Higher magnitude accumbal phasic firing changes among core neurons exhibiting tonic firing increases during cocaine self-administration

Studies using i.v. cocaine self-administration in rats have documented rapid-phasic changes in the firing rate of nucleus accumbens neurons within seconds of cocaine-reinforced lever presses, as well as changes that occur over the course of the cocaine self-administration experiment, i.e. tonic changes in firing rate. During the self-administration period of the experiment, individual neurons exhibit either a tonic increase, a tonic decrease, or no tonic change in firing rate, relative to the neuron's firing rate during the pre-drug period. We evaluated whether rapid-phasic changes in firing were differentially associated with tonically reduced or tonically elevated firing of nucleus accumbens core and shell neurons in cocaine self-administering rats. Rapid-phasic firing patterns within seconds of the cocaine-reinforced lever press were exhibited predominantly by core neurons that also exhibited tonic increases in firing. Conversely, core neurons that did not exhibit such rapid-phasic firing patterns were more likely to show tonically reduced firing. Moreover, core neurons were more likely than shell neurons to exhibit: 1) tonic increases in firing and 2) rapid-phasic increases in firing preceding the cocaine-reinforced lever press. These differences between accumbens subterritories may be related to their distinct involvement in operant responding; the present findings are consistent with an emerging literature which implicates shell in contextual stimulus-induced responding, and core in processing the instrumental response via its discrete output to classic basal ganglia structures. The distinct tendency of the core to exhibit increased firing, coupled with its dichotomous firing outputs (i.e. tonic decreases without rapid phasic responses or tonic increases with rapid phasic responses), may reflect particular sensitivity of these neurons to excitatory limbic afferent signaling involved in instrumental responding. Enhanced phasic responsivity in the core may be an integral component of the mechanism inherent in normal reward processing which is subverted by chronic drug exposure.

Fluctuations in somatosensory responsiveness and baseline firing rates of neurons in the lateral striatum of freely moving rats: effects of intranigral apomorphine

Somatosensory responsiveness and baseline firing rates of 102 striatal neurons were studied in freely moving rats. For individual neurons, mean levels of responsiveness and baseline firing fluctuated unpredictably in direction and magnitude and independently of each other throughout an experiment. Following microinjections of apomorphine into the substantia nigra, which were used as a means of reducing nigral output activity, the magnitude of fluctuations in striatal somatosensory responsiveness significantly increased, while the magnitude of fluctuations in baseline firing was unaltered. The receptive zones of 54 neurons studied in control experiments remained stable, whereas receptive zones changed in 12 of 25 neurons studied after apomorphine microinjection. Normal nigrostriatal dopamine transmission appears to selectively restrict the magnitude of fluctuations in responsiveness of striatal neurons to corticostriatal synaptic input and may exert additional control over afferent projections from cutaneous receptive zones to these neurons.

Tonic firing of rat nucleus accumbens neurons: changes during the first 2 weeks of daily cocaine self-administration sessions

Activity of single neurons in the nucleus accumbens (NAcc) of rats was recorded extracellularly on the 2nd and 15th days of intravenous cocaine self-administration. Each of the two electrophysiological recording sessions consisted of three successive phases: a pre-drug baseline recording period, a cocaine self-administration session, and a post-drug recording period. Firing of individual neurons was typically inhibited during the self-administration session, relative to the pre-drug period. The inhibition was greater on the 15th day relative to the 2nd day. Additionally, firing rates during the pre-drug period and the self-administration session were typically lower on the 15th day as compared to the 2nd day. The present data are consistent with previous acute electrophysiological findings and are in line with the hypothesis that repeated drug self-administration engenders changes in the mesoaccumbens pathway that contribute to drug addiction.

Anesthetics eliminate somatosensory-evoked discharges of neurons in the somatotopically organized sensorimotor striatum of the rat

