Limitations of Photosynthesis in Pinus taeda L. (Loblolly Pine) at Low Soil Temperatures

The relative importance of stomatal and nonstomatal limitations to net photosynthesis (A) and possible signals responsible for stomatal limitations were investigated in unhardened Pinus taeda seedlings at low soil temperatures. After 2 days at soil temperatures between 13 and 7 degrees C, A was reduced by 20 to 50%, respectively. The reduction in A at these moderate root-chilling conditions appeared to be the result of stomatal limitations, based on the decrease in intercellular CO(2) concentrations (c(i)). This conclusion was supported by A versus c(i) analysis and measurements of O(2) evolution at saturating CO(2), which suggested increases in stomatal but not biochemical limitations at these soil temperatures. Nonuniform stomatal apertures, which were demonstrated with abscisic acid, were not apparent 2 days after root chilling, and results of our A versus c(i) analysis appear valid. Bulk shoot water potential (psi) declined as soil temperature dropped below 16 degrees C. When half the root system of seedlings was chilled, shoot psi and gas-exchange rates did not decline. Thus, nonhydraulic root-shoot signals were not implicated in stomatal limitations. The initial decrease in leaf conductance to water vapor after root chilling appeared to precede any detectable decrease in bulk fascicle psi, but may be in response to a decrease in turgor of epidermal cells. These reductions in leaf conductance to water vapor, which occurred within 30 minutes of root chilling, could be delayed and temporarily reversed by reducing the leaf-to-air vapor-pressure deficit, suggesting that hydraulic signals may be involved in initiating stomatal closure. By independently manipulating the leaf-to-air vapor-pressure deficit of individual fascicles, we could induce uptake of water vapor through stomata, suggesting that nonsaturated conditions occur in the intercellular airspaces. There was an anomaly in our results on seedlings maintained for 2 days at soil temperatures below 7 degrees C. Lower A appeared primarily the result of nonstomatal limitations, based on large increases in calculated c(i) and A versus c(i) analysis. In contrast, measurements of O(2) evolution at saturating CO(2) concentrations implied nonstomatal limitations per se did not increase at these temperatures. One explanation for this paradox is that calculations of c(i) are unreliable at very low gas-exchange rates because of inadequate measurement resolution, and limitations of A are predominantly stomatal. An alternative interpretation is that increases in c(i) are real and the results from O(2)-evolution measurements are in error. The high CO(2) concentration used in O(2)-evolution measurements (15%) may have overcome nonstomatal limitations by enzymes that were down-regulated by a feedback mechanism. In this scenario, carbohydrate feedback limitations may be responsible for nonstomatal reductions in A after 2 days at soil temperatures below 7 degrees C.

Studies on muscle cells isolated from Schistosoma mansoni: a Ca(2+)-dependent K+ channel

Muscle cells from adult male Schistosoma mansoni have been isolated using a combination of papain digestions and mechanical dissociation procedures. The muscle fibres isolated in this way lacked nuclei but they did contract and relax in response to high [K+], a response which was blocked in the presence of Co2+. From this we conclude that the isolation procedure yields viable muscle fibres useful for physiological studies. Patch-clamp recordings taken from the isolated fibres show a variety of discrete ionic conductances. In inside-out patches one prominent channel was a Ca(2+)-activated K+ channel with a conductance of 195 pS and a selectivity greater than 10:1 for K+ over Na+, Cs+ or NH4+. Percentage open time was dependent on [Ca2+] at the intracellular face. With [Ca2+] at 1 microM or greater percentage open time was greater than 95%; at 0.1 microM it was less than 2%. No voltage sensitivity could be detected in the voltage range from -50 to -10 mV membrane potential. Ba2+ (10 mM), but neither tetraethylammonium nor 3,4-diaminopyridine blocked the channel from the intracellular face. This Ca(2+)-activated K+ channel in the muscle membrane of this acoelomate animal is similar in most respects to the maxi-K+ channels which have been described in a variety of cells from more highly evolved animals.

