Electrophysiological examination of the ventral tegmental (A10) area in the rat

Ventral tegmental area interneurons revisited: GABA and glutamate projection neurons make local synapses

The ventral tegmental area (VTA) contains projection neurons that release the neurotransmitters dopamine, GABA, and/or glutamate from distal synapses. VTA also contains GABA neurons that synapse locally on to VTA dopamine neurons, synapses widely credited to a population of so-called VTA interneurons. Interneurons in cortex, striatum, and elsewhere have well-defined morphological features, physiological properties, and molecular markers, but such features have not been clearly described in VTA. Indeed, there is scant evidence that local and distal synapses originate from separate populations of VTA GABA neurons. In this study we tested whether several markers expressed in non-dopamine VTA neurons are selective markers of interneurons, defined as neurons that synapse locally but not distally. Challenging previous assumptions, we found that VTA neurons genetically defined by expression of parvalbumin, somatostatin, neurotensin, or mu-opioid receptor project to known VTA targets including nucleus accumbens, ventral pallidum, lateral habenula, and prefrontal cortex. Moreover, we provide evidence that VTA GABA and glutamate projection neurons make functional inhibitory or excitatory synapses locally within VTA. These findings suggest that local collaterals of VTA projection neurons could mediate functions prior attributed to VTA interneurons. This study underscores the need for a refined understanding of VTA connectivity to explain how heterogeneous VTA circuits mediate diverse functions related to reward, motivation, or addiction.

Dopamine and Beyond: Implications of Psychophysical Studies of Intracranial Self-Stimulation for the Treatment of Depression

Major depressive disorder is a leading cause of disability and suicide worldwide. Consecutive rounds of conventional interventions are ineffective in a significant sub-group of patients whose disorder is classified as treatment-resistant depression. Significant progress in managing this severe form of depression has been achieved through the use of deep brain stimulation of the medial forebrain bundle (MFB). The beneficial effect of such stimulation appears strong, safe, and enduring. The proposed neural substrate for this promising clinical finding includes midbrain dopamine neurons and a subset of their cortical afferents. Here, we aim to broaden the discussion of the candidate circuitry by exploring potential implications of a new “convergence” model of brain reward circuitry in rodents. We chart the evolution of the new model from its predecessors, which held that midbrain dopamine neurons constituted an obligatory stage of the final common path for reward seeking. In contrast, the new model includes a directly activated, non-dopaminergic pathway whose output ultimately converges with that of the dopaminergic neurons. On the basis of the new model and the relative ineffectiveness of dopamine agonists in the treatment of depression, we ask whether non-dopaminergic circuitry may contribute to the clinical efficacy of deep brain stimulation of the MFB.

The Convergence Model of Brain Reward Circuitry: Implications for Relief of Treatment-Resistant Depression by Deep-Brain Stimulation of the Medial Forebrain Bundle

Deep-brain stimulation of the medial forebrain bundle (MFB) can provide effective, enduring relief of treatment-resistant depression. Panksepp provided an explanatory framework: the MFB constitutes the core of the neural circuitry subserving the anticipation and pursuit of rewards: the “SEEKING” system. On that view, the SEEKING system is hypoactive in depressed individuals; background electrical stimulation of the MFB alleviates symptoms by normalizing activity. Panksepp attributed intracranial self-stimulation to excitation of the SEEKING system in which the ascending projections of midbrain dopamine neurons are an essential component. In parallel with Panksepp’s qualitative work, intracranial self-stimulation has long been studied quantitatively by psychophysical means. That work argues that the predominant directly stimulated substrate for MFB self-stimulation are myelinated, non-dopaminergic fibers, more readily excited by brief electrical current pulses than the thin, unmyelinated axons of the midbrain dopamine neurons. The series-circuit hypothesis reconciles this view with the evidence implicating dopamine in MFB self-stimulation as follows: direct activation of myelinated MFB fibers is rewarding due to their trans-synaptic activation of midbrain dopamine neurons. A recent study in which rats worked for optogenetic stimulation of midbrain dopamine neurons challenges the series-circuit hypothesis and provides a new model of intracranial self-stimulation in which the myelinated non-dopaminergic neurons and the midbrain dopamine projections access the behavioral final common path for reward seeking via separate, converging routes. We explore the potential implications of this convergence model for the interpretation of the antidepressant effect of MFB stimulation. We also discuss the consistent finding that psychomotor stimulants, which boost dopaminergic neurotransmission, fail to provide a monotherapy for depression. We propose that non-dopaminergic MFB components may contribute to the therapeutic effect in parallel to, in synergy with, or even instead of, a dopaminergic component.

Psychophysical inference of frequency-following fidelity in the neural substrate for brain stimulation reward

The rewarding effect of electrical brain stimulation has been studied extensively for 60 years, yet the identity of the underlying neural circuitry remains unknown. Previous experiments have characterized the directly stimulated ("first-stage") neurons implicated in self-stimulation of the medial forebrain bundle. Their properties are consistent with those of fine, myelinated axons, at least some of which project rostro-caudally. These properties do not match those of dopaminergic neurons. The present psychophysical experiment estimates an additional first-stage characteristic: maximum firing frequency. We test a frequency-following model that maps the experimenter-set pulse frequency into the frequency of firing induced in the directly stimulated neurons. As pulse frequency is increased, firing frequency initially increases at the same rate, then becomes probabilistic, and finally levels off. The frequency-following function is based on the counter model which holds that the rewarding effect of a pulse train is determined by the aggregate spike rate triggered in first-stage neurons during a given interval. In 7 self-stimulating rats, we measured current- vs. pulse-frequency trade-off functions. The trade-off data were well described by the frequency-following model, and its upper asymptote was approached at a median value of 360 Hz (IQR = 46 Hz). This value implies a highly excitable, non-dopaminergic population of first-stage neurons. Incorporating the frequency-following function and parameters in Shizgal's 3-dimensional reward-mountain model improves its accuracy and predictive power.

Brain stimulation reward is integrated by a network of electrically coupled GABA neurons

The neural substrate of brain stimulation reward (BSR) has eluded identification since its discovery more than a half-century ago. Notwithstanding the difficulties in identifying the neuronal integrator of BSR, the mesocorticolimbic dopamine (DA) system originating in the ventral tegmental area (VTA) of the midbrain has been implicated. We have previously demonstrated that the firing rate of a subpopulation of gamma-aminobutyric acid (GABA) neurons in the VTA increases in anticipation of BSR. We show here that GABA neurons in the VTA, midbrain, hypothalamus, and thalamus of rats express connexin-36 (Cx36) gap junctions (GJs) and couple electrically upon DA application or by stimulation of the internal capsule (IC), which also supports self-stimulation. The threshold for responding for IC self-stimulation was the threshold for electrical coupling between GABA neurons, the degree of responding for IC self-stimulation was proportional to the magnitude of electrical coupling between GABA neurons, and GJ blockers increased the threshold for IC self-stimulation without affecting performance. Thus, a network of electrically coupled GABA neurons in the ventral brain may form the elusive neural integrator of BSR.

