A dual moveable stimulating electrode and its application to the behavioral version of the collision test

Mapping the Neural Substrate Underlying Brain Stimulation Reward with the Behavioral Adaptation of Double-Pulse Methods

Behavioral adaptations of double-pulse methods--primarily collision and refractory period tests--have been employed to unveil the electrophysiological and anatomical characteristics of neural networks of known function. These paradigms are based on trade-off functions: a determination of different combinations of stimuli that yield the same behavioral output. A detailed explanation of the logic and methodology underlying these techniques is elaborated in this paper. The implementation of such approaches to the study of brain stimulation reward (BSR) has provided a means of discriminating between the neurons underlying this behavior from other cells activated by the stimulating electrode, endowing them with a particularly powerful scientific scope. An increasingly detailed portrait of the BSR substrate, both within and outside the medial forebrain bundle, has been emerging as a result of these investigations and is reviewed in this paper. Finally, the challenges associated with these paradigms are discussed and potential solutions as well as future experimental ventures proposed. Attention is drawn to the major contribution of these methods to our understanding of the neural pathways and characteristics underlying BSR.

The substrate for brain-stimulation reward in the lateral preoptic area. II. Connections to the ventral tegmental area

This experiment investigated the existence of a direct anatomical connection between lateral preoptic and ventral tegmental areas that mediate brain stimulation reward using the behavioral adaptation of the collision test. This test is a double-pulse, two-electrode technique based on the axonal conduction failure that occurs when two separate sites in the same axon bundle are concurrently stimulated. This anatomical arrangement is inferred from the shape of the function relating the effectiveness of double-pulse stimulation to the interval between pulses. In this study, nine rats with a total of 44 pairs of sites were examined. In two pairs only was there a profile suggestive of an axonal collision effect, while the double-pulse effectiveness curve consistent with the properties of transynaptic collision was apparent for a single pair of sites; the remaining 93% were associated with relatively flat effectiveness curves. While electrode misalignment could be responsible for these results, there was adequate sampling to suggest that the preponderance of first stage signals that give rise to the rewarding effects mediated by the lateral preoptic and ventral tegmental areas do not travel along the same fiber bundle. However, stimulation applied to both sites concurrently produces a summation that is roughly 40% greater than stimulation at either site alone, suggesting reasonable integration of the reward signals generated by lateral preoptic and ventral tegmental area stimulation.

Medial forebrain bundle lesions fail to structurally and functionally disconnect the ventral tegmental area from many ipsilateral forebrain nuclei: implications for the neural substrate of brain stimulation reward

Lesions in the medial forebrain bundle rostral to a stimulating electrode have variable effects on the rewarding efficacy of self-stimulation. We attempted to account for this variability by measuring the anatomical and functional effects of electrolytic lesions at the level of the lateral hypothalamus (LH) and by correlating these effects to postlesion changes in threshold pulse frequency (pps) for self-stimulation in the ventral tegmental area (VTA). We implanted True Blue in the VTA and compared cell labeling patterns in forebrain regions of intact and lesioned animals. We also compared stimulation-induced regional [14C]deoxyglucose (DG) accumulation patterns in the forebrains of intact and lesioned animals. As expected, postlesion threshold shifts varied: threshold pps remained the same or decreased in eight animals, increased by small but significant amounts in three rats, and increased substantially in six subjects. Unexpectedly, LH lesions did not anatomically or functionally disconnect all forebrain nuclei from the VTA. Most septal and preoptic regions contained equivalent levels of True Blue label in intact and lesioned animals. In both intact and lesioned groups, VTA stimulation increased metabolic activity in the fundus of the striatum (FS), the nucleus of the diagonal band, and the medial preoptic area. On the other hand, True Blue labeling demonstrated anatomical disconnection of the accumbens, FS, substantia innominata/magnocellular preoptic nucleus (SI/MA), and bed nucleus of the stria terminalis. [14C]DG autoradiography indicated functional disconnection of the lateral preoptic area and SI/MA. Correlations between patterns of True Blue labeling or [14C]deoxyglucose accumulation and postlesion shifts in threshold pulse frequency were weak and generally negative. These direct measures of connectivity concord with the behavioral measures in suggesting a diffuse net-like connection between forebrain nuclei and the VTA.