The somatotopic organization of the lateral striatum has been demonstrated by anatomical studies of corticostriatal projections from somatosensory and motor cortices and by single-cell recordings in awake animals. The functional organization in the rat, characterized thus far in the freely moving rat preparation, could be mapped more precisely if a stereotaxic, and possibly an anesthetized, preparation could be used. Because striatal discharges evoked by innocuous somatosensory stimulation are used in mapping, this study tested whether such discharges can be observed during anesthesia, encouraged by responsiveness during anesthesia in somatosensory cortical layers projecting to the striatum. Electrode tracks through lateral striatum of anesthetized rats (pentobarbital or ketamine) revealed spontaneously discharging neurons but no discharges evoked by somatosensory examination (passive manipulation and cutaneous stimulation of 14 body parts). Similar tracks in chronically implanted rats showed evoked firing at numerous sites during wakefulness but not during anesthesia (pentobarbital or urethane). Comparisons of the activity of individual neurons between wakefulness and anesthesia showed that pentobarbital, ketamine, chloral hydrate, urethane, or metofane eliminated evoked firing and suppressed spontaneous firing. Recovery time was greater for neural than for behavioral measures. Thus, mapping as proposed is ruled out, and more importantly, the data show that somatotopically organized lateral striatal neurons stop discharging in response to natural stimulation during anesthesia. Available data indicate they do not reach threshold in response to depolarizations produced by glutamatergic corticostriatal synaptic transmission projected from the somatosensory cortex. These data and demonstrations of anesthetic-induced imbalances in most striatal neurotransmitters emphasize that many results regarding striatal physiology and pharmacology during anesthesia cannot be extrapolated to behavioral conditions, thus indicating the need for more empirical testing in conscious animals.

Phasic firing time locked to cocaine self-infusion and locomotion: dissociable firing patterns of single nucleus accumbens neurons in the rat

The activity of single nucleus accumbens (NAcc) neurons of rats was extracellularly recorded during intravenous cocaine self-administration sessions (0.7 mg/kg per infusion, fixed ratio 1). We reported previously that NAcc neurons showed a change, usually a decrease, in firing rate during the first 1 min after the cocaine-reinforced lever press. This postpress change was followed by a progressive reversal of that change, which began within the first 2 min after the press and was not complete until the last 1 min before the next lever press (termed the change + progressive reversal firing pattern). In the present study we documented a regular pattern of locomotion that occurred in parallel with the change + progressive reversal firing pattern. This observation suggested that discharges time locked to locomotion may determine the change + progressive reversal firing pattern. However, 55% of the neurons failed to show firing time locked to locomotion that could have contributed to the change + progressive reversal firing pattern. Moreover, for all neurons, the change + progressive reversal firing pattern was apparent even if the calculation of firing rate excluded all periods of locomotion. The present data showed that the change + progressive reversal firing pattern is not solely attributable to phasic changes in firing time locked to the execution of locomotion. The change + progressive reversal firing pattern closely mirrors changes in drug level and dopamine overflow observed by previous researchers and may thus be a component of the neurophysiological mechanism by which drug level regulates drug-taking behavior during an ongoing self-administration session.

Tonic inhibition of single nucleus accumbens neurons in the rat: a predominant but not exclusive firing pattern induced by cocaine self-administration sessions

Inhibition of nucleus accumbens neurons is hypothesized to be a mechanism that contributes to the reinforcing (addictive) effects of cocaine and other drugs. To test this hypothesis, the activity of single nucleus accumbens neurons of rats was recorded extracellularly during cocaine self-administration sessions. Fifty-eight percent of neurons were tonically inhibited during cocaine self-administration relative to predrug baseline; thirty-one percent were tonically excited. A majority of both excited and inhibited neurons showed phasic increases in firing time-locked to self-infusion. The high percentage of tonically inhibited neurons is in line with the strong inhibitory effects of cocaine and amphetamine observed in previous anesthetized and slice recording studies; however, the prevalence of inhibition, relative to excitation, was less than might have been expected on the basis of the earlier recording studies. The present results support the hypothesis that accumbal (tonic) inhibition contributes to drug taking. However, they also suggest that changes in firing that are distinct from the tonic inhibition may additionally contribute to accumbal mediation of drug taking and drug addiction. The uniform observation of predominant inhibition among the various electrophysiology studies is consistent with the heuristic value of anesthetized and slice recording methods in identifying potential neurophysiological correlates of drug taking; however, the existence of firing patterns (e.g., phasic increases) uniquely associated with self-administration behavior (and thus absent in anesthetized and slice studies), as well as the unique presence of the primary behavior of interest in studies such as the present one, underscores the importance of conducting electrophysiological investigations of drug taking and drug addiction in the self-administering animal in parallel with anesthetized and slice studies whenever possible.

A removable peripheral device for intracerebral microinjection in freely moving rats

This report describes an original, removable, peripheral device: a microinjector that injects a single dose of fluid chemical into a particular site in the brain of freely moving rats. The microinjector was constructed especially for simultaneous use with a microdrive for microelectrode recording of single neuron activity but can be utilized alone. The microinjector is simple, lightweight, small, and easy to operate and maintain.

