David R. Liskowsky

Parabrachial area as mediator of bradycardia in rabbits

This study examined the role of the parabrachial nucleus (PBN) in the mediation of bradycardia and in the reception of barosensory information. The 82 rabbits in the investigation were anesthetized with ethyl carbamate. Train stimulation of medial or lateral PBN produced primary bradycardia (mean peak change: -74 beats/min) associated with a pressor response (average peak mean change: +10 mm Hg) of longer latency. Section of the cervical vagus nerves indicated that the bradycardia was mediated primarily by the parasympathetic nervous system. Heart rate and blood pressure responses to train stimulation did not vary systematically as a function of respiratory pattern; paralyzing animals with decamethonium hydrochloride and artificially ventilating them also did not influence the cardiovascular responses to stimulation. Single-pulse stimulation of PBN in conjunction with extracellular single neuron recording established that neurons originating in or projecting through PBN project to the commissural region of nucleus tractus solitarius (NTS) where synapse is made neurons receiving barosensory input. In addition to establishing the existence of descending functional projections passing from PBN to NTS, injections of HRP into PBN revealed direct descending anatomical projections to PBN from regions of the forebrain previously implicated in the mediation of bradycardia. These included central nucleus of amygdala, lateral preoptic region, medial, forebrain bundle, bed nucleus of stria terminalis, anterior and lateral hypothalamus and zona incerta. The present investigation also indicated that PBN receives barosensory information. Single-pulse electrical stimulation of the aortic nerve (AN) activated neurons in NTS at an average latency of 7.5 ms and in PBN at a mean latency of 12.1 ms. Mean latency of 9 neurons in NTS activated antidromically by PBN stimulation was 3.4 ms. Conduction velocity of the monosynaptic fibers projecting from NTS to PBN was approximately 3.5 m/s, which would be characteristic of finely myelinated fibers. Injection of HRP into PBN confirmed the existence of direct ascending projections to PBN from regions of NTS (e.g. lateral commissural area) previously shown to receive primary barosensory input. However, the finding that only 1 of 9 NTS neurons antidromically activated by PBN stimulation also received barosensory stimulation, indicates that additional study is needed of the mono- and oligosynaptic functional projections from NTS to PBN. The present study did provide evidence that PBN both receives barosensory information at short latency over a direct route, and serves as a relay for descending projections mediating bradycardia.

Auditory cortex lesions prevent the extinction of Pavlovian differential heart rate conditioning to tonal stimuli in rabbits

The present study examined the role of the auditory cortex in the extinction of differentially conditioned heart rate (HR) responses in rabbits. Lesions were placed bilaterally in either the auditory cortex or the visual cortex. Three days after recovery from surgery, the auditory cortex lesion group and the visual cortex lesion control group were habituated to the tonal conditioned stimuli (CSs), and then given 2 days of Pavlovian differential conditioning (60 trials per day) in which one tone (CS+) was always paired with the unconditioned stimulus and another tone (CS-) was never paired with the unconditioned stimulus. Animals that had demonstrated reliable differential conditioning (CS+ response at least 5 beats greater than the CS- response) were placed on an extinction schedule for 7 days. The extinction schedule was identical to the differential conditioning schedule with the exception that shock never followed the CS+. The results of the study indicate that auditory cortex lesions prevent the extinction of differential bradycardia conditioned responses (CRs) to tonal CSs. Whereas the bradycardia responses to the CS+ quickly extinguished in the group that had control lesions in the visual cortex, the auditory cortex lesion group continued to exhibit significantly larger bradycardiac HR CRs to the CS+ relative to the CS- during all 7 days of extinction. These results suggest that the animals in the auditory cortex lesioned group did not inhibit responses to a previously reinforced stimulus (i.e., CS+) as well as animals with control lesions in the visual cortex.

