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The intensity, direction and dopamine dependence of circling behaviour were determined following stereotaxic injections of 40 ng muscimol (in 0.2 microliter over 3 min) into different regions of the rat's substantia nigra (SN). Weak, haloperidol-sensitive ipsilateral postural or locomotor asymmetries were invariably obtained from the rostral SN zona compacta (SNC), whilst robust contraversive rotational behaviour was always initiated from the SN zona reticulata (SNR) and caudal SNC. This was most rapid from the central SNR and was markedly attenuated by i.p. pretreatment with haloperidol (0.1 mg/kg) or pimozide (0.25 mg/kg), or by week-old 6-OHDA lesions of the nigrostriatal dopamine (DA) tract. Turning was significantly weaker from the lateral and ventral areas of the SNR, where it was not susceptible to DAergic blockade. Lower circling rates were also obtained if the SNR injections were made rapidly (in 30 sec), in a large volume (0.5 microliter) or at a supramaximal dose level (400 ng), possibly due to increased spread of the drug to remote neurones having an opposite effect on directional behaviour, or to exaggerated stereotypy. The variable action of muscimol at multiple sites in the SN is suggested to account for the earlier conflicting data in the literature.

Extracellular recordings were made from neurones in the ventromedial and parafasicular nuclei of the rat thalamus, many of which had demonstrable capsular or caudate projections. These cells responded to electrical stimulation of the ipsilateral substantia nigra with a short latency (4 ms) inhibition presumed to be monosynaptic. This inhibitory response was often preceded by a brief period of increased excitability (latency approximately 3 ms) attributed to activation of corticofugal collaterals. Longer latency, presumably oligosynaptic excitations (latency approximately 8 ms) and inhibitions (approximately 18 ms) were also obtained, but were more commonly evoked in non-projection neurones. All units were inhibited by iontophoretically applied GABA, glycine or 5-HT. Short and long latency synaptic and GABA-induced inhibitions were selectively blocked by bicuculline. Strychnine only antagonised glycine, while 5-HT was not affected by either convulsant. Intranigral injection of muscimol greatly elevated the spontaneous discharge rate of thalamic neurones, particularly those with a striatal projection. These data are compatible with nigrothalamic neurones maintaining a tonically active, GABA-mediated inhibition of cells in the ventromedial and parafascicular nuclei of the thalamus. It is speculated that intranigral muscimol indirectly activates these thalamic cells and thereby initiates contraversive circling behaviour by suppressing this inhibitory system.

Focal, unilateral injection of muscimol into the rat substantia nigra, pars reticulata evoked two outwardly different forms of contraversive rotation. In the first (and most common) type of circling, animals adopted a tight curvature of the whole head and body about the longitudinal axis and propelled themselves primarily with their forelimbs. In other cases, little if any postural asymmetry was present during rotation in equivalent diameter circles created by use of both fore- and hindlimbs. Each profile correlated with the nigral locus injected. It is reiterated that postural asymmetry is not essential for rotation.

Experiments employing a rodent circling model were conducted to test the predictive capacity of the theory which states that striatonigral gamma-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 gamma-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 gamma-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 gamma-aminobutyrate levels in other brain areas, since similar ipsiversive rotations occurred if gamma-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 gamma-aminobutyrate and thereby impede striatonigral outflow ipsilaterally.

Stereotaxic injection of muscimol into a restricted region of one substantia nigra of the rat provoked robust circling and a concomitant rise in ipsilateral nigrostriatal dopamine activity, as revealed by a greater accumulation of 3,4-dihydroxyphenylacetic acid and homovanillic acid in the caudate-putamen together with depleted nigral dopamine concentrations. Considered with earlier evidence, these data are taken to indicate that dopamine may be involved in the mediation of this particular rotational behaviour. On the other hand, focal application of bicuculline to the substantia nigra or ventromedial thalamus, or intrathalamic kainate, all evoked a closely similar and vigorous hypermotility (not circling) that could not be correlated with the assorted changes in dopamine utilisation occurring in the substantia nigra, caudate-putamen or nucleus accumbens, either uni- or bilaterally. These changes were therefore probably casually rather than causally related to the mechanisms underlying the behaviour of the animals. Whilst the regional concentrations of noradrenaline were unaltered by these focal drug treatments, the induction of halothane anaesthesia coupled with a unilateral intranigral saline injection produced bilateral elevations in regional dopamine utilisation when assessed 15 min after injection. Such changes were not apparent in tissue taken 30 or 60 min post-injection. We conclude that dopamine cell activity and/or other indices of dopamine utilisation cannot be used to predict the behavioural state of the individual and that an imbalance between the dopamine systems in the two hemispheres does not per se lead to postural or locomotor asymmetry.

The gamma-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 gamma-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 gamma-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 greater than medial sites). Contraversive circling to unilateral 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 micrograms) 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.

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 pretreatments, 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 microgram) 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 intrahalamic injection of picrotoxin. Muscimol (40 ng--4 microgram) 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.