No effect of morphine on ventral tegmental dopamine neurons during withdrawal

Substantial evidence indicates that the ventral tegmental area (VTA) of the mesocorticolimbic dopaminergic (DA) system has a key role in mechanisms of opiate dependence. Although DA neurons have been studied extensively, little is known about their activity and their response to acute morphine during morphine dependence. We recorded the activity of VTA DA neurons in five groups of anesthetized rats: drug-naive (naive) rats, morphine-dependent [(MD) implanted with pellets] rats, and three groups of withdrawn rats. Withdrawals either were precipitated by naltrexone or occurred spontaneously 24 h or 15 d after pellet removal. We confirmed that acute morphine in naive rats produced a marked increase in the firing of VTA DA neurons. We also found that the basal firing rate of VTA DA neurons was markedly higher in MD than in naive rats; however, in MD rats, acute morphine failed to increase DA activity. We confirmed inhibition of VTA DA activity in MD rats in response to precipitated withdrawal; however, this inhibition resulted only in a normalization of the firing rate to that of naive animals. In rats that had spontaneous withdrawal after 24 h or 15 d, the activity of VTA DA neurons was similar to that of naive rats, and an acute injection of morphine failed to alter their activity. Our results indicate that VTA DA neurons show long-lasting tolerance to the acute effect of morphine after withdrawal. These findings show that VTA DA neural activity is unlikely to be a factor in the altered behavioral responses that occur with acute morphine or naltrexone administration after chronic opiate exposure.

Tracing the Neuroanatomical Profiles of Reward Pathways with Markers of Neuronal Activation

Functional neuroanatomical tools have played an important role in proposing which structures underlie brain stimulation reward circuitry. This review focuses on studies employing metabolic markers of neuronal and glial activation, including 2-deoxyglucose, cytochrome oxidase, and glycogen phosphorylase, and a marker of cellular activation, the immediate early gene c-fos. The principles underlying each method, their application to the study of brain stimulation reward, and their strengths and limitations are described. The usefulness of this strategy in identifying candidate structures, and the degree of overlap in the patterns of activation arising from different markers is addressed in detail. How these data have contributed to an understanding of the organization of reward circuitry and directed our thinking towards an alternative framework of neuronal arrangement is discussed in the final section.

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.

Responses of ventral tegmental area GABA neurons to brain stimulation reward

Dopamine neurons in the ventral tegmental area (VTA) have been implicated in rewarded behaviors, including intracranial self-stimulation (ICSS). We demonstrate, in unrestrained rats, that the discharge activity of a homogeneous population of presumed VTA GABA neurons, implicated in cortical arousal, increases before ICSS of the medial forebrain bundle (MFB). These findings suggest that VTA GABA neurons may be involved in the attentive processes related to brain stimulation reward (BSR).

Heterogeneity of ventral tegmental area neurons: single-unit recording and iontophoresis in awake, unrestrained rats

Single-unit recording combined with iontophoresis of dopamine, GABA, and glutamate was used in awake, unrestrained rats to characterize the electrophysiological and receptor properties of neurons in the ventral tegmental area under naturally occurring behavioural conditions. All isolated ventral tegmental area units (n=90) were analysed and compared with cells (n=58) recorded from dorsally adjacent areas of the pre-rubral area and red nucleus. Two distinct neuronal groups were identified in the ventral tegmental area: units with triphasic, long-duration spikes (78/90) and units with biphasic, short-duration spikes (12/90). Although all long-spike units discharged in an irregular, bursting pattern with varying degrees of within-burst decrements in spike amplitude, they could be further subdivided into at least three distinct subgroups. Type I long-spike units (36/78) discharged at a relatively slow and stable rate (mean: 6.03 imp/s; range: 0.42-15.78) with no evident fluctuations during movement. These cells were inhibited by dopamine and GABA and responded to glutamate with a low-magnitude excitation accompanied by a pronounced decrement in spike amplitude and a powerful rebound inhibition. Type II long-spike units (23/78) had relatively high and unstable discharge rates (mean: 22.82 imp/s; range: 4.42-59.67) and showed movement-related phasic activations frequently followed by partial or complete cessation of firing. Some Type II cells (4/9) were inhibited by dopamine, but all were excited by glutamate at very low currents (0-10 nA). With an increase in current, the glutamate-induced excitation often (18/22) progressed into a cessation of firing. All these cells were inhibited by GABA followed by a strong rebound excitation (8/9), which also frequently (6/8) resulted in cessation of firing. Type III long-spike units (19/78) had properties that differed from either Type I or Type II cells, including a lack of spontaneous firing (5/19). Short-spike ventral tegmental area units were either silent (4/12) and unresponsive to dopamine and GABA or spontaneously active (range: 0.89-34.13 imp/s) and inhibited by GABA and, in some cases (2/8). by dopamine; all were phasically activated during movement and glutamate iontophoresis. It appears that ventral tegmental area neurons, including those with long-duration spikes, do not comprise a uniform population in awake, unrestrained rats. Type I, long-spike units match the characteristics of histochemically-identified dopamine neurons, and they appear to express dopamine autoreceptors, which may explain the relatively slow, stable rate of activity and the limited responsiveness to excitatory inputs. Although the nature of the other long-spike units in our sample is unclear, they may include dopamine neurons without autoreceptors as well as non-dopamine cells. The heterogeneity of ventral tegmental area neurons is an important consideration for further attempts to assess the role of the mesocorticolimbic dopamine system in motivated behaviour.

Basal ganglia organization in amphibians: catecholaminergic innervation of the striatum and the nucleus accumbens

The aim of the present study was to determine the origin of the catecholaminergic inputs to the telencephalic basal ganglia of amphibians. For that purpose, retrograde tracing techniques were combined with tyrosine hydroxylase immunohistochemistry in the anurans Xenopus laevis and Rana perezi and the urodele Pleurodeles waltl. In all three species studied, a topographically organized dopaminergic projection was identified arising from the posterior tubercle/mesencephalic tegmentum and terminating in the striatum and the nucleus accumbens. Although essentially similar, the organization of the mesolimbic and mesostriatal connections in anurans seems to be more elaborate than in urodeles. The present study has also revealed the existence of a noradrenergic projection to the basal forebrain, which has its origin in the locus coeruleus. Additional catecholaminergic afferents to the striatum and the nucleus accumbens arise from the nucleus of the solitary tract, where catecholaminergic neurons appear to give rise to the bulk of the projections to the basal forebrain. In other regions, such as the olfactory bulb, the anterior preoptic area, the suprachiasmatic nucleus, and the thalamus, retrogradely labeled neurons (after basal forebrain tracer-applications) and catecholaminergic cells were intermingled, but none of these centers contained double-labeled cell bodies. It is concluded that the origin of the catecholaminergic innervation of the striatum and the nucleus accumbens in amphibians is largely comparable to that in amniotes. The present study, therefore, strongly supports the existence of a common pattern in the organization of the catecholaminergic inputs to the basal forebrain among tetrapod vertebrates.

Bicuculline microinjections into the ventral tegmental area of the rat: alteration of self-stimulation thresholds and of cytochrome oxidase activity in the brain

Abuse of drugs that potentiate GABAergic neurotransmission, namely benzodiazepines, is difficult to understand because this potentiation should elicit, among other effects, a decrease in activity within the mesolimbic system. Abuse of benzodiazepines is difficult to understand since the opposite, namely an increase in mesolimbic activity, has been implicated in drug abuse as well as in the rewarding effect of direct mesolimbic stimulation. In order to evaluate how the activity of the mesolimbic system depends on mesolimbic GABAergic influence, a GABAA receptor antagonist, bicuculline methiodide, was unilaterally injected into the ventral tegmental area and its effect on self-stimulation thresholds derived from stimulations applied to the same area was evaluated. Microinjection of 15, 20 and 30 ng increased the stimulation threshold. This decrease in stimulation efficiency lasted no more than 15 min after which baseline levels were obtained. Such a decrease is paradoxical considering that the manipulation should have released the ventral tegmentum from a tonic inhibitory influence. The metabolic consequences of repeated injections of 30 ng bicuculline were furthermore evaluated by cytochrome oxidase histochemistry. The staining was found to be weak around the injection site and dense in the ipsilateral nucleus accumbens. Release of a tonic GABAergic inhibition added to some cytotoxic damage probably resulted in an increased metabolic activity of this system. The presently reported paradoxical response of the ventral tegmentum and mesolimbic system to a GABAergic challenge may account for the paradoxical relationship between some behavioral properties of the mesolimbic system and GABAergic drugs.

