Population coding by electrosensory neurons

Sensory stimuli typically activate many receptors at once and therefore should lead to increases in correlated activity among central neurons. Such correlated activity could be a critical feature in the encoding and decoding of information in central circuits. Here we characterize correlated activity in response to two biologically relevant classes of sensory stimuli in the primary electrosensory nuclei, the electrosensory lateral line lobe, of the weakly electric fish Apteronotus leptorhynchus. Our results show that these neurons can display significant correlations in their baseline activities that depend on the amount of receptive field overlap. A detailed analysis of spike trains revealed that correlated activity resulted predominantly from a tendency to fire synchronous or anti-synchronous bursts of spikes. We also explored how different stimulation protocols affected correlated activity: while prey-like stimuli increased correlated activity, conspecific-like stimuli decreased correlated activity. We also computed the correlations between the variabilities of each neuron to repeated presentations of the same stimulus (noise correlations) and found lower amounts of noise correlation for communication stimuli. Therefore the decrease in correlated activity seen with communication stimuli is caused at least in part by reduced noise correlations. This differential modulation in correlated activity occurred because of changes in burst firing at the individual neuron level. Our results show that different categories of behaviorally relevant input will differentially affect correlated activity. In particular, we show that the number of correlated bursts within a given time window could be used by postsynaptic neurons to distinguish between both stimulus categories.     

Clustered burst firing in FMR1 premutation hippocampal neurons: amelioration with allopregnanolone

Premutation CGG repeat expansions (55-200 CGG repeats; preCGG) within the fragile X mental retardation 1 (FMR1) gene cause fragile X-associated tremor/ataxia syndrome (FXTAS). Defects in neuronal morphology and migration have been described in a preCGG mouse model. Mouse preCGG hippocampal neurons (170 CGG repeats) grown in vitro develop abnormal networks of clustered burst (CB) firing, as assessed by multielectrode array recordings and clustered patterns of spontaneous Ca(2+) oscillations, neither typical of wild-type (WT) neurons. PreCGG neurons have reduced expression of vesicular GABA and glutamate (Glu) transporters (VGAT and VGLUT1, respectively), and preCGG hippocampal astrocytes display a rightward shift on Glu uptake kinetics, compared with WT. These alterations in preCGG astrocytes and neurons are associated with 4- to 8-fold elevated Fmr1 mRNA and occur despite consistent expression of fragile X mental retardation protein levels at ～50% of WT levels. Abnormal patterns of activity observed in preCGG neurons are pharmacologically mimicked in WT neurons by addition of Glu or the mGluR1/5 agonist, dihydroxyphenylglycine, to the medium, or by inhibition of astrocytic Glu uptake with dl-threo-β-benzyloxyaspartic acid, but not by the ionotropic Glu receptor agonists, α-2-amino-3-(5-methyl-3-oxo-1,2-oxazol-4-yl) propanoic acid or N-methyl-d-aspartic acid. The mGluR1 (7-(hydroxyimino)cyclopropa [b]chromen-1a-carboxylate ethyl ester) or mGluR5 (2-methyl-6-(phenylethynyl)pyridine hydrochloride) antagonists reversed CB firing. Importantly, the acute addition of the neurosteroid allopregnanolone mitigated functional impairments observed in preCGG neurons in a reversible manner. These results demonstrate abnormal mGluR1/5 signaling in preCGG neurons, which is ameliorated by mGluR1/5 antagonists or augmentation of GABA(A) receptor signaling, and identify allopregnanolone as a candidate therapeutic lead.     

