Effects of desipramine on neuronal responses to dopamine, noradrenaline, 5-hydroxytryptamine and acetylcholine in the caudate nucleus of the rat

Neonatal medial prefrontal cortex lesion enhances the sensitivity of the mesoaccumbal dopamine system

Neurodevelopmental models of schizophrenia posit that early brain damage leads to dys- or misconnection effects possibly altering synaptic transmission in brain sites distal of the lesion. We tested the hypothesis that neonatal medial prefrontal cortex (mPFC) lesions affect the sensitivity of the mesoaccumbal dopamine (DA) system. Using extracellular single-unit recordings combined with systemic application of the DA agonist apomorphine, followed by the D2 receptor antagonist haloperidol or the D1 receptor antagonist SCH23390, we compared electrophysiological properties of nucleus accumbens core and shell neurons after bilateral excitotoxic lesions of mPFC induced at postnatal day 7 or in adult rats. Whereas animals with adult mPFC lesions showed an altered discharge pattern within the core region, neonatal mPFC lesions altered the discharge pattern within the shell region. Subcutaneous administration of apomorphine (4 mg/kg) reduced accumbal firing rate in 77% of all neurons. Onset and magnitude of apomorphine-induced inhibition of neuronal activity was faster and stronger in rats with neonatal but not adult mPFC lesions in both core and shell regions. Apomorphine-induced inhibition was partially reversed by 0.1 mg/kg haloperidol only in core region of neonatal lesioned rats. Apomorphine-induced excitation of neuronal activity (in 21% of all neurons) was reversed by the D1 receptor antagonist SCH23390 (0.1 mg/kg) in all excited neurons. These data support the hypothesis that neonatal but not adult lesions of mPFC alter cortico-striatal networks and suggest that disturbance of mPFC development leads to neurodevelopmental changes in mesoaccumbal DA system during adulthood.

In vivo electrochemical measurements and electrophysiological studies of rat striatum following neonatal 6-hydroxydopamine treatment

The effects of neonatal treatment (one day after birth) with the neurotoxin, 6-hydroxydopamine (75 micrograms/10 microliters intracisternal), were studied in the striatum of normal adult and treated Sprague-Dawley rats. Measurements of monoamine levels in the dorsal striatum and nucleus accumbens, by high-performance liquid chromatography coupled with electrochemical detection, showed that neonatal 6-hydroxydopamine treatment produced a permanent and massive destruction of striatal dopamine. The effects were more pronounced in the dorsal striatum than in the nucleus accumbens. In addition, serotonin levels were elevated in the rat striatum as a consequence of the neonatal treatment. Rapid chronoamperometric recordings of K(+)-evoked monoamine overflow using Nafion-coated recording electrodes were investigated in both the dorsal and ventral striatum of control and neonatally lesioned rats. The potassium-evoked responses recorded from the dorsal striatum of the 6-hydroxydopamine-treated rats were significantly reduced in amplitude as compared to controls. In addition, the reduction/oxidation current ratios of the responses were more serotonin-like, in contrast to the dopamine-like current ratios measured in the striatum of untreated animals. In ventral striatum, the amplitudes of the K(+)-evoked responses were not significantly reduced versus control. However, the K(+)-evoked signals were more serotonin-like in their electrochemical characteristics as compared to controls. In addition to the release studies, extracellular single-unit electrophysiological recordings were performed in normal and neonatally 6-hydroxydopamine-treated rats. The spontaneous discharge rate of striatal neurons in the neonatally 6-hydroxydopamine-treated rats was similar to that of control rats. This is in contrast to dopamine lesions in adult animals, where a marked elevation of the discharge rate is observed. Local applications of dopamine and serotonin into the striatum of neonatally 6-hydroxydopamine-treated rats elicited excitations of striatal cells rather than the normal inhibitory effects seen in control animals. Taken together, these data suggest that loss of striatal dopamine terminals at birth leads to both pre- and postsynaptic alterations in monoamine pathways.

Serotonin-evoked modifications of the neuronal firing rate in the superior vestibular nucleus: a microiontophoretic study in the rat

Microiontophoretic ejection (10-100 nA) of serotonin (5-hydroxytryptamine) into the superior vestibular nucleus induced modifications of the mean firing rate in 87% of the neurons examined. The responses to 5-hydroxytryptamine application were excitatory in 48% of the cells, inhibitory in 29%, and biphasic (inhibitory/excitatory) in the remaining 10%. The excited neurons were scattered throughout the nucleus; the units inhibited or characterized by biphasic responses were distinctly more numerous in the ventrolateral sector of the nucleus. The magnitude of both excitatory and inhibitory effects was dose-dependent. The excitatory responses to 5-hydroxytryptamine were blocked or greatly reduced by two 5-hydroxytryptamine antagonists, methysergide and ketanserin, or even reversed in many cases. Inhibitory responses were enhanced by simultaneous application of 5-hydroxytryptamine antagonists in half of the units studied. In the remaining units, ketanserin left the response unmodified, whereas methysergide reduced but never quite blocked it. The application of 5-methoxy-N,N- dimethyltryptamine, a 5-hydroxytryptamine agonist more effective on 5-hydroxytryptamine1 than on 5-hydroxytryptamine2 receptors, and of 8-hydroxy-2(di-n-propyl-amino) tetralin, a 5-hydroxytryptamine1A-specific agonist, induced a decrease in the firing rate which was unaffected by methysergide. These results support the hypothesis that 5-hydroxytryptamine exerts various functions throughout the superior vestibular nucleus by various receptors and that the inhibitory action is limited to an area of it.(ABSTRACT TRUNCATED AT 250 WORDS)