Mesencephalic substrate of reward: axonal connections

The behavioral version of the collision technique was used to study the existence of axonal linkage between reward-relevant sites in the ventral tegmental area (VTA) and posterior mesencephalon (PM) in six rats trained to self-administer trains of electrical brain stimulation. The combined use of fixed and moveable stimulation electrodes allowed us to carry out collision tests at a total of 46 different combinations of VTA-PM sites, and collision-like effects were observed at 24 of these. Stimulation of the VTA and the most caudal PM sites generally resulted in collision curves that were characterized by a single increase in paired-pulse effectiveness (E-values), whereas recovery in those collision curves obtained from stimulation of the VTA and more rostral PM sites was generally slower, and often characterized by a double rise. Despite little variability in interelectrode distances (1.0-3.8 mm), collision intervals varied widely, ranging from 1.5 to 7.3 msec. Recovery from refractoriness (initial 25%) was also estimated and ranged from 0.7 to 1.0 msec, resulting in conduction-time estimates of 0.7-6.3 msec. The lack of correspondence between interelectrode distances and conduction times suggests the presence of axonal branching. Results of this study constitute the first behavioral evidence of a reward-relevant axonal link between the VTA and the PM. In addition, the finding that in one animal the anterior electrode was located within the posterior portion of the lateral hypothalamus (LH) suggests that the reward-relevant axonal bundle linking the LH and VTA may extend as far back as the caudal regions of the PM.

Electrically evoked behaviors: axons and synapses mapped with collision tests

The properties of many axon bundles mediating electrically evoked responses have been described using double-pulse methods in behaving animals. The directly stimulated axons whose activation leads to the behavior are defined by their refractory periods, conduction velocities and trajectories in those studies. In this review, new collision effects (asymmetric collision) are described that locate synapses mediating turning and startle responses. The direction, time and reliability of transmission are determined by the direction, time and strength of the asymmetry. In systems mediating turning, five axon bundles have been localized with symmetric collision effects and evidence for two synapses has been provided with asymmetric collision effects. In systems mediating startle responses, three fast, reliable synapses have been located with asymmetric collision. Asymmetric collision effects between a loud acoustic stimulus and a single electrical pulse in hindbrain sites define the timing and location of the acoustic volley that produces acoustic startle. Therefore, when synapses are strong, circuit diagrams can be constructed by use of collision tests in behaving animals.

An uncrossed tectopontine pathway mediates ipsiversive circling

Stimulation of the superior colliculus (SC) of rodents, following knife cuts to the predorsal bundle decussation, evokes ipsiversive circling along with "cringing" or avoidance responses. A major uncut SC output is the uncrossed tectopontine pathway that projects heavily to the ventrolateral pons (VLP). Stimulation of this pathway in the VLP also evokes ipsiversive circling, but the circling is smoother, lacks the avoidance components, and begins with a shorter latency than SC circling. To determine whether continuous tectopontine axons mediate ipsiversive circling in both sites, the collision method of Shizgal et al. was used. Pairs of stimulating pulses were presented to the two sites, conditioning (C) pulses to one site and testing (T) pulses to the other site. Collision was evidenced when the frequencies required to evoke circling were higher at short conditioning-testing (C-T) intervals than at long C-T intervals. Between SC and VLP, collision varied from 25 to 64%. Refractory periods ranged from 0.4 to 1.0 ms in most VLP sites, and from 0.45 to roughly 3 ms in SC sites. Conduction velocities ranged from 1.2 to 19 m/s, but most were concentrated in two ranges, 1.2 to 2.7 m/s and 10 to 19 m/s. The contribution of the slower population was higher in electrode pairs where the percent collision was higher. Therefore, continuous axons from colliculus to ventrolateral pons mediate most of the ipsiversive circling produced by collicular stimulation. Slight asymmetries in the collision were observed between 3 pairs with high threshold colliculus electrodes, suggesting transsynaptic collisions across colliculus synapses transmitting from dorsal to ventral.

Electrically evoked turning: asymmetric and symmetric collision between anteromedial cortex and striatum

Electrical stimulation of the anteromedial cortex (AMC) or striatum of rats evoked contraversive eye, head and body movements. In these experiments we test which neurons and which pathways are responsible for the turning by delivering conditioning (C) pulses to one site and test (T) pulses to the second site, and measuring the frequency of pulse pairs required to evoke a full turn in 10 s. Decreases in the required frequency were usually found at C-T intervals from 0.6 to 1.0 ms, whether the C pulses were delivered to the AMC or to the striatum. This symmetric effect is attributed to collision in fast-conducting axons connecting cortex and striatum. Symmetric collision at C-T intervals of 2-4 ms was observed between cortex and 3 dorsal striatal sites, suggesting slower axons from cortex to these dorsal striatal sites. In several animals, asymmetric changes in required frequency also occurred. When the C pulses were presented via the striatal electrode, the recovery in required frequency occurred at C-T intervals of 1-4 ms, but when the C pulses were presented via the cortical electrode, recovery occurred at C-T intervals of 2-50 ms. This asymmetry is attributed to indirect (i.e., transynaptic) activation of corticostriatal or striatal output axons. These results suggest that in both cortex and striatum there are synapses, transmitting from rostral to caudal, which are important for electrically evoked turning. When C and T pulses were delivered to the same site, decreases in required frequency occurred at C-T intervals from 0.4 to 4 ms, attributable to recovery from refractoriness. In 3 striatal sites, however, large changes were also seen at C-T intervals from 6 to 50 ms. In all 3 sites, asymmetric collision occurred at these same intervals. The recovery at long C-T intervals could be due to transynaptic collision also, resulting from the simultaneous activation of presynaptic and postsynaptic axons by a single striatal electrode.

