Dopaminergic involvement in nociceptive sensitivity/behavioral reactivity regulation during aversive states of different nature in the rat

Neurobiology of aversive states

Hoebel and colleagues are often known as students of reward and how it is coded in the CNS. This article, however, attempts to focus on the significant advances by Hoebel and others in dissecting out behavioral components of distinct aversive states and in understanding the neurobiology of aversion and the link between aversive states and addictive behaviors. Reward and aversion are not necessarily dichotomous and may reflect an affective continuum contingent upon environmental conditions. Descriptive and mechanistic studies pioneered by Bart Hoebel have demonstrated that the shift in the reward-aversion spectrum may be, in part, a result of changes in central dopamine/acetylcholine ratio, particularly in the nucleus accumbens. The path to aversion appears to include a specific neurochemical signature: reduced dopamine release and increased acetylcholine release in "reward centers" of the brain. Opioid receptors may have a neuromodulatory role on both of these neurotransmitters.

I.v. cocaine induces rapid, transient excitation of striatal neurons via its action on peripheral neural elements: single-cell, iontophoretic study in awake and anesthetized rats

Cocaine's (COC) direct interaction with the dopamine (DA) transporter is usually considered the most important action underlying the psychomotor stimulant and reinforcing effects of this drug. However, some physiological, behavioral and psycho-emotional effects of COC are very rapid and brief and they remain intact during DA receptor blockade, suggesting possible involvement of peripheral non-DA neural mechanisms. To assess this issue, single-unit recording with microiontophoresis was used to examine changes in impulse activity of dorsal and ventral striatal neurons to i.v. COC (0.25-0.5 mg/kg) in the same rats under two conditions: awake with DA receptor blockade and anesthetized with urethane. In the awake preparation approximately 70% striatal neurons showed rapid and transient (latency approximately 6 s, duration approximately 15 s) COC-induced excitations. These effects were stronger in ventral than dorsal striatum. During anesthesia, these phasic effects were fully blocked and COC slowly decreased neuronal discharge rate. Cocaine-methiodide (COC-M), a derivative that cannot cross the blood-brain barrier, also caused phasic excitations in the awake, but not anesthetized condition. In contrast to regular COC, COC-M had no tonic effect on discharge rate in either preparation. Most striatal neurons that were phasically excited by both COC forms also showed short-latency excitations during tail-touch and tail-pinch in the awake preparation, an effect strongly attenuated during anesthesia. Finally, most striatal neurons that in awake conditions were phasically excited by somato-sensory stimuli and COC salts were also excited by iontophoretic glutamate (GLU). Although striatal neurons were sensitive to GLU in both preparations, the response magnitude at the same GLU current was higher in awake than anesthetized conditions. These data suggest that in awake animals i.v. COC, like somato-sensory stimuli, transiently excites striatal neurons via its action on peripheral neural elements and rapid neural transmission. While the nature of these neuronal elements needs to be clarified using other analytical techniques, they might involve voltage-gated K(+) and Na(+) channels, which have a high affinity for COC and are located on terminals of visceral sensory nerves that densely innervate peripheral vessels. Therefore, along with direct action on specific brain substrates, central excitatory effects of COC may occur via indirect action, involving afferents of visceral sensory nerves and rapid neural transmission. By providing a rapid sensory signal and triggering transient neural activation, such a peripherally triggered action might play a crucial role in the sensory effects of COC, thus contributing to learning and development of drug-taking behavior.

Cocaine enhances the changes in extracellular dopamine in nucleus accumbens associated with reinforcing stimuli: a high-speed chronoamperometric study in freely moving rats

Numerous data suggest that the mesocorticolimbic dopamine (DA) system is critically involved in the organization and regulation of goal-directed behaviours of various types as well as in the mediation of the psychogenic effects of cocaine. To test the hypothesis that cocaine not only alters levels of extracellular DA within the mesolimbic DA system, but in addition changes the response of this system to reinforcing environmental stimuli, a study using high-speed chronoamperometry was done to evaluate the effects of cocaine (15 mg/kg, i.p.) on extracellular DA in the nucleus accumbens and to assess the effects of cocaine on the response evoked by the presentation of tail-pinch and palatable food. Cocaine was found to induce long-term biphasic changes in extracellular DA (an increase followed by a decrease) and, more importantly, to enhance DA increases evoked by both tail-pinch and food. The powerful enhancing action of cocaine on DA release, triggered by significant environmental stimuli and associated with behaviours of different types, is considered to be a possible primary mechanism of its rewarding or euphorigenic effect.

