Single neuronal responses in medial prefrontal cortex during cocaine self-administration in freely moving rats.

Chronic single neuronal recording techniques were applied to investigate the involvement of the medial prefrontal cortex (mPFC) during cocaine self-administration in the rat. Rats were trained to press a lever for cocaine under continuous reinforcement and fixed ratio schedules. Different patterns of phasic neuronal activity changes were found to be associated with lever-pressing for cocaine. The neuronal responses could be classified into five categories: 1) increases in neuronal firing before the lever press (15 out of 121 neurons, 12.4%); 2) decreases in neuronal firing before the lever press (13 neurons, 10.7%); 3) increases in neuronal firing after cocaine infusion (4 neurons, 3.3%); 4) decreases in neuronal firing after cocaine infusion (32 neurons, 26.4%); and 5) no alteration of neuronal activity throughout the self-administration session (67 neurons, 55.4%). The anticipatory responses, i.e., neuronal activity appearing before the lever press, were observed for both the continuous reinforcement and fixed ratio schedules. In a few cases, alteration of firing rate was not observed for the first lever press but appeared before subsequent lever presses in fixed ratio schedules. Eliminating cocaine abolished the inhibitory neuronal responses observed after lever press, suggesting that these inhibitory responses after cocaine self-administration were attributable to the pharmacologic effect of cocaine. The data provide initial electrophysiological evidence that the mPFC may play a role in mediating the task sequencing which leads to cocaine self-administration.

Neuronal spike activity in rat nucleus accumbens during cocaine self-administration under different fixed-ratio schedules.

Chronic ensemble recording techniques were used to investigate neuronal activity in the nucleus accumbens in freely moving rats during different cocaine self-administration schedules. The issue of concern in this study was the role of nucleus accumbens in initiating and sustaining cocaine self-administration. Specifically, to determine the nature of the neuronal activity, either motor or motivational, which precedes the multiple bar presses required to self-administer cocaine and of the post-lever press neuronal response, we used conventional fixed ratio-5, fixed ratio-10, and modified fixed ratio-3 schedules. In the modified fixed ratio-3 schedule, the first lever press resulted in retraction of the lever for 2 s; the second lever press retracted the lever and turned on a cue light; the third lever press turned off the cue light and delivered cocaine (1.0 mg/kg) intravenously. In the fixed ratio-5 and -10 schedules, rats continuously pressed the lever 5 or 10 times, respectively, to obtain a single infusion of cocaine. Phasic alterations in neural spike activity were observed in 50% of nucleus accumbens neurons before (termed "anticipatory" responses) and after lever pressing for cocaine self-administration. Neurons with anticipatory responses typically exhibited such responses for all lever presses in the modified fixed ratio-3, fixed ratio-5, and fixed ratio-10 schedules, but instances were found when the activity correlate was absent. In addition, some neurons had a prominent alteration in firing rate lasting 1-5 min after cocaine self-administration, and some of these neurons also had anticipatory responses. When cocaine was eliminated during self-administration sessions, the post-lever press inhibitory responses were largely abolished or even reversed, whereas anticipatory responses were not markedly changed when rapid lever presses occurred before behavior ceased. Post-cocaine inhibitory responses compared between self-administered and passively administered cocaine were not significantly different between these two conditions. The results suggest that nucleus accumbens may be involved in initiating general reward-seeking behaviors and action which are not exclusively associated with cocaine self-administration. Moreover, the neuronal responses in the nucleus accumbens to cocaine self-administration may play an essential role in maintaining cocaine reinforcement.

Cerebellar-responsive neurons in the thalamic ventroanterior-ventrolateral complex of rats: in vivo electrophysiology.

