Interactions between dopamine and γ-aminobutyrate in the substantia nigra: Implications for the striatonigral output hypothesis

Experiments employing a rodent circling model were conducted to test the predictive capacity of the theory which states that striatonigral γ-aminobutyrate neurones transmit striatal information influencing the animal's locomotion and orientation. In agreement with this proposal, blocking nerve conduction in one substantia nigra with procaine, or nigral γ-aminobutyrate receptors with bicuculline administered stereotaxically, frequently forced rats to move ipsiversively to systemic apomorphine, as though the treatment had impaired striatonigral transmission on that side of the brain. Attempts to reverse the direction of apomorphine circling by stimulating γ-aminobutyrate receptors with muscimol, by facilitating the amino acid's action with flurazepam, or by increasing its synaptic concentration either with a breakdown inhibitor (ethanolamine O-sulphate or 4-amino-hex-5-enoic acid) or an uptake blocker (cis-1,3-aminocyclohexane carboxylic acid) in one nigra, proved unsuccessful. In fact, ethanolamine O-sulphate, flurazepam and muscimol all gave the appearance of hindering rather than enhancing the passage of striatal-derived motor information through the nigra. Broadly speaking, these drugs gave predictable behavioral responses from the ventromedial thalamus, suggesting they were acting in accordance with known mechanisms.The anomalous behaviour with ethanolamine O-sulphate may be attributed to its elevating γ-aminobutyrate levels in other brain areas, since similar ipsiversive rotations occurred if γ-aminobutyrate catabolism was prevented at a wide variety of extranigral sites. A simple explanation for the paradoxical ipsiversive behaviours produced by intranigral flurazepam or muscimol in combination with systemic or intracerebral injection of dopamine agonists, is that they act via presynaptic receptors to inhibit the release of endogenous γ-aminobutyrate and thereby impede striatonigral outflow ipsilaterally.     

Role of the ventromedial nucleus of the thalamus in motor behaviour—II. Effects of lesions

Rotational behaviour was initiated in naive rats by injecting muscimol into one substantia nigra pars reticulata, or in unilaterally 6-hydroxydopamine-treated rats with systemic or intracaudate apomorphine. Electrolytic or kainic acid lesions were made in one or both ventromedial nuclei of the thalamus and their effects on the components of circling studied. A unilateral ventromedial electrolesion imposed a weak ipsilateral posture and occasionally elicited weak ipsiversive circling acutely, but not chronically. Challenging these rats with a large subcutaneous dose of apomorphine invariably provoked ipsiversive circling, however old was the lesion. Bilateral electrolesions caused slight hypoactivity. Kainic acid treatments of one or both ventromedial thalami produced uncontrolled hypermotility initially, with subsequent loss of ventromedial neurones and recovery of normal motor behaviour. No form of ventromedial lesion affected the incidence of stereotypy. Acute (but not chronic) contralateral or ipsilateral ventromedial electrolesions, or both, slowed muscimol and apomorphine-induced circling (often in different ways) through complex changes in posture and/or locomotor drive. Animals lesioned during the course of a circling episode often showed the biggest changes in circling to begin with, only to recover minutes later. Rapidly circling rats were sometimes more readily inhibited than slowly circling rats. Toxin injury of the ventromedial nucleus appeared to suppress muscimol and not apomorphine circling. Any ventromedial lesion (electrical or chemical, acute or chronic), if positioned opposite a contraversive circling stimulus, intensified the associated posture. Ipsilateral lesions tended to abolish posture altogether or, like bilateral treatments, to suppress locomotion. Sham operations had none of these effects. Acute electrical lesions and drug-induced inhibition of one or both ventromedial thalami were more or less identical in their effects on rat circling behaviour, save that bilateral muscimol injection caused profound catalepsy while lesions did not.

It is suggested that the ventromedial thalamus is more concerned with the registration of striatal dopamine-mediated behaviours in drug-stimulated than in spontaneously behaving rats, and that other output pathways may rapidly compensate for any impairment of function in the ventromedial nuclei.     

Antithrombic activity of some polysaccharide resins

Crosslinked dextrans (Sephadex®) bearing essentially car☐ymethyl, benzylsulphonate and -amino acid groups have been synthesized. The antithrombic activity of the resins may be the result of a cooperative effect between the functional groups. The binding of benzylsulphonate to car☐ymethylated Sephadex endows these materials with activity. However the highest activity is obtained when the resins contain simultaneously benzylsulphonate, -amino acid and car☐ylic acid groups.     