Noxious somatic stimuli excite neurosecretory vasopressin cells via A1 cell group

Activation of nociceptive somatic afferents excites hypothalamic neurosecretory cells and stimulates the release of vasopressin. To investigate the possibility that relevant afferent information is relayed through the A1 norepinephrine cell group of the caudal ventrolateral medulla, single-unit recording experiments were performed in pentobarbital sodium-anesthetized rats. The effects of somatic nerve stimulation, application of noxious somatic stimuli, and A1 region stimulation on the activity of putative vasopressin-secreting neurosecretory cells of the supraoptic nucleus were compared. The predominant effect of femoral and sciatic nerve stimulation on these cells was excitation, 54% (n tested = 113) displaying a marked increase in discharge probability, which had a mean onset latency of 72 +/- 3 ms and a mean duration of 114 +/- 9 ms. Almost all cells (96%) responding to somatic nerve stimulation were also excited by pinching of the ipsilateral or contralateral hindlimb paw, and the majority (84%) displayed a matching but shorter latency response to A1 region stimulation (mean onset 35 +/- 4 ms, duration 55 +/- 9 ms). A1 region injections of the inhibitory neurotransmitter gamma-aminobutyric acid reversibly blocked the effects of both somatic nerve stimulation (n = 14) and paw pinch (n = 9) on putative vasopressin cells. These results indicate that excitation of vasopressinergic neurosecretory cells by noxious somatic stimuli requires activation of neurons of the caudal ventrolateral medulla and hence are consistent with the proposal of a role for the A1 norepinephrine cell group.

Involvement of the A1 cell group in baroreceptor inhibition of neurosecretory vasopressin cells

Extracellular recording experiments were done to investigate the proposal that arterial baroreceptor inhibition of vasopressinergic (AVP) neurosecretory cells involves the A1 noradrenaline cell group of the caudal medulla. The responsiveness of functionally identified AVP cells of the rat supraoptic nucleus to baroreceptor activation was not discernibly altered by A1 region lesions even though AVP cell responses to certain other stimuli were abolished. Acute blockade of A1 region function by injection of gamma-aminobutyric acid sometimes impaired AVP cell responsiveness to baroreceptor stimulation, although more commonly there was no effect. These data suggest that the A1 cell group is not an essential component in pathways mediating baroreceptor inhibition of neurosecretory AVP cells, but may contribute to the sensitivity of AVP cells to this input.

Excitation of supraoptic vasopressin cells by stimulation of the A1 noradrenaline cell group: failure to demonstrate role for established adrenergic or amino acid receptors

The effects of adrenergic and excitatory amino acid antagonists on supraoptic nucleus (SON) neurosecretory cell responses to stimulation of the A1 noradrenaline (NA) cell group were examined in anaesthetized male rats. As in previous studies, delivery of cathodal pulses (100 microA, 1 ms pulses, 1 Hz) to the A1 region of the caudal ventrolateral medulla excited spontaneously active, antidromically identified neurosecretory cells, the majority of which were identified as arginine vasopressin (AVP) secreting on the basis of basal discharge patterns and responses to abrupt increases in arterial blood pressure. Administration of alpha- and beta-adrenoreceptor antagonists, by systemic or intracerebroventricular delivery of a bolus, or by direct pressure injection into the SON, did not alter neurosecretory cell responses to A1 stimulation, even when doses applied exceeded that required for blockade of excitations elicited by local application of NA. Application of the broad spectrum excitatory amino acid antagonist kynurenic acid (5-40 mM) blocked the excitatory effects of locally applied glutamate (100 microM) and transiently inhibited spontaneous activity, but failed to alter the excitatory effects of A1 region stimulation on SON cells. Identical effects were obtained with a selective kainate/quisqualate receptor antagonist. These data indicate that neurosecretory cell responses to activation of the A1 cell group are unaltered by antagonists of alpha- and beta-adrenoreceptors, or excitatory amino acid receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Oxytocin localization and function in the A1 noradrenergic cell group: ultrastructural and electrophysiological studies

