Midline thalamic reuniens lesions improve executive behaviors

The role of the thalamus in complex cognitive behavior is a topic of increasing interest. Here we demonstrate that lesions of the nucleus reuniens (NRe), a midline thalamic nucleus interconnected with both hippocampal and prefrontal circuitry, lead to enhancement of executive behaviors typically associated with the prefrontal cortex. Rats were tested on four behavioral tasks: (1) the combined attention-memory (CAM) task, which simultaneously assessed attention to a visual target and memory for that target over a variable delay; (2) spatial memory using a radial arm maze, (3) discrimination and reversal learning using a touchscreen operant platform, and (4) decision-making with delayed outcomes. Following NRe lesions, the animals became more efficient in their performance, responding with shorter reaction times but also less impulsively than controls. This change, combined with a decrease in perseverative responses, led to focused attention in the CAM task and accelerated learning in the visual discrimination task. There were no observed changes in tasks involving either spatial memory or value-based decision making. These data complement ongoing efforts to understand the role of midline thalamic structures in human cognition, including the development of thalamic stimulation as a therapeutic strategy for acquired cognitive disabilities (Schiff, 2008; Mair et al., 2011), and point to the NRe as a potential target for clinical intervention.

Non-opioid antinociceptive effects of supraspinal histogranin and related peptides: possible involvement of central dopamine D(2) receptor

The antinociceptive effects of intracerebroventricular (ICV) administration of histogranin (HN) and related peptides were assessed in the mouse writhing and tail-flick assays. In the writhing test, the peptides displayed dose-dependent analgesic effects with an AD(50) of 23.9 nmol/mouse for HN and the following order for other peptides: HN-(7-15)<histone H4-(86-100) approximately HN approximately HN-(7-10)<[Ser(1)]HN<osteogenic growth peptide (OGP) approximately HN-(1-10). HN-(6-9) and HN-(8-10) did not show any significant analgesic activity at 50 nmol/mouse. The importance of the C- and N-terminal amino acids in the analgesic activity of the peptides was demonstrated by the prolonged effects of HN and [Ser(1)]HN ( approximately 30 min) compared with those of HN fragments (HN-(7-15), HN-(1-10) and HN-(7-10): 5-10 min). The analgesic activity of [Ser(1)]HN (50 nmol/mouse) was not affected by the coadministration of opioid (naloxone, 1 nmol/mouse), NMDA (CPP, 0.3 and MK-801, 0.3 nmol/mouse) and D(1) (SCH-23390, 0.5 nmol/mouse) receptor antagonists, but it was significantly antagonized by the coinjection of the D(2) receptor antagonist raclopride (0.5 nmol/mouse). In the mouse tail-flick assay, HN and related peptides (50 nmol/mouse) also showed significant analgesic activity (15-35% MPE). The analgesic effect of [Ser(1)]HN was dose-dependent and, at 75 nmol/mouse, lasted for up to 45 min, and was partially blocked by the coadministration of raclopride (1 nmol/mouse), but not naloxone (2 nmol/mouse). In the mouse rotarod assay, relative high doses (75-100 nmol/mouse) of HN and related peptides did not significantly affect motor coordination. These results indicate that supraspinal administration of HN and related peptides induce significant non-opioid analgesic effects devoid of motor activity by a mechanism that involves the participation of central dopamine D(2) receptors.

Nonopioid motor effects of dynorphin A and related peptides: structure dependence and role of the N-methyl-D-aspartate receptor

Dynorphin (Dyn) A and related opioid and nonopioid peptides were tested for their ability to produce motor effects in mice. Central (intracerebroventricular) administration of Dyn A in mice produced marked motor effects characterized by wild running, jumping, circling and/or barrel rolling with an ED50 value of 14.32 (95% confidence limits, 10.09-20.32) nmol/mouse. The order of potency of the various Dyn A-related peptides and fragments in producing motor effects was Dyn A approximately Dyn A-(1-13) > [Ala1]Dyn A-(1-13) approximately Dyn A-(2-13) > alpha-Neo-End > Dyn A-(1-8) approximately Dyn B approximately Dyn A-(2-8) >>> Dyn A-(3-8). Dyn A-(1- 5) (or Leu-Enk) and Dyn A-(6-10) displayed no motor effect at doses up to 100 nmol/mouse. The potencies of Dyn A and Dyn A-(2-13) were not affected by preadministration of naloxone (5 mg/kg s.c.), but the motor effects of Dyn A-(1-13) (20 nmol/mouse i.c.v.) were significantly reduced by coadministration of low doses (0.2-0.6 nmol/mouse) of the N-methyl-D-aspartate (NMDA) receptor antagonists dextrorphan, MK-801 and CPP. Dyn A was also a potent inhibitor of the binding of the phencyclidine receptor ligand, [3H]MK-801, to rat brain membranes, with a Ki value of 0.41 microM. However, the order of potency of the various Dyn A-related peptides and fragments in inhibiting [3H]MK-801 binding did not correlate with their ability to produce motor effects. On the other hand, Dyn A and related peptides produced a significant potentiation of the binding of the competitive NMDA antagonist [3H]CGP-39653 to rat brain membranes, an effect that correlated well (r = 0.91) with their potency in producing motor effects. These results indicate that the nonopioid motor effects of Dyn A and related peptides are structure dependent, with Dyn A-(2-8) being the minimal core peptide for motor activity. In addition, these effects most likely involve the participation of the excitatory amino acid binding domain on the NMDA receptor complex.

