Transient contralateral rotation following unilateral substantia nigra lesion reflects susceptibility of the nigrostriatal system to exhaustion by amphetamine

Amphetamine-evoked rotation requires newly synthesized dopamine at 14 days but not 1 day after intranigral 6-OHDA and is consistently dissociated from sensorimotor behavior

Immediately after unilateral, intranigral 6-hydroxydopamine (6-OHDA), amphetamine (AMPH) evokes "paradoxical" contraversive rotation, whereas 14 days later, AMPH evokes the traditional ipsiversive rotation used to model the chronic Parkinsonian state. In this study, the hypothesis was that accelerated dopamine (DA) synthesis ipsilateral to the lesion augments cytoplasmic DA to produce paradoxical rotation. Therefore, the sensitivity to synthesis inhibition of AMPH-evoked rotation at 1 or 14 days after 6-OHDA was assessed. To determine the functional status that might be reflected by paradoxical rotation, sensorimotor abilities were examined at 1 and 14 days following unilateral 6-OHDA using the elevated swing, paw placement, grip strength, ladder walking, somatosensory neglect, and cylinder tests. At 14 days after 6-OHDA when AMPH-evoked ipsiversive rotation is mediated by the intact hemisphere, rotation was dose-dependently reduced by tyrosine hydroxylase (TH) inhibition with alpha-methyl-p-tyrosine (alpha-MPT) or dopa decarboxylase (DDC) inhibition with 3-hydroxybenzyl hydrazine (NSD-1015), indicating dependence upon newly synthesized DA. Conversely, at 1 day after 6-OHDA, paradoxical rotation, presumably mediated by the treated hemisphere, was completely resistant to synthesis blockade, indicating an abundant supply of intracellular DA that is independent from synthesis rates. Sensorimotor behaviors were not correlated with AMPH-evoked rotation. The present data do not support the hypothesis that enhanced DA synthesis is required to express paradoxical rotation. Therefore, alternative mechanisms that may enhance cytoplasmic DA to produce paradoxical rotation are discussed.

Temporally limited role of substantia nigra-central amygdala connections in surprise-induced enhancement of learning

Prediction error plays an important role in modern associative learning theories. For example, the omission of an expected event (surprise) can enhance attention to cues that accompany those omissions, such that subsequent new learning about those cues is more rapid. Many studies from our laboratories have demonstrated that circuitry that includes the amygdala central nucleus (CeA), the cholinergic neurons in the substantia innominata/nucleus basalis region and their innervation of the posterior parietal cortex is critical for this surprise-induced enhancement of attention in learning. We recently showed that midbrain dopamine neurons, known to code prediction error, are also important for surprise-induced enhancement of learning through their interaction with CeA. The present study examined whether in rats the communication between the substantia nigra pars compacta (SNc) and CeA is critical only at the time of surprise, for example to detect prediction error information, or is also needed to maintain and later express that information as enhanced learning. All animals received unilateral CeA lesions and unilateral cannula implants targeting the SNc located contralateral to the lesioned CeA. As the SNc-CeA connections are mainly ipsilateral, inactivating SNc contralateral to the lesioned CeA provided transient blockage of SNc and CeA communication. The results show that SNc-CeA communication is critical for processing prediction error information at the time of surprise, but neither SNc nor SNc-CeA communication is necessary to express that information as enhanced learning later.

Orphan neurones and amine excess: the functional neuropathology of Parkinsonism and neuropsychiatric disease