Neurons in accumbens subterritories of the rat: phasic firing time-locked within seconds of intravenous cocaine self-infusion

Individual neurons were recorded extracellularly in the nucleus accumbens (NAcc) of rats during cocaine self-administration sessions. NAcc neurons exhibited a variety of phasic changes in firing rate within the few seconds before and/or after cocaine self-infusion. Analysis of the topographical distribution of the phasic firing patterns showed that there were no differences between NAcc subterritories in the nature of phasic changes in firing exhibited by neurons in relation to cocaine self-infusion. However, the prevalence of phasic firing was lower in the border regions of the caudal shell and within the caudal shell itself relative to the remainder of the NAcc.

Firing rate dependent effect of cocaine on single neurons of the rat lateral striatum

Cocaine's effects on striatal neurons related to vertical head movement were studied during a task requiring vertical head movement. The proportion of long-distance head movements was increased by low doses but decreased by the high dose, which produced stereotypic head bobbing. At all doses, normally low firing rates related to movement were elevated to a greater degree than were normally high firing rates. At the high dose, normally high firing rates were strongly suppressed, a restriction which may contribute to the decreased behavioral diversity characteristic of stereotypy.

Operant behavior during sessions of intravenous cocaine infusion is necessary and sufficient for phasic firing of single nucleus accumbens neurons

The activity of individual accumbens neurons in rats was recorded in relation to intravenous cocaine infusions that were either response (i.e., lever press) contingent or response non-contingent. Neural firing was additionally recorded in relation to non-reinforced lever presses. Comparisons of firing under the three conditions showed that operant behavior was necessary and sufficient for preinfusion firing to occur. Surprisingly, the same was true, in many cases, for firing that occurred during the infusion. For other neurons, firing during the infusion was unrelated to operant behavior and possibly related to infusion stimuli. The relationship to operant behavior exhibited by the majority of NAcc neurons is consistent with previous studies that demonstrated a necessary relationship between NAcc neurons and cocaine reinforced operant behavior.

Distributions of single neurons related to body parts in the lateral striatum of the rat

Single unit recordings in awake, unrestrained rats confirmed and extended previous findings regarding the functional organization of the lateral striatum. In individual electrode tracks, clusters of neurons related functionally to an individual body part were interspersed with clusters related to other body parts. The overlapping distributions of these neurons were arranged somatotopically in the dorsal-ventral dimension. The distribution of hind limb neurons was most dorsal and showed no overlap with the distribution of neurons related to oral sensorimotor activity. Oral representation was most ventral of all body parts and extended to the ventral boundary of the lateral striatum. Representations of other body parts overlapped with that of the hind limb dorsally but differed primarily in the degree to which they extended ventrally. Forelimb representation extended farther ventrally than that of the hind limb, but did not extend as far ventrally as that of the neck. Despite substantial overlap in the dorsal-to-ventral order of hind limb-forelimb-neck-face representations, single neurons showed no evidence of overlap, or convergence, of body parts. These data provide a more complete description of the dorsal-ventral somatotopy in the lateral striatum of the rat, which as shown previously, extends throughout the medial-lateral, and much of the anterior-posterior dimensions of the lateral striatum.

Loss of lever press-related firing of rat striatal forelimb neurons after repeated sessions in a lever pressing task

Lateral striatal neurons that fire phasically in relation to active movement of the contralateral forelimb (determined via daily sensorimotor examination) were studied during acquisition of cued lever pressing. Rats were trained to lift the contralateral forepaw from the floor to press a lever in the presence of a tone. The tone was presented 70 times per day (session) for 18 consecutive days. All animals acquired the task, evidenced by gradual improvements across sessions and eventual asymptotic levels in tone discrimination, reaction time, and efficiency of the lever press. Forelimb neurons fired in relation to the lever press during early sessions of acquisition but not after repeated sessions on the task. This difference in firing could not be attributed to differences in forelimb movements during lever pressing or to sampling from different populations of neurons in early versus late sessions. In view of evidence that striatal damage impairs acquisition of motor skills, the change in firing suggests that the striatal activity present in early sessions may be necessary for the acquisition of, but not the automatic performance of, learned motor responses.

