Lawrence J. Ryan

Histological and ultrastructural evidence thatd-amphetamine causes degeneration in neostriatum and frontal cortex of rats

D-Amphetamine sulfate, continuously administered for 3 days subcutaneously via an implanted minipump, induced neural degeneration in Long-Evans and Sprague-Dawley rats at doses between 20 and 60 mg/kg/day. Using Fink-Heimer silver staining, axonal degeneration was detected in the neostriatum and the dorsal agranular insular cortex and degenerating pyramidal cells were observed in portions of the somatosensory neocortex in both strains. In contrast, dense axonal degeneration largely confined to layers 2 and 3 of frontal motor areas (Fr1, Fr2 and Fr3 of Zilles36) with occasional degenerating cells was seen reliably in Long-Evans rats but rarely in Sprague-Dawley rats. In the electron microscope, cortical degeneration consisted mainly of disrupted cell bodies and dark processes, including axons making asymmetric synapses. Damage in all cortical areas represents damage to non-monoamine neurons and processes since tyrosine hydroxylase and serotonin immunolabeling were normal. In contrast, the damage in neostriatum probably includes damage to dopamine axonal terminals since tyrosine hydroxylase immunolabeling was patchy with many swollen and distorted labeled axons. Serotonin and Leu-enkephalin labeling were normal. Electron microscopy confirmed that the neostriatum contained many tyrosine hydroxylase-labeled axons that were swollen and disrupted, although other labeled processes made normal symmetric synapses onto spines and dendrites. Additional degeneration found only in amphetamine-treated rats included many dark, shrunken profiles. Some of these appeared to be astrocytic processes and a few were myelinated axons, suggesting that some non-monoamine, possibly cortical afferents, are also degenerating in the neostriatum. Since similar degrees of behavioral activation, weight loss and lethality were seen in both strains, a genetic predisposition constrain amphetamine-induced motor cortex damage but not neostriatal damage.

Effects of STN lesions on simple vs choice reaction time tasks in the rat: preserved motor readiness, but impaired response selection

The subthalamic nucleus (STN) is a key structure within the basal ganglia, inactivation of which is a current strategy for treating parkinsonism. We have previously shown that bilateral lesions of the STN or pharmacological inactivation of this structure in the rat induce multiple deficits in serial reaction time tasks. The aim of the present study was to investigate further a possible role for the STN in response preparatory processes by using simple (SRT) and choice (CRT) reaction time tasks. In contrast to the CRT procedure, the information related to the location of where the response had to be made was given in advance in the SRT procedure. Accurate performance on these tasks requires not only the selection of the correct response (i.e. which response), but also preparation in order to perform when required. A comparison between the two tasks allows assessment of whether STN lesions affect which response (‘which’) or when to perform it (‘when’). As previously observed in these procedures, the responses were faster as a function of the variable foreperiod preceding the trigger stimulus. This well‐known effect, termed ‘motor readiness’, was maintained after STN lesions, suggesting that STN lesions did not affect the ‘when’ phase of action preparation. However, while performance on the SRT was faster than on the CRT task preoperatively, STN lesions slowed RTs and abolished the beneficial effect of advance information, suggesting a deficit in the selection (‘which’) phase of response preparation. This deficit in the selection phase was further supported by deficits in accuracy of responding after STN lesions, as well as increases in mislocated premature responding in the SRT condition. Together, these results suggest that the STN plays an important role in response preparatory processes, including response selection and inhibitory control processes.

The role of the subthalamic nucleus in the response of globus pallidus neurons to stimulation of the prelimbic and agranular frontal cortices in rats

SummaryWe investigated how the cerebral cortex can influence the globus pallidus by two routes: the larger, net inhibitory route through the neostriatum and the separate, smaller, net excitatory route through the subthalamic nucleus. Stimulation (0.3 and 0.7 mA) of two regions of frontal agranular (motor) cortex and of the medial orbitofrontal cortex centered in the prelimbic cortex typically elicited one or more of the following extracellularly recorded responses in over 50% of tested cells: an initial excitation (approximately 6 ms latency), a short inhibition (15 ms latency) and a late excitation (29 ms latency). Some other cells responded with an excitatory response only (18 ms latency). The excitatory responses largely arise from the subthalamic route. Kainic acid or electrolytic lesion of the subthalamic nucleus eliminated most excitatory responses and greatly prolonged the duration (16 vs 50 ms) of the inhibition. Subthalamic neurons typically showed one or more of the following responses to cortical stimulation: an early excitatory response (4 ms latency), an inhibitory period (9 ms) and a late excitatory response (16 ms). The early response was seen after motor cortex but not prelimbic stimulation. The timing of the globus pallidus and subthalamic responses suggest the operation of a reciprocal inhibitory/excitatory pathway. Two reciprocal interactions were indicated. First, pallidal inhibition may disinhibit the subthalamus and, via a feedback pathway onto the same pallidal cells, act to terminate the neostriatal-induced inhibition. Second, there may be a feedforward pathway from pallidal cells to subthalamic neurons to a different group of pallidal cells. This pathway could act to suppress competing responses. Thus the subthalamus may have three actions: 1) an early direct cortical and 2,3) later reciprocal feedforward and feedback excitatory antagonism of the neostriatal mediated inhibition of globus pallidus.