The N-methyl-D-aspartate receptor and burst firing of CA1 hippocampal pyramidal neurons

Previous intracellular investigations in the rat hippocampus have demonstrated that N-methyl-D-aspartate, ibotenate and 2,3-pyridine dicarboxylate (quinolinate) all evoke burst firing of CA1 pyramidal neurons, whereas kainate and quisqualate, which are thought to react with different receptors, do not. The purpose of the present study has been to investigate the ability of a series of compounds either to trigger burst firing or to antagonize this pattern of excitation. We report here that N-methyl-L-aspartate, 1,2-benzene dicarboxylate (phthalate) and methylene succinate (itaconate) are also capable of evoking burst firing. The results of this investigation suggest that since both quinolinate and phthalate are rigid planar molecules and only the 2 and 3 positioning of the carboxylates of pyridine was active, a cis configuration of the carboxyls with respect to the 2,3 carbon bond appears to be necessary for excitation. While a nitrogen atom is not necessary for activity (this is absent in phthalate and itaconate) a third functional group, bearing at least a partial positive charge, and in a position alpha to one of the carboxyl groups is required. The requirements for pyridine derivatives to trigger burst firing is similar to that reported as necessary for evoking convulsions and neurotoxicity after intrahippocampal infusion and a correlation between N-methyl-D-aspartate-like burst firing and depolarization and this neuropathology is considered. An important observation has been that the addition of a benzene ring to either quinolinate or phthalate to yield 2,3-quinoline dicarboxylate and 2,3-napthalene dicarboxylate, respectively, converted these excitants into antagonists of burst firing.(ABSTRACT TRUNCATED AT 250 WORDS)     

Functional implications of burst firing and single spike activity in lateral geniculate relay neurons

Guinea-pig thalamocortical relay neurons can intrinsically generate action potentials in two distinct patterns: as high frequency bursts or as relatively independent single spikes. The burst firing mode is due to the presence of a low threshold Ca2+ current and imposes a marked non-linear transformation on depolarizing or hyperpolarizing inputs. In the burst firing mode, thalamic neurons respond to increasing frequencies of depolarizing inputs with progressively fewer action potentials such that they fail to respond to inputs arriving at rates greater than approximately 15 Hz. In this manner, the amplitude of the burst discharge relays little information concerning the characteristics of phasic excitatory postsynaptic potentials which may trigger them, but rather is determined by the membrane potential preceding the burst and the time interval since the last burst. In contrast to the behavior of neurons in the burst firing mode, the pattern of action potentials generated after depolarization into the single spike mode is a more faithful representation of the characteristics of incoming excitatory postsynaptic potentials or depolarizing inputs. The pattern of action potentials generated in the single spike mode is determined by the intensity, duration, and frequency of incoming excitatory inputs even when they arrive at rates in excess of 100 Hz. These, and other properties, allow thalamic neurons to possess two distinct states of neuronal activity: an oscillatory mode in which rhythmic bursts of action potentials are generated and in which responsiveness to stimulation of peripheral receptive fields is greatly reduced, and a transfer mode in which action potentials are generated in relative independence of one another and in which the ability to respond to barrages of phasic excitatory inputs is greatly enhanced. The presence of the rhythmic burst firing mode may therefore facilitate the filtering of sensory information during periods of drowsiness, inattentiveness, and slow wave sleep.     

Neurochemical and electrophysiological studies on the functional significance of burst firing in serotonergic neurons