Antibodies to oxytocin and noradrenalin were utilized in an immunocytochemical study of the caudal ventrolateral medulla of the rat brainstem. Noradrenalin was visualized by using antibodies to noradrenalin and by means of a silver-gold intensification of diaminobenzidine, whereas oxytocin could be demonstrated in the same section by using the diaminobenzidine precipitate as a marker. At the light microscopic level, oxytocin fibers were densely distributed around the A1 cell bodies. At the ultrastructural level, oxytocin-containing fibers were seen to terminate synaptically onto noradrenalin-containing neurons. Previous studies have shown that electrical stimulation of A1 neurons selectively activates vasopressin-secreting neurons in the supraoptic nucleus. Therefore, separate electrophysiological studies were set up, in which we observed that oxytocin infusions (100-200 pg) into the A1 area enhanced the activity of 16 out of 19 putative vasopressin-secreting neurons and elicited no response from any of 10 oxytocin-secreting neurons. This finding suggests that some of the parvicellular neurons in the paraventricular nucleus of the hypothalamus, from which the A1 neurons derive their oxytocin innervation, can activate the A1 cell group via this peptidergic neurotransmitter. One of the consequences of A1 neuronal activation is enhanced firing of hypothalamic supraoptic (and paraventricular) vasopressin-secreting neurons, and a consequent rise in plasma vasopressin.

A1 cell group mediates solitary nucleus excitation of supraoptic vasopressin cells

Stimulation of the nucleus tractus solitarius (NTS) excites putative vasopressin-secreting cells of the supraoptic nucleus (SON) via a catecholaminergic projection to hypothalamus. Despite recent evidence of a direct catecholaminergic projection from NTS to SON, we have performed single-unit recording experiments in pentobarbital sodium-anesthetized rats to investigate the possibility that NTS stimulation effects on SON vasopressin cells are indirect, being relayed via the A1 noradrenergic cell group of the caudal ventrolateral medulla. The effects of single-pulse NTS and A1 region stimulation on the activity of antidromically identified SON neurosecretory cells that had been functionally characterized as vasopressin secreting were compared. NTS stimulation excited 81% of all putative vasopressin-secreting cells tested (n = 83), with a mean onset latency of 51 +/- 1 ms. A1 region stimulation excited 76% of all cells tested and 90% of units responsive to NTS stimulation, with a mean latency of 39 +/- 1 ms. Consistent with previous work NTS stimulation excited only a minority of oxytocin cells tested (3/13), and of these two-thirds also responded to A1 stimulation. Bilateral electrolytic lesions of the A1 region abolished the effects of NTS stimulation on putative vasopressin cells. Ipsilateral A1 region injections of the inhibitory neurotransmitter gamma-amino-butyric acid reversibly blocked NTS stimulation effects on putative vasopressin cells in animals where the contralateral A1 region had already been lesioned. These results support the proposal that excitation of SON vasopressin-secreting cells after NTS stimulation is due to activation of a relay projection through the A1 noradrenergic cell group of the caudal ventrolateral medulla.

Neuropeptide Y potentiates excitation of supraoptic neurosecretory cells by noradrenaline

The effects of neuropeptide Y (NPY) and noradrenaline (NA) on the activity of rat supraoptic nucleus (SON) neurosecretory cells were examined using perfused hypothalamic slices. Bath application of either NPY (10(-9)-10(-6) M) or NA (10(-6)-10(-3) M) excited SON cells, although only NA elicited consistent, dose-dependent effects. Application of NPY at a dose having virtually no direct effects (10(-8) M) produced a 5-fold increase in SON cell responsiveness to NA at the sub-maximal response dose of 10(-5) M, but did not alter the minimum concentration of NA required to excite SON cells or increase the maximal response elicited by higher NA concentrations. The effects of NA, alone or in combination with NPY, were abolished by alpha-adrenoreceptor blockade. These data suggest that NPY has only weak direct effects on neurosecretory cells, but may have important neuromodulatory actions, significantly enhancing the excitatory effects of NA.