Cardiovascular responses elicited by electrical and chemical stimulation of the rostral medullary raphe of the rabbit

Electrical stimulation of the rostral medullary raphe (RMR) of the rabbit elicited pressor responses that were accompanied by tachycardia or bradycardia. Stimulation of dorsal sites (the dorsal raphe obscurus) evoked a pressor/tachycardia response and stimulation of ventral sites (the ventral raphe obscurus, raphe magnus and raphe pallidus) produced a pressor/bradycardia response. Electrical stimulation of the RMR after sinoaortic denervation led to an increase in the magnitude of the pressor response elicited from all stimulation sites, a decrease in the magnitude of the bradycardia produced by stimulation at the ventral sites, but had no effect upon the magnitude of the tachycardia observed from stimulation of the dorsal sites. These findings suggest that electrical stimulation of the dorsal sites leads to inhibition of the cardiomotor component of the baroreceptor reflex. The results of vagal blockade experiments demonstrated that baroreceptor attenuation of the pressor responses at ventral sites was mediated primarily by parasympathetic input to the heart. Chemical stimulation of the RMR with L-glutamate also led to a pressor/tachycardia response at the dorsal sites and a pressor/brachycardia response at the ventral sites. This finding provides evidence that neuronal cell bodies, not axon of passage, mediated the responses elicited by electrical stimulation.

Hypothalamic, midbrain and bulbar areas involved in the defense reaction in rabbits ☆

The present study mapped neuroanatomical sites in the hypothalamus and periaqueductal gray (PAG) of the rabbit which, when stimulated electrically, evoked the cardiorespiratory components of the defense reaction (CRDR). This included increases in heart rate, blood pressure, hindlimb blood flow and respiration rate. All of the components of the CRDR were elicited by electrical stimulation of the posterior hypothalamus, at sites dorsal and medial to the fornix. Although there were regions throughout the PAG in which electrical stimulation elicited concomitant increases in blood pressure, hindlimb blood flow and respiration rate, only stimulation of the dorsal PAG evoked tachycardia. Injection of horseradish peroxidase into the rostral ventrolateral medulla (RVLM) led to heavy retrograde and anterograde labeling in the region of the hypothalamus that yielded the CRDR when stimulated electrically. Heavy labeling was also observed in the dorsal and ventral PAG. The results of this study provide evidence that the posterior hypothalamus and the dorsal PAG are nodal structures in the mediation of the CRDR and that cells in posterior hypothalamus, dorsal PAG and ventral PAG make monosynaptic connections with the RVLM.

Role of auditory cortex in the acquisition of differential heart rate conditioning.

Previous findings from our laboratory indicate that lesions of the auditory cortex disrupt the retention of differentially conditioned bradycardiac responses to tonal stimuli in rabbits. In the present experiment, the effect of lesions of the auditory cortex on the acquisition of differential bradycardiac conditioning was examined. The effect of lesions in the auditory cortex were compared to the effect produced by control lesions in the visual cortex. After 7 days of recovery, animals received 7 days of differential Pavlovian bradycardiac conditioning in which one tone (CS+) was paired with the unconditioned stimulus, and another tone (CS-) was never paired with the unconditioned stimulus. All animals demonstrated differential conditioning during the first 3 days of conditioning. On days 4-7, however, auditory cortex lesioned animals did not exhibit significant differential heart rate (HR) conditioning, whereas control animals with lesions in the visual cortex showed no loss of conditioning during this period. The loss of differential conditioning in animals with lesions in the auditory cortex appears to be due to an increase in the magnitude of the response to the CS-. These data support the hypothesis that the auditory cortex serves to inhibit the response to the CS- in differential conditioning of bradycardia to acoustic stimuli, and that the inhibition may be mediated by a descending corticothalamic or corticolimbic pathway.

D-2 dopamine receptors in the frontal cortex of rat and human

D-2 dopamine receptors and serotonin receptors in the frontal cortex of rat and human were labelled with 3H-spiroperidol. The D-2 receptors were then distinguished in 4 ways. Dissociation of spiroperidol was biphasic, indicating two populations of sites. Cinanserin in competition with 3H-spiroperidol exhibited high (75%) and low (25%) affinity sites. Dopamine and LY 141865 in competition with 1.25 nM 3H-spiroperidol exhibited high (20-25%) and low (80-75%) affinity sites in the absence of cinanserin, while in the presence of 300 nM cinanserin only the high affinity sites remained. Lesioning of the dopaminergic meso-cortical pathway increased the number of cinanserin-resistant sites by 26%. Thus 3H-spiroperidol binding in the presence of cinanserin can be used to selectively label D-2 receptors in the frontal cortex.

