Stability of substantia nigra pars reticulata neuronal discharge rates during dopamine receptor blockade and its possible mechanisms

The hidden side of drug action: brain temperature changes induced by neuroactive drugs

RATIONALE

Most neuroactive drugs affect brain metabolism as well as systemic and cerebral blood flow, thus altering brain temperature. Although this aspect of drug action usually remains in the shadows, drug-induced alterations in brain temperature reflect their metabolic neural effects and affect neural activity and neural functions.

OBJECTIVES

Here, I review brain temperature changes induced by neuroactive drugs, which are used therapeutically (general anesthetics), as a research tool (dopamine agonists and antagonists), and self-administered to induce desired psychic effects (cocaine, methamphetamine, ecstasy). I consider the mechanisms underlying these temperature fluctuations and their influence on neural, physiological, and behavioral effects of these drugs.

RESULTS

By interacting with neural mechanisms regulating metabolic activity and heat exchange between the brain and the rest of the body, neuroactive drugs either increase or decrease brain temperatures both within (35-39 °C) and exceeding the range of physiological fluctuations. These temperature effects differ drastically depending upon the environmental conditions and activity state during drug administration. This state-dependence is especially important for drugs of abuse that are usually taken by humans during psycho-physiological activation and in environments that prevent proper heat dissipation from the brain. Under these conditions, amphetamine-like stimulants induce pathological brain hyperthermia (>40 °C) associated with leakage of the blood-brain barrier and structural abnormalities of brain cells.

CONCLUSIONS

The knowledge on brain temperature fluctuations induced by neuroactive drugs provides new information to understand how they influence metabolic neural activity, why their effects depend upon the behavioral context of administration, and the mechanisms underlying adverse drug effects including neurotoxicity.

Striatal and extrastriatal dopamine in the basal ganglia: an overview of its anatomical organization in normal and Parkinsonian brains

Degeneration of the nigrostriatal dopaminergic system is the characteristic neuropathological feature of Parkinson's disease and therapy is primarily based on a dopamine replacement strategy. Dopamine has long been recognized to be a key neuromodulator of basal ganglia function, essential for normal motor activity. The recent years have witnessed significant advances in our knowledge of dopamine function in the basal ganglia. Although the striatum remains the main functional target of dopamine, it is now appreciated that there is dopaminergic innervation of the pallidum, subthalamic nucleus, and substantia nigra. A new dopaminergic- thalamic system has also been uncovered, setting the stage for a direct dopamine action on thalamocortical activity. The differential distribution of D1 and D2 receptors on neurons in the direct and indirect striato-pallidal pathways has been re-emphasized, and cholinergic interneurons are recognized as an intermediary mediator of dopamine-mediated communication between the two pathways. The importance and specificity of dopamine in regulating morphological changes in striatal projection neurons provides further evidence for the complex and multifarious mechanisms through which dopamine mediates its functional effects in the basal ganglia. In this review, the role of basal ganglia dopamine and its functional relevance in normal and pathological conditions will be discussed.

Dopamine action in the substantia nigra pars reticulata: iontophoretic studies in awake, unrestrained rats

Dopamine (DA) neurons located in the substantia nigra pars compacta release DA not only via axonal terminals, affecting neurotransmission within the striatum, but also via dendrites, some of which densely protrude into the substantia nigra pars reticulata (SNr). Although the interaction of dendritically released DA with somatodendritic autoreceptors regulates DA cell activity, released DA may also affect SNr neurons. These cells, however, lack postsynaptic DA receptors, making it unclear how locally released DA modulates their activity. Although previous work in brain slices suggests that DA might modulate the activity of GABA inputs, thus affecting SNr neurons indirectly, it remains unclear how increased or decreased DA release might affect these cells exposed to normal afferent inputs. To explore this issue, we examined the effects of iontophoretic DA and amphetamine on SNr neurons in awake, unrestrained rats. DA had no consistent effects on SNr cells but amphetamine, known to induce DA release, dose-dependently inhibited most of them. This effect was blocked by SCH23390, a selective D1 receptor blocker, which itself strongly increased neuronal discharge rate. As GABA input is a major factor regulating the activity of SNr neurons, our data suggest that dendritically released DA, by interacting with D1 receptors on striato-nigral and pallido-nigral afferents, is able to decrease this input, thus releasing SNr neurons from tonic, GABA-mediated inhibition. Surprisingly, a full DA receptor blockade (SCH23390 + eticlopride) did not result in the expected increase in SNr discharge rate, suggesting that other mechanisms are responsible for behavioral abnormalities following acute disruption of DA transmission.