Inhibitory effect of bile acids on renin angiotensinogen reaction system

The ventral pallidum: Subregion-specific functional anatomy and roles in motivated behaviors

The ventral pallidum (VP) plays a critical role in the processing and execution of motivated behaviors. Yet this brain region is often overlooked in published discussions of the neurobiology of mental health (e.g., addiction, depression). This contributes to a gap in understanding the neurobiological mechanisms of psychiatric disorders. This review is presented to help bridge the gap by providing a resource for current knowledge of VP anatomy, projection patterns and subregional circuits, and how this organization relates to the function of VP neurons and ultimately behavior. For example, ventromedial (VPvm) and dorsolateral (VPdl) VP subregions receive projections from nucleus accumbens shell and core, respectively. Inhibitory GABAergic neurons of the VPvm project to mediodorsal thalamus, lateral hypothalamus, and ventral tegmental area, and this VP subregion helps discriminate the appropriate conditions to acquire natural rewards or drugs of abuse, consume preferred foods, and perform working memory tasks. GABAergic neurons of the VPdl project to subthalamic nucleus and substantia nigra pars reticulata, and this VP subregion is modulated by, and is necessary for, drug-seeking behavior. Additional circuits arise from nonGABAergic neuronal phenotypes that are likely to excite rather than inhibit their targets. These subregional and neuronal phenotypic circuits place the VP in a unique position to process motivationally relevant stimuli and coherent adaptive behaviors.

Basal forebrain and frontal cortex neuron responses during visual discrimination in the rat

Using a classical conditioning procedure in urethane-anesthetized rats, a light applied to one eye (CS+) was paired with medial forebrain bundle (MFB) stimulation, whereas a light applied to the other eye (CS-) was not paired. Basal forebrain neurons in the substantia innominata, medial globus pallidus, and nucleus basalis magnocellularis responded differentially to CS+ and CS-, with larger responses to CS+. Some neurons were excited by CS+, and others were inhibited. Fifty percent of these neurons responded in the same direction to CS+ and MFB stimulation, and 38% responded in opposite directions. Frontal cortex neurons exhibited similar differential responses; 47% of the differential responses to CS+ were in the same direction as the response to MFB stimulation, and 29% were in the opposite direction. When light to either eye was paired with MFB stimulation, conditioning-related basal forebrain neuron responses of comparable magnitude to left and right eye illumination were observed, providing evidence that association of CS and UCS rather than the eye to which light was applied determined the differential response to CS+. Also, two different intensities of light induced comparable basal forebrain responses when both were paired with the UCS. These experiments provide support for a role of the basal forebrain in conditioning-related neural activity. Furthermore, this preparation can be utilized to investigate transmitter systems that mediate conditioning-related responses of basal forebrain neurons.

A role for acetylcholine in conditioning-related responses of rat frontal cortex neurons: microiontophoretic evidence

In an associative conditioning paradigm, an auditory stimulus (CS+) was paired with rewarding medial forebrain bundle stimulation or a tone of different frequency (CS-) was presented without pairing. After training, slow potential (SP) and single neuron responses were recorded from rat frontal cortex. When cortical SP responses indicated the development of discrimination between CS+ and CS- tones, single neurons could be isolated that exhibited a discriminative response to CS+. Seventy-three percent of the 56 neurons which discriminated between CS+ and CS- were excited by the paired tone while the remainder were inhibited. Iontophoretically applied acetylcholine increased spontaneous firing rate in 90% of the excited cells and 87% of the inhibited cells. Iontophoretic administration of a muscarinic receptor antagonist, either atropine or tropicamide, during trial presentation attenuated the conditioning-related response to CS+ as well as the response to acetylcholine in the majority of neurons. The largest group of discriminating neurons were excited by both CS+ and acetylcholine, and both responses were suppressed by the antagonists. The results provide evidence that conditioning-related responses of a major population of frontal cortex neurons are modulated by cholinergic input, a portion of which may originate in the basal forebrain area. There also may be a significant non-cholinergic influence on these neuronal responses.

Haloperidol antagonism of amphetamine-induced effects on event-related slow potentials from rat cortex

Slow potential (SP) responses to a click followed after 2 sec by rewarding stimulation of the medial forebrain bundle were recorded from the frontal cortex of rats with permanently implanted electrodes. The SP responses were frequently enhanced by haloperidol (0.0125 to 0.1 mg/kg). Both haloperidol and alpha-methyltyrosine antagonized amphetamine-induced suppression of the SP responses. The results are consistent with the idea that dopamine (and possibly norepinephrine) exerts an inhibitory influence on the mechanisms involved in generation of event-related slow potentials.