Microiontophoretic studies of the effects of D-1 and D-2 receptor agonists on type I caudate nucleus neurons: Lack of synergistic interaction

Several lines of evidence have suggested there may be a physiologically relevant form of synergistic interaction between D-1 and D-2 dopamine (DA) receptors located on postsynaptic neurons in the forebrain that receive a dopaminergic innervation. Because of the theoretical importance of such an interaction with respect to understanding the normal physiology of dopaminergic systems, we evaluated effects of D-1 and D-2 selective agonists, applied microiontophoretically, on the spontaneous electrical activity of a single, identifiable subpopulation of neurons within the caudate nucleus, the type I striatal neuron, in locally anesthetized, gallamine-paralyzed rats. It was observed that the D-2 receptor agonist quinpirole (QUIN) produced biphasic effects on cell firing rate. Low ejection currents significantly increased firing rate, while higher currents produced an inhibition. Similar effects were observed for the D-1 agonists SKF 38393; however, the overall excitations observed at low ejection currents were far less than those observed for QUIN. When these two agonists were applied concurrently, a simple additive effect (but not synergism) was always observed. The acute reduction of striatal levels of DA, by as much as 84% (with pretreatment with alpha-methyl-p-tyrosine, AMPT), did not alter the responsiveness of type I striatal neurons to the DA receptor agonists applied alone or in combination. These observed effects were not altered either by chloral hydrate anesthesia (in which glutamate-driven activity was studied) or by a more severe depletion of striatal DA levels (98% depletion produced by combined pretreatment with AMPT and reserpine).

Effects of p-chloroamphetamine on release of [3H]gamma-aminobutyric acid from slices of rat caudate-putamen

The effect of endogenous serotonin on the release of [3H]gamma-aminobutyric acid ([3H]GABA) from slices of rat caudate-putamen was studied. p-Chloroamphetamine was used to release endogenous serotonin. p-Chloroamphetamine (100 nM) enhanced the release of [3H]GABA induced by 20 mM K+, while 1000 nM p-chloroamphetamine decreased it. The 5-HT3 receptor antagonists ICS 205-930 (50 nM) and MDL 72222 (100 nM) prevented this facilitation caused by 100 nM p-chloroamphetamine. ICS 205-903 (50 nM), when used alone, reduced the release of [3H]GABA caused by 23 mM K+. This finding confirmed the hypothesis that endogenous serotonin can enhance the release of [3H]GABA via 5-HT3 receptors. In contrast, an effect of 5-HT1 and 5-HT2 receptors could not be clearly established. It is likely that the release of endogenous GABA from striatonigral GABA neurons may also be affected by serotonin via 5-HT3 receptors.

Dopaminergic transmission in the hypothalamic arcuate nucleus to produce acupuncture analgesia in correlation with the pituitary gland

Acupuncture analgesia (AA) caused by low frequency stimulation of the acupuncture point (AP) was abolished by hypophysectomy and adrenalectomy. Termination of the AA producing pathway from the AP to the pituitary gland was in the medial hypothalamic arcuate nucleus (M-HARN). The origin of the descending pain inhibitory system associated with AA was in the posterior HARN (P-HARN). AA in the hypophysectomized rats, and enhanced neuronal activity in the P-HARN that were abolished during acupuncture stimulation, were both restored by intraperitoneal microinjection of 0.5 mg/kg morphine or 0.1 micrograms beta-endorphin into the P-HARN during acupuncture stimulation. Of the analgesia produced by dopamine or beta-endorphin injected into the P-HARN, that caused by beta-endorphin disappeared after denervation of the M-HARN. The P-HARN neurons that responded to acupuncture stimulation also responded to iontophoretic dopamine, but not to iontophoretic morphine nor ultramicroinjected beta-endorphin. The transmission between the M-HARN and P-HARN may be dopaminergic, and beta-endorphin might presynaptically modulate this transmission. Reduction of sodium ions may have been the reason for abolition of AA after adrenalectomy.

Immunohistochemical study of the serotoninergic innervation of the basal ganglia in the squirrel monkey

