The purpose of this study was to identify the afferent link in the neural pathway which mediates emotional responses coupled to auditory stimuli. We evaluated whether autonomic and behavioral responses elicited by acoustic conditioned emotional stimuli are based on afferent information derived from the auditory cortex or from the auditory thalamic relay station, the medial geniculate nucleus (MG), in rats. The rat auditory cortex was defined through anterograde neuroanatomical tracing studies involving the injection of HRP into MG. Lesions were then placed in the auditory cortex or in MG. After 10 to 20 days the rats were subjected to classical fear conditioning trials involving the pairing of a pure tone with electric footshock. Changes in mean arterial pressure and heart rate and the duration of immobilization ("freezing") and drink suppression elicited by presentation during extinction trials (no footshock) of the acoustic conditioned emotional stimulus were measured. Auditory cortex lesions did not affect the magnitude of the mean arterial pressure or heart rate conditioned responses nor the duration of freezing or drink suppression. In contrast, lesions of MG suppressed the magnitude of both the autonomic and somatomotor (behavioral) conditioned emotional responses but did not affect either autonomic or somatic responses elicited by the footshock unconditioned stimulus. Lesions of the inferior colliculus, the primary source of afferent input to MG, replicated the effects of MG lesions. These findings demonstrate that lesions of MG and lower auditory centers, but not lesions of the auditory cortex, block autonomic and behavioral conditioned emotional responses coupled to acoustic stimuli and indicate that subcortical rather than cortical efferents of MG sustain these responses. Our concurrent observation that MG projects to several subcortical areas (central and lateral amygdala; caudate-putamen; ventromedial hypothalamus) involved in emotional behavior and autonomic function suggests hypotheses concerning subsequent links in this emotional processing pathway.

Neurons projecting from the rostral ventrolateral medulla (RVL) to the spinal cord were antidromically identified in rats anesthetized with urethane, paralyzed, and ventilated. The sites of lowest antidromic threshold were concentrated in the intermediolateral nucleus (IML). Their axonal conduction velocities were distributed bimodally, with the mean of the rapidly conducting fibers (greater than 1 m/sec) being 3.1 +/- 0.1 m/sec (n = 105), and of the slower axons being 0.8 +/- 0.03 m/sec (n = 25). Single-shock electrical stimulation of RVL elicited 2 bursts of excitation in splanchnic sympathetic nerve activity (SNA), which resulted from activation of 2 descending pathways with conduction velocities comparable to those of antidromically excited RVL-spinal neurons. The probability of discharge of RVL-spinal cells was synchronized both with the cardiac-related bursts in SNA with functional baroreceptor reflexes and with the free-running 2–6 Hz bursts in SNA following baroreceptor afferent denervation. On the average, their spontaneous discharges occurred 67 +/- 2 msec (n = 31) prior to the peak of the spontaneous bursts in splanchnic SNA. This time corresponded to the latency to the peak of the early excitatory potential in splanchnic SNA following electrical stimulation of RVL. Baroreceptor reflex activation inhibited RVL-spinal neurons. The recording sites of RVL-spinal vasomotor neurons were consistently located within 100 micron of cell bodies (C1 neurons) immunoreactive for the adrenaline-synthesizing enzyme phenylethanolamine N- methyltransferase (PNMT). Ultrastructural analysis of the lateral funiculus of the cervical and thoracic spinal cord demonstrated PNMT immunoreactivity within myelinated (0.6–2.1 micron diameter) and unmyelinated (0.1–0.8 micron diameter) axons. Estimated conduction velocities of these fibers were comparable to the antidromic conduction velocities of the rapidly and slowly conducting populations of RVL- spinal vasomotor neurons. We conclude that in rat, the discharge of RVL- spinal vasomotor neurons strongly influences SNA: the baroreceptor- mediated inhibition of these neurons is reflected in the cardiac locking of SNA, while, in the absence of baroreceptor input, the synchronous discharge of RVL-spinal neurons maintains a free-running 2- 6 Hz bursting pattern in SNA. RVL-spinal neurons are located within, and may be elements of, the C1 adrenergic cell group, and they provide a sympathoexcitatory drive to neurons in the IML over rapidly and slowly conducting pathways that correspond to myelinated and unmyelinated spinal axons containing PNMT.

Imidazoline (I) receptors constitute a family of nonadrenergic high-affinity binding sites for clonidine, idazoxan, and allied drugs. One major subclass, the I1 receptors, whose subcellular distribution and signal transduction mechanisms are uncertain, partly mediates the central hypotensive actions of clonidine-like drugs. The I2 receptors, another subclass, are mitochondrial, not G protein coupled, and have diversified functions. Several endogenous ligands for I receptors, collectively termed clonidine-displacing substances (CDSs), have been detected in tissues and serum, but the structure of only one, agmatine (decarboxylated arginine), is known. Agmatine, widely distributed and bioactive, binds, like clonidine, to alpha 2-adrenergic and I receptors of all subclasses. The presence of agmatine and its biosynthetic enzyme in synaptosomes and specific neuronal pathways as well as serum suggests that it may be a novel neurotransmitter/hormone. Another CDS that binds to I receptors and to antibodies to imidazoline drugs has been detected, but its structure is unknown.

Interrupting neuronal activity in the caudal ventrolateral medulla (CVL) abolishes baroreceptor-mediated sympathoinhibition and vasodepressor responses. In the present study we identified CVL neurons that function as interneurons in the baroreceptor reflex arc. The mean antidromic onset latency of CVL neurons (5 ms) after stimulation in the rostral ventrolateral medulla (RVL) suggests that they transmit information to the RVL via unmyelinated axons. Stimulation of baroreceptor afferents in the aortic depressor nerve (ADN) excited CVL neurons projecting to the RVL with onset latencies between 15 and 45 ms. The short-latency ADN stimulus-evoked excitation of CVL barosensory interneurons preceded the onset of the ADN stimulus-evoked inhibition of RVL-spinal sympathoexcitatory neurons. Longer-latency ADN-evoked CVL neuronal responses may arise from activation of unmyelinated ADN afferents that can prolong the inhibition of RVL sympathoexcitatory neurons. The excitation of CVL barosensory interneurons resulting from the systolic pressure rise was followed by a period of decreased excitability of RVL sympatho-excitatory neurons, which was of comparable duration to the increase in the discharge probability of CVL neurons. Our characterization of baroreceptor reflex interneurons in the CVL suggests that transmission of baroreceptor information involves a complex activation of medullary interneurons responding selectively to different aspects of the arterial pressure stimulus.

Neurons containing the enzyme aromatic-L-amino-acid decarboxylase (AADC) but lacking either tyrosine hydroxylase or serotonin were found in the spinal cord of neonatal and adult rats by light and electron microscopic immunocytochemistry. The majority of these neurons localized to area X of Rexed contact ependyma. Thus, spinal AADC neurons have the enzymatic capacity to catalyze directly the conversion of the amino acids tyrosine, tryptophan, or phenylalanine to their respective amines tyramine, tryptamine, or phenylethylamine. These amines normally present in the central nervous system may be of potential clinical significance as endogenous psychotomimetics.