Control of neurosecretory vasopressin cells by noradrenergic projections of the caudal ventrolateral medulla

Activation of noradrenergic afferents arising from the A1 cell group of the caudal VLM excites neurosecretory AVP cells of both the supraoptic and paraventricular nuclei, thus stimulating the release of this potent vasoconstrictor into the circulation. Although this effect is mimicked by application of alpha 1-adrenoreceptor agonists to AVP cells, the excitatory effects of A1 afferents may not be mediated by activation of post-synaptic alpha 1-receptors. Evidence has also been obtained that the actions of A1 afferents are not dependent upon the release of excitatory amino acids or NPY, although the latter is co-stored with NA in A1 cells and potentiates the actions of low concentrations of NA on AVP cells. Although a projection to AVP and OXY neurosecretory cells from the A2 NA cell group of the NTS has been established, this projection does not appear to contribute directly to the control of SON AVP cell activity. Rather, NTS stimulation excites SON AVP cells via a relay projection through the A1 cell group. This pathway is likely to correspond to that involved in the stimulatory effects of haemorrhage and caval constriction on AVP secretion, although it is uncertain whether the effects of these particular stimuli are contingent upon unloading of arterial baroreceptors and atrial stretch receptors, as commonly presumed, or upon the activation of other receptors such as ventricular mechanoreceptors or chemoreceptors. On balance, current evidence suggests that the A1 projection is unlikely to be critically involved in mediating the effects of arterial baroreceptor, arterial chemoreceptor, or atrial stretch receptor activation on AVP cells.

Direct catecholaminergic projection from nucleus tractus solitarii to supraoptic nucleus

To determine whether the supraoptic nucleus (SON) receives a direct projection from catecholamine cells of the nucleus tractus solitarii (NTS), retrograde transport of rhodamine-tagged latex microspheres was combined with a procedure for the fluorescence histochemical visualization of catecholamines. SON tracer injections, made via transpharyngeal approach, retrogradely labelled cells at all levels of NTS, although the majority were located caudal to obex with an ipsilateral predominance. Approximately half of these cells were also identified as catecholaminergic; the relatively caudal level in the dorsomedial medulla of most of these cells suggests that they probably correspond to the A2 catecholamine cell group.

Solitary nucleus excitation of supraoptic vasopressin cells via adrenergic afferents

Recent studies confirm that the supraoptic nucleus (SON) receives a direct projection from the nucleus of the solitary tract (NTS). We have examined the effect of NTS stimulation on antidromically identified SON neurosecretory cells that were classified as arginine vasopressin (AVP) or oxytocin (OXY) secreting in accord with basal activity patterns and responsiveness to arterial baroreceptor activation. Medial NTS stimulation at the level of the obex or area postrema excited 78% (59 of 76) of putative AVP cells (onset latency 52 +/- 1 ms) but only 20% (3 of 15) of putative OXY cells. Commissural NTS stimulation did not excite AVP cells (n = 13). After complete SON catecholamine afferent disruption, achieved by local injection of 6-hydroxydopamine, AVP cells tested were unresponsive to medial NTS stimulation (12 of 13), although arterial baroreceptor activation inhibited four of four cells. These data indicate that medial NTS stimulation preferentially excites SON AVP cells and that this effect involves an adrenergic input to SON. A direct projection from the A2 catecholamine cell group of the NTS may be involved, although the long latency to excitation and the poor correspondence between effective NTS stimulation sites and the location of the A2 group within NTS raise the possibility that a relay projection, possibly through the A1 catecholamine cell group of the ventrolateral medulla, may be involved.