A specific antibody raised against 5-hydroxytryptamine (5-HT) conjugated to bovine serum albumin was used to study the serotoninergic innervation of the basal ganglia in the squirrel monkey (Saimiri sciureus). At midbrain level, numerous fine 5-HT-immunoreactive axons were seen to arise from the immunopositive neurons of the dorsal raphe nucleus and less abundantly from those of the nucleus centralis superior. The bulk of these axons formed a rather loosely arranged bundle that arched ventrorostrally through the central portion of the midbrain tegmentum and ascended toward the ventral tegmental area. Several fascicles detached themselves from this bundle to reach the substantia nigra where they arborized into a multitude of heterogeneously distributed 5-HT terminals. The 5-HT innervation was particularly dense in the pars reticulata but much less so in the pars compacta of the substantia nigra. More rostrally other 5-HT fibers swept dorsolaterally and formed a remarkably dense network of varicose fibers within the subthalamic nucleus. A multitude of 5-HT axons continued their ascending course within the lateral hypothalamic area, and many of them swept laterally to invade the lenticular nucleus. At pallidal levels, the 5-HT axons arborized much less profusely in the external segment than in the internal segment, which contained numerous 5-HT varicose fibers and terminals arranged in a typical bandlike pattern. At striatal levels, the 5-HT terminals were particularly abundant in the ventral striatum, including the nucleus accumbens and deep layers of the olfactory tubercle. They also abounded in the ventrolateral region of the putamen and the ventromedial aspect of the caudate nucleus. Overall, the number of 5-HT fibers and terminals decreased progressively along the rostrocaudal axis of the striatum and several large and elongated zones rather devoid of 5-HT immunoreactivity were visualized, particularly in the caudate nucleus and the dorsal putamen. These zones of poor 5-HT immunoreactivity were in register with similar areas devoid of tyrosine hydroxylase immunoreactivity as seen on contiguous sections. These findings reveal that all the core structures of the basal ganglia in primates receive a significant serotoninergic input, but that the densities and patterns of innervation vary markedly from one structure to the other.

Electrophysiological properties of nigrothalamic neurons after 6-hydroxydopamine lesions in the rat

Extracellular recordings were made from electrophysiologically identified nigrothalamic cells in the substantia nigra pars reticulata of anaesthetized rats. The firing rate, firing pattern and responses to striatal stimulation were investigated in normal animals and in animals in which dopamine concentration in the ipsilateral striatum was reduced by more than 90%. At relatively short times after the lesion (less than 10 days) the mean firing rate of the spontaneously active cells in the population was significantly reduced and there was an increase in the occurrence of bursting activity. There was also a significant increase in the number of silent cells, located by antidromic stimulation from the thalamus. In spite of this reduction in mean firing rate the responses of neurons to stimulation of either the ipsilateral striatum or ventromedial thalamus was much larger in cells from lesioned animals. At longer times after the lesion (more than six months) the average firing rate of the neurons had returned to normal but there was still a prevalence of bursting activity and a consequent reduction in mean inter-spike intervals. There was little evidence of the previous hyper-responsiveness to thalamic stimulation but the responsiveness to striatal stimulation was still significantly elevated.

Serotonin innervation in adult rat neostriatum. II. Ultrastructural features: a radioautographic and immunocytochemical study

High-resolution radioautography after cerebroventricular administration of tritiated serotonin (5-HT) and PAP immunocytochemistry with an antiserum against 5-HT-glutaraldehyde conjugate (kindly donated by M. Geffard) were used in parallel to investigate the intrinsic and relational fine structural features of 5-HT axon varicosities (terminals) in the neostriatum of the adult rat. The uptake-labeled varicosities were examined in single thin sections from a paraventricular sector of neostriatum, whereas their immunostained counterparts were viewed in serial thin sections from the same paraventricular sector plus a dorsal neostriatal sector. The two approaches yielded complementary results in terms of varicosity dimensions, synaptic features and appositional relationships. Serotonin axon terminals were generally small and, as measured in immunostained material, even smaller in the dorsal than in the paraventricular neostriatum. Their internal features, best viewed in radioautographs, included small pleomorphic synaptic vesicles with occasional large granular vesicles and mitochondria. Junctional 5-HT terminals from both the paraventricular and the dorsal neostriatal sectors synapsed exclusively, and with equal frequency, on dendritic spines or shafts, almost always with asymmetrical membrane differentiations. The proportion of junctional varicosities, however, was very low in serial (immunocytochemical) as well as single (radioautographic) thin sections. Only 10-13% of 5-HT varicosities from either the paraventricular or the dorsal neostriatum exhibited a synaptic junction, in contrast with a junctional incidence of at least 70% for randomly selected axonal varicosities similarly sampled in the surrounding neuropil. Serotonin axon terminals, whether or not synaptic, were closely apposed to a variety of structures comprising mostly other axon terminals, dendritic spines and branches, but rarely neuronal somata. The synaptic and appositional features of immunostained 5-HT varicosities were similar for both the dorsal and the paraventricular neostriatum. In this context, it is likely that the effects of 5-HT in the neostriatum are exerted upon a multiplicity of cellular target sites in addition to the restricted number of dendritic spines and shafts synaptically contacted by this type of monoamine terminal.

Low doses of accumbens dopamine modulate amygdala suppression of spontaneous exploratory activity in rats

The effect of pharmacological stimulation of the amygdala on spontaneous locomotor activity in the rat and its modulation by accumbens dopamine were investigated. Bilateral injection of N-methyl-D-aspartic acid into the basolateral nucleus of the amygdala produced a dose-dependent suppression of spontaneous locomotor activity in the rat. The suppression of locomotor activity was reversed completely by injection of L-glutamic acid diethyl ester, a putative glutamatergic antagonist, into the nucleus accumbens but partially enhanced by injection of nipecotic acid, a GABA uptake inhibitor, into the ventral pallidum. Furthermore, low doses of dopamine injected into the accumbens, which by itself did not elicit hyperactivity in the animals, completely reversed the suppression of locomotor activity following amygdala stimulation. These results show that the projection from the amygdala to nucleus accumbens has an inhibitory effect on spontaneous locomotor activity in rats and that dopamine in the accumbens attenuated this suppression effect possibly due to its neuromodulatory action as demonstrated in previous electrophysiological experiments.