Long-term changes of striatal D-2 receptors in rats chronically exposed to morphine under aversive life conditions

Chronic morphine treatment has been shown to cause the development of hyperreactivity of the dopamine system detected as the increased behavioral and biochemical responses to the action of specific dopamine agonists. Furthermore, inverted changes in animal behavioral reactivity to the stimulation of presynaptic, proposed D-2 receptors by apomorphine in a low dose was found in our previous study when morphine was chronically used in animals under conditions of restraint. To estimate the nature and proposed receptor mechanisms of changes found in behavioral reactivity due to chronic morphine administration in aversive life conditions at the level of highly sensitive D-2 receptors, the density and affinity of [3H] spiroperidol binding sites was studied in these animals two weeks after the last opiate administration. Increased density and affinity of D-2 receptors probably indicating their hypersensitivity was found in animals chronically exposed to two-hour restraint stress, while a significant decrease in density accompanied by increase in affinity of these receptors was typical to rats chronically exposed to morphine under conditions of restraint. The data are discussed in aspects of quantitative and qualitative changes in D-2 receptors, and their proposed mechanisms and functional significance in the mediation of modified organism's functional state due to chronic opiate administration in different environmental conditions.

Dopaminergic involvement in the mediation of morphine effects on vocalization and movement reactivity in the rat

To understand the role of dopamine in the mediation of opiate's analgesic and reinforcing activity, the influence of pharmacologically-induced increase (apomorphine in a high dose) and decrease (haloperidol and apomorphine in a low "presynaptic" dose) of dopamine neurotransmission on morphine-induced depression of vocalization and movement reactivity was investigated in freely behaving rats. Strong enhancement of morphine-induced vocalization reactivity depression and a weakening of movement reactivity depression typical to concomitant pharmacological increase of dopamine neurotransmission may represent the enhancement of its reinforcing and analgesic properties and inhibition of its depressive/sedative activity. Enhancement of MOR-induced depression of movement and vocalization reactivity characteristic of drug-induced decrease of dopamine neurotransmission may be the consequence of the blockade of opiate's reinforcing and stimulatory activity and summation of depressive/sedative actions typical to these drugs administered alone.

Activation-induced changes in evoked and slow brain potentials: effects of cocaine in awake rabbit

To study the interrelations between event-related potentials and brain activation, evoked responses to visual stimulation registered from occipital cortex and hippocampal CA1 region were investigated in awake animals administered cocaine (2 mg/kg, SC) or saline. Administration of COCA resulted in longterm significant changes in heart and breathing rates and spontaneous EEG (desynchronization in the occipital cortex and slow rhythmic theta activity in the hippocampus), inhibition of animal responses to noxious electroshock stimuli and modification in different amplitudes of evoked responses (mainly in the cortex, the enhancement of negativity and bidirectional changes in late slow phase). Principally identical but more or less changes were observed in response to administration of saline. Obtained results are discussed in respect to their interrelations with the general activation organism's response and the modification of brain metabolism induced by cocaine in restrained rabbits.

Neurobiological background of pain and analgesia: the attempt at revaluation according to position of the organism's adaptive activity

The most adequate and successful way to understand the essence of any complex psychophysiological phenomenon, including pain, is obviously the study of its origin, its genesis, i.e., its biological background. Based on critical analysis of recent literature and our own electrophysiological, biochemical and pharmacological data we tried to overcome the difficulties and contradictions derived from the traditional reflex approach and analytical orientation in understanding the experimental investigation of pain-related problems and to determine the neurobiological background of pain and analgesia through the notion of the organism's adaptive activity. Interrelations between the notion of pain and other biological and psychological ideas, the place and functional significance of pain and endogenous analgesic mechanisms in the organization, maintenance and regulation of the organism's adaptive activity, characterization of the involvement of endogenous opioid peptides and monoamines in central processes associated with pain and analgesia, the essence and mechanisms of pain-depressing activity of the opiates are the main stages in our neurobiological consideration of the phenomenon of pain and its natural and pharmacological regulation.