but not (-)-3-PPP nor the specific D1 DA receptor agonist SKF 38393, were as effective as apomorphine when injected into the PVN. Apomorphine-induced penile erection and yawning were antagonized by pretreatment with neuroleptic drugs, such as haloperidol, (-)-sulpiride, a specific D2 DA antagonist, and SCH 23390, a specific D1 DA antagonist. The present results suggest that the PVN is the brain area where D2 DA agonists act to induce penile erection and yawning. Moreover, since the PVN contains the cell bodies of a group of incerto-hypothalamic DA neurons, the above results suggest for the first time a possible involvement of the incerto-hypothalamic DA system in the expression of penile erection and yawning.

Dopaminergic Modulation of Glutamate Release in Striatum as Measured by Microdialysis

Glutamate and aspartate are the primary neurotransmitters of projections from motor and premotor cortices to the striatum. Release of glutamate may be modulated by dopamine receptors located on corticostriatal terminals. The present study used microdialysis to investigate the dopaminergic modulation of in vivo striatal glutamate and aspartate release in the striatum of awake-behaving rats. Local perfusion with a depolarizing concentration of K+ through a dialysis probe into the rat striatum produced a significant increase in the release of glutamate, aspartate, and taurine. The D2 agonist LY171555 blocked the K(+)-induced release of glutamate and aspartate, but not taurine, in a concentration-dependent manner. The D1 agonist SKF 38393 did not alter K(+)-induced release of glutamate and taurine, but did significantly decrease aspartate release. Neither agonist had any effect on basal amino acid release. The D2 antagonist (-)-sulpiride reversed the inhibitory effects of LY 171555 on K(+)-induced glutamate release. These results provide in vivo evidence for a functional interaction between dopamine, the D2 receptor, and striatal glutamate release.

Behavioral correlations of dopamine receptor activation

According to the classification scheme of Kebabian and Calne, there are two types of dopamine (DA) receptors: D1 (activation of which causes increased cyclic AMP formation) and D2 (activation of which causes no increment in cyclic AMP). It is not clear what role the different receptors play in mediating motor behavior. Using drugs that act selectively at only one receptor site, we studied the effects of D1 and D2 receptor activation in two different models of parkinsonism--the rotating rat and the reserpinized mouse. Neither the D1 agonist nor the D2 agonist, given alone, could overcome reserpine akinesia, but together they restored locomotor activity. In rats with unilateral nigrostriatal lesions, both drugs induced a rotational response, each with a distinct temporal pattern. Pretreatment with alpha-methyl-paratyrosine (an inhibitor of DA synthesis) led to decrements in the rotational response induced by D2 agonists, but not that induced by D1 agonists. The mechanism by which these DA agonists induce motor activity is different; activation of both types of DA receptors seems to be necessary for normal motor behavior.

Independent in vitro regulation by the D-2 dopamine receptor of dopamine-stimulated efflux of cyclic AMP and K+-stimulated release of acetylcholine from rat neostriatum

Two types of dopamine receptors whose stimulation affect cAMP efflux (and by inference formation) could be identified in rat neostriatum. One type of receptor, called D-1 receptor, increased cAMP efflux whereas stimulation of a second type of dopamine receptor, called D-2 receptor, was followed by a reduction in cAMP efflux induced by stimulation with a D-1 receptor agonist. D-2 receptor agonists inhibited the effects of D-1 receptor agonists on cAMP efflux in a non-competitive way. These inhibiting effects of D-2 receptor agonists occurred also in the absence of Ca2+-ions which could imply that some of the D-2 receptors are located on cells possessing D-1 receptors. The dopamine receptor mediating inhibition of the release of radiolabeled acetylcholine (ACh) in the neostriatum appeared to have the same pharmacological characteristics as the D-2 dopamine receptor mediating the inhibition of the D-1 receptor agonist induced cAMP efflux. Selective D-2 receptor agonists like LY 141865 and RU 24926 stimulated this receptor while the D-1 receptor agonist SKF 38393 was inactive. Effects of the selective D-2 receptor agonists could be antagonized by (-)-sulpiride, a selective D-2 receptor antagonist. Although the pharmacological characteristics of the dopamine receptors mediating inhibition of both ACh release and (D-1 dopamine receptor agonist induced) cAMP efflux appeared to be similar, drugs stimulating cAMP efflux did not affect ACh release or LY 141865 induced inhibition of ACh release from rat neostriatum. Therefore it is still questionable whether the dopamine receptor mediating inhibition of both ACh release and cAMP efflux is one and the same functional entity.