Excitatory and inhibitory effects of dopamine on neuronal activity of the caudate nucleus neurons in vitro

Effects of dopamine on the rat caudate nucleus neurons were examined in a slice preparation using an intracellular recording technique. Perfusion of the bath with a low concentration (1 microM) of dopamine produced a depolarization concomitant with an increase in the spontaneous firing and the number of action potentials evoked by a depolarizing pulse applied into the cells. In contrast, higher concentrations (100-500 microM) of dopamine inhibited the spontaneous and current-induced firings without apparent effects on the resting membrane potential. In addition, during application of a high concentration (100 microM) of dopamine there was a marked elevation of the threshold potential of the action potential elicited by a higher depolarizing current. Simultaneous application of haloperidol (0.5-5 microM) antagonized both excitatory and inhibitory effects induced by the low and high concentrations of dopamine, respectively. In addition, the excitatory effect induced by a low concentration (1 microM) of dopamine was antagonized by domperidone (0.5 microM), a selective D2 receptor antagonist, while the inhibitory effect by a high concentration (100 microM) was blocked by SCH 23390, a selective D1 receptor antagonist. These results strongly suggest that the postsynaptic sites of caudate nucleus neurons have at least two subtypes of dopamine receptors (D1 and D2 receptors) that mediate inhibitory and excitatory responses of the neuron to dopamine, respectively.

Transcallosal evoked potentials in relation to behavior in the rat: effects of atropine, p-chlorophenylalanine, reserpine, scopolamine and trifluoperazine

Single pulse electrical stimulation of the sensorimotor cortex in waking rats produced an evoked response in the contralateral sensorimotor cortex. The slow wave response consisted of: (1) an early component that was negative at the pial surface and in layer V, and was associated with multiunit discharge; and (2) a late component that was mainly negative at the surface, positive in layer V, and was associated with multiunit suppression. Previous research suggests that the early component represents summed excitatory postsynaptic potentials; the late component summed inhibitory postsynaptic potentials. Both components could be elicited by direct stimulation of the corpus callosum and both were abolished by midline callosal section. The amplitude and duration of the late component varied with concurrent motor activity in a striking manner. It was large during waking immobility and also during face-washing, licking the paws, chewing food and drinking water, but was much reduced or absent during head movements, walking and changes in posture. Only minor changes were associated with the transition from waking immobility to slow wave sleep. A series of pharmacological experiments indicated that the behavior-related variation in the late component of the transcallosal evoked response was dependent on both cholinergic and serotonergic transmission.

Coexistence of inhibitory dopamine D-1 and excitatory D-2 receptors on the same caudate nucleus neurons

Microiontophoretic studies using cats anesthetized with alpha-chloralose were performed to determine whether or not dopamine D-1 and D-2 receptors co-exist in the same caudate nucleus (CN) neurons that receive inputs from the substantia nigra (SN), and in which spikes elicited by SN stimulation were blocked by domperidone, a selective D-2 antagonist. Iontophoretic application of dopamine produced a dose-dependent inhibition of spontaneous firing in 2 of 4 spontaneously active CN neurons and an increase in firing in the remaining 2 neurons. However, dopamine inhibited the glutamate-induced firing in 31 of 32 CN neurons that were not spontaneously active. Similar inhibition with iontophoretically applied SKF 38393, a selective D-1 agonist, was observed in 33 of 34 spontaneously inactive neurons tested. When the effects of dopamine, SKF 38393 and bromocriptine (D-2 agonist) were examined on the same CN neurons, the inhibitory effects of both dopamine and SKF 38393 were seen in 14 of 15 neurons, and both an inhibition by SKF 38393 and an excitation by bromocriptine were observed in 15 of 17 neurons. The inhibitory effects of dopamine and SKF 38393 were antagonized by haloperidol and SCH 23390 (D-1 antagonist) without being affected by domperidone. Furthermore, the dopamine-induced inhibition was converted to an excitation during simultaneous application of SCH 23390 in 6 of 10 CN neurons, and this excitation was antagonized by domperidone. These results strongly suggest that the inhibitory D-1 and excitatory D-2 receptors co-exist on the same CN neurons receiving inputs from the SN.

Excitation by dopamine D-2 receptor agonists, bromocriptine and LY 171555, in caudate nucleus neurons activated by nigral stimulation

Electrophysiological studies using cats anesthetized with alpha-chloralose were carried out to determine whether or not the dopamine D-2 receptor mediates the excitation of the caudate nucleus (CN) neurons activated by stimulation of the substantia nigra (SN). Microiontophoretic application of domperidone (D-2 antagonist) produced a significant inhibition of spikes elicited by SN stimulation in 20 of 27 CN neurons. When bromocriptine and LY 171555 (D-2 agonists) were iontophoretically applied to the CN neurons in which the SN-induced spikes were inhibited by domperidone, an increase in spontaneous firing rate was observed in 18 of 20 neurons and all of 10 neurons tested, respectively. However, no alterations of firing occurred with bromocriptine or LY 171555 in any 7 neurons in which the SN-induced spikes were not affected by domperidone. The increase in firing rate by the D-2 agonists was apparently antagonized during simultaneous application of domperidone and haloperidol, but not affected during application of SCH 23390 (D-1 antagonist). These results strongly suggest that the spike generation of the CN neurons upon SN stimulation is mediated by the dopamine D-2 receptor.