In vivo intracellular recordings were obtained from identified thalamocortical neurons in the ventroanterior-ventrolateral complex in urethane-anesthetized rats. This thalamic nucleus has few interneurons. Neurons that responded to cerebellar stimulation were injected intracellularly with horseradish peroxidase or biocytin and examined with light and electron microscopy (see companion paper). Intrinsic membrane properties and voltage-dependent rhythmic activity of cerebellar-responsive ventroanterior-ventrolateral neurons were similar to those described previously for thalamic neurons. Thus, in addition to conventional "fast" Na(+)-dependent spikes, rat ventroanterior-ventrolateral neurons had "slow" Ca(2+)-mediated low-threshold spikes and membrane conductances that supported rhythmic oscillations. Two modes of spontaneous activity were observed: (i) a tonic firing pattern that consisted of irregularly occurring fast spikes that predominated when the membrane potential was more positive than about -60 mV, and (ii) a rhythmic firing pattern, observed when the membrane potential was more negative than about -65 mV, composed of periodic (4-8 Hz) membrane hyperpolarizations and ramp depolarizations that often produced a low-threshold spike and a burst of fast spikes. In some neurons, spontaneous fast prepotentials were also observed, often with a relatively constant rate (up to 70 Hz). Cerebellar stimulation elicited excitatory postsynaptic potentials that in some cases appeared to be all-or-none and were similar in form to fast prepotentials. Stimulation of ipsilateral motor cortex elicited a short-latency antidromic response followed by a monosynaptic excitatory postsynaptic potential, which had a slower rise time than excitatory postsynaptic potentials evoked from cerebellum, suggesting that cortical inputs were electrotonically distal to cerebellar inputs. In the presence of moderate membrane hyperpolarization, the cortically evoked excitatory postsynaptic potential was followed by a long-lasting hyperpolarization (100-400 ms duration), a rebound depolarization and one or two cycles resembling spontaneous rhythmic activity. Membrane conductance was increased during the initial component of the long hyperpolarization, much of which was probably due to an inhibitory postsynaptic potential. In contrast, membrane conductance was unchanged or slightly decreased during the latter three-quarters of the long hyperpolarization. The amplitude of this component of the long hyperpolarization usually decreased when the membrane was hyperpolarized with intracellular current injection. Thus, both disfacilitation and an inhibitory postsynaptic potential may have contributed to the latter portion of the cortically-evoked long hyperpolarization. The cortically-evoked inhibitory postsynaptic potentials likely originated predominantly from feedforward activation of GABAergic neurons in the thalamic reticular nuclei.(ABSTRACT TRUNCATED AT 400 WORDS)

Cerebellar-responsive neurons in the thalamic ventroanterior-ventrolateral complex of rats: light and electron microscopy.

The morphology and synaptic organization of neurons in the ventroanterior-ventrolateral nucleus of rats was examined using in vivo intracellular staining techniques. Neurons were characterized electrophysiologically based on intrinsic membrane properties and synaptic responses to stimulation of motor cortex and cerebellar nuclei, as described in the companion paper. Cerebellar-responsive neurons were stained intracellularly with either horseradish peroxidase or biocytin. All stained ventroanterior-ventrolateral nucleus neurons were identified as thalamocortical neurons on anatomical (and often electrophysiological) grounds, consistent with previous findings that rat ventroanterior-ventrolateral nucleus is interneuron-sparse. Ventroanterior-ventrolateral nucleus neurons had three to eight thick primary dendrites. Proximal dendrites often exhibited a tufted branching pattern, from which many thinner, higher order dendrites arose. Dendrites branched to form a funnel-like infiltration of the neuropil that resulted in a spherical, roughly homogeneous dendritic field. The axon originated from the cell body or a proximal dendrite and coursed laterally and dorsally to innervate motor cortex. One to five axon collaterals were emitted in the rostral dorsolateral sector of the thalamic reticular nucleus; collaterals were not observed in the ventroanterior-ventrolateral nucleus or other nuclei in dorsal thalamus. The synaptic organization of the ventroanterior-ventrolateral nucleus was examined with electron microscopy, including two intracellularly labeled ventroanterior-ventrolateral nucleus neurons that were shown electrophysiologically to receive monosynaptic inputs from the cerebellum. The neuropil of rat ventroanterior-ventrolateral nucleus lacked the complexity and diversity found in corresponding thalamic nuclei of felines and primates, due to the paucity of interneurons. Vesicle-containing dendrites, dendrodendritic synapses and glomeruli were not observed. Three broad classes of presynaptic terminals were identified. (1) Small round boutons: small boutons containing densely-packed, small round vesicles that formed asymmetric synapses predominantly with the distal dendrites of thalamocortical neurons. These were the most prevalent type of bouton in the ventroanterior-ventrolateral nucleus (78% of presynaptic elements) and likely arose from the cerebral cortex. (2) Large round boutons: large terminals with loosely packed small round vesicles that made multiple asymmetric synapses with proximal and intermediate dendrites. Large round boutons comprised 8% of the neuropil, and likely arose from the cerebellar nuclei. (3) Medium size boutons with pleomorphic vesicles: medium-sized profiles containing pleomorphic vesicles that formed symmetric synapses with proximal, intermediate and distal dendrites and, less frequently, with cell bodies.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiological and pharmacological evidence for the role of the nucleus accumbens in cocaine self-administration in freely moving rats.

