Do antidromic latency jumps indicate axonal branching in nigrostriatal and hypothalamo-neurohypophysial neurons?

In vivo discharge properties of hypothalamic paraventricular nucleus neurons with axonal projections to the rostral ventrolateral medulla

The hypothalamic paraventricular nucleus (PVN) and rostral ventrolateral medulla (RVLM) are key components of a neural network that generates and regulates sympathetic nerve activity (SNA). Although each region has been extensively studied, little is presently known about the in vivo discharge properties of individual PVN neurons that directly innervate the RVLM. Here extracellular recording was performed in anesthetized rats, and antidromic stimulation was used to identify single PVN neurons with axonal projections to the RVLM (n = 94). Neurons were divided into two groups that had either unbranched axons terminating in the RVLM (i.e., PVN-RVLM neurons, n = 65) or collateralized axons targeting both the RVLM and spinal cord [i.e., PVN-RVLM/intermediolateral cell column (IML) neurons, n = 29]. Many PVN-RVLM (32/65, 49%) and PVN-RVLM/IML (17/29, 59%) neurons were spontaneously active. The average firing frequency was not different across groups. Spike-triggered averaging revealed that spontaneous discharge of most neurons was temporally correlated with renal SNA (PVN-RVLM: 12/21, 57%; PVN-RVLM/IML: 6/9, 67%). Time histograms triggered by the electrocardiogram (ECG) R-wave indicated that discharge of most cells was also cardiac rhythmic (PVN-RVLM: 25/32, 78%; PVN-RVLM/IML: 10/17, 59%). Raising and lowering arterial blood pressure to increase and decrease arterial baroreceptor input caused a corresponding decrease and increase in firing frequency among cells of both groups (PVN-RVLM: 9/13, 69%; PVN-RVLM/IML: 4/4, 100%). These results indicate that PVN-RVLM and PVN-RVLM/IML neurons are both capable of contributing to basal sympathetic activity and its baroreflex modulation.

Age-dependent changes in dopaminergic projections from the substantia nigra pars compacta to the neostriatum

Age-dependent changes in dopaminergic (DA) innervation of the neostriatum (Str) were studied in male F344/N rats. Projections from the substantia nigra pars compacta (SNc) to the neostriatum were quantified using electrophysiological methods at age points from 6 to 24 months. The percentage of DA neurons activated antidromically by electrical stimulation (P-index) of Str increased between 18 and 24 months. Additionally, the percentage of DA neurons showing multiple antidromic latencies from striatal stimulation (M-index), which suggests axonal branching of individual DA neurons, increased significantly between 6 and 12 months and 6 and 24 months. These results suggest that DA neurons exhibit increased axonal branching in the aged brain.

Excitatory and inhibitory local circuit input to the rat dorsal motor nucleus of the vagus originating from the nucleus tractus solitarius

The nucleus tractus solitarius (NTS) and dorsal motor nucleus of the vagus nerve (DMV) constitute sensory and motor nuclei of the dorsal vagal complex, respectively. We used whole-cell patch-clamp recordings from DMV neurons in rat brain slices and three methods of stimulation (electrical, glutamate microdrop, glutamate photostimulation) to test the hypothesis that convergent excitatory and inhibitory inputs to DMV neurons originate from intact neurons in multiple NTS areas. Electrical stimulation of the NTS resulted in evoked excitatory and inhibitory postsynaptic currents (eEPSCs and eIPSCs) in DMV neurons. Stimulation of the dorsal NTS with glutamate microdrops, which selectively stimulates the soma and dendrites of intact neurons, resulted in 31% of DMV neurons receiving eEPSCs, 44% receiving eIPSCs, and 6% receiving convergent excitatory and inhibitory inputs. Glutamate photostimulation allowed selective activation of intact neurons in multiple, discrete areas of the NTS and resulted in 36% of DMV neurons receiving eEPSCs, 65% receiving eIPSCs and 20% receiving both inputs. Data obtained by stimulation of multiple NTS areas support the hypothesis that there are anatomically convergent inputs to DMV neurons originating from intact neurons within the NTS. These data support the hypothesis that there is transfer of convergent information from the NTS to the DMV, implying that significant sensory-motor processing occurs within the brainstem.