Rolipram, a stereospecific inhibitor of calmodulin-independent phosphodiesterase, causes beta-adrenoceptor subsensitivity in rat cerebral cortex

Prolonged pretreatment of rats with the atypical antidepressant rolipram attenuates noradrenaline (NA) sensitivity of the cerebral cortical cAMP generating system. The development of this down-regulation is time (7 d treatment required) and dose dependent (EC50 = 0.35 mg/kg). Density of beta-adrenoceptor as measured by (-)-3H-dihydroalprenolol [(-)-3H-DHA] binding is also reduced by rolipram pretreatment. The effect of rolipram is absolutely stereospecific for the (-)-enantiomer (ED50 = 0.18 mg/kg). In addition, only with this isomer, a reduction in daily weight gain was found compared to sham treated controls. Presynaptic denervation using intracerebroventricular (i.c.v.) injections of 6-hydroxydopamine (6-OHDA) prior to or during rolipram treatment did not completely block the effect of a rolipram treatment on down-regulation of cerebral cortical beta-adrenoceptors. The data favor a pre- and postsynaptic action of rolipram different from all other antidepressants studied so far in this experimental setting. Rolipram is known as inhibitor of brain phosphodiesterase. Using partially purified calmodulin-independent phosphodiesterase from brain it is shown that exclusively the (-)-enantiomer of rolipram inhibits phosphodiesterase with an IC50 of 1.25 mumol/l whereas the (+)-isomer possesses little potency. Since a marked stereospecificity for the (-)-isomer of rolipram was displayed in all pharmacological parameters tested so far with (+)- and (-)-rolipram, it is suggested that stereospecific and isozyme specific inhibition of cAMP-phosphodiesterase is, at least in part, related to the mechanism of action of the potential antidepressant drug rolipram and possibly of other antidepressants as well.

Dopamine D-2 receptor-mediated excitation of caudate nucleus neurons from the substantia nigra

Microiontophoretic studies using cats anesthetized with alpha-chloralose were performed to elucidate whether the excitatory response of caudate nucleus (CN) neurons upon stimulation of the pars compacta of the substantia nigra (SN) is mediated by the dopamine D-1 or D-2 receptor. There were rare convergent inputs from the SN and motor cortex (MC) in the CN neurons. Iontophoretic application of haloperidol and domperidone (dopamine D-2 receptor antagonist) produced dose-dependent inhibition of spikes elicited by SN stimulation in 25 of 42 and 50 of 82 CN neurons, respectively, however, no alterations of spikes elicited by MC stimulation occurred in any 11 neurons tested. Iontophoretically applied SCH 23390 (D-1 antagonist) did not inhibit the SN-induced spikes in any CN neurons, of which spikes were inhibited by domperidone. These results suggest that the SN-induced spikes are mediated by dopamine, which acts on postsynaptic D-2 receptors.

Treatment of pathologic laughing and weeping with amitriptyline

Patients with bilateral forebrain disease may commonly manifest the syndrome of pathologic laughing and weeping. We investigated the efficacy of low-dose amitriptyline in 12 patients in whom this syndrome was a consequence of multiple sclerosis. In a double-blind crossover study comparing amitriptyline with placebo, eight patients experienced dramatic and significant improvement with amitriptyline (P = 0.02). The mean dose of amitriptyline was 57.8 mg per day and did not exceed 75 mg per day in any patient. Concurrent measurements of depression showed no change during the study. We conclude that amitriptyline is effective in the treatment of this disturbance of affective expression, and that this effect is distinct from the antidepressant effect of the medication.

alpha-Noradrenergic potentiation of neurotransmitter-stimulated cAMP production in rat striatal slices

The present study examines the possible involvement of alpha-adrenergic receptors in catecholamine-stimulated cAMP production in intact slices of rat striatum. Norepinephrine (NE) produces a greater stimulation of cAMP levels than does the beta-adrenergic agonist, isoproterenol (ISO), and the NE response is inhibited by both the beta-adrenergic antagonist, propranolol, and the alpha-adrenergic antagonist, phentolamine. The alpha-adrenergic agonist, 6-fluoronorepinephrine (6-FNE), has little or no effect on basal cAMP levels; however, 6-FNE causes a marked potentiation of the cAMP response to ISO. Hence, NE stimulation of cAMP levels in striatal slices appears to involve a synergistic interaction between alpha- and beta-adrenergic receptors. alpha-Receptors also potentiate adenosine stimulation of cAMP levels in striatal slices. However, in contrast to results previously reported in cerebral cortical slices, the alpha-adrenergic component of the NE response in striatal slices is not dependent on endogenous adenosine. Finally, 6-FNE interactions with adenylate cyclase in striatal homogenates differ from those observed in the slice preparation. In homogenates, 6-FNE appears to directly stimulate adenylate cyclase through a D-1 receptor. D-1 receptor involvement in catecholamine responses in the striatal slice preparation, on the other hand, appears to be minimal.