Mesolimbic dopaminergic reduction outlasts ethanol withdrawal syndrome: evidence of protracted abstinence

Rats chronically administered with ethanol every six hours for six consecutive days show, upon suspension of treatment, a marked somatic withdrawal syndrome characterized by classical neurological signs. The emergence of the behavioral syndrome coincides with a profound decline of dopaminergic mesolimbic neuronal activity which corresponds to a reduction of dopamine outflow in the nucleus accumbens [Diana et al. (1993) Proc. natn. Acad. Sci. U.S.A. 90, 7966-7969]. However, while the behavioral manifestation of the ethanol withdrawal syndrome recedes in about 48 h, electrophysiological indices of mesolimbic dopaminergic function are still reduced 72 h after ethanol discontinuation, thus outlasting the physical signs of ethanol withdrawal syndrome. Dopaminergic neuronal activity is reintegrated by anti-craving drugs such as ethanol itself and gamma-hydroxybutyric acid. It is postulated that the reduced spontaneous activity of mesolimbic dopaminergic neurons may form the neural basis of the dysphoric state which accompanies abrupt interruption of chronic ethanol administration. Pharmacological manipulations of dopaminergic activity targeted at restoring "normal" dopaminergic function after ethanol withdrawal may lead to way to the experimental basis of new therapeutic strategies of alcoholism.

Evidence implicating both slow- and fast-conducting fibers in the rewarding effect of medial forebrain bundle stimulation

A behavioral version of the collision test was used to determine whether reward-relevant neurons directly link self-stimulation sites in the lateral hypothalamic (LH) and ventral tegmental (VTA) areas. Five male rats served as subjects. Trains of conditioning (C) and test (T) pulses were delivered to the two stimulation sites, each site receiving one of the pulses from each pair. The C-T interval was varied from 0.2-17.3 ms, and the effectiveness of the paired pulse stimulation was estimated by comparing the rate-number curve obtained at each C-T interval to rate-number curves obtained with trains of evenly spaced single pulses delivered via one electrode. For 4 of the subjects, stimulation effectiveness increased with the C-T interval, and the form of this increase was similar regardless of which electrode delivered the C-pulses. These increases in effectiveness are consistent with recovery from collision block in reward-relevant fibers stimulated at both sites. The domain of the rising portion of the effectiveness versus C-T interval curve spanned 2.2-7.7 ms. Such a gradual rise suggests that the directly stimulated substrate is composed of fibers with a wide range of conduction velocities and/or refractory periods. The discrepancy between these gradually rising collision curves and the steeply rising curves obtained in previous collision studies may have been due to inadequate sampling of the rate-number function in the earlier studies.

Profound decrement of mesolimbic dopaminergic neuronal activity during ethanol withdrawal syndrome in rats: electrophysiological and biochemical evidence

Activity of the mesolimbic dopaminergic system was investigated in rats withdrawn from chronic ethanol administration by single-cell extracellular recordings from dopaminergic neurons of the ventrotegmental area, coupled with antidromic identification from the nucleus accumbens, and by microdialysis-technique experiments in the nucleus accumbens. Spontaneous firing rates, spikes per burst, and absolute burst firing but not the number of spontaneously active neurons were found drastically reduced; whereas absolute and relative refractory periods increased in rats withdrawn from chronic ethanol treatment as compared with chronic saline-treated controls. Consistently, dopamine outflow in the nucleus accumbens and its acid metabolites were reduced after abruptly stopping chronic ethanol administration. All these changes, as well as ethanol-withdrawal behavioral signs, were reversed by ethanol administration. This reversal suggests that the abrupt cessation of chronic ethanol administration plays a causal role in the reduction of mesolimbic dopaminergic activity seen in the ethanol-withdrawal syndrome. Results indicate that during the ethanol-withdrawal syndrome the mesolimbic dopaminergic system is tonically reduced in activity, as indexed by electrophysiological and biochemical criteria. Considering the role of the mesolimbic dopaminergic system in the reinforcing properties of ethanol, the depressed activity of this system during the ethanol-withdrawal syndrome may be relevant to the dysphoric state associated with ethanol withdrawal in humans.

Electrophysiological evidence for the existence of NMDA and non-NMDA receptors on rat ventral tegmental dopamine neurons

In vitro extracellular single-unit recordings from rat midbrain slices were used to assess the effects of excitatory amino acid agonists on the activity of A10 dopamine neurons. N-methyl-D-aspartic acid (NMDA), kainic acid (KA), and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) elicited dose-dependent increases in firing rates. The relative potencies for the 3 compounds was AMPA > KA > NMDA. None of the excitations was accompanied by burst firing, but frequently periods of nonrecordable activity occurred following pronounced stimulation. Concurrent application of the excitatory amino acid antagonist CGS 19755 (cis-4-phosphonomethyl-2-piperidine carboxylate) selectively blocked the excitations elicited by NMDA but not by KA or AMPA. Likewise the selective non-NMDA antagonist NBQX [2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(F)quinoxaline] blocked only the excitatory effects of AMPA and KA but not those elicited by NMDA. NBQX appeared to be less potent at antagonizing KA than AMPA. These results suggest that mesolimbic-mesocortical dopamine neurons possess both NMDA and non-NMDA receptors, and possibly distinct AMPA and KA recognition sites.

Na+ current kinetics are not the determinants of the action potential duration in neurons of the rat ventral tegmental area

Na+ currents were recorded from two morphological subpopulations of neurons acutely dissociated from the rat ventral tegmental area (VTA). About 45% of 56 VTA cells examined possessed only the ordinary type of Na+ current which was blocked by a low concentration (0.2 microM) of TTX. However, the remaining 55% had the Na+ current which contained a small fraction of the TTX-insensitive component, irrespective of morphological variations and action potential durations of VTA cells. The peak amplitude of the TTX-insensitive component was less than 10% of the peak amplitude of the total Na+ current. The activation and inactivation kinetics of the TTX-insensitive component were much the same as those of the overwhelming TTX-sensitive component of the Na+ current in VTA cells but differed from those of the TTX-insensitive Na+ current reported in peripheral sensory neurons. Thus, it was concluded that the well-known different action potential durations found for subpopulations of VTA cells are not due to multiplicity of Na+ channel kinetics.

Behaviourally derived estimates of excitability in striatal and medial prefrontal cortical self-stimulation sites

The refractory periods of the substrate underlying brain-stimulation reward were investigated in three rats with moveable electrodes implanted in the rostral caudate-putamen and the medial prefrontal cortex. Acquisition of caudate-putamen self-stimulation occurred within the first session, while self-stimulation for medial prefrontal cortex was observed only after three sessions of caudate-putamen stimulation. The currents required for self-stimulation ranged from 300 to 800 microA (0.1 ms pulse duration) across animals; the maximum response rates averaged roughly 40 bar presses per minute for both structures. Refractory period estimates were obtained from ten caudate-putamen and four medial prefrontal cortex sites. The time course of recovery had the following profile: the curves began to rise at 0.65 ms and 0.95 ms for caudate-putamen and medial prefrontal cortex stimulation, respectively, thereafter increasing to approach an asymptote at 6.00 ms for the caudate-putamen and 6.25 ms for the medial prefrontal cortex. The mean effectiveness value corresponding to the asymptotic portion of the curves was 73% for the caudate-putamen and 69% for the medial prefrontal cortex. Like other forebrain structures, the behaviourally derived refractory periods underlying caudate-putamen and medial prefrontal cortex stimulation, at least at these particular sites, are significantly longer than those observed in most medial forebrain bundle areas, both beginning and ending later. One interpretation for the similarity in their refractory period profiles and the apparent facilitating effect of caudate-putamen stimulation on acquisition of medial prefrontal cortex self-stimulation is that these two regions form part of the same reward substrate.