We have previously described a population of 5-hydroxytryptamine neurons which repetitively fires bursts of usually two (but occasionally three or four) action potentials, with a short (<20 ms) interspike interval within a regular low-frequency firing pattern. Here we used a paradigm of electrical stimulation comprising twin pulses (with 7- or 10-ms inter-pulse intervals) to mimic this burst firing pattern, and compared the effects of single- and twin-pulse electrical stimulations in models of pre- and postsynaptic 5-hydroxytryptamine function. Firstly, we measured the effect of direct electrical stimulation (2 Hz for 2 min) of rat brain slices on efflux of preloaded [3H]5-hydroxytryptamine. In this in vitro model, twin-pulse stimulation increased the efflux of tritium by about twice as much as did single-pulse stimulation. This effect was evident in the medial prefrontal cortex (area under the curve: 2. 59+/-0.34 vs 1.28+/-0.22% relative fractional release), as well as in the caudate-putamen (3.93+/-0.65 vs 2.17+/-0.51%) and midbrain raphe nuclei (5.42+/-1.05 vs 2.51+/-0.75%). Secondly, we used in vivo microdialysis to monitor changes in endogenous extracellular 5-hydroxytryptamine in rat medial prefrontal cortex in response to electrical stimulation (3 Hz for 10 min) of the dorsal raphe nucleus. In this model, twin-pulse stimulation of the dorsal raphe nucleus increased 5-hydroxytryptamine by approximately twice as much as did single-pulse stimulation at the same frequency (area under the curve: 50.4+/-9.0 vs 24.2+/-4.4 fmol). Finally, we used in vivo extracellular recording to follow the response of postsynaptic neurons in the rat medial prefrontal cortex to 5-hydroxytryptamine released by dorsal raphe stimulation. Electrical stimulation of the dorsal raphe nucleus (1 Hz) induced a clear-cut poststimulus inhibition in the majority of cortical neurons tested. In these experiments, the duration of poststimulus inhibition following twin-pulse stimulation was markedly longer than that induced by single-pulse stimulation (200+/-21 vs 77+/-18.5 ms).Taken together, the present in vitro and in vivo data suggest that in 5-hydroxytryptamine neurons, short bursts of action potentials will propagate along the axon to the nerve terminal and will enhance both the release of 5-hydroxytryptamine and its postsynaptic effect.     

Withdrawal from intermittent ethanol exposure increases probability of burst firing in VTA neurons in vitro

Changing the activity of ventral tegmental area (VTA) dopamine neurons from pacemaker to burst firing is hypothesized to increase the salience of stimuli, such as an unexpected reward, and likely contributes to withdrawal-associated drug-seeking behavior. Accordingly, pharmacological, behavioral, and electrophysiological data suggest an important role of the VTA in mediating alcohol-dependent behaviors. However, the effects of repeated ethanol exposure on VTA dopamine neuron ion channel function are poorly understood. Here, we repeatedly exposed rats to ethanol (2 g/kg ethanol, ip, twice per day for 5 days), then examined the firing patterns of VTA dopamine neurons in vitro after 7 days withdrawal. Compared with saline-treated animals, the function of the small conductance calcium-dependent potassium channel (SK) was reduced in ethanol-treated animals. Consistent with a role for SK in regulation of burst firing, NMDA applied during firing facilitated the transition to bursting in ethanol-treated but not saline-treated animals; NMDA consistently induced bursting only in saline-treated animals when SK was inhibited. Also, enhanced bursting in ethanol-treated animals was not a result of differences in NMDA-induced depolarization. Further, I(h) was also reduced in ethanol-treated animals, which delayed recovery from hyperpolarization, but did not account for the increased NMDA-induced bursting in ethanol-treated animals. Finally, repeated ethanol exposure and withdrawal also enhanced the acute locomotor-activating effect of cocaine (15 mg/kg, ip). Thus withdrawal after repeated ethanol exposure produced several alterations in the physiological properties of VTA dopamine neurons, which could ultimately increase the ability of VTA neurons to produce burst firing and thus might contribute to addiction-related behaviors.     

Nitric oxide is involved in nicotine-induced burst firing of rat ventral tegmental area dopamine neurons

In the present study, using single cell recordings in vivo and intracellular recordings in vitro from midbrain slices, the role of N-methyl-d-aspartate (NMDA) receptor signaling on firing activity in ventral tegmental area dopamine neurons elicited by nicotine was investigated in the rat. In accordance with previous studies, systemic nicotine (0.5 mg/kg s.c.) increased both firing rate and burst firing of dopamine neurons in vivo, and bath-applied nicotine (10 microM) increased firing rate in vitro. The competitive NMDA receptor antagonist CGP39551 (2.5 mg/kg i.p.) inhibited nicotine's effects on burst firing and also attenuated the nicotine-induced increase in firing rate. Moreover, although the nitric oxide (NO)-synthase inhibitor N-nitro-l-arginine-methyl-ester (l-NAME; 5.0 mg/kg i.p.) had no effect on cell firing by itself, it prevented the response to nicotine in vivo. In contrast, l-NAME (100 microM) did not influence nicotine's effect on dopamine cell firing in vitro, suggesting that the effect of l-NAME seen in vivo is dependent on presynaptic afferent input. The present study confirms previous results suggesting that the effect of systemically administered nicotine is in part presynaptic and mediated via NMDA receptors. The data also indicate that NO plays an important role in the previously demonstrated, indirect, glutamate-mediated excitation of these neurons by nicotine. By inference, our results provide additional support for the involvement of NO in nicotine dependence.     