Synergistic effects of D1 and D2 dopamine agonists on turning behaviour in rats

In rats with unilateral 6-hydroxydopamine lesions of the substantia nigra, a specific D1 dopamine receptor agonist, SKF 38393A, at a dose that does not itself produce turning, significantly increased the contralateral rotation observed following a low dose of the specific D2 agonist LY 171555. Doses of SKF 38393A or the D2 agonist bromocriptine, which would themselves not induce turning, in combination produced a high rate of turning. These results suggest a synergistic interaction between D1 and D2 dopamine receptors in this system.

Dopamine D3 receptor mRNA and binding sites in human brain

Dopamine D3 receptors (Sokoloff et al., 1990) have been shown to be related to dopamine D2 receptors and have been suggested to play a role in mediating the antipsychotic effects of neuroleptics. So far studies on the expression of D3 mRNA and of binding sites with pharmacological characteristics of D3 receptors have been restricted to rat brain. Using in situ hybridization histochemistry, we demonstrate that D3 mRNAs are enriched in human n, accumbens and in the islands of Calleja. In addition, D3 mRNA was detected at very low levels in anterior caudate and putamen with a rostro-caudally decreasing gradient and in hypothalamic mammillary nuclei. In receptor autoradiographic binding studies, the islands of Calleja were found to be labeled by [125I]iodosulpride and [3H]CV 205 502 but not by [3H]raclopride and [3H]YM 09151-2. Pharmacological analysis of binding of the D2/D3 ligand [3H]CV 205 502 in n. accumbens and caudate-putamen is consistent with the presence of D3 receptor sites in ventral striatum. Overall distribution and pharmacology of D3 sites in human and rat brain appear to be similar. Presence and distribution of D3 receptors in human brain are compatible with the notion that D3 receptors might be involved in mediating the clinical effects of antipsychotics.

Is stimulation of both D1 and D2 receptors necessary for the expression of dopamine-mediated behaviors?

Recent electrophysiological findings have indicated that D1 dopamine (DA) receptor stimulation by SKF 38393 enables the inhibitory effects of the D2 receptor agonist quinpirole on nucleus accumbens neurons. In the present study, a similar interaction was shown for quinpirole-induced stereotyped behaviors. In control rats, SKF 38393 enhanced the stereotyped responses induced by quinpirole, converting lower-level stereotypies (sniffing and rearing) to more intense oral behaviors (licking and gnawing). In rats depleted of DA (79% reduction) by the tyrosine hydroxylase inhibitor alpha-methyl-p-tyrosine (AMPT), the behavioral effects of quinpirole were abolished. However, quinpirole-induced stereotyped responses were reinstated by SKF 38393 suggesting that D1 receptor stimulation by endogenous DA is necessary for D2 receptor-mediated stereotyped responses (sniffing, rearing). In support of this suggestion, stereotyped behaviors produced by the non-selective D1/D2 agonist apomorphine were not affected by AMPT pretreatment. In contrast to the effects of quinpirole, the ability of SKF 38393 to induce grooming responses was not abolished by AMPT pretreatment or by combined pretreatment with AMPT and reserpine (greater than 99% DA depletion). These results indicate that D1 receptor stimulation enables D2 receptor-mediated stereotyped responses, but that this relationship is not reciprocal since D2 receptor stimulation is not necessary for the grooming response elicited by SKF 38393.