Enhanced dopamine metabolism in accumbens leads to motor activity and concurrently to increased output from nondopamine neurons in ventral tegmental area and substantia nigra

We previously have reported that nondopamine (non-DA) neurons in substantia nigra (SN) and ventral tegmental area (VTA) of the rat show increased discharge rates during amphetamine (AMPH) and apomorphine (APO)-induced motor activity. The present study represents an attempt to determine the contribution of nucleus accumbens (ACC) dopaminergic activity to these effects, and to ascertain whether the effects in VTA differ from those seen in SN when dopaminergic activity is enhanced locally in ACC. The experiments were carried out in male albino rats (300-400 g) chronically implanted with multiple fine wire electrodes (62 microns) aimed at the pars reticulata of SN (SNR) and VTA. Unit activity was recorded extracellularly in the behaving rat, from neurons identified on the basis of the properties of their action potentials as representing the output of the non-DA neurons in these two structures. In each drug session, unit activity was recorded in parallel from several probes, while motor activity was measured with the open-ended wire technique. But with the recording technique used, a unit represented in most instances the output of a small family of neurons (3-10). Each animal underwent a series of tests given on consecutive days. During these tests, motor and unit activity were measured for 90 min before the drug was administered, and for 135 min after. The first test was of the effects of AMPH, 5 mg/kg, given by the systemic route. The second was of the effects of saline containing 0.1% ascorbic acid (the vehicle) injected bilaterally in ACC, in a volume of 2 microliters per side. The third and all subsequent tests were of the effects of a mixture containing 40 micrograms AMPH, 20 micrograms DA, and 20 micrograms pargyline (P) dissolved in 2 microliters of the vehicle, injected bilaterally in ACC. The results showed that systemic AMPH made the animal hyperactive and at the same time, increased the discharge rate of the non-DA neurons. The bilateral injections of the vehicle in ACC, increased motor activity for about 7 min, an effect interpreted as a rebound from the restraint of the animal during the intracerebral injections, and then depressed motor throughout the 135 min of the postinjection recording period. The effect of the vehicle was to depress unit activity. The effects of injecting the mixture in ACC was to increase motor activity, but with the magnitude and duration of the increase depending on the number of treatments received.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Amphetamine-induced and spontaneous release of dopamine from A9 and A10 cell dendrites: an in vitro electrophysiological study in the mouse

d-Amphetamine (d-AMP) is a potent releaser of dopamine (DA), and its central nervous system stimulant action is mediated primarily through its effect on the substantia nigra and ventral tegmental area dopaminergic neurons (nuclei A9 and A10, respectively). The purpose of the present experiment was to use electrophysiological techniques to examine dendritic release of DA in the in vitro slice preparation, and determine whether: (1) d-AMP inhibits the firing rates of both A9 and A10 cells; (2) the d-AMP-induced inhibition is mediated via the dendritic release of DA; and (3) there is spontaneous dendritic release of DA. Superfusion with d-AMP (2-100 microM) produced identical inhibitory dose-response curves for A9 and A10 cells, and a dose of 6.25 microM caused more than 50% inhibition in the cell firing rates. The d-AMP-induced inhibition was attenuated by blocking DA synthesis. Either D2 receptor blockade (sulpiride, 1 microM), or DA synthesis inhibition (alpha-methylparatyrosine, 50 microM) resulted in a marked increase in the firing rates of dopaminergic cells. These data suggest that d-AMP comparably releases DA from both A9 and A10 cell dendrites, that it releases newly-synthesized DA to inhibit cell firing, and that DA is tonically released to regulate cell firing rates via interactions with inhibitory D2 autoreceptors.

Cocaine effects in the ventral tegmental area: evidence for an indirect dopaminergic mechanism of action

Behavioral studies have implicated central dopaminergic systems, especially the ventral tegmental area of Tsai (VTA), in the mediation of the reinforcing effects of drugs of abuse such as cocaine. A brain slice preparation of the VTA was used to assess the direct effects of cocaine on the spontaneous activity of dopamine-type neurons. When superfused with 1-10 microM cocaine the firing rate of spontaneously active VTA neurons was decreased, with no corresponding change in spike height. While there was a considerable variability in the response to a given concentration of cocaine among the individual units, every cell inhibited by dopamine was also inhibited by cocaine. The local anesthetic lidocaine had variable effects on firing rate, but never potentiated the inhibitory effects of dopamine. Inhibitory responses to either dopamine or cocaine were blocked by the specific D2 dopamine receptor antagonist sulpiride. Small concentrations of cocaine (0.1-0.5 microM), which by themselves had little or no effect on spontaneous activity, potentiated the inhibitory effect of exogenously applied dopamine. Furthermore, the inhibitory action of apomorphine on spontaneous activity in the VTA was not potentiated by cocaine. These observations suggest that in low concentrations, cocaine can act as a dopamine reuptake inhibitor in the VTA, and that the resultant increase in extracellular dopamine acts upon dopamine autoreceptors to inhibit cellular activity.

The notion of response invariance in trade-off studies of self-stimulation

Trade-off profiles, displaying the co-variations of 2 electrical parameters required to maintain a constant magnitude of brain stimulation reward (BSR), have been used extensively in order to characterize the self-stimulation (SS) neurons. It has often been assumed that constancy in the magnitude of BSR can be achieved more accurately by holding SS at a constant proportion of the maximum rate, rather than at a constant rate. The validity of this assumption was tested in 2 experiments using rats. In Exp. 1, we first computed the function that relates SS barpressing rate to pulse frequency (RF function) for two different pulse intensities, separately. The peak SS rate was found to be lower in the low current than in the high current RF function. The rats were then placed in a 2-lever box and were allowed to select either a fixed frequency of the high current pulses or a variable frequency of the low current pulses. In Exp. 2, the RF function was first computed for 2 different lever weights, separately. The peak SS rate was found to be lower in the heavy-lever RF function than in the light-weight lever RF function. The rats were then allowed to select either a fixed pulse frequency delivered by the heavy lever or a variable pulse frequency delivered by the light-weight lever. Isopreference was noted in both experiments, for pulse frequencies which, in the single-lever box, elicited the same proportion of the maximum SS rate, not the same SS rate. The data thus validate the idea that a constant magnitude of BSR is translated into a constant proportion of the maximum SS rate, not a constant SS rate.