Fast burst firing and short-term synaptic plasticity: a model of neocortical chattering neurons

We present an ionic conductance model of chattering neurons in the neocortex, which fire fast rhythmic bursts in the gamma frequency range (approximately 40 Hz) in response to stimulation [Gray C. M. and McCormick D. A. (1996) Science 274, 109-113]. The bursting mechanism involves a "ping-pong" interplay between soma-to-dendrite back propagation of action potentials and an afterdepolarization generated by a persistent dendritic Na+ current and a somatic Na+ window current. The oscillation period is primarily determined by a slowly inactivating K+ channel and passive membrane properties. The model behavior is compared quantitatively with the experimental data. It is shown that the cholinergic muscarinic receptor activation can transform the model cell's firing pattern from tonic spiking to rapid bursting, as a possible pathway for acetylcholine to promote 40-Hz oscillations in the visual cortex. To explore possible functions of fast burst firing in the neocortex, a hypothetical neural pair is simulated, where a chattering cell is presynaptic to an inhibitory interneuron via stochastic synapses. For this purpose, we use a synapse model endowed with a low release probability, short-term facilitation and vesicle depletion. This synapse model reproduces the behavior of certain neocortical pyramid-to-interneuron synapses [Thomson A. M. et al. (1993) Neuroscience 54, 347-360]. We showed that the burstiness of cell firing is required for the rhythmicity to be reliably transmitted to the postsynaptic cell via unreliable synapses, and that fast burst firing of chattering neurons can provide an exceptionally powerful drive for recruiting feedback inhibition in cortical circuits. From these results, we propose that the fast rhythmic burst firing of neocortical chattering neurons is generated by a calcium-independent ionic mechanism. Our simulation results on the neural pair highlight the importance of characterizing the short-term plasticity of the synaptic connections made by chattering cells, in order to understand their putative pacemaker role in synchronized gamma oscillations of the visual cortex.     

Ocular counterroll modulates the preferred direction of saccade-related pontine burst neurons in the monkey

Saccade-related burst neurons in the paramedian pontine reticular formation (PPRF) of the head-restrained monkey provide a phasic velocity signal to extraocular motoneurons for the generation of rapid eye movements. In the superior colliculus (SC), which directly projects to the PPRF, the motor command for conjugate saccades with the head restrained in a roll position is represented in a reference frame in between oculocentric and space-fixed coordinates with a clear bias toward gravity. Here we studied the preferred direction of premotor burst neurons in the PPRF during static head roll to characterize their frame of reference with respect to head and eye position. In 59 neurons (short-lead, burst-tonic, and long-lead burst neurons), we found that the preferred direction of eye displacement of these neurons changed, relative to head-fixed landmarks, in the horizontal-vertical plane during static head roll. For the short-lead burst neurons and the burst-tonic group, the change was about one-fourth of the amount of ocular counterroll (OCR) and significantly different from a head-centered representation. In the long-lead burst neurons, the rotation of the preferred direction showed a larger trend of about one-half of OCR. During microelectrical stimulation of the PPRF (9 sites in 2 monkeys), the elicited eye movements rotated with about one-half the amount of OCR. In a simple pulley model of the oculomotor plant, the noncraniocentric reference frame of the PPRF output neurons could be reproduced for recently measured pulley positions, if the pulleys were assumed to rotate as a function of OCR with a gain of 0.5. We conclude that the saccadic displacement signal is transformed from a representation in the SC with a clear bias to gravity to a representation in the PPRF that is closely craniocentric, but rotates with OCR, consistent with current concepts of the oculomotor plant.     

