Intracranial Self Stimulation

Reprinted from: Brain and Behavior. Raju TR, Kutty BM, Sathyaprabha TN and Shanakranarayana Rao BS (eds.), National Institute of Mental Health and Neuro Sciences, Bangalore, India. 2004:121-126.

INTRACRANIAL SELF-STIMULATION Ramkumar K, Raju TR and Shankaranarayana Rao BS Electrical stimulation of the brain is an important tool in the study of the neural basis of behavior. It has, often, the opposite effects of a lesion to the same site. It can elicit some specific types of behavior that would have been normally induced by stimuli naturally sought or attended to: eating, drinking, copulating, attacks or sleep. The elicited responses depend on the location of the electrode in the brain, the parameters of the current and the test environment in which the stimulation is administered. Electrical stimulation of the brain continues to be a fruitful method for exploring brain-behavior relationships. The important applications of brain stimulation are; a. Study of the electrical self-stimulation phenomenon

Using the electrical stimulation method, James Olds and Peter Milner (1954) in McGill University, observed that some animals seem to behave in a manner that increased the amount of intracranial stimulation that they received. Further investigation demonstrated that rats will press a lever as rapidly as 2000 times in fifteen minutes to obtain electrical brain stimulation (Shankaranarayana Rao et al 1993, 1994), and they will continue responding at this rate for several hours. They will ignore other rewards, such as water or food, and continue working for electrical stimulation. These very powerful results led to the adoption of the intracranial self-stimulation method for investigating the “reward system” in the brain and remains up to now the principal tool.

d. The use of electrical brain stimulation as the conditioned and/or unconditioned stimulus in classical conditioning studies

Olds and his collaborators have conducted extensive mapping of the neuronal substrates of reward in the late 1950s. They reported that the highest rate of intracranial self-stimulation (ICSS) was obtained in the septal area, the amygdala and the anterior hypothalamus. Moderate but substantial rates of ICSS were observed in related limbic structures, particularly the hippocampus, cingulate gyrus, anterior thalamus and posterior hypothalamus.

e. The elicitation of speech or recall of experience by brain stimulation in humans and

General methodology

b. Elicitation of complex behaviors by subcortical stimulation c. The mapping of “motor” areas in the cerebral cortex from which limb movements can be elicited by stimulation

f. The use of brain stimulation to induce amnesia, or conversely to facilitate learning Brain electrical stimulation is a method by which the functions of some parts of the brain (limbic system) have been investigated. The method consists of introducing stimulating electrodes into specific areas of the brain. Weak pulses of current produce an immediate increase in the firing of neurons near the tip of the electrode. The effects of passing electrical currents through these electrodes is measured directly from the observation or recording of some aspects of behavior.

Subjects: Adult male Wistar rats weighing in the range of 250-275 grams are used for the stereotaxic implantation of bipolar electrodes in substantia nigra-ventral tegmental area (SN-VTA) bilaterally. Apparatus: Stereotaxic table, surgical instruments, stimulator (programmable pulse generator), Skinner’s box modified for ICSS and connecting cables. Electrodes: Either monopolar or bipolar electrodes can be used for the purpose. Bipolar electrodes, spaced closely together (0.5-1.0 mm between tips), are widely used, since the spread of 121

the stimulation current can be precisely defined, the maximally stimulated tissue lying between the electrode tips. Although platinum is a superior metal for chronic brain stimulation due to its non-corrosive property, stainless steel or nichrome wire is commonly used since it is cheaper and readily available. The diameter of the wire used is 28G that should be insulated, except across the tip, with some inert material (Epoxylite resin). Surgical procedure: The rat is anesthetized with sodium pentabarbitone (40mg/kg b.w) and then positioned in the stereotaxic frame with rat adaptor. Following injection of lignocaine anesthesia into the scalp region, the skull surface was exposed for landmarks. Flatskull coordinates adapted from Paxinos and Watson rat atlas are marked with bregma as reference point and burr holes are drilled through the skull. The bipolar electrodes are implanted chronically into substantia nigraventral tegmental area (SN-VTA). The stereotaxic coordinates for SN-VTA are: (see Figure 2) Antero-posterior (AP) : -3.5 to – 4.5 mm Medio-lateral (ML) : 1.1 to 1.8 mm Dorso-ventral (DV) : 8.5 ± 0.2 mm