Effects of renal receptor activation on neurosecretory vasopressin cells

Electrical stimulation of afferent renal nerves (ARN) has been shown to excite neurosecretory vasopressin (AVP) cells of the supraoptic nucleus (SON). To investigate the sensory modality of the ARN involved, the present study examined in pentobarbital-anesthetized rats the responses of putative AVP cells to procedures intended to differentially activate renal receptor populations. Neurosecretory SON cells were identified by antidromic invasion from the neurohypophysis and classified as AVP secreting on the basis of spontaneous activity patterns and responses to arterial baroreceptor activation. Neither elevation of systemic arterial pressure (50-100 mmHg, 9 cells) following sinoaortic and cardiopulmonary afferent nerve transection nor renal venous occlusion (15 cells) altered AVP cell discharge. Renal ischemia, produced by renal arterial occlusion (50-120 s, 14 cells), and renal arterial infusion of adenosine (1-50 micrograms, 8 cells) were also without effect. However, infusions into the renal artery of bradykinin (1-3 micrograms) excited 9/15, of capsaicin (1-3 micrograms) excited 13/15, and of sodium cyanide (5-40 micrograms) excited 1/11 AVP cells examined. These data demonstrate that, in the anesthetized rat, putative neurosecretory AVP cells in the SON are responsive to activation of bradykinin- and capsaicin-sensitive renal receptors and suggest that activation of these receptors contributes to the hormonal regulation of the circulation.

Neuropharmacology of supraoptic nucleus neurons: norepinephrine and gamma-aminobutyric acid receptors

The neurosecretory neurons in the mammalian hypothalamic supraoptic nucleus receive prominent GABAergic and noradrenergic projections arising from local interneurons and the A1 cells in the ventrolateral medulla, respectively. Intracellular recordings in in vitro perfused hypothalamic explants reveal an abundance of spontaneous inhibitory postsynaptic potentials (IPSPs) and a compound IPSP after electrical stimulation in the diagonal band of Broca area. The sensitivity of both spontaneous and evoked IPSPs to intracellular chloride injection, bicuculline, and pentobarbital is consistent with a GABA-activated, chloride-mediated inhibitory synaptic input. Parallel changes in membrane voltage and conductance are present during applications of GABA and muscimol, with similar sensitivity to ionic manipulation, bicuculline, and pentobarbital. These observations contrast with the consistently excitatory effects that follow either the stimulation of A1 cells or the application of norepinephrine and alpha 1-adrenergic agonists. Norepinephrine induces membrane depolarizations and bursting activity patterns that are blocked by the selective alpha 1 antagonist prazosin. Membrane response to norepinephrine is voltage dependent and is associated with little change in conductance. GABA and norepinephrine are proposed as transmitters in the final central pathways that mediate information to supraoptic vasopressinergic neurons from peripheral baroreceptors and chemoreceptors, respectively.

Adrenoceptors in the preoptic-anterior hypothalamic area stimulate secretion of prolactin but not growth hormone in the male rat

Plasma growth hormone (GH) and prolactin concentrations were measured by radioimmunoassay in unanesthetized male rats after stereotaxic microinjection of adrenergic agents and 6-hydroxydopamine into the preoptic-anterior hypothalamic area (PO/AHA). Norepinephrine, epinephrine, isoprenaline and clonidine failed to stimulate GH, moreover, 16 nanomoles norepinephrine produced a decrease. However, these agents stimulated prolactin secretion and the mixed alpha antagonist phentolamine, administered systemically, inhibited the stimulatory action of epinephrine on prolactin secretion. GH and prolactin secretory patterns were not affected by 6-hydroxydopamine disruption of catecholamine terminals in the PO/AHA. GH responses to adrenergic agonists and the failure of 6-hydroxydopamine to affect GH secretory patterns indicate that PO/AHA norepinephrine afferents do not facilitate GH secretion. Taken in conjunction with previous studies, the results suggest that there must be an extra-hypothalamic site at which norepinephrine is stimulatory for GH. Prolactin responses suggest that alpha adrenoceptors in the PO/AHA may participate in prolactin secretion.