Age-dependent changes in projections from locus coeruleus to hippocampus dentate gyrus and frontal cortex

Age-dependent changes in noradrenergic innervations of the hippocampal dentate gyrus (DG) and the frontal cortex (FC) have been studied in male F344 rats. The projections from the nucleus locus coeruleus (LC) to DG or FC with advancing age (from 7 to 27 months) in rats have been quantified by electrophysiological and immunohistochemical methods. In the electrophysiological study, we observed that the percentage of LC neurons activated antidromically by electrical stimulation (P-index) of DG or FC decreased with age. We found that the percentage of LC neurons showing multiple antidromic latencies (M-index), which suggests axonal branching of individual LC neurons, increased markedly between 15 and 17 months in DG or FC. In DG, the M-index increased steadily between 15 and 24 months. In contrast, the increased M-index in FC was maintained until 24 months. The increased M-index in both targets declined at 27 months. These results suggest that LC neurons give rise to axonal branching following the loss of projections to DG or FC with age. In the immunohistochemical study, the density of dopamine-beta-hydroxylase-positive axonal varicosities was measured in molecular, granule cell and polymorphic layers of DG. The density in the polymorphic layer significantly decreased in the earlier stage of ageing (7-19 months), whilst the density in the molecular and granule cell layers decreased in the later stage (27 months). These findings suggested that a layer-specific decline occurred with age in the noradrenergic axon terminals in DG.

Midbrain dopamine system electrophysiological functioning: a review and new hypothesis

The pathogenesis of a variety of neurological and mental disorders has been attributed to the malfunctioning of central dopaminergic systems. Twenty years of investigation concerning the way that dopamine systems function in the brain has yielded a great deal of information about the control and behavior of these systems. This collection of seemingly disparate facts is reviewed. A hypothesis is then presented that attempts to synthesize from these facts a view of the electrophysiological functioning of midbrain dopamine systems that may be helpful in understanding brain mechanisms underlying the pathology of such disorders as Parkinson's disease and schizophrenia.

Modulation of dopaminergic terminal excitability by D1 selective agents: further characterization

We have previously shown that stimulation of striatal D1 receptors affects dopaminergic nigrostriatal terminal excitability, which is thought to be an index of biophysical events resulting from the activation of receptors on the presynaptic membrane. The experiments presented here further examine the locus and bases of these D1 effects in the rat. We now report that striatal administration of the D1 receptor selective antagonist R-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazapine+ ++-7-ol-HCl (SCH 23390) produces a paradoxical agonist-like decrease in dopaminergic terminal excitability. This effect is blocked by pretreatment with the dopamine synthesis inhibitor, alpha-methyl-paratyrosine, suggesting that the action of SCH 23390 is dependent upon endogenous dopamine. Further, haloperidol pretreatment also prevents the SCH 23390-induced decrease in terminal excitability, confirming that dopamine, acting through a dopamine receptor, is responsible for this agonist-like action. Striatal application of the active R-(+) enantiomer of the dopaminergic D1-selective agonist 1-phenyl-2,3,4,5-tetrahydrol-(1H)-3-benzazepine-7,8-diol-HCl (R-SKF 38393) decreases terminal excitability in the alpha-methyl-paratyrosine pretreated animal, indicating that dopamine is not required for the agonist action. In an effort to ascertain the presynaptic or postsynaptic location of these actions, an extensive destruction of postsynaptic neurons in the neostriatum was produced by local administration of the neurotoxin, kainic acid. It was observed that the neurotoxin-induced neostriatal neuronal loss did not disrupt the action of R-SKF 38393 nor its reversal by SCH 23390.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine-containing neurons in the mammalian central nervous system: electrophysiology and pharmacology

A decade of research culminated in the late 1950's with the demonstration that dopamine was a chemical neurotransmitter within the mammalian brain. Since this time, dopaminergic neuronal systems have been extensively studied using numerous techniques. This paper will review the last 14 years of electrophysiological investigation on neurochemically identified dopamine-containing neurons in the central nervous system. This will include an examination of both the electrophysiological and pharmacological characteristics in these cells, as well as the resulting insights into the regulation of dopamine cell electrical activity which is derived from this work.