The goal of this study was to clarify the role of the nucleus accumbens septi (NAS) in the expression of cocaine self-administration behaviors. Rats were trained in a continuous reinforcement schedule to press a lever to activate a pump that provided an intravenous injection of cocaine. Once the rats were trained, neuronal activity in the NAS was monitored during cocaine self-administration with chronic recording techniques using permanently implanted microwires. In the NAS, 19% of 181 neurons exhibited either increased or decreased firing rates seconds prior to lever pressing (termed "anticipatory responses"), and 48% had altered, predominantly decreased, firing rates for a few minutes after lever pressing ("postcocaine responses"). Two-thirds of the neurons with anticipatory responses had postcocaine responses. Neurons with either of these response patterns were localized histologically to both core and shell regions of the NAS, with no statistically significant differences in the proportion of response types in either area. Analysis of videotaped cocaine self-administration behaviors revealed that anticipatory responses were specifically associated either with the animal orienting toward and pressing the lever or only with movements directly related to pressing the lever. Anticipatory-like phasic spike activity was not observed during similar movements unrelated to lever pressing. In some animals, D1 (SCH 23390) or D2 (pimozide) receptor antagonists were injected systemically prior to or during self-administration sessions to assess the effects of dopamine receptor blockade on anticipatory and postcocaine responses. Each antagonist, given separately, often produced extinction of lever pressing. Both antagonists blocked the post-cocaine inhibitory response of neurons that had anticipatory responses. Neither antagonist modified anticipatory unit responses, nor did they affect postcocaine inhibitory responses in neurons that did not exhibit anticipatory responses. Taken together, these results suggest that the role of the NAS in cocaine self-administration may consist of two different mechanisms: (1) An initiation or trigger mechanism, as represented by the anticipatory neuronal responses, in which the NAS participates in triggering or mediating the goal-directed behaviors (e.g., lever pressing) that lead to the acquisition of the reinforcing agent (e.g., cocaine).(ABSTRACT TRUNCATED AT 400 WORDS)

A GABA immunocytochemical study of rat motor thalamus: light and electron microscopic observations.

A light and electron microscopic study of GABA-immunoreactive neurons and profiles in the ventroanterior-ventrolateral and ventromedial nuclei of rat dorsal thalamus was conducted using antiserum raised against GABA. Less than 1% of the neurons in these motor-related nuclei exhibited GABA immunoreactivity, confirming previous reports that these nuclei are largely devoid of interneurons. Immunoreactive neurons in the ventral anterior-ventral lateral complex and ventromedial nucleus were bipolar or multipolar in shape, and tended to be smaller than non-immunoreactive neurons. GABA immunoreactivity in the neuropil consisted of labeled axon terminals and myelinated and unmyelinated axons, and was lower in the ventral anterior-ventral lateral complex and ventromedial nucleus than in neighboring thalamic nuclei. The density of neuropil immunolabeling was slightly higher in ventral anterior-ventral lateral complex than in ventromedial nucleus. GABA-immunoreactive axon terminals, collectively termed MP boutons for their medium size and pleomorphic vesicles (and corresponding to "F" profiles of some previous studies of thalamic ultrastructure), formed symmetric synapses and puncta adhaerentia contacts predominantly with large and medium-diameter (i.e. proximal) non-immunoreactive dendrites. Approximately 12 and 18% of boutons in the ventral anterior-ventral lateral complex and ventromedial nucleus, respectively, were GABA-immunopositive. Many of these immunoreactive profiles probably arose from GABAergic neurons in the thalamic reticular nucleus, substantia nigra pars reticulata and entopeduncular nucleus. Two types of non-immunoreactive axon terminals were distinguished based on differences in morphology and synaptic termination sites. Boutons with small ovoid profiles and round vesicles that formed prominent asymmetric synapses onto small-diameter dendrites were observed. Mitochondria were rarely observed within these boutons, which arose from thin unmyelinated axons. These boutons composed approximately 82 and 74% of boutons in the ventral anterior-ventral lateral complex and ventromedial nucleus, respectively, and were considered to arise predominantly from neurons in the cerebral cortex. In contrast, boutons with large terminals that contained round or plemorphic vesicles and formed multiple asymmetric synapses predominantly with large-diameter dendrites were also observed. Puncta adhaerentia contacts were also common. Mitochondria were numerous within large boutons with round vesicles, which arose from myelinated axons. Many of the large boutons were likely to have originated from neurons in the cerebellar nuclei. Approximately 6% of the boutons in the ventral anterior-ventral lateral complex and 8% in ventromedial nucleus were of the large type.(ABSTRACT TRUNCATED AT 400 WORDS)

Quantitative Golgi study of anatomically identified subdivisions of motor thalamus in the rat.