Anatomical and functional connections of neurons of the rostral medullary raphe of the rabbit

Single cell recordings were made from neurons in the rostral medullary raphe (RMR) of the rabbit. The recording sites were ones that had been shown to yield pressor responses from electrical stimulation and by pressure injections of glutamate. Electrical stimulation of the intermediolateral (IML) region of the spinal cord led to antidromic activation of 12 of the 100 cells studied. Eleven of these cells were located in raphe pallidus or raphe magnus, and one cell was located in raphe obscurus. These findings were consistent with the results of horseradish peroxidase (HRP) histochemistry experiments. Injections of HRP into the IML led to heavy cell body labeling in raphe pallidus and raphe magnus, but sparse labeling in raphe obscurus. Cells in the RMR could be orthodromically activated by electrical stimulation of the putative defense area of the periaqueductal (PAG) but not by stimulation of putative defense areas in the hypothalamus. Most of these cells were located in raphe pallidus or raphe magnus. Similarly, HRP injections into raphe pallidus and raphe magnus led to heavy cell body labeling in the PAG but not the hypothalamus; no cell body labeling was found in the PAG when injections were made into raphe obscurus.

The location of chronotropic cardioinhibitory vagal motoneurons in the medulla of the rabbit

Vagal preganglionic motoneurons originating in nucleus ambiguus (NA) and dorsal vagal nucleus (DVN) were identified via retrograde labeling with horseradish peroxidase (HRP). DVN and NA were then explored for cardiovascular responsive sites using microstimulation. Stimulation within DVN from slightly caudal to obex to 3.00 mm rostral to obex produced a primary bradycardia (n = 15, X = -123 bpm). Stimulation within NA from slightly rostral to obex to 1.5 mm caudal to obex produced a similar primary bradycardia (n = 15, X = -127 bpm). Extracellular recordings were made from 7 cells in DVN and 10 cells in NA in regions producing maximal bradycardia to electrical stimulation. These cells were antidromically activated by cervical vagus nerve (VN) stimulation, increased their firing rates to systemic injection of phenylephrine (PE), revealed an expiratory rhythm, showed an increase in firing rate coinciding with spontaneous and elicited decreases in heart rate, had conduction velocities in the A-delta and B-fiber range, and produced bradycardia upon stimulation through the recording electrode with thresholds as low as 4 microA. The data indicate that in rabbits, chronotropic cardioinhibitory vagal motoneurons are discretely localized on the lateral, caudal portions of DVN and NA between 0.5 mm caudal and 1.5 mm rostral to obex.

Muscarine-binding sites localized to cortical dopamine terminals

The effects of lesions to the mesocortical dopaminergic system on D2 dopamine receptors and muscarine receptors in the frontal cortex of the rat was examined. Four weeks following 6-hydroxydopamine lesioning of the ventral tegmental area, there was a 26% increase in the number of [3H]spiroperidol sites, and a 13% decrease in the number of [3H]oxotremorine-M sites in the frontal cortex, indicating a development of D2 receptor supersensitivity, as a result of deafferentation, and a loss of acetylcholine sites, as result of terminal degeneration. This demonstrates that in the frontal cortex, as in the striatum and nucleus accumbens, the activity of dopaminergic terminals may be partially modulated by cholinergic inputs.

A pre-positron emission tomography study of l-3,4-dihydroxy-[3H]phenylalanine distribution in the rat

The distribution of L-3,4-dihydroxy-[3H]phenylalanine (L-[3H]DOPA) was examined in rats following i.v. injection, to ascertain the possible usefulness of using this ligand to image dopaminergic systems using positron emission tomography. It was found that L-DOPA and its metabolites were preferentially localized in the basal ganglia as compared to other brain regions, and that this preferential localization could be abolished by lesioning of the nigro-striatal tract. The parameters of the L-DOPA uptake and the sensitivity of this uptake to alterations in dopaminergic pathways indicate that this ligand may be useful in visualizing aberrations in dopaminergic pathways in various pathological conditions.