Alteration of neuronal responses in the subthalamic nucleus following globus pallidus and neostriatal lesions in rats.

Kainic acid (2-4 days) or ibotenic acid (7-9 days) lesions of the globus pallidus or neostriatum altered the responsiveness of subthalamic nucleus neurons to electrical stimulation of the agranular frontal cortex. Three changes in responsiveness were seen following pallidal lesion: a) An increase in the proportion of responding cells as compared to controls (approximately 90% vs. 60%); b) an increase in the total duration of the evoked response (62.5 ms vs. 28.6 ms); 3) an increase in magnitude of response (9.76 spikes per stimulus vs. 3.24). Both an increase in firing rate (17.94 spikes/s vs. 8.23) and a change to a bursty spontaneous firing pattern were seen. Lesion of the neostriatum had fewer but opposite effects including decreased firing rate (7.21 spikes/s) and decreased total response duration (18.9 ms). These results suggest that the normal tonic inhibition of the subthalamic nucleus by the globus pallidus may play an important role in controlling subthalamic neuronal spontaneous activity and responsiveness. The neostriatum may influence the subthalamic nucleus via the globus pallidus. Globus pallidus lesions may have important consequences on the specificity of cortical control of the subthalamic nucleus and may alter subthalamic influence on basal ganglia output.

Antidromically identified striatonigral projection neurons in the chronically implanted behaving rat: Relations of cell firing to amphetamine-induced behaviors.

The effects of systematically administered amphetamine (0.25-5.0 mg/kg, sc) on neostriatal neurons recorded in chronically implanted behaving rats were studied. Projection neurons, identified by antidromic activation from the substantia nigra, fired very infrequently during most predrug behaviors (e.g., median rate, 0.02 spikes per second during locomotion; 17 of 18 fired less than 1 spike per second during all rated behaviors). Nonantidromic cells also tended to fire slowly (median rate, 0.02 spikes per second during locomotion; 20 of 24 cells fired less than 1 spike per second). Cells of both type showed up to 10-fold variations in firing rate across behaviors. For most neurons, amphetamine caused a reduction in the firing rate during related pre- and postdrug behaviors. For instance, the firing rate of 28 of 42 neurons was reduced during the initial amphetamine-induced locomotion as compared with the rate during predrug locomotion. Moreover, with the higher doses of amphetamine, there was a further reduction in firing rate corresponding to the transition from locomotion to stereotypies. In contrast to previous studies, which suggest that amphetamine generally increases neostriatal firing rate in behaving animals, these results suggest that amphetamine inhibits the numerous slowly firing neostriatal neurons, many of which were identified as projection neurons. Thus amphetamine alters the magnitude and pattern of neostriatal control of its neural targets.

Cocaine, in contrast to D-amphetamine, does not cause axonal terminal degeneration in neostriatum and agranular frontal cortex of long-evans rats

Continuous three day administration via implanted minipumps of cocaine hydrochloride (50-450 mg/kg/day, sc and 100-250 mg/kg/day, iv) did not produce axonal degeneration in frontal agranular cortex or neostriatum that was detectable by Fink-Heimer silver staining or tyrosine hydroxylase immunolabeling. This is in contrast to the extensive axonal degeneration detectable in these regions following d-amphetamine sulfate (10-60 mg/kg/day) administered following an identical protocol. Doses of cocaine and amphetamine were equated using three measures: 1) weight loss, 2) lethality and 3) behavioral activation. Thus, cocaine resembles other catecholamine reuptake blockers and does not cause the neurodegenerative changes characteristic of other abused drugs that interact with the brains dopamine systems.