Re-evaluation of the role of dopamine in intracranial self-stimulation using in vivo microdialysis

Rats were implanted with an electrode-microdialysis assembly in order to test the hypothesis that the reward signal elicited by medial forebrain bundle stimulation is relayed by the meso-accumbens dopamine cells. We first obtained the strength-duration function of self-stimulation, that is, a family of behaviorally equivalent stimuli (pulse intensity and pulse duration pairs yielding a constant self-stimulation rate). We then collected the self-stimulation-bound intra-accumbens dopamine for several pairs of intensity and duration, selected from within the strength-duration function. Our reasoning was that if the reward signal travels along the meso-accumbens dopaminergic neurons, the release of dopamine should not depend on the stimulus parameters because behaviorally equivalent stimuli should produce a constant output in all neural stages carrying the reward signal. The results showed that short duration/high intensity pulses induced considerably larger increases in dopamine levels than long duration/low intensity pulses, despite the fact that these stimuli maintained a constant self-stimulation rate. Among the interpretations envisaged, the most parsimonious one seems to be that the MFB rewarding signal is not relayed exclusively by meso-accumbens dopaminergic cells and that the latter may play a permissive-facilitator role at some transmission stage of the reward signal.

Turning responses evoked by stimulation of visuomotor pathways

Lateral eye, head, and body movements are produced by electrical stimulation of many brain regions from frontal cortex to pons. A new collision method shows that at least 5 separate axon bundles mediate stimulation-elicited lateral head and body movements in rats. One bundle passes between the rostromedial tegmentum and medial pons, with conduction velocities of 0.8-18 m/s. A second bundle passes between the superior colliculus and contralateral medial pons, with conduction velocities of 1.7-13 m/s. A third bundle passes between the superior colliculus and ventrolateral pons, with conduction velocities of 1.3-20 m/s. A fourth bundle passes between the internal capsule and medial substantia nigra, with conduction velocities of 0.9-4.4 m/s. A fifth bundle passes between the anteromedial cortex and rostral striatum, with conduction velocities of 2.4-36 m/s. Collision effects have not been observed between the anteromedial cortex and the internal capsule, medial substantia nigra, superior colliculus, rostromedial tegmentum, or medial pons, which suggests that these sites are not connected by axons mediating turning. Possible synaptic linkages between the 5 bundles and possible transmitters are discussed.

Two converging brainstem pathways mediating circling behavior

Ipsiversive circling results from stimulation of the rostromedial tegmentum (RMT) or medial pons (PONS), and contraversive circling results from stimulation of the superior colliculus (SC). To determine whether these sites are functionally connected, the collision method of Shizgal, Bielajew, Corbett, Skelton and Yeomans (1980) was used in rats. Pairs of stimulation pulses were presented to two sites, and the degree of collision between stimulation-evoked action potentials was assessed by measuring the frequency required for circling at short and long intrapair conditioning-testing (C-T) intervals. Collision was evidenced when the required frequencies were higher at short C-T intervals than at long C-T intervals. Collision of 46-62% was observed between RMT and PONS, and collision of 15-29% was observed between SC and PONS. Sites from which collision was obtained were located along the trajectories of the medial tegmental tract and the crossed tectospinal pathway. Refractory periods in all sites were similar, ranging from 0.3 to 1.7 ms. Conduction velocities of axons connecting RMT and PONS and SC and PONS were comparable, ranging from 0.8 to 13.3 m/s and 1.7 to 13.8 m/s, respectively, with lower conduction velocities associated with more ventral pontine sites. Thus, RMT and PONS, and SC and PONS are connected by myelinated axons that mediate circling.