Dysfunction of the midbrain angular complex can accentuate or attenuate circling behaviour in the rat

Summary The role of the midbrain angular complex (AC) in the execution of motor behaviours was investigated in the rat. In an automated holeboard apparatus bilateral AC electrolesions attenuated exploration and increased locomotor performance of drug-free rats on the first and second test occasions respectively; the latter result may signify a retarding of between-session habituation. Apomorphine also decreased locomotion and almost abolished head dipping and rearing in the holeboard; bilateral AC lesions reinstated locomotion to a normal level without modifying the other behavioural parameters. An electrolesion of one AC did not affect the animal's posture or spontaneous locomotion in the open field, but gave rise to pronounced ipsiversive circling when coupled with systemic administration of apomorphine. In unilaterally 6-hydroxydopamine (6-OHDA) treated rats subcutaneous injection of apomorphine evoked robust contraversive circling. A concomitant lesion of the ipsilateral AC introduced an additional ipsilateral bias to these animals' movements; contraversive circling was initially curtailed and posture reduced (or reversed), while stereotyped activities (particularly grooming) were suppressed. Contralateral orientation and circling were restored by subsequently lesioning the contralateral AC as well; bilateral AC lesions significantly potentiated circling to systemic apomorphine. Contralateral locomotor asymmetry was also produced by depositing apomorphine stereotaxically into the supersensitive caudate, or by microinjecting one substantia nigra zona reticulata with muscimol (in naive rats). Both rotational responses were facilitated by injury to the ipsilateral AC. The effects of electrocoagulating the AC were generally duplicated by discrete microinjection of muscimol or -vinyl GABA into this area, suggesting GABA-mediated synapses are normally operative in this part of the brain. These results do not support the claim that the AC is specifically engaged in mediating postural asymmetry in the unilaterally 6-OHDA denervated rat. Instead, we believe that impairment of neurotransmission through one AC imposes an independent and reciprocal tendency to move towards that side of the brain, as well as attenuating stereotypy and facilitating locomotion. The resultant behavioural response to systemic apomorphine shown by animals bearing these two types of lesion embodies these separate actions.     

Evidence for the participation of nigrotectal γ-aminobutyrate-containing neurones in striatal and nigral-derived circling in the rat

The γ-aminobutyrate-containing nature of nigrotectal neurones and the possible involvement of the tectum in circling behaviour were investigated in the rat. Electrolytic or kainic acid lesions of the substantia nigra reduced γ-aminobutyrate levels on average by 19–29% in intermediate and deep, but not superficial superior colliculus. Placement of lesions or injection of muscimol (40 ng) into these γ-aminobutyrate-innervated layers of superior colliculus gave only weak ipsilateral posturing or circling that was intensified by apomorphine, but which strongly antagonized contraversive apomorphine-induced circling in 6-hydroxydopamine pretreated rats (lateral > medial sites). Contraversive circling to unilateral intranigral muscimol (40 ng) was significantly attenuated by lesions or muscimol injections placed in the ipsi- or contralateral superior colliculus. Picrotoxin (40 ng) and tetanus toxin (30 mouse LD₅₀ doses) evoked explosive motor behaviour from medial colliculus and vigorous contraversive circling when injected into the lateral colliculus. The latter offset ipsiversive asymmetries to kainate (0.8 μg) in the corresponding substantia nigra. Bilateral intratectal picrotoxin produced hyperactivity that reversed haloperidol catalepsy. Similar bilateral administration of muscimol did not produce catalepsy but a state of frozen immobility. Kainic acid introduced into the superior colliculus gave mixed excitatory-inhibitory responses initially followed by ipsiversive circling only and loss of tectal perikarya. None of these drug effects occurred from the overlying cerebral cortex or subjacent tegmentum.We propose that separate medial ‘non-postural’ and lateral ‘postural’ tectal locomotor regions may exist in the superior colliculus that are situated within a striato-nigrotectal outflow system capable of influencing the animal's motor activity and posture.     