The indwelled electrodes should be firmly fixed with acrylic dental cement and one or two anchoring screws can be fixed for the firm fixation of the electrodes. Following 5-7 days of post surgical recovery from surgical trauma the rats can be tested for the ICSS behavioral response. Electrical stimulation: The electrodes of one side of the head should be connected to the output socket of the pulse generator (stimulator) and the rat placed in the testing chamber (the Skinner’s box modified for ICSS behavior). The Skinner’s box has a pedal (lever) on one of the walls of the chamber that is connected to the micro switch, which has connection with the pulse generator. Pressing the lever completes the circuit and delivers a pulse of current (Figure 1). Once a current level is found which serves as a reward, shaping can be used to quickly establish lever pressing; i.e., by reinforcing first movements, then locomotion towards the lever, then sniffing around the lever, then touching and finally pressing it. Soon the rat learns to predict the relationship of pedal pressing and the rewarding stimulation. The current strength and frequency are monitored by observing the rat’s response to the stimulus. Thus, the animal self-stimulates reliably on a continuous reinforcement schedule resulting in stable self-stimulation behavior.

OSCILLOSCOPE

MODIFIED SKINNER CHAMBER FOR ICSS TIMER (STOP WATCH)

Figure 1: Diagrammatic representation of the rat performing ICSS behavior (Shankaranarayana Rao 1996). 122

Histological Placement

verification

of

Electrode

After the experiments, the position of the electrode tips can be marked with a small lesion by passing current through the stimulating

electrodes. Perfuse the brain and section it and stain the sections with cresyl violet. Under a light microscope establish the location of the electrode tips. Reconstruct the position of the tips on plates taken from a brain atlas.

Figure 2. Plate from Paxinos and Watson (1982) stereotaxic atlas of the rat brain. The arrow indicates the placement of electrodes in the SN-VTA where electrical stimulation should serve as a reinforcer. The vertical mm scale on the right indicates depth from the surface of the brain.

However, the method of ICSS suffers several drawbacks: 1. The current would spread over a wider area of the brain than the targeted neuronal nucleus. 2. It does not discriminate between different fibers of passage crossing trough and nearby the targeted nuclei. Mesocorticolimbic system a major neural substrate for ICSS Research investigating the rewarding properties of intracranial self-stimulation (ICSS) has identified dopamine as the neurotransmitter involved in reward. Regions in the brain that support ICSS, known as pleasure centers, overlap with known dopamine systems in the brain. The dopamine system involved in reward is the mesotelencephalic dopamine system (referred to as the midbrain dopamine system). It is constituted of dopaminergic

neurons projecting from the mesencephalon (the midbrain) into various regions of the telencephalon (Figure 3). The neurons that compose this system have their cell bodies in the substantia nigra and the ventral tegmental area. These are two closely related nuclei composed of dopamine-containing neurons, which projects to a number of forebrain sites. These include regions of the limbic cortex, prefrontal neocortex, the lateral hypothalamus, the preoptic area, the olfactory tubercle, the amygdala, striatum and in particular the nucleus accumbens. It is dopamine activity in the nucleus accumbens that is involved with the experience of pleasure. In bypassing much of the input side of these neuronal circuit(s), ICSS provides a unique tool in neuropharmacological research to investigate the influence of various substances on reward and reinforcement processes. Intracranial selfstimulation differs significantly from drug selfadministration in that, in this procedure, the animal is working to directly stimulate presumed 123

reinforcement circuits in the brain and the effects of the drugs are assessed on these reward thresholds. Drugs of abuse decrease thresholds for ICSS, and there is a good correspondence between the ability of drugs to decrease ICSS thresholds and their abuse potential.

Figure 3. Schematic representation of mesocorticolimbic dopaminergic system.

ICSS, a Model to Study Neuronal Plasticity Recently, the electrical self-stimulation paradigm has proved to be very useful in delineating the neural substrate involved in learning and drug abuse. Self-stimulation involves operant learning, which can induce changes in the neuronal cytoarchitecture. Accordingly, we have used this experimental para-digm to study learning related neuronal plasticity. We have conducted a series of experiments to determine the self-stimulation (SS) rewarding experience induced neuronal plasticity in hippocampal and motor cortical neurons (Shankaranarayana Rao and Raju, 2001a, Shankaranarayana Rao et al 2001b). The electrical self-stimulation has been considered as one of the intensely rewarding behavioral experience, perhaps even more influential than feeding or sexual rewards (Olds 1962). Selfstimulation rewarding experience for 10 days resulted in an long-lasting increase in the dendritic branching and dendritic length in CA3 hippocampal and layer V motor cortical pyramidal neurons (Shankaranarayana Rao et al. 1993, 1994). The dendritic growth is associated with cytoskeletal changes in the hippocampal and