Current-distance relations of axons mediating circling elicited by midbrain stimulation

Stimulation of mediocaudal midbrain in rats produces ipsiversive circling due to the stimulation of longitudinal axons. The refractory periods of these axons were measured by delivering trains of conditioning and testing pulses via a single electrode at various conditioning-testing (C-T) intervals. As C-T interval increased from 0.3 to 2.0 ms, the frequency required to produce a constant amount of circling halved. The current-distance relations of these axons were measured by placing two electrodes lateral to one another, and delivering conditioning pulses via one electrode and testing pulses via the second electrode. The required frequency decreased less at C-T intervals in the refractory period range using two electrodes rather than using a single electrode. This partial refractoriness suggests that only part of the axons were stimulated by both electrodes. The refractoriness increased as current increased or as interelectrode distance decreased. The overlap in the fields of stimulation at each current was calculated from the refractoriness observed in single and double electrode experiments. The results suggest that the axons mediating circling have a wide range of thresholds rather than a single threshold. The current required to activate an axon is roughly equal to K X r2, were K is a constant and r is the radial distance from electrode to axon. K must range from 400 to at least 3000 microA/mm2, to account for the circling data. For axons mediating medial forebrain bundle self-stimulation3, K must range from 1000 to at least 6400 microA/mm2. Estimation of the K distribution allows calculation of the effects of electrode size, placement and current on the recruitment of axons with different thresholds.

Longitudinal brainstem axons mediating circling: behavioral measurement of conduction velocity distributions

Current applied near the midline brainstem elicits rapid ipsiversive circling. Pontine and midbrain sites were stimulated concurrently with paired pulses and the number of pulse pairs required to produce 3 complete circles in 10 s was measured at various intrapair (C-T) intervals. The same results were obtained when C pulses were presented via the midbrain electrode and T pulses via the pontine electrode, or vice versa: as C-T interval increased from 0.4 to 2.0 ms, the number of pulse pairs required decreased gradually. These decreases occurred at longer C-T intervals than the refractory period decreases observed in single-electrode tests. These results imply that collision occurred in the directly stimulated axons and that a longitudinal bundle of uninterrupted axons mediates the circling behavior. The sites from which collision was obtained overlap with both medial longitudinal fasciculus and crossed tectobulbar and tectospinal tracts. The axons mediating circling appear to have conduction velocities from 2 to roughly 20 m/s. By comparison, superior colliculus units antidromically driven from contralateral electrode sites that produce circling had conduction velocities from 0.7 to 40 m/s.

Current-distance relation for rewarding brain stimulation

A novel approach to estimating the current density required to directly activate axons involved in the reinforcing effect of brain stimulation is described. Self-stimulating rats received trains of cathodal pulse pairs via two adjacent, stimulating electrodes which were positioned in the lateral hypothalamus and oriented to lie in a plane transverse to the medial forebrain bundle. The first pulse of each pair was delivered through one electrode and, after varied delays, the second was applied to the other electrode in an attempt to detect a loss of stimulation effectiveness attributable to refractoriness of axons that penetrated the intersection of the two stimulation fields. In low-current tests, no change in the psychophysically scaled effectiveness was observed as the interval separating the pulses was varied but, at higher currents, the stimulation effectiveness rose when the delay between the pulses surpassed the refractory periods of these cells. Our inference was that greater currents produced progressively overlapping stimulation fields. Moreover, evidence of overlap was seen at lower currents in rats that had been prepared with smaller separations between the electrodes. Our estimate of the threshold current density for the most sensitive of the reward units is 1300 microA/mm2 when 0.1 ms pulses are used.

The pontine substrate of circling behavior

Single or twin movable stimulating electrodes were implanted in 9 rats in order to investigate the pontine substrate of circling behavior. The region located between the caudal part of the interpeduncular nucleus and the fourth ventricle was examined. The electrodes were implanted 6 mm below the surface of the skull and subsequently moved down by steps of 0.13 or 0.16 mm. The stimulating current consisted of trains of cathodal rectangular pulses of constant intensity and width (100 microA and 0.1 ms respectively) and of variable frequency. The effectiveness of the stimulation in eliciting a circling reaction was inferred from a psychophysical determination of the pulse period required at each site in order for the animal to maintain a criterion rotation speed. In the average, 48 brain sites were investigated per animal. Stimulation of 166 out of a total of 387 sites elicited ipsiversive rotation. Depending on the coronal plane of implantation, the dorsal boundary of the circling substrate was located within the pedunculus cerebellaris superior or the floor of the substantia grisea centralis. In addition, the positive region extended 1.7-2 mm ventrally and 1.9 mm from the midline. The distribution of the positive sites seems to suggest that the circling substrate is a large bundle which runs sagittally through the medial part of the reticular formation.