The electrophysiology of dopamine (D2) receptors: a study of the actions of dopamine on corticostriatal transmission

Electrophysiological recordings from the cells of the neostriatum in rats anaesthetised with halothane revealed only inhibitory actions of dopamine applied iontophoretically close to the cells. Inhibition of cortical driving seemed to have a slightly higher threshold in most cells but dopamine inhibited spontaneous action potentials, glutamate-induced responses, and cortical driving in the cells studied. Fluphenazine applied iontophoretically blocked the actions of dopamine but was itself without effect on the neuronal responses. Sulpiride, in contrast, was without effect on the spontaneous activity of the cells and was ineffective in blocking the action of applied dopamine. Sulpiride, nevertheless, increased the response to cortical stimulation though it had no action on the response to applied glutamate. These results suggest that the sub-class of dopamine receptors on the terminals of the corticostriatal pathway may be inhibitory on glutamate release and preferentially sensitive to blockade by sulpiride.

Shift from inhibition to excitation in the neostriatum but not in the nucleus accumbens following long-term amphetamine

Neuronal responses to 1.0 or 5.0 mg/kg D-amphetamine were recorded simultaneously in the neostriatum and nucleus accumbens of rats pretreated twice daily with these doses or with saline for 6 consecutive days. In all groups, the number of neurons responding to a challenge injection of either dose of amphetamine with an overall excitation or inhibition was not significantly different. During the first 30-60 min of the drug response, however, neurons in the neostriatum of amphetamine-pretreated rats responded with a significant increase in firing rate compared to saline controls. In the nucleus accumbens, on the other hand, tolerance developed to the inhibition produced by 1.0 mg/kg D-amphetamine, whereas the responses produced by 5.0 mg/kg were not significantly altered by long-term treatment. Liquid chromatography with electrochemical detection revealed that pretreatment with 5.0 mg/kg D-amphetamine produced a slight, but significant, reduction of dopamine and norepinephrine levels in the neostriatum. Catecholamine levels were not significantly altered in the nucleus accumbens by either dose. These electrophysiological and neurochemical changes are discussed in relation to the known involvement of these sites in the dose-dependent behavioral alterations that accompany repeated amphetamine injections.

Effects of dopamine on spontaneous and evoked activity of caudate neurons

This study examined the effects of dopamine (DA), pressure ejected from multi-barrelled micropipettes, on the spontaneous and evoked activity of caudate neurons, recorded extracellularly in rats anesthetized with urethane. Neurons were categorized according to their discharge latencies in response to supramaximal cortical stimulation: neurons which fired with latencies less than 13 msec were classified "short-latency-discharge neurons", while neurons with latencies greater than or equal to 13 msec were classified "long-latency-discharge neurons". This procedure also allowed the detection of neurons with low levels of spontaneous activity. The predominant effect of DA on both neuronal types was inhibition of spontaneous activity. However, DA exerted a modulatory effect in that spontaneous activity was inhibited at "doses" which did not affect activity evoked by cortical stimulation. Although DA-induced excitation was infrequent, it was significantly more prevalent among long-latency neurons than among short-latency-discharge neurons. Long-latency-discharge neurons were also significantly more spontaneously active than were short-latency neurons. In rats depleted of endogenous DA by treatment with reserpine, caudate neurons had significantly increased rates of spontaneous and evoked activity, shorter duration of stimulus-evoked inhibition, and longer latency for evoked discharges than in control rats. These results suggest that DA exerts modulatory effects on caudate neuronal activity. Furthermore, these results suggest that short- and long-latency-discharge neuronal groups may consist of pharmacologically, as well as physiologically, distinct neuronal types.

Serotonergic excitation from dorsal raphe stimulation recorded intracellularly from rat caudate-putamen

Similar to other afferents to rat caudate-putamen, stimulation of the dorsal raphe nucleus evokes a series of 3 responses which can be recorded intracellularly. An initial depolarization is followed by a long-lasting inhibition which is, in turn, terminated by another period of depolarization. Pharmacological manipulations demonstrate that the initial depolarizing potential is serotonergic. Depletion of serotonin by means of prior treatment with para-chlorophenylalanine leads to a reduction in the amplitude of the depolarization which can be evoked by maximal stimulation of dorsal raphe. Neither the long-lasting hyperpolarization nor the late excitation which follow the initial depolarization is affected. Replacement of serotonin in levels by injection of 5-hydroxytryptophan results in a restoration of the amplitude of the depolarizing response. The latency of the initial depolarization is, however, unchanged in serotonin-depleted animals. This together with the observation in some cells of a component of the initial depolarization resistant to para-chlorophenylalanine treatment, suggests that there is a non-serotonergic excitation which precedes that mediated by serotonin.