Spontaneous synchronized burst firing of subthalamic nucleus neurons in rat brain slices measured on multi-electrode arrays

The current study presents an organotypic rat midbrain slice culture that served as a consistent and informative framework, where the STN neurons and their interconnectivity were closely examined with respect to electrophysiological and pharmacological properties. From multi-electrode array recordings, it was found that the majority of STN neurons spontaneously fired in bursts rather than tonically under control conditions, and the neural activity between pairs of burst-firing STN neurons was tightly correlated. This spontaneous synchronized burst firing was also affected by a glutamate receptor antagonist, yet unaffected by a GABA receptor antagonist. Moreover, even when the STN was isolated from all its known external inputs, spontaneous synchronized burst firing was still observed under control conditions and consistently switched to tonic firing following the application of a glutamate receptor antagonist. Therefore, the results indicated the existence of glutamatergic projections to the STN in the slice preparation, and these excitatory synaptic connections appeared to originate from axon collaterals within the STN rather than other basal ganglia nuclei. It could be concluded that the STN neurons and their interconnectivity are essential requirements in the rat brain slice preparation to produce spontaneous synchronized burst firing.     

Calcium dynamics underlying pacemaker-like and burst firing oscillations in midbrain dopaminergic neurons: a computational study

A mathematical model of midbrain dopamine neurons has been developed to understand the mechanisms underlying two types of calcium-dependent firing patterns that these cells exhibit in vitro. The first is the regular, pacemaker-like firing exhibited in a slice preparation, and the second is a burst firing pattern sometimes exhibited in the presence of apamin. Because both types of oscillations are blocked by nifedipine, we have focused on the slow calcium dynamics underlying these firing modes. The underlying oscillations in membrane potential are best observed when action potentials are blocked by the application of TTX. This converts the regular single-spike firing mode to a slow oscillatory potential (SOP) and apamin-induced bursting to a slow square-wave oscillation. We hypothesize that the SOP results from the interplay between the L-type calcium current (I(Ca,L)) and the apamin-sensitive calcium-activated potassium current (I(K,Ca,SK)). We further hypothesize that the square-wave oscillation results from the alternating voltage activation and calcium inactivation of I(Ca,L). Our model consists of two components: a Hodgkin-Huxley-type membrane model and a fluid compartment model. A material balance on Ca(2+) is provided in the cytosolic fluid compartment, whereas calcium concentration is considered constant in the extracellular compartment. Model parameters were determined using both voltage-clamp and calcium-imaging data from the literature. In addition to modeling the SOP and square-wave oscillations in dopaminergic neurons, the model provides reasonable mimicry of the experimentally observed response of SOPs to TEA application and elongation of the plateau duration of the square-wave oscillations in response to calcium chelation.     

Do neurons in the motor cortex encode movement direction? An alternative hypothesis

Previous investigations by Georgopoulos et al. of cell activities in the primate motor cortex during the execution of voluntary arm movements have shown that these cells are characterized by tuning properties related to the direction of hand trajectories. Here, it is demonstrated that these findings do not necessarily imply that cortical cells encode spatial features of hand movements and an alternative hypothesis is considered according to which cortical cells encode muscle state variables. It is shown that this hypothesis would lead both to the single-cell activities and to the population behaviors observed by Georgopoulos et al.     