Increased midbrain dopaminergic cell activity following 2'CH3-MPTP-induced dopaminergic cell loss: an in vitro electrophysiological study

Several days after the administration of 1-methyl-4-(2'-methylphenyl)-1,2,3,6-tetrahydropyridine (2'CH3-MPTP) to the BALB/cJ mouse there is a loss of midbrain dopaminergic neurons, a reduction of forebrain dopamine (DA) content, and an elevation in forebrain DA turnover. The purpose of the present study was to determine whether the increase in forebrain DA turnover is related to an increase in dopaminergic neuronal activity. In vitro extracellular single unit recordings were made from midbrain dopaminergic neurons in the substantia nigra pars compacta (nucleus A9) and ventral tegmental area (nucleus A10) of BALB/cJ mice. The experimental animals were treated intraperitoneally with 40, 50 or 55 mg/kg 2'CH3-MPTP and killed 7-15 days later. Forebrain DA concentrations were decreased below control values by the two higher toxin doses in the caudate-putamen (67% and 78%, respectively), but not in the nucleus accumbens. DA turnover increased more than 2-fold in the caudate-putamen, but was unchanged in the nucleus accumbens. Nucleus A9 cells, in the 2'CH3-MPTP-treated animals, exhibited a 3-fold increase in the number of spontaneously active cells, and an 84% increase in basal firing rates. There was also a positive correlation between the A9 cell firing rates, and the DA turnover in the striatum of the toxin-treated mice. Nucleus A10 cells, in the 2'CH3-MPTP-treated animals, exhibited neither changes in number of spontaneously active cells nor changes in firing rates.(ABSTRACT TRUNCATED AT 250 WORDS)

Cocaine increases extracellular dopamine in rat nucleus accumbens and ventral tegmental area as shown by in vivo microdialysis

In vivo microdialysis measurements of extracellular dopamine (DA) were made simultaneously in nucleus accumbens (NAS) and ventral tegmental area (VTA) in anesthetized rats. Intravenous cocaine (1 mg/kg) resulted in rapid increases in DA in both NAS and VTA. The DA increase in NAS was greater in magnitude and persisted longer than that in VTA. The relevance of the observed effects to reported effects of cocaine on firing rate of mesoaccumbens DA neurons is discussed.

Two substrates for medial forebrain bundle self-stimulation: myelinated axons and dopamine axons

The directly activated substrates for medial forebrain bundle (MFB) self-stimulation are primarily low threshold, myelinated axons with absolute refractory periods of 0.4 to 1.2 msec, conduction velocities of 1 to 8 m/sec and current-distance constants of 1000 to 3000 microA/mm2. When small electrode tips or high currents are used, however, a second population of long refractory period (1.2 to 5 msec) axons is added. The excitability properties of this second population are almost identical with those of dopamine (DA) axons. Furthermore, the long-refractory period effects of MFB self-stimulation are reduced, but not completely blocked, by peripheral injections of alpha-flupenthixol, suggesting that dopamine axons make small contributions to MFB self-stimulation when small tips are used. Collision data, strength-duration data and refractory period data in various self-stimulation experiments are compared. Asymmetric collision effects, recently observed in cortical and striatal sites mediating electrically evoked turning, may help determine where synapses are located in circuits mediating electrically evoked behaviors. A neural model of symmetric, asymmetric and mixed collision is proposed.

Intracellular recording from putative dopamine-containing neurons in the ventral tegmental area of Tsai in a brain slice preparation

Coronal slices of rat mesencephalon containing the ventral tegmental area of Tsai (VTA) and the substantia nigra were prepared. Stable intracellular recordings were obtained from presumed dopamine (DA)-containing neurons in the VTA. Both silent and spontaneously active cells were encountered; spontaneously active neurons fired in an extremely regular pacemaker-like fashion. These neurons had resting membrane potentials ranging from -45 to -75 mV and input resistances ranging from 80-400 M omega. DA-containing neurons in the VTA demonstrated marked anomalous rectification in response to hyperpolarizing current pulses. Application of DA or the GABAB agonist, baclofen, to the bathing medium produced suppression of spontaneous firing, sometimes accompanied by membrane hyperpolarization. Neuronal input resistance was not changed consistently by DA and was generally reduced by baclofen.

Mesocortical dopaminergic neurons. 1. Electrophysiological properties and evidence for soma-dendritic autoreceptors

Mesencephalic dopaminergic neurons were electrophysiologically identified by a variety of criteria, including antidromic activation from prefrontal or cingulate cortex, neostriatum, or nucleus accumbens in urethane-anesthetized rats. The mean firing rate of 98 mesocortical dopaminergic neurons was 2.9 +/- 0.3 spikes/sec and did not differ from the mean firing rate found for nigrostriatal or nucleus accumbens dopaminergic neurons. Spontaneously active mesocortical dopaminergic neurons were inhibited by intravenous administration of either apomorphine (6 micrograms/kg) or amphetamine (0.25 mg/kg). Whereas most antidromic responses of nigrostriatal and mesoaccumbens neurons consisted of the initial segment spike only, cortically-elicited antidromic responses typically consisted of a full initial segment-soma-dendritic spike. These findings are discussed with regard to the presence of soma-dendritic autoreceptors on mesocortical dopaminergic neurons.

Effect of noxious tail pinch on the discharge rate of mesocortical and mesolimbic dopamine neurons: selective activation of the mesocortical system

The effects of noxious tail pinch on the activity of mesocortical and mesolimbic dopamine (DA) neurons located in the ventromedial mesencephalic tegmentum were analyzed in ketamine-anesthetized rats. The great majority of mesocortical DA neurons responded to tail pinch, either by an excitation (65%), or by an inhibition (25%). In contrast, most DA neurons projecting either to the nucleus accumbens or the septum remained unaffected. These results demonstrate that noxious tail pinch selectively influences the firing rate of mesocortical DA neurons.

Dopamine agonists at repeated "autoreceptor-selective" doses: effects upon the sensitivity of A10 dopamine autoreceptors

Previous reports have established the ability of dopamine (DA) agonists to stimulate inhibitory DA autoreceptors at doses which minimally stimulate postsynaptic DA receptors, suggesting that hyperactive DA transmission may be controlled clinically by treatment with DA agonists. Little is known, however, about the possible loss of autoreceptor sensitivity that may occur after repeated treatment with low doses of DA agonists. Extracellular single cell recording and microiontophoretic techniques were used to measure the sensitivity of impulse-regulating DA autoreceptors on A10 DA cells in the ventral tegmental area (VTA) of chloral hydrate-anesthetized rats pretreated for seven days with repeated subcutaneous (s.c.) doses of the DA agonist apomorphine (APO). The ability of intravenous (i.v.) administration of the potent D2 DA agonist quinpirole (QUIN) to inhibit the firing of A10 cells was not attenuated in rats pretreated with repeated low doses (2 x 50 micrograms/kg/day, s.c.) of APO for 7 days, although higher doses (2 x 250 or 500 micrograms/kg/day) did cause subsensitive responses to QUIN. In rats pretreated with repeated low doses of APO, microiontophoretic application of DA on A10 cells revealed somewhat subsensitive responses. However, ibotenic acid lesions of postsynaptic cells in the nucleus accumbens (NAc) prior to initiation of APO treatment (2 x 50 micrograms/kg/day) did not alter the response of A10 cells to systemic QUIN, contradicting the possibility that the feedback projection from the NAc to the VTA was compensating for autoreceptor down-regulation during systemic challenge with QUIN. In contrast, administration of the irreversible DA antagonist EEDQ (2 mg/kg, i.p.) to control and APO-treated rats (2 x 50 micrograms/kg/day) 24 hr prior to recording did reveal a difference in A10 cell sensitivity to systemic QUIN and to microiontophoretic DA between the two groups, suggesting that "spare" DA autoreceptors may have concealed the down-regulation of autoreceptors induced by repeated low doses of APO. Challenge of A10 DA cells with the partial DA autoreceptor agonist (-)-3-(3-hydroxyphenyl)-N-n-propylpiperidine [(-)3-PPP], for which an autoreceptor reserve should not exist, produced slightly attenuated responses in APO-treated rats (2 x 50 micrograms/kg/day). These findings provide evidence for the existence of spare somatodendritic DA autoreceptors on A10 DA cells with respect to potent DA agonists, suggesting that repeated administration of "autoreceptor-selective" doses of DA agonists may not result in a diminished inhibition of DA neuronal activity.