Bilateral asymmetrical changes in the nigral release of [3H]GABA induced by unilateral application of acetylcholine in the cat caudate nucleus

In halothane anaesthetized cats, a push-pull cannula was implanted into the right caudate nucleus (CN) and in each substantia nigra (SN). The release of [3H]GABA continuously formed from [3H]glutamine was estimated in each structure. Acetylcholine (ACh, 5 x 10(-5) M) added in presence of eserine (5 x 10(-5) M) for 50 min in the right caudate nucleus 2 h after the onset of superfusion with [3H]glutamine, stimulated the [3H]GABA release locally. The effect was biphasic when ACh application was made in the median two-thirds of the structure and it was monophasic and transient when the ACh application was restricted to the lateral part. ACh application in the right caudate nucleus also induced changes in [3H]GABA released in the anterior (pars reticulata) and posterior (pars compacta) parts of both SN. While [3H]GABA release was enhanced in the ipsilateral anterior SN, it was reduced in the contralateral anterior SN. Respective opposite effects were observed in the posterior parts of the ipsi- and contralateral SN. These bilateral asymmetrical changes in [3H]GABA release were not dependent on the site of ACh application in the right caudate nucleus. These results indicate that the facilitation of cholinergic transmission in one caudate nucleus influences in an opposite way the striato-nigral GABA neurones on both sides of the brain.

Inhibition of the electrically induced release of [3H]dopamine by serotonin from superfused rat striatal slices

The effect of serotonin (5-hydroxytryptamine) on the electrically induced release of [3H]dopamine from superfused slices of the rat striatum has been studied. It was observed that serotonin produced a concentration dependent decrease in the field stimulation-induced release of [3H]dopamine with the threshold concentration being 10(-6) M or lower. Methysergide, in a concentration which did not alter the evoked release, antagonized the inhibitory effect of serotonin. The present results suggest that serotonin should be added to the list of endogenous substances that can influence dopaminergic transmission in the striatum.

Characterisation of inhibitory 5-hydroxytryptamine receptors that modulate dopamine release in the striatum

The effect of a series of indoleamines on the potassium-evoked tritium release of previously accumulated [3H]dopamine from rat striatal slices has been investigated. The indoleamines 5-hydroxytryptamine, 5-methoxytryptamine, 5-methoxy-N,N'-dimethyltryptamine and tryptamine (10(-7) to 10(-5) M) all reduced potassium-evoked release of tritium, to a maximum of 50%. The uptake of [3H]dopamine was unaffected by these compounds. A series of 5-hydroxytryptamine antagonists were examined for their ability to reduce the inhibition of potassium-evoked tritium release induced by 5-methoxytryptamine. The relative order of antagonist potency obtained was methysergide greater than metergoline greater than methiothepin greater than cinanserin greater than cyproheptadine greater than mianserin, and was consistent with an action on 5-hydroxytryptamine receptors. It is concluded that there are inhibitory 5-hydroxytryptamine receptors located on the terminals of dopaminergic neurones in the striatum.

Tricyclic anti-depressant induced mania

The authors report four cases of Monopolar Depressed patients who developed manic features for the first time while on tricyclic antidepressant therapy. The phenomenon is viewed as tricyclic antidepressant induced mania. The manic symptoms occurred late in therapy, with moderate doses of tricyclic antidepressants (125-150 mg per day) and were easily controlled with antipsychotic medication. Factors relating to age of patients, metabolic products of tricyclic antidepressants and central nor-adrenergic and dopaminergic systems are considered important and their role has been discussed.

Recurrent inhibition in the rat neostriatum

During intracellular recording, in the neostriatum of rats anesthetized with urethane, the triggering of an action potential in the recorded neuron by a depolarizing pulse of current resulted in inhibition in that same neuron. This inhibition was evident through its ability to reduce the amplitude of EPSPs evoked from stimulation of substantia nigra. The shunting of SN EPSPs was shown not to be due to action potential currents. The inhibition is antagonized by the GABA blocking agent bicuculline. Intracellular labeling of recorded neurons revealed them as medium spiny neurons. It is concluded that the extensive axon collaterals of spiny projection neurons mediate recurrent inhibition, a portion of which involves autaptic synapses of a neuron back onto itself.

"Classical" and "atypical" antipsychotic drugs: differential antagonism of amphetamine- and apomorphine-induced alterations of spontaneous neuronal activity in the neostriatum and nucleus accumbens

The ability of clozapine and haloperidol to antagonize the depression of firing rate produced by d-amphetamine and apomorphine in the neostriatum and nucleus accumbens was tested in immobilized, locally anesthetized rats. In the neostriatum, an intraperitoneal injection of 2.5 mg/kg d-amphetamine or 1.0 mg/kg apomorphine produced a prolonged inhibition of neuronal activity that was reversed by a subjsequent injection of either 20 mg/kg clozapine or 2.0 mg/kg haloperidol. An analysis of the onset and magnitude of the blockade revealed that clozapine was more effective than haloperidol in reversing the amphetamine response but that both antipsychotic drugs produced a comparable blockade of the apomorphine-induced depression. Similar results were obtained in the nucleus accumbens. The data indicate that although clozapine acts equieffectively in the neostriatum and nucleus accumbens, this atypical antipsychotic drug, aside from blocking postsynaptic dopamine receptors, may exert at least some of its effects by preventing dopamine release.