Antidromic firing occurs spontaneously on thalamic relay neurons: triggering of somatic intrinsic burst discharges by ectopic action potentials

Eighty-two identified thalamocortical relay neurons were recorded extracellularly in the ventral posterior thalamic nucleus in 29 urethane-anaesthetized rats. Electrical stimulations were applied to the contralateral vibrissae or to the ipsilateral neocortex for ortho- or antidromic activation. The critical period following a known somatic action potential and during which no antidromic response could reach the soma was systematically determined for each cell using a collision test. Thus, the possible ectopic axonal origin of a given impulse could be determined. Thalamic neurons displayed either tonic or phasic firing modes, the latter characterized by episodes of rhythmic high-frequency burst discharges. The present results suggest that such bursts were generated at the soma and probably involved an intrinsic mechanism, since: (1) a modulation of the somatic excitability with an excitatory or inhibitory amino acid affected the intra-burst structure; (2) an antidromic test action potential collided with the second or any of the later impulses of such bursts; (3) an orthodromic activation could evoke a burst structurally similar to a natural one; and (4) the duration of the first interval of such an evoked burst was always inferior to the sum of the critical period plus the antidromic conduction time, ruling out the possibility that it might have been entirely ectopically generated on thalamic terminals. The results further show that a spontaneous ectopic axonal impulse could trigger a somatic burst, since: (1) an electrically-evoked antidromic action potential could trigger a burst structurally similar to a spontaneous one; (2) on 42% of the tested thalamic cells, a known antidromic action potential delivered during the critical period following a spontaneous single impulse could not collide with it: in many cases such non-collisions were seen with the first action potential of a burst; and (3) with increasing ionophoretic doses, GABA could: (i) convert bursts to single action potentials, while the ortho- but not the antidromic responses were abolished, (ii) block these single impulses at similar doses than those which abolished known antidromic ones, and (iii) multiply by a factor of 3 the probability of testing an ectopic action potential. On 70% of the cells tested, such GABA-isolated impulses could be proved to have been ectopically generated. Finally, ectopic impulses have never been observed during periods of tonic firing, indicating that such a feature was not an experimental artifact.(ABSTRACT TRUNCATED AT 400 WORDS)     

Antidromic firing occurs spontaneously on thalamic relay neurons: triggering of ectopic action potentials by somatic intrinsic burst discharges

Possible dynamic relationships between orthodromically conducted somatic bursts and antidromic impulses arising from presynaptic endings of thalamocortical neurons were explored. Evoked or spontaneous bursts were recorded from 125 identified thalamic relay neurons in 36 anesthetized rats using extracellular microelectrodes. Evoked bursts were obtained by electrical stimulation of either the neocortex or the peripheral activation field. Spontaneous antidromic firing appeared only during periods of (or an expected) rapid somatic intrinsic burst discharge. Ectopic axonal impulses occurred either separately, or clustered in doublets or triplets having relatively long-lasting intervals; these slow bursts represented a proportion of about 12% of evoked and 20% of spontaneous whole bursts. Separate ectopic action potentials could also appear several milliseconds after rapid bursts, producing peculiar long last-interval bursts; about 15% of the whole bursts were of this long interval type. The probability that an ectopic axonal impulse will occur after a rapid burst increases with the number of its action potentials, suggesting that the duration of orthodromic burst firing might contribute to the triggering of ectopic impulses. For 52% of the neurons tested, the activation threshold of their axon terminals decreased just before or immediately after rapid somatic bursts. Since no excitability changes were observed in thalamocortical axons of the white matter, the ectopic action potential generators were probably located on presynaptic endings. During a transient deafferentation of thalamic neurons induced by intrathalamic microinjection of a magnesium solution, neither burst activity nor spontaneous antidromic firing were observed, suggesting that thalamic orthodromic burst discharges are required for presynaptic impulse generation. In conclusion, somatic intrinsic bursts traveling orthodromically along thalamocortical axons might be involved in triggering presynaptic impulses on parent and possibly on nearby thalamic cells. Since a spontaneous antidromic action potential is able to trigger a rapid burst [Pinault (1988) Eur. J. Neurosci. Suppl. P. 246; Pinault and Pumain (1989) Neuroscience 31, 625-637], it is postulated that excitatory interactions between presynaptic endings might be involved in intrinsic burst synchronization processes.     