Photoreceptors of mouse retinas possess D4 receptors coupled to adenylate cyclase

In the mouse, the light-sensitive pool of cAMP can be eliminated in the dark by application of the dopamine D2-like receptor agonists LY 171555 (quinpirole), (+)-N0437 (2-[N-(n-propyl)-N-2-(thienylethylamino)-5-hydroxytetralin]) , or (+)-3-PPP [3-(3-hydroxyphenyl)-N-propylpiperidine hydrochloride]. The rank-order affinity of the ability of the D2-like antagonists to block the action of LY 171555 matched that of the rat D4 receptor. Reverse transcription of retina mRNA followed by DNA amplification using D4-specific nucleotides demonstrates the presence of D4 mRNA in retina. In situ hybridization studies using D4-specific digoxygenin-labeled oligonucleotides or 35S-labeled UTP RNA probes demonstrate the presence of D4 mRNA in the photoreceptor cell layer and in the inner nuclear and ganglion cell layers. The modulation by D4 ligands of the dark level of light-sensitive cAMP in photoreceptors demonstrates the physiological coupling of the D4 receptor subtype.

Hippocampal signal transmission to the pedunculopontine nucleus and its regulation by dopamine D2 receptors in the nucleus accumbens: an electrophysiological and behavioural study

The integrative role of the nucleus accumbens and subpallidal area in relaying hippocampal signals to the mesencephalic locomotor region in the brainstem was investigated electrophysiologically in urethan-anaesthetized rats. A behavioural study of the functional connections was also performed in freely moving rats. In the electrophysiological experiments, subpallidal output neurons to the pedunculopontine nucleus and the adjacent ventral gray were first identified by their antidromic responses to electrical stimulation of the pedunculopontine nucleus. Hippocampal stimulation was then shown to inhibit orthodromically some of these subpallidal neurons. The inhibitory response was attenuated following microinjection of a dopamine D2 agonist (LY 171555), but not a D1 agonist (SKF 38393), into the accumbens. This suggests that signal transmission from the hippocampus to the subpallidal output neurons to the pedunculopontine nucleus is modulated by a D2 receptor-mediated mechanism in the nucleus accumbens. Injections of N-methyl-D-aspartate into the ventral subiculum of the hippocampus resulted in a threefold increase in locomotor responses. Injection of a D2 agonist into the accumbens reduced the hyperkinetic response dose-dependently and suggests that D2 receptors regulate locomotor responses initiated by the hippocampal-accumbens pathway. Injection of nipecotic acid, a GABA uptake inhibitor, into the subpallidal area or of procaine, a neural transmission blocker, into the region of the pedunculopontine nucleus, also reduced significantly the hippocampal-induced hyperkinetic response. These results provide evidence of limbic (e.g. hippocampus) influences on locomotor activity by way of nucleus accumbens-subpallidal-pedunculopontine nucleus connections which may contribute to adaptive behaviour. Signal transmission from the hippocampus may be regulated by a dopamine D2 receptor mechanism in the accumbens, presumably mediated by the converging mesolimbic dopaminergic input from the ventral tegmental area.

Sleep deprivation in the rat: an animal model of mania

The model of sleep deprivation in rats by the platform method has been extensively studied in our laboratory as a possible animal model of mania. At the end of the period of sleep deprivation, the rat does not fall asleep as soon as it is returned to its home cage, but shows a period of wakefulness of about 30 min, during which the animal presents a cohort of symptoms that appear to mimic those present in idiopathic mania. In particular, during this period the animal displays insomnia, a high degree of hyperactivity, irritability, aggressiveness, hypersexuality and stereotypy. Haloperidol (0.2 mg/kg) was effective in reducing latency to sleep, while L-sulpiride was much weaker (< 50 mg/kg). The dopamine D1 receptor antagonist SCH 23390 exhibited an extremely high potency and efficacy in reducing sleep latency, a significant effect being observed with 3 micrograms/kg. The administration of the specific D1 receptor agonist SKF 38393 markedly prolonged the period of insomnia with the correlated behavioral syndrome. When lithium was added to the diet and consumed during the sleep deprivation period in adequate amounts to produce serum lithium levels of 0.7-1.0 mEq/l, sleep latency and locomotor activity were significantly reduced. The administration of naloxone (1-10 mg/kg) reduced the latency to sleep in a dose-related manner. By contrast, morphine (1 and 5 mg/kg, i.p.), beta-endorphin and [D-Ala2,D-Leu5]enkephalin (i.c.v., 2 and 1 micrograms, respectively) markedly prolonged the insomnia. The model not only represents a confirmation in the rat that sleep loss often precedes and may trigger a manic episode in man, but suggests that an opioid-dopamine interaction may play a pathogenetic role in mania.