cortical neurons. In addition to the dendritic growth, a significant increase is observed in the thickness of lacunosum, radiatum and lucidum laminae in the CA3 region of the hippocampus in selfstimulation experienced rats (Shankaranarayana Rao et al. 1993). ICSS also resulted in a significant increase in numerical density of dendritic spines in different categories of both apical and basal dendrites in CA3 hippocampal and layer V motor cortical pyramidal neurons (Shankaranarayana Rao et al. 1999a) and also an increase in the density of thorny excrescences in apical dendrites of CA3 neurons of the hippocampus (Shankaranarayana Rao et al. 1998a). ICSS caused synaptogenesis in CA3 region, which included moleculare, radiatum and lucidum layers of the hippocampus and molecular layer of the motor cortex (Shankaranarayana Rao et al. 1999b). In addition, SS results in an increase in the levels of glutamate, noradrenaline, dopamine and enhancement of AChE activity in the hippocampus and the motor cortex (Shankaranarayana Rao et al 1998b). Furthermore, the prior SS experience is known to facilitate the acquisition of operant and spatial learning tasks in rats (Yoganarasimha et al 1998). Such facilitation might be due to an increase in the dendritic arborization associated with neurochemical changes in the hippocampus. In summary, the facilitatory effects of ICSS on operant and spatial learning led us to suggest that rewarding electrical stimulation could affect learning and memory by inducing an adequate activation of the neural system and the pathways that were able to affect a wide variety of conditioned responses. Further experiments are thus necessary to demonstrate a more specific effect of reinforcing brain stimulation on learning and memory process. ICSS, an Animal Model to study Drug Addiction From the basic neuroscience perspective, study of the neurobiology of drug addiction offers a novel opportunity to establish the biological basis of a complex and clinically relevant behavioral abnormality. Many prominent aspects of drug 124

addiction in people can be clearly produced in laboratory animals, in striking contrast to most of other forms of neuropsychiatric illness. Thus, advances made in the study of drug addiction should provide important insights into the mechanisms underlying some of the psychiatric disorders. In recent conceptualizations of drug reinforcement, the positive reinforcing properties of drugs have been thought to play an important role in drug dependence. It is amply clear that animals and humans will readily self-administer drugs in a dependent state and that drugs have powerful reinforcing properties in that animals will perform many different tasks to obtain drugs. The drugs that have positive reinforcing effects correspond well with the drugs that have high abuse potential in humans. Electrical selfstimulation of certain brain areas is rewarding for animals and humans as demonstrated by the fact that subjects will readily self-administer the stimulation (Olds & Milner 1954). The powerful nature of the reward effect produced by ICSS is indicated by the behavioral characteristics of the ICSS response, which include rapid learning and vigorous execution of the stimulation producing behavior (Shankaranarayana Rao and Raju, 2001). ICSS provides a unique tool in neuropharmacological research to investigate the influence of various substances on reward and reinforcement processes. Drugs of abuse decrease thresholds for ICSS, and there is a good correspondence between the ability of drugs to decrease ICSS thresholds and their abuse potential. ICSS thresholds have been used to assess changes in systems mediating reward and reinforcement processes during the course of drug dependence. Acute administration of psychostimulant drugs lower ICSS threshold (i.e., increases ICSS reward) and withdrawal from chronic administration of these drugs elevate ICSS thresholds (i.e., decrease ICSS reward). Similar results have been observed with precipitated withdrawal in opiate-dependent rats. The advantage of the ICSS paradigm as a model of drug effect on motivation and reward is that by directly stimulating the putative reward systems,

