Reward contingent positive variation (RCPV) and post-reinforcement EEG synchronization (PSR) in the cat: physiological aspects, the effect of morphine and LSD-25, and a new interpretation of cholinergic mechanisms

Effects of acute lysergic acid diethylamide on intermittent ethanol and sucrose drinking and intracranial self-stimulation in C57BL/6 mice

BACKGROUND

Psychedelics, like lysergic acid diethylamide (LSD), are again being studied as potential therapies for many neuropsychiatric disorders, including addictions. At the same time, the acute effects of psychedelics on rewarding behaviours have been scarcely studied.

AIMS

The current study aimed to clarify if LSD decreases binge-like ethanol drinking in mice, and whether the observed acute effects on ethanol consumption are generalizable to a natural reinforcer, sucrose, and if the effects resulted from aversive or reward-attenuating effects caused by LSD.

METHODS

The effects of acute LSD were examined using 2-bottle choice intermittent ethanol (20%) and sucrose drinking (10%), discrete-trial current-intensity threshold method of intracranial self-stimulation and short-term feeding behaviour assay in C57BL/6 male mice.

RESULTS

The results showed that acute 0.1 mg/kg, but not 0.05 mg/kg, dose (i.p.) of LSD reduced 2-h intermittent ethanol drinking transiently without any prolonged effects. No effects were seen in intermittent 2-h sucrose drinking. The tested LSD doses had neither effect on the intracranial self-stimulation current-intensity thresholds, nor did LSD affect the threshold-lowering, or rewarding, effects of simultaneous amphetamine treatment. Furthermore, LSD had small, acute diminishing effects on 2-h food and water intake.

CONCLUSIONS

Based on these results, LSD decreases binge-like ethanol drinking in mice, but only acutely. This effect is not likely to stem from reward-attenuating effects but could be in part due to reduced consummatory behaviour.

Slow cortical DC-potential responses to sweet and bitter tastes in humans

Processing of hedonic stimulus quality is assumed to be accompanied by a tuning of cortical arousal and excitability. In this pilot study in 11 healthy humans scalp-recorded DC potentials were assessed during application of a sweet (sucrose) and bitter (quinine hydrochloride) taste, i.e., primary reinforcers of positive and negative quality. Muscular, ocular, and skin potential activity were controlled. Application of sucrose induced a widespread positive DC-potential shift with an amplitude of 40-50 microV and persisting for more than 120-s post-stimulus onset. Following administration of quinine hydrochloride, this positive shift was reduced, most distinctly between 48- and 88-s post-stimulus onset. The reduction appeared to be most consistent at anterior midline recording sites (Fz, Cz). It is assumed that the higher DC-potential positivity during sweetness than during bitterness points to a differential tuning of cortical excitability by a widespread decrease in depolarization of apical dendrites.

The magnitude of post-reinforcement EEG synchronization (PRS) in cats reflects learning ability

Electrocortical alpha-like activity, termed post-reinforcement synchronization (PRS), observed during operantly behavior, was quantified in 25 cats and plotted against the number of training sessions required for each to learn to press a lever for milk reward. A significant correlation (r = 0.77; P less than 0.001) was found between PRS magnitude and learning ability. The time required for learning correlated with the amount of perseveration in partially reinforced, i.e. ineffective, behavioral modes.

Pulvinar and lateral geniculate neuronal activity in the cat during operantly conditioned appetitive behavior

Cats were trained to press a lever for 0.5--1.0 ml of milk reward both in the presence and absence of ambient light. 'Floating' microelectrodes were implanted in the pulvinar and lateral geniculate nuclei to allow chronic recording from single or multiple neurons over a period of 1--16 days. Thirty-one of 36 pulvinar neurons (86%) showed a consistent periodicity in the firing rate that was clearly related to certain phases of operant behavior. In the presence of ambient light, the firing rate was maximal shortly prior to or immediately after a rewarded bar press, particularly during the animal's initial contact with reward (the first lap). This burst of activity was followed by an abrupt and strong inhibition of firing which was, in most instances, associated with high voltage slow wave (6--9 c/sec) postreinforcement synchronization (PRS) of the electroencephalogram over the visual and association cortices as well as in the pulvinar. Postreinforcement inhibition of neuronal activity was also observed in the lateral geniculate. Neuronal firing in both nuclei dropped significantly below the respective mean rate observed for each nucleus during relaxed wakefulness. In the dark, however, the reward-induced inhibition was abolished in both nuclei. Moreover, some pulvinar neurons in the dark showed a conspicuous and significant increase in firing during the consummatory response (dark reversal pattern). These findings suggest that a transient but powerful inhibitory process triggered by positive reinforcement depends on visual input. Firing patterns of some pulvinar neurons were noted to be entrained not only by the rhythm of PRS oscillations (inhibitory phasing and postinhibitory discharges) but also by the much slower 3--4 c/sec rhythm of lapping and licking. Furthermore, the patterns of entrainment by initial lapping appear to be different from those observed during the later phase of the consummatory response, thus indicating that the pulvinar may receive information regarding the content of the delivery cup. No entrainment of neuronal activity was observed in the lateral geniculate. These findings reveal remarkable plasticity in the pulvinar neurons that is consistent with their role in polysensory integration.

