Globus pallidus lesions depress the excitatory responses to apomorphine but not amphetamine in the subthalamic nucleus of the behaving rat with a 6-OHDA nigra lesion

Behavioral/neurophysiological investigation of effects of combining a quinolinic acid entopeduncular lesion with a fetal mesencephalic tissue transplant in striatum of the 6-OHDA hemilesioned rat

Behavioral and electrophysiological methods were used to investigate the effects of combining a unilateral quinolinic acid lesion of the entopeduncular nucleus (QA/EP) with a striatal transplant of fetal ventral mesencephalic tissue in the 6-hydroxydopamine (6-OHDA) hemilesioned rat model for Parkinson's disease. The subjects were 6-OHDA-lesioned rats selected because their response to amphetamine treatment was a strongly biased rotation toward the side of the 6-OHDA lesion in the substantia nigra, at the expense of other evoked behaviors associated with amphetamine. Two experiments were performed. In the first, the motor effects of the QA/EP lesion alone and of the combination of QA/EP lesion with striatal transplant were determined by measuring six aspects of the motor response. In the second the electrophysiological effects of the two interventions on the responses of neurons in the subthalamic nucleus to amphetamine and apomorphine were determined in the 6-OHDA-lesioned rats. The results of the first experiment show that the QA/EP lesion by itself produced an attenuation of the rotation response and, simultaneously, an increase of oral stereotypy. They also show that the combination of QA/EP lesion with striatal transplant was more effective than the single intervention, inducing more attenuation of rotation and more oral stereotypy. The results of the second experiment show that the responses of subthalamic neurons to amphetamine in the behaving hemiparkinsonian rat with combined QA/EP lesion and transplant were larger than the responses in the hemiparkinsonian rat with the QA/EP lesion alone. However, even these larger responses in the rats with combined intervention were not as large as those recorded at the same time in the subthalamus in the opposite, intact, hemisphere. The results of the two experiments, both of which show enhanced motor and neuronal sensitivity to amphetamine after the combined intervention, suggest that such a multiple approach might prove more beneficial than a one-site intervention targeting either the EP or the striatum.

Cocaine-induced psychomotor activity is associated with its ability to induce c-fos mRNA expression in the subthalamic nucleus: effects of dose and repeated treatment

Factors that modulate the psychomotor activating effects of amphetamine and cocaine, such as environmental novelty and dose, also regulate the ability of these drugs to induce c-fos mRNA expression in the subthalamic nucleus (STN). We hypothesized therefore that engagement of the STN may be important for stimulant-induced psychomotor activation. To further test this hypothesis we examined whether repeated treatment with cocaine, which enhances its psychomotor activating effects (i.e. produces behavioural sensitization), also enhances its ability to induce c-fos expression in the STN. In addition, given that STN activity is thought to be influenced by preproenkephalin mRNA-containing (ENK+) neurons in the caudate-putamen, we also examined whether repeated cocaine treatment alters c-fos expression in ENK+ cells. We report that: (i) cocaine pretreatment enhances the ability of a cocaine challenge to induce c-fos mRNA expression in the STN, and this effect is most robust at challenge doses where behavioural sensitization is observed; (ii) the ability of cocaine to induce c-fos in the STN is independent of the ability of cocaine to engage ENK+ cells. These results support the idea that the STN is involved in stimulant-induced psychomotor activation and sensitization, but suggest that stimulant-induced engagement of the STN is not dependent on ENK+ cells in the caudate-putamen. These findings may have implications concerning the neurobiological mechanisms underlying the behavioural effects of psychostimulant drugs.

Behavioral and subthalamic effects of combining a fetal ventral mesencephalic transplant in striatum with an electrolytic lesion of the entopeduncular nucleus in the rat with a unilateral 6-OHDA lesion of substantia nigra

Behavioral and electrophysiological methods were used to determine whether a transplant of dopamine-rich fetal tissue in striatum combined with an electrolytic lesion of the entopeduncular nucleus have additive effects in the unilaterally lesioned rat model for Parkinson's disease. The subjects were rats with the left substantia nigra lesioned with 6-hydroxydopamine (6-OHDA) and responding to systemic amphetamine with rotation toward the side of the lesion (ipsilateral rotation). The motor response to amphetamine was fractionated into six aspects, half reflecting the unilateral deafferentation in striatum and half representing those aspects of the response evoked in normal rats. After collection of baseline values, 25 rotators received a transplant of fetal ventral mesencephalic tissue in the left striatum and 20 received a transplant and, at the same time, an electrolytic lesion of the left entopeduncular nucleus. Testing for the motor response to amphetamine resumed after 4 weeks of recovery and continued at weekly intervals for 5 weeks. Upon completion of these tests, each rotator was implanted with multiple electrodes in the subthalamic nucleus. After recovery, multiunit responses to amphetamine and apomorphine were recorded from several electrodes in parallel during the motor response to the drugs. In rotators with transplant only, treatment with amphetamine evoked oral stereotypy and an attenuated ipsilateral rotation response. In rotators with combined transplant and entopeduncular lesion, ipsilateral rotation did not change or increased. Subthalamic responses to amphetamine and apomorphine were larger in rotators with combined transplant and entopeduncular lesion than in rotators with transplant alone. These findings indicate that the combination of transplant and pallidotomy in the 6-OHDA rat model for parkinsonism does not lead to additive benefits, an effect that may have been due to the nonselectivity of the electrolytic damage and/or of the lesion extending beyond the entopeduncular nucleus into the lateral hypothalamus.