one presumably bypasses the input side of the system and eliminates the nonspecific effects of consummatory behaviors, such as feeding, that can complicate data interpretation. Also, the behavioral threshold measure provided by ICSS procedures is easily quantifiable, because ICSS threshold estimates are very stable over periods of several months. Another considerable advantage of the ICSS technique is the high reliability with which it predicts the abuse liability of drugs (Shankaranarayana Rao and Raju, 2001a). ICSS, an Animal Model for Depression ICSS paradigm provides an operational measure of anhedonia, a core feature of depression. Because the anhedonia experienced by depressed patients suggest that these individuals might exhibit alterations in reward processes, the ICSS paradigm has been proposed as a model of depression. Substantial evidence suggests that ICSS thresholds are reliable measures of reward that reflect the whole continuum from hedonia to anhedonia. Because it appears that ICSS acts directly on some of the same neuronal substrates that mediate the rewarding effects of natural reinforcers, it is considered to be a valuable tool for the investigation of brain-reward systems (Shankaranarayana Rao and Raju, 2001a). The study of neural substrates of ICSS behavior following experiential or pharmacological manipulations promises to promote our understanding of reward mechanisms that seem to be altered in several psychiatric disorders, including depression and schizophrenia. Two manipulations have been used to produce an anhedonic state in animals, as operationally defined by decreases in ICSS response rates or elevations of thresholds, (i) exposure to uncontrollable stress and (ii) withdrawal from long-term exposure to psychomotor stimulants. Tricyclics appear to be effective in reversing the effects of withdrawal from amphetamine on ICSS. Although many more studies are needed, the evidence to date suggests that ICSS model has good construct and etiological validity and exhibits pharmacological isomorphism as a model for the drug induced anhedonia. Nevertheless, the precise 125

relationship between drug-induced and nondrug-induced depression in humans is not known. Thus, the possible etiological validity of the ICSS paradigm as a model of non-druginduced depression remains unclear. Future clinical and preclinical research needs to address this issue further. References: 1. Olds, J. and Milner, P. (1954) Positive reinforcement produced by electrical stimulation of septal area and other regions of the rat brain. J. Comp. Physiol. Psychol., 47:419-429. 2. Paxinos G and Watson C (1982) The rat brain in stereotaxic coordinates. Academic Press. Australia. 3. Shankaranarayana Rao BS (1996) Neuronal plasticity induced by self-stimulation rewarding behavioural experience: Morphological, biochemical and immunocytochemical evaluation. Ph.D. Thesis, NIMHANS Deemed University, Bangalore. India. 4. Shankaranarayana Rao BS and Raju TR (2001a) Intracranial self-stimulation : An animal model to study drug addiction, depression and neuronal plasticity – A Review. Proc Indian Natnl Sci Acad B67: 155-188. 5. Shankaranarayana Rao BS, Desiraju T and Raju TR (1993) Neuronal plasticity induced by selfstimulation rewarding experience in rats - a study on alterations in dendritic branching in pyramidal neurons of hippocampus and motor cortex. Brain Res 627:216-224. 6. Shankaranarayana Rao BS, Desiraju T, Meti BL and Raju TR (1994) Plasticity of hippocampal and motor cortical pyramidal neurons induced by selfstimulation experience. Indian J Physiol Pharmacol 38:23-28.

7. Shankaranarayana Rao BS, Meti BL and Raju TR (2001b) Neuronal plasticity - A unique property for Neurorehabilitation. In : Neurorehabilitation : Principles and Practice, (Taly AB, Nair KPS and Murali T eds) 2nd edition, Ahuja Book Company, New Delhi, India, pp.27-37. 8. Shankaranarayana Rao BS, Raju TR and Meti BL (1998a) Alterations in the density of excrescences in CA3 neurons of hippocampus in rats subjected to selfstimulation experience. Brain Res 804:320-324. 9. Shankaranarayana Rao BS, Raju TR and Meti BL (1998b) Self-stimulation of lateral hypothalamus and ventral tegmentum increases the levels of noradrenaline, dopamine, glutamate and AChE activity, but not 5-hydroxytryptamine and GABA levels in hippocampus and motor cortex. Neurochem Res 23:1053-1059. 10. Shankaranarayana Rao BS, Raju TR and Meti BL (1998e) Long-lasting structural changes in CA3 hippocampal and layer V motor cortical pyramidal neurons associated with self-stimulation - a quantitative Golgi study. Brain Res Bull 47:95-101. 11. Shankaranarayana Rao BS, Raju TR and Meti BL (1999a) Self-stimulation rewarding experience induced alterations in dendritic spine density in CA3 hippocampal and layer V motor cortical pyramidal neurons. Neuroscience 89:1067-1077. 12. Shankaranarayana Rao BS, Raju TR and Meti BL (1999b) Increased numerical density of synapses in CA3 region of hippocampus and molecular layer of motor cortex after self-stimulation rewarding experience. Neuroscience 91:799-803. 13. Yoganarasimha D, Shankaranarayana Rao BS, Raju TR and Meti BL (1998) Facilitation of acquisition and performance of operant and spatial learning tasks in self-stimulation experienced rats. Behav Neurosci 112:725-729.

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