Auto- and cross-correlation analysis of subthalamic nucleus neuronal activity in neostriatal- and globus pallidal-lesioned rats

Statistical analyses (autocorrelation and first-order interstimulus interval) were conducted on the spontaneous activity of over 420 subthalamic neurons recorded in 5 groups (control, large globus pallidus kainic acid lesion, partial globus pallidus kainic acid lesion, partial globus pallidus ibotenic acid lesion and neostriatal lesion) of anesthetized rats. Cross-correlation and peristimulus time histogram (to frontal motor cortex stimulation at 0.7 mA) analyses were conducted on pairs (n = 58) of subthalamic neurons recorded simultaneously on a single microelectrode. Lesion of the globus pallidus increased spontaneous firing rate as compared to controls and shifted the pattern of spontaneous activity from either a regular or irregular pattern to a markedly bursting pattern. Neostriatal lesion reduced firing rate and reduced the likelihood of highly regular firing. In control, neostriatal and partial lesioned animals, approximately 1 in 3 pairs of neurons showed correlated firing. The correlations were joint increased probabilities of firing over intervals of 200-400 ms, suggesting a shared excitatory input. No short-interval (less than 10 ms) correlations were seen. Large globus pallidus lesion increased the likelihood of correlated firing (12 of 16 pairs). In all groups of animals the peristimulus time histograms (PSTHs) to motor cortex stimulation were more similar than would be expected by chance and pairs of neurons showed the same increases in response following globus pallidus lesion. Thus adjacent neurons share common cortical inputs and responsiveness to those inputs. These changes indicate that the globus pallidus influences the spontaneous firing rate and pattern of subthalamic neurons as well as the degree of correlated firing of adjacent neurons.

Subthalamic nucleus lesion regularizes firing patterns in globus pallidus and substantia nigra pars reticulata neurons in rats

Subthalamic nucleus lesion altered the statistical properties of the firing patterns of globus pallidus and substantia nigra pars reticulata neurons recorded in urethane anesthetized rats by increasing the proportion of cells in both structures that fired with a very highly regular pattern (from approximately 25% to approximately 50%). In all cases, the most regularly firing neurons fired at a higher mean rate than did more slowly firing neurons. In contrast, globus pallidus lesion shifted the pattern of substantia nigra neurons towards more irregular firing and induced a bursty pattern in two neurons.

Subthalamic nucleus and globus pallidus lesions alter activity in nigrothalamic neurons in rats

Lesions of the subthalamic nucleus or the globus pallidus altered the response of substantia nigra pars reticulata neurons (antidromically identified as projecting to the thalamus) to electrical stimulation of the frontal agranular cortex. In intact animals, cortical stimulation evokes three independent responses (excitation, inhibition, excitation) that may occur singly or in various combinations. The independence of the various responses, especially the temporally coincident excitatory and inhibitory responses, suggests that the net inhibitory and excitatory pathways carrying these signals from the cortex may converge to varying degrees on individual nigrothalamic neurons. Subthalamic lesions increased total response duration (from 28.4 to 39.7 ms), increased the duration of inhibition (from 18 to 30 ms), decreased the occurrence of excitatory responses, and decreased the intensity of the second excitation (from 1.1 to 0.6 spikes/s). Lesion of the globus pallidus also increased total response duration (up to 38 ms), but by increasing the duration of the second excitation (from 15.1 up to 23.8 ms). The intensity of the second excitation (from 1.1 to 1.5 spikes/stimulus) and the number of cells showing the first and second excitations also increased. The incidence, but not the duration, of the inhibition increased. The mean firing rate increased after subthalamic nucleus lesion (34.2 spikes/s) as compared to intact (27.0) or globus pallidus lesion (25.6). These changes may reflect changes in the relative contribution of the five different pathways transmitting information from the cortex to the substantia nigra. In all cases the cortico-striato-nigral pathway is largely intact.(ABSTRACT TRUNCATED AT 250 WORDS)

Substantia nigra stimulation evoked antidromic responses in rat neostriatum

SummaryElectrical stimulation of the substantia nigra of rats elicits a burst of small amplitude waves with a latency of 4–6 ms that may last for 10–15 ms throughout much of the neostriatum. Frontal cortex stimulation also elicits a burst response, which can occlude the substantia nigra response. The substantia nigra evoked burst response was still present after chronic neocortical ablation or thalamic transection or both treatments combined. The response corresponds to the first sharp negative wave of the substantia nigra evoked neostriatal field potential. Single substantia nigra evoked action potentials were recorded in neostriatum with a mean latency of 9.8 ms, ranging from 4–22 ms. These action potentials were considered to be antidromic because 1) they were occluded during appropriate collision intervals by orthodromic action potentials elicited by frontal cortex stimulation. Subthreshold frontal cortex conditioning stimulation did not alter the threshold for activation from substantia nigra. The refractory period for the axon was at least as long as that for the soma and ranged between 0.8–2.0 ms. The antidromic responses failed to follow low frequency stimulation (< 40 Hz for 3000 ms). This failure occurred in the axon between substantia nigra and globus pallidus. The burst response and first sharp negative wave of the field potential probably represent the antidromic activation of the ubiquitous and densely packed medium spiny neostriatal projection neurons. These responses 1) occur at the same latency, 2) respond in the same manner to twin pulse and repetitive stimulation and 3) are occluded by frontal cortex stimulation in the same manner as antidromic action potentials.