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.

Neuromodulatory action of dopamine in the nucleus accumbens: an in vivo intracellular study

Intracellular recordings were made from neurons in the nucleus accumbens in situ to determine how dopamine produces the selective neuromodulatory action in the accumbens observed in previous studies. Electrical stimulation of the basolateral nucleus of the amygdala was found to produce monosynaptically evoked depolarizing and hyperpolarizing postsynaptic potential sequences in a large proportion of the accumbens neurons sampled. Dopamine applied iontophoretically or released endogenously by stimulation of the ventral tegmental area produced consistent membrane depolarization and an increase in membrane conductance but not an increase in spontaneous activity of the accumbens neurons. Stimulation of the ventral tegmental area with trains of 10 pulses at 10 Hz prior to stimulation of the amygdala produced 8-58% reduction in the amplitude of the depolarizing postsynaptic potential but no change in the late hyperpolarizing postsynaptic potential. Although attenuation of the depolarizing postsynaptic potential amplitude from ventral tegmental area stimulation was often accompanied by membrane depolarization, it appeared that the two responses were not causally related. The effect of ventral tegmental area stimulation on the evoked depolarizing postsynaptic potential and the membrane potential were blocked by haloperidol indicating the involvement of dopamine. Iontophoretically applied dopamine produced responses similar to ventral tegmental area stimulation with two exceptions: (i) iontophoretically applied dopamine produced consistently stronger maximal attenuation of the depolarizing postsynaptic potential than did ventral tegmental area stimulation; and (ii) iontophoretically applied dopamine always attenuated both the depolarizing postsynaptic potential and hyperpolarizing postsynaptic potential whereas ventral tegmental area stimulation produced selective attenuation of the depolarizing postsynaptic potential only. These electrophysiological results are complementary to those from pharmacological experiments and suggest that one of several physiological functions of dopamine in the nucleus accumbens is a neuromodulatory one involving presynaptic action on non-dopaminergic terminals.

Effects of electrical stimulation of brain reward sites on release of dopamine in rat: an in vivo electrochemical study

Behavioral studies suggest that mesencephalic dopamine neurons mediate the rewarding effects of electrical stimulation of the medial forebrain bundle. Yet there is little direct evidence that rewarding electrical stimulation actually activates dopamine-containing neurons. The purpose of the present study was to determine, using in vivo electrochemistry, if electrical stimulation applied to lateral hypothalamic or ventral tegmental reward sites would elicit changes in extracellular levels of dopamine. In vivo high speed chronoamperometric recordings were performed in anesthetized rats that had been previously trained to respond for rewarding electrical stimulation of the medial forebrain bundle or ventral tegmental area. We found that a single 500 msec train of pulses elicited a small transient electrochemical signal, the magnitude of which was dependent on the pulse duration and frequency. This signal was potentiated by inhibition of dopamine reuptake. Prolonged electrical activation with a self-stimulation-like regimen resulted in the gradual accumulation of an electroactive compound, tentatively identified as dihydroxyphenylacetic acid (DOPAC). Taken together, the data reported here support the idea that rewarding electrical stimulation causes the release of dopamine.

The response of non-dopamine neurons in substantia nigra and ventral tegmental area to amphetamine and apomorphine during hypermotility: the striatal influence

The effects of haloperidol pretreatment in striatum on the motor response, and on concurrently recorded unit responses of nondopamine (DA) neurons in substantia nigra (SN) and ventral tegmental area (VTA) to systemic amphetamine and apomorphine, were investigated with the objective of determining the role of the striatum in the output of putative DA output neurons. Unit and motor activity were recorded in the male rat, chronically implanted with 9 electrodes in SN and VTA and with two cannulae for bilateral injections into striatum. The recording electrodes were 3 bundles of 3 wires, each wire in the bundle of a different length, but all 3 aimed at SN, pars reticulata, or VTA. In each recording session, unit activity was derived from 7 wires while gross motor activity was recorded with the open-ended wire technique. The subjects were tested under two conditions. In the first, the vehicle was injected bilaterally into striatum 90 min before one of the DA agonists was injected by the intraperitoneal route. In the second, the DA antagonist haloperidol was injected bilaterally into striatum before the systemic treatment with the DA agonist. In subjects which received injections of the vehicle into striatum, amphetamine induced a large motor response, and concurrently, a large increase in the rate of discharge of a portion of the identified non-DA neurons in SN and VTA. In subjects which received injections of haloperidol into striatum, amphetamine induced a smaller behavioral response, a smaller increase in the rate of discharge of these neurons in SN but not in VTA where the increase was of the same magnitude as controls. In control subjects, apomorphine induced an increase in motor activity and concurrently, an increase in the rate of firing of the identified non-DA neurons in SN and VTA. But the increases were of somewhate smaller magnitude and much shorter duration than the increases induced by amphetamine. In subjects which had been pretreated with haloperidol in striatum, apomorphine induced an increase in motor activity that was of the same magnitude as the insion that the striatum has the capacity to influence the output of non-DA neurons only in SN but also in VTA, indicating that, if there is a specialization of function, it is only relative.(ABSTRACT TRUNCATED AT 400 WORDS)

Excitability properties of medial forebrain bundle axons of A9 and A10 dopamine cells

A9 and A10 units identified as dopaminergic were recorded with extracellular micropipettes. The units were antidromically activated by electrical stimulation at the level of the preoptic area. The absolute refractory periods ranged from 1.2 to 2.5 ms. During the 2-8 ms of the relative refractory period, conduction was slower than normal by up to 1.5 ms. The time constant, C, of the strength-duration curve ranged from 0.4 to 0.6 ms. The current (I)-distance (D) relationship, tested by moving the stimulating electrode past the axon, was approximately parabolic (I = K D exp 2), with the constant of the equation, K, ranging from 900 to 2000 microA/mm exp 2, for 0.5 ms pulses. This relationship allows calculation of the radius of the field of dopamine axon excitation at any current. These high K values show that axons of dopamine cells cannot be activated unless high current densities are delivered, even when electrodes are placed near the axons. These data allow determination of the extent to which dopamine axons can be the directly activated substrates for behaviors, such as self-stimulation and circling, which are evoked by electrical stimulation of the medial forebrain bundle or internal capsule.

Dopamine-containing neurons in the mammalian central nervous system: electrophysiology and pharmacology

A decade of research culminated in the late 1950's with the demonstration that dopamine was a chemical neurotransmitter within the mammalian brain. Since this time, dopaminergic neuronal systems have been extensively studied using numerous techniques. This paper will review the last 14 years of electrophysiological investigation on neurochemically identified dopamine-containing neurons in the central nervous system. This will include an examination of both the electrophysiological and pharmacological characteristics in these cells, as well as the resulting insights into the regulation of dopamine cell electrical activity which is derived from this work.