Potentiation of responses to monoamines by antidepressants after destruction of monoamine afferents

1 Stereotaxic lesioning and microiontophoretic techniques were used to study the effects of lesions of the medial forebrain bundle (MFB) on the potentiation by antidepressant drugs of responses to monoamines of cortical neurones.2 Active uptake of noradrenaline (NA) and 5 hydroxytryptamine (5-HT) by synaptosomes from the motor and somatosensory cortex was reduced to approximately 20%, 10 to 14 days following lesion of the MFB in rats.3 Unilateral lesions of the MFB caused changes in responsiveness of neurones to NA and 5-HT, applied by iontophoresis, in the cortex ipsilateral to the lesion. Excitatory responses to both amines were observed less frequently and depression was the predominant response. Excitatory responses on the lesioned side were significantly smaller than on the unlesioned side, but the size of depressant responses was unaltered.4 Viloxazine strongly potentiated responses of cortical neurones to NA and 5-HT on both sides of the brain of MFB-lesioned rats. There were no significant differences in the potentiation of responses to monoamines on the lesioned or unlesioned sides of the brain.5 Desipramine potentiated responses to NA of neurones in the cortex ipsilateral to MFB lesions.6 Chlorimipramine potentiated responses to 5-HT of neurones in the cortex ipsilateral to MFB lesions.7 It is concluded that antidepressants can potentiate responses to monoamines despite a profound reduction in presynaptic terminals. The potentiation is unlikely to be the result of blockade of monoamine uptake into presynaptic terminals, and is probably a postsynaptic effect of the antidepressant drugs.

Effects of viloxazine, its optical isomers and its major metabolites on biogenic amine uptake mechanisms in vitro and in vivo

Viloxazine hydrochloride (ICI 58,834, VIVALAN) a chemically novel antidepressant, shows selective inhibition of noradrenaline uptake into mouse heart in vivo and into rat brain in vitro. The noradrenaline uptake inhibitory activity resides primarily in one of the two optically active isomers, and it is suggested that in the conformation adopted for uptake by noradrenaline, the aryl and the amino groups are trans. In a comparison of in vivo and in vitro potency, tri- and tetracyclic antidepressants exhibit a good correlation. However, viloxazine possesses higher in vivo activity than would be expected from in vitro studies. The latter finding cannot be readily explained on the basis of known pharmacokinetic or metabolic factors.

Comparison of the responses of single cortical neurones to tyramine and noradrenaline: effects of desipramine

1 The technique of microelectrophoresis was used in order to compare the actions of tyramine and noradrenaline on single neurones in the cerebral cortex of the rat.2 Tyramine could both excite and depress cortical neurones. Each tyramine-sensitive cell was also sensitive to noradrenaline. There was a high correlation between the directions of responses to tyramine and noradrenaline, most cells excited by tyramine being excited by noradrenaline, and most cells depressed by tyramine being depressed by noradrenaline.3 In the case of both excitatory and depressant responses, tyramine appeared to be less potent than noradrenaline.4 Tyramine evoked ;slower' responses than noradrenaline, both the latencies to onset and the recovery times being longer for responses to tyramine than for responses to noradrenaline.5 When the rates of release of tyramine and noradrenaline from micropipettes were measured in vitro, no significant difference could be observed between the transport numbers of the two drugs. Thus the difference in potency between the two drugs, and the difference in the time courses of responses to the two drugs, are presumably of biological origin.6 Desipramine could discriminate between neuronal responses to tyramine and noradrenaline: responses to tyramine were antagonized, while responses to noradrenaline were either potentiated or unaffected. Responses to DL-homocysteic acid were not affected by desipramine.7 The results are consistent with the hypothesis that tyramine is an indirectly acting sympathomimetic amine in the brain, and desipramine acts by blocking the uptake of both tyramine and noradrenaline into presynaptic noradrenergic nerve terminals.

Dose-dependent biphasic alterations in the spontaneous activity of neurons in the rat neostriatum produced by d-amphetamine and methylphenidate

A dose-response analysis was performed on D-amphetamine- and methylphenidate-induced changes in neuronal activity in the neostriatum of immobilized, phenidate-induced changes in neuronal activity in the neostriatum of immobilized, locally anesthetized rats. Whereas a marked depression of firing rate characterized the response to intraperitoneal injections of 2.5 mg/kg D-amphetamine or 10 mg/kg methylphenidate, increasing the dose (5.0--7.5 mg/kg D-amphetamine and 20--25 mg/kg methylphenidate) shifted this response pattern to a prolonged increase in activity. Both stimulant-induced increases and decreases in neostriatal activity are reversed by 2.0 mg/kg haloperidol. In contrast to the response of neostriatal neurons, the firing rate in the substantia nigra pars compacta was inhibited by both stimulants, even at doses that increased the activity of neurons in the neostriatum. The possible mechanisms underlying these drug-induced changes in firing rate are discussed along with the behavioral implications of a stimulant-induced dose-dependent shift in neostriatal unit activity.

The effect of some neuroleptics on the interaction of cortically and nigrally evoked potentials in the rat striatum

The interaction of cortico- and nigrofugal inputs to the striatum of the rat was investigated using the technique of evoked potentials. Repetitive, unilateral stimulation of the substantia nigra inhibited potentials which were evoked from the ipsilateral rostral cortex and recorded from the ipsilateral striatum. The inhibition was antagonized by low doses of various intraperitoneally administered neuroleptics such as: pimozide (0.1 - 0.2 mg/kg), haloperidol (0.1 - 0.5 mg/kg), chlorpromazine (0.5 - 2.0 mg/kg) and thioridazine (0.5 - 4.0 mg/kg). These findings are discussed in view of the existing controversy regarding the mode of action of dopamine in the striatum.