Distribution of γ-aminobutyrate in the rat thalamus: Specific decreases in thalamic γ-aminobutyrate following lesion or electrical stimulation of the substantia nigra

Various subnuclei of the rat's thalamus were punched out or finely microdissected from frozen coronal sections of brain and assayed for γ-aminobutyrate by microdansylation. Highest levels were recorded in the ventromedial and parafascicular regions, corresponding to the sites of termination of nigrothalamic neurones. One week after placing a unilateral electrolytic lesion in the zona reticulata of the substantia nigra there were significant decreases in γ-aminobutyrate levels in the ipsilateral ventromedial (?14%) and parafascicular (?11%) nuclei, but not in other nuclei tested. Falls in the level of γ-aminobutyrate of 19–26% were found to be confined to the ventromedial region in microdissections of brain slices containing this nucleus, two weeks after injecting 1 μg kainic acid into the substantia nigra. The concentrations of glutamate, aspartate, glutamine, glycine and alanine were not altered in the deafferented ventromedial thalamus. Pathological effects of kainate were registered as a complete loss of neuronal perikarya at the injection site coupled with intense gliosis. Mild gliosis, but no postsynaptic neuronal damage, was visible in the corresponding ventromedial and parafascicular nuclei in the thalamus, although degenerating electron-dense terminal boutons were occasionally seen in electron micrographs of this tissue.In rats in which thalamic glutamate decar?ylase activity was inhibited by systemic injection of 3-mercaptopropionic acid, trains of biphasic electrical pulses delivered to the nigra (100 Hz, 0.1 mA, width 0.6 ms for 15 min) evoked a stimulus-dependent decrease in ventromedial (?17%) and parafascicular (?10%) γ-aminobutyrate content, but not in that of other thalamic nuclei. Similar post-stimulus falls in the level of γ-aminobutyrate in the ventromedial nucleus were noted after direct stereotaxic injection of this nucleus with the enzyme inhibitor.These findings are consistent with the existence of γ-aminobutyrate-containing nigrothalamic fibres whose cell bodies lie in the medial part of the substantia nigra zona reticulata and which terminate in the ventromedial and parafascicular nuclear regions of the thalamus.     

Differential effects of sodium-free media on γ-aminobutyrate and muscimol-evoked conductance increases recorded from lobster muscle fibres

Membrane conductance increases evoked by γ-aminobutyric acid and muscimol were compared on single lobster muscle fibres. Muscimol was approximately 4 times more potent than γ-amino-butyric acid. The log-dose/conductance curves for γ-aminobutyrate and muscimol were parallel (with similar apparent maximum) and were fitted by a two independent binding-site receptor model (apparent dissociation constants were, respectively, 27 μM and 7 μM). Combination of γ-aminobutyrate with a fixed dose of muscimol did not reveal any mutual hindrance between agonists. Dose/conductance curves for 4,5,6,7-tetrahydroisoxazolo-pyridin-3-ol or isoguvacine also paralleled the curves for γ-aminobutyrate, but these agonists were about half as potent as γ-aminobutyrate. In 1 mM nipecotic acid, responses to γ-aminobutyrate (but not muscimol) were very slightly enhanced. In contrast, replacement of extracellular Na⁺ with Li⁺ or glucosamine consistently depressed responses to muscimol but had little or no effect on responses to γ-aminobutyrate. Use of choline as a Na⁺ substitute depressed the effects of both muscimol and γ-aminobutyrate, although a preferential depression of muscimol responses was still apparent. Two possible interpretations of our data are discussed: (1) that there may be two populations of inhibitory amino acid receptors on lobster muscle, one preferring muscimol (modulated by Na⁺), the other preferring γ-aminobutyrate (less affected by Na⁺ removal); (2) that Na⁺ can affect differentially the affinity of the same receptor site for a range of γ-aminobutyrate receptor agonists. Our results in Na⁺-free solutions therefore raise the possibility that γ-aminobutyrate and muscimol might not share an identical mechanism of action.     

Role of thalamic gamma-aminobutyrate in motor functions: catalepsy and ipsiversive turning after intrathalamic muscimol