A Golgi study of neurons in the ventroanterior-ventrolateral complex (VAL) and ventromedial (VM) nucleus in the dorsal thalamus of rats was performed. To facilitate the delineation of subdivisions of these nuclei, some animals received injections of horseradish peroxidase (HRP) into the afferent and efferent fields of VAL and VM, and alternate sections were processed for the histochemical detection of HRP. As an adjunct to subjective observations, a multivariate statistical analysis of morphometric variables was performed to provide an objective assessment of neuronal morphology. All Golgi-stained neurons in VAL and VM were tentatively identified as projection neurons; no cells with morphological features commonly ascribed to thalamic interneurons were impregnated. Four classes of morphologically distinct neurons were identified in VAL. Type 1 neurons, the most commonly impregnated cell, were found throughout the extent of VAL and resembled "tufted" or "multipolar bush" neurons described previously in many thalamic nuclei. The remaining three neuronal types differed in a number of morphometric parameters and were differentially distributed throughout VAL. Type 2 neurons, distinguished in part by dendritic spine morphology and elongated bipolar dendritic fields, were found only in the rostral sector of the dorsal division of VAL (VALD). Type 3 neurons, characterized by a large and evenly distributed dendritic field, were situated in rostral VAL (all subdivisions). Type 4 neurons had small soma and dendritic dimensions and were located in the ventromedial aspect of the ventral division of VAL (VALV) adjacent to VM. In contrast, the vast majority of neurons in VM were considered to be a single morphological class (similar in form to type 4 neurons in VAL), although a rarely impregnated second type of neuron was also observed. The apparent scarcity of interneurons in VAL and VM is consistent with previous evidence that the synaptic organization of motor thalamus in the rat is markedly different from that of higher-order mammals. Speculation about the functional attributes of the neuronal types in VAL and VM is necessarily restricted to considerations of afferent and efferent relations, since "motor modality" functions of neurons in these nuclei have yet to be elucidated.

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.

Mesocortical dopaminergic neurons. 2. Electrophysiological consequences of terminal autoreceptor activation.

Measurement of drug- and stimulation-induced changes in the electrical excitability of dopaminergic terminals was employed to assess the effects of stimulation of dopamine terminal autoreceptors in the prefrontal cortex in urethane-anesthetized rats. Systemic or local administration of amphetamine decreased, whereas systemic administration of haloperidol increased the excitability of prefrontal cortical dopaminergic terminals of ventral tegmental area dopaminergic neurons. Mesoprefrontal dopaminergic terminal excitability was also responsive to spontaneous and stimulation-induced alterations in the rate of impulses reaching the terminal fields. These results are comparable to those previously reported for nigrostriatal and mesoaccumbens dopaminergic neurons, and are discussed with regard to the operational characteristics of autoinhibition in the mesocortical dopaminergic system.

Electrophysiologically identified nigral dopaminergic neurons intracellularly labeled with HRP: light-microscopic analysis.

Intracellular recordings were obtained in vivo from neurons of the rat substantia nigra, pars compacta. Neurons that were identified as dopaminergic by a variety of electrophysiological criteria, including antidromic activation from ipsilateral neostriatum or globus pallidus, were microiontophoretically injected with horseradish peroxidase and examined at the level of the light microscope. Dopaminergic neurons were of medium size and had ovoid, polygonal, or fusiform cell bodies that emitted from 3-6 primary dendrites. Much of the sparse and relatively unbranched dendritic arborization of these neurons remained within pars compacta, except for 1 or 2 large dendrites that were directed ventrally or ventrolaterally into pars reticulata, roughly perpendicular to the plane of the pars compacta. In coronal sections, the dendrites of ovoid- or polygonal-shaped pars compacta neurons were oriented mainly along the dorsoventral axis, whereas fusiform-shaped neurons had dendrites that were oriented primarily mediolaterally. Although some of the dendrites of dopaminergic neurons exhibited variations in diameter, most were not markedly varicose. Dendrites were sometimes sparsely invested with spinelike appendages or other dendritic extrusions, particularly along their distal portions. The axons of dopaminergic pars compacta neurons were emitted from primary or proximal secondary dendrites, and were extremely fine processes, 0.5 micron or less in diameter. No local axon collaterals were observed.

Autoreceptor-mediated changes in dopaminergic terminal excitability: effects of potassium channel blockers.