Auditory cue preceding intracranial stimulation induces event-related potential in rat frontal cortex: alterations by amphetamine

Slow potential (SP) responses to a click followed by rewarding stimulation of the medial forebrain bundle (MFB) were recorded from the frontal cortex of rats with permanently implanted electrodes. Trials were presented at variable intervals and the final interval between the click and MFB stimulation was 2 seconds. Negative SP responses developed rapidly with training, as the interstimulus interval was gradually increased from 0.5 sec to 2 sec. No decrement of the SP response was observed during recording sessions consisting of more than 300 trials. The SP responses remained stable over several weeks of recording. Dextroamphetamine produced a dose-related depression of the SP responses. Since the effect of amphetamine in this study was similar to the effect in studies using food reinforcement, the results suggest that amphetamine-induced depression of event-related slow potentials in rat frontal cortex is not dependent on the type reinforcement. Advantages of the use of intracranial stimulation as reinforcement for event-related potential studies are discussed.

Brain stimulation as a cue for event-related potentials in rat cortex: amphetamine effects

Slow potential (SP) responses were recorded bilaterally from the frontal cortex of rats with permanently implanted silver-silver chloride electrodes. Trials were presented at variable intervals from 15 to 50 sec and the interval between the pulse cue and onset of the rewarding train was 2 sec. The cue stimulus was a single 0.5 msec monophasic square wave pulse of the same current intensity as rewarding stimulation (100 Hz, 500 msec train). Appropriate current strength was determined by prior testing for self-stimulation. The single pulse by itself failed to evoke an SP response but after repeated pairings, large negative SP responses developed which were bilaterally equal. These responses extinguished rapidly when rewarding stimulation was discontinued. d-Amphetamine (0.125, 0.25 and 0.5 mg/kg, SC) produced a dose-related depression of the SP response to the pulse cue, an effect comparable to that observed using an auditory cue with either food or MFB stimulation reinforcement. The results indicate that frontal cortex SP responses which develop in anticipation of a meaningful event do not require a peripheral sensory cue. Furthermore, amphetamine suppression of these SP responses is not produced via a disruption of the auditory system.

Event-related slow potentials in rat cortex during a reaction time task: cortical area differences

Event-related slow potentials were recorded from frontal and visual cortex of rats during a reaction time task in which rats were initiated at variable intervals by an auditory warning stimulus. Transcortical slow potential (SP) responses during the two-second period between onset of the warning stimulus and extension of a retractable lever were analyzed, using single trial and averaged response data. SP responses recorded from the two cortical areas differed significantly with respect to waveform and amplitude. The results indicate that event-related slow potentials from rat cortex are area dependent and that the small size of the rat brain does not preclude recording of SP changes with differential cortical distribution.

Amphetamine effects on brain slow potentials associated with discrimination in the rat

Slow potentials were recorded from the anterior cortex of rats during discrimination conditioning and the effects of various doses of d-amphetamine on these responses were examined. In the discrimination paradigm one tone (Sd) was followed at three see after the onset by food reinforcement while another tone (S delta) indicated that no reinforcement would follow. Slow potential (SP) responses were measured during the three-sec period following onset of the stimulus. For the first several training sessions the SP responses demonstrated a phase of generalization during which responses were the same to both stimuli. Thereafter, the responses to Sd were significantly greater than responses to S delta. d-Amphetamine produced a dose-related depression of SP responses to the reinforced stimulus in doses of 0.25 to 2.0 mg/kg. The effect of amphetamine on SP responses to S delta was biphasic; the lower doses (0.25 and 0.5 mg/kg) enhanced responses, no change was seen after 1.0 mg/kg and the high dose (2.0 mg/kg) depressed responses. This study demonstrates that the rat develops differential slow potential responses to reinforced and nonreinforced stimuli in a discrimination paradigm and that d-amphetamine produces a differential and dose-related alteration of these SP responses. It is suggested that the actions of amphetamine may be produced through interference with mechanisms of discrimination, by an effect on subcortical activating systems involving norepinephrine, and/or by activation of inhibitory dopamine receptors on cortical neurons.