Bilateral intrathalamic microinjections of nanogram amounts (5–50 ng) of muscimol, a γ-aminobutyrate (GABA) receptor agonist, elicited catalepsy in rats. Like neuroleptic-treated rats, those injected with muscimol in the thalamus remained suspended on a vertical grid but, unlike opioid-treated rats, they failed to remain horizontal on two book-holders. The righting reflex was present, while ptosis was absent. The areas with the highest sensitivity to the cataleptogenic effects of muscimol were the ventromedial and ventral-anterior nuclei of the thalamus. These thalamic areas were also characterized by the shortest latency for the induction of catalepsy. Injection of up to 50 ng of muscimol into the caudate, globus pallidus or entopeduncular nucleus failed to produce catalepsy. Catalepsy was also obtained after intrathalamic microinjection of other GABA analogs, such as 3-aminopropanesulphonic and imidazolacetic acid, which are known to be potent GABA receptor agonists, and β-p-chlorophenyl-GABA , a compound which has GABA mimetic activity. The catalepsy produced by 10 ng of muscimol was reversed by an intrathalamic microinjection of picrotoxin, a GABA receptor antagonist. Muscimol-induced catalepsy, unlike neuroleptic-induced catalepsy, was not reversed by systemic administration of high doses of apomorphine, a dopamine receptor agonist, or of scopolamine, a muscarine antagonist, or by intranigral injection of muscimol, and was not prevented by kainic acid-induced lesions of the striatum or of the nigra. Vice versa, injection of cataleptogenic doses of muscimol in the thalamus failed to prevent the stereotyped gnawing produced by systemic apomorphine or intranigral muscimol. Therefore, in these animals, catalepsy and stereotyped gnawing coexisted. The unilateral intrathalamic microinjection of muscimol resulted in a postural asymmetry consisting of turning towards the injected side. This ipsilateral posturing was converted into an ipsilateral circling by systemic administration of apomorphine.The results indicate that thalamic GABAergic mechanisms play an important role in the regulation of posture and in the mediation of certain motor responses arising in the striatum.     

Enhanced GABA function in the angular complex (lateral periaqueductal grey matter and adjacent reticular formation) alters the postural component of striatal- or nigral-derived circling

Summary Unilateral injection of muscimol into the angular complex (lateral periaqueductal grey matter and adjacent reticular formation) caused ipsiversive rotation. Focal injection of picrotoxin into the same site produced contraversive rotation. Administration of apomorphine to animals with a unilateral 6OHDA lesion of the left medial forebrain bundle caused contraversive rotation. Focal injection of muscimol into the angular complex reversed the direction of rotation such that apomorphine administration now produced ipsiversive circling. Unilateral injection of muscimol into substantia nigra zona reticulata caused contraversive rotation. Focal injection of picrotoxin into the same site produced ipsiversive rotation. The prior injection of muscimol into the ipsilateral angular complex prevented the contraversive rotation induced by intranigral administration of muscimol such that animals now showed ipsiversive circling. In both 6-OHDA-lesioned animals and animals receiving intranigral muscimol, focal injection of muscimol into the angular complex caused a reversal in the direction of circling through loss of the postural component with no obvious change in locomotor activity. Bilateral electrolytic lesions of the angular complex overall had no effect on amphetamine-induced locomotion. Manipulation of GABA function in the angular complex alters circling behaviour initiated from the striatum or substantia nigra by altering the postural component without affecting the locomotor response of the animals. The data suggest a critical role for the angular complex as an outflow station from basal ganglia.     

GABAA receptors in the mediodorsal thalamus play a crucial role in rat shell-specific acetylcholine-mediated,but not dopamine-mediated,turning behaviour

The role of GABA_A receptors in the mediodorsal thalamus (mdT) in turning behaviour of rats was studied. Neither the GABA_A receptor agonist muscimol (50 ng) nor the antagonist bicuculline (200 ng) unilaterally injected into the mdT elicited any behavioural change. Unilateral injection of the acetylcholine receptor agonist (carbachol, 5 μg) into the nucleus accumbens shell has been found to elicit contraversive circling while unilateral injection of a mixture of dopamine D₁ ((±)-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine-7,8-diol [SKF 38393], 5 μg) and D₂ (quinpirole, 10 μg) receptor agonists into the same site is known to elicit contraversive pivoting. The contraversive circling induced by unilateral injection of carbachol (5 μg) into the nucleus accumbens shell was dose-dependently inhibited by muscimol (25 and 50 ng) injected into the mdT. This inhibitory effect of muscimol (50 ng) was antagonised by co-administration of bicuculline (200 ng), which alone did not modify the contraversive circling induced by carbachol (5 μg). The contraversive pivoting induced by unilateral injection of a mixture of SKF 38393 (5 μg) and quinpirole (10 μg) into the nucleus accumbens shell was inhibited by muscimol (25 and 50 ng) injected into the mdT, whereas bicuculline (200 ng) injected into the mdT did not significantly modify the pivoting. The inhibitory effect of muscimol (50 ng) on the pivoting induced by a mixture of SKF 38393 (5 μg) and quinpirole (10 μg) was not dose-dependent and not antagonised by bicuculline (200 ng). The present study suggests that GABA_A receptors in the mdT play a limited role in spontaneously occurring locomotor activity. Secondly, this study demonstrates that GABA_A receptors in the mdT transmit accumbens-dependent cholinergic circling, but not accumbens-dependent dopaminergic pivoting, to other brain structures. Finally, the present study shows that muscimol-sensitive, non-GABA_A receptors in the mdT influence the accumbens-dependent dopaminergic pivoting. To what extent GABA_B receptors in the mdT mediate the muscimol-induced effects upon the dopaminergic pivoting behaviour requires additional research.     