The effects of the potassium channel blockers, 4-aminopyridine (4-AP) and tetraethylammonium (TEA), on autoreceptor-mediated changes in dopaminergic terminal excitability were examined in urethane-anesthetized rats. Local infusions of 4-AP or TEA into neostriatal terminal fields of nigral dopaminergic neurons led to marked decreases in terminal excitability, as measured by the increase in stimulating current required to activate the neurons antidromically from the site of the infusion. The decreased excitability resulting from 4-AP could be reversed by subsequent i.v. injection of haloperidol, and was blocked in rats that had been depleted of endogenous dopamine by prior treatment with alpha-methyl-p-tyrosine (AMpT). Thus, the decrease in excitability elicited by the potassium channel-blockers was indirect, and apparently due to increased autoreceptor stimulation resulting from enhanced transmitter release. In addition, co-infusion of 4-AP and apomorphine in AMpT-treated animals led to decreased terminal excitability that did not differ from the effects of apomorphine alone, indicating that 4-AP did not block the effects of exogenous autoreceptor agonist administration. These results provide in situ electrophysiological evidence that autoreceptor-mediated processes occurring at dopaminergic terminals are not mediated by 4-AP- or TEA-sensitive potassium channels. Furthermore, our findings suggest that, as in other types of presynaptic terminals, blockade of voltage-sensitive potassium channels in dopamine terminals leads to enhanced release of transmitter.

Improving survival in the treatment of congenital diaphragmatic hernia.

Thirty-two infants were treated for congenital diaphragmatic hernia at our institution from 1979 to 1984. Eight were in no or minimal distress at birth and had operative intervention when they were more than 24 hours old; survival was 100%. The remaining 24 neonates required immediate intubation and ventilation followed by operation at less than 12 hours of age. Overall survival was 54%; survival was 31% (4 of 13 patients, Group 1) in the first three years of the series and 82% (9 of 11 patients, Group 2) in the last three years (p less than 0.001). Apgar score, gestational age, birth weight, and incidence of associated congenital heart disease were equal for the two groups (all, p greater than 0.05). The two groups also were examined with reference to alveolar-arterial oxygen differences P(A-a)O2 and mean airway pressure (MAP). The best preoperative P(A-a)O2 was greater than 600 mm Hg for 7 neonates in Group 1 and 6 in Group 2, and survival was 0% and 71%, respectively (p less than 0.001). Infants with a postoperative MAP of 13 cm H2O or greater had a higher mortality (100% in Group 1 and 50% in Group 2, p greater than 0.05). Our treatment protocol was studied to determine those methods related to improved survival. Sodium bicarbonate infusion was used earlier in Group 2 as a prophylaxis against persistent fetal circulation (PFC) (p greater than 0.05). The incidence of severe PFC dropped from 85 to 54% (p greater than 0.05). Higher ventilator rates rather than pressures were used to achieve equally effective ventilation.(ABSTRACT TRUNCATED AT 250 WORDS)

Antidromic activation of dorsal raphe neurons from neostriatum: physiological characterization and effects of terminal autoreceptor activation.

Three types of neurons, distinguished on the basis of their spontaneous firing rates and patterns, extracellularly recorded waveforms and responses to neostriatal stimulation, were observed in the dorsal raphe nucleus in urethane-anesthetized rats. Type 1 neurons (presumed to be serotonergic) fired spontaneously from 0.1 to 3 spikes/s in a regular pattern, with initial positive-going bi- or triphasic action potentials. Type 1 cells exhibited long-latency antidromic responses to neostriatal stimulation (mean +/- S.E.M. 24.9 +/- 0.3 ms) that sometimes occurred at discrete multiple latencies, and supernormal periods persisting up to 100 ms following spontaneous spikes. Type 2 cells fired spontaneously in an irregular, somewhat bursty pattern from 0 to 2 spikes/s with initial negative-going biphasic spikes, and were antidromically activated from neostriatal stimulation at shorter latencies than Type 1 cells (21.8 +/- 0.9 ms). Type 3 cells were characterized by initial positive-going biphasic waveforms and displayed a higher discharge rate (5-30 spikes/s) than Type 1 or Type 2 cells. Type 3 cells could not be antidromically activated from neostriatal stimulation. The relatively long conduction time to neostriatum of the Type 1 presumed serotonergic neuron is discussed with respect to previous interpretations of the synaptic action of serotonin in the neostriatum. In conjunction with these antidromic activation studies, the neurophysiological consequences of serotonergic terminal autoreceptor activation were examined by measuring changes in the excitability of serotonergic terminal fields in the neostriatum following administration of the serotonin autoreceptor agonist, 5-methoxy-N,N-dimethyltryptamine (5-MeODMT). The excitability of serotonergic terminal fields was decreased by intravenous injection of 40 micrograms/kg 5-MeODMT, and by infusion of 10-50 microM 5-MeODMT directly into the neostriatum. These results are interpreted from the perspective of mechanisms underlying autoreceptor-mediated regulation of serotonin release.