Anatomical and electrophysiological characterization of presumed dopamine-containing neurons within the supramammillary region of the rat

A combination of immunocytochemical, electrophysiological and pharmacological techniques were employed to study the properties of neurons within the supramammillary (SUM) complex of the rat. The SUM region contains a small, but dense, population of tyrosine hydroxylase immunoreactive neurons. Following injection of the orthograde neuroanatomical tracer, Phaseolus Vulgaris leucoagglutinin, into the SUM region, heavy terminal labeling was observed in the lateral septal nucleus, diagonal band of Broca and bed nucleus of the stria terminalis. The electrophysiological and pharmacological properties of antidromically-activated SUM neurons revealed evidence of two neuronal populations. Both groups of neurons exhibited long duration action potentials (greater than 2 msec) and slow conduction velocities (less than 0.5 m/sec). However, cells in one group were characterized by slow and erratic firing rates and insensitivity to dopamine (DA) autoreceptor agonists. Cells in the other group typically exhibited no spontaneous activity but could be induced to discharge by iontophoretic application of glutamate. These latter cells were sensitive to DA autoreceptor stimulation. Of the two populations of mammilloseptal SUM neurons, the silent population exhibited several properties similar to those of midbrain DA neurons.

Contraversive circling elicited from the internal capsule and substantia nigra: evidence for a continuous axon bundle mediating circling

Electrical stimulation of many brain sites (e.g., anteromedial cortex, internal capsule, substantia nigra, superior colliculus, rostro-medial tegmentum, and medial pons) evokes circling. The collision method of Shizgal et al. (J. Comp. Physiol. Psychol., 94 (1980) 227-237) was used to determine whether these sites are functionally connected for the production of circling in rats. If connectivity was evidenced, then refractory period and conduction velocity distributions were determined for axons passing through the connected stimulation sites. Collision of up to 90% was found between electrodes placed in internal capsule and substantia nigra, suggesting that these sites are connected by continuous axons that mediate circling. The refractory periods of these axons ranged from 0.5 to 4.5 ms, and the conduction velocities of these axons ranged from 0.9 to 4.4 ms. These velocities are similar to those of striatonigral axons. No collision was found between anteromedial cortex and any other sites tested, nor between pontine sites and internal capsule or substantia nigra.

Amphetamine-induced increase in motor activity is correlated with higher firing rates of non-dopamine neurons in substantia nigra and ventral tegmental area

The responses of non-dopamine neurons in substantia nigra and ventral tegmental area to systemic amphetamine were investigated in the behaving rat chronically implanted with multiple fine-wire electrodes. The neurons were identified with electrophysiological criteria requiring that the signals be of biphasic shape, short duration (less than 2.0 ms), and show high and regular rates of discharge (greater than 20 spikes/s). In recording sessions lasting 240 min, single and multiple unit activity was recorded from seven electrodes, and motor activity was measured automatically with the open-ended wire technique. The movement counts provided an index of gross motor activity, not of the specific movements occurring during DA behaviors. D-Amphetamine, 5.0 mg/kg, given by the intraperitoneal route at 90 min into the session, induced an increase in motor activity and in the firing rate of some non-dopamine neurons. The behavioral and neural responses were correlated for magnitude, latencies and duration. But not all non-dopamine neurons in ventral tegmental area, and substantia nigra showed responses to amphetamine. When unit responses were obtained, they were obtained in subjects which showed large motor responses. In substantia nigra, responsive and non-responsive units were interdigitated and found mainly in the pars reticulata subdivision. In the ventral tegmental area, responsive and non-responsive neurons were interdigitated throughout this structure. The effects of amphetamine were dose-responsive, doses of 1.0, 2.0 and 3.0 mg/kg inducing smaller behavioral and unit responses than 5.0 mg/kg. D-Amphetamine, 5.0 mg/kg, was more effective than L-amphetamine, given at the same dose, in inducing these changes. In rats pretreated with systemic haloperidol, 1.5 mg/kg, the behavioral and neural responses to D-amphetamine, 5.0 mg/kg, were greatly attenuated. In rats pretreated with a subanesthetic dose of urethan, 600 mg/kg, to prevent changes in gross motor activity, the response to D-amphetamine in ventral tegmental area was attenuated, but it was of normal magnitude in substantia nigra. In rats with bilateral electrolytic lesions of nucleus accumbens, D-amphetamine induced a smaller motor response than in controls, but the neural responses in ventral tegmental area and substantia nigra were the same as in controls. These findings support the notion that non-dopamine neurons in ventral tegmental area and substantia nigra, pars reticulata, play a role in the motor function of the A9 and A10 dopamine neurons, and in the behavioral effects of amphetamine.(ABSTRACT TRUNCATED AT 400 WORDS)

Correlation between the discharge rate of non-dopamine neurons in substantia nigra and ventral tegmental area and the motor activity induced by apomorphine

The effects of systemic apomorphine on the discharge rates of non-dopamine neurons of the ventral tegmental area and the substantia nigra were investigated in the behaving rat to determine the relationship between the neural responses and the motor activity induced by the dopamine agonist. Apomorphine, 3.0 mg/kg, induced large increases in motor activity and in the rate of firing of non-dopamine neurons in both ventral tegmental area and substantia nigra. The effects were similar in both structures, but only a portion of the non-dopamine neurons sampled were sensitive to the dopamine agonist. The motor and unit responses were correlated for latencies, magnitude and duration. These effects were dose-responsive, 0.75 mg/kg and 1.5 mg/kg inducing smaller behavioral and neural responses than 3.0 mg/kg. Apomorphine, 3.0 mg/kg, given to rats pretreated with haloperidol, 1.5 mg/kg, 60 min before the recording session, induced smaller behavioral and neural responses than in controls. The dopamine agonist given to rats in which gross motor activity was prevented through light anesthesia with urethan, 600 mg/kg, led to a decrease in the magnitude of the unit response in ventral tegmental area, and to a potentiation of the response in substantia nigra. In rats with bilateral electrolytic lesions of nucleus accumbens given one week earlier, apomorphine induced a smaller behavioral response than in controls, and differential effects on the neural responses. In ventral tegmental area the response was the same as in controls, but in substantia nigra it was blocked. These results indicate the presence in substantia nigra and ventral tegmental area of subpopulations of non-dopamine neurons responding with excitation to experimental manipulations that activate dopamine receptors. The dissociation between the motor effects of apomorphine and the neural effects in the subjects prevented from expressing gross motor activity, and in the lesioned animals, indicates that the neural responses were not the result of behavioral feedback. And the differential effects of apomorphine in ventral tegmental area and substantia nigra in these two groups of subjects suggest that the dopamine motor influence, at this brain level, may be fractionated, different groups of non-dopamine neurons conveying different aspects of the dopamine influence on motor activity to premotor neurons. The results, taken together, support the notion that non-dopamine efferent neurons in ventral tegmental area and substantia nigra function as dopamine output neurons, their output being critical for the behavioral effects of dopamine agonists.

Cocaine and central monoaminergic neurotransmission: a review of electrophysiological studies and comparison to amphetamine and antidepressants

Psychomotor stimulants (e.g. cocaine and amphetamine) and many antidepressants are believed to elicit their psychotropic actions by interacting primarily with central monoaminergic neurons. The acute central neuronal effects of amphetamine and antidepressants have been extensively investigated in rats utilizing extracellular single unit electrophysiological and microiontophoretic techniques in vivo. In recent years the chronic effects of these compounds on the above neuronal systems have also been reported. Such investigations have proliferated because of the realization that the mechanisms underlying the psychotomimetic effects (e.g. amphetamine and cocaine) and mood elevation (i.e. antidepressants) observed with the administration of these drugs are more accurately reflected in chronic studies. For many years it has been assumed that cocaine and amphetamine produce very similar if not identical psychotropic effects through their actions on central monoaminergic neurotransmission. In terms of effects on single monoaminergic neurons, this assumption had gone by untested until two years ago, when the first report of the electrophysiological effects of cocaine on central monoaminergic (locus ceruleus) neurons appeared in the literature (61). This review discusses recent electrophysiological studies with cocaine at the level of single identified monoaminergic neurons and compares such data with that previously reported for amphetamine and antidepressants. In addition to identifying some of the similarities and differences between these compounds, this review also highlights some of the gaps in our knowledge regarding the effects of these drugs on central monoaminergic neurotransmission.