Cerebral ventricular infusion of excess calcium in the rat: effects on sleep states, behavior and cortical EEG

Sleep and behavior of the rat were recorded during cerebroventricular infusion of artificial cerebrospinal fluid (aCSF) containing regular or excess concentrations of calcium. Three different types of aCSF were used for control infusions. Depending on the ionic composition, paradoxical sleep (PS) was reduced by 6--52% during a 1-h aCSF infusion period, whereas the total amount of sleep was not altered. The depression of PS by aCSF could be prevented by increasing the concentration of Ca in the infusate by a factor of 2--5 (2.6--9.1 mM). Infusions of high concentrations of Ca (9.1--54.6 mM) caused feeding and wet-dog shakes. A slow-wave cortical EEG pattern prevailed during feeding elicited by either infusion of excess Ca or systemic administration of a small dose of pentobarbital. It is concluded (a) that the ionic composition of the CSF may selectively influence the occurrence of a sleep state, and (b) that Ca-induced feeding may be related to a covert sedative action of this cation.

Electrical activation of visual pathways substitutes for tonic light input in triggering EEG correlates of food reward during conditioned behavior in cats

Cats trained to press a lever for 1 cc of milk reward normally show during the consummatory response high voltage 6--8 c/sec EEG synchronization associated with epicortical positive steady potential shift over the primary and secondary visual projections. The emergence of this postreinforcement synchronization (PRS) and Reward Contingent Positive Variation (RCPV) is known to depend upon appropriate gustatory input and presence of ambient light, although visual perception of reward and/or environment is not essential as shown in cats wearing translucent "milky" contact lenses. Training the animals in a paradigm in which a "light-off" cue signaled the availability of reward, and thus assigned positively reinforcing quality to the dark condition, also failed to restore the PRS-RCPV phenomenon in the absence of light. However, brief electrical stimuli applied to either the optic tract or the lateral geniculate nucleus substituted for unpatterned light input and fully restored the PRS-RCPV in the dark. The suprathreshold stimuli were effective only during the consummatory response. Maximum effect could be produced during a brief time period between 0.8 and 1.5 sec after the onset of consummatory response as judged by lapping activity, thus showing the specificity of the effect of visual input. Even during a relaxed wakefulness after satiation or during slow wave sleep or REM sleep the same electric stimuli were ineffective although they produced well-developed evoked potentials with all characteristic wave components. The results indicate that brain pathways utilize unpatterned i.e. noisy visual input in complex integrative processes involving gustatory input.

Dependence of reward contingent positive variation (RCPV) and cortical synchronization on visual input in the cat

Cats trained to press a lever for milk reward display high amplitude low frequency electrocorticographic post-reinforcement synchronization (PRS) associated with epicortical steady potential shift, termed Reward Contingent Positive Variation (RCPV), over the parieto-occipital cortex. Previously, it was found that the PRS-RCPV phenomenon depends on unpatterned light input devoid of any conceivable information or conditional property. Although training in the dark most likely eliminated the "novelty" factor and the resulting orienting reaction to "light off" as a cause of PRS-RCPV suppression, a possibility could not be excluded that the absence of light had a negatively reinforcing property preventing the emergence of PRS-RCPV. Hence, in the present study an experimental paradigm was used in which "light off" and/or "tone on" cues signalled the availability of reward, the "light off" periods extending through the consummatory response. Seven out of 9 cats, despite 2-4 months of daily training, and 85% correct timing of bar press performance, never showed a PRS-RCPV response (one animal showed only sporadic and poorly developed patterns.) However, when the light was turned on during consumption, in 85% of such tests a fully developed PRS-RCPV emerged in a stimulus-bound manner. One animal was an exception and showed PRS-RCPV both in the absence and presence of light. It was concluded that light input plays a crucial role in the emergence of the PRS-RCPV phenomenon and therefore also participates in the integration of gustatory input.