Opposing alpha- and beta-adrenergic mechanisms mediate dose-dependent actions of noradrenaline on supraoptic vasopressin neurones in vivo

The effects of pressure-applied noradrenaline (NA) on the activity of neurosecretory cells of the supraoptic nucleus (SON) were examined in anaesthetized male rats. Spontaneously active, antidromically identified neurosecretory cells were classified as vasopressin (VP)-secreting on the basis of activity patterns and responsiveness to baroreceptor activation. The probability of encountering VP units was enhanced by confining electrode penetrations to the caudal aspect of the SON. Application of low concentrations of NA (50-150 microM) excited 75% of putative VP neurones tested (n = 45), while very high concentrations (1-100 mM) were inhibitory (79%, n tested = 14). The excitatory effects of NA were blocked by the alpha 1 antagonist prazosin (0.1-5 microM, n = 9) and mimicked by application of the alpha 1 agonist methoxamine (300 microM-1 mM, n = 29). The alpha 2 agonist clonidine (800 microM-1 mM) also frequently elicited mild excitations (92%, n tested = 13); however, this was commonly followed by an extended period of quiescence. Neither the alpha 2 antagonist yohimbine (5 microM, n = 4) nor the beta-adrenoreceptor antagonist timolol (5-20 microM, n = 6) blocked NA-induced excitations. The inhibitory effects of high concentrations of NA, however, were blocked by the application of timolol (5-20 microM, n = 5). It is suggested that the excitatory effect of low concentrations of NA on VP neurones reflects the actions of this substance when endogenously secreted at normal sites of release within the SON.

Comparison between the actions of avian pancreatic polypeptide, neuropeptide Y and norepinephrine on the excitability of rat supraoptic vasopressin neurons

Effects of pressure-applied avian pancreatic polypeptide (APP), neuropeptide Y (NPY) and norepinephrine (NE) on the activity of putative vasopressin-synthesizing neurosecretory cells of the supraoptic nucleus were studied in pentobarbitone-anesthetized male rats. APP (17-170 microM) excited 80% (20/25), NPY (20 microM-2 mM) excited 23% (6/26) and NE (100-200 microM) excited 76% (26/34) of cells tested; no depressant actions were observed. However, simultaneously applied APP appeared to reduce the NE-evoked excitation in 4/4 cells tested. These data indicate that an endogenous pancreatic polypeptide-like peptide may mimic the excitatory actions of NE on supraoptic vasopressin-synthesizing neurosecretory cells but only at high concentrations. These peptides do not potentiate but rather appear to interfere with NE's excitatory effects.

Afferent renal nerve stimulation excites supraoptic vasopressin neurons

Single-unit recording experiments were performed in pentobarbital-anesthetized rats to investigate the effects of afferent renal nerve (ARN) stimulation on the activity of neurosecretory vasopressin cells of the supraoptic nucleus (SON). Neurosecretory SON cells were identified by antidromic invasion from the neurohypophysis and classified either as vasopressin (AVP) or oxytocin (OXY) secreting on the basis of their spontaneous activity patterns and response to activation of arterial baroreceptors. Fifty-three spontaneously active units were identified bilaterally in the SON: 40 putative AVP and 13 putative OXY neurons. Most putative AVP neurons (14/14 contralateral, 18/26 ipsilateral) were excited by ARN stimulation (mean onset latency 189 +/- 5 ms, mean response duration 237 +/- 17 ms). In contrast, ARN stimulation had no effect on the firing frequency of the 13 putative OXY neurons. These data indicate that sensory information originating in the kidney selectively alters the activity of SON AVP neurons and suggest that afferent information from the kidney is important in the coordination of neural and hormonal activity concerned with body fluid balance and the regulation of arterial pressure.