Electrophysiological and pharmacological characterization of identified nigrostriatal and mesoaccumbens dopamine neurons in the rat

Extracellular single-unit recording techniques were used to compare the basal activity and pharmacological responsiveness of identified nigrostriatal and mesoaccumbens dopamine (DA)-containing neurons. The projection area of each DA cell was determined by antidromic activation techniques. The forebrain stimulation used for the cell identification procedure did not alter the pharmacological responsiveness of DA neurons; the inhibitory effect of apomorphine (and d-amphetamine) was identical when stimulation was applied either prior to or following drug administration. Analysis of the spike discharge pattern revealed that a higher proportion of mesoaccumbens DA cells exhibited burst-firing activity. Although the firing pattern of the two populations of burst-firing DA cells was similar in many regards, mesoaccumbens DA cells exhibited a longer postburst inhibition than did nigrostriatal DA cells. Each of the DA agonists, apomorphine, pergolide, B-HT 920, and d-amphetamine, inhibited nigrostriatal and mesoaccumbens DA neuronal activity in a similar fashion. However, there was a marked population difference in the recovery of cell firing in the 10 minutes following apomorphine-induced inhibition; the recovery of mesoaccumbens spike discharges was considerably slower. Although this population difference was apparent to some extent following administration of pergolide or B-HT 920 (but not d-amphetamine), it was considerably less marked. The present findings are discussed with respect to the known regulatory control of midbrain DA neurons.

The bed nucleus of the stria terminalis: electrophysiological properties and responses to amygdaloid and hypothalamic stimulation

Single unit activity was recorded in the bed nucleus of the stria terminalis (BNST) in response to amygdaloid and hypothalamic stimulation. BNST neurons were identified by their spontaneous discharge rate, spike duration, orthodromic and antidromic responses and convergence properties. The results indicate that these neurons have characteristic electrophysiological features and that they are organized to receive amygdaloid and hypothalamic inputs with increasing convergence along a dorsal to ventral continuum.

Cholecystokinin potentiates dopamine inhibition of mesencephalic dopamine neurons in vitro

Cholecystokinin octapeptide sulfate (CCK-S) is a neuropeptide that is co-localized with dopamine (DA) in some neurons of the ventral tegmental area (VTA). A functional role for this peptide/monoamine co-localization has not been firmly established; however, behavioral and in vivo electrophysiological studies indicate that CCK-S modifies the action of DA in some brain areas. A brain slice preparation of the rat VTA was developed in order to examine primary effects of CCK-S on DA-containing neurons, and to determine whether CCK-S modulates the inhibitory action of DA on these neurons. Spontaneously active DA neurons of the VTA were identified on the basis of their characteristic spike waveforms and firing rate as determined with extracellular recording techniques. These cells were inhibited by perfusion with DA in a dose-dependent, sulpiride-reversible manner. CCK-S produced brief excitatory increases in firing rate in 83% of these cells tested. This excitation was dose-dependent, and the excitatory responses frequently diminished even in the continued presence of CCK-S. Prior administration of CCK-S to these cells markedly potentiated DA-induced inhibition of spontaneous firing; the magnitude of this effect ranged from a 24 to 376% increase in the inhibitory response. This CCK-induced potentiation of DA inhibition was not blocked by low calcium, high magnesium superfusion medium, indicating that this effect is a direct consequence of a postsynaptic action on the VTA neurons from which recordings were made. These results suggest that co-localized CCK-S may significantly affect neuronal sensitivity to synaptically released DA.

Dopaminergic modulation of bulbofugal projections in the rat olfactory tubercle

Neuronal activities following olfactory bulb electrical stimulation were examined before and after administration of dopamine and dopamine antagonist in the rat olfactory tubercle. The inhibitory response to olfactory bulb stimulation was attenuated by systemic haloperidol administration, but the excitatory response to olfactory bulb stimulation rarely was modulated. Topical application of dopamine by iontophoresis extended the duration of inhibition in 56% of the neurons sampled and diminished it in 25%; the excitatory response was modulated in 42% of neurons, most of which were attenuated. These findings suggest that dopamine in the olfactory tubercle could be involved in modulations of neuronal activities related to olfactory transduction.

Recording of mouse ventral tegmental area dopamine-containing neurons

We examined the electrophysiologic and pharmacologic properties of dopamine-containing ventral tegmental area neurons in the mouse using extracellular single-unit recording techniques in both chloral hydrate-anesthetized mice and in vitro mouse midbrain slices. In vivo the ventral tegmental area neurons had long-duration action potentials (2 to 5 ms) and discharged at 1 to 9 spikes/s with either a decremental burst pattern or a regular pattern. Systemic administration of the dopamine agonist, apomorphine, decreased their firing rate, and the dopamine receptor blocker, haloperidol, reversed this effect. Similarly, systemic administration of the dopamine-releasing agent, d-amphetamine, suppressed their discharge rate, an effect blocked by pretreatment of the animals with alpha-methyl-p-tyrosine. When recorded in vitro from midbrain slices, ventral tegmental area neurons showed electrophysiologic properties similar to those found in vivo; however, the neurons recorded in vitro fired at a significantly faster rate and their firing pattern tended to be more pacemaker-like, especially when recordings were made in an incubation medium that blocked synaptic transmission (i.e., low calcium/high magnesium). The activity of most of these neurons was suppressed by addition of apomorphine to the incubation medium, an effect reversed by haloperidol. Pretreatment with alpha-methyl-p-tyrosine produced no significant change in the discharge pattern or rate for cells recorded in vitro. These data indicate that mouse ventral tegmental area dopamine neurons in vivo exhibit the same electrophysiologic and pharmacologic properties as do rat and cat dopamine-containing neurons and that in vitro they fire with pacemaker regularity in a low-calcium/high-magnesium medium. The in vitro preparation offers an approach to examining the fundamental properties of ventral tegmental area dopamine-containing neurons in the absence of afferent inputs.

Refractoriness of neurons mediating intracranial self-stimulation in the anterior basal forebrain

The post-stimulation excitability of neurons mediating electrical self-stimulation of the anterior basal forebrain was evaluated psychophysically in the rat. Rats with electrodes in the nucleus accumbens, caudate nucleus, lateral preoptic area, diagonal band, or anterior medial forebrain bundle pressed a lever to earn 0.5-s trains of conditioning (C) and test (T) pulse pairs. The C-T interval was systematically varied and the effectiveness of the T-pulse was estimated by measuring the frequency of pulse pairs required to sustain criterion responding. All sites tested demonstrated similar recovery; T-pulse effectiveness, normalized against the effect of the C-pulse, was lowest at delays of 0.4-0.8 ms and it rose monotonically until 5 ms when it achieved an effectiveness plateau of one. Increasing the current of the T-pulse by 50 or 60% failed to hasten recovery, suggesting that the recovery profiles primarily reflect the activation of neurons very soon after emergence from absolute refractoriness. Compared to lateral hypothalamic and ventral tegmental self-stimulation, the neurons that support self-stimulation in the ventral basal forebrain recover more slowly; recovery here is only about half done by the time lateral hypothalamic placements demonstrate complete recovery.