Role of EPSPs in initiation of spontaneous synchronized burst firing in rat hippocampal neurons bathed in high potassium

1. Spontaneous discharges that resemble interictal spikes arise in area CA3 b/c of rat hippocampal slices bathed in 8.5 mM [K+]o. Excitatory postsynaptic potentials (EPSPs) also appear at irregular intervals in these cells. The role of local synaptic excitation in burst initiation was examined with intracellular and extracellular recordings from CA3 pyramidal neurons. 2. Most (70%) EPSPs were small (less than 2 mV in amplitude), suggesting that they were the product of quantal release or were evoked by a single presynaptic action potential in another cell. It is unlikely that most EPSPs were evoked by a presynaptic burst of action potentials. Indeed, intrinsic burst firing was not prominent in CA3 b/c pyramidal cells perfused in 8.5 mM [K+]o. 3. The likelihood of occurrence and the amplitude of EPSPs were higher in the 50-ms interval just before the onset of each burst than during a similar interval 250 ms before the burst. This likely reflects increased firing probability of CA3 neurons as they emerge from the afterhyperpolarization (AHP) and conductance shunt associated with the previous burst. 4. Perfusion with 2 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a potent quisqualate receptor antagonist, decreased the frequency of EPSPs in CA3 b/c neurons from 3.6 +/- 0.9 to 0.9 +/- 0.3 (SE) Hz. Likewise, CNQX reversibly reduced the amplitude of evoked EPSPs in CA3 b/c cells. 5. Spontaneous burst firing in 8.5 mM [K+]o was abolished in 11 of 31 slices perfused with 2 microM CNQX.(ABSTRACT TRUNCATED AT 250 WORDS)     

Stimulation of the pedunculopontine tegmental nucleus in the rat produces burst firing in A9 dopaminergic neurons.

Stimulation of the medial prefrontal cortex in the rat produces events in midbrain dopaminergic neurons which resemble natural bursts, and which are closely time-locked to the stimulation, albeit with a very long latency. As a consequence, we have previously argued that such bursts are polysynaptically generated via more proximal excitatory amino acidergic afferents, arising, for example, from the pedunculopontine tegmental nucleus. In the present study, single-pulse electrical stimulation applied to this nucleus (and other sites in the rostral pons) was found to elicit responses in the majority of substantia nigra (A9) dopaminergic neurons. Responses usually consisted of long-latency, long-duration excitations or inhibition-excitations. Thirty-seven percent of responses (currents combined) elicited by stimulation of the pedunculopontine tegmental nucleus contained time-locked bursts, the bursts being embedded in the long-duration excitatory phases of excitation and inhibition-excitation responses. Stimulation sites located within 0.5 mm of the pedunculopontine tegmental nucleus were also effective at eliciting time-locked bursts (although less so than sites located in the nucleus itself), whereas more distal sites were virtually ineffective. For responses containing time-locked bursts, a higher percentage of stimulations produced a burst when the response was elicited from within the pedunculopontine tegmental nucleus than when it was elicited from outside: the bursts themselves having a very long latency (median of 96.2 ms; shorter than that of medial prefrontal cortex-induced bursts). Finally, although there was no difference in the distribution within the substantia nigra pars compacta of cells which exhibited time-locked bursting and those which did not, stimulation-induced bursts were elicited more frequently in dopaminergic neurons which were classified as "bursting" on the basis of their basal activity. The pedunculopontine tegmental nucleus appears to be a critical locus in the rostral pons for the elicitation of time-locked bursts in A9 dopaminergic neurons. Since time-locked bursts were more often elicited from cells which exhibited bursting under basal conditions, this suggests that rostral pontine sites, in particular the pedunculopontine tegmental nucleus, may play a role in the natural burst activity of dopaminergic neurons. Given that bursts in dopaminergic neurons are generated in response to primary and secondary reinforcers, the projection from the pedunculopontine tegmental nucleus could be one means by which motivationally relevant information (arising, for example, from the medial prefrontal cortex) reaches these cells.