Electrophysiology of the subfornical organ and its hypothalamic connections--an in-vivo study in the rat

The connections of the subfornical organ to a defined population of hypothalamic neurons have been explored with extracellular recording methods in the rat. Electrical stimulation in the subfornical organ has a predominantly excitatory action on a majority of oxytocin and vasopressin-secreting neurosecretory neurons in the supraoptic and paraventricular nuclei. Subfornical organ stimulation also enhances the excitability of paraventricular nucleus neurons that project to the median eminence, and to the dorsomedial medulla. These observations provide initial evidence of functional connectivity of subfornical organ neurons with other hypothalamic cells that are engaged in central regulation of pituitary secretions and autonomic activities.

Noradrenergic afferents facilitate the activity of tuberoinfundibular neurons of the hypothalamic paraventricular nucleus

The role of ascending noradrenergic projections of medullary origin in regulating the activity of tuberoinfundibular neurons of the hypothalamic paraventricular nucleus (PVN) was examined in pentobarbital-anesthetized male Sprague-Dawley rats. Discrete electrical stimulation of either the A1 or the A2 noradrenaline cell group areas of the caudal medulla enhanced the probability of firing in a substantial proportion of antidromically identified tuberoinfundibular PVN cells tested. Notably, no inhibitory effects were observed. Destruction of the PVN noradrenergic terminal plexus by local application of the neurotoxin 6-hydroxydopamine 1 day prior to electrophysiological experiments abolished the effects of both A1 and A2 stimulation. These findings indicate that noradrenergic afferents can exert a facilitatory influence on the activity of a population of tuberoinfundibular PVN neurons, thus supporting earlier suggestions that central noradrenergic structures can enhance the release of certain anterior pituitary hormones.

Subfornical organ stimulation excites paraventricular neurons projecting to dorsal medulla

Electrical stimulation in the subfornical organ (SFO) of pentobarbital-anesthetized Sprague-Dawley rats was noted to influence the excitability of paraventricular nucleus (PVN) neurons antidromically identified as projecting to the dorsomedial medulla. Extracellular recordings indicated that 60% (n = 34) of these caudally projecting PVN neurons increased activity in response to single shock stimuli delivered to the SFO. Short-latency [30.0 +/- 2.7 (SE) ms] and long-latency (162.5 +/- 32.5 ms) responses were observed. The remaining neurons were either unaffected (38%) or inhibited (2%) by SFO stimulation. These data suggest functional connectivity between the SFO and the dorsomedial medulla. It is proposed that such a pathway may mediate pressor responses observed to follow electrical stimulation in the SFO.

Subfornical organ efferents influence the excitability of neurohypophyseal and tuberoinfundibular paraventricular nucleus neurons in the rat

Electrical stimulation in the subfornical organ (SFO) alters the excitability of antidromically identified paraventricular nucleus neurons. Extracellular recordings demonstrate that the dominant effect of single stimuli delivered to the SFO on neurohypophyseal oxytocin and vasopressin containing neurons is an increase in excitability. In 35% of cells tested, this excitation showed a long latency (44.3 +/- 3.4 ms) prolonged duration (208.7 +/- 23.5 ms), while in 16% of the neurons the excitation observed may be described as short latency (24.7 +/- 1.8 ms) short duration (11.6 +/- 1.4 ms). Of the remaining cells antidromically identified as projecting to the posterior pituitary, 12% showed initial decreases in excitability following SFO stimulation while the remaining 37% were unaffected. Evidence is presented demonstrating that stimulation in the region of the SFO results in short latency (27.9 +/- 2.4 ms) short duration (7.8 +/- 0.7 ms) increases in excitability in 22% of antidromically identified PVN tuberoinfundibular neurons tested. These data provide electrophysiological evidence in support of the proposed role of the subfornical organ in the control of posterior and anterior pituitary function.