cis-Dichlorodiammineplatinum alters GABAergic structures in the immature rat cerebellum

It has been reported that injection of the antitumoral drug cis-dichlorodiammineplatinum at 10 days of life affects cerebellar development in rats. After a single dose of 5 μg/g of body weight, the formation of granule cells is decreased and the maturation of postmitotic neurons is slowed down. A substantial time after treatment, reduced cell packing density of the internal granule layer and atrophy of the molecular layer can be observed. In addition, there is degeneration of some Purkinje cells and Golgi neurons. In spite of all these alterations, the regular architecture of the cerebellar folia is retained in many places. In the present study, we used immunocytochemistry with an immune serum raised against glutamic acid decar☐ylase to further characterize the cis-dichlorodiammineplatinum-induced alterations of GABAergic neurons. The aim was to examine cerebellar development and to test for factors controlling the settling of GABAergic circuits. At all post-treatment intervals, most of the Purkinje and Golgi neurons and molecular layer interneurons showed stronger anti-glutamic acid decar☐ylase immunoreactivity than in controls; this may have been due to altered fixation because of cis-dichlorodiammineplatinum-induced damages to the blood vessels; but could also reflect cellular retention of the enzyme, maybe due to cis-dichlorodiammineplatinum-induced damage of the microtubular apparatus.

After seven days, large roundish immunoreactive varicosities were present in the molecular layer adjacent to the Purkinje cell dendritic poles. These varicosities, which were not observed in control animals, may be terminals of Purkinje cell axon recurrent collaterals contributing to the supraganglionic plexus, whose abnormal development would compensate for the reduced inhibitory inputs from inhibitory interneurons and/or Purkinje cells, which degenerated at early post-treatment intervals. At later post-treatment intervals (15 and 21 days), there were also alterations in the pericellular basket at the Purkinje cell axon hillock, which was poorly developed in or absent from the majority of cells. The finding was confirmed by morphological observation of basket cells in Golgi-Cox preparation and immunocytochemistry with an antibody raised against 200,000 mol. wt phosphorylated neurofilaments.

It is concluded that early changes in anti-glutamic acid decar☐ylase immunoreactivity of neurons may be due to a direct interference of the drug with the cellular metabolic pathways. The late anomalies in the anti-glutamic acid decar☐ylase immunoreactivity appear to be secondary to changes in the tissue cytoarchitecture rather than being primary cis-dichlorodiammineplatinum-induced lesions of the cells.     

GABA(A) agents injected into the ventral pallidum differentially affect dopaminergic pivoting and cholinergic circling elicited from the shell of the nucleus accumbens

The ability of GABA(A) receptors in the ventral pallidum to modulate shell-specific behavior was studied. Injections of the non-selective acetylcholine receptor agonist, carbachol (5 microg), into the shell of the nucleus accumbens elicited contraversive circling, namely turning marked by normal stepping; in contrast, injections of a mixture of dopamine D(1) (SKF 38393, 5 microg) and D(2) (quinpirole, 10 microg) receptor agonists into this brain structure elicited contraversive pivoting, namely turning marked by abnormal hindlimb stepping. Unilateral injections of the GABA(A) receptor agonist muscimol (10, 25 and 50 ng) into the ventral pallidum dose-dependently mimicked shell-specific circling, especially when given at a level +8.6mm anterior to the interaural line; this effect was GABA(A) receptor specific, because it was prevented by the GABA(A) receptor antagonist bicuculline (150 ng). Unilateral pallidal injections of a dose of muscimol that was ineffective per se (10 ng) abolished contraversive pivoting elicited by shell injections of dopamine receptor agonists; instead, it elicited moderate ipsiversive pivoting. Pallidal injections of bicuculline (150 ng) replaced the contraversive pivoting elicited by dopamine receptor agonist with ipsiversive circling. In contrast, unilateral pallidal injections of 10 ng muscimol (anterior +8.6mm level) suppressed the contraversive circling elicited by shell injections of carbachol; instead, it elicited moderate ipsiversive pivoting. Pallidal injections of bicuculline (150 ng) produced short-lasting ipsiversive circling that was followed by contraversive pivoting.We conclude that the ventromedial portion of the ventral pallidum contains GABA(A) receptors that are crucial for the transmission of information from the shell of the nucleus accumbens via the ventral pallidum towards other brain structures; this holds especially for information about shell-specific circling elicited by carbachol. The same portion of the ventral pallidum also contains GABA(A) receptors that control the transfer of information from the nucleus accumbens towards structures outside the ventral pallidum; this holds especially for information about shell-specific pivoting elicited by dopaminergic agonists.     