Synthesis and biological activity of histogranin and related peptides

Histogranin (HN) was first isolated from bovine adrenal medulla and shown to be a pentadecapeptide displaying N-methyl-D-aspartate (NMDA) receptor antagonist activity. To determine the active pharmacophore of HN, fragments of the peptide were synthesized and their structure-activity relationships studied by measuring their ability to displace the binding of [125I][Ser1]HN to rat brain membrane preparations and to block NMDA-induced convulsions in mice. In the binding assay, only the full length peptide HN and HN(1-10) displayed a high affinity (Ki of 72 and 162 nM, respectively). All other tested fragments with deletions at the N- and (or) C-terminals of the molecule showed large (16- to 2500-fold) decreases in potency. The least active peptide fragment tested was HN(6-10) (Ki of 164 microM). In vivo, HN and HN(2-15) (100 nmol; i.c.v.) produced 94 and 40% protection against NMDA-induced convulsions in mice, respectively. None of the other peptide fragments displayed significant anticonvulsant activity. The protective activity of HN (60 and 100 nmol) was markedly antagonized by coadministration of HN(1-10) (100 nmol). The results indicate that the in vivo anti-NMDA and in vitro binding activities of HN and related peptides, with the exception of HN(1-10), depend upon the integrity of the molecule. On the other hand, the high affinity of HN(1-10) for HN binding sites correlates well with its antagonist effects towards the activity of the parent peptide.

Acetylcholine, melatonin, and potassium depolarization stimulate release of luteinizing hormone-releasing hormone from rat hypothalamus in vitro

We have examined the release of radioimmunoassayable luteinizing hormone-releasing hormone (LH-RH) from fragments of rat medial basal hypothalamus. These fragments were cultured overnight in medium containing serum and then preincubated in groups of three for 10 min in medium resembling cerebrospinal fluid in its electrolyte constituents and containing bacitracin. This was followed by 30-min incubation periods during which some of the hypothalami were exposed to test substances. Potassium depolarization, effected by the addition of 56 mM potassium chloride to the incubation medium, caused a marked stimulation in LH-RH release, but only in the presence of calcium. Acetylcholine at 10 nM and the parasympathomimetic anticholinesterase agent neostigmine at 1 microM markedly stimulated LH-RH release. Hexamethonium, a nicotinic antagonist, at 1 microM abolished the acetylcholine-induced increment in LH-RH release. Melatonin, a pineal indolamine, caused significant stimulation of LH-RH release at a concentration as low as 10 nM. Bacitracin (21 microM) was employed in all these experiments. It had no effect on LH-RH release but did prevent the degradation of LH-RH in this system. We conclude that acetylcholine and melatonin are capable of inducing LH-RH release from the rat medial basal hypothalamus. These actions may account for some of the progonadotropic properties previously ascribed to these agents.

Somatostatin release from rat hypothalamus in vitro: effects of melatonin and serotonin

The release of immunoreactive somatostatin (SRIF) from explants of rat medial basal hypothalamus, which were maintained in culture for 24 h, was quantitated by a sensitive RIA. Validity of the specific SRIF assay has been previously established by chromatographic criteria using gel and high pressure systems and by the demonstration of immunological parallelism. After 24 h of culture in medium containing heat-inactivated fetal calf serum, hypothalamic fragments were incubated in serum-free medium, and the release of immunoreactive SRIF was quantitated. Melatonin at concentrations of 10(--8) and 10(--7) M stimulated SRIF release, and no significant increases were observed at concentrations of 10(--9) M or less or at concentrations of 10(--6) M or greater. Serotonin oxalate at concentrations of 10(--8)--10(--5) M significantly inhibited SRIF release. The serotonin antagonist cyproheptadine at a concentration of 10(--5)M had no effect on basal SRIF release but abolished the inhibitory effect of 10(--7) M serotonin. Finally, when hypothalami were incubated with melatonin and serotonin, each at 10(--7) M, SRIF release was unchanged compared to control values. The results suggest that the brain indoleamines, melatonin and serotonin, may modulate GH secretion by effects on SRIF release at a hypothalamic level.