Facilitatory influence of noradrenergic afferents on the excitability of rat paraventricular nucleus neurosecretory cells

The role of the A1 and A2 noradrenergic cell groups of the caudal medulla in regulating the activity of paraventricular nucleus neurosecretory cells was examined with electrophysiological methods in anaesthetized male Sprague-Dawley rats. Antidromically identified neurosecretory cells were classified as vasopressin or oxytocin secreting on the basis of spontaneous firing patterns and responsivity to baroreceptor activation. The effect on cell firing of single pulses (25-200 microA) delivered to either the A1 or A2 cell group areas was then examined using peri-stimulus histograms. Stimulation of the A1 region enhanced the activity of 78% of putative vasopressin-secreting neurones tested (n = 18), but failed to affect the activity of the majority (73%) of putative oxytocin-secreting units (n = 15). A2 stimulation enhanced the firing rate of both putative vasopressin- (60%, total n = 14) and putative oxytocin-secreting (70%, total n = 27) neurones. Destruction of the paraventricular nucleus catecholamine terminal plexus by pre-treatment with the neurotoxin 6-hydroxydopamine abolished the facilitatory effects of both A1 and A2 stimulation. These findings suggest that noradrenergic afferents of medullary origin facilitate the activity of paraventricular nucleus neurosecretory cells. The role of the projection from the A1 cell group appears to differ from that of the A2 group, however, in that its effects are specific to putative vasopressin-secreting units.

Electrophysiological evidence that noradrenergic afferents selectively facilitate the activity of supraoptic vasopressin neurons

The functional role of the ascending projection from A1 noradrenergic neurons of the caudal ventrolateral medulla to the supraoptic nucleus of the hypothalamus was investigated by examining the effects of electrical stimulation of the A1 region on the activity of supraoptic neurons deemed to be vasopressinergic or oxytocinergic on the basis of basal firing patterns and responsivity to baroreceptor activation. A1 stimulation enhanced the activity of all putative vasopressin-secreting supraoptic neurons tested. This effect appeared to be selective in that no putative oxytocin-secreting neurons were excited by A1 stimulation. Destruction of the supraoptic noradrenergic terminal plexus by local application of the neurotoxin 6-hydroxydopamine abolished the facilitatory effects of A1 stimulation but did not noticeably alter basal activity patterns, nor the influence of baroreceptor inhibitory pathways. These findings suggest a facilitatory role for noradrenergic afferents in regulating the activity of neurohypophysially-projecting vasopressin neurons of the supraoptic nucleus.

Connections of hypothalamic paraventricular neurons with the dorsal medial thalamus and neurohypophysis: an electrophysiological study in the rat

Connections between the hypothalamic paraventricular nucleus (PVN) and thalmic paraventricular nucleus ( PVT ) were examined using electrophysiological methods. Efferent projections of adjacent PVN cells were defined on the basis of antidromic activation from either PVT (n = 12) or neurohypophyseal (n = 38) stimulation; antidromic activation from both sites (n = 3) suggested that some PVN cells project both to the PVT and to the neurohypophysis. PVT stimulation evoked only weak orthodromic responses from 21% of PVN neurohypophyseal neurons, whereas short latency, high probability orthodromic responses were observed from 43% of PVN non-neurosecretory neurons. These data indicate reciprocal PVN- PVT connections and suggest that PVT afferents preferentially innervate non-neurosecretory PVN cells.

CNS regulation of reproduction: peptidergic mechanisms

The coordinated activities of several networks of peptidergic neurons contribute to the overall neural regulation of reproduction and associated behaviours. Key elements are rostral hypothalamic cells that synthesize and release LHRH from their median eminence nerve terminals, subject to modulation by a variety of endogenous transmitters and neuropeptides including VIP, CCK, opioids and somatostatin. In the CNS, LHRH may also participate in intercellular communication to facilitate the expression of estrogen-sensitive sexual behavior. Vasopressin and oxytocin also appear to modulate maternal and reproductive behavior. In addition, 'oxytocinergic' neurons recorded during lactation and milk ejection display unique bursting activity patterns deemed important for efficient release of hormone. However, endogenous opioid peptides appear able to dissociate this stimulus-secretion coupling mechanism, possibly through interference with a calcium sensitive component in nerve terminals.