Septal driving of hippocampal theta rhythm: A role for γ-aminobutyrate in the effects of minor tranquillizers?

In male rats, when the hippocampal theta rhythm is artificially driven by stimulation in the septum at frequencies between 5 and 10 Hz, the function relating frequency to the threshold current required to drive the theta rhythm at that frequency has a minimum at 7.7 Hz. The minor tranquillizers chlordiazepoxide and amobarbital sodium abolish this minimum and it has been proposed that this action correlates with their anxiolytic effects. The flattening of the curve produced by chlordiazepoxide (5 mg/kg) was antagonized by picrotoxin (0.75 mg/kg) but not by bicuculline (2.5 mg/kg). The flattening produced by amobarbital sodium (15 mg/kg) was not antagonized by either picrotoxin (2 mg/kg) or bicuculline (2.5 mg/kg).Muscimol, a γ-aminobutyrate agonist, acted like a minor tranquillizer on the theta-driving curve. The dose response-curve for this effect was an inverted-U and the muscimol still produced significant flattening at a dose of less than 0.001 mg/kg. The effect of muscimol was antagonized by bicuculline (2.5 mg/kg) but not by picrotoxin (2 mg/kg).The results are discussed in relation to the hypothesis that γ-aminobutyrate may be involved in anxiety.     

Comparative study of the release of glutamate and GABA, newly synthesized from glutamine, in various regions of the central nervous system

The in vitro release of [³H]glutamate and [³H]γ-aminobutyric acid, both newly synthesized from [³H]glutamine, were studied in various regions of the rat and pigeon brain. The amount of transmitter released from slices during stimulation by excess K⁺, as well as the ratio of the amounts of glutamate and γ-aminobutyrate released, varied considerably from region to region. Very high levels of release of glutamate were observed in rat striatum and in rat and pigeon hippocampus whereas only moderate release of glutamate was found in rat and pigeon cerebellum, rat cochlear nucleus and rat substantia nigra. The highest levels of release of γ-aminobutyrate were observed in the rat substantia nigra, hippocampus and striatum. Particularly low levels of release of glutamate was observed in the pigeon optic tectum. The release of γ-aminobutyrate and glutamate in all structures was largely calciumdependent.These results suggest that the release of glutamate and γ-aminobutyrate newly synthesized from glutamine reflects the neurotransmitter function of the two amino acids within various CNS regions. They further suggest that the glutamate synthesizing and releasing pool is different from the γ-aminobutyrate synthesizing and releasing pool. The similarity between the release of glutamate in rat and pigeon cerebellum and hippocampus suggest some structural homology in both species.     

Gaba(A) receptors in the pedunculopontine tegmental nucleus play a crucial role in rat shell-specific dopamine-mediated, but not shell-specific acetylcholine-mediated, turning behaviour

The role of GABA(A) receptors in the pedunculopontine tegmental nucleus in turning behaviour of rats was studied. Unilateral injection of the GABA(A) receptor agonist, muscimol (25-100 ng), into the pedunculopontine tegmental nucleus dose-dependently produced contraversive pivoting, namely tight head-to-tail turning marked by abnormal hindlimb backward stepping. This effect was GABA(A) receptor specific, since it was prevented by the GABA(A) receptor antagonist, bicuculline (50 ng), which alone did not elicit turning behaviour. Unilateral injection of a mixture of dopamine D(1) ((+/-)-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine-7,8-diol [SKF 38393], 5 microg) and D(2) (quinpirole, 10 microg) receptor agonists into the nucleus accumbens shell has been found to elicit contraversive pivoting, whilst unilateral injection of the acetylcholine receptor agonist (carbachol, 5 microg) into the same site is known to elicit contraversive circling, namely turning marked by normal stepping. The pivoting induced by a mixture of SKF 38393 (5 microg) and quinpirole (10 microg) injected into the nucleus accumbens shell was significantly inhibited by bicuculline (50 ng) injected into the pedunculopontine tegmental nucleus, whereas muscimol (25 ng) had no effect. Neither muscimol (25 ng) nor bicuculline (50 ng) modulated the contraversive circling induced by carbachol (5 microg) injected into the nucleus accumbens shell. It is therefore concluded that unilateral stimulation of GABA(A) receptors in the pedunculopontine tegmental nucleus can elicit contraversive pivoting and that the pedunculopontine tegmental nucleus is one of the output stations of the accumbens region that mediates shell-specific, dopaminergic pivoting, but not of the accumbens region that mediates shell-specific, cholinergic circling.     