The aetiology and treatment of Parkinsonism is currently conceptualised within a dopamine (DA) deficiency-repletion framework. Loss of striatal DA is thought to cause motor impairment of which tremor, bradykinaesia and rigidity are prominent features. Repletion of deficient DA should at least minimise parkinsonian signs and symptoms. In Section 2, based on extensive pre-clinical and clinical findings, the instability of this approach to Parkinsonism is scrutinised as the existing negative findings challenging the DA deficiency hypothesis are reviewed and reinterpreted. In Section 3 it is suggested that Parkinsonism is due to a DA excess far from the striatum in the area of the posterior lateral hypothalamus (PLH) and the substantia nigra (SN). This unique area, around the diencephalon/mesencephalon border (DCMCB), is packed with many ascending and descending fibres which undergo functional transformation during degeneration, collectively labelled 'orphan neurones'. These malformed cells remain functional resulting in pathological release of transmitter and perpetual neurotoxicity. Orphan neurone formation is commonly observed in the PLH of animals and in man exhibiting Parkinsonism. The mechanism by which orphan neurones impair motor function is analogous to that seen in the diseased human heart. From this perspective, to conceptualise orphan neurones at the DCMCB as 'Time bombs in the brain' is neither fanciful nor unrealistic [E.M. Stricker, M.J. Zigmond, Comments on effects of nigro-striatal dopamine lesions, Appetite 5 (1984) 266-267] as the DA excess phenomenon demands a different therapeutic approach for the management of Parkinsonism. In Section 4 the focus is on this novel concept of treatment strategies by concentrating on non-invasive, pharmacological and surgical modification of functional orphan neurones as they affect adjacent systems. The Orphan neurone/DA excess hypothesis permits a more comprehensive and defendable interpretation of the interrelationship between Parkinsonism and schizophrenia and other related disorders.

Relationship between asymmetries in striatal dopamine release and the direction of amphetamine-induced rotation during the first week following a unilateral 6-OHDA lesion of the substantia nigra

In animals with a large unilateral 6-hydroxydopamine (6-OHDA) lesion of the nigrostriatal dopamine (DA) system the traditional "rotational behavior model" states that amphetamine will induce circling behavior towards the denervated striatum (ipsiversive), that is, away from the side where there is greater amphetamine-stimulated DA release and greater DA receptor stimulation. It is puzzling, therefore, why amphetamine induces contraversive rotation in rats tested 4 days after a unilateral 6-OHDA lesion, despite a 90-95% loss of the dopaminergic input to the striatum by this time. Rats reverse their direction of amphetamine-induced rotation by 8 days post-lesion and turn in the ipsiversive direction thereafter. To try and resolve this paradox, bilateral striatal microdialysis was used to estimate the effects of amphetamine on DA neurotransmission on Day 4 and Day 8 following a large unilateral 6-OHDA lesion of the substantia nigra. On Day 4 post-lesion, amphetamine produced a moderate (around 50% of control) increase in the extracellular concentration of DA in the denervated striatum. This amphetamine-releasable pool of DA was exhausted by a single amphetamine-challenge, because a second injection of amphetamine given 3 h after the first did not produce a comparable increase in DA. It is suggested that on Day 4 post-lesion the amount of DA released by amphetamine in the denervated striatum is sufficient to produce greater DA receptor stimulation on that side, because of DA receptor supersensitivity, and this leads to contraversive rotation. On Day 8 post-lesion, amphetamine induced DA release in the intact striatum but had no effect on extracellular DA in the denervated striatum (DA was nondetectable). On Day 8, therefore, DA receptor stimulation would be greatest in the intact striatum, leading to ipsiversive rotation. In conclusion, it is suggested that the seemingly paradoxical reversal in the direction of amphetamine-induced rotation that occurs over the first week following a unilateral 6-OHDA lesion is consistent with the traditional rotational model, and is due to time-dependent changes in the ability of amphetamine to release DA in the denervated striatum.

Factors in amphetamine-induced contralateral rotation in the unilateral 6-OHDA lesion rat model during the first-week postoperative: implications for neuropathology and neural grafting