Low-dose amphetamine elevates movement-related firing of rat striatal neurons

To study the striatal role in amphetamine's stimulant effects on motor behavior, single neurons were recorded in the dorsolateral striatum of unrestrained rats before and after amphetamine injection (0.5 or 1.0 mg/kg, i.p.). Comparisons of firing were made between similar motor behaviors before and after injection. Mean locomotor firing rates increased 5% to 276% within 30 min after injection and reversed within 2 h. Firing related to specific head- or forelimb-movements, which were similar in all measured parameters before and after injection, was elevated several hundred percent after injection and then reversed, the time course paralleling that of the stimulant effect on these movements. Elevation of movement-related striatal firing rates by low doses of the psychomotor stimulant is in line with established increases in firing rate normally observed for striatal neurons related to motor behavior.

Phasic firing of single neurons in the rat nucleus accumbens correlated with the timing of intravenous cocaine self-administration

To examine potential neural mechanisms involved in cocaine self-administration, the activity of single neurons in the nucleus accumbens of rats was recorded during intravenous cocaine self-administration. Lever pressing was reinforced according to a fixed-ratio 1 schedule. On a time base comparable to the interinfusion interval, half the neurons exhibited phasic firing patterns time locked to the cocaine reinforced level press. For almost all neurons, this pattern consisted of a change in firing rate postpress, typically a decrease, followed by a reversal of that change. The postpress change was closely related to the lever press. Typically, it began within the first 0.2 min postpress and culminated within the first 1.0 min postpress. For a small portion of responsive neurons, the reversal of the postpress change was punctate and occurred within 1-3 min of either the last lever press or the next lever press so that firing was stable during much of the interinfusion interval. For the majority of neurons, the reversal was progressive; it began within 2 min after the previous level press, and it was not complete until the last 0.1-2.0 min before the next lever press. The duration of this progressive reversal, but not of the postpress change, was positively correlated with the interinfusion interval. Thus, in addition to exhibiting changes in firing related to the occurrence of self-infusion, the majority of neurons also exhibited progressive changes in firing related to the spacing of infusions. In a structure that has been shown to be necessary for cocaine self-administration, such a firing pattern is a likely neurophysiological component of the mechanism that transduces declining drug levels into increased drug-related appetitive behavior. It is, thus, a neural mechanism that may contribute to compulsive drug-maintained drug taking.

Nationalism, race, and gender: the politics of family planning in Zimbabwe, 1957-1990

In line with a general tendency of nationalists to hold pro-natalist views, African nationalists in Zimbabwe took a hostile position to family planning upon its introduction in 1957, arguing that it was part of a conspiracy to control the black population. However, it was only after the unilateral declaration of independence in 1965 by the white settlers under Ian Smith that an official policy aimed at reducing African fertility emerged. The African nationalists waged a consistent propaganda campaign against this policy, and the facilities that were established under it, as well as their personnel, became military targets during the guerrilla war in the late 1970s. After independence in 1980, the triumphant nationalists tried to maintain their pro-natalist position. But, with a postwar 'baby boom' pushing the birth rate close to four per cent by the mid-1980s, the officials in charge of economic and social development concluded that society could not sustain such a high fertility rate. Consequently, there was a reversal of policy, and by 1990 Zimbabwe had become an internationally recognized leader of family planning among developing countries. For the most part, however, these changes have taken place without any real input by African women, who remained largely excluded from the male-dominated circles in which decisions about family planning were made.

Representation of the body in the lateral striatum of the freely moving rat: single neurons related to licking

This study examined the relationship of single-neuron activity (n = 739), recorded from the lateral striatum of freely moving rats, to oral movements involved in licking single drops of liquid. Certain neurons (n = 74) fired specifically in relation to licking. Their firing rates increased during licking, but remained near zero in the absence of licking, throughout a full sensorimotor examination of the remainder of the orofacial area and all other body parts. Another category of neurons (n = 17) fired during licking but also fired in the absence of licking, during one or more other orofacial sensorimotor function(s). Lick-related neurons were located in the lateral striatum, throughout the entire anterior-posterior range studied (from +1.5 to -1.5 mm anterior-posterior, A-P, bregma = 0). Summed over the full A-P range, they were located significantly ventral to representations of the trunk and limbs. These findings extend the characterization of the somatotopic organization exhibited by lateral striatal neurons in the rat, to include representation of oral functions, consistent with converging evidence regarding the functional organization of the striatum.