Bilateral cerebral metabolic effects of pharmacological manipulation of the substantia nigra in the rat: unilateral intranigral application of the inhibitory GABAA receptor agonist muscimol.

Rates of cerebral glucose utilization were measured by means of the autoradiographic 2-deoxy-d[1-¹⁴C]glucose technique in the rat brain in order to determine the metabolic effects of unilateral intranigral application of the GABA_A agonist muscimol upon the substantia nigra and its targets. Intranigral injection of 1.5 μl 0.3 M muscimol (52 μg total dose) induced local metabolic activation in the injected substantia nigra reticulata (by 87% as compared to the control group), and distal metabolic depressions in the nucleus accumbens, striatum, globus pallidus and subthalamic nucleus only ipsilaterally to the injected nigra. The remaining basal ganglia components, including the substantia nigra compacta and the entopeduncular nucleus were bilaterally unaffected. Among the principal efferent projections of the substantia nigra reticulata, the ventromedial and centrolateral thalamic nuclei as well as the deep layers of the superior colliculi were metabolically depressed bilaterally, whereas the ventrolateral, parafascicular and mediodorsal thalamic nuclei as well as the pedunculopontine nucleus displayed metabolic depressions ipsilateral to the muscimol-injection nigra. The ventromedial and centrolateral thalamic nuclei were depressed by 41 and 42%, respectively, in the ipsilateral side, and by 30 and 26% in the contralateral side, when compared to the respective values of the control group of rats. Furthermore, unilateral intranigral injection of 0.3 M muscimol induced metabolic depressions in reticular, intralaminar and prefrontal thalamocortical areas mostly ipsilateral to the injected nigra, as well as in limbic areas bilaterally.

It is suggested that the present findings are due to a postsynaptic effect of muscimol on the nigral GABAergic cells and to a consequent metabolic depression of the basal ganglia and associated thalamocortical areas, in contrast to an earlier suggested presynaptic nigral effect of lower doses of intranigrally injected muscimol which induced metabolic activations within the same network.¹⁸ This suggestion is further supported by the fact that intranigrally injected substrate P¹⁹ induced similar effects to those elicited by the lower doses of intranigral muscimol and opposite to those induced at present by the higher muscimol dose. Moreover, it is further substantiated that the nigrothalamic GABAergic system is responsible for considerable transfer of information from one substantia nigra reticulata to the ipsilateral basal ganglia and associated thalamocortical components as well as to bilateral motor, intralaminar and limbic areas.     

Evidence for a GABAergic nigrothalamic pathway in the rat

Summary Unilateral stereotaxic microinjection of muscimol into the caudal region of the substantia nigra (SN) evoked tight, dose-related contralateral locomotor asymmetry and stereotypy. These behaviours were partially attenuated by various pre-treatments, including 6-hydroxydopamine lesions of the nigrostriatal dopamine pathway, intraperitoneal (i.p.) haloperidol, and inhibition of thalamic GABA-transaminase activity by local intrathalamic injection of ethanolamine-O-sulphate. Electrolytic or kainic acid lesions of the medial thalamic nuclei (MTN) partially reduced the contraversive rotation to intranigral muscimol, and completely abolished the similar behaviour elicited by apomorphine (25 g) injected into the ipsilateral caudate nucleus. Contraversive turning to intranigral muscimol was completely inhibited by kainic acid lesions of the ipsilateral SN, but potentiated by intrathalamic injection of picrotoxin. Muscimol (40 ng-4 g) administered to the MTN complex in one hemisphere stimulated rats to move in ipsilateral circles that were unaffected by haloperidol. The results of these behavioural experiments suggest that the nigrostriatal dopamine pathway, the nigrothalamic projection and possibly other non-dopaminergic SN efferents all play important roles in mediating the influences of the SN on motor and stereotyped behaviours. Disruption of the nigrothalamic pathway following electrical or chemical injury to the SN was accompanied by falls in GABA and its synthesising enzyme in the corresponding MTN. These data, together with the findings of our electrophysiological study presented in the following paper, are consistent with the nigrothalamic system having a GABAergic inhibitory function.This work was partly supported by an M.R.C. programme grant awarded to Prof. D.W. StraughanI.C. Kilpatrick and A. Fletcher are respectively M.R.C. and S.R.C. scholars     

Topography and functional role of dopaminergic projections from the ventral mesencephalic tegmentum to the ventral pallidum.