Amphetamine induced ipsilateral rotation in rats with chronic unilateral 6-hydroxydopamine (6-OHDA) lesions is a widely accepted line of evidence supportive of dopaminergic mediation of amphetamine effects on motoric behavior. However, there is literature indicating that amphetamine induces contralateral rotation, in the early postoperative phase of a unilateral 6-OHDA lesion. In an attempt to reconcile these opposite amphetamine effects on rotation in terms of dopaminergic mechanisms, a series of 4 experiments were conducted. These studies showed that amphetamine reliably elicits contralateral rotation for up to 7 days postoperative but only ipsilateral rotation thereafter. The amphetamine induced contralateral rotation differed behaviorally in several respects from subsequent ipsilateral rotation induced by amphetamine. It was comparatively more intense; and, while onset of peak rotation was dose dependent, rate of rotation was independent of dose level (0.5, 1.0 and 2.5 mg/kg). Dopamine and dopamine metabolite analyses by HPLC-EC after 3 postoperative intervals (days 3, 6, and 9) indicated a progressive and severe depletion of striatal dopamine in conjunction with elevated dopamine turnover. Importantly, after 6 days postoperative, dopamine was reduced to less than 0.06% after intact hemisphere but yet, amphetamine (1.0 mg/kg) elicited contralateral rotation. It was proposed that amphetamine could release a small amount of dopamine present in a sparse number of residual degenerating terminals and this dopamine, unrestricted by reuptake, could widely access supersensitive dopamine receptors to elicit contralateral rotation. This possibility calls into question amphetamine tests for neural graft efficacy in animal models which use amphetamine induced contralateral rotation as the criterion response.

Heterogeneous rotational responsiveness in 6-hydroxydopamine-denervated rats: pharmacological and neurochemical characterization

Qualitative differences in pharmacological responsiveness to various types of dopamine agonists have been reported in rats that have undergone unilateral 6-hydroxydopamine (6-OHDA)-induced denervation of the nigro-striatal pathway. The present experiments further characterize these differences, pharmacologically and neurochemically. Rats were classified as having high rotational sensitivity (0.03 mg/kg SC apomorphine sufficient to induce more than 100 rotations/20 min) or low sensitivity (0.3 mg/kg SC apomorphine required to meet this criterion). High sensitivity rats showed marked contralateral rotational behavior (approximately 150 rotations/20 min) in response to apomorphine (ED50 = 0.08 mg/kg IP), CGS 15855A (ED50 = 0.07 mg/kg), CGS 15873A (ED50 = 0.43 mg/kg), (+)-3-PPP (ED50 = 2.3 mg/kg), (-)-3-PPP (ED50 = 0.87 mg/kg) and quinpirole (peak effective dose, 0.03 mg/kg). In low sensitivity rats, 3- to 10-fold higher doses of apomorphine induced a maximal rate of rotational behavior, but only partial effects were produced by quinpirole, CGS 15855A, CGS 15873A, (+)-3-PPP, and (-)-3-PPP (40-80 rotations/20 min). Because apomorphine is a nonselective D1 and D2 agonist, it is proposed that activation of either D1 or D2 receptors suffices to induce high rates of rotation in high sensitivity rats, whereas in low sensitivity rats, D1 or D2 agonism alone induces submaximal rotation rates. The ipsilateral rotational behavior induced by d-amphetamine was more pronounced and occurred at lower doses in the high-sensitivity rats. Striatal dopamine depletion on the lesioned side did not differ between the groups, but low sensitivity rats showed two-fold higher DOPAC/DA ratios on the lesioned side than did high-sensitivity rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Exposure to amphetamine after substantia nigra lesion interferes with the process of behavioral recovery

Recovery from unilateral substantia nigra lesion may be indicated by re-emergence of circling in the pre-lesion preferred direction. Following 6-OHDA-induced lesion of the dominant SN, we examined: (a) The effect of the delay from lesioning on amphetamine-induced rotation asymmetry, and (b) The effect of early post-lesion exposure to amphetamine on later rotation asymmetry. d-Amphetamine was initially injected either 7, 14, 21, or 30 days after lesioning. Transient circling in pre-lesion preferred direction (contralateral to lesioned side) was more frequently encountered on days 7 and 30 after lesioning, as compared to days 14 and 21. The contralateral rotation observed on day 7 is attributed to degeneration-induced DA release, whereas contralateral rotation noted on day 30 is believed to reflect the operation of post-lesion compensatory processes within the spared DA neurons. In response to subsequent amphetamine administration 30 days after lesioning, rats with previous exposure to the drug circled ipsilaterally, whereas most rats given amphetamine for the first time in that session rotated contralaterally to the lesion. These findings suggest that post-lesion administration of amphetamine interferes with the process of recovery.