Activation of single neurons in the rat nucleus accumbens during self-stimulation of the ventral tegmental area

Single neurons (n = 76) were recorded in the nucleus accumbens septi (NAS) of rats self-stimulating the ipsilateral medial forebrain bundle (MFB) at the level of the ventral tegmental area (VTA). Responses evoked by rewarding trains of stimulus pulses fell into five categories. The first category (40% of the sample) was characterized by a single discharge at invariant latency in response to individual pulses of the train, and hence was termed "tightly time locked" (TTL). Two TTL neurons were collision tested, and both showed collision, suggesting that self-stimulation of the VTA may involve antidromic, and thus direct, activation of a substantial number of NAS axons. The second category (26%) was characterized by discharges that varied in latency from pulse to pulse and hence was termed "loosely time locked" (LTL). Responses of the remainder of the sample showed no coupling to individual pulses but were categorized based on general firing patterns during the train: excited (7%), inhibited (4%), and no change (23%). Irrespective of category, immediately after the self-stimulation session, the likelihood of evoked discharge at monosynaptic latency by single pulse stimulation of the ipsilateral fimbria was reduced (relative to pre-session level), concurrent with elevations in mean firing rate and motor activity. NAS neurons thus exhibit vigorous activation, apparently both antidromically and orthodromically, in response to VTA self-stimulation. The responses of certain LTL and TTL neurons increased as a function of pulse number in the train, suggestive of integrative mechanisms important for brain stimulation reward. Conduction velocities of directly activated (TTL) axons (0.41-0.65 m/sec) were slower than those previously reported for first-stage, reward-relevant axons. Nonetheless, an implication of direct activation of NAS (and other MFB) axons is that rewarding stimulation triggers action potentials that could invade all axonal branches, including those between the stimulation site and the soma, and send synaptic signals to target neurons. Such signals from NAS neurons could contribute to the increased motor behavior accompanying MFB self-stimulation, and/or could interact with dopamine-mediated signals projected to the NAS from reward circuitry.

Representation of the body by single neurons in the dorsolateral striatum of the awake, unrestrained rat

Single cell recordings in awake monkeys and cats have demonstrated that individual body parts are represented within striatal subregions receiving projections from somatic sensorimotor cortex. Literature indicating that the lateral striatum of the rat receives similar cortical inputs and subserves sensorimotor functions prompted a study of whether this subregion contains similar representations of the body. Single cell recordings were obtained from 923 neurons of 24 awake, unrestrained rats. Of 788 neurons categorized according to body part, 264 (34%) discharged in relation to active movement, passive manipulation, and/or cutaneous stimulation of a particular part of the body; the remainder were related to global, whole body movement (38%) or were unresponsive (28%). Neurons related to individual body parts were recorded throughout the entire anterior-posterior extent of the dorsolateral striatum (+1.60 to -2.12 mm A-P, from bregma), intermingled among each other in all 3 dimensions. Two topographic arrangements were observed. First, neurons that fired rhythmically, in phase with low frequency (5-6 Hz) whisking of the vibrissae were segregated in the caudal striatum (-0.2 to -2.12 mm A-P) from neurons related to other body parts, which were distributed from +1.6 to -0.8 mm A-P. Second, representations of the head and face were located ventral to those of the limbs, despite substantial overlap in their overall distributions. A prominent feature of individual electrode tracks was the clustering together of cells related to the same body part. Neurons related to body parts exhibited substantial diversity, which took several distinct forms. Some neurons fired during movement or sensory stimulation in any direction, whereas others showed selectivity for a particular direction. Certain neurons responded to sensory stimulation of a large unilateral region of the body (e.g., all vibrissae or the entire forelimb), whereas others responded to stimulation of highly restricted regions (e.g., a single vibrissa or a single forepaw digit). Finally, neurons differed in the extent to which they exhibited active and passive properties. Among vibrissae-related neurons, one group fired rhythmically during whisking but did not respond to sensory stimulation of the vibrissae; a second group responded to sensory stimulation of the vibrissae but did not fire rhythmically during whisking; a third group showed both properties. Among limb-related neurons, firing during active movement was a property of every cell; none showed sensory responsiveness without showing a relation to active movement of one limb. Of the limb-related neurons, 89% tested responded to passive manipulation of the limb to which the neuron was actively related, and 71% also responded to cutaneous stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

A region in the dorsolateral striatum of the rat exhibiting single-unit correlations with specific locomotor limb movements