A dopaminergic projection from the ventral tegmental area to the ventral pallidum was identified in the rat using anterograde tract tracing and combined retrograde tracing-immunocytochemistry. The projection was found to be topographically organized such that fibers innervating the ventromedial ventral pallidum arose from neurons located along the midline nuclei of the ventral mesencephalon, including the nucleus interfascicularis and nucleus linearis caudalis. Ventral tegmental neurons situated more laterally, in the nucleus parabrachialis pigmentosus and nucleus paranigralis, projected to the ventromedial and dorsolateral ventral pallidum. The substantia nigra did not supply a major contribution to this projection. The proportion of ventral tegmental area dopaminergic neurons projecting to the ventral pallidum ranged from approximately 30% to 60%. The functional significance of the projection is indicated since intra-ventral pallidum microinjections of dopamine elicited a dose-dependent increase in locomotor activity. Furthermore, whereas pretreatment of the ventral pallidum with the GABAA agonist muscimol has been shown to attenuate opioid-induced locomotor activity elicited from the ventral pallidum, it did not attenuate the dopamine-induced motor response. Thus, while mu-opioids in the ventral pallidum may presynaptically regulate GABAergic efferents from the nucleus accumbens, it appears that the dopaminergic input directly influences the ventral pallidal output neuron which is involved in locomotion.     

Electrical activity and feeding correlates of intracranial hypothalamic injection of GABA, muscimol and picrotoxin in the rats

Assemblies of electrodes and a cannula were stereotaxically implanted in the ventromedial (VMH), lateral (LHA) and paraventricular (PVH) hypothalamic areas in male albino rats. Electrical activity of these regions was recorded electrographically before and following intracranial injection (ICI) of GABA, muscimol and picrotoxin. In another set of animals, food intake and water intake were also measured. The activity of the ventromedial hypothalamus changed from slow to fast after ICI of GABA and picrotoxin and fast to slow after muscimol. The activity of the lateral hypothalamus changed from slow to fast with ICI of muscimol and picrotoxin and from fast to slow with GABA, while that of the paraventricular hypothalamic nucleus changed from slow to fast with ICI GABA and fast to slow with muscimol and picrotoxin. ICI of GABA into VMH and LHA and muscimol in VMH, LHA and PVH caused a decrease in food intake. Water intake was also decreased after ICI of GABA in PVH and muscimol in LHA and PVH. On the opposite picrotoxin increased food intake in VMA and LHA and water intake in PVH. The possible interaction of GABAergic drugs with the areas of the brain controlling feeding and drinking is being discussed.     

The role of the substantia nigra in a striatally-evoked motor response in the rat: effects of intranigral drugs and lesions

Turning of the head towards the contralateral side was induced in the conscious unrestrained rat by electrical stimulation of the neostriatum through permanent indwelling electrodes. The importance of the substantia nigra in mediating this response was investigated using electrolytic and kainic acid lesions. Contralateral head-turning was attenuated by small electrolytic lesions of the substantia nigra on the stimulated side and abolished by large electrolytic lesions that destroyed the entire nucleus. The injection of kainic acid into the substantia nigra caused persistent circling behaviour but did not modify the head-turn response to striatal stimulation; these injections destroyed some of the neurons in the pars compacta but the majority of neurons in the pars reticulata were undamaged. Drugs that alter the function of nigral neurotransmitters were injected through an indwelling cannula in the zona reticulata. Interference with the function of γ-aminobutyrate in the nigra produced small changes in the latency of the motor response. However the following drugs did not modify the response: substance P, nicotine, atropine, apomorphine, glutamic acid diethylester, glycine, strychnine, met-enkephalin and 5-methoxy-N,N-dimethyltryptamine.It is concluded that descending projections from the basal ganglia which pass through or very close to the nigra, probably without forming synapses in the pars reticulata, may be important for the mediation of the striatally-evoked response.