Amphetamine-induced c-fos mRNA expression in the caudate-putamen and subthalamic nucleus: interactions between dose, environment, and neuronal phenotype

When administered in a novel environment relatively low doses of amphetamine induce c-fos mRNA in the subthalamic nucleus (STN) and in preproenkephalin mRNA-containing (ENK+) neurons in the caudate-putamen (CPu). When administered at home, however, low doses of amphetamine do not produce these effects. Environmental novelty also facilitates the behavioral effects of acute and repeated amphetamine, but this is dose-dependent. The purpose of the present experiment therefore was to determine if the effect of context on amphetamine-induced c-fos expression is also dose-dependent. It was found that: (i) No dose of amphetamine tested (1-10 mg/kg) induced c-fos in many ENK+ cells when given at home. (ii) When given in a novel environment low to moderate doses of amphetamine (1-5 mg/kg) induced c-fos in substantial numbers of ENK+ cells, but the highest dose examined (10 mg/kg) did not. (iii) Environmental novelty enhanced the ability of low to moderate doses of amphetamine to induce c-fos in the STN, but the highest dose of amphetamine induced robust c-fos mRNA expression in the STN regardless of context. The results do not support the idea that engaging ENK+ cells, at least as indicated by c-fos mRNA expression, is critical to produce robust behavioral sensitization, but do suggest a possible role for the STN. Furthermore, the results highlight the importance of drug-environment interactions on the neurobiological effects of drugs, and have implications for thinking about the circuits by which context modulates the acute and long-lasting consequences of amphetamine treatment.

D2-mediated modulation of N-type calcium currents in rat globus pallidus neurons following dopamine denervation

We have studied the effects of dopamine and the D2-like agonist quinpirole on calcium currents of neurons isolated from the striatum and the globus pallidus (GP). Experiments were performed in young adult rats, either in control conditions or following lesion of the nigrostriatal pathway by the unilateral injection of 6-hydroxydopamine (6-OHDA) in the substantia nigra. Apomorphine-driven contralateral turning, 15 days after lesioning, assessed the severity of the dopamine denervation. In addition, the loss of tyrosine hydroxylase immunohistochemistry confirmed the extent of the toxin-induced damage. In both striatal medium spiny (MS) and GP neurons of control animals dopamine and quinpirole promoted a very modest inhibition of calcium conductance. Following 6-OHDA, the inhibition was unaltered in MS (from 10 to 12%), but significantly augmented in GP neurons (21% vs. 9%). Interestingly, analogous inhibition was observed in GP neurons dissociated 20 h after reserpine treatment. Further features of the D2 response were thus studied only in neurons isolated from 6-OHDA-lesioned GP. The D2 modulation was G-protein-mediated but not strictly voltage-dependent. omega-Conotoxin-GVIA occluded the response implying the involvement of N-type calcium channels. The effect of quinpirole developed fast and was insensitive to alterations of cytosolic cAMP. The incubation in phorbol esters or OAG blocked the D2 effect, supporting the involvement of PKC. These findings suggest that postsynaptic D2-like receptors are functionally expressed on GP cell bodies and may supersensitize following dopamine-denervation. A direct D2 modulation of calcium conductance in GP may alter GP firing properties and GABA release onto pallidofugal targets.

Entopeduncular lesions facilitate and thalamic lesions depress spontaneous and drug-evoked motor behavior in the hemiparkinsonian rat

Pallidotomy is a neurosurgical procedure designed to ameliorate the akinesia and bradykinesia associated with Parkinson's disease. In the present study, the effects of pallidal-like lesions on motor behavior in the hemiparkinsonian rat were compared to the effects of lesions in the ventrolateral thalamus, a target of entopeduncular projections feeding motor-related information to motor cortex. Six aspects of spontaneous and evoked behavior induced by amphetamine and apomorphine in the hemiparkinsonian rat with either bilateral electrolytic entopeduncular lesions or bilateral electrolytic ventrolateral thalamic lesions were measured for 60 min. Saline or amphetamine, 5 mg/kg, or apomorphine, 0.3 mg/kg, were administered IP 5 min before the tests. The results show that on all measures except time spent resting the hemiparkinsonian rats with the entopeduncular lesions were more active than the hemiparkinsonian rats with the thalamic lesions. The asymmetrical rotation responses to dopamine receptor stimulation evoked by amphetamine and apomorphine were influenced by the general effect on gross motor behavior, as shown by the response being very large in the entopeduncular group and very small in the thalamic group. These results are consistent with current thinking about the functional organization of the basal ganglia according to which damage of the entopeduncular nucleus reduces its inhibitory control of the thalamic motor regions, thereby promoting thalamic facilitation of motor cortex, and damage to the thalamic motor regions has the opposite effect. These effects of the lesions translate, respectively, into hyperactivity and hypoactivity without blocking the asymmetrical rotation response of the hemiparkinsonian rat.