1. To examine the activity of single units in the lateral striatum of the awake rat with respect to sensorimotor function, 788 units were recorded during locomotion and passive testing. The focus of this report is on 138 units (18%) that fired in relation to sensorimotor activity of a single limb. The remaining units were related to other body parts (16%), to general body movement (38%), or were unresponsive (28%). 2. Firing rates of limb-related units were near zero during resting behavior but increased markedly during treadmill locomotion. Each of the 138 units exhibited a rhythmic pattern of discharge in phase with the locomotor step cycle. Passive testing revealed that 86/97 units tested (89%) responded to passive manipulation of a single limb, exhibiting increased firing rates. Of these, 77 (90%) were related to contralateral and 9 (10%) to ipsilateral limbs. Sixty-one units (71%) were related to a forelimb and 25 (29%) to a hindlimb. Of the 86 units responding to passive manipulation. 34/48 units tested (71%) also responded to cutaneous stimulation of the same limb but no other part of the body. 3. To study in greater detail the rhythmic unit discharges in phase with the locomotor step cycle, computer-synchronized videotape recordings were used to generate perimovement time histograms constructed around discrete locomotor movements of each limb (n = 17 units). Activity of each unit was shown to be restricted to a specific portion of a particular limb's step cycle. The majority of units discharged throughout (8 units) or during a portion of (3 units) the swing phase, whereas other units fired during a portion of stance (3 units), footfall (2 units), or foot off (1 unit). 1. The specificity of unit firing was further demonstrated by the finding that rhythmic discharges, related to discrete locomotor limb movements in the forward direction, were completely absent during spontaneous deviations such as backward or disrupted locomotion. 5. Units related to limb movement were located in the far lateral, especially the dorsolateral, subregion of the striatum. This subregion extend rostrocaudally from A-P +1.6 to -1.0 mm relative to bregma. No clear somatotopic organization was observed, but this issue requires further study. 6. These results show that functional representations of individual limbs can be demonstrated in the lateral striatum of the rat, within a subregion containing terminals of projections from somatic sensorimotor cortex.(ABSTRACT TRUNCATED AT 400 WORDS)

Control of sensory activation of granule cells in the fascia dentata by extrinsic afferents: septal and entorhinal inputs

Three groups of rats were trained to perform a differential discrimination task in a 2-tone operant conditioning paradigm. One group received electrolytic lesions of the medial septal nuclei, another received electrolytic or knife cut lesions of the entorhinal cortex. These groups were compared with a normal control group. Recordings of granule cells in the fascia dentata were obtained in all animals during criterion performance of the behavioral task. Both lesions produced disruption of behavioral discrimination in the form of increased error and intertrial responding. Granule cell discharges to the tone stimuli were disrupted by each type of lesion. Septal lesions reduced the differential discharge tendency to CS+ and CS- and changed granule cell firing on all trials to statistically resemble firing on CS- trials in normal animals. Extensive lesions in the entorhinal cortex or knife cuts that severed the perforant path caused near elimination of the tone-evoked discharges to both the CS+ and CS-tones. Septal and entorhinal lesions caused marked changes in the sequential dependence of the granule cell discharge compared with intact animals. Results are discussed in terms of the control of the granule cell discharge by the remaining afferent pathways in each type of lesion condition.

Sequence-related changes in sensory-evoked potentials in the dentate gyrus: a mechanism for item-specific short-term information storage in the hippocampus

Sensory-evoked potentials were recorded from the dentate gyrus of the rat hippocampus during performance of a differential auditory discrimination task. The short latency (20 ms) component (N1) of the sensory-evoked potential showed systematic amplitude fluctuations dependent upon the sequence of positive and negative trials preceding the presentation of a given trial and did not depend on the associated reward values of the individual tone stimuli which evoked the potential. The amplitude fluctuations could be accurately depicted by a model which retained the sequence for the five preceding trials in a "buffer" with exponentially decaying influence as a function of time of trial occurrence within the sequence. The results provide evidence that the hippocampus encodes accurate short-lasting representations of sensory events which can provide the basis for storage of information pertaining to past experiences.

Opiates and opioid peptides modify sensory evoked potentials and synaptic excitability in the rat dentate gyrus

The effects of morphine and the synthetic opioid peptide D-Ala2-MePhe4-Met-O-ol-enkephalin (FK 33-824) on averaged (AEPS) potentials evoked by a tone and extracellular synaptic potentials (EPSs) in the perforant path, recorded from the outer molecular layer (OM) of the dentate gyrus, were examined in rats trained to respond in an auditory discrimination task. Potentials evoked by a tone were systematically altered by both peripheral (intraperitoneal) and central (intracerebroventricular) administration of opioids. The short-latency negative (N1) component of the average evoked potential was increased in amplitude and the longer-latency negative (N2) component was decreased in amplitude by administration of opioids. At the same time, perforant path extracellular synaptic potentials were enhanced after administration of opioids. The changes in the average evoked potential and extracellular synaptic potentials in the perforant path were reversed by subsequent administration of naloxone. The significance of these results is discussed in terms of a possible role of endogenous opioid peptides in modulating the synaptic efficacy of the perforant path during the transmission of sensory information to the hippocampus from the entorhinal cortex.

A technique for microiontophoretic study of single neurons in the freely moving rat

An adaptation of a miniature microdrive allows unit recording and microiontophoresis in freely moving rats. Multibarrel glass micropipets are inserted into the detachable microdrive daily, and advanced manually into superficial or deep brain structures. The demonstration of stability in recording neuronal responses to iontophoretically applied drugs suggests that this technique will be useful in studying drug actions on single neurons across a wide range of conscious behaviors.

Evoked potentials in the dentate gyrus reflect the retention of past sensory events

Averaged sensory evoked potentials were recorded from the outer molecular layer of the dentate gyrus (OM AEP) in freely moving rats trained to criterion performance in a two-tone differential discrimination paradigm. OM AEPs showed consistent fluctuations in amplitude based on the acquired significance of the tone through their association with reward (CS+) or non-reward (CS-). However, the relationship between tone significance and OM AEP configuration was dependent upon the sequence of trials preceding the evoked potential, irrespective of whether the potential was evoked by a CS+ or CS-. Specifically, when the preceding sequence contained an increased number of CS- trials, the amplitude of the initial negative component (N1) of the OM AEP was increased, while that of the slower (N2) component was reduced; conversely, when the preceding sequence contained an increased number of CS+ trials, the amplitude of N1 was reduced, while that of N2 was increased. The results suggest that the transmission of sensory information into the dentate gyrus is biased by prior experience.

Dentate granule cell discharge during conditioning. Relation to movement and theta rhythm

The activity of dentate granule cells (g-cells) was recorded in chronic rats during single-tone sensory discrimination performance. Attempts to correlate the discharge pattern of dentate g-cells with (1) the onset of the conditioned tone stimulus, (2) movements of the animals during the conditioning trials, (3) the occurrence of the reinforced response, and (4) the presence or absence of theta rhythm during the trial revealed several features of g-cell discharge during performance of operant sensory discrimination. The most consistent finding was that g-cells were driven at short latencies (40-60 ms) by the onset of the tone stimulus in the absence of detectable body movements. A subsequent, less intense sustained discharge of g-cells was shown to be unrelated to the onset of (1) conditioned movements during the trial, or (2) the execution of the conditioned response. Thus neither phase of g-cell discharge to the tone stimulus appeared to be a direct correlate of inadvertently or directly conditioned movements in this experimental paradigm. Theta rhythm, which accompanied certain movements prior to the execution of the operant response, did not correlate with the initial phase of g-cell discharge but appeared to modulate the subsequent phase of sustained g-cell activity.

Entorhinal and septal inputs differentially control sensory-evoked responses in the rat dentate gyrus

Averaged sensory-evoked potentials were recorded from the outer molecular layer of the dentate gyrus in naïve rats and in rats conditioned to respond in the presence of an auditory stimulus. Two components (negative peaks) of the potentials were functionally distinguished in terms of responsiveness to unique or conditioned auditory stimuli. Each component was independently generated by a separate input pathway to the dentate gyrus: The perforant path provided an "insignificance" or "unexpected" feature of the sensory stimulus when appropriate, and the septum controlled the development of a second component as a function of the behavioral significance of the stimulus during the acquisition of auditory discrimination behavior. A reciprocal relationship between the peak amplitudes of both components of the average evoked potentials dependent on the relative behavioral significance of the sensory stimulus was observed in all animals during extinction and reconditioning of the sensory discrimination task. The findings indicate that the entorhinal and septal projections to the dentate granule cells are activated differentially by sensory stimuli